Ras Algethi

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Triple
star Ras Algethi (α Herculis)
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Observation
data epoch : J2000.0
AladinLite
Constellation Hercules
Vis. Brightness
(total) 		3.06var mag
Astrometry
Radial velocity −32.09 ± 0.22 km / s
parallax  9.07 ± 1.32 mas
distance  360 ± 60 ly
(110 ± 16 pc )
Absolute visual
brightness (total) M vis 		−2.15var mag
period 3,600 y
Age 0.41 to 1.25 billion years
Proper movement
Rec. Share: −7.32 ± 0.92 mas / a
Dec. portion: 36.07 ± 0.97 mas / a
Individual data
Names α 1 , α 2 A, α 2 B
Observation data:
Right ascension α 1 17 h 14 m 38.85 s
α 2 A 17 h 14 m 39.18 s
α 2 B like α 2 A
declination α 1 2142325.3+ 14 ° 23 ′ 25.3 ″
α 2 A 2142324+ 14 ° 23 ′ 24.0 ″
α 2 B like α 2 A
Apparent
brightness 		α 1 3.35 var m
α 2 A 5.6 m
α 2 B 6.6 m
Typing:
Spectral class α 1 M5 Ib-II
α 2 A G8 III
α 2 B A9 IV-V
BV color index α 1 1.44
α 2 A 0.8
α 2 B 0.5
UB color index α 1 1.01
α 2 A not specified
α 2 B not specified
Astrometry:
Absolute
visual
brightness M vis 		α 1 −1.86var mag
α 2 A 0.4 mag
α 2 B 1.4 mag
Physical Properties:
Dimensions α 1 2.175 to 3.25 M ☉
α 2 A 2.175 to 3 M ☉
α 2 B 1.6 to 2.3 M ☉
radius α 1 284 ± 60 R ☉
α 2 A 13 R ☉
α 2 B 1.5 R ☉
Luminosity α 1 ≈ 8,300 + 3,200−2,300 L ☉
α 2 A 126 ± 13 L ☉
α 2 B 26 L ☉
Effective temperature α 1 3,280 ± 87 K
α 2 A 4,900 ± 150 K
α 2 B 7,350 ± 150 K
Designations and catalog entries
Bayer-Bez. α Herculis
Flamsteed-Bez. 64 Herculis
Bonn diam. BD + 14 ° 3207
HD catalog HD 156014
Hipparcos catalog HIP 84345
WDS catalog WDS 17146 + 1423
ADS catalog ADS 10418
Bright Star Cat. α 1 HR 6406
α 2 A HR 6407 (with α 2 B)
α 2 B MR -
SAO catalog α 1 SAO 102680
α 2 A SAO 102681
(with α 2 B)
α 2 B SAO -
Tycho catalog α 1 TYC 990/2133/1
α 2 A TYC 990/2134/1
(with α 2 B)
α 2 B TYC -

Ras Algethi or Rasalgethi ( Arabic رأس الجاثي, DMG raʾs al-ǧāṯī  'head of the kneeling man'; Bayer name : α Herculis , short: α Her) is the main star of the constellation Hercules .

Ras Algethi is not a single star, but a triple star system . In addition, it is a variable star with a mean apparent total brightness (all three components added together) of 3.06 m . Thus Ras Algethi is - contrary to the Greek letter α  - only the third brightest star in Hercules (the two even brighter stars are β Herculis and ζ Herculis).

The star system is about 360 light years from the Sun and is between 0.41 and 1.25 billion years old.

Ras Algethi is an interesting double star for amateur astronomers , as it can be resolved in small instruments from 6 to 7 cm lens aperture and shows a nice color contrast.

Name and structure of the star system

Ras Algethi is a triple star system and consists of the stars Ras Algethi A (Bayer designation: α 1 Herculis) and Ras Algethi B (Bayer designation: α 2 Herculis), of which component B is now double. The single stars of Ras Algethi B are called α 2 Herculis A (or Ras Algethi Ba) and α 2 Herculis B (or Ras Algethi Bb).

