O-type star
An O-type star is a hot, blue-white
Stars of this type are very rare, but because they are very bright, they can be seen at great distances; out of the 90
O-type stars are typically found in regions of active
Classification
O-type stars are classified by the relative strength of certain spectral lines.
The luminosity classes of O-type stars are assigned on the relative strengths of the He+
Star types O3 to O8 are classified as luminosity class sub-type "Vz" if they have a particularly strong 468.6 nm ionised helium line. The line's presence is thought to indicate extreme youth; the "z" stands for zero-age.[4]
To help with the classification of O-type stars, standard examples are listed for most of the defined types. The following table gives one of the standard stars for each spectral type. In some cases, a standard star has not been defined. For spectral types O2 to O5.5, supergiants are not split into Ia / Iab / Ib sub-types:
Characteristics
O-type stars are hot and luminous. They have characteristic surface temperatures ranging from 30,000–52,000
Other stars in the same temperature range include rare
O-type stars represent the highest masses of stars on the main sequence. The coolest of them have initial masses of around 16 times the Sun.[8] It is unclear what the upper limit to the mass of an O-type star would be. At solar metallicity levels, stars should not be able to form with masses above 120–150M☉, but at lower metallicity this limit is much higher. O-type stars form only a tiny fraction of main-sequence stars and the vast majority of these are towards the lower end of the mass range. The most massive and hottest types O3 and O2 are extremely rare, were only defined in 1971[9] and 2002[2] respectively, and only a handful are known in total. Giant and supergiant stars are somewhat less massive than the most massive main sequence O-type stars due to mass loss, but are still among the most massive stars known.
The formation rate of class O stars cannot be observed directly, but initial mass functions (IMF) can be derived that model observations of existing star populations and particularly young star clusters. Depending on the chosen IMF, class O stars form at a rate of one in several hundred main sequence stars.[10] Because the luminosity of these stars increases out of proportion to their masses, they have correspondingly shorter lifespans. The most massive spend less than a million years on the main sequence and explode as supernovae after three or four million years. The least luminous O-type stars can remain on the main sequence for around 10 million years, but cool slowly during that time and become early B-type stars. No massive star remains with spectral class O for more than about 5–6 million years.[6][8] Although sdO and CSPNe stars are low-mass stars billions of years old, the time spent in this phase of their lives is extremely short, of the order of 10,000,000 years.[11] The
It has been estimated that there are around 20,000 massive O-type stars in the Milky Way. The low-mass sdO and CSPNe O-type stars are probably more common, although less luminous and therefore harder to find. Despite their short lifetimes, they are thought to be normal stages in the evolution of common stars only a little more massive than the Sun.
Structure
O-type main-sequence stars are fueled by
Type "sdO" stars and CSPNe-type have a substantially different structure. Although they have a wide range of distinct characteristics, it is not fully understood how they all form and develop; they are thought to have degenerate cores that will eventually be exposed as a white dwarf. Before then, the material outside that core, is mostly helium with a thin layer of hydrogen, which is rapidly being lost due to the strong stellar wind. There may be several different origins for this type of star, but at least some of them have an internal shell-like layer where helium is being fused. That shell-burning enlarges the core and provides the power for these small stars' high luminosities.[14]
Evolution
In the lifecycle of O-type stars, different metallicities and rotation rates introduce considerable variation in their evolution, but the basics remain the same.[8]
O-type stars start to move slowly from the zero-age main sequence almost immediately, gradually becoming cooler and slightly more luminous. Although they may be characterised spectroscopically as giants or supergiants, they continue to burn hydrogen in their cores for several million years and develop in a very different manner from low-mass stars such as the Sun. Most O-type main-sequence stars will evolve more or less horizontally in the
The more-massive stars, initially main-sequence stars hotter than about O9, never become red supergiants because strong convection and high luminosity blow away the outer layers too quickly. 25–60M☉ stars may become
Low to intermediate-mass stars age in a very different way, through
At certain masses or chemical makeups, or perhaps as a result of binary interactions, some of these lower-mass stars become unusually hot during the horizontal branch or AGB phases. There may be multiple reasons, not fully understood, including stellar mergers or very late thermal pulses re-igniting post-AGB stars. These appear as very hot OB stars, but only moderately luminous and below the main sequence. There are both O (sdO) and B (sdB) hot subdwarfs, although they may develop in entirely different ways. The sdO-type stars have fairly normal O spectra but luminosities only around a thousand times the Sun.
Examples
O-type stars are rare but luminous, so they are easy to detect and there are a number of naked eye examples.
Supergiants - 29 Canis Majoris
- Alnitak
- Alpha Camelopardalis
- Cygnus X-1
- Tau Canis Majoris
- Zeta Puppis
Giants - Iota Orionis
- LH 54-425
- Meissa
- Plaskett's star
- Xi Persei
- Mintaka
- HD 164492 A
Subdwarfs - HD 49798 (sdO6p)
Main-sequence
Location
Spiral arms
O-type main-sequence stars tend to appear in the arms of spiral galaxies. This is because, as a spiral arm moves through space, it compresses any
O/OB associations
Stellar associations are groups of stars that are gravitationally unbound from the beginning of their formation. The stars in stellar associations are moving from one another so rapidly that gravitational forces cannot keep them together. In young stellar associations, most of the light comes from O- and B-type stars, so such associations are called OB associations.
