Bright Stars: Sirius

By Peter Forrester | February 13, 2019

The brightest star in the night sky, as seen from Earth, is called Sirius. It is the second brightest overall (obviously the Sun is the brightest star we can see).

Sirius is larger than the Sun, but weak and relatively dim compared to many other stars, but it is so close to the Sun (8.6 light years) that its apparent magnitude (-1.46) is almost twice as bright as any other star. It has an absolute magnitude of 1.42.

Being so bright, Sirius was important in the mythology of many ancient cultures. For example, its appearance in the sky shortly before sunrise (its “heliacal rising”) occurred just before the summer flooding in ancient Egypt and was an important omen in their religion. Their civil calendar was originally based on the return of Sirius, but wandered off within 4 years. However, they continued to observe its return and discovered a cycle that became part of the Julian and Alexandrian calendars.

The Ancient Greeks associated Sirius with hot weather, men becoming weak, and other bad consequences, terming the time right after its return “dog days” because of its constellation, Canis Major. Its association with dogs also concerned the way dogs pant in hot weather, which made some ancient people fear they might have a disease, even in some cases rabies. For more on the mythologies associated with Sirius, see the Wikipedia link below.

Sirius is in Canis Major, or the Great Dog, which is just to the left of Orion as seen in the Northern Hemisphere. Being the brightest star in this constellation, its abbreviation is α CMa. Canis Major is often depicted as a dog of Orion who is helping him hunt the Bull, Taurus.

Sirius is actually made up of two separate stars, Sirius A and Sirius B. Sirius A has an absolute magnitude of +1.42, while Sirius B (nicknamed “the Pup”) is much, much dimmer at +11.18. The distance between them is about 20 AU, or about the distance of Uranus from the Sun. 1 Astronomical Unit (AU), the distance between the Earth and the Sun, is about 93 million miles. Sirius A is blue-white, while Sirius B is a white dwarf, the second one ever discovered.

The existence of a second star was first hypothesized in 1844, and observed in 1862. A third companion star has been suggested to explain small changes in the movement of the star system but its existence has never been verified.

Sirius is one of the three stars in a shape called the “Winter Triangle”, along with Procyon in Canis Minor, and Betelgeuse in Orion. Those who know the constellation Orion should be able to find Sirius easily. You just follow the belt down to the left, the star is about 8 times the width of the belt away from it. These directions will be opposite in the Southern Hemisphere.

There are four planets brighter than Sirius (two of them, Jupiter and Venus, are always brighter when visible, with Mercury and Mars being brighter some of the time). Since it is known to be the brightest night-time star, knowing its location can help you identify planets in the sky.

It is possible to see Sirius in daylight, though there are several conditions that have to be met, such as the sky being clear, the Sun being low on the horizon, Sirius being overhead, and the observer being at a high altitude. These conditions are met more easily in the Southern Hemisphere, since the star gets higher in the sky there. Sirius is gradually moving to the south and will no longer be visible in northern or central Europe in about 7,000 years.

Sirius contains 2 of the 8 nearest stars to our solar system, and is the fifth closest stellar system to us. The Voyager 2 spacecraft launched in 1977 is expected to pass about 4 light years from Sirius in about 300,000 years. No planets have been detected in the Sirius system, and because of differences in the stars, the conditions for life being met would be much more difficult. In particular, Sirius B used to be a red giant and thus would have swallowed up any planets. Also Sirius A is much brighter than the Sun and thus habitable planets would have to be much further away than the Earth is from the Sun. Sirius A is also much younger than the Sun and thus there would have been much less time for life to have evolved on any planet there.

There are a couple of historical controversies concerning Sirius. The first is about the color of the star, which ancient authors described as red, but the star is now known to be white to blue-white in color (various colors can be seen in its twinkling, because of effects of the light passing through Earth’s atmosphere).

The second controversy concerns a native tribe in Mali, Africa, which has been claimed to have known about the second star before Western astronomers (they also said there was a third star with a planet). But the accounts have been disputed and it seems the author who wrote about it in 1938 may have been the one to tell them, or they may have learned about it from a French expedition to view an eclipse in 1893.

This is the best month of the year to observe Sirius in the early evening, although it can also be seen in the early morning during summer. Happy skywatching!


References:

Byrd, Deborah. (2019, February 7). Sirius is Dog Star and brightest star. Retrieved from https://earthsky.org/brightest-stars/sirius-the-brightest-star on February 13, 2019.

Howell, Elizabeth. (2018, October 24). Sirius: Brightest Star in Earth’s Night Sky. Retrieved from https://www.space.com/21702-sirius-brightest-star.html on February 13, 2019.

Wikipedia. (2018, December 31). Heliacal rising. Retrieved from https://en.wikipedia.org/wiki/Heliacal_rising.

Wikipedia. (2019, February 12). Sirius. Retrieved from https://en.wikipedia.org/wiki/Sirius.

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

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