February's Star is Sirius

This month look for the star Sirius. Sirius is the lucida of the constellation of Canis Major, the Big Dog and its half way up in the sky when you face towards the southwest during February nights. It’s the brightest star in the heavens and only the planets Venus, Mars, and Jupiter can surpass its nighttime brilliance. Sirius is a bright white star that appears to throw off sparks of color when it’s low to the horizon. That’s because there’s more atmosphere low in the horizon to refract its starlight. If you were born in 2006, then Sirius is your birthday star this year because the light you see tonight left Sirius 9 years ago. The name Sirius comes from the Greek word for scorching. During the Dog Days of summer, which occurs in early August, the sun and Sirius are close together in the sky. The Greeks believed that the heat of Sirius added to the sun’s heat, making these days especially hot.  

Sirius has a strange companion. Back in the early 19th century, astronomers discovered that the star wobbled back and forth as it slowly drifted across the sky. It was as if the gravity of a massive star was tugging on it. However, try as they might, no astronomer could discover a star close to Sirius to account for its wobbly motion. It wasn’t until 1854 when Alvan Clark turned a new 18 inch refracting telescope to the star that he discovered a tiny spark of a star next to Sirius. The star was not really that faint, but its closeness to Sirius made it impossible to see with previously telescopes.

The companion to Sirius, called the Pup Star, orbits Sirius with a period of fifty years. From the amount of tugging Sirius experiences from this star, we know that the Pup Star has a mass equal to our sun. What’s so surprising is that if we viewed the sun from nine light years away, we would easily see it with the unaided eye. However, the Pup Star isn’t. What gives?  Stars like the Pup Star have the spectrum of a very hot star. Therefore, every square foot of these stars is more intense than an equal area of our sun. The low total brightness but high surface intensity of these types of star tells astronomers that they must be very tiny, about the size of our planet. We call them White Dwarf stars.

Once each white dwarf was like our sun, moderately bright and moderately large. Once its nuclear fuel ran out, the star collapsed on itself because it could no longer support its weight using nuclear fusion. Only the repulsion between electrons keeps white dwarf stars from collapsing any smaller. The compression of a star into a white dwarf makes it incredibly dense. A billiard ball of white dwarf weighs as much as a tank, or about the same weight as 100 family cars.