FREE ESSAY ON A GALAXY

A GALAXY

A Galaxy: Characteristics, Contents and Types
Galaxies are huge and contain billions of stars. They contain all of the stars of the
Herzspring-Russell diagram, open cluster, emission, nubula, reflection nubula, nova
remnants and supernova remnants. We have three different and distinct types of galaxies,
which are spiral galaxies, eluptical galaxies, and last but not least irregular
galaxies.
The Milky Way galaxy, which is a representative of a spiral galaxy, is basically
lens-shaped. Its circumference is about 120,000 light years across. On a dark summer
night, we are able to see a few of the Milky Way's arm. The Milky Way, as all spiral
galaxies, rotates on its axis. However, it does not rotate uniformly. The budge rotates
much faster than the arms and it takes about 250 million years to make a single rotation.
In the Milky Way galaxy, as in all spiral galaxies, there are two star populations.
Population I Stars are similar to our sun. Those stars are relatively young and orbit in
circular orbits in the galactic disk. The small mass of these stars consists of heavy
elements. These elements are heavier than helium and include carbon, oxygen, silicon and
iron. The Population II Stars are nearly pure hydrogen and helium with a small percentage
of heavy elements. These stars are in the center of our galaxy. Their orbits are random
around the bulge. These stars as a group usually have less mass and are redder in color.
Eliptica Galaxies are mostly made of old stars and contain almost no dust. These stars
can range from dwarf size to very massive. These galaxies can be up to two or three times
larger than the Milky Way Galaxy. They are smooth shaped and have no spiral arms.
Irregular Galaxies come in many shapes and are usually smaller than the Milky Way Galaxy.
There are several different types of Irregular Galaxies like Type I, Type II, Peculiar.
Some common properties that galaxies are measured by are distance and size. Distance,
which is the most basic property, is not an exact science. Galaxies do not have sharp
edges and vary in size, so it's very hard to get an exact measurement. Galaxies may be as
few as thousands of light years in diameter and as much as 600,000 light years. Size is
measured by luminosity. Two issues must be addressed to calculate luminosity. First, one
must measure the total range of radiation given off by the galaxy. Second, one must
compensate for the fact that the light given off is not evenly distributed across its
diameter. With these two issues in consideration astronomers can make a model of light
distribution and use it to calculate luminosity.
Color and rotation are two other properties used to classify galaxies. But the most
important and hardest to measure is mass. There is a relationship between mass and
luminosity. The more mass the brighter the star and the reverse is also true. Also, about
90% of the mass of a galaxy is dark matter.
Galaxies also can do other interesting exhibits. They can collide, interact and merge
with each other. They can also form clusters and super clusters. These big blobs of gas
and dust are magnificent and almost have a life of their own.