Helium star

Last updated

A helium star is a class O or B star (blue), which has extraordinarily strong helium lines and weaker than normal hydrogen lines, indicating strong stellar winds and a mass loss of the outer envelope. Extreme helium stars (EHe) entirely lack hydrogen in their spectra. Pure helium stars lie on or near a helium main sequence, analogous to the main sequence formed by the more common hydrogen stars. [1]

Previously, a helium star was a synonym for a B-type star, but this usage is considered obsolete. [2]

A helium star is also a term for a hypothetical star that could occur if two helium white dwarfs with a combined mass of at least 0.5 solar masses merge and subsequently start nuclear fusion of helium, with a lifetime of a few hundred million years. This may only happen if these two binary masses share the same type of envelope phase. It is believed this is the origin of the extreme helium stars.

The helium star's great capability of transforming into other stellar objects has been observed over the years. The blue progenitor system of the type-Iax supernova 2012Z in the spiral galaxy NGC 1309 is similar to the progenitor of the Galactic helium nova V445 Puppis, suggesting that SN 2012Z was the explosion of a white dwarf accreting from a helium-star companion. It is observed to have caused a growing helium star that has the potential to transform into a red giant after losing its hydrogen envelope in the future. [3]

The helium main sequence is a line in the HR diagram where unevolved helium stars lie. It lies mostly parallel and to the left (ie. higher temperatures) of the better-known hydrogen main sequence, although at high masses and luminosities it bends to the right and even crosses the hydrogen main sequence. Therefore pure helium stars have a maximum temperature, between about 100,000  K and 150,000 K depending on metallicity, because high luminosity causes dramatic inflation of the stellar envelope. [4]

See also

Related Research Articles

<span class="mw-page-title-main">Supernova</span> Explosion of a star at its end of life

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">Stellar evolution</span> Changes to stars over their lifespans

Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

<span class="mw-page-title-main">Supergiant</span> Type of star that is massive and luminous

Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spans from about 3,400 K to over 20,000 K.

<span class="mw-page-title-main">Red supergiant</span> Stars with a supergiant luminosity class with a spectral type of K or M

Red supergiants (RSGs) are stars with a supergiant luminosity class of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Antares A are the brightest and best known red supergiants (RSGs), indeed the only first magnitude red supergiant stars.

<span class="mw-page-title-main">Blue supergiant</span> Hot, luminous star with a spectral type of B9 or earlier

A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier.

<span class="mw-page-title-main">Giant star</span> Type of star, larger and brighter than the Sun

A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905.

<span class="mw-page-title-main">Red-giant branch</span> Portion of the giant branch before helium ignition

The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass stars. Red-giant-branch stars have an inert helium core surrounded by a shell of hydrogen fusing via the CNO cycle. They are K- and M-class stars much larger and more luminous than main-sequence stars of the same temperature.

<span class="mw-page-title-main">Subgiant</span> Type of star larger than main-sequence but smaller than a giant

A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star.

<span class="mw-page-title-main">Luminous blue variable</span> Type of star that is luminous, blue, and variable in brightness

Luminous blue variables (LBVs) are massive evolved stars that show unpredictable and sometimes dramatic variations in their spectra and brightness. They are also known as S Doradus variables after S Doradus, one of the brightest stars of the Large Magellanic Cloud. They are extraordinarily rare, with just 20 objects listed in the General Catalogue of Variable Stars as SDor, and a number of these are no longer considered LBVs.

<span class="mw-page-title-main">Type Ia supernova</span> Type of supernova in binary systems

A Type Ia supernova is a type of supernova that occurs in binary systems in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.

<span class="mw-page-title-main">Yellow hypergiant</span> Class of massive star with a spectral type of A to K

A yellow hypergiant (YHG) is a massive star with an extended atmosphere, a spectral class from A to K, and, starting with an initial mass of about 20–60 solar masses, has lost as much as half that mass. They are amongst the most visually luminous stars, with absolute magnitude (MV) around −9, