Spending Time with Dogs
by Drew Farwell
The constellation Canis Major contains within it one of the most storied celestial objects in the heavens. Commonly known as the “dog star”, Sirius is the brightest star in the night sky. It is so bright in fact, that at times it shines brighter than all other bodies, save the Moon, Venus and Jupiter. But what if I told you this bright star is not really a star at all? It is in fact two stars. Or more accurately, a binary star system with a massive primary star and a smaller secondary star locked in orbit around an invisible shared center of mass.
The primary star, Sirius A, is a white main-sequence star twice as massive as our Sun and twenty five times more luminous. The secondary star, Sirius B, is a white dwarf star nearly equal to the Sun in mass but only the size of the Earth in volume. The distance from Sirius A to B varies between 753 million miles and 2.9 billion miles. At their closest, the two stars are closer together than the Sun is to Jupiter. At their most distant they are further apart than the Sun is from Neptune. In our solar system, that difference in orbital distance is large enough to house the two gas giants Saturn and Uranus.
What this means for amateur astronomers is that there is enough angular separation between Sirius A and Sirius B for them to be observed as a double star in a large (twelve inch or larger) telescope. That is no small feat considering we are viewing them from 50.6 trillion miles away. While that distance seems unfathomable, this binary system is contains two of the eight nearest stars to our solar system. And they’re getting closer. They will continue to grow brighter over the next 60,000 years until they finally recede and begin their journey away from us. Even after they have passed, they will remain the brightest star in the night sky for another 150,000 years.
Sirius has played a crucial role in astronomy throughout mankind’s history. In ancient Egypt, the arrival of Sirius meant the coming of the flood waters of the Nile. With Sirius’ arrival farmers could anticipate and redirect the mineral rich waters to canals allowing them to fertilize and irrigate the arid desert land. While a bringer of bounty to the Egyptians, Sirius was not so kind to the people of ancient Greece. Its name, derived from the Greek word seirios, meant “scorcher” and meant the “dog days” of summer would soon be at hand. Its sweltering cruelty was lamented in Homer’s Illiad:
“Sirius rises late in the dark, liquid sky
On summer nights, star of stars,
Orion's Dog they call it, brightest
Of all, but an evil portent, bringing heat
And fevers to suffering humanity.”
Half a world away, Sirius belonged to the constellation Manu and marked the arrival of winter to the peoples of Polynesia. The ocean going people of the islands looked to its light to navigate across the open waters of the Pacific.
Today, Sirius is a popular target amongst amateur and professional astronomers. With little more than binoculars it shines like a diamond in the night sky. Its colors are brilliant and ever changing. In 150 AD the Greek Astronomer Claudius Ptolemy described Sirius as red in color while the 1st century poet Marcus Manilius claimed it was as blue as the sea. Still, Chinese astronomers of the same period described the star as a bright white light. And so it is today. It is not uncommon for observers to report seeing all three colors brilliantly flashing at them as the star rises from the horizon during a single night of observation.
For professional astronomers, observation of Sirius has also been beneficial. Beyond its beauty and history, it serves a very useful purpose in modern astronomy. Because it is a binary star system, its luminosity drops in intensity when one eclipses the other (from our vantage point here on Earth). The luminosity of the star does not drop as significantly during an occult of Sirius B as it does when the secondary star transits across the face of the larger Sirius A. By measuring the fundamental parameters of the binary star during an eclipse, we have been able to calculate the mass of both Sirius A and B (as well as the Large Magellanic Cloud, the Small Magellanic Cloud, the Andromeda Galaxy and the Triangulum Galaxy).
Whether this beautiful binary brings with it the sweltering heat of summer or the icy chill of winter, it is a perfect night to spend time with the dogs.
Drew Farwell is an amateur astronomer focused on urban outreach and is the author of the blog Front Yard Astronomy.
***Author's Note: As correctly pointed out by David W. Knisely, while the use of binary stars is in fact to provide a standard candle for astronomers and astrophysicists, the eclipses of Sirius A and B are not visible from Earth and thus, were not used to calculate the distances of the Large Magellanic Cloud, the Small Magellanic Cloud, the Andromeda Galaxy or the Triangulum Galaxy. Additionally, the photo that appears in the article was taken by the Hubble Space Telescope and can be found on Wikipedia. I apologize for these oversights.