The first thing that we will discuss will be galaxies. Galaxies are very large collections of gas, dust, and stars orbiting a common mass. When most people think of galaxies, they have this picture in mind (click on the link to view the picture), however, this spiral galaxy is only one of three main types. The three main types of galaxies are Elliptical Galaxies, spiral Galaxies, and Irregular Galaxies. They actually are quite different, and not just in their appearance.
Spiral
Galaxies
contain spiral arms made of hot, blue
stars, clouds of dust, gas, and young star clusters. The three main parts of
this galaxy are the disk, the halo, and the spiral arms. We know that these
spiral arms are not connected to the disk because if they were, the galaxy would
destroy them within only a billion of years because of the strong differential
rotation. The spiral arms must be very stable, however, because they retain
relatively the same shape even though everything is always changing. The reason
is that the spiral arms are moving around relatively slow around the galaxy
compared to everything else in space. Spiral galaxies contain both hot, young population I stars and cooler, old population II stars.
Some examples of Spiral galaxies are M31, M32 and M87.
These are two spiral galaxies with distinct spiral arms.
Elliptical Galaxies
are round shaped and have no visible gas
or dust. They also lack hot, bright stars that spiral galaxies have. They range
in sizes and are really old. They aren't always a perfect sphere shape, but some
can be flat, oval, or elongated. They are actually seen more commonly in the
night sky than spiral galaxies, but they are hard to recognize. If you saw one,
it would most likely look like a white fuzzy dot unless your telescope was
really powerful. Elliptical galaxies contain mostly population II stars.
Some examples of elliptical galaxies are M102, M84, and M85.
The first picture is a bunch of elliptical galaxies in the Virgo Cluster (I will also mention it on my Cluster page!) The second picture is a close-up of an elliptical galaxy and from first glance, it almost looks like a massive star!
Irregular Galaxies
are all the other galaxies that aren't classified in any of the other groups
yet. Irregular galaxies do not have a disks, spiral arms, or any traditional
shape, but they can get extremely large. The gravity from an Irregular galaxy
sometimes interacts with other galaxies with is why they are such an odd shape.
These galaxies are not as common in our visible universe as the other two types.
One example of an Irregular galaxy is M82.
These are two irregular galaxies. AS you can see, their isn't a particular shape that holds true to all irregular galaxies, but they do have a lot of stars.
Sometimes galaxies can
collide. The galaxies are so big and the stars are so far apart, that a two
galaxies can pass right through each other without a single star colliding.
Sometimes, however, the galaxies have a strong gravitational pull and can
"unwind" each other.
Go to a website that has a video simulation of two galaxies colliding! (when you get to the site, click on the link that says "galaxy merger animation")
Where do we live? Lets
find out!
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