Galaxies are huge collections of stars, dust and gas and usually contain several million to over a trillion stars. Galaxies can range in size from a few thousand to several hundred thousand light-years accross. 90% of galaxies are actually composed of largely unknown substance called dark matter. Individual galaxies are separated by distances in excess of millions of light years. Every galaxy is controlled and held together by one central gravitational force. The Earth and its solar system are contained within the Milky Way galaxy, a large disk-shaped barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in width.
The Milky Way galaxy is believed to contain more than three hundred billion stars, and has a total mass of six hundred billion times the gross mass of the sun. Intergalactic space, or the space between the galaxies of the universe, is filled with an unsubstantiated plasma matter that has an average density less than one atom per cubic meter. There are believed to be more than one hundred billion galaxies within the total expanse of the universe. Galaxies with less than a billion stars are considered "small galaxies." In our own galaxy, the sun is just one of about 100 billion stars. There are three distinct types of galaxies: ellipticals, irregulars and spirals .
Elliptical galaxies make up around 10 - 15% of all the galaxies in the Universe and mainly comprise of older low-mass stars. Typically, an Elliptical galaxy will be found close to the centre of a galaxy cluster. Elliptical galaxies are also named because of their shapes. Elliptical galaxies range from circular (remember, a circle is an ellipse!) to long, narrow, and cigar-shaped. Elliptical galaxies are denoted by the letter E. They are also given a number from 0 to 7. An E0 galaxy looks like a circle. An E7 galaxy is very long and thin.
Elliptical galaxies have a broader range in size than other types of galaxies. The smallest are dwarf elliptical galaxies, which can be less than 10 percent the size of the Milky Way and contain only ten million times the mass of the sun. But ellipticals can also stretch to more than a million light-years across, and contain more than ten trillion stars. M87, identified as one of the largest galaxies in the universe, is classified as an elliptical galaxy — E0, to be exact. Scientists recently announced that the largest galaxy, IC 1101, was 50 times the size of the Milky Way and 2000 times as massive.
Most galaxies fit one of the three described types, but about 3% of the galaxies we observe are very different. These so-called "irregular galaxies" do not have a lot of common features. Many of them are the results of galaxy collisions or near misses. One type of irregular galaxy is called a "starburst galaxy." Starburst galaxies shine brightly as many new stars are born in a short period of time. Irregular galaxies are denoted by the letters Irr.
Irregular galaxies are usually found in groups or clusters, where collisions and near-misses between galaxies are common. In a few irregular galaxies, astronomers can't figure out why they look so strange!
So, how do irregulars form? It seems that they are typically formed through gravitational interactions and mergers of other galaxies. Most, if not all of them began life as some other galaxy type. Then through interactions with each other they became distorted and lost some, if not all of their shape and features.
Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the centre into the disk. These arms are brighter in appearence than the centre of the galaxy and this is due to the continuing formation of stars. Our own Milky Way has long been considered to be a Spiral galaxy but it is very difficult to tell from our position. It is beleived that spiral galaxies are the most common galaxies found in the Universe.
Most spiral galaxies contain a central bulge surrounded by a flat rotating disk of stars. Made up of older, dimmer stars, the bulge in the center is thought to contain a supermassive black hole, though observing it can be a challenge. The dim light from the older stars can make the bulge difficult to pinpoint, and there are some spirals that lack this characteristic. The supermassive black hole thought to exist at the center is often blocked by dust and gas surrounding it.
Orbiting the bulge, a rotating disk gives the galaxy its distinctive classification. The disk separates itself into arms that circle the galaxy. These spiral arms contain young stars that shine brightly before their quick demise, as well as a wealth of gas and dust. The brilliant stars are the reason the arms are so well defined.
The Milky Way
For hundreds of years scientists beleived that the Milky Way was the entire Universe. This beleif was shattered in the 1920's by Edwin Hubble when he peered through his 100" telescope at Mount Wilson Observatory in California. He noticed that distant stars were contained in nebulae that were too distant to be part of our galaxy. The Milky Way is home to our Solar System and 400 billion other stars. These stars, like our sun, have the potential to supply the right conditions for orbiting planets to sustain life. The size of the Milky Way has been a topic of debate for many years but astronomers estimate its size to be 100,000 light years accross and approximately 1,000 light years thick. To get a fair idea of the vast size of the Milky Way, take a look up at the sky at night - every object visible to the naked eye is located inside our galaxy.
Our solar system is made up of the sun and the nine planets that orbit it. The sun is situated about 26,000 light years from the centre of the galaxy on the outer edge of one of the spiral arms and it takes 250 million years for our solar system to complete one lap of the galaxy. In this lap, we (the nine planets and the sun) are travelling at 155 miles/sec. The arms of the Milky Way are named for the constellations that are seen in those directions. The major arms of the Milky Way galaxy are the Perseus Arm, Sagittarius Arm, Centaurus Arm, and Cygnus Arm; our Solar System is in a minor arm called the Orion Spur. The central hub (or central bulge) contains old stars and at least one black hole; younger stars are in the arms, along with dust and gas that form new stars. The Milky Way is located reletivly near other galaxies that make up the Local Group. Within this Local Group can be found the Milky Way's largest neighbour, the Andromeda Galaxy.