The Definition of Color in Astronomy

Stars

As you can see, the magnitude of a star through any given filter has little bearing on the star’s color. Stars with similar magnitudes in the g filter, for example, might have very different colors.

But when you look at the difference in magnitudes between two filters, then you should see that stars with similar values have similar colors. Red stars have similar values for magnitude difference, blue stars have similar values for magnitude difference, and white stars have similar values for magnitude difference. The order is not perfect, but nothing in nature ever is.

You have just discovered how astronomers classify color! Astronomers look at the five magnitudes of a star and measure the difference between any two. The g-r value is one way to find color, but astronomers have other options as well because the SDSS uses five filters. Astronomers could also use u-r, r-i, or i-z to measure color.

When an astronomer talks about a star’s “color,” then, he or she is talking about these magnitude difference measurements: g-r, u-r, i-z, and so on. If you asked an astronomer what the color of a star is, he or she wouldn’t say “red” or “white”; he or she would say something like “this star has a g-r color of 1.3.”

Magnitude

A star’s color is measured by its magnitude, which tells how bright a star or galaxy appears from Earth. Astronomers have used versions of the magnitude scale for thousands of years, so they keep using it even though the scale is a bit confusing. In the magnitude scale, higher numbers correspond to fainter objects, lower numbers to brighter objects; the very brightest objects have negative magnitudes.

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An increase of one number in magnitude corresponds to a decrease in brightness by a factor of about 2.51 – a magnitude five object is 2.51 times fainter than a magnitude four object. The sun has magnitude -26. The brightest star in the Northern sky, Sirius, has magnitude -1.5. The faintest object you can see with your eyes has a magnitude of about 6; the faintest object the SDSS telescope can see has a magnitude of about 23. If you’re curious about the magnitudes of other famous stars, take a look at this table of the 314 brightest stars.

Question 1. In the magnitude scale, increasing the measurement by one means decreasing the quantity measured by a factor of 2.51. Scales where the measurement increases by addition when the quantity measured increases or decreases by multiplication are called “logarithmic” scales. Can you think of any other quantities that are measured using logarithmic scales?

Now you know how astronomers measure a star’s color. But what exactly is “color,” the quantity they are measuring? Not surprisingly, a star’s color is caused by the color of light the star gives off. But what does it mean for light to have a certain color? Click Next to find out.