Fundamental questions

In astrophysics, the questions of galaxy formation and evolution are:

* How, from a homogeneous universe, did we obtain the very heterogeneous one we live in?
* How did galaxies form?
* How do galaxies change over time?

After the Big Bang, the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Microwave Background, the fluctuations of which are less than one part in one hundred thousand.

The most accepted view today is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations. The primordial fluctuations caused gas to be attracted to areas of denser material, hierarchically forming superclusters, clusters, galaxies, star clusters and stars. One consequence of this model is that the location of galaxies indicates areas of higher density of the early universe. Hence the distribution of galaxies is closely related to the physics of the early universe.

The observed components of galaxies (including our own Milky Way) that must be explained in, or at least not be at odds with, a theory of galactic evolution, include:

* the stellar disk is quite thin, dense, and rotates
* the stellar halo is very large, sparse, and does not rotate (or has perhaps even a slight retrograde rotation), with no apparent substructure
* halo stars are typically much older and have much lower metallicities than disk stars (there is a correlation, but there is no absolute connection between these data)
* some astronomers have identified an intermediate population of stars, variously called the "metal weak thick disk", the "intermediate population II", et al. If these are indeed a distinct population, they would be described as metal-poor (but not as poor as the halo stars), old (but not as old as the halo stars), and orbiting very near the disk, in a sort of "puffed-up", thicker disk shape.
* globular clusters are typically old and metal-poor as well, but there are a few which are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods)!
* in each globular cluster, all the stars were born at virtually the same time (except for a few globulars that show multiple epochs of star formation)
* globular clusters with smaller orbits (closer to the galactic center) have orbits which are somewhat flatter (less inclined to the disk), and less eccentric (more circular), while those further out have orbits in all inclinations, and tend to be more eccentric.
* High Velocity Clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (these would be the necessary source of a gas disk from which the disk stars formed).

On the 11th July 2007, using the 10 metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasedena and his team found six star forming galaxies about 13.2 billion light years away and therefore created when the universe was only 500 million years old [1].

Recent research as a part of the Galactic Zoo project suggests that there is an unexplained parity violation, with a greater proportion of the galaxies rotating in an anticlockwise direction when seen from the Earth[2].

Research outcomes

As of 2007, there is no direct evidence of extraterrestrial life.[16] Although examination of the ALH84001 meteorites, which were recovered in Antarctica and are thought to have originated from the planet Mars have provided what some scientists suggested to be microfossils of extraterrestrial life, the interpretation is disputed.[17] In 2004, the spectral signature of methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express probe. Methane is predicted to have a relatively short half-life in the Martian atmosphere, so the gas must be actively replenished. Since one possible source, active volcanism, has thus far not been detected on Mars, this has led scientists to speculate that the source could be (microbial) life - as terrestrial methanogens are known to produce methane as a metabolic byproduct.

Missions to other planetary bodies, such as Mars Science Laboratory, ExoMars, Beagle 2: Evolution to Mars, the Cassini probe to Saturn's moon Titan), and the "Ice Clipper" mission to Jupiter's moon Europa hope to further explore the possibilities of life on other planets in our solar system.

Efforts to answer secondary questions, such as the abundance of potentially habitable planets in habitable zones and chemical precursors, have had much success. Numerous extrasolar planets have been detected using the "wobble method" and transit method, showing that planets around other stars are more diverse than previously postulated. The first Earth-like extrasolar planet to be discovered within its star's habitable zone is Gliese 581 c, which was found using radial velocity.[18]

Due to technological limitations, most of the planets so far discovered have been hot gas giants, thought to be inhospitable to any life. It is possible that some of these planets may have moons with solid surfaces or oceans that are more hospitable. It is not yet known whether our solar system, with rocky, metal-rich inner planets ideal for life, is of an aberrant composition. Improved detection methods and increased observing time will undoubtedly discover more planetary systems, and possibly some more like ours. For example, NASA's Kepler Mission seeks to discover Earth-sized planets around other stars, by measuring minute changes in the star's light curve as the planet passes between the star and the spacecraft. Research into the environmental limits of life and the workings of extreme ecosystems is also ongoing, enabling researchers to predict what planetary environments might be most likely to harbor life.

Progress in infrared astronomy and submillimeter astronomy has revealed the constituents of other star systems. Infrared searches have detected belts of dust and asteroids around distant stars, underpinning the formation of planets. Some infrared images purportedly contain direct images of planets, though this is disputed. Infrared and submillimeter spectroscopy has identified a growing number of chemicals around stars which underpin the origin or maintenance of life.

Overview Astrobiology

Although astrobiology is an emerging field, and still a developing subject, the question of whether life exists elsewhere in the universe is a verifiable hypothesis and thus a valid line of scientific inquiry. Astrobiology is a multidisciplinary field utilizing physics, biology, and geology as well as philosophy to speculate about the nature of life on other worlds. One commentator on the field, planetary scientist David Grinspoon, calls astrobiology a field of natural philosophy, grounding speculation on the unknown in known scientific theory (Grinspoon 2003). Since we have only one example of a planet with life (the Earth), most of the work is speculative and based on current understanding of physics, biochemistry, and biology.[11][12]

Though once considered outside the mainstream of scientific inquiry, astrobiology has become a formalized field of study. NASA now hosts an Astrobiology Institute.[13] Additionally, a growing number of universities in the United States (e.g., University of Arizona, Penn State University, and University of Washington) Canada, Britain, and Ireland now offer graduate degree programs in astrobiology.

A particular focus of current astrobiology research is the search for life on Mars.[14] There is a growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface; water is considered to be an essential precursor to the development of life, although this has not been conclusively proven.[15] At the present, the creation of theory to inform and support the exploratory search for life may be considered astrobiology's most concrete practical application.

Missions specifically designed to search for life include the Viking program and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive and Beagle 2 failed to transmit from the surface and is assumed to have crashed. A future mission with a strong astrobiology role would have been the Jupiter Icy Moons Orbiter, designed to study the frozen moons of Jupiter—some of which may have liquid water—had it not been canceled. In 2009, NASA plans to launch the Mars Science Laboratory Rover which will continue the search for past or present life on Mars using a suite of scientific instruments.

Astrobiology

This article is about the scientific discipline. For the journal, see Astrobiology (journal).

Astrobiology (from Greek: ἀστρο, astro, "constellation"; βίος, bios, "life"; and λόγος, logos, "knowledge") is the interdisciplinary study of life in space, combining aspects of astronomy, biology and geology.[2] It is focused primarily on the study of the origin, distribution and evolution of life. It is also known as exobiology (from Greek: έξω, exo, "outside").[3][4][5] The term "Xenobiology" has been used as well, but this is technically incorrect because its terminology means "biology of the foreigners".[6]

Some major astrobiological research topics include:[2][7][8][9] What is life? How did life arise on Earth? What kind of environments can life tolerate? How can we determine if life exists on other planets? How often can we expect to find complex life? What will life consist of on other planets? Will it be DNA/Carbon based or based on something else?[1] What will it look like?

