Humankind has always wondered about the vast expanse of space and everything in it. Perhaps most interesting are the celestial objects that stand out in some way in the dark canvas that is our night sky. In this article, we take a look at one such object: the star Sirius.
What is Sirius?
Sirius is a star located in the constellation Canis Major, also known as the Big Dog. Sirius itself makes up the neck of the constellation and is therefore sometimes referred to as “the Dog Star.” It’s even rumored that the expression “the dog days of summer” comes from the period in July and August when Sirius rises at the same time as the Sun.
The star Sirius stands out in the nighttime sky because of its apparent brightness compared to the other stars that we see. In fact, Sirius is the brightest star we can see at night from anywhere on Earth. Sure, the Sun is brighter, but if the Sun is visible, it’s not nighttime!
Two Stars in One
What we know of as Sirius is actually two different stars that orbit around each other. Such a phenomenon is known as a double or binary star. The stars average a distance of 20 astronomical units (AU) from each other, approximately the distance from the Sun to Uranus. Sirius A is the primary star of the duo, with Sirius B being the lesser star.
Sirius A is the celestial body we’re able to see when we look up at the night sky. It is a whitish blue star larger than our Sun and has been viewed from the dawn of time. For the record, Sirius A is the main focus of this article. If we mention ‘Sirius’ as you read on, this is the star we’re referring to.
Sirius B, however, is an entirely different body. This star was once the same beautiful blue as its brother but unfortunately ran out of steam and shrunk to a white dwarf. As a result, Sirius B is over 10,000 times dimmer than Sirius A and is even smaller than our Earth. Its small stature has earned it the nickname “The Pup.”
Where is Sirius in the Sky?
At the right time of year, they can be seen from just about anywhere in the world. The only location that the star is impossible to see are the far northern reaches of Earth.
The constellation and star first make an appearance in the early mornings during the last days of summer, whether you’re in the Northern or Southern Hemisphere. They peak in the evening sky during January and February, and by April they disappear until the end of summer approaches again.
When visible, the star is easy to find in the night sky if you’re able to locate Orion and his belt of three stars. Drawing a line through those belt stars down and to the left will lead you to Sirius every time.
Brighter Than the Rest
Sirius’s luminosity does put it in a league of its own among celestial bodies. There are only a few objects that surpass Sirius’s level of brightness. The Moon, Venus, Jupiter, and the International Space Station are brighter. The name ‘Sirius’ actually comes from the Greek word “Seirous,” meaning “glowing or scorching.”
Because Sirius glows so brightly, it frequently appears to shimmer through the entire spectrum of colors as its light passes through our atmosphere. This flickering effect has earned Sirius the name “the rainbow star.”
Sirius is as bright as it is to us on Earth partially because of its proximity. Our Sun aside, Sirius is listed as the fifth closest star to our planet, at a distance of just 8.58 light-years. That’s still some 4.7029003 x 1013 miles (7.5685844 x 1013 kilometers) away!
That’s not to say Sirius isn’t bright on its own. If placed in space right next to our Sun, Sirius would be twice as big, nearly twice as hot, and over 25 times more luminous. With Sirius, day and night would be significantly different!
Magnitude: The Measure of Celestial Brightness
To put all this brightness into perspective, we must look at the magnitude scale. Magnitude is a unitless scale that measures the brightness of celestial objects as we perceive them from Earth. This is different from an object’s luminosity, or how bright it would be compared to another entity if they were side by side.
The ancient Greeks first came up with the concept of magnitude, using a range from 1 to 6 to categorize the stars they were able to see. Stars listed as magnitude 1 were the brightest, with magnitude 6 stars faintly visible to the naked eye. Therefore, the smaller the number, the brighter the object.
The magnitude scale was exacted in modern times, with each celestial body given a specific value to two decimal places. There is also a specific correlation between magnitude values. For instance, a magnitude 2 star is two and a half times dimmer than a magnitude 1 star. Stars higher than magnitude 6 are not visible to the naked eye.
Because of its closeness to Earth, the Sun’s apparent magnitude is -26.72! When full, the Moon is bright enough to reach -12.74. Sirius A, in all its brightness, has a magnitude of -1.46.
We’ve also been able to categorize stars only seen through telescopes. The dwarf star Sirius B is invisible to the naked eye and has an apparent magnitude of 8.44.
Conclusion
As you set out to capture amazing starscapes, take a moment to think about how vast our universe really is. A star like Sirius catches our eye due to its brightness, but that light takes nearly nine years to reach us here on Earth! Each star out there has its own special characteristics that make every celestial photo a unique work of art.