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  The position of Sirius (circled). | |
| Observation data Epoch J2000.0 Equinox J2000.0 (ICRS) | |
|---|---|
| Constellation | Canis Major |
| Pronunciation | /ˈsɪriəs/[1] |
| Right ascension | 06h 45m 08.9173s[2][note 1] |
| Declination | −16° 42′ 58.017″[2][note 1] |
| Apparent magnitude (V) | −1.47 (A)[2] / 8.30 (B)[3] |
| Characteristics | |
| Spectral type | A1V (A)[2] / DA2 (B)[3] |
| U−B color index | −0.05 (A)[4] / −1.04 (B)[3] |
| B−V color index | 0.01 (A)[2] / −0.03 (B)[3] |
| Astrometry | |
| Radial velocity (Rv) | −7.6[2] km/s |
| Proper motion (μ) | RA: −546.05[2][note 1] mas/yr Dec.: −1223.14[2][note 1] mas/yr |
| Parallax (π) | 379.21 ± 1.58[2][5] mas |
| Distance | 8.60 ± 0.04 ly (2.64 ± 0.01 pc) |
| Absolute magnitude (MV) | 1.42 (A)[note 2] / 11.18 (B)[3] |
| Orbit[6] | |
| Companion | α CMa B |
| Period (P) | 50.090 ± 0.055 yr |
| Semi-major axis (a) | 7.50 ± 0.04" |
| Eccentricity (e) | 0.5923 ± 0.0019 |
| Inclination (i) | 136.53 ± 0.43° |
| Longitude of the node(Ω) | 44.57 ± 0.44° |
| Periastron epoch (T) | 1894.130 ± 0.015 |
| Argument of periastron(ω) (secondary) | 147.27 ± 0.54° |
| Details | |
| α CMa A | |
| Mass | 2.02[7] M☉ |
| Radius | 1.711[7] R☉ |
| Luminosity | 25.4[7] L☉ |
| Surface gravity (log g) | 4.33[8] cgs |
| Temperature | 9,940[8] K |
| Metallicity [Fe/H] | 0.50[9] dex |
| Rotation | 16 km/s[10] |
| Age | 2–3 × 108[7] years |
| α CMa B | |
| Mass | 0.978[7] M☉ |
| Radius | 0.0084 ± 3%[11] R☉ |
| Luminosity | 0.026[note 3] L☉ |
| Surface gravity (log g) | 8.57[11] cgs |
| Temperature | 25,200[7] K |
| Other designations | |
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright asCanopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Σείριος Seirios ("glowing" or "scorcher"). The star has the Bayer designation Alpha Canis Majoris (α CMa). What the naked eye perceives as a single star is actually a binary star system, consisting of a white main-sequence star of spectral type A1V, termed Sirius A, and a faintwhite dwarf companion of spectral type DA2, called Sirius B. The distance separating Sirius A from its companion varies between 8.1 and 31.5 AU.
Sirius appears bright because of both its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs (8.6 ly), as determined by the Hipparcos astrometry satellite, the Sirius system is one of Earth's near neighbors; for Northern-hemisphere observers between 30 degrees and 73 degrees of latitude (including almost all of Europe and North America), it is the closest star (after the Sun) that can be seen with the naked eye. Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to recede, but it will continue to be the brightest star in the Earth's sky for the next 210,000 years.
Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun but has a significantly lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old. It was originally composed of two bright bluish stars. The more massive of these, Sirius B, consumed its resources and became a red giant before shedding its outer layers and collapsing into its current state as a white dwarf around 120 million years ago.
Sirius is also known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major (Greater Dog). The heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the "dog days" of summer for theancient Greeks, while to the Polynesians it marked winter and was an important star for navigation around the Pacific Ocean
Sirius is Dog Star and brightest star
Sirius A and B
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How to See the Dog Star
Most people in the Northern Hemisphere notice Sirius in the southeast – south – or southwest on evenings from winter to mid-spring. February evenings are a grand time to see it. It’s also fun to spot Sirius as it ascends in the east before dawn on late summer mornings. Whenever you see Sirius, you’ll recognize it easily because it is our sky’s brightest star.
Image Credit: Yuuji Kitahara
Although white to blue white in color, Sirius might be called a rainbow star, as it often flickers with many colors.