Amateur astronomy

Collectively, amateur astronomers observe a variety of celestial objects and phenomena sometimes with equipment that they build themselves. Common targets of amateur astronomers include the Moon, planets, stars, comets, meteor showers, and a variety of deep-sky objects such as star clusters, galaxies, and nebulae. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Many amateurs like to specialize in the observation of particular objects, types of objects, or types of events which interest them.[44][45]

Most amateurs work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum. This includes the use of infrared filters on conventional telescopes, and also the use of radio telescopes. The pioneer of amateur radio astronomy was Karl Jansky who started observing the sky at radio wavelengths in the 1930s. A number of amateur astronomers use either homemade telescopes or use radio telescopes which were originally built for astronomy research but which are now available to amateurs (e.g. the One-Mile Telescope).[46][47]

Amateur astronomers continue to make scientific contributions to the field of astronomy. Indeed, it is one of the few scientific disciplines where amateurs can still make significant contributions. Amateurs can make occultation measurements that are used to refine the orbits of minor planets. They can also discover comets, and perform regular observations of variable stars. Improvements in digital technology have allowed amateurs to make impressive advances in the field of astrophotography.[48][49][50]

Major questions in astronomy

Although the scientific discipline of astronomy has made tremendous strides in understanding the nature of the universe and its contents, there remain some important unanswered questions. Answers to these may require the construction of new ground- and space-based instruments, and possibly new developments in theoretical and experimental physics.

* What is the origin of the stellar mass spectrum? That is, why do astronomers observe the same distribution of stellar masses—the initial mass function—apparently regardless of the initial conditions?[51] A deeper understanding of the formation of stars and planets is needed.
* Is there other life in the Universe? Especially, is there other intelligent life? If so, what is the explanation for the Fermi paradox? The existence of life elsewhere has important scientific and philosophical implications.[52][53]
* What is the nature of dark matter and dark energy? These dominate the evolution and fate of the cosmos, yet we are still uncertain about their true natures.[54]
* Why did the universe come to be? Why, for example, are the physical constants so finely tuned that they permit the existence of life? Could they be the result of cosmological natural selection? What caused the cosmic inflation that produced our homogeneous universe?[55]
* What will be the ultimate fate of the universe?[56]

Interdisciplinary studies

Astronomy and astrophysics have developed significant interdisciplinary links with other major scientific fields. These include:

* Astrobiology: the study of the advent and evolution of biological systems in the universe.
* Archaeoastronomy: the study of ancient or traditional astronomies in their cultural context, utilizing archaeological and anthropological evidence.
* Astrochemistry: the study of the chemicals found in space, usually in molecular clouds, and their formation, interaction and destruction. It represents an overlap of the disciplines of astronomy and chemistry.
* Cosmochemistry: the study of the chemicals found within the Solar System, including the origins of the elements and variations in the isotope ratios.

Cosmology

Cosmology (from the Greek κοσμος "world, universe" and λογος "word, study") could be considered the study of the universe as a whole.

Observations of the large-scale structure of the universe, a branch known as physical cosmology, have provided a deep understanding of the formation and evolution of the cosmos. Fundamental to modern cosmology is the well-accepted theory of the big bang, wherein our universe began at a single point in time, and thereafter expanded over the course of 13.7 Gyr to its present condition. The concept of the big bang can be traced back to the discovery of the microwave background radiation in 1965.

In the course of this expansion, the universe underwent several evolutionary stages. In the very early moments, it is theorized that the universe experienced a very rapid cosmic inflation, which homogenized the starting conditions. Thereafter, nucleosynthesis produced the elemental abundance of the early universe. (See also nucleocosmochronology.)

When the first atoms formed, space became transparent to radiation, releasing the energy viewed today as the microwave background radiation. The expanding universe then underwent a Dark Age due to the lack of stellar energy sources.[41]

A hierarchical structure of matter began to form from minute variations in the mass density. Matter accumulated in the densest regions, forming clouds of gas and the earliest stars. These massive stars triggered the reionization process and are believed to have created many of the heavy elements in the early universe.

Gravitational aggregations clustered into filaments, leaving voids in the gaps. Gradually, organizations of gas and dust merged to form the first primitive galaxies. Over time, these pulled in more matter, and were often organized into groups and clusters of galaxies, then into larger-scale superclusters.[42]

Fundamental to the structure of the universe is the existence of dark matter and dark energy. These are now thought to be the dominant components, forming 96% of the density of the universe. For this reason, much effort is expended in trying to understand the physics of these components.[43]

Extragalactic astronomy

The study of objects outside of our galaxy is a branch of astronomy concerned with the formation and evolution of Galaxies; their morphology and classification; and the examination of active galaxies, and the groups and clusters of galaxies. The latter is important for the understanding of the large-scale structure of the cosmos.

Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies.[38]

As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction. These galaxies contain little or no interstellar dust; few star-forming regions; and generally older stars. Elliptical galaxies are more commonly found at the core of galactic clusters, and may be formed through mergers of large galaxies.

A spiral galaxy is organized into a flat, rotating disk, usually with a prominent bulge or bar at the center, and trailing bright arms that spiral outward. The arms are dusty regions of star formation where massive young stars produce a blue tint. Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and the Andromeda Galaxy are spiral galaxies.

Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.

An active galaxy is a formation that is emitting a significant amount of its energy from a source other than stars, dust and gas; and is powered by a compact region at the core, usually thought to be a super-massive black hole that is emitting radiation from in-falling material.

A radio galaxy is an active galaxy that is very luminous in the radio portion of the spectrum, and is emitting immense plumes or lobes of gas. Active galaxies that emit high-energy radiation include Seyfert galaxies, Quasars, and Blazars. Quasars are believed to be the most consistently luminous objects in the known universe.[39]

The large-scale structure of the cosmos is represented by groups and clusters of galaxies. This structure is organized in a hierarchy of groupings, with the largest being the superclusters. The collective matter is formed into filaments and walls, leaving large voids in between.[40]

Galactic astronomy

Our solar system orbits within the Milky Way, a barred spiral galaxy that is a prominent member of the Local Group of galaxies. It is a rotating mass of gas, dust, stars and other objects, held together by mutual gravitational attraction. As the Earth is located within the dusty outer arms, there are large portions of the Milky Way that are obscured from view.