The IAU has on 30 June 2016 the name Rasalgethi as standardized proper name for the star α 1 set Herculis. α 2 Herculis therefore has no proper name.

The angular distance between Ras Algethi A and Ras Algethi B is 4.8 ″, the position angle of component B to A is 104 °. The French astronomer Paul Baize published a provisional orbital calculation in 1980 and determined the orbital period to be 3,600 years, with the lower limit at 2,000 and the upper limit at 4,800 years. He determined the major semi-axis to be 4.68 ″, which corresponds to an actual distance of about 520  AU . He indicates the eccentricity of the path as 0 (circular).

The Ras Algethi B subsystem is a spectroscopic binary star . Both stars orbit each other on an approximately circular orbit with a period of 51.578 days.

Track elements of the individual systems:

Track element Main system (α 1 - α 2 ) Subsystem (α 2 A - α 2 B)
Period of circulation 3,600 y 51,578 T
Epoch of the periastron Year 3685 JD 2,434,791.026 ± 0.012
eccentricity 0 0.0220 ± 0.0022
Major semi-axis 4.68 ″ not specified
Orbit inclination 155.8 ° not specified
Argument of the node 119.6 ° not specified
Argument of the periapsis 5.0 ° 67.5 ± 5.7 °

Ras Algethi A (α 1 Herculis)

Ras Algethi A is a semi-regularly variable star of the subgroup SRc. Its apparent brightness is on average 3.35 m and can rise to a maximum of 2.74 m and fall to a minimum of 4.00 m . The fluctuation of the apparent brightness is due to the pulsation of the star and has a period of about 125 days. This (main) period is overlaid by a secondary period, which has an average of 1,343 days. The star belongs to the spectral class M5 and the luminosity class Ib to II and can therefore be classified as a bright giant . It is also one of the AGB stars , which is why it is located on the asymptotic giant branch in the Hertzsprung-Russell diagram . Its surface temperature is about 3,280 K .

For a long time the mass of Ras Algethi A was unclear, which was due to the fact that its exact development phase was not known. The information in the specialist literature fluctuated between 1.7 and 15 solar masses. More recent studies have confirmed the correctness of the lower mass range and give the star's initial mass between 2.175 and 3.25 solar masses . The mass loss caused by the stellar wind is 0.00000011 or 0.00000015 solar masses per year. Due to this outflow of matter, the Ras Algethi system (including component B) is surrounded by an extensive, thin dust cover . In addition, a 518 K warm, spherical shell- shaped material shell around Ras Algethi A was detected by interferometric observations . It is interpreted as the result of a major eruption that must have occurred around 1990 and ejected about a millionth of the solar mass at 75 km / s into space.

Determinations of the star radius sometimes lead to very different results. The star disk diameter measured by interferometry in the near infrared (K-band, λ  = 2.2 µm) and based on the darkening of the edges is 33.15 ± 0.75 milli-angle seconds, which corresponds to a real diameter of around 390 ± 80 solar diameter. In the mid-infrared (λ = 11.15 µm), this is significantly larger at 39.32 ± 1.04 milli-angle seconds, or about 470 ± 100 solar diameters. The reason for the size difference between near and mid infrared is not known. However, the cause is assumed to be an envelope of hot water vapor surrounding the star, as calculations show that such an envelope can influence the optical depth of the star between the wavelengths in such a way that the observations coincide with the calculations. However, these two size determinations contradict newer theoretical models, according to which Ras Algethi A has only 284 ± 60 solar diameters. Apart from that, the diameter of the star globe fluctuates with an amplitude of up to 14% because the star pulsates. ( Note: For the particular problem of defining the surface of giant stars, see article: Star surface . )

Ras Algethi B (α 2 Herculis)

Ras Algethi B is a spectroscopic double star and has an apparent magnitude of 5.39 m . The apparent brightness of the individual components is estimated to be 5.6 m (α 2 Herculis A) and 6.6 m (α 2 Herculis B).

