Sirius
- "Sirius B" redirects here. For Therion's musical album, see Sirius B (album).
- For information on Sirius satellite radio, see Sirius Satellite Radio.
 The position of Sirius. | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Canis Major |
| Right ascension | 06h 45m 08.9s |
| Declination | −16° 42' 58" |
| Apparent magnitude (V) | −1.47 (A) / 8.44 (B) |
| Characteristics | |
| Spectral type | A1V (A) / DA2 (B) |
| U−B color index | −0.08 (A) / −1.04 (B) |
| B−V color index | 0.01 (A) / −0.03 (B) |
| Variable type | None |
| Astrometry | |
| Radial velocity (Rv) | −7.6 km/s |
| Proper motion (μ) | RA: −546.01 mas/yr Dec.: −1223.08 mas/yr |
| Parallax (π) | 380.02 ± 1.28 mas |
| Distance | 8.58 ± 0.03 ly (2.631 ± 0.009 pc) |
| Absolute magnitude (MV) | 1.47 (A) / 11.35 (B) |
| Details | |
| Mass | 2.14 (A) / 0.98 (B) M☉ |
| Radius | 1.68 (A) / 0.008 (B) R☉ |
| Luminosity | 26.1 (A) / 0.0024 (B) L☉ |
| Temperature | 9,900 (A) / 25,200 (B) K |
| Metallicity | 190% Sun (A) |
| Age | 2-3 × 108 years |
| Other designations | |
| Database references | |
| SIMBAD | data |
Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) is the brightest star in the night-time sky, with a visual apparent magnitude of −1.47. It is located in the constellation Canis Major. Its name comes from the Latin Sīrius, from Greek Σείριος (Seirios, "glowing" or "scorcher"). As the major star of the "Big Dog" constellation, it is often called the "Dog Star".
Sirius can be seen from every inhabited region of the Earth's surface and, in the Northern Hemisphere, is known as a vertex of the Winter Triangle.
At a distance of 2.6 pc or 8.6 light years, Sirius is one of the nearest stars to Earth.[citation needed] The best time of year to view it is around January 1, when it reaches the meridian at midnight.
It is a main sequence star of spectral type A0 or A1 and has a mass about 2.1 times that of the Sun.[citation needed]
It is also known by the Latin name Canicula ("little dog") and Arabic: الشعرى aš-ši‘rā in Islamic astronomy, from which the alternate name Al Shira derives.
In Chinese the star is known as 天狼星 (Tiānláng xīng literally, “heavenly wolf star”). The Sino-Japanese pronunciation is Tenrōsei and the Sino-Korean pronunciation 천랑성 Cheonlangseong.
Its closest large neighbour star except Sirius B is Procyon, 1.61 pc or 5.24 ly away.
In 1909, Ejnar Hertzsprung suggested that Sirius was part of the Ursa Major Moving Group; however, more recent research by Jeremy King et al. at Clemson University in 2003 questions whether that is true, since the two components of Sirius appear to be too young.
The diameter of Sirius was first measured by Robert Hanbury Brown and Richard Q. Twiss in 1959 at Jodrell Bank using their stellar intensity interferometer.[citation needed]
Under the right conditions, Sirius can be observed in daylight with the naked eye. Ideally the sky must be very clear, with the observer at a high altitude, the star passing overhead, and the sun low down on the horizon. (Henshaw, C.,1984)[1]
Binary system
In 1844 Friedrich Wilhelm Bessel deduced that Sirius was actually a binary star. In 1862 Alvan Graham Clark discovered the companion, which is called Sirius B, or affectionately "the Pup". The visible star is now sometimes known as Sirius A. The two stars orbit each other with a separation of about 20 AU (roughly the distance between the Sun and Uranus) and a period of close to 50 years.
In 1915 astronomers at the Mount Wilson Observatory determined that Sirius B was a white dwarf, the first to be discovered.
This means that Sirius B must have originally been by far the more massive of the two, since it has already evolved off the main sequence. Robert Hanbury Brown measured the diameter of Sirius for the first time in 1956. In 2005, using the Hubble Space Telescope, astronomers determined Sirius B to be 12,000 kilometers (7,500 miles) in diameter, with a mass that is 98% of the Sun. [2]
History
Historically, many cultures have attached special significance to Sirius. Sirius was worshipped as Sothis in the valley of the Nile long before Rome was founded, and many ancient Egyptian temples were oriented so that light from the star could penetrate to their inner altars.[citation needed] The Egyptians based their calendar on the heliacal rising of Sirius, which occurred just before the annual flooding of the Nile and the Summer solstice. In Greek mythology, Orion's dog became Sirius. The Greeks also associated Sirius with the heat of summer: they called it Σείριος Seirios, often translated "the scorcher." This also explains the phrase "dog days of summer".[citation needed]
In the astrology of the Middle Ages, Sirius was a Behenian fixed star, associated with beryl and juniper. Its kabbalistic symbol File:Agrippa1531 Canismaior.png was listed by Heinrich Cornelius Agrippa.
Mysteries
There are unsolved mysteries regarding Sirius.
- Apparent orbital irregularities in Sirius B have been observed since 1894, suggesting a third very small companion star, but this has never been definitely confirmed.[1]
- Ancient observations of Sirius describe it as a red star.[citation needed] To the Romans this meant an angry god, and they are known to have sacrificed red dogs to this star. Today, Sirius A is bluish white. The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy is rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when it was observed rising left the viewer with the impression that it was red. To the naked eye, it often appears to be flashing with red/white/blue hues when near the horizon.
The Dogon
The Dogon people are a tribal people living in Africa who are said to have known that Sirius had a companion star before its discovery in the 1840's. Of course, it is impossible to see Sirius's companion except with very powerful telescopes and at the right time of Sirius B's orbit, which lasts several years. They also knew that Saturn had rings and that Jupiter had 4 moons (Jupiter actually has many more, but only 4 can be seen with simple telescopes). This knowledge, which it seems impossible for them to have acquired without outside assistance, is a source of speculation for UFO enthusiasts and was the subject of the book The Sirius Mystery by Robert Temple, who linked this to the legends of the Dogon as well with the legends and traditions of the ancient Egyptians and Sumerians, conjecturing that aliens from Sirius had visited Earth and influenced early civilization. This work has been challenged by astronomer Carl Sagan, among others, as based on selective evidence. Careful research reveals there may have been cultural contamination on the part of visiting astronomers who went to the region to observe a transit of Venus, though this is still a matter of some dispute.[2]
- see also The Dogon Controversy
Sirius in popular culture
Being the brightest star in the night sky, Sirius is often mentioned in science fiction and other popular culture as well.
See also
References and external links
- ^ Benest, D., & Duvent, J. L. (1995, July). Is Sirius a triple star? Astronomy and Astrophysics, 299, 621-628. (available at The NASA Astrophysics Data System)
- ^ Ridpath, Ian (1978) Investigating the "Sirius Mystery". The Skeptical Inquirer. Retrieved July 11, 2005.
- Details regarding mass of Sirius B: Monthly Notices of the Royal Astronomical Society, October 2005.[3]
- Hubble News Release 2005
- Detailed information on Sirius
- Getting Sirius about time