In the center of the Milky Way is the core, a bar-shaped bulge with what is believed to be a supermassive black hole at the center. This is surrounded by four primary arms that spiral from the core. This is a region of active star formation that contains many younger, population II stars. The disk is surrounded by a spheroid halo of older, population I stars, as well as relatively dense concentrations of stars known as globular clusters.^[34][35]

Between the stars lies the interstellar medium, a region of sparse matter. In the densest regions, molecular clouds of molecular hydrogen and other elements create star-forming regions. These begin as irregular dark nebulae, which concentrate and collapse (in volumes determined by the Jeans length) to form compact protostars.^[36]

As the more massive stars appear, they transform the cloud into an H II region of glowing gas and plasma. The stellar wind and supernova explosions from these stars eventually serve to disperse the cloud, often leaving behind one or more young open clusters of stars. These clusters gradually disperse, and the stars join the population of the Milky Way.

Kinematic studies of matter in the Milky Way and other galaxies have demonstrated that there is more mass than can be accounted for by visible matter. A dark matter halo appears to dominate the mass, although the nature of this dark matter remains undetermined.^[37]

Stellar astronomy

The study of stars and stellar evolution is fundamental to our understanding of the universe. The astrophysics of stars has been determined through observation and theoretical understanding; and from computer simulations of the interior.

Star formation occurs in dense regions of dust and gas, known as giant molecular clouds. When destabilized, cloud fragments can collapse under the influence of gravity, to form a protostar. A sufficiently dense, and hot, core region will trigger nuclear fusion, thus creating a main-sequence star.[32]

Almost all elements heavier than hydrogen and helium were created inside the cores of stars.

The characteristics of the resulting star depend primarily upon its starting mass. The more massive the star, the greater its luminosity, and the more rapidly it expends the hydrogen fuel in its core. Over time, this hydrogen fuel is completely converted into helium, and the star begins to evolve. The fusion of helium requires a higher core temperature, so that the star both expands in size, and increases in core density. The resulting red giant enjoys a brief life span, before the helium fuel is in turn consumed. Very massive stars can also undergo a series of decreasing evolutionary phases, as they fuse increasingly heavier elements.

The final fate of the star depends on its mass, with stars of mass greater than about eight times the Sun becoming core collapse supernovae; while smaller stars form planetary nebulae, and evolve into white dwarfs. The remnant of a supernova is a dense neutron star, or, if the stellar mass was at least three times that of the Sun, a black hole.[33] Close binary stars can follow more complex evolutionary paths, such as mass transfer onto a white dwarf companion that can potentially cause a supernova. Planetary nebulae and supernovae are necessary for the distribution of metals to the interstellar medium; without them, all new stars (and their planetary systems) would be formed from hydrogen and helium alone.

Planetary science

This astronomical field examines the assemblage of planets, moons, dwarf planets, comets, asteroids, and other bodies orbiting the Sun, as well as extrasolar planets. The solar system has been relatively well-studied, initially through telescopes and then later by spacecraft. This has provided a good overall understanding of the formation and evolution of this planetary system, although many new discoveries are still being made.[27]

The solar system is subdivided into the inner planets, the asteroid belt, and the outer planets. The inner terrestrial planets consist of Mercury, Venus, Earth, and Mars. The outer gas giant planets are Jupiter, Saturn, Uranus and Neptune.[28] Beyond Neptune lie the Kuiper Belt, and finally the Oort Cloud, which may extend as far as a light-year.

The planets were formed by a protoplanetary disk that surrounded the early Sun. Through a process that included gravitational attraction, collision, and accretion, the disk formed clumps of matter that, with time, became protoplanets. The radiation pressure of the solar wind then expelled most of the unaccreted matter, and only those planets with sufficient mass retained their gaseous atmosphere. The planets continued to sweep up, or eject, the remaining matter during a period of intense bombardment, evidenced by the many impact craters on the Moon. During this period, some of the protoplanets may have collided, the leading hypothesis for how the Moon was formed.[29]

Once a planet reaches sufficient mass, the materials with different densities segregate within, during planetary differentiation. This process can form a stony or metallic core, surrounded by a mantle and an outer surface. The core may include solid and liquid regions, and some planetary cores generate their own magnetic field, which can protect their atmospheres from solar wind stripping.[30]

A planet or moon's interior heat is produced from the collisions that created the body, radioactive materials (e.g. uranium, thorium, and 26Al), or tidal heating. Some planets and moons accumulate enough heat to drive geologic processes such as volcanism and tectonics. Those that accumulate or retain an atmosphere can also undergo surface erosion from wind or water. Smaller bodies, without tidal heating, cool more quickly; and their geological activity ceases with the exception of impact cratering.[31]

Solar astronomy

The most frequently studied star is the Sun, a typical main-sequence dwarf star of stellar class G2 V, and about 4.6 Gyr in age. The Sun is not considered a variable star, but it does undergo periodic changes in activity known as the sunspot cycle. This is an 11-year fluctuation in sunspot numbers. Sunspots are regions of lower-than- average temperatures that are associated with intense magnetic activity.[23]
An ultraviolet image of the Sun's active photosphere as viewed by the TRACE space telescope. NASA photo.

The Sun has steadily increased in luminosity over the course of its life, increasing by 40% since it first became a main-sequence star. The Sun has also undergone periodic changes in luminosity that can have a significant impact on the Earth.[24] The Maunder minimum, for example, is believed to have caused the Little Ice Age phenomenon during the Middle Ages.[25]

The visible outer surface of the Sun is called the photosphere. Above this layer is a thin region known as the chromosphere. This is surrounded by a transition region of rapidly increasing temperatures, then by the super-heated corona.

At the center of the Sun is the core region, a volume of sufficient temperature and pressure for nuclear fusion to occur. Above the core is the radiation zone, where the plasma conveys the energy flux by means of radiation. The outer layers form a convection zone where the gas material transports energy primarily through physical displacement of the gas. It is believed that this convection zone creates the magnetic activity that generates sun spots.[23]

A solar wind of plasma particles constantly streams outward from the Sun until it reaches the heliopause. This solar wind interacts with the magnetosphere of the Earth to create the Van Allen radiation belts, as well as the aurora where the lines of the Earth's magnetic field descend into the atmosphere.[26]

Theoretical astronomy

Theoretical astronomers use a wide variety of tools which include analytical models (for example, polytropes to approximate the behaviors of a star) and computational numerical simulations. Each has some advantages. Analytical models of a process are generally better for giving insight into the heart of what is going on. Numerical models can reveal the existence of phenomena and effects that would otherwise not be seen.[21][22]

Theorists in astronomy endeavor to create theoretical models and figure out the observational consequences of those models. This helps allow observers to look for data that can refute a model or help in choosing between several alternate or conflicting models.

Theorists also try to generate or modify models to take into account new data. In the case of an inconsistency, the general tendency is to try to make minimal modifications to the model to fit the data. In some cases, a large amount of inconsistent data over time may lead to total abandonment of a model.

