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O-type stars are the hot, blue-white stars of spectral type O in the Yerkes classification system employed by astronomers. Temperatures in excess of approximately 33,000 Kelvin (K) place this class furthest to the left on the Hertzsprung–Russell diagram. Stars of this type are identified by their strong He II absorption (sometimes emission) lines and their He I absorption lines that are slightly weaker than stars of spectral type B.
O-type stars are typically located in regions of active star formation, such as the spiral arms of a spiral galaxy. These stars illuminate any surrounding material and are largely responsible for the distinct coloration of a galaxy's arms. Furthermore, O-type stars are often in systems with other stars, where one star may accumulate material from another star, acquiring enough mass to achieve the conditions necessary to be considered type O.
Stars of this type are particularly rare; only 0.00003% of the main sequence are O-type stars. Due to their hot and massive nature, O-type main-sequence stars end their lives rather quickly in violent supernova explosions, resulting in black holes or neutron stars. Otherwise, many more O-type stars reside as dwarfs or giants moving through the latter stages of their evolution off of the main sequence.
- 1 Main sequence
- 2 Dwarfs
- 3 Giants
- 4 Location
- 5 References
O-type main-sequence stars represent the highest masses and temperatures of stars on the main sequence. With characteristic surface temperatures ranging from 30,000 - 52,000 K, they emit intense ultraviolet light and appear in the visible spectrum as bluish-white. Spectral analysis of O-type main-sequence stars usually shows strong ionized helium absorption (and sometimes emission) lines with weaker lines of neutral helium as well. Spectral identification of other lines becomes complicated due to the intense heat of their atmospheres. Despite this, ionized silicon, carbon and nitrogen lines remain identifiable along with prominent hydrogen Balmer lines. Due to the inherent complexity of generating high-mass stars and the comparatively shortened life-spans of O-type main-sequence stars, they are accordingly the rarest with only around 20,000 suspected to exist in the entire Milky Way.
O-type main-sequence stars are fueled by nuclear fusion, as all main-sequence stars are. However, the high mass of O-type main-sequence stars results in extremely high core temperatures. At these temperatures, the CNO cycle dominates the production of the star’s energy and consumes its nuclear fuel at a much higher rate than low-mass hydrogen-fusing stars. The intense amount of energy generated by O-type main-sequence stars cannot be radiated away from the core efficiently enough, and consequently O-type main-sequence stars experience convection in their cores, a notable difference from other main-sequence spectral types whose convective zones are generally found outside the core. The radiative zones of O-type main-sequence stars occur between the core and photosphere. 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.
Another consequence of the extremely high nuclear-fusion-reaction rates present in O-type main-sequence stars is that they experience a dramatically shortened life-span compared to other stars by consuming their hydrogen fuel at an increased rate. As such, they are the first to leave the main sequence and undergo stellar evolution. Upon fusing all available material in their cores, high-mass stars such as O-type main-sequence stars can result in supernovae, become neutron stars and some may collapse into black holes.
- 10 Lacertae
- AE Aurigae
- AFGL 2591
- BI 253
- Delta Circini
- HD 35619
- HD 93205
- LY Aurigae
- Mu Columbae
- Sigma Orionis
- Theta1 Orionis C
- Upsilon Sagittarii
- V560 Carinae
- V838 Monocerotis
- VFTS 102
- Zeta Ophiuchi
In the final stages of a low-mass star, the star sheds its outer layers in the form of a planetary nebula. Eventually, all that is left is a white dwarf. These stars are very hot when they first form, placing them in the O-star category. However, as time goes on, white dwarfs cool off by emitting photons, causing them to shift to the right in the Hertzsprung–Russell diagram.
Blue supergiants and blue giants are hot stars that have luminosity class I and III, respectively. The giants or supergiants that have hot enough surface temperatures (greater than 30,000K) are classified as spectral type O. Lower-mass O-type stars (2–8 solar masses) expand into giants due to hydrogen-shell burning around an inert helium core after the exhaustion of the supply of hydrogen in the core. Higher-mass O-type stars (10–85 solar masses) go through a similar process, but instead become supergiants. In some cases, after becoming supergiants, they can evolve into Wolf–Rayet stars or luminous blue variables. Lower-mass blue supergiants expand further and become red supergiants. Blue supergiants are usually deemed unstable, due to their extremely high luminosities and high mass-loss rates.
After the core of hydrogen fuel in a massive O star is depleted, it develops a hydrogen-burning shell, becoming a supergiant. At the same time, the core of the star contracts, which causes the temperature to rise until it is hot enough to begin fusing helium into carbon. This process of core contraction is repeated again until it is hot enough for carbon to be fused into heavier elements. Basically, each time the core depletes its supply of a certain element, it shrinks and heats until it is hot enough for fusion of other elements to begin. This gives an onion shell structure to the star at this stage in its life. During the final the days of the star, iron begins to accumulate inside the core. However, because nuclear energy cannot be generated by iron, the only thing keeping the core of the star from collapsing under gravity is electron degeneracy pressure. After nuclear fusion comes to a stop, the core cools, leading to a reduction in the thermal pressure in the interior of the star. As this happens, the outward pressures become insufficient to counter the inward force of gravity. Now the core shrinks to a size much smaller than a white dwarf in a fraction of a second. This forces the electrons and protons in the remaining core to form neutrons, which have their own degeneracy pressure. A huge amount of energy is released due to this quick gravitational collapse in the form of a supernova. If the interior is only about 3 solar masses or less, the neutron degeneracy pressure is sufficient to counter inward pull of gravity; this results in the formation of a neutron star. However, if the interior is greater than 3 solar masses, gravity wins the battle and the core continues to collapse until it becomes a black hole.
- 29 Canis Majoris
- Alpha Camelopardalis
- Cygnus X-1
- Eta Carinae
- Gamma Velorum
- HD 93129B
- IRS 8*
- Tau Canis Majoris
- Zeta Puppis
O-type main-sequence stars tend to appear in the spiral arms of spiral galaxies. This is because of the fact that, as a spiral arm moves through space, it compresses any molecular clouds in its way. The initial compression of these molecular clouds leads to the formation of stars, some of which are O- and B-type stars. Also, as these stars have shorter lifetimes, they cannot move great distances before their death and so they stay in or relatively near to the spiral arm in which they formed. On the other hand, less massive stars live longer and thus are found throughout the galactic disc, including in between the spiral arms.
Stellar associations are groups of stars that are gravitationally unbound from the beginning of their formation. The stars in stellar associations are moving away 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.
As a star dies, it may explode in a supernova. As the core collapses, it releases vast amounts of particles and energy that blow the star apart as they blast through space. The massive explosion produces shock waves that compress the gas surrounding the dying star. This compression leads to a new round of star birth. The stars that form from this round of compression are not as massive as the original O- and B-type stars that formed from the first round of compression.
- Carroll, Bradley; Ostlie, Dale (2007). An Introduction to Modern Astrophysics (2 ed.).
- Hester, Jeff; Blumenthal, George; Smith, Bradford; Burstein, David; Greeley, Ronald; Voss, Howard (2007). 21st Century Astronomy: Stars and Galaxies (2 ed.).
- Freedman, Roger A.; Kaufmann III, William J. (2005). Universe: Stars and Galaxies (2nd ed.). New York: W. H. Freeman and Company.