Wednesday, April 25, 2012

How Old is that Star?


Determining the age of a star is not as easy as you might think. Since we can't ask a star how old it is, we have to guess the stars age by its appearance. And just like with humans sometimes looks can be deceiving! 





 

There are many ways to determine the age of a star, and today we will discuss stellar models. Like we've discussed before, stars can be placed on an Hertzsprung-Russel (HR) diagram. To do this, you need to measure the stars brightness, or luminosity, and you also need to know what type of star it is. Is it a big, hot blue star,? A cool, small, red? Somewhere in between? Astronomers can determine this by looking at a star's spectrum, or distribution of light, with a telescope. Once we know these two things, we can place the star at the proper position on the HR diagram. Astronomers have been hard at work modeling how stars form, and how their size, temperature, and brightness changes as they age. They have developed paths or lines that are placed on the HR diagram which show a stars path on the graph as it ages. There are models for before the star has reached the main sequence, and after. Basically what you do, is place the star on the HR diagram, see which line it is closest too, and that tells you the stars size and age. Here is an example of how this works. The graph above shows brightness vs. temperature, and models (solid lines) for stars of different masses. Stars, in theory, follow one solid line path going right to  left as it ages. The star represents the spot on the diagram where some arbitrary star's properties are. Based on its position, the star is probably about 4 times the mass of the sun, and about 200,000 years old! This is before it has started hydrogen burning, and is still a "baby" star. You can follow the same method with different models and estimate the age of a star that is burning hydrogen, or on its way towards death.