Topics studied by theoretical astronomers include: stellar dynamics and evolution; galaxy formation; large-scale structure of matter in the Universe; origin of cosmic rays; general relativity and physical cosmology, including string cosmology and astroparticle physics. Astrophysical relativity serves as a tool to gauge the properties of large scale structures for which gravitation plays a significant role in physical phenomena investigated and as the basis for black hole (astro)physics and the study of gravitational waves.

Some widely accepted and studied theories and models in astronomy, now included in the Lambda-CDM model are the Big Bang, Cosmic inflation, dark matter, and fundamental theories of physics.

A few examples of this process:
Physical process Experimental tool Theoretical model Explains/predicts
Gravitation Radio telescopes Self-gravitating system Emergence of a star system
Nuclear fusion Spectroscopy Stellar evolution How the stars shine and how metals formed
The Big Bang Hubble Space Telescope, COBE Expanding universe Age of the Universe
Quantum fluctuations
Cosmic inflation Flatness problem
Gravitational collapse X-ray astronomy General relativity Black holes at the center of Andromeda galaxy
CNO cycle in stars



Dark matter and dark energy are the current leading topics in astronomy, as their discovery and controversy originated during the study of the galaxies.

Astrometry and celestial mechanics

Main articles: Astrometry and Celestial mechanics

One of the oldest fields in astronomy, and in all of science, is the measurement of the positions of celestial objects. Historically, accurate knowledge of the positions of the Sun, Moon, planets and stars has been essential in celestial navigation.

Careful measurement of the positions of the planets has led to a solid understanding of gravitational perturbations, and an ability to determine past and future positions of the planets with great accuracy, a field known as celestial mechanics. More recently the tracking of near-Earth objects will allow for predictions of close encounters, and potential collisions, with the Earth.^[18]

The measurement of stellar parallax of nearby stars provides a fundamental baseline in the cosmic distance ladder that is used to measure the scale of the universe. Parallax measurements of nearby stars provide an absolute baseline for the properties of more distant stars, because their properties can be compared. Measurements of radial velocity and proper motion show the kinematics of these systems through the Milky Way galaxy. Astrometric results are also used to measure the distribution of dark matter in the galaxy.^[19]

During the 1990s, the astrometric technique of measuring the stellar wobble was used to detect large extrasolar planets orbiting nearby stars.

Observational astronomy 2

[edit] Ultraviolet astronomy

Ultraviolet astronomy is generally used to refer to observations at ultraviolet wavelengths between approximately 100 and 3200 Å (10 to 320 nm).^[14] Light at these wavelengths is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space. Ultraviolet astronomy is best suited to the study of thermal radiation and spectral emission lines from hot blue stars (O stars and B stars) that are very bright in this wave band. This includes the blue stars in other galaxies, which have been the targets of several ultraviolet surveys. Other objects commonly observed in ultraviolet light include planetary nebulae, supernova remnants, and active galactic nuclei.^[14] However, ultraviolet light is easily absorbed by interstellar dust, and measurement of the ultraviolet light from objects need to be corrected for extinction.^[14]

[edit] X-ray astronomy

X-ray astronomy is the study of astronomical objects at X-ray wavelengths. Typically, objects emit X-ray radiation as synchrotron emission (produced by electrons oscillating around magnetic field lines), thermal emission from thin gases (called bremsstrahlung radiation) that is above 10⁷ (10 million) kelvins, and thermal emission from thick gases (called blackbody radiation) that are above 10⁷ Kelvin.^[14] Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be done from high-altitude balloons, rockets, or spacecraft. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.^[14]

[edit] Gamma-ray astronomy

Gamma ray astronomy is the study of astronomical objects at the shortest wavelengths of the electromagnetic spectrum. Gamma rays may be observed directly by satellites such as the Compton Gamma Ray Observatory or by specialized telescopes called atmospheric Cherenkov telescopes.^[14] The Cherenkov telescopes do not actually detect the gamma rays directly but instead detect the flashes of visible light produced when gamma rays are absorbed by the Earth's atmosphere.^[16]

Most gamma-ray emitting sources are actually gamma-ray bursts, objects which only produce gamma radiation for a few milliseconds to thousands of seconds before fading away. Only 10% of gamma-ray sources are non-transient sources. These steady gamma-ray emitters include pulsars, neutron stars, and black hole candidates such as active galactic nuclei.^[14]

[edit] Fields of observational astronomy not based on the electromagnetic spectrum

Other than electromagnetic radiation, few things may be observed from the Earth that originate from great distances.

In neutrino astronomy, astronomers use special underground facilities such as SAGE, GALLEX, and Kamioka II/III for detecting neutrinos. These neutrinos originate primarily from the Sun but also from supernovae.^[14]

Cosmic rays consisting of very high energy particles can be observed hitting the Earth's atmosphere.^{[citation needed]} Additionally, some future neutrino detectors will also be sensitive to the neutrinos produced when cosmic rays hit the Earth's atmosphere.^[14]

A few gravitational wave observatories have been constructed, but gravitational waves are extremely difficult to detect.^[17]

Planetary astronomy has benefited from direct observation in the form of spacecraft and sample return missions. These include fly-by missions with remote sensors; landing vehicles that can perform experiments on the surface materials; impactors that allow remote sensing of buried material, and sample return missions that allow direct, laboratory examination.

Observational astronomy

Main article: Observational astronomy

In astronomy, information is mainly received from the detection and analysis of light and other forms of electromagnetic radiation.^[13] Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or space. Specific information on these subfields is given below.

[edit] Radio astronomy

Radio astronomy studies radiation with wavelengths greater than approximately one millimeter.^[14] Radio astronomy is different from most other forms of observational astronomy in that the observed radio waves can be treated as waves rather than as discrete photons. Hence, it is relatively easier to measure both the amplitude and phase of radio waves, whereas this is not as easily done at shorter wavelengths.^[14]

Though some radio waves are produced by astronomical objects in the form of thermal emission, most of the radio emission that is observed from Earth is seen in the form of synchrotron radiation, which is produced when electrons oscillate around magnetic fields.^[14] Additionally, a number of spectral lines produced by interstellar gas, particularly the hydrogen spectral line at 21 cm, are observable at radio wavelengths.^[7][14]

A wide variety of objects are observable at radio wavelengths, including supernovae, interstellar gas, pulsars, and active galactic nuclei.

Infrared astronomy

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. Infrared astronomy is particularly useful for observation of galactic regions cloaked by dust, and for studies of molecular gases.

[edit] Optical astronomy

The Subaru Telescope (left) and Keck Observatory (center) on Mauna Kea, both examples of an observatory that operates at near-infrared and visible wavelengths. The NASA Infrared Telescope Facility (right) is an example of a telescope that operates only at near-infrared wavelengths.