α 2 Herculis A is a giant star belonging to the spectral class G8 III. Its initial mass is between 2.175 and 3 solar masses and its surface temperature is around 4,900 K. According to theoretical models, it is located in the Hertzsprung-Russell diagram either on the giant red branch at the point shortly after the helium burn began ( see also: horizontal branch ) or at the beginning of the AGB phase.

α 2 Herculis B has the spectral class A9 IV – V and is either at the end of the main sequence phase with hydrogen burning still taking place in the star core or at the beginning of the phase as a sub-giant . Its initial mass is between 1.6 and 2.3 solar masses.

Web links

Individual evidence

  1. ↑ a b Jorge R. Ducati: VizieR Online Data Catalog: Catalog of Stellar Photometry in Johnson's 11-color system . In: CDS / ADC Collection of Electronic Catalogs, 2237 . 2002. bibcode : 2002yCat.2237 .... 0D . Corresponding database entry on VizieR .
  2. ↑ Benoit Famaey et al .: VizieR Online Data Catalog: Radial velocities for 6691 K and M giants (Famaey +, 2005) . In: VizieR On-line Data Catalog: J / A + A / 430/165 (originally published in: bibcode : 2005A & A ... 430..165F ). 2004. bibcode : 2004yCat..34300165F . Corresponding database entry on VizieR .
  3. ↑ Derived from parallax (0.00907 ± 0.00132 ″ according to van Leeuwen, 2007). Based on this, Moravveji et al. (2013) give a rounded value of 110 ± 16 pc.
  4. ↑ a b c d e f g h i j k l m n o p Ehsan Moravveji et al .: The age and mass of the α Herculis triple-star system from a MESA grid of rotating stars with 1.3 ≤ M / M ☉ ≤ 8.0 . In: The Astronomical Journal , Volume 146, Issue 3, 2013, Article ID 148. bibcode : 2013AJ .... 146..148M , doi: 10.1088 / 0004-6256 / 146/6/148 , arxiv : 1308.1632 . The luminosities of α 2 Her A and B were taken from the publication by Thiering / Reimers (1993) and from Moravveji et al. (2013) corrected upwards because the distance information used by Thiering / Reimers (70 pc) was out of date.
  5. ↑ a b c d e f Armin J. Deutsch: The Circumstellar Envelope of Alpha Herculis . In: The Astrophysical Journal , Volume 123, 1956, pp. 210-227. bibcode : 1956ApJ ... 123..210D , doi: 10.1086 / 146152 .
  6. ↑ a b c d N. J. Woolf: The masses of Alpha Herculis and Eta Geminorum . In: The Observatory , Vol. 83, 1963, pp. 260-262. bibcode : 1963Obs .... 83..260W . Based on this, Moravveji et al. the apparent magnitude for α 2 Her B is 6.6 m .
  7. ^ Erik Høg et al .: The Tycho-2 catalog of the 2.5 million brightest stars . In: VizieR Online Data Catalog: The Tycho-2 Catalog (Hog + 2000) (originally published in: bibcode : 2000A & A ... 355L..27H ). 2000. bibcode : 2000yCat.1259 .... 0H . Corresponding database entry on VizieR : α 1 Herculis , α 2 Herculis .
  8. ↑ a b c d e f g h i Inge Thiering, Dieter Reimers: Ultraviolet Observations of the Circumstellar Envelope of α 1 Her in the Line-Of of α 2 Her . In: Astronomy and Astrophysics , Volume 274, Issue 3, 1993, pp. 838-846. bibcode : 1993A & A ... 274..838T .
  9. ^ A b c Dorrit Hoffleit, Wayne H. Warren Jr: VizieR Online Data Catalog: Bright Star Catalog, 5th Revised Ed. (Hoffleit +, 1991) . In: VizieR On-line Data Catalog: V / 50 (originally published in bibcode : 1964BS .... C ...... 0H ). 1995. bibcode : 1995yCat.5050 .... 0H . Corresponding database entry on VizieR : α 1 Herculis , α 2 Herculis .
  10. ↑ To calculate the absolute brightness, subtract the distance module (here 5.21 m ) from the apparent brightness. The distance module is calculated as −5 mag - (5 mag · lgπ) , where π indicates the parallax in arc seconds (0.00907 ″ according to van Leeuwen, 2007).
  11. ↑ Bulletin of the IAU Working Group on Star Names, No. 1, July 2016. (PDF) Retrieved November 9, 2016 (English, 184 KiB).
  12. ^ Brian D. Mason et al .: VizieR Online Data Catalog: The Washington Visual Double Star Catalog (Mason + 2001-2014) , Version 2017-05-02. In: VizieR On-line Data Catalog: B / wds (originally published in: bibcode : 2001AJ .... 122.3466M ). 2017. bibcode : 2017yCat .... 102026M . Corresponding database entry on VizieR (WDS 17146 + 1423) .
  13. ^ A b Paul Baize: Les masses des étoiles variables a lonuge période . In: L'Astronomie , Volume 94, 1980, pp. 78-79. bibcode : 1980LAstr..94 ... 71B .
  14. ↑ a b c The linear expansion across the line of sight is calculated as 2r tan (α / 2) , where r is the distance in m (360 Lj =  3.41e18 m) and α the expansion inangular degrees(4.68 ″ = 0.0013 °; 33.15 mas = 9.21ste-6 °; 39.32 mas = 1.09e-5 °). That makes7.74e13 m ≈ 520 AU,5.48e11 m ≈ 390 R☉and6.49e11 m ≈ 470 R☉. When specifying the tolerance, the measurement error of the distance (± 60 ly) and the star disc diameter was explored (i.e. the smallest possible distance in combination with the smallest possible star disc diameter and vice versa) and the result rounded.
  15. ↑ a b Nikolai N. Samus et al .: General Catalog of Variable Stars (Samus + 2007-2013) . In: VizieR On-line Data Catalog: B / gcvs , Version 2013-04-30. bibcode : 2009yCat .... 102025S . Corresponding database entry on VizieR .
  16. ↑ Ehsan Moravveji et al .: Investigating the semi-regular light variations of the bright M5 supergiant: α Herculis . In: Astrophysics and Space Science , Volume 328, Ed. 1-2, 2010, pp. 113-117. bibcode : 2010Ap & SS.328..113M , doi: 10.1007 / s10509-009-0261-x .
  17. ↑ Ken Tatebe et al .: Observation of a Burst of High-Velocity Dust from α Herculis . In: The Astrophysical Journal , Volume 658, Issue 2, 2007, L103-L106. bibcode : 2007ApJ ... 658L.103T , doi: 10.1086 / 515569 .
  18. ↑ Guy Perrin et al .: Interferometric observations of the supergiant stars α Orionis and α Herculis with FLUOR at IOTA . In: Astronomy and Astrophysics , Volume 418, 2004, pp. 675-685. bibcode : 2004A & A ... 418..675P , doi: 10.1051 / 0004-6361: 20040052 , arxiv : astro-ph / 0402099 .
  19. ^ Jonathon Weiner, David DS Hale, Charles H. Townes: Asymptotic Giant Branch and Supergiant Stellar Diameters in the Mid-Infrared . In: The Astronomical Journal , Volume 589, Ed. 2, 2003, p. 978. bibcode : 2003ApJ ... 589..976W , doi: 10.1086 / 374779 .
  20. ↑ Keiichi Ohnaka: Warm water vapor envelope in the supergiants α Ori and α Her and its effects on the apparent size from the near-infrared to the mid-infrared . In: Astronomy and Astrophysics , Volume 421, 2004, pp. 1.149-1.158. bibcode : 2004A & A ... 421.1149O , doi: 10.1051 / 0004-6361: 20035668 .