The flashing Dog Star

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On a winter's evening, with the snow crunching beneath your boots and your breath hanging in frosty clouds in the air, your eye may catch upon a single star glimmering like a multifaceted diamond through the dark silhouettes of bare trees in the southeast. As you gaze upon this scene, you may imagine a kind of stillness settling upon the world, or perhaps within yourself. At such contemplative times it is hard to tell. But something about 'lone stars' strikes an emotional chord within us all. Can there be no greater symbol of solitude and yet dignified defiance than a single star shouting down the night?

Perhaps no star in our hemisphere evokes such feelings more than Sirius the 'Dog Star,' which is the star you see in the southeastern sky during the early evenings this week. Sirius is not only the brightest star in the constellation Canis Major (the Greater Dog), it is also the brightest star in the night sky. The only other star seen from Earth that is brighter than Sirius is the Sun. But though the Sun may outshine Sirius from our perspective, if you could place the two side-by-side in the sky an equal distance away, Sirius would be the brighter star - 23 times brighter, in fact. The reason: Sirius is not only nearly twice as large as the Sun, but it is a much hotter type of star and hence more intrinsically luminous.

Although Sirius may look like a single star, it isn't alone in space. Orbiting Sirius is a small companion star, which was found in 1862 by Alvan George Clark, an eccentric but venerated lens maker, who, at the time of the discovery, was testing the optics of one of his telescopes for a Chicago observatory. (I'd say the test was a success.) Unfortunately, this companion, called Sirius B, is not readily apparent in small telescopes; it takes a powerful, well-tuned set of optics to catch its glint next to Sirius A, which is 10,000 times brighter.

Nevertheless, a window of opportunity to glimpse this tiny sun is opening for observers with larger refracting and reflecting telescopes. Remember that Sirius B orbits Sirius A in an elliptical orbit, and as such there are times when it lies farther from the blinding star than others. Sirius B completes one orbit around Sirius A every 50 years. In 1994, the little star reached a minimum separation distance of about 3 arcse-conds. Since then, this distance has been increasing. During the first 25

years of the twenty-first century, the separation distance between the two stars will grow to a maximum separation (in 2025) of 11.5 arcse-conds. By then a 4-inch refractor or 6- to 8-inch reflector should be able to split the pair.

Until then, you're going to need access to a high quality 12-inch reflector or 6- to 7-inch refractor to see Sirius B. (If your local astronomy club or college observatory conducts a public star party or starwatch during the winter months, you can ask them to try and split Sirius A and B for you.) Look southeast of Sirius for the companion shining in the brighter star's glare. By 2002, Sirius B should be about 5.5 arcseconds from Sirius A and should just be visible in telescopes of this size and quality. The task becomes easier in 2005, when the separation widens to 7 arcseconds, and in 2010, the separation is about 9 arcseconds, with the companion due east of Sirius.

Things just keep getting better until 2025 when the star reaches maximum separation, called apastron. (The star will then be northeast of Sirius.) Thereafter, the separation decreases until the next minimum, called periastron, in 2043.

Astronomers classify the companion as a white dwarf, the core of a dying star that has collapsed so much under its own gravity that it is very dense. This companion has roughly twice the diameter of Earth, but its mass nearly equals the Sun's: a cupful would weigh over 50 tons.

Sirius' intense brightness, and the fact that it doesn't rise very high in the sky as seen from the Northern Hemisphere, conspire to trick people unfamiliar with the stars into believing they are witnessing a UFO. Whenever it is seen near the horizon, be it in the east or west, the star's brightness with respect to the landscape endows Sirius with the illusion of nearness. Furthermore, the agitated mixing of warm and cool air cells in the atmosphere bends the rays of starlight in random directions like a prism, making Sirius appear to shift rapidly in color from red to blue as well as jiggle around in position. The rapidly changing colors and the apparent jiggling animate the star to such an extent that it seems to be a nearby UFO maneuvering anomalously in the sky. Some terrified individuals have even reported being 'pursued' by Sirius, a phenomenon not unlike the illusion of being followed by the Moon.

The Dog Star is indeed nearby, though not in terrestrial terms. At 8.6 light-years, or a little over 51 trillion miles, Sirius is the fifth-nearest star to the Sun. Astronomically speaking, this is a mere hop, skip, and a jump away. Nonetheless, it would take tens of thousands of years to cross this distance using conventional spacecraft. Astronomical velocities are required to traverse astronomical distances.

So, when you look at Sirius this week, consider it a neighbor - like a farm on the far horizon with the house lights on. Truth is, no star that we can see this time of year is so far from Earth that its light can't warm our spirits on an otherwise cold winter night.

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