Historically, optical astronomy, also called visible light astronomy, is the oldest form of astronomy.^[15] Optical images were originally drawn by hand. In the late nineteenth century and most of the twentieth century, images were made using photographic equipment. Modern images are made using digital detectors, particularly detectors using charge-coupled devices (CCDs). Although visible light itself extends from approximately 4000 Å to 7000 Å (400 nm to 700 nm),^[15] the same equipment used at these wavelengths is also used to observe some near-ultraviolet and near-infrared radiation.

Scientific revolution

During the Renaissance, Nicolaus Copernicus proposed a heliocentric model of the solar system. His work was defended, expanded upon, and corrected by Galileo Galilei and Johannes Kepler. Galileo innovated by using telescopes to enhance his observations.

Kepler was the first to devise a system that described correctly the details of the motion of the planets with the Sun at the center. However, Kepler did not succeed in formulating a theory behind the laws he wrote down. It was left to Newton's invention of celestial dynamics and his law of gravitation to finally explain the motions of the planets. Newton also developed the reflecting telescope.

Further discoveries paralleled the improvements in the size and quality of the telescope. More extensive star catalogues were produced by Lacaille. The astronomer William Herschel made a detailed catalog of nebulosity and clusters, and in 1781 discovered the planet Uranus, the first new planet found. The distance to a star was first announced in 1838 when the parallax of 61 Cygni was measured by Friedrich Bessel.

During the nineteenth century, attention to the three body problem by Euler, Clairaut, and D'Alembert led to more accurate predictions about the motions of the Moon and planets. This work was further refined by Lagrange and Laplace, allowing the masses of the planets and moons to be estimated from their perturbations.

Significant advances in astronomy came about with the introduction of new technology, including the spectroscope and photography. Fraunhofer discovered about 600 bands in the spectrum of the Sun in 1814-15, which, in 1859, Kirchhoff ascribed to the presence of different elements. Stars were proven to be similar to the Earth's own Sun, but with a wide range of temperatures, masses, and sizes.^[11]

The existence of the Earth's galaxy, the Milky Way, as a separate group of stars, was only proved in the 20th century, along with the existence of "external" galaxies, and soon after, the expansion of the universe, seen in the recession of most galaxies from us. Modern astronomy has also discovered many exotic objects such as quasars, pulsars, blazars, and radio galaxies, and has used these observations to develop physical theories which describe some of these objects in terms of equally exotic objects such as black holes and neutron stars. Physical cosmology made huge advances during the 20th century, with the model of the Big Bang heavily supported by the evidence provided by astronomy and physics, such as the cosmic microwave background radiation, Hubble's law, and cosmological abundances of elements.

History

Main article: History of astronomy

Further information: Archaeoastronomy

A celestial map from the 17th century, by the Dutch cartographer Frederik de Wit.

In early times, astronomy only comprised the observation and predictions of the motions of objects visible to the naked eye. In some locations, such as Stonehenge, early cultures assembled massive artifacts that likely had some astronomical purpose. In addition to their ceremonial uses, these observatories could be employed to determine the seasons, an important factor in knowing when to plant crops, as well as in understanding the length of the year.^[8]

Before tools such as the telescope were invented early study of the stars had to be conducted from the only vantage points available, namely tall buildings, trees and high ground using the bare eye.

As civilizations developed, most notably Mesopotamia, Egypt, Persia, Maya, Greece, India, China, and the Islamic world, astronomical observatories were assembled, and ideas on the nature of the universe began to be explored. Most of early astronomy actually consisted of mapping the positions of the stars and planets, a science now referred to as astrometry. From these observations, early ideas about the motions of the planets were formed, and the nature of the Sun, Moon and the Earth in the universe were explored philosophically. The Earth was believed to be the center of the universe with the Sun, the Moon and the stars rotating around it. This is known as the geocentric model of the universe.

A few notable astronomical discoveries were made prior to the application of the telescope. For example, the obliquity of the ecliptic was estimated as early as 1000 BC by the Chinese. The Chaldeans discovered that lunar eclipses recurred in a repeating cycle known as a saros.^[9] In the 2nd century BC, the size and distance of the Moon were estimated by Hipparchus.^[10]

During the Middle Ages, observational astronomy was mostly stagnant in medieval Europe, at least until the 13th century. However, observational astronomy flourished in the Islamic world and other parts of the world. Astronomers during that time introduced many Arabic names that are now used for individual stars.

Lexicology

The word astronomy literally means "law of the stars" (or "culture of the stars" depending on the translation) and is derived from the Greek αστρονομία, astronomia, from the words άστρον (astron, "star") and νόμος (nomos, "laws or cultures").

Use of terms "astronomy" and "astrophysics"

Generally, either the term "astronomy" or "astrophysics" may be used to refer to this subject.^[2][3][4] Based on strict dictionary definitions, "astronomy" refers to "the study of objects and matter outside the earth's atmosphere and of their physical and chemical properties"^[5]and "astrophysics" refers to the branch of astronomy dealing with "the behavior, physical properties, and dynamic processes of celestial objects and phenomena".^[6] In some cases, as in the introduction of the introductory textbook The Physical Universe by Frank Shu, "astronomy" may be used to describe the qualitative study of the subject, whereas "astrophysics" is used to describe the physics-oriented version of the subject.^[7] However, since most modern astronomical research deals with subjects related to physics, modern astronomy could actually be called astrophysics.^[2] Various departments that research this subject may use "astronomy" and "astrophysics", partly depending on whether the department is historically affiliated with a physics department,^[3] and many professional astronomers actually have physics degrees.^[4] Even the name of the scientific journal Astronomy & Astrophysics reveals the ambiguity of the use of the term.

Astronomy

From Wikipedia, the free encyclopedia

Astronomy Portal

Astronomy is the scientific study of celestial objects (such as stars, planets, comets, and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

Astronomy is one of the oldest sciences. Astronomers of early civilizations performed methodical observations of the night sky, and astronomical artifacts have been found from much earlier periods. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and even astrology, but professional astronomy is nowadays often considered to be synonymous with astrophysics. Since the 20th century, the field of professional astronomy split into observational and theoretical branches. Observational astronomy is focused on acquiring and analyzing data, mainly using basic principles of physics. Theoretical astronomy is oriented towards the development of computer or analytical models to describe astronomical objects and phenomena. The two fields complement each other, with theoretical astronomy seeking to explain the observational results, and observations being used to confirm theoretical results.

Amateur astronomers have contributed to many important astronomical discoveries, and astronomy is one of the few sciences where amateurs can still play an active role, especially in the discovery and observation of transient phenomena.

