Spot the Brightest Star in the Sky

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 daylight 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 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.

Dog days

Everyone talks about "dog days" but few know what the expression means. Some will say that it signifies hot sultry days "not fit for a dog"; others say it's the weather in which dogs go mad.

But the dog days are defined as the period from July 3 through Aug. 11 when the Dog Star, Sirius, rises in conjunction (or nearly so) with the sun. As a result, some felt that the combination of the brightest luminary of the day (the sun) and the brightest star of night (Sirius) was responsible for the extreme heat that is experienced during the middle of the summertime. Other effects, according to the ancients, were droughts, plagues and madness.

A more sensible view was put forward by the astronomer Geminus around 70 B.C. He wrote: "It is generally believed that Sirius produces the heat of the Dog Days, but this is an error, for the star merely marks a season of the year when the sun's heat is the greatest."

Star of the Nile

In ancient Egypt, the New Year began with the return of Sirius. It was, in fact, the "Nile Star" or the "Star of Isis" of the early Egyptians.

Interestingly, some 5000 years ago, this "heliacal rising" (appearing to rise just prior to the sun) occurred not in August, as is the case today, but rather on, or around June 25. When they saw Sirius rising just before the sun, they knew that the "Nile Days" were at hand. Its annual reappearance was a warning to people who lived along the Nile River. The star always returned just before the river rose, and so announced the coming of floodwaters, which would add to the fertility of their lands. People then opened the gates of canals that irrigated their fields.

Priests, who were the calendar keepers, sighted the first rising of the Dog Star from their temples.

At the temple of Isis-Hathor at Denderah is a statue of Isis, which is located at the end of an aisle lined by tall columns. A jewel was placed in the goddess' forehead. The statue was oriented to the rising of Sirius, so that the light from the returning Dog Star would fall upon the gem. When the priests saw the light of the star shining upon the gem for the first time, they would march from the temple and announce the New Year.

In the temple appears the inscription: "Her majesty Isis shines into the temple on New Year's Day, and she mingles her light with that of her father Ra on the horizon."

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.