Messier 81

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Messier 81
Messier 81 HST.jpg
The HST's view of M81, with its open star clusters, globular star clusters, and regions of fluorescent gas.
Observation data (J2000 epoch)
Constellation Ursa Major [1]
Right ascension 09h 55m 33.2s [2]
Declination +69° 3′ 55″ [2]
Redshift −0.000113
Heliocentric radial velocity −34
Galactocentric velocity73
Distance 11.8  Mly (3.6  Mpc)
Apparent magnitude  (V)6.94 [3] [4]
Characteristics
Type SA(s)ab, [2] LINER [2]
Size29.44 kiloparsecs (96,000 light-years)
(diameter; 25.0 mag/arcsec2 B-band isophote) [2] [5]
Apparent size  (V)26.9 × 14.1 moa [2]
Other designations
NGC 3031, UGC 5318, MCG+12-10-010, PGC  28630, Bode's Galaxy [3]

Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a grand design spiral galaxy about 12 million light-years away in the constellation Ursa Major. It has a D25 isophotal diameter of 29.44 kiloparsecs (96,000 light-years ). [2] [5] Because of its relative proximity to the Milky Way galaxy, large size, and active galactic nucleus (which harbors a 70 million M☉ [6] supermassive black hole), Messier 81 has been studied extensively by professional astronomers. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers. [7] In late February 2022, astronomers reported that M81 may be the source of FRB 20200120E, a repeating fast radio burst. [8] [9]

Contents

Discovery

Messier 81 was first discovered by Johann Elert Bode on 31 December 1774. [10] Thus, it is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, hence listed it in the Messier Catalogue. [10]

Visibility

The galaxy is to be found approximately 10° northwest of Alpha Ursae Majoris (Dubhe) along with several other galaxies in the Messier 81 Group. [7] [11] Its apparent magnitude due to its distance means it requires a good night sky and only rises very briefly and extremely low at its southernmost limit from Earth's surface, about the 20th parallel south.

Messier 81 and Messier 82 are considered ideal for viewing using binoculars and small telescopes. [7] [11] The two objects are generally not observable to the unaided eye, although highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a very dark sky. [7] [12] Telescopes with apertures of 8 inches (20 cm) or larger are needed to distinguish structures in the galaxy. [11]

An infrared image of Messier 81 taken by the Spitzer Space Telescope. The blue colors represent stellar emission observed at 3.6 mm. The green colors represent 8 mm emission originating primarily from polycyclic aromatic hydrocarbons in the interstellar medium. The red colors represent 24 mm emission originating from heated dust in the interstellar medium. Messier81 highres.jpg
An infrared image of Messier 81 taken by the Spitzer Space Telescope. The blue colors represent stellar emission observed at 3.6 μm. The green colors represent 8 μm emission originating primarily from polycyclic aromatic hydrocarbons in the interstellar medium. The red colors represent 24 μm emission originating from heated dust in the interstellar medium.

The galaxy is best observed during April. [15]

Interstellar dust

Most of the emission at infrared wavelengths originates from interstellar dust. [14] [16] This interstellar dust is found primarily within the galaxy's spiral arms, and it has been shown to be associated with star formation regions. [14] [16] The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and thus enhancing the infrared dust emission from these regions.

Globular clusters

It is estimated M81 has 210 ± 30 globular clusters. [17] In late February 2022, astronomers reported that M81 may be the source of FRB 20200120E, a repeating fast radio burst. [8] [9]

Supernovae

M81 (left) and M82 (right). M82 is one of two galaxies strongly influenced gravitationally by M81. The other, NGC 3077, is located off the top edge of this image. Bode and Cigar galaxies.jpg
M81 (left) and M82 (right). M82 is one of two galaxies strongly influenced gravitationally by M81. The other, NGC 3077, is located off the top edge of this image.
M81 with satellite galaxy Holmberg IX in the top center-right corner M81.jpg
M81 with satellite galaxy Holmberg IX in the top center-right corner

