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].

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]