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Moon and Antares

August 18, 2018

Antares, the heart of the scorpion, is the 11th brightest star that’s visible from most of the United States. But if we could line up those stars at the same distance, Antares would zoom way up the list.

How bright a star looks depends on two things. First is its true brightness — how much light it beams out into space. And Antares is quite high in that category. It’s hundreds of times the diameter of the Sun, so it’s quite bright — it radiates about 10,000 times more visible light than the Sun does.

The other factor is distance. Among those 11 brightest stars, the distance ranges from about nine light-years up to about 850. There’s a good bit of uncertainty among the farthest stars, but even so, we know they’re a long way off. Antares, for example, probably is about 550 to 600 light-years away. That makes it appear fainter than many of the other stars on the list.

When astronomers compare true brightness, they use a number called absolute magnitude. It’s how bright the star would look at a distance of 10 parsecs — about 33 light-years. On that scale, the brightest of those 11 stars would shine more than 10,000 times brighter than the faintest. And Antares would rank third, looking more than 300 times brighter than it is now — even brighter than Venus, the brilliant “evening star.”

Look for bright orange Antares to the lower left of the Moon as darkness falls tonight, and to the lower right of the Moon tomorrow night.

Script by Damond Benningfield

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