Diane Bobkowski, Age 13, Of Bridgeport, Conn., for her question:
How hot is the brightest star?
When we look at the night sky, it is plain that some stars are much brighter than others. But our view of the heavens tends to fool us. A brilliant star at a great distance may look paler than a star which happens to be but a short sky distance away. And the stars which look brightest arc not always the hottest.
Our sun, of course, is thousands of times more brilliant than the brightest of the stars which appear in the night sky. But the sun is actually a medium type star in size, temperature and brightness. All the other stars look ditmner because they arc billions of miles away. And, no star, other than the sun is near enough to the earth for us to feel any of its warmth.
The brightest star we see in the sky is sirius, the dog star. But sirius looks only a few shades brighter than betelguese, the giant star which is some 40 times farther away. The brightness of a star is called magnitude, and each star has two magnitudes. Its apparent magnitude is the brightness it seems to have in our sky. Its absolute magnitude is its true brightness compared with other stars, regardless of distance.
A star of first magnitude is 2.5 times brighter than a star of second magnitude, which is 2.5 times brighter than a star of third magnitude. The brightest stars have minus magnitudes, and the apparent magnitude of the sun is minus 27. The apparent magnitude of Sirius is minus 1.6. Its absolute or true magnitude is 1.3. The absolute magnitude of our sun is 5, which means that it is actually about 50 times dimmer than Sirius,
The surface temperature of the sun is estimated to be around 6000 centigrade degrees, which is hot enough to melt iron and. Turn it to vapor. Sirius is 11,000 degrees, almost twice as hot as the sun. This brightest of stars is also almost twice the diameter of the sun. Its brilliant sparkle is due partly to its size and partly to its seething heat. But the most important factor is its nearness to us. Sirius is only 8.6 light years away, which places it among the nearest starry neighbors of the sun.
The dog star appears in the winter sky north of the equator. It is in the constellation Canis major, the big dog. This constellation follows the brilliant constellation of orion, the hunter, over the sky. Sirius., the brightest of stars we see from the earth, rises a little later than Orion, and it seems to sparkle like a white diamond at the heels of the celestial hunter.