On our first day of the project, we talked about Color-Magnitude Diagrams. (For our purposes, a CMD is the same as an H-R diagram, named after Hertzprung and Russell.) To make a CMD, you need to gather the brightnesses and colors of stars. Astronomers gather this information in a funny system which uses magnitudes. Bright stars have small magnitudes. If we measure a star's magnitude through a bluer filter and a redder filter, then we can tell what color a star is by which filter it appears brighter in. So the difference in magnitudes through two filters gives us information about the color. Then you plot the brightness versus the color on a diagram like this:
It turns out that stars which are bluer are relatively hot, while stars that are redder are relatively cool. If you make a CMD for stars near the Sun, you find something like this:
Most normal stars appear on sort of a line in this diagram, called the Main Sequence. It turns out that the hot blue stars on the main sequence are very massive while the cool red stars on the main sequence are relatively lightweight. A few stars are not on the main sequence; this is because as stars age they evolve away from the main sequence. More about this later.
From now on, I will just use lines (like the line through the main sequence above) to show where the stars appear on these diagrams.
We discussed how we can use the main sequence to determine the distance to a group of stars. If the the main sequence for nearby stars has a certain brightness, the main sequence for a faraway cluster will appear fainter because of the extra distance. Since brighter is UP in the CMD, the comparison might look something like this:
We discussed how stars age, and how we can use this knowledge and a CMD to determine the age of a globular cluster. When a globular cluster forms, all of the stars basically form at the same time, and start their lives on the main sequence. So, a young cluster's CMD has just a main sequence. But the massive, blue stars are so bright that they burn their fuel up quickly, and start to evolve away from the main sequence. They become much redder and eventually explode as supernovae. As the cluster ages, more and more stars leave the blue side of the diagram and evolve up the red giant branch. By looking at what blue stars are left on a CMD, we can determine the age of the cluster. Here's a sequence of CMDs to illustrate all this:
So, to summarize: In this project you'll find the age and distance to a globular cluster, and you'll do it by finding how much fainter your cluster's main sequence is compared to stars of a known distance, and by noting the shape of the CMD to determine the age. Like this:
This cluster has a fainter main sequence than nearby stars
because of its distance, and it has no very blue
stars on its main sequence because of its age.