Only one supernova has been detected in Messier 81. [18] The supernova, named SN 1993J, was discovered on 28 March 1993 by F. García in Spain. [19] At the time, it was the second brightest type II supernova observed in the 20th century, [20] peaking at an apparent magnitude of 10.7. The spectral characteristics of the supernova changed over time. Initially, it looked more like a type II supernova (a supernova formed by the explosion of a supergiant star) with strong hydrogen spectral line emission, but later the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib. [20] [21]

Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernovae, [22] [23] but did resemble the variations observed in type Ib supernovae. [24] Hence, the supernova has been classified as a type IIb, a transitory class between type II and type Ib. [21] The scientific results from this supernova suggested that type Ib and Ic supernovae were formed through the explosions of giant stars through processes similar to those taking place in type II supernovae. [21] [25] Despite the uncertainties in modeling the unusual supernova, it was also used to estimate a very approximate distance of 8.5 ± 1.3 Mly (2.6 ± 0.4 Mpc) to Messier 81. [20] As a local galaxy, the Central Bureau for Astronomical Telegrams (CBAT) tracks novae in M81 along with M31 and M33. [26]

SMBH

In the center of M81 there exists a supermassive black hole (SMBH) with a mass of about 7×107 M☉. The SMBH is active, having an accretion disk and one-sided relativistic jet. The observation also demonstrate that there may exist a second SMBH that orbits the primary SMBH with a period of around 30 years. The mass of the secondary SMBH is estimated at 0.1 of the primary. [27]

Environment

Messier 81 is the largest galaxy in the M81 Group, a group of 34 in the constellation Ursa Major. [28] At approximately 11.7 Mly (3.6 Mpc) from the Earth, it makes this group and the Local Group, containing the Milky Way, [28] relative neighbors in the Virgo Supercluster.

Gravitational interactions of M81 with M82 and NGC 3077 [29] have stripped hydrogen gas away from all three galaxies, forming gaseous filamentary structures in the group. [29] Moreover, these interactions have allowed interstellar gas to fall into the centers of M82 and NGC 3077, leading to vigorous star formation or starburst activity there. [29]

Distance

The distance to Messier 81 has been measured by Freedman et al [30] to be 3.63 ± 0.34 Megaparsecs (11.8 ± 1.1 million light years) by using the Hubble Space Telescope to identify classical Cepheid variables and measure their periods using the period-luminosity relation discovered by Henrietta Swan Leavitt.

See also

Related Research Articles

<span class="mw-page-title-main">M81 Group</span> Galaxy group in Ursa Major and Camelopardalis

The M81 Group is a galaxy group in the constellations Ursa Major and Camelopardalis that includes the galaxies Messier 81 and Messier 82, as well as several other galaxies with high apparent brightnesses. The approximate center of the group is located at a distance of 3.6 Mpc, making it one of the nearest groups to the Local Group. The group is estimated to have a total mass of ×1012M☉. The M81 Group, the Local Group, and other nearby groups all lie within the Virgo Supercluster.

<span class="mw-page-title-main">Supernova</span> Astrophysical phenomenon

A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

<span class="mw-page-title-main">Messier 82</span> Starburst galaxy in the constellation Ursa Major

Messier 82 (also known as NGC 3034, Cigar Galaxy or M82) is a starburst galaxy approximately 12 million light-years away in the constellation Ursa Major. It is the second-largest member of the M81 Group, with the D25 isophotal diameter of 12.52 kiloparsecs (40,800 light-years). It is about five times more luminous than the Milky Way and its central region is about one hundred times more luminous. The starburst activity is thought to have been triggered by interaction with neighboring galaxy M81. As one of the closest starburst galaxies to Earth, M82 is the prototypical example of this galaxy type. SN 2014J, a type Ia supernova, was discovered in the galaxy on 21 January 2014. In 2014, in studying M82, scientists discovered the brightest pulsar yet known, designated M82 X-2.

<span class="mw-page-title-main">Pinwheel Galaxy</span> Galaxy in the constellation Ursa Major

The Pinwheel Galaxy is a face-on, unbarred, and counterclockwise spiral galaxy located 21 million light-years from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 and was communicated that year to Charles Messier, who verified its position for inclusion in the Messier Catalogue as one of its final entries.

