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January 28, 2014

The big dog trots across the southern evening sky this week. It’s quite easy to spot because it’s home to Sirius, the brightest star in all the night sky. It’s in the southeast in early evening, with the rest of the constellation below it and to the sides.

Sirius really is a fairly bright star, but it’s a mere firefly compared to some of the other stars of Canis Major. One example stands close to the right or upper right of Sirius during the early evening.

Mirzam looks much fainter than Sirius, but that’s a bit of a trick. The star is really thousands of times brighter than Sirius, but it’s also much farther — about 500 light-years, versus less than nine light-years for Sirius.

That means that Mirzam is quite a stunner. When you add up all forms of energy, it’s thousands of times brighter than Sirius, and tens of thousands of times brighter than the Sun.

The key to its brightness is the star’s great mass. It’s so heavy that its gravity squeezes its core tightly. That’s like mashing your foot down on the gas pedal of your car — it revs up the nuclear reactions in the star’s core, making it produce much more energy. But also like your car, mashing down the pedal uses up the fuel much more quickly. So despite its great size, Mirzam will live less than one percent as long as the Sun will. When its time is up, it’ll explode as a supernova — a brilliant ending for a brilliant star.

We’ll have more about the big dog tomorrow.


Script by Damond Benningfield, Copyright 2013

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