Sirius

Sirius (/ˈsɪriəs/) is the brightest star in the night sky. Its name is derived from the Greek word Σείριος (Seirios, lit. 'glowing' or 'scorching'). The star is designated α Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated Alpha CMa or α CMa. With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years.[25]

Sirius

The position of Sirius (circled)
Observation data
Epoch J2000.0      Equinox ICRS
Constellation Canis Major
Pronunciation /ˈsɪriəs/[1]
Sirius
Right ascension 06h 45m 08.91728s[2]
Declination −16° 42 58.0171[2]
Apparent magnitude (V) −1.46[3]
A
Right ascension 06h 45m 08.917s[4]
Declination −16° 42 58.02[4]
Apparent magnitude (V) −1.47[5]
B
Right ascension 06h 45m 09.0s[6]
Declination −16° 43 06[6]
Apparent magnitude (V) 8.44[5]
Characteristics
Sirius A
Evolutionary stage Main sequence
Spectral type A0mA1 Va[7]
U−B colour index −0.05[3]
B−V colour index +0.00[3]
Sirius B
Evolutionary stage White dwarf
Spectral type DA2[5]
U−B colour index −1.04[8]
B−V colour index −0.03[8]
Astrometry
Radial velocity (Rv)−5.50[9] km/s
Proper motion (μ) RA: −546.01[2] mas/yr
Dec.: −1223.07[2] mas/yr
Parallax (π)374.4896 ± 0.2313[10] mas
Distance8.709 ± 0.005 ly
(2.670 ± 0.002 pc)
Sirius A
Absolute magnitude (MV)+1.42[11]
Sirius B
Absolute magnitude (MV)+11.18[8]
Orbit[12]
Primaryα Canis Majoris A
Companionα Canis Majoris B
Period (P)50.1284 ± 0.0043 yr
Semi-major axis (a)7.4957 ± 0.0025″
Eccentricity (e)0.59142 ± 0.00037
Inclination (i)136.336 ± 0.040°
Longitude of the node (Ω)45.400 ± 0.071°
Periastron epoch (T)1994.5715 ± 0.0058
Argument of periastron (ω)
(secondary)
149.161 ± 0.075°
Details
Sirius A
Mass2.063 ± 0.023[12] M
Radius1.711[13] R
Luminosity25.4[13] L
Surface gravity (log g)4.33[14] cgs
Temperature9,940[14] K
Metallicity [Fe/H]0.50[15] dex
Rotation16 km/s[16]
Age242 ± 5[12] Myr
Sirius B
Mass1.018 ± 0.011[12] M
Radius0.0084 ± 3%[17] R
Luminosity0.056[18] L
Surface gravity (log g)8.57[17] cgs
Temperature25,000 ± 200[13] K
Age228+10
−8
[12] Myr
Other designations
Dog Star, Aschere, Canicula, Al Shira, Sothis,[19] Alhabor,[20] Mrgavyadha, Lubdhaka,[21] Tenrōsei,[22] α Canis Majoris (α CMa), 9 Canis Majoris (9 CMa), HD 48915, HR 2491, BD−16°1591, GJ 244, LHS 219, ADS 5423, LTT 2638, HIP 32349[23]
Sirius B: EGGR 49, WD 0642-166, GCTP 1577.00[24]
Database references
SIMBADThe system
A
B

Sirius appears bright because of its intrinsic luminosity and its proximity to the Solar System. At a distance of 2.64 parsecs (8.6 ly), the Sirius system is one of Earth's nearest neighbours. Sirius is gradually moving closer to the Solar System, so it is expected to slightly increase in brightness over the next 60,000 years. After that time, its distance will begin to increase, and it will become fainter, but it will continue to be the brightest star in the Earth's night sky for approximately the next 210,000 years.[26]

Sirius A is about twice as massive as the Sun (M) and has an absolute visual magnitude of +1.42. It is 25 times as luminous as the Sun,[13] 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.[13] 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.[13]

Sirius is known colloquially as the "Dog Star", reflecting its prominence in its constellation, Canis Major (the Greater Dog).[19] The heliacal rising of Sirius marked the flooding of the Nile in Ancient Egypt and the "dog days" of summer for the ancient Greeks, while to the Polynesians, mostly in the Southern Hemisphere, the star marked winter and was an important reference for their navigation around the Pacific Ocean.