<span class="mw-page-title-main">Messier 83</span> Barred spiral galaxy in the constellation Hydra

Messier 83 or M83, also known as the Southern Pinwheel Galaxy and NGC 5236, is a barred spiral galaxy approximately 15 million light-years away in the constellation borders of Hydra and Centaurus. Nicolas-Louis de Lacaille discovered M83 on 17 February 1752 at the Cape of Good Hope. Charles Messier added it to his catalogue of nebulous objects in March 1781.

<span class="mw-page-title-main">Messier 61</span> Galaxy in the constellation Virgo

Messier 61 is an intermediate barred spiral galaxy in the Virgo Cluster of galaxies. It was first discovered by Barnaba Oriani on May 5, 1779, six days before Charles Messier discovered the same galaxy. Messier had observed it on the same night as Oriani but had mistaken it for a comet. Its distance has been estimated to be 45.61 million light years from the Milky Way Galaxy. It is a member of the M61 Group of galaxies, which is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster.

<span class="mw-page-title-main">Messier 59</span> Elliptical galaxy in the constellation Virgo

Messier 59 or M59, also known as NGC 4621, is an elliptical galaxy in the equatorial constellation of Virgo. It is a member of the Virgo Cluster, with the nearest fellow member 8′ away and around 5 magnitudes fainter. The nearest cluster member of comparable brightness is the lenticular galaxy NGC 4638, which is around 17′ away. It and the angularly nearby elliptical galaxy Messier 60 were both discovered by Johann Gottfried Koehler in April 1779 when observing comet seeming close by. Charles Messier listed both in the Messier Catalogue about three days after Koehler's discovery.

<span class="mw-page-title-main">Messier 60</span> Elliptical galaxy in the constellation Virgo

Messier 60 or M60, also known as NGC 4649, is an elliptical galaxy approximately 57 million light-years away in the equatorial constellation of Virgo. Together with NGC 4647, it forms a pair known as Arp 116. Messier 60 and nearby elliptical galaxy Messier 59 were discovered by Johann Gottfried Koehler in April 1779, observing a comet in the same part of the sky. Charles Messier added both to his catalogue about three days after this.

<span class="mw-page-title-main">Messier 66</span> Intermediate spiral galaxy in the constellation Leo

Messier 66 or M66, also known as NGC 3627, is an intermediate spiral galaxy in the southern, equatorial half of Leo. It was discovered by French astronomer Charles Messier on 1 March 1780, who described it as "very long and very faint". This galaxy is a member of a small group of galaxies that includes M65 and NGC 3628, known as the Leo Triplet or the M66 Group. M65 and M66 are a common object for amateur astronomic observation, being separated by only 20′.

<span class="mw-page-title-main">Messier 74</span> Face-on spiral galaxy in the constellation Pisces

Messier 74 is a large spiral galaxy in the equatorial constellation Pisces. It is about 32 million light-years away from Earth. The galaxy contains two clearly defined spiral arms and is therefore used as an archetypal example of a grand design spiral galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur astronomers to observe. Its relatively large angular size and the galaxy's face-on orientation make it an ideal object for professional astronomers who want to study spiral arm structure and spiral density waves. It is estimated that M74 hosts about 100 billion stars.

<span class="mw-page-title-main">Messier 99</span> Spiral galaxy in the constellation Coma Berenices

Messier 99 or M99, also known as NGC 4254 or St. Catherine's Wheel, is a grand design spiral galaxy in the northern constellation Coma Berenices approximately 15,000,000 parsecs from the Milky Way. It was discovered by Pierre Méchain on 17 March 1781. The discovery was then reported to Charles Messier, who included the object in the Messier Catalogue of comet-like objects. It was one of the first galaxies in which a spiral pattern was seen. This pattern was first identified by Lord Rosse in the spring of 1846.