Old or even ancient astronomy is not to be confused with astrology, the belief system that claims that human affairs are correlated with the positions of celestial objects. Although the two fields share a common origin and a part of their methods (namely, the use of ephemerides), they are distinct.^[1]

Astronomers Discover Record Fifth Planet Around Nearby Star 55 Cancri

ScienceDaily (Nov. 6, 2007) — Astronomers have discovered a record-breaking fifth planet around the nearby star 55 Cancri, making it the only star aside from the sun known to have five planets.

The discovery comes after 19 years of observations of 55 Cancri and represents a milestone for the California and Carnegie Planet Search team, which this year celebrates the 20th anniversary of its first attempts to find extrasolar planets by analyzing the wobbles they cause in their host star.

The team's long history of measurements - more than 300 for 55 Cancri alone - made the discovery of a five-planet system possible, said UC Berkeley astronomy professor Geoffrey Marcy, who with Paul Butler, now at the Carnegie Institution of Washington, began observations of many nearby stars at the University of California Lick Observatory in 1987.

The unique 55 Cancri system, located 41 light-years away in the direction of the constellation Cancer, is notable also because its clutch of four inner planets and one giant outer planet resembles our own solar system, though without an Earth or Mars.

"This system is interesting because there's a giant planet at 6 AU and four smaller planets inward of 0.8 AU, with a huge remaining gap in between, right where we would expect to find an Earth-sized planet," Marcy said.

An AU, or astronomical unit, is the average distance between the Earth and the sun, about 93 million miles.

According to lead author Debra Fischer, assistant professor of astronomy at San Francisco State University, the fifth planet is within the star's habitable zone in which water could exist as a liquid. Though the planet is a giant ball of gas, liquid water could exist on the surface of a moon or on other, rocky planets that may yet be found within the zone. "Right now, we are looking at a gap between the 260-day orbit of the new planet and the 14-year orbit of another gas giant, and if you had to bet, you'd bet that there is more orbiting stuff there."

Fischer noted that what occupies this gap has to be another planet around the size of Neptune or smaller, because anything larger would have destabilized the orbits of the other planets. All of the planets around 55 Cancri are in stable, nearly circular orbits, like the eight planets in our solar system. Jupiter is located at 5.2 AU from the sun, while Mercury and Venus are closer than 0.72 AU. Earth and Mars are in the gap at 1 AU and 1.5 AU.

"We haven't found a twin of our solar system, because the four planets close to the star are all the size of Neptune or bigger," Marcy said, but he added that he's optimistic that continued observations will reveal a rocky planet within five years.

The new discovery, using data from the Lick Observatory and the W. M. Keck Observatory in Hawaii, has been accepted for publication in the Astrophysical Journal. The authors are Fischer, Marcy and their colleagues at the Carnegie Institution, San Francisco State University, UC Santa Cruz, Tennessee State University and UC Berkeley.

Fischer and Marcy also discussed their findings today during a media teleconference hosted by NASA.

"This work marks an exciting next step in the search for worlds like our own," said Michael Briley, an astronomer at the National Science Foundation. "To go from the first detections of planets around sun-like stars to finding a full-fledged solar system with a planet in a habitable zone in just 12 years is an amazing accomplishment and a testament to the years of hard work put in by these investigators."

In 1996, when Marcy and Butler found a Jupiter-sized planet orbiting close to 55 Cancri and circling every 14.6 days, it was only the fourth known star with an exoplanet. The second planet discovered in 2002 around the star turned out to circle in a more distant orbit, like our own Jupiter does, although the planet was four times the weight of Jupiter. The third, also discovered in 2002, was smaller, about half the size of Saturn, and was orbiting near the star with an orbit of 44 days, slightly farther than the first planet. The fourth planet, found in 2004, was so close to the star as to be hellishly hot - a Neptune-sized planet (14 times Earth's mass) with a 2.8 day period discovered in collaboration with a team led by Barbara McArthur of the University of Texas.

Although astronomers have found nearly 250 exoplanets, only one other star, mu Ara in the southern sky, is known to have four planets.

The newly-found fifth planet around 55 Cancri is also large - around half the size of Saturn, or at least 45 times the mass of Earth - and orbiting at about 0.785 AU in 260.8 days. Because the star 55 Cancri is older and dimmer than our sun, the habitable zone - the region in which planetary temperatures can be favorable for liquid water - is closer to the star than is our sun's habitable zone, and includes the new planet.

Finding multiple planets around a star is difficult because each planet produces its own stellar wobble. Marcy compares detecting the wobble within wobbles that are caused by one of several planets to picking out a single musical note from many played simultaneously. While the ear can do that, it took Marcy more than 10 months to convince himself that a fifth wobble was buried in the data.

The Doppler technique used by the search team sees this wobble as a change in the speed with which a star moves toward or away from us. The search team can detect velocities as small as 1 meter per second, which is walking speed.

55 Cancri has produced "a rat's nest of radial velocity data," Fischer said. "We probably still don't have all the planets. We are pulling out one thread at a time, disentangling all these orbits, and it has taken a lot more data and time than we predicted. I think it's amazing what we have been able to do with the system."

Coauthors with Fischer, Marcy and Butler are Steven S. Vogt and Greg Laughlin of UC Santa Cruz; Jason T. Wright, John A. Johnson and Kathryn M. G. Peek of UC Berkeley; Gregory W. Henry of Tennessee State University's Center of Excellence in Information Systems; and David Abouav, Chris McCarthy and Howard Isaacson of San Francisco State University.

The work was supported by the University of California, NASA and the National Science Foundation.

Adapted from materials provided by University of California - Berkeley.

An artist's concept of the star 55 Cancri showing the newly discovered planet in the foreground -- a gas giant half the mass of Saturn -- and three already known inner planets (the planet farthest from the star is not pictured). All the inner planets are the size of Neptune or bigger, unlike our solar system's rocky inner planets. The colors of the planets in this illustration were chosen to resemble those of our own solar system. Astronomers do not know what the planets look like. (Credit: NASA/JPL-Caltech)

Catch a glimpse of Comet Holmes

The "flaring comet" continues to impress observers.

Francis Reddy

How long can you track Comet 17P/Holmes? This map shows its course in Perseus through the year's end. Astronomy: Roen Kelly [larger image]

November 14, 2007 Comet 17P/Holmes remains a striking target for binoculars, small telescopes, and even the unaided eye. Take the time to see this unusual visitor, which leapt from obscurity to celebrity October 23. For reasons astronomers don't entirely understand, the cosmic iceball flared in brightness by a million times in just 2.5 days. This outburst propelled the comet from a faint-fuzzy best viewed in a large amateur telescope to a star-like object observers throughout the Northern Hemisphere could easily see in a moonlit sky. The comet subsequently expanded into a fuzzy patch and now rivals the Moon in size. Holmes has faded relatively little in terms of astronomers' brightness scale, where it now hovers near magnitude 3, but its light is spread out over a larger area.

