Thursday, April 16, 2009

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.

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:

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



  1. Thanks for the response. I know some kid at school that has a green laser pointer. Remember when Murphy told us that if you point the laser at a star/planet, the laser will eventually reach it? That's what brought up my question. We don't really use the green laser pointer for astronomy purposes here at school though. Just to shoot it at buildings and things across the Mississippi. It's pretty cool.

  2. Have you talked to Murphy? When we get back, we should try to say hello.

  3. What would you like to know about Galaxies?