Sorting Stars by Color
Sorting Stars by Color
Goals: Contrast the differences in stellar color.
Before starting, turn on: stars
You will be using: galaxy, see and showbox commands
The color of a star is a measure of its temperature and provides astronomers with a means of classifying stars. With color as a discriminator, two stellar populations are apparent, revealing the underlying structure of our Galaxy.
Color Is Derived from Two Brightness Measurements at Different Wavelengths
In the Milky Way Atlas, we use the (B - V) observed color to assign the stars their visual colors. This is the subtraction of the B and V filter brightnesses. Astronomers observe a star through many filters, then combine them to get an overall color for the star or object. In this case, the B filter is a blue light filter that sees light between about 400 nm and 480 nm. The V filter is roughly equivalent to the human eye and only allows light between about 500 and 600 nm to pass through. The difference between the measured magnitudes from each filter is the (B - V) color of the star. For this color, blue stars will be high in the B filter, resulting in a lower magnitude (remember, the brighter the star, the lower its magnitude) and a negative (B - V) color. Red stars will be brighter in the V filter, resulting in positive (B - V) colors. Let's see this in the Atlas.
In the Milky Way Atlas, move away from the Sun so that you see most of the stars (be sure to turn the mwVis group off). If the stars are too dim, feel free to use the Slum Slider to increase their brightness. As you orbit the Sun from this great distance, you will select stars based on their (B - V) color. The datavar command reports that the colorb_v variable ranges from -0.63 to 2.057, with an average value of 0.535.
We've set up two selection expressions to show the color differences among stars. These selection expressions are defined in the mw.cf file and are called blue and red.
Blue Stars Are Young, Hot Stars
First let's explore the bluer stars by typing
see blueYou should see a complete change in the data with a flat distribution along the Galactic disk. This might make sense, since the blue stars are hotter stars that have shorter lifetimes, burning their nuclear fuel at a high rate. These are the gas-guzzlers of stars. Because star formation takes place in the disk of the Galaxy, it makes sense that these short-lived stars would be found here.
As an aside, culling the blue stars better reveals a feature of larger Galactic structure called Gould's Belt. It is a local grouping of stars inclined about 16o to the Galactic plane. If you type showbox 1 in stars and turn the boxes on, a box will appear around the bright blue stars. Press the Box Toggle Button to turn the boxes off.
Red Stars Are Cooler Stars
Now let's see the redder stars by typing
see redThe red stars are more numerous (more than 80% of the stars) and are distributed in a spherical shape. Of course, as these are observed from Earth, the distribution is centered on the Sun. But one can imagine the entire Galaxy filled with these dim stars.
Red stars are cooler stars that form throughout the Galaxy. They burn their fuel slowly and have long lifespans; some stars are as old as the Galaxy. The Sun is among these red stars, a relatively cool star with a (B - V) = 0.65. It has been burning its fuel for about 5 billion years and has about another 5 billion to go before entering the end stages toward a red giant and a white dwarf star.
Stellar Color Traces Galactic Structure
These two contrasting views tell us about the structure of the stars in our Galaxy. The red stars are typically known as Population II stars and revolve around the center of the Galaxy in elliptical, sometimes highly inclined orbits. Bluer stars are referred to as Population I stars and lie in the Galactic disk, orbiting in orderly paths around the Galactic center. While we plot only the stars we have accurate distances for, you can imagine the disk [turn on the Milky Way image] filled with blue stars with many red stars as well. Also imagine a spherical distribution around the entire Galaxy filled with cool, red stars. This is called the Galactic halo, and we'll talk about it soon.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott