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Sirius, the brightest star in the night sky

Sirius is the brightest star in the night sky and one of the closest: its light takes about 8.6 years to reach us. It belongs to Canis Major, the Great Dog, and NASA's headlines still call it the Dog Star.

Where is it from where I am?

Where Sirius is among the stars NASA's star map around Sirius, about 52 degrees across, north up and east to the left, with this site's chart lines and labels over it. Labelled: Sirius, Betelgeuse, Rigel. A dashed line shows the pointer described in the text. SiriusBetelgeuseRigel NE

Show in my sky

North is up and east is to the left, the way the sky looks from underneath. The pattern turns through the night, so match it by shape, not angle. Lines are this site's own drawing, a convention: the IAU defines constellations as areas of sky with official boundaries, not as stick figures. The sky map draws the fuller IAU figures. Sky image: NASA/GSFC Scientific Visualization Studio, Deep Star Maps 2020.
About this picture

NASA/Goddard Space Flight Center Scientific Visualization Studio. Gaia DR2: ESA/Gaia/DPAC. Constellation figures based on those developed for the IAU by Alan MacRobert of Sky and Telescope magazine (Roger Sinnott and Rick Fienberg). NASA's page (opens in a new tab).

The map is a composite of 1.7 billion catalogued stars, not a photograph from one place: it shows stars far too faint for the eye, and no weather, Moon or city light. It is drawn here in this chart's projection, with each star where it was in 2000; the fastest-moving bright star has shifted about a minute of arc since, far less than a pixel here.

How to spot it: Orion's three belt stars point at it. Source (opens in a new tab)

Why it is so bright

Sirius outshines every other star mainly because it is close, not because it is the most powerful star you can see.

NASA puts the Sirius system 8.6 light-years away (opens in a new tab), the fifth-closest star system known. In the Hipparcos satellite's measurements it shines at magnitude −1.44, first on ESA's list (opens in a new tab) of the brightest stars. On the magnitude scale smaller numbers mean brighter, and the NASA Night Sky Network (opens in a new tab) gives its yardstick: a first-magnitude star looks 100 times brighter than a sixth-magnitude one.

Distance flatters it. NASA's skywatching FAQ (opens in a new tab) reckons that if the Sun and Sirius sat at the same distance, Sirius would be around 25 times brighter: a strong star, but nothing like the supergiants nearby in the sky. Jim Kaler (opens in a new tab) puts Rigel, in Orion, at about 85,000 times the Sun's light. Sirius tops our sky because, in cosmic terms, it is next door.

The main star is an ordinary star in the prime of its life. NASA lists Sirius (opens in a new tab) among the main-sequence stars, which shine by fusing hydrogen into helium in their cores, and gives it (opens in a new tab) about twice the Sun's mass and a surface temperature of about 10,500 kelvin. Hotter stars look blue or white (opens in a new tab), which is why Sirius looks white rather than yellow.

Keep in mind Brightest star is not brightest point of light. Venus can shine brighter than magnitude −4 (opens in a new tab), far brighter than Sirius, so a dazzling “star” low in the sky after sunset or before dawn may be a planet. How brightness and distance work.

Sources for this section NASA Science, Astronomers Use Hubble to 'Weigh' Dog Star's Companion (2005) (opens in a new tab); ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab); Kat Troche (Astronomical Society of the Pacific), Check Your Sky Quality with Orion!, NASA Night Sky Network (2023) (opens in a new tab); NASA Science, Skywatching FAQ (2024) (opens in a new tab); Jim Kaler, Rigel, STARS (University of Illinois) (opens in a new tab); NASA Science, Star Types (opens in a new tab); NASA Science, Stars in an Exoplanet World (opens in a new tab)

The companion you cannot see

Sirius is a pair: the bright star, and a white dwarf about the size of Earth hidden in its glare.

Sirius B is a white dwarf (opens in a new tab), the leftover core of a star that has used up its nuclear fuel. NASA gives it a diameter of about 7,500 miles (12,000 km), a little smaller than Earth, yet a mass 98 percent of the Sun's, measured with the Hubble Space Telescope in 2004. The two stars circle each other every 50 years.