Molecular clouds
The birth of an O-type star in a molecular cloud has a destructive effect on the cloud, but also may trigger the formation of new stars. O-type stars emit copious amounts of ultraviolet radiation, which ionizes the gas in the cloud and pushes it away.[15] O-type stars also have powerful stellar winds, with velocities of thousands of kilometers per second, which can blow a bubble in the molecular cloud around the star.[16] O-type stars explode as supernovae when they die, releasing vast amounts of energy, contributing to the disruption of a molecular cloud.[17] These effects disperse the remaining molecular material in a star-forming region, ultimately stopping the birth of new stars, and possibly leaving behind a young open cluster.
Nevertheless, before the cloud is disrupted, the sweeping up of material by an expanding bubble (called collect and collapse) or the compression of existing cloudlets (called radiation driven implosion) may lead to the birth of new stars. Evidence of
Footnotes
- ^ The 4 visibly bright O-type stars are Gamma Velorum, Alnitak (Zeta Orionis), Mintaka (Delta Orionis), and Zeta Puppis.
- ^ The corona present in other spectral types are also exhibited by O-type main-sequence stars, however the coronae of O-type main-sequence stars extend out much further and generate stellar winds many times stronger. The intense radiation and solar winds from O-type main-sequence stars are strong enough to strip away the atmospheres from planets that form inside the radius of the habitable zone of the star via photoevaporation.[citation needed]
References
- S2CID 122609922.
- ^ S2CID 122127697.
- S2CID 118686731.
- S2CID 119259952.
- ^ a b
Maíz Apellániz, J.; Sota, A.; Arias, J.I.; Barbá, R.H.; Walborn, N.R.; Simón-Díaz, S.; Negueruela, I.; Marco, A.; Leã o, J.R.S.; Herrero, A.; Gamen, R.C.; Alfaro, E.J. (2016). "The Galactic O-Star Spectroscopic Survey (GOSSS). III. 142 additional O-type systems". S2CID 55658165.
- ^ a b
Weidner, Carsten; Vink, Jorick (2010). "The masses, and the mass discrepancy of O-type stars". Astronomy & Astrophysics. 524: A98. S2CID 118836634.
- ^
Aller, A.; Miranda, L.F.; Ulla, A.; Vázquez, R.; Guillén, P.F.; Olguín, L.; et al. (2013). "Detection of a multi-shell planetary nebula around the hot subdwarf O-type star 2MASS J19310888+4324577". S2CID 59036773.
- ^ a b c d
Meynet, G.; Maeder, A. (2003). "Stellar evolution with rotation". S2CID 17546535.
- ^
Walborn, N.R. (1971). "Some extremely early O stars near η Carinae". The Astrophysical Journal. 167: L31. doi:10.1086/180754.
- ^
Kroupa, Pavel; Weidner, Carsten; Pflamm-Altenburg, Jan; Thies, Ingo; Dabringhausen, Jörg; Marks, Michael; Maschberger, Thomas (2013). "The stellar and sub-stellar initial mass function of simple and composite populations". Planets, Stars, and Stellar Systems. pp. 115–242. S2CID 204934137.
- ^
Yu, S.; Li, L. (2009). "Hot subdwarfs from the stable Roche lobe overflow channel". S2CID 15336878.
- ^
Ledrew, Glenn (February 2001). "The real starry sky". Bibcode:2001JRASC..95...32L.
- ^ Mamajek, Eric (ed.). "Number densities of stars of different types in the solar vicinity" (faculty webpage). University of Rochester. Retrieved 2018-10-31.
- ^
Landstreet, John D.; Bagnulo, Stefano; Fossati, Luca; Jordan, Stefan; O'Toole, Simon J. (2012). "The magnetic fields of hot subdwarf stars". S2CID 118474970.
- ^
Dale, J.E.; et al. (2013). "Ionizing feedback from massive stars in massive clusters – III. Disruption of partially unbound clouds". S2CID 118480561.
- ^
Dale, K.V.; et al. (2008). "The effect of stellar winds on the formation of a protocluster". S2CID 16227011.
- ^
Dekel, A.; Krumholz, M.R. (2013). "Steady outflows in giant clumps of high-z disc galaxies during migration and growth by accretion". S2CID 32488591.
- ^
Getman, K.V.; et al. (2009). "Protoplanetary disk evolution around the triggered star-forming region Cepheus B". S2CID 18149231.
- ^
Getman, K. V.; et al. (2012). "The Elephant Trunk Nebula and the Trumpler 37 cluster: Contribution of triggered star formation to the total population of an HII region". S2CID 49528100.