Celestial Hierarchy: From Atoms to Galaxy Clusters
Due to the expansion of the universe, its temperature was 3000 K (Weinberg, 1977).
Evolution of Stellar Clusters and Galaxies
Matter in the universe is organized in a hierarchy of celestial bodies, listed below in descending order of size:
- Clusters of galaxies
- Galaxies
- Stars, pulsars, and black holes
- Planets and satellites
- Comets
- Asteroids
- Meteoroids
- Dust
- Molecules
- H and He atoms
On the subatomic scale, the space between stars and galaxies is filled with cosmic rays (nuclear particles) and photons (light).
Stars are the basic units in the hierarchy of celestial bodies. Nuclear reactions occur within them. Many billions of stars group together to form a galaxy, and a large number of galaxies are associated in galactic clusters. Stars may have stellar companions or orbiting planets. The planets in our solar system have their own satellites. The space between Mars and Jupiter contains asteroids, most of which are fragments of larger bodies broken by collisions and the gravitational forces of Jupiter and Mars. Pieces of asteroids have struck planets and satellites as meteorites, leaving a record of these events in craters.
On an even smaller scale, the space between the stars contains clouds of gas and solid particles. The gas is composed primarily of hydrogen and helium produced during the initial expansion of the universe. Moreover, the interstellar medium contains higher atomic number elements synthesized by nuclear reactions inside exploded stars. A third component consists of hydrogen and carbon compounds that are the precursors of life.
These gas and dust clouds can contract to form new stars whose evolution depends on their mass and the H/He ratio of the gas cloud from which they formed.
The evolution of stars can be described by specifying their luminosities and surface temperatures. The brightness of a star is proportional to its mass, and the surface temperature (or color) is an indicator of volume. When a cloud of interstellar gas contracts, its temperature rises and it begins to radiate in the infrared and visible spectrum. When the core temperature of the gas cloud is about 2 x 107 K, the production of energy by the fusion of hydrogen becomes possible, and a star is born. Most of a typical galaxy star’s power is derived from this process and therefore falls in a band called the main sequence in the Hertzsprung-Russell diagram (Figure 2.1).
Massive stars, called blue giants, have high brightness and high surface temperatures. The Sun is an intermediate-mass star and has a surface temperature of about 5800 K. Stars less massive than the Sun are called red dwarfs and plot at the lower end of the main sequence.
As a star five times as massive as the Sun converts hydrogen to helium while on the main sequence, the core density increases, causing the interior of the star to contract. The core temperature therefore rises slowly during the combustion of hydrogen. The higher temperature accelerates the fusion reaction and causes the outer envelope of the star to expand. However, when the core runs out of hydrogen, the rate of energy production decreases, and the star contracts, raising the core temperature. The site of energy production now moves from the nucleus to a shell. The changes in brightness and surface temperature cause the star to move away from the main sequence into the realm of the red giants (Figure 2.1).
The helium produced by the fusion of hydrogen accumulates in the nucleus, which continues to contract, so it becomes even hotter. The resulting expansion of the envelope lowers the surface temperature and makes the color red. At the same time, the shell in which hydrogen…