Putting a star's mass into a planet's size makes it extraordinarily dense. NASA's guide to star types (opens in a new tab) says a teaspoon of white-dwarf material would weigh more than a pickup truck, and that a white dwarf makes no new heat of its own but cools slowly over billions of years. Hubble measured (opens in a new tab) the surface of Sirius B at about 25,200 kelvin, hotter than Sirius A.

Its existence was worked out before anyone saw it. The Night Sky Network (opens in a new tab) notes that the companions of Sirius and of Procyon were both inferred in the 1800s from careful study of how their bright stars move. Sirius B was first glimpsed in 1862 (opens in a new tab), through one of the most powerful telescopes of the day.

Keep in mind Do not expect to see it. Sirius B is about 10,000 times fainter (opens in a new tab) than Sirius A and swamped by its glare; the Night Sky Network (opens in a new tab) calls it a challenge even for a large telescope.

Sources for this section NASA Science, Astronomers Use Hubble to 'Weigh' Dog Star's Companion (2005) (opens in a new tab); NASA Science, Star Types (opens in a new tab); NASA Night Sky Network, Sirius and Procyon (opens in a new tab)

How to find it

Follow Orion's belt: its three stars point at Sirius.

Find Orion first: three bright stars close together in an almost straight line (opens in a new tab) make his belt. NASA's sky guide for December 2024 (opens in a new tab) tells northern skywatchers to follow the belt to the left and slightly downward, where it points to Sirius.

Sirius lies well south of the celestial equator, so the two hemispheres see it differently. By this site's calculation (opens in a new tab), at its highest it stands about 33° up in the south from 40°N, and about 72° up in the north from 35°S. Either way it is highest at about 9 p.m. local time in mid-February; the block further down this page gives the latitudes where it rises at all.

How to see it From the Northern Hemisphere, once you have Sirius, look up and to its left for Procyon, then back toward Orion for reddish Betelgeuse at his shoulder: the three make the Winter Triangle (opens in a new tab), which stays in the evening sky through the northern winter.

Keep in mind Low in the sky Sirius often flashes with colour. That is our turbulent air (opens in a new tab) acting on a very bright point of light near the horizon, not the star changing. Why stars twinkle.

Sources for this section NASA Science, Discovering the Universe Through the Constellation Orion (opens in a new tab); NASA Science, What's Up: December 2024 (opens in a new tab); Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab); NASA Night Sky Network, Sirius and Procyon (opens in a new tab)

Often confused with

A very bright “star” that the sky finder does not list is probably a planet.

Venus can reach magnitude −4 (opens in a new tab) or brighter, well beyond Sirius. This site's sky finder covers selected stars and constellations and leaves the planets out, so a brilliant point it does not mention is worth checking against a planet guide.

Planets also tend to shine more steadily. A star is effectively a point of light (opens in a new tab), so turbulence in the air makes it flicker in brightness and colour, while a planet's tiny disc is a broader bundle of light and is, on average, less affected.

From the Southern Hemisphere a second brilliant star is often up at the same time: Canopus, at magnitude −0.62 second only to Sirius (opens in a new tab), about 36° to its south (opens in a new tab).

Sources for this section Kat Troche (Astronomical Society of the Pacific), Check Your Sky Quality with Orion!, NASA Night Sky Network (2023) (opens in a new tab); Robert Nemiroff and Jerry Bonnell, Scintillating, Astronomy Picture of the Day, NASA (28 April 2011) (opens in a new tab); ESA, Hipparcos: The 50 Brightest Stars (opens in a new tab); Bright Star Catalogue, 5th Revised Ed. (Hoffleit & Warren 1991), as computed by this site's star engine (opens in a new tab)

Where on Earth, and when

It rises from anywhere south of about 73°N.

South of about 73°S, it never sets.

It climbs at least 20° high, the finder's threshold for a suggestion, from latitudes between 87°S and 53°N.

It is highest in the evening sky, at about 9 p.m. local time, in mid-February, from either hemisphere, wherever it rises. With daylight saving time, that is nearer 10 p.m. on the clock.

Worked out by this site from the Bright Star Catalogue, for a flat, clear horizon. Hills, buildings, haze and city light all cut into this; the air also lifts objects near the horizon by about half a degree. For your own place and night, use the sky finder.

How Sirius compares with the Sun

Sources

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