The brightness, twinkling and color changes sometimes prompt first-time observers to report Sirius as a UFO. But these changes have nothing to do with Sirius. Rather, they are what happens when such a bright star as Sirius shines through the blanket of Earth’s atmosphere. The light from Sirius, which often appears fairly low in the sky from the mid-north latitudes, passes through a long column of air before it reaches our eyes. Changes in density and temperature of this air affect the light and cause the flickering and shimmering we see when we gaze at this star. This happens for other stars, too, but it is more noticeable for Sirius because it is so bright, and because it appears low in the sky.
Orion's Belt points to Sirius
From the mid-northern latitudes such as most of the U.S., Sirius rises in the southeast, arcs across the southern sky, and sets in the southwest. In December, you’ll find Sirius rising in mid-evening. By mid-April, Sirius is setting in the southwest in mid-evening.
Sirius is always easy to find. It’s the sky’s brightest star! Plus, anyone familiar with the constellation Orion can simply draw a line through Orion’s Belt, to the left. This line will point to Sirius, which is roughly 8 times as far from the Belt as the Belt is wide.
History and Myth
Sirius is also well known as the Dog Star, because it is the chief star in the constellation Canis Major, the Big Dog. Have you ever heard anyone speak of the dog days of summer? Sirius is behind the sun as seen from Earth in Northern Hemisphere summer. In late summer, it appears in the east before sunrise – near the sun in our sky. The early stargazers might have imagined that the double-whammy of Sirius and the sun caused the hot weather, or dog days.
The Egyptian god Osiris
Sirius has been known since ancient times, and its name signified its nature as “scorching” or “sparkling.” It was associated with the Egyptian god Osiris and other gods. Ancient Egyptians noted that Sirius rose just before the sun each year immediately prior to the annual flooding of the Nile River. Although the floods could bring destruction, they also brought new soil and new life. Fittingly, Osiris, whom Sirius may have represented, was a god of life, death, fertility and rebirth of plant life along the Nile.
In India, Sirius is sometimes known as Svana, the dog of Prince Yudhistira. The prince and his four brothers, along with Svana, set out on a long and arduous journey to find the kingdom of heaven. However, one by one the brothers all abandoned the search until only Yudhistira and Svana were left. At long last they came to the gates of heaven. The gatekeeper, Lord Indra, welcomed the prince but denied Svana entrance. Yudhistira was aghast and told Lord Indra that he could not forsake his good and faithful servant and friend. His brothers, Yudhistira told the Lord, had abandoned the journey to heaven to follow their hearts’ desires. But Svana, who had given his heart freely, chose to follow none but Yudhistira. The prince told the Lord that without his dog, he would forsake even heaven. This is what Lord Indra had wanted to hear, and then he welcomed both the prince and the dog through the gates of heaven.
Science
Magnitude is a star’s brightness expressed by a number. The smaller the number, the brighter the star. The visual magnitude of Sirius is -1.44, lower than any other star. It is 3.5 times brighter than Arcturus in Bootes, the next brightest star easily visible from the northern hemisphere. There are brighter stars in terms of actual energy and light output, but they are farther away and hence dimmer. Normally, the only objects that outshine Sirius in our heavens are the sun, moon, Venus, Jupiter, Mars and Mercury (and usually Sirius outshines the latter two!).
The next brightest star (Canopus) after Sirius, and the closest major star (Alpha Centauri) are both too far south in the sky to be easily seen from mid-north latitudes.
This Hubble Space Telescope image shows Sirius A, the brightest star in our night sky, along with Sirius B, its faint, tiny stellar companion. Sirius B is the faint, tiny dot at the lower left. Sir. Image credit: H. Bond (STScI) and M. Barstow (University of Leicester)
At 8.6 light-years distance, Sirius is one of the nearest stars to us after the sun. (A light year is nearly 6 trillion miles!) In fact it is the nearest star easily visible to the unaided eye from most of the northern hemisphere. Classified by astronomers as an “A” type star, it is much hotter than our sun, with about surface about 17,000 degrees F (the sun is about 10,000 degrees F). With slightly more than twice the mass of the sun and just less than twice its diameter, Sirius still puts out 26 times as much energy. It is considered a normal (main sequence) star, meaning that it produces most of its energy by converting hydrogen into helium through nuclear fusion.
Sirius has a small, faint companion star appropriately called The Pup. That name signifies youth, but in fact the companion to Sirius is a dead star called a white dwarf. Once a mighty star, today The Pup is an Earth-sized ember too faint to be seen without a telescope.
The position of Sirius is RA: 06h 45m 08.9s, dec:-16° 42′ 58″.
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