Bright Comet 17P/Holmes lies in the constellation Perseus, in the northeastern sky after darkness falls. Astronomy: Roen Kelly [larger image] Some observers dubbed 17P/Holmes the ultimate "urban comet." While it lacks a spectacular tail, the comet initially was easy to spot from urban locations, and it can still be seen visually in suburban areas. Take the time to observe it carefully. Visual observers may notice that it appears distinctly un-starlike. Low-power binoculars reveal a ghostly disk surrounding a bright center. Amateurs who image Comet Holmes are finding detailed structures related to its recent blast of dust and gas. And, although Holmes is an old, relatively inactive comet, many observers now report just a hint of a bluish gas tail.

Astronomy Senior Editor Francis Reddy made this composite of Comet 17P/Holmes from images captured Nov. 11. He used infrared, green, and blue filters to create the view. Click here to see a Google maps view that places this image in the sky. Francis Reddy Comet Holmes currently lies 151 million miles (244 million km) from Earth and 234 million miles (377 million km) from the Sun. In early May, the comet reached its closest point to the Sun in its 6.88-year orbit. At that time, Holmes was about twice as far from the Sun as Earth. Since then, the comet has been increasing its solar distance. Earth, traveling on an inside lane of the solar system, passed Holmes November 5. The comet lies about 50° high — halfway from the horizon to straight overhead — at 8 p.m. local time. It then appears about twice as high as the bright star Capella. For observers at mid-northern latitudes, the comet climbs directly overhead around local midnight.

These images taken by NASA's Hubble Space Telescope reveal Comet Holmes's bright core. The images show that the coma, the cloud of dust and gas encircling the comet, is getting fainter over time. This is happening because the coma is expanding as dust particles ejected in the October 23 outburst move outward. The nucleus, however, is still active and is producing a significant amount of new dust. NASA/ESA/H. Weaver (JHUAPL)

Interference from the waxing crescent Moon will worsen throughout the next week. The First Quarter Moon, which sets around local midnight, occurs Saturday, November 17, and the Moon is Full the following Saturday. Read our earlier coverage of the comet Naked-eye comet continues to shine, October 30, 2007 Naked-eye comet bursts into view, October 24, 2007 Listen to Michael Bakich's guide to observing this comet

Comet 17P comet_17p.mp3 [file size 4614K] Zoom into the heart of Comet Holmes heic0718a.mov [file size 5142K]

Ingenious Pursuits by Lisa Jardine ISBN: 0385720017 (Book Reviewed)

Did you know that Sir Christopher Wren was a professor of astronomy? Strictly speaking, designing St. Paul’s Cathedral was an extra-curricular activity. Did you know that Edmund Halley’s greatest contribution to astronomy was not his predicting the return of the comet bearing his name? He was the one who persuaded Newton to publish his momentous discoveries concerning gravity and optics. Did you know that Christian Huygens’ discovery of Titan and understanding of what exactly Saturn’s rings were, are almost modest compared to his construction of the first really accurate clock? Modern science depends on it!

Well maybe you did, but I didn't; at least not until I read Lisa Jardine's very detailed but eminently readable book about the scientific discoveries of the 17th and 18th centuries and the people who made them. In her introduction, Jardine makes the case for scientists to be viewed with more sympathy and less suspicion. We have unprecedented access to information allowing us to educate ourselves as never before. Yet many of the populace view science with a suspicion based on ignorance. Ignorance not only of even the most basic facts of science but also of the huge importance it plays in our lives. This is one of a number of books that try to popularize science by telling its history as a story; a story full of drama, human failings and heroics, petulant patriotism and patronage. In other words, it says that far from scientists being the soulless automatons of popular ignorance they are all too human, both in their failings and their greatness.

The book starts with the story of why the Royal Observatory in Greenwich was built (in order to help establish longitude at sea) and also who built it and how. Sir Jonas Moore was the driving force behind both funding and construction. Wren and Hooke designed it and Moore installed John Flamsteed as the first Astronomer Roy- al. Great…except Moore discovered that Flamsteed was an observer who took devotion to duty to ludicrous extremes. Although his measurements were more than enough to satisfy the most demanding sailor in locating his position, the new Astronomer Royal stubbornly refused to publish his observations until he was happy with them. Not only did Moore die without ever seeing these observations in print, Flamsteed did too. Quite remarkable when you realize that he started his observing in 1675 and kept them up until his death in 1719!

The story moves on to one of the main characters of the book –- Robert Hooke. On the debit side, he emerges as the type of guy that would argue with Christ up on the cross (he had a long running dispute with Huygens regarding the invention of the balance wheel in clocks).

On the credit side, however, we learn that not only was he involved with Wren in designing St. Paul's but he was the designer and maker of the most superb scientific instruments. A pioneer in the development of compound microscopes, even he could not match the work of another of scientific history’s (and therefore history’s) great figures – Anthony Von Leeuwenhoek. This Dutch civil servant never used anything other than a simple microscope but made and used them to such an exceptional standard that not only did he discover bacteria but it was another hundred years before anyone else managed to see them!

Jardine eventually moves into the territory of botany, concentrating on the medical and huge commercial implications of expeditions to far-flung places. I was less interested in these chapters but only because I am not that interested in plants. Nevertheless, I felt they took up too much of the book (which, by the way, is well illustrated both in color and black-and-white).

Eventually the author leaves botany behind and tells the story of Henry Oldenburg who set up voluminous correspondence between various scientists and contributed to the dissemination of information around Europe. The last chapter deals with Crick and Watson’s historic discovery of the structure of the DNA molecule, giving due credit to the now (largely forgotten) Rosalind Franklin, whose excellent X-Ray diffraction photos led her to suspect what Crick and Watson used those same images to prove. Franklin died tragically young and surprisingly is omitted from the very useful thumbnail descriptions of the work of the main characters.

Crash Test Chickens

Scientists at NASA had built a gun specifically to launch dead chickens at the windshields of airliners, military jets and the space shuttle, all traveling at maximum velocity. The idea was to simulate the frequent incidents of collisions with airborne fowl to test the strength of the windshields.

British engineers heard about the gun and were eager to test it on the windshields of their new high-speed trains. Arrangements were made, and a gun was sent to the British engineers.

When the gun was fired and the chicken hurtled out of the barrel, the engineers stood shocked as it crashed into the shatterproof windshield, smashed it to smithereens, blasted through the control console, snapped the engineer's backrest in two and embedded itself in the back wall of the cabin.

The Britons sent NASA a detailed report covering the exact steps they had followed arid the disastrous results of the experiment, along with the design of the windshield, and asked the U.S. scientists for suggestions.

NASA responded with a three-word memo: 'Thaw the chicken."