<span class="mw-page-title-main">Messier 106</span> Galaxy in the constellation Canes Venatici

Messier 106 is an intermediate spiral galaxy in the constellation Canes Venatici. It was discovered by Pierre Méchain in 1781. M106 is at a distance of about 22 to 25 million light-years away from Earth. M106 contains an active nucleus classified as a Type 2 Seyfert, and the presence of a central supermassive black hole has been demonstrated from radio-wavelength observations of the rotation of a disk of molecular gas orbiting within the inner light-year around the black hole. NGC 4217 is a possible companion galaxy of Messier 106. Besides the two visible arms, it has two "anomalous arms" detectable using an X-ray telescope.

<span class="mw-page-title-main">NGC 6946</span> Galaxy in the constellations Cepheus & Cygnus

NGC 6946, sometimes referred to as the Fireworks Galaxy, is a face-on intermediate spiral galaxy with a small bright nucleus, whose location in the sky straddles the boundary between the northern constellations of Cepheus and Cygnus. Its distance from Earth is about 25.2 million light-years or 7.72 megaparsecs, similar to the distance of M101 in the constellation Ursa Major. Both were once considered to be part of the Local Group, but are now known to be among the dozen bright spiral galaxies near the Milky Way but beyond the confines of the Local Group. NGC 6946 lies within the Virgo Supercluster.

<span class="mw-page-title-main">NGC 4526</span> Lenticular galaxy in the constellation Virgo

NGC 4526 is a lenticular galaxy with an embedded dusty disc, located approximately 55 million light-years from the Solar System in the Virgo constellation and discovered on 13 April 1784 by William Herschel. Herschel observed it again on 28 December 1785, resulting in the galaxy being entered twice into the New General Catalogue.

<span class="mw-page-title-main">SN 1994D</span> Type Ia supernova

SN 1994D was a Type Ia supernova event in the outskirts of galaxy NGC 4526. It was offset by 9.0″ west and 7.8″ south of the galaxy center and positioned near a prominent dust lane. It was caused by the explosion of a white dwarf star composed of carbon and oxygen. This event was discovered on March 7, 1994 by R. R. Treffers and associates using the automated 30-inch telescope at Leuschner Observatory. It reached peak visual brightness two weeks later on March 22. Modelling of the light curve indicates the explosion would have been visible around March 3-4. A possible detection of helium in the spectrum was made by W. P. S. Meikle and associates in 1996. A mass of 0.014 to 0.03 M☉ in helium would be needed to produce this feature.

<span class="mw-page-title-main">SN 1993J</span> Supernova in the spiral galaxy Messier 81

SN 1993J is a supernova observed in Bode's Galaxy. It was discovered on 28 March 1993 by F. Garcia in Spain. At the time, it was the second-brightest type II supernova observed in the twentieth century behind SN 1987A, peaking at a visible apparent magnitude of 10.7 on March 30, with a second peak of 10.86 on April 18.

SN 1990U was a type Ic supernova event in the nucleus of the galaxy NGC 7479. It was discovered July 27, 1990 by the Berkeley Automated Supernova Search after reaching magnitude 16±0.5. Initially this was classified as a Type Ib supernova, but the weakness of the neutral helium absorption lines led to a reclassification.

<span class="mw-page-title-main">SN 2014J</span> Supernova in Messier 82

SN 2014J was a type-Ia supernova in Messier 82 discovered in mid-January 2014. It was the closest type-Ia supernova discovered for 42 years, and no subsequent supernova has been closer as of 2023. The supernova was discovered by chance during an undergraduate teaching session at the University of London Observatory. It peaked on 31 January 2014, reaching an apparent magnitude of 10.5. SN 2014J was the subject of an intense observing campaign by professional astronomers and was bright enough to be seen by amateur astronomers.

<span class="mw-page-title-main">NGC 1084</span> Galaxy in the constellation Eridanus

NGC 1084 is an unbarred spiral galaxy in the constellation Eridanus. It is located at a distance of about 63 million light-years away from the Milky Way. The galaxy was discovered by William Herschel on 10 January 1785. It has multiple spiral arms, which are not well defined. It belongs in the same galaxy group with NGC 988, NGC 991, NGC 1022, NGC 1035, NGC 1042, NGC 1047, NGC 1052 and NGC 1110. This group is in turn associated with the Messier 77 group.

