stars

This is a great time to go out and look at stars. It is not as cold as February and March but the sky is still clear. Go out about 9 p.m. and look up. If you are not familiar with the stars you may look around, point, and ask, “What’s that star?”

Chances are the “star” you are pointing to isn’t a single star. It is probably a double or even a triple star. Take Sirius for example. You can’t miss Sirius, it is the brightest star in the sky, fully 10 times brighter than any other star (though not as bright as Jupiter, Venus, or occasionally Mars). It is located about mid-sky, slightly southwest at dark. It is in the constellation Canis Major, or in English, “The Big Dog”, hence its nickname, “The Dog Star.”

Three things determine how bright a star appears. First is distance. Sirius is close. Only six other stars are closer. Most people know Alpha Centauri is the closest star to our own sun, but it isn’t just one star, it is actually three; two stars the size of our sun, and a red dwarf all revolving around each other. The red dwarf is actually closer to us than the other two and it is named accordingly, Proxima Centauri. We can’t see the Alpha Centauri system from the Northern Hemisphere, and anyway, with the naked eye the three stars look like one.

The next three stars closest to us are red dwarfs that are very hard to see and you probably haven’t heard of them anyway. Then you have Sirius. The brightest of all the stars as seen from Earth. OK, not just Sirius, but Sirius A and Sirius B. Sirius is a double star.

The second thing that determines brightness is size. As you would guess, larger stars are brighter than the dwarf stars. Sirius A is more than double the size of the sun and Sirius B, a white dwarf, is about the size of the earth, but with a lot more mass. Sirius B is a dying star. It will eventually burn out to become a black dwarf, or a dead star.

Bigger than the sun and close, the third thing is luminosity. Sirius is a Type A star that burns hot, about twice the surface temperature as our sun, and because of that it is bright white. Our Sun is a Type G star that is not as hot and hence has a yellow color.

Colors can be confusing however when you are looking at a star from earth because of our atmosphere. Sirius burns bright white in the sky, but at times, due to our atmosphere, it will have a hint of red or blue, and even green. It may change colors while you watch. Sirius is the most often reported “UFO” by the general public.

The ancient Egyptians called Sirius “Sopdet” and to them it represented the goddess of fertility. In late August Sirius rises just before the sun in the eastern sky. Late August is also the time of year that all the rains from the wet season in the Ethiopian highlands of present day Ethiopia, Uganda, and Sudan make their way down the Nile to the Mediterranean. When the Egyptians saw Sirius rising just before the sun they knew to prepare for the annual flooding of the Nile. Those floods replenished the soils of the valley and are the reason for its fertility. The timing of the floods was so important that the Egyptian calendar was based upon the rising of Sirius.

So step out right after sundown and take a quick look. The moon will rise later and later, giving us dark skies. Locate Sirius, trust me, it won’t be difficult, and then look around at the other stars, or groups of stars. Chances are that more than half of the “stars” you see are actually double or triple stars.