Galaxies

Sorry for the long hiatus from posting on my blog. I'm gonna try to be more regular with this. Jennifer wanted me to explain galaxies after my last post so I will try to do just that.

Too fully understand galaxies you have to start with something even more mind boggling: the Big Bang. When the big bang erupted roughly 13 to 14 billion years ago it sent all of the contents and mass of our universe flying outwards in every direction. This mass, the entire existence of our universe, was no where near as spread out as it is now and was instead very condensed and hot, almost entirely gaseous in nature (almost all hydrogen at the very beginning). The widely excepted belief is that this superheated gas had pockets where it was more massive and condensed than in other places. After what is believed to be about 300,000 years these areas called proto-galaxies, due to gravity, had accumulated enough mass (and coincidentally heat) for the process of nuclear fusion to begin and making them the first stars. Now these stars were nothing like our sun. They were enormous, much, much larger and hotter than our quaint little star. The problem with stars this big and hot is that they live very short lives (only a few million years, unlike a star the size of our Sun which will exist for many billions of years).

To understand this next part you need to understand a little about nuclear fusion. Nuclear fusion occurs in a star when it reaches a high enough temperature to fuse the element hydrogen into helium. After it has used up all of it's hydrogen if it's massive enough it can fuse helium into carbon. If it is still even larger it can fuse carbon into neon, then neon into oxygen, and then oxygen into silicon. However the process ends when the star tries to fuse silicon into iron. This is impossible and the star will gravitationally collapse, resulting in a massive explosion (the largest in the universe) called a supernova.

These short lives end in extremely violent ways, that of a supernova. When these first stars went supernova they created the heavier elements that we are familiar with, like the carbon that we are mostly made out of and the nitrogen and oxygen that we breath.

To understand how this process resulted in the first galaxies being formed you have to understand a little bit about the most mysterious things in the universe: Black Holes.

When a super-massive star explodes into a supernova not all of that mass is expelled into the space around it. Some of that star reached such a critical mass that its very own gravity sucked inwards and became so dense that it formed a black hole. A black hole is something that is so densely pact into one place that its gravity has reached a point where nothing in the universe, not even light can escape.

These enormous black holes that were created continued to gather material and gas from the space around them until they had accumulated matter in an orbit around it (not unlike the Earth's orbit around the sun) that stretched for thousands of light years in each direction. This matter eventually formed into more stars and planets revolving around those stars until eventually, millions and billions of stars orbited these supermassive black holes. This is how galaxies were formed, at least in some theories.

Currently astronomy is being completely overhauled and changed to encompass the theory of dark matter which is a whole other topic that I have not read up on enough to fully understand at this time. However, this theory could completely change all current notions of what the universe is composed of and how we perceive it to work. Current estimation put dark matter as high 90% of the makeup of galaxies (mindblowing right?!?!).

Over time galaxies have combined and become larger. Some of them have burned out and died and some of them are producing new stars on a regular basis. Some of them are rich in plenty of elements and some of them are absent of many heavier elements like metals.

There are believed to be over 100 billion galaxies in the known universe. Of these galaxies almost all of them (95%) are close neighbors to another galaxy. For instance, we (the milky way galaxy) share the neighborhood with the galaxy Andromeda and our little cluster is a part of a much larger neighborhood of galaxies as well. All of these galaxies close by are the same age as the milky way, showing that we all came from the same enormous cloud of matter.

If I didn't answer what you wanted to know just ask me and I would be happy to add to this. :)

Determining Cosmic Distances

So I got my first astronomy question! Thanks pitrowski89 for asking!

He asked: How do Astronomers know how far away the farthest galaxy is?

I'll try my best to answer the question.

The truth is they don't have an exact estimate on how far away the galaxies are because of how extremely far away they are. This only becomes more difficult when astronomers start to view galaxies on the edge of the universe. Since our universe is expanding and is so incredibly large any galaxy viewed on the edge of our universe would be almost as old as the universe itself. However, with the use of powerful telescopes and computers capable of performing immense mathematical calculations (for instance general relativity) they have come up with a pretty good idea of how far away the farthest galaxies are. This is mainly through the technique of measuring a galaxy's red shift in the light spectrum, or how fast it is moving away from us.

http://www.universetoday.com/2004/03/01/record-for-furthest-galaxy-is-broken-again/

Closer galaxies are a bit easier to determine the distance to. There are many techniques astronomers have used to measure these distances and most of them give us accurate and proven answers. The most popular way for measuring any cosmic distance is by first finding the distance to nearby stars. Stars are easy to find the distance to because they can tell the size and brightness of a star by its color (an indication of how hot it is). The measurement of a star's brightness is called its magnitude. The brighter the object, the lower its magnitude (the Sun appears to us on Earth as -26.73). Since we are on Earth we can easily measure how bright a star's magnitude appears to us. This is called the apparent magnitude. The problem with this is that a really big, hot star that is far away might appear to have the same magnitude as a relatively normal star that is closer to us. For this reason there is also absolute magnitude, or a measure of how bright a star would be if viewed from one light year away (the Sun is actually -4.83). Like mentioned earlier, the color of the star is then used to determine how bright the star should be. Knowing both how bright a star appears, and how bright it would be if viewed from a standard distance, astronomers can then figure out how far away the star is.

The problem with galaxies is that they are much to big to be given an absolute magnitude. For this reason astronomers need to use different techniques to find out how far away galaxies are. There are tons of methods that have proven to be successful in helping us determine these distances. Here are most of them:
http://www.astro.ucla.edu/~wright/distance.htm

One of the most popular and proven methods for determining the distance to galaxies is actually based on the same principal as the one used for finding stars. There are some kinds of stars that don't have a standard brightness, they fluctuate. These stars are called variable stars. Though there are several different types of variable stars, only one of them, Cepheid variable stars, change brightness on regular intervals. Because Cepheid variable stars are very large stars (5-20 times bigger than the Sun) they are luminous enough to be seen in other galaxies, with the use of a powerful telescope that is. Since all Cepheid variable stars are about the same temperature, the only determinant left in how far away it is based on how big the star is. This is determined by measuring how long the variation in brightness is of the star. The longer the period, the larger the Cepheid variable star. And voila! The distance to the questioned galaxy can be determined.

Unfortunately, the farthest galaxies from us are so faint that we cannot see any individual stars in them. Sometimes we get lucky and a supernova of a star occurs (the brightest event known) which can shed some light on a galaxy's distance. Unfortunately, like I mentioned in the beginning, the galaxy is expanding. Anything we see now is what happened billions of years ago. How far away those galaxies are now is totally left up to theory. The current estimate is about 13 billion light years away or about 80,000,000,000,000,000,000,000 miles! Because this galaxy is so far away, on the edge of the universe per se, it is almost as old as the universe itself. This galaxy would be one of the first things created after the Big Bang.

If you have any questions about anything I have said feel free to ask! I tried to be as simple in explaining the question as possible. If you don't understand something I have said and want clarification I'd be happy to help you. My ability to explain this stuff in a logical fashion could make or break my decision to become an Astronomy teacher. :-D

Drew