Spot the brightest star in the sky

By Joe Rao

updated 1:07 p.m. ET March 14, 2008

An increasingly bright gibbous moon will obscure many of the dimmer stars in our sky during this week, but certainly not Sirius, the Dog Star, the brightest star in the night sky.

Many astronomy books suggest you can locate Sirius by using the belt of Orion; that the belt points southeast directly toward Sirius. That's absolutely true, although all anyone needs to do is simply cast a glance toward the southern sky these cold late winter evenings after it gets dark and you'll immediately see Sirius. It will be due south between 7:30 and 8 p.m. local time all of this week, and won't set in the southwest until around 1 a.m.

Sirius is the brightest star of the constellation Canis Major, the "Greater Dog," in Latin. According to Burnham's Celestial Handbook other names for it include "The Sparkling One" or "The Scorching One."

This star appears a brilliant white with a tinge of blue, but when the air is unsteady, or when it is low to the horizon it seems to flicker and splinter with all the colors of the rainbow. At a distance of just 8.7 light years — just over 50 trillion miles — Sirius is the fifth-nearest known star. Among the naked-eye stars, it is the nearest of all, with the sole exception of Alpha Centauri. This also explains why the Dog Star is one of a handful of stars that seemed to have shifted in relation to their neighbor stars since people first started making records of the sky: its direction in the sky changes as much as the apparent width of the full moon over a span of 1,500-years.

The Pup
Friedrich Wilhelm Bessel (1784-1846) was a German mathematician and astronomer, who went on to obtain precise positional measurements of Sirius. His observations revealed that Sirius was slowly moving in a wiggly line across the sky. In 1844, Bessel had a sufficient number of precise observations to announce that Sirius must have an unseen companion. The orbital period of the two stars around each other turned out to be about fifty years.

In 1862, Alvan G. Clark (1832-1897) became the first to sight Sirius B, also known as "the Pup," the traveling companion star responsible for the wiggle. Sirius B is only one ten-thousandth as bright as Sirius A, but by 1914, spectroscopic observations had demonstrated that its temperature was about the same. From physical laws it follows that B emits the same amount of light per unit surface area as A, therefore to be so dim it must be very small.

Later calculations have shown that A has just over twice the mass as our sun, but B has nearly one solar mass. Since it is so small, B must be exceedingly dense. In fact, it packs 98 percent of one solar mass into a body just 2 percent of the sun's diameter. To do that, Sirius B must have a density 90,000 times that of the sun.

A teaspoon of this star material would weigh about 2 tons.

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