Epoch J2000.0 Equinox J2000.0
|Right ascension||18h 50m 04.79525s|
|Declination||+33° 21′ 45.6100″|
|Apparent magnitude (V)||3.52 (3.4–4.3)|
|Spectral type||B6–8II / B|
|U−B color index||−0.56|
|B−V color index||+0.00|
|Variable type||β Lyr|
|Radial velocity (Rv)||−19.2 km/s|
|Proper motion (μ)||RA: 1.90 mas/yr
Dec.: −3.53 mas/yr
|Parallax (π)||3.39 ± 0.17 mas|
|Distance||960 ± 50 ly
(290 ± 10 pc)
|Absolute magnitude (MV)||−3.91|
|–6.3 ± 0.2 / –4.7 ± 0.1|
|Companion||Beta Lyrae B|
|Period (P)||12.9414 days|
|Semi-major axis (a)||0.865 ± 0.048"|
|Inclination (i)||92.25 ± 0.82°|
|Longitude of the node (Ω)||254.39 ± 0.83°|
|β Lyr A|
|Mass||13.16 ± 0.3 M☉|
|Radius||6.0 ± 0.2 R☉|
|Surface gravity (log g)||4.0 ± 0.1 cgs|
|Temperature||30,000 ± 2,000 K|
|β Lyr B|
|Mass||2.97 ± 0.2 M☉|
|Radius||15.2 ± 0.2 R☉|
|Surface gravity (log g)||2.5 ± 0.1 cgs|
Beta Lyrae has the traditional name Sheliak (occasionally Shelyak or Shiliak), derived from الشلياق šiliyāq or Al Shilyāk, the Arabic name of the constellation of Lyra. The meaning of the name Sheliak is debated. It has been suggested to derive from السلحفاة al-sulḥafāt, the Arabic word for "tortoise." This, however, is likely an error related to a fellow star in Lyra: Sulafat. Sheliak has also been linked to the meaning "eagle." Again, this is likely due to another star in the same constellation, Vega. Its name has been linked to the Arabic word wāqi‘, meaning "falling" or "landing", via the phrase an-nasr al-wāqi‘, "the falling eagle". It is possible the name Sheliak is of non-Arabic origin, as it has no other meaning in Arabic and cannot be identified with an Arabic triliteral root.
In China, the star was named Tsan Tae., from 漸台 (Jiāntāi), meaning Clepsydra Terrace, refers to an asterism consisting of this star, δ2 Lyrae, γ Lyrae and ι Lyrae. Consequently, β Lyrae itself is known as 漸台二 (Jiāntāièr, English: the Second Star of Clepsydra Terrace.)
The Bayer designation for this star was given by the German astronomer Johann Bayer with the publication of his star atlas Uranometria in 1603. It was given the Flamsteed designation 10 Lyrae by John Flamsteed in 1712 with the first publication of his star catalogue. The variable luminosity of this system was discovered in 1784 by the British amateur astronomer John Goodricke.
Beta Lyrae is a semidetached binary system made up of a stellar class B7II primary star and a secondary that is probably also a B-type star. The fainter, less massive star (B7II) in the system was once the more massive member of the pair, which caused it to evolve away from the main sequence first and become a giant star. Because the pair are in a close orbit, as this star expanded into a giant it filled its Roche lobe and transferred most of its mass over to its companion. The secondary, now more massive star is surrounded by an accretion disk from this mass transfer, with bipolar, jet-like features projecting perpendicular to the disk. This accretion disk blocks our view of the secondary star, lowering its apparent luminosity and making it difficult for astronomers to pinpoint what its stellar type is. The amount of mass being transferred between the two stars is about 2 × 10−5 solar masses per year, or the equivalent of the Sun's mass every 50,000 years, which results in an increase in orbital period of about 19 seconds each year.
The orbital plane of this system is nearly aligned with the line of sight from the Earth, so the two stars periodically eclipse each other. This causes Beta Lyrae to regularly change its apparent magnitude from +3.4 to +4.6 over an orbital period of 12.9414 days. The two components are so close together that they cannot be resolved with optical telescopes, forming a spectroscopic binary. In 2008, the primary star and the accretion disk of the secondary star were resolved and imaged using the CHARA Array interferometer and the Michigan InfraRed Combiner (MIRC) in the near infrared H band (see video below), allowing the orbital elements to be computed for the first time.
Two other stars appear close to the line of sight of Beta Lyrae, but are not part of the binary system. One, at an angular distance of 45.7", is of spectral type B7V, has an apparent magnitude of +7.2, and can easily be seen with binoculars. It is about 80 times as luminous as the Sun and is also a spectroscopic binary with a period of 4.34 days. The other is magnitude 9.9v separation 86" and with a luminosity 7 times that of the Sun.
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- (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 3 日
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- Philippe Stee's homepage: Hot and Active Stars Research
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