<span class="mw-page-title-main">NGC 4242</span> Galaxy in constellation Canes Venatici

NGC 4242 is a spiral galaxy in the northern constellation of Canes Venatici. The galaxy is about 18 million light years away. It was discovered on 10 April 1788 by William Herschel, and it was described as "very faint, considerably large, irregular, round, very gradually brighter in the middle, resolvable" by John Louis Emil Dreyer, the compiler of the New General Catalogue.

References

  1. ↑ Dreyer, J. L. E. (1988). Sinnott, R. W. (ed.). The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters. Sky Publishing Corporation / Cambridge University Press. ISBN   978-0-933346-51-2.
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  6. ↑ Devereux, N.; Ford, H.; Tsvetanov, Z.; Jocoby, J. (2003). "STIS Spectroscopy of the Central 10 Parsecs of M81: Evidence for a Massive Black Hole". Astronomical Journal . 125 (3): 1226–1235. Bibcode:2003AJ....125.1226D. doi:10.1086/367595. S2CID   121093306.
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  14. 1 2 3 Gordon, K. D.; Pérez-González, P. G.; Misselt, K. A.; Murphy, E. J.; Bendo, G. J.; Walter, F.; Thornley, M. D.; Kennicutt Jr., R. C.; Rieke, G. H.; Engelbracht, C. W.; Smith, J.-D. T.; Alonso-Herrero, A.; Appleton, P. N.; Calzetti, D.; Dale, D. A.; Draine, B. T.; Frayer, D. T.; Helou, G.; Hinz, J. L.; Hines, D. C.; Kelly, D. M.; Morrison, J. E.; Muzerolle, J.; Regan, M. W.; Stansberry, J. A.; Stolovy, S. R.; Storrie-Lombardi, L. J.; Su, K. Y. L.; Young, E. T. (2004). "Spatially Resolved Ultraviolet, Hα, Infrared, and Radio Star Formation in M81". Astrophysical Journal Supplement Series . 154 (1): 215–221. arXiv: astro-ph/0406064 . Bibcode:2004ApJS..154..215G. doi:10.1086/422714. S2CID   17283721.
  15. ↑ "Messier 81 - NASA Science".
  16. 1 2 Pérez-González, P. G.; Kennicutt Jr., R. C.; Gordon, K. D.; Misselt, K. A.; Gil De Paz, A.; Engelbracht, C. W.; Rieke, G. H.; Bendo, G. J.; Bianchi, L.; Boissier, S.; Calzetti, D.; Dale, D. A.; Draine, B. T.; Jarrett, T. H.; Hollenbach, D.; Prescott, M. K. M. (2006). "Ultraviolet through Far-Infrared Spatially Resolved Analysis of the Recent Star Formation in M81 (NGC 3031)". Astrophysical Journal . 648 (2): 987–1006. arXiv: astro-ph/0605605 . Bibcode:2006ApJ...648..987P. doi:10.1086/506196. S2CID   13901458.
  17. ↑ Chandar, Rupali; Whitmore, Bradley; Lee, Myung Gyoon (10 August 2004). "The Globular Cluster Systems of Five Nearby Spiral Galaxies: New Insights from Hubble Space TelescopeImaging". The Astrophysical Journal. 611 (1): 220–244. arXiv: astro-ph/0407460 . Bibcode:2004ApJ...611..220C. doi: 10.1086/421934 . ISSN   0004-637X.
  18. ↑ "NASA/IPAC Extragalactic Database". Results for extended name search on NGC 3031. Retrieved 27 February 2007.
  19. ↑ Ripero, J.; Garcia, F.; Rodriguez, D.; Pujol, P.; Filippenko, A. V.; Treffers, R. R.; Paik, Y.; Davis, M.; Schlegel, D.; Hartwick, F. D. A.; Balam, D. D.; Zurek, D.; Robb, R. M.; Garnavich, P.; Hong, B. A. (1993). "Supernova 1993J in NGC 3031". IAU Circular . 5731: 1. Bibcode:1993IAUC.5731....1R.
