Sirius is the brightest star in the night sky, shining at magnitude −1.46 in the constellation Canis Major. It looks so bright because it is both a powerful star, about 25 times more luminous than the Sun, and one of our nearest neighbours, just 8.6 light-years away.
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Sirius at a glance
| Other names | Alpha Canis Majoris, the Dog Star |
|---|---|
| Constellation | Canis Major (the Great Dog) |
| Brightness | Magnitude −1.46, brightest star in the night sky |
| Distance | 8.6 light-years (5th-nearest star system) |
| Type | Sirius A: white main-sequence star (A1V); Sirius B: white dwarf |
| Mass | Sirius A about 2 Suns; Sirius B about 1 Sun |
| Size | Sirius A about 1.7 times the Sun’s diameter; Sirius B about Earth-sized |
| Temperature | About 9,900 K (Sirius A) |
| Orbit of the pair | About 50 years |
| Best time to see it | Evenings around February (highest at about 9 p.m.) |
| Where on Earth | Visible from everywhere south of about 73°N |
Why is Sirius so bright?
Two things make a star look bright from Earth: how much light it really gives out and how close it is. Sirius scores well on both. It is a hot, white star of spectral type A1, about twice the mass of the Sun and roughly 25 times more luminous. And at 8.6 light-years it is one of the Sun’s closest neighbours; only a handful of star systems, led by Alpha Centauri, are nearer. Stars such as Rigel or Deneb give out tens of thousands of times more light than Sirius, but they are hundreds or thousands of light-years away, so Sirius wins in our sky.
Sirius is also getting slightly brighter. It is moving towards the Solar System, and over the next 60,000 years or so it will come a little closer and brighten further before it starts to move away again.
Sirius B: the white dwarf companion
Sirius is not one star but two. In 1844 the German astronomer Friedrich Bessel noticed that Sirius wobbled slightly as it moved across the sky and concluded that an unseen companion was pulling on it. In 1862 the American telescope maker Alvan Graham Clark spotted that companion while testing a new lens. Sirius B turned out to be a white dwarf: the leftover core of a star that once was more massive than Sirius A. It packs about the mass of the Sun into a ball roughly the size of Earth, so a teaspoon of its material would weigh several tonnes.
The two stars orbit each other every 50 years or so. Sirius B is about 10,000 times fainter than Sirius A and is lost in its glare in small telescopes, but around the mid-2020s the pair is near its widest apparent separation, which gives experienced observers with larger telescopes and very steady air their best chance in decades to glimpse it.
Why is Sirius called the Dog Star?
Sirius is the brightest star in Canis Major, the Great Dog, one of the two hunting dogs that follow Orion across the sky. Its name comes from the ancient Greek seirios, meaning “glowing” or “scorching”. In ancient Greece and Rome, the hottest weeks of summer, when Sirius rose in the dawn sky just before the Sun, were called the “dog days”, a phrase still used today. In ancient Egypt, that first dawn appearance of Sirius (its heliacal rising) roughly coincided with the start of the annual Nile flood, and it was important in their calendar.
Why does Sirius flash different colours?
People often report a star “flashing red, green and blue” low in the sky, and it is very often Sirius. The star itself is white. Because it is so bright, the twinkling caused by Earth’s turbulent atmosphere is very noticeable, and when Sirius is low the atmosphere also acts like a weak prism, splitting its light into colours. The effect fades as Sirius climbs higher. It is a natural effect, not a UFO or an aircraft.
How to find Sirius
Sirius is easy to find once Orion is up. Follow the three stars of Orion’s Belt down and to the left (from the Northern Hemisphere) and they point almost straight at Sirius, the brightest star in that part of the sky. From the Southern Hemisphere, follow the belt up and to the right. Sirius also forms the Winter Triangle with Betelgeuse in Orion and Procyon in Canis Minor.
From mid-northern latitudes, Sirius rises in the late evening in November, is well placed by mid-evening from December to March, and sinks into the evening twilight in April and May. Because it lies south of the celestial equator, it climbs higher and stays up longer the farther south you live; from the Southern Hemisphere it rides high on summer evenings. Use the live tool above for exact times where you are.
Frequently asked questions about Sirius
Is Sirius the brightest star in the sky?
Yes. At magnitude −1.46, Sirius is the brightest star in the night sky, almost twice as bright as the second brightest, Canopus. Only the Moon and some planets outshine it.
How far away is Sirius?
Sirius is about 8.6 light-years away, or roughly 81 trillion kilometres. The light you see tonight left Sirius around 2018.
Is Sirius bigger than the Sun?
Yes. Sirius A is about twice the Sun’s mass, 1.7 times its diameter and about 25 times as luminous.
Why does Sirius twinkle so much?
Earth’s atmosphere bends the light of every star; Sirius is so bright that the effect is easy to see, especially when it is low, where it can flash several colours.
Can you see Sirius from both hemispheres?
Yes. Sirius is visible from almost everywhere on Earth except the far Arctic, north of about 73°N.
Sources: Sirius overview and references. Star data from standard catalogues; positions and rise/set times are calculated by SpaceTracker.live with the Astronomy Engine library.