Starting out in Astronomy, reviewed from: Fintan Sheerin

Dressing properly for an observing session is vital if you want to stay warm and enjoy the experience. This article tells you how to do just that.

First Mistakes

Most people, at some time in their lives, look up at the night sky. Some are awe-struck, others mildly interested, and still others are not even aware that they have looked up in the first place, and consequently go about their lives oblivious to the 99.99% of the universe which is not Earth-based! Those who are filled with wonder usually develop their interest fully, and join an astronomical society, while those with a mild interest may venture out again, perhaps to a Star Party. What happens? They arrive in suits and dresses - the most inappropriate clothing - freeze, and then decide that astronomy is not for them. It cannot be emphasized enough that one must be adequately prepared for ob serving. Clothing is all-important, and as seen in fig. 1, incorporates several layers of loose-fitting and warm items. You may vary in the make-up of your protective clothing, but ensure that you are warm. Always wear a hat as it is estimated that up to 60% of body heat is lost through the head.

What to Take With You

It is normal to take breaks during an observing session in order to warm up and to rest the eyes. After 30 minutes or so, in -2C, a cup of hot soup, coffee or cocoa is very welcome, and only involves a few minutes of preparation before setting out. So, bring a flask. If you are going out on a general naked-eye or meteor ob serving session, bring a plastic groundsheet to lie on, or a reclining deck-chair (if you can carry it). These reduce the risk of neck strain due to cold muscles, and prolonged neck flexion if standing.

On the astronomy side, a star map is a must. This can take the form of a single sheet (such as the Philip's Chart of the Stars), or a multi-chart format, as in Norton's Star Atlas. A plan sphere can also be useful in helping the beginner to locate individual constellations and stars. This, and the star maps, may be obtained from good bookshops. Bring a red torch, such as a bicycle back light, for reading maps and charts. Red light places less strain on the eyes than white, and so causes little disruption to dark adapted eyes. A clip board with blank, white paper, and a HB pencil, should be brought for recording details of objects seen, and drawings of meteors, etc. These can be transferred into a formal observing log later on, after the watch.

What to Look For

Many of the nights spent observing will merely involve you in becoming familiar with the night sky. This must be the first step on the road towards proficiency in astronomical observing, as it is only by becoming familiar with the "unchanging sky" that one can begin to notice those objects which move. Learning constellation names and stars takes many hours of practice and observing. A series of articles on constellations has been featured in this and prior issues of Orbit and these will form a basis on which inexperienced astronomers can develop that required learning. In the meantime though, identify the most famous constellations such as the Plough (Ursa Major), Orion and Cassiopeia. As you observe, you will notice several interesting differences between particular stars: First, some are brighter than others. This relative brightness of stars (and other celestial objects) is called Magnitude, and is dependant on the luminosity of the star and its distance from Earth. It should be noted that, on the magnitude scale, the brighter the star, the lower its assigned magnitude (hence - 1 is brighter than +1).

Second, you will see some stars that appear to be made up of two components. These are double stars, and may involve a pair of stars revolving about a common centre of gravity under the influence of their mutual gravitational attraction (a Binary star), or two stars which appear close together because they lie nearly in the same direction from Earth, but are in fact many light years apart (an Optical double star). Look at the second star in the "handle" of the Plough and you will see the optical double, Mizar and Alcor.

Thirdly, you may notice that not all stars are the same color. For example, Betelgeuse, at the top left corner of Orion, appears vividly red when contrasted with Rigel, at the bottom right corner, which seems bluish white. This is due to the fact that these stars belong to different spectral types, and so possess different characteristics.

Scattered throughout the sky are closely-knit groups of stars, called star clusters. These may be wide open, as with the Hyades, or closer as with the Pleiades, both of which are in Taurus. Other groups of stars take on what appears to be a much, much tighter grouping. This may be due to the vast distance between them and us, perhaps placing them outside our own galaxy. These are themselves galaxies, and one naked-eye example is the Andromeda Galaxy, which is estimated to be some 2.2 million light years away, and appears as a faint "fuzzy star". Our own galaxy - the Milky Way - is more observable, stretching across the sky as a wide band of luminosity. Mention should also be made of nebulae. These clouds of interstellar dust, often associated with stellar development, are difficult to see with the unaided eye, though one - the Great Nebula in Orion, may be observed as a luminous patch in the centre of Orion's Sword. A useful method for observing faint objects is the Averted Vision technique. All that is involved to locate an object is to look at it and then avert your eyes slightly away from it. Often the object will be noticed "through the corner of your eye", so to speak.

All of the above objects are unchanging in the night sky. Other star like bodies will be noticed, in the sky, though, which are not on the star charts and which appear to change their positions nightly. These are the planets, and six are visible to the naked-eye - Mercury, Venus, Mars, Jupiter, Saturn and sometimes Uranus.

The planets are different to stars in several ways. A star may be defined as a luminous, gaseous body that generates energy by means of nuclear fusion reactions in its core. A planet on the other hand, is a body which orbits the Sun or another star and shines by reflected light. To the naked-eye observer, the main interest in planets lies in tracing their paths across the sky, and noting how some tend to backtrack as the months pass by.

The Moon displays little to those without optical aid, but does go through a monthly cycle of phases. An interesting exercise is to judge the positions of the Earth and Moon with respect to the Sun during each phase. Look out also for occultation - that is when the Moon passes over a star or planet. Predictions for these are listed in the Sky notes.

Very occasionally, a bright comet may grace our skies. This may appear either as a "fuzzy star" or as a classic comet, with a prominent head and streaming tail. More often though, the only sight of cometary’s matter which we can see every night, is observed to whiz across the sky in less than a second. These star like objects are, of course, meteors or shooting stars, and are usually caused when the Earth passes through dust spread out along the orbit of a comet. Rates during these meteor showers may rise to as many as 100 per hour. Full de- tails of current meteor showers are given in the Sky notes. Often, new observers mistake slow-moving objects which cross the sky in a matter of minutes, for meteors. These are in fact, manmade artificial satellites in Earth orbit. One of the most interesting of these at present is the International Space Station, which is regularly seen over Ireland.

Is Light Pollution A Problem?

Frankly, yes. But this does not rule out observing in urban areas. Light pollution, while reducing the number and quality of observable objects, does not block out all stars or planets. A colleague once suggested the use of a cardboard box, placed over the head and resting on the shoulders, as a method for dealing with troublesome light. This blinkers the lateral views of the observer and I can vouch for its effectiveness. You may think that you will look ridiculous with such a contraption on your head, but let's face it, people generally consider astronomers to be weird anyway!

Conclusion

The most significant way of getting started in astronomy, whether young or old, is to join an astronomical association. There, you will find other ordinary people who have been amateur astronomers for many years, and who will be more than pleased to advise and help you on the road to becoming proficient in the science of amateur astronomy.