Sirius: The Dog Star

Sirius, the brightest star in the sky, accompanied by the prominent constellation Orion, the heavenly hunter, is sparkling over a snow-covered winter landscape.
H. Raab herbraab/ Flickr CC

Sirius, also known as the Dog Star, is the brightest star in our night-time sky. It's also the sixth closest star to Earth, at a distance of 8.6 light-years. (A light-year is the distance that light travels in a year). The name "Sirius" comes from the ancient Greek word for "scorching" and it has fascinated observers throughout human history because of its brightness and colorful twinkling.

Astronomers began seriously studying Sirius in the 1800s, and continue to do so today. It is usually noted on star maps and charts as alpha Canis Majoris, the brightest star in the constellation Canis Major (the Big Dog). Sirius is visible from most parts of the world (except for very northerly or southerly regions), and can sometimes be seen during the day if conditions are right. 

The Science of Sirius

The astronomer Edmond Halley observed Sirius in 1718 and determined its proper motion (that is, its actual motion through space). More than a century later, astronomer William Huggins measured the actual velocity of Sirius by taking a spectrum of its light, which revealed data about its speed. Further measurements showed that this star is actually moving toward the Sun at a velocity of about 7.6 kilometers per second. 

Astronomers long suspected that Sirius might have a companion star. It would be hard to spot since Sirius itself is so bright. But, they kept looking for it. In 1844, F.W. Bessel used analysis of its motion to determine that Sirius really did have a companion. That discovery was finally confirmed by telescope observations in 1862. The companion is called Sirius B, and it is the first white dwarf (an aged type of star) with a spectrum to show a gravitational redshift as predicted by the general theory of relativity

There are stories floating around that some early civilizations saw this companion without the aid of a telescope. It would have been very hard to see unless the companion was very bright. So, it's not clear what the ancients saw. However, current scientists are quite interested in learning more about Sirius A and B. More recent observations with Hubble Space Telescope have measured both of the stars, and revealed that Sirius B is only about the size of Earth, but has the mass close to that of the Sun. 

Comparing Sirius Itself to the Sun

Sirius A, which is what we see with the naked eye, is about twice as massive as our Sun. It is also 25 times more luminous than our star. Over time, and as it gets closer to the solar system in the far distant fugure, it will also increase in brightness. That's part of its evolutionary path. While our Sun is about 4.5 billion years old, Sirius A and B are thought to be no more than 300 million years old and so their story is yet to be told.

Why is Sirius Called the "Dog Star"? 

This star has earned the name "Dog Star" from an interesting time in Earth's past. One reason it's called that is that it's the brightest star in Canis Major. However, there's a more interesting idea about its name: it was also incredibly important to stargazers in the ancient world for its prediction of seasonal change. For example, in the time of the Pharoahs in Egypt, people watched for Sirius to rise just before the Sun did. That marked the season when the Nile would flood, and bathe the nearby farms with mineral-rich silt. The Egyptians made a ritual of looking for Sirius at the right time—it was that important to their society. The rumor goes that this time of year, typically late summer, came to be known as the "Dog Days" of summer, particularly in Greece, when people began looking for the Dog star just before sunrise.

The Egyptians and Greeks weren't the only ones interested in this star. Ocean-going explorers also used it as a celestial marker, helping them navigate around the world's seas. For example, to the Polynesians, who have been accomplished navigators for centuries, Sirius was known as "A'a" and it was part of a complex set of navigational star lines the islanders used to voyage up and down the Pacific between Tahitian islands and Hawai'i. 

Today, Sirius is a favorite of stargazers, and enjoys many mentions in science fiction, song titles, and literature. It appears to twinkle madly, although that's really a function of its light passing through Earth's atmosphere, particularly when the star is low on the horizon. 

 

Edited and updated by Carolyn Collins Petersen.