.
There are many galaxies in the Universe. A Galaxy is a large scale aggregate of stars plus gas and dust, held together by gravity. It is now believed that all galaxies started with a Massive Black Hole. Gasses rotated very rapidly around this black hole. This rapid speed caused them to glow and form Quasars. This also resulted in the stars being pushed outward beyond the reach of the black hole. Eventually the black hole consumed the Quasar and become dormant, but still alive. The larger the black hole, the further the stars were pushed and the faster they rotated. Hence there is a direct relationship among the size of the black hole, the size of the galaxy and the speed of the stars rotating in the outer reaches of the galaxy. As can be expected, the stars closer to the center or the galaxy and hence closer to the black hole rotate faster. The black holes will still consume any gas or stars that come close enough, and could start again on a feeding frenzy.
They have spiral, elliptical, or irregular, structure, and contain, on the average, one hundred billion solar masses. Types Sa have their spiral arms wound to the central region. Sb have arms spread out more from the center. Sc spirals have very loose spiral arms. The SO type links the normal spirals to the smooth ellipticals. Barred spirals have a broad bar that extends outwards from opposite sides of the central region.
For figures of the different types of galaxies, click below
What is the origin of galaxies? Did they form from hugh clouds of gas and dust? There are two theories.
1. Bottom-up theory states that the Universe started from small particles of mass that came together to form galaxies, which evolved into clusters and then superclusters.
2. Top-down theory states vast pancaked shaped clouds of matter provided the seeds for the formation of galaxies. In December 1991, radio astronomers in New Mexico reported the detection of a hugh mass of primordial hydrogen gas near the edge of the observable universe that has the structure of a flat disk or pancake., with a mass of 500 trillion solar masses and measuring 5 million light years across.
The distance to remote galaxies is calculated by means of the red shift in the galaxy's spectrum. When a photograph is taken of an excited element in the gaseous phase by a spectrograph located in an Earth-based laboratory, a normal line emission spectrum is obtained. When a similar photograph is taken of a galaxy, containing this same element, the spectrum shows a displacement of the normal lines towards the red end of the spectrum (longer wavelengths).
Since every thing is moving away from us, does that mean that we are at the center of the Universe? Not really. Think of a balloon with many dots painted on the surface. When the balloon is filled with gas, and expands, every dot is moving away from every other dot. We are on of the dots.
The galaxy we live in is called the Milky
Way Galaxy , a Hubble type Sb spiral galaxy, containing about 100 billion
stars. It is about 100,000 light years in diameter and has a thickness
of about 2,000 light years. It is rotating eastwards or counterclockwise.
The period of rotation for our region of the galaxy is 200,000,000 years.
Like other galaxies, the Milky way has a bulge, a disk, and a halo.
Pictures below shows the Milky Way Galaxy from the side and from the top.
.
In the immediate vicinity of the Milky Way and confined to an ellipsoidal volume of space 900,000 pc along the major axis, and 800,000 pc along the minor axis is located a group of 21 known members of galaxies known as the local group. Other groups of galaxies are clustered together with more members with diameters of 300 million light years and having 1015 solar masses, which are known as superclusters.
Another close galaxy to us is the Andromeda Galaxy. Picture is shown below. For statistics: Statistics about Andromeda
For photographs of more galaxies, click below
The Andromeda Galaxy is moving towards the Milky Way Galaxy at a rate
of 200,000 miles per hour. The two galaxies will clash in about 3
billion years. Both galaxies are at the same time moving towards
the Constellation
Virgo
|
|
|
|
|
|
|
|
|
 Continue to the Stars |
|