The Sun Is Absolutely Not The Brightest Star

The setting sun shines through a cluster of red pine needles north of Duluth recently. Photo: Bob King

Good to see the sun today. Closest of all the stars, the sun is 7 billion times brighter than the next brightest star in the sky, Sirius in the constellation Canis Major the Greater Dog. But is it really so brilliant?

Star brightness is ranked by magnitude. A first magnitude star is 2.512 times brighter than a second magnitude star, making 6th magnitude stars, the dimmest visible with the naked eye from a dark sky site, 100 times fainter than 1st magnitude stars like Deneb in the Northern Cross. For even brighter objects like the most brilliant stars and planets, the moon and sun, magnitudes are given as negative numbers. Here are some examples:

Sirius  -1.5
Jupiter -2.5
Mars at brightest -3.0
Venus -4.4
Full moon -12.7
Sun -27

As viewed from Alpha Centauri the sun would be a bright, first magnitude star in the W of Cassiopeia. Created with Celestia

Of course we know the sun is intensely bright because it’s so close. The nearest star system beyond ours is Alpha Centauri, located 25,809, 062,400,000 miles away. Imagine having to write long strings of numbers when describing star distances. That’s why astronomers use the more convenient “light year”, the distance a beam of light moving at 186,000 miles per second travels in a year’s time. Calculated this way, Alpha Centauri is 4.4 light years from Earth. Very tidy.

Astronomers have to have some way of comparing stars’ true brightnesses to one another. After all, just because a star is faint doesn’t necessarily mean it’s intrinsically dim. Likewise a dim star can appear bright if close by.

To correct for distance they measure a star’s absolute magnitude or how bright it would appear at a set distance of 32.6 light years or 10 parsecs. This seemingly odd number was chosen because a parsec is equal to 3.26 light years, the distance at which a celestial object appears to shift 1 arc second (a tiny amount!) against the background stars as viewed from either end of Earth’s orbit. By measuring a star’s apparent brightness and knowing its distance – determined by other methods – astronomers can compute its intrinsic brightness or absolute magnitude.

Seen from 32.6 light years, the sun would shine meekly at magnitude 4.8. You’d never see it from a city or most suburbs, and while visible from the country, it would be an undistinguished faint star. Sirius at 8.7 light years from Earth fairs better. Its absolute magnitude is 1.4, making it one of the brighter stars despite its loss of stature. Returning to Deneb in the Northern Cross, we’re in for a surprise. Here distance really makes a difference. Deneb is a blue-white supergiant star some 1425 light years away and shines at magnitude 1.2, but at 10 parsecs it would blaze at -7.0, bright enough to rival the light of a thick crescent moon. There’s a reason for its radiance; Deneb is 108 times the size of the sun and 55,000 times brighter.

Sirius and the sun are both hydrogen burning stars, but Sirius is hotter and larger. Credit: Wikipedia

So I got to wondering how far away the sun would have to be to shine as brightly as Sirius, now rising to the lower left of Orion around 9 o’clock. A quick check using the handy sun magnitude calculator revealed that our star would match Sirius if removed to just 1.8 light years. Since Sirius is 8.7 light years or nearly five times farther away, we know it must be intrinsically larger and brighter than the sun. Indeed, astronomers have determined it’s about twice the sun’s diameter and 25 times more luminous. At Deneb’s distance, the sun would be 13th magnitude and require a 6-inch telescope to see. Absolute magnitudes allow us to make real comparisons between one star.

Use Orion to find Sirius, the Winter Triangle and Lepus the next clear night. This map shows the sky facing southeast around 10 p.m. Created with Stellarium

You can add a dimension of reality to this discussion by stepping outside tonight around 9:30-10 p.m. and facing southeast. Orion’s Belt of three-stars-in-a-row will be halfway up in the sky. If you shoot a line through the Belt down to the lower left or southeastern horizon, you’ll arrive at twinkling, white Sirius. That’s not all. Connect Sirius with Orion’s bright orange-pink star Betelgeuse and Procyon in Canis Minor the Lesser Dog and voila, you’ve got the Winter Triangle. Now direct your gaze below brilliant Rigel for a fainter group of stars outlining the rabbit Lepus the Hare. Those bunny ears really make the constellation click.

4 Responses

  1. Great post!!!

    I believe it was in Guy Ottewell’s (incredible, indispensible) Astronomical Companion that I was clued into a star in Orion’s shield that is a near-match for the sun. It’s the one just below the center line of the shield, and is, as I recall, about 25 light years away. So there’s a little mirror for us…..

    1. astrobob

      Thanks Jim. I have more than a few of Ottewell’s giant booklets in my library. I haven’t bought one for a few years now, but remember well how his diagrams really gave the sky depth and dimension.

  2. Sira Sudhindranath

    How far away do we have to move from the sun (in any direction) when it becomes the second brightest star in the sky?

    1. astrobob

      From a distance of two light years the sun would be magnitude -1.23 or just a little fainter than Sirius the brightest star.

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