  20. 1 2 3 Schmidt, B.P.; Kirshner, R.P.; Eastman, R.G.; Grashuis, R.; Dell'Antonio, I.; Caldwell, N.; Foltz, C.; Huchra, John P.; Milone, Alejandra A. E. (1993). "The unusual supernova SN1993J in the galaxy M81". Nature . 364 (6438): 600–602. Bibcode:1993Natur.364..600S. doi:10.1038/364600a0. S2CID   4304547.
  21. 1 2 3 Filippenko, A. V.; Matheson, T.; Ho, L. C. (1993). "The "Type IIb" Supernova 1993J in M81: A Close Relative of Type Ib Supernovae". Astrophysical Journal Letters . 415: L103–L106. Bibcode:1993ApJ...415L.103F. doi: 10.1086/187043 .
  22. ↑ Benson, P. J.; Herbst, W.; Salzer, J. J.; Vinton, G.; Hanson, G. J.; Ratcliff, S. J.; Winkler, P. F.; Elmegreen, D. M.; Chromey, F.; Strom, C.; Balonek, T. J.; Elmegreen, B. G. (1994). "Light curves of SN 1993J from the Keck Northeast Astronomy Consortium". Astronomical Journal . 107: 1453–1460. Bibcode:1994AJ....107.1453B. doi: 10.1086/116958 .
  23. ↑ Wheeler, J. C.; Barker, E.; Benjamin, R.; Boisseau, J.; Clocchiatti, A.; De Vaucouleurs, G.; Gaffney, N.; Harkness, R. P.; Khokhlov, A. M.; Lester, D. F.; Smith, B. J.; Smith, V. V.; Tomkin, J. (1993). "Early Observations of SN 1993J in M81 at McDonald Observatory". Astrophysical Journal . 417: L71–L74. Bibcode:1993ApJ...417L..71W. doi:10.1086/187097.
  24. ↑ Richmond, M. W.; Treffers, R. R.; Filippenko, A. V.; Palik, Y.; Leibundgut, B.; Schulman, E.; Cox, C. V. (1994). "UBVRI photometry of SN 1993J in M81: The first 120 days". Astronomical Journal . 107: 1022–1040. Bibcode:1994AJ....107.1022R. doi: 10.1086/116915 .
  25. ↑ Filippenko, A. V.; Matheson, T.; Barth, A. J. (1994). "The peculiar type II supernova 1993J in M81: Transition to the nebular phase". Astronomical Journal . 108: 2220–2225. Bibcode:1994AJ....108.2220F. doi:10.1086/117234.
  26. ↑ Bishop, David. "Extragalactic Novae". supernovae.net (International Supernovae Network). Archived from the original on 8 April 2010. Retrieved 11 September 2010.
  27. ↑ Jiang, Wu; Shen, Zhiqiang; Martí-Vidal, Ivan; Yan, Zhen; Huang, Lei; Gold, Roman; Li, Ya-Ping; Xie, Fuguo; Kawaguchi, Noriyuki (1 December 2023). "Observational Evidence of a Centi-parsec Supermassive Black Hole Binary Existing in the Nearby Galaxy M81". The Astrophysical Journal. 959 (1): 11. arXiv: 2312.01328 . doi: 10.3847/1538-4357/ad063d .
  28. 1 2 Karachentsev, I. D. (2005). "The Local Group and Other Neighboring Galaxy Groups". Astronomical Journal . 129 (1): 178–188. arXiv: astro-ph/0410065 . Bibcode:2005AJ....129..178K. doi:10.1086/426368. S2CID   119385141.
  29. 1 2 3 Yun, M. S.; Ho, P. T. P.; Lo, K. Y. (1994). "A high-resolution image of atomic hydrogen in the M81 group of galaxies". Nature . 372 (6506): 530–532. Bibcode:1994Natur.372..530Y. doi:10.1038/372530a0. PMID   7990925. S2CID   4369085.
  30. ↑ Freedman, Wendy (1994). "The Hubble Space Telescope Extragalactic Distance Scale Project. I. The Discovery of Cepheids and a New Distance to M81". The Astrophysical Journal. 427 (June): 628–655. Bibcode:1994ApJ...427..628F. doi:10.1086/174172 – via Astrophysics Data System.
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