Let's Talk About: The Bright stuff

July 5, 2012 12:19 am

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Pick a star in the sky and ask yourself how bright it is. Brighter than the one next to it, perhaps? And why is it so bright? If it sounds simple, think again. It's a puzzle that astronomers say our eyes alone cannot solve. In order to truly understand the bright stuff, we need physics at the ready.

The measure of a star's brightness is referred to as its magnitude. Astronomers arrange magnitudes on a scale in which dim stars are signified by high numbers and bright stars by low numbers. A star with a magnitude of less than zero is considered exceptionally bright.

Think of our sun. Nothing in the sky is brighter than the light from our parent star. That is, if you're talking about its apparent magnitude, which is the brightness of a star as viewed from Earth. Because the sun is so close to us, it has the highest apparent magnitude of all celestial objects: --26.7.

But while the sun is king by apparent magnitude, by absolute magnitude it's not. Absolute magnitude refers to the brightness of an object as it would be viewed 10 parsecs (about 32.6 light-years) from Earth. Consider the star Rigel in the constellation Orion. It may appear as a minor sparkle in the sky, but if all stars were the same distance from Earth, Rigel would outshine them all.

We can further understand a star's brightness by determining its luminosity, or the amount of energy it radiates per second at all wavelengths -- not just visible light. But if physical contact with stars is impossible, how do we measure this inherent property of theirs? The answer lies in clever math. Because absolute magnitude provides a measurement of brightness at a standard distance, astronomers can insert its value into a formula and, thereby, convert it to units of luminosity. Thus, we have crucial criteria for classifying stars.

First Published July 5, 2012 12:00 am