Astronomy:AM Herculis

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AM Herculis
Observation data
Equinox J2000.0]] (ICRS)
Constellation Hercules
Right ascension  18h 16m 13.255s[1]
Declination +49° 52′ 04.76″[1]
Apparent magnitude (V) 12.30-15.7[2]
Spectral type pec + M4.5V[2]
Variable type AM/XRM+E[2]
Radial velocity (Rv)-19.0[3] km/s
Proper motion (μ) RA: −45.957[1] mas/yr
Dec.: +28.046[1] mas/yr
Parallax (π)11.3953 ± 0.0179[1] mas
Distance286.2 ± 0.4 ly
(87.8 ± 0.1 pc)
Period (P)3.094 hours (variable)[4]
Eccentricity (e)0.47±0.21[5]
Inclination (i)50[4]°
White dwarf
Mass0.6 - 0.7[4] M
Red dwarf
Mass0.26[5] M
Radius0.32[5] R
Other designations
AN 1923.0028, GSC 3533.01105, X 18149+498[2]
Database references

AM Herculis is a binary variable star located in the constellation Hercules. This star, along with the star AN Ursae Majoris, is the prototype for a category of cataclysmic variable stars called polars, or AM Her type stars.


AM Herculis was first cataloged in 1923 by Max Wolf and was listed at the time as Veränderlicher 28.1923, which is now AN 28.1923 in the General Catalogue of Variable Stars. It was observed to be an irregular variable star ranging from 12 to 14 in apparent magnitude.[6] In 1976, the astronomer S. Tapia discovered that light from the star is both linearly and circularly polarized, showing that there was a strong magnetic field surrounding the system and revealing that the system was more complex than previously thought.[7][8][9]


The AM Herculis binary system contains a white dwarf and a red dwarf. The white dwarf is accreting material directly from the red dwarf without an accretion disk. The white dwarf primary is highly magnetic and the infalling material is channelled towards the magnetic poles. The accretion rate is unstable, at times decreasing dramatically and reducing the brightness of the whole system. There are also periodic variations thought to be caused by the appearance and eclipse of the accreting regions during rotation of the white dwarf.[4][5]


  1. 1.0 1.1 1.2 1.3 1.4 Brown, A. G. A. (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics 616: A1. doi:10.1051/0004-6361/201833051. Bibcode2018A&A...616A...1G.  Gaia DR2 record for this source at VizieR.
  2. 2.0 2.1 2.2 2.3 "Query= AM Her". General Catalogue of Variable Stars. Centre de Données astronomiques de Strasbourg. 
  3. Duflot, M.; Figon, P.; Meyssonnier, N. (1995). "Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue". Astronomy and Astrophysics Supplement Series 114: 269. Bibcode1995A&AS..114..269D. 
  4. 4.0 4.1 4.2 4.3 Gawroński, M. P.; Goździewski, K.; Katarzyński, K.; Rycyk, G. (2018). "Another look at AM Herculis – radio-astrometric campaign with the e-EVN at 6 cm". Monthly Notices of the Royal Astronomical Society 475 (1): 1399–1409. doi:10.1093/mnras/stx3175. Bibcode2018MNRAS.475.1399G. 
  5. 5.0 5.1 5.2 5.3 Dai, Zhibin; Qian, Shengbang; Li, Linjia; Rycyk, G. (2013). "Updated Photometry and Orbital Period Analysis for the Polar Am Herculis on the Upper Edge of the Period Gap". The Astrophysical Journal 774 (2): 153. doi:10.1088/0004-637X/774/2/153. Bibcode2013ApJ...774..153D. 
  6. S. Seliwanow (1923). "Mitteilungen über Veränderliche - Veränderlicher 28.1923 Herculis - M. Wolf - December 1923" (in de). Astronomische Nachrichten 220 (15): 255–256. doi:10.1002/asna.19232201505. Bibcode1924AN....220..249H. 
  7. Tapia, S. (March 15, 1977). "Discovery of a magnetic compact star in the AM Herculis/3U 1809+50 system". The Astrophysical Journal Letters 212: L125–L129. doi:10.1086/182390. Bibcode1977ApJ...212L.125T. 
  8. Hessman, F.V.; Gansicke, B.T.; Mattei, J.A. (September 2000). "The history and source of mass-transfer variations in AM Herculis". Astronomy & Astrophysics 361: 952–958. Bibcode2000A&A...361..952H. 
  9. Krzeminski, W.; Serkowski, K. (August 1977). "Extremely high circular polarization of AN Ursae Majoris". The Astrophysical Journal Letters 216: L45. doi:10.1086/182506. Bibcode1977ApJ...216L..45K. 

Further reading

External links