There is persuasive evidence that the Universe was created in ``the Big Bang'', in which space and time were created in a simple hot energetic, state, about 15 billion (15 x 109 ) years ago.
About 300,000 years after the Big Bang, there was the era of recombination in which protons and electrons combined to form neutral Hydrogen. At this point, baryonic matter in the Universe consisted of about 75% Hydrogen and 25% Helium (by mass), with some small amounts of heavy elements (elements starting from Lithium). The distribution of this material was very close to, but not quite, uniform. These slight over- and under-densities were observed for the first time by the COBE satellite (launched in 1989) and amount to only a few parts in 100,000. The variations were mapped out over the whole sky (see figure 1.2) on scales of greater than about 7 degrees.
After recombination, the Universe entered a period called the ``Dark Ages'', until gravitational attraction had operated on very slight over-densities in the matter distribution, leading to the formation of light emitting stars and galaxies. The Universe was optically observable again!
Exactly how stars and galaxies formed, when the process started and how long it took is currently a major area of research. A simple picture runs like this: about 1 billion years after the big bang the first star forming regions, conglomerates of perhaps 106 to 109 solar masses began to develop. Over the next several billion years, most of these merge to form larger units or are partially destroyed by the energetic supernovae which develop as a natural part of star formation. Within a few billion years most of these have developed into stable configurations of stars and gas and are recognisable as ``galaxies''.
The faintest galaxies so far observed were seen in the ``Hubble Deep Field'', a tiny patch of sky which was imaged for more than a week by the Hubble Space Telescope (figure 1.3).
Galaxies have been identified in this image with redshifts which indicate that the light reaching us has been traveling for about 90 % of the age of the Universe - or about 12-15 billion years. One such galaxy is shown in figure 1.4. Such galaxies (and the stars in them) are likely part of the very first generation of stars and galaxies to have formed.
These galaxies are at present too few in number and too faint to be study in the kind of detail which we would like - to allow us to answer basic questions concerning exactly when and and what conditions the stars formed. However, a new generation of 10 meter class telescopes is currently coming on line all around the world so these issues are among those which these telescopes will concentrate.
The last 10 billion years or so of the developments in galaxies and in their stellar content is now quite well studied because of the Hubble Space Telescope, which was able to obtain clear images of these distant galaxies for the first time. Figure 1.5 shows early galaxies in the (confused) process of forming. One shouldn't forget that these knots of light are due to billions of stars forming, more or less at the same time.
The space telescope has recently allowed the construction of a sequence of typical galaxy images over time (see figure 1.6). The general picture is that galaxies have been forming over quite a few billion years, are continuing to form and develop still, and seem to have been assembled from many smaller sub-galaxies.
In the proto-galactic units the dominant process is that of star formation itself. We know that star formation takes place in giant collapsing clouds of gas, and can take place under a wide range of circumstances which result in stars being formed at a slow rate (such as most nearby regions of star formation in our own galaxy) or hundreds to thousands of times faster (such as in the compressed gas clouds which result when galaxies are disturbed or actually collide). However, the process by which clouds actually fragment and collapse into individual stars has long been a very poorly understood area of Astronomy, and remains almost as obscure today as it was 30 years ago. This is an area of research whose time has not yet come!
Hubble has revealed higher resolution images of star forming regions, which at least show some of the complexities of the physical processes involved.
Figure 1.7 shows columns of cool interstellar hydrogen gas and dust that are also incubators for new stars. The pillars protrude from the interior wall of a dark molecular cloud like stalagmites from the floor of a cavern. They are part of the ``Eagle Nebula'' (also called M16), a nearby star-forming region 7,000 light-years away in the constellation Serpens. The pillars of gas are dense gas which has survived being ``eroded'' away by the light of hot UV stars nearby, and the small blobs are even denser regions where it is very likely that stars themselves are actually forming. They'll emerge from the cocoons in which they are incubating millions of years from now, and for the moment are so heavily shrouded by dust that we cannot see them at all with optical light. Infrared light (heat) which does escape from the regions is the main clue that energetic processes are taking place. Figure 1.8 shows how different images in optical (on the left) and infrared (on the right) of the same region in the Orion Nebula can be.
The star forming regions can be very large, sometimes occupying a major section of an entire galaxy, such as seen in figure 1.9.
Out of these star forming regions star eventually emerge - often ``en mass'' in giant clusters of up to a million stars at once. An example of this is seen in the giant star forming complex 30Dor in the Large Magellanic Cloud, a galaxy nearby to our own (see figure 1.10).
Clusters of stars which we can see forming like this might end up as what we know today as ``globular clusters'' of our own Galaxy - millions of stars tightly bound by their own gravity which have survived from the earliest times when our galaxy was forming.
A timeline of the processes of gravitational collapse in the Universe, the formation of galaxies and stars, and the formation of our own Galaxy (the Milky Way), Sun and Earth is shown in figure 1.11.
Encyclopedia of Applied Physics, Chapter 5,
Encyclopedia of Applied Physics (The Structure of the Universe)
Cosmic Microwave Background Explorer
The Hubble Deep Field
The Hubble Space Telescope