Galaxies
During the early 1900's, which is not very long ago, astronomers were unaware that there were other galaxies outside our own Milky Way Galaxy. When they saw a small fuzzy patch in the sky through their telescopes, they called it a nebula. When examined closely, some of the nebulae had a spiral shape. So astronomers at first called these "spiral nebulae". These nebulae were all believed to be part of our Galaxy, our community of stars.
Edwin Hubble studied the "spiral nebulae" and found that they were composed of stars. These nebulae were not nebulae at all, but other communities of billions of stars held together by gravity - galaxies! Suddenly, our universe was much bigger. We realized that our Galaxy was just one of many billions of galaxies in the universe.
Hubble studied galaxies for a very long time, and after seeing many, many galaxies, he realized that he could put them into groups based on their shape: spirals, ellipticals, or irregulars. His work helped us to understand that the appearance of galaxies depends on our point of view, and on what's happening in the galaxies.
Nebulae - The Dust of Stars
Why would we call nebulae stardust? Because the gas in nebulae is used to make new stars, and dying stars create nebulae from their gas. While stars are made of very hot, dense gas, the gas in nebulae is cool and spread out. Water is at least 1,000,000,000,000,000 times as dense as the gas found in nebulae.
Nebulae come in a variety of shapes. But their appearance is usually due to the type of energy source which is lighting them up. There is lot of gas that we can't see in the Galaxy. Something must happen for the nebulae to reveal themselves.
So, what happens to make nebulae glow? It depends on the type of nebulae. Some are involved in the formation of new stars. These are stellar nurseries called HII regions. The young stars found in HII regions are extremely hot and provide a lot of energy for lighting up nebulae. Some nebulae are created by dying stars: supernova remnants and the planetary nebulae surrounding white dwarfs.
The Milky Way
When you look at the night sky, all the stars you see are a small part of our own Milky Way galaxy. The Milky Way is a spiral galaxy, and the sun is located about 30,000 light-years from the Milky Way’s nucleus on the Orion arm. If you see a creamy strip across the sky on a moonless night, you’re looking out at the outer edge of the Milky Way. (If you lived in the southern hemisphere, you’d be looking into the center of the Milky Way.)
Like all galaxies, the Milky Way is held together by gravity. This gravity also holds the stars, gas, and dust in orbit around the center of the galaxy. Just as the planets orbit around the sun, the sun orbits around the center of the Milky Way. Of course, it’s a long trip around---about 225 million years!
This map shows the full extent of the Milky Way galaxy - a spiral galaxy of at least two hundred billion stars. Our Sun is buried deep within the Orion Arm about 26 000 light years from the centre. Towards the centre of the Galaxy the stars are packed together much closer than they are where we live. Notice also the presence of small globular clusters of stars which lie well outside the plane of the Galaxy, and notice too the presence of a neighbouring dwarf galaxy - the Sagittarius dwarf - that is slowly being swallowed up by our own galaxy.
This is a drawing of the Milky Way looking down from above. The evidence for this picture is provided below. The Sun is just one of 200 billion stars in this typical barred spiral galaxy that is about 90 000 light years in diameter.
It should be emphasized that there are almost as many stars between the spiral arms as in the spiral arms. The reason why the arms of spiral galaxies are so prominant is that the brightest stars are found in the spiral arms. Spiral arms are the major regions of star formation in spiral galaxies and this is where most of the major nebulae are found.
These four galaxies look like the Milky Way. NGC 3953 (top left) is 55 million light years away and 95000 light years in diameter. NGC 5970 (top right) is 105 million light years away and 85000 light years in diameter. NGC 7329 (bottom right) is even further at a distance of 140 million light years but it is larger with a diameter of 140000 light years. NGC 7723 (bottom right) is 80 million light years away with a diameter of 90000 light years.
Click the pictures to enlarge !
