January's Star is Sirius

This month look for the star, Sirius. Sirius is the lucida of the constellation of Canis Major, the Big Dog. It’s half way up the sky when you face towards the southeast on January evenings. Sirius is the brightest star in the heavens and significantly brighter than any other star. In fact, the second brightest star, Canopus, is only half as bright as Sirius. Of the planets, only Venus, Mars, and Jupiter can surpass Sirius in brilliance. Sirius appears as a bright white spark of light. When it is low to the horizon, the star throws off shards of color because there’s thicker atmosphere 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 nine years ago. The name Sirius comes from the Greek word for scorching. During the Dog Days of summer (early August), the sun and Sirius reside close together in the sky. The Greeks believed that the additional heat of Sirius added to the sun’s heat to make these days especially hot.

A companion named Sirius B accompanies Sirius. Astronomer’s hypothesized the existence of Sirius B as early as 1844 due to the wobbling they observed in Sirius’s slow motion across the sky (a movement called the star’s proper motion). From the amount of measured wobble, astronomers deduced that the companion star had a mass equal to the sun. This created a problem, however. A star like the sun nine light years away would be easily visible. As the companion of Sirius was not visible in telescopes, the star had to be incredibly faint for its mass; and an astronomical mystery. It wasn’t until 1862 that astronomers finally observed the star through an 18.5 inch refractor telescope. Later measurements of the star’s spectrum indicated that it was incredibly hot. So hot in fact, that its surface brightness was greater than the sun’s surface brightness. The star’s high surface brightness but low total brightness was a sign that this star is a white dwarf star. Some 120 million years ago, Sirius B, or the Pup Star was a star like the sun. Today however, the star’s nuclear core has shut down, letting the star collapse into a smaller sphere. Since atoms consist of mostly empty space, there’s enough room for the atoms to squish down to one millionth their original volume. That compression is needed to convert a sun-like star into a white dwarf. White dwarf stars like Sirius B have a density in far excess of any material known on earth. Their typical densities are around 200,000 times greater than the density of Earth. Therefore, a sugar cube sized piece of white dwarf weighs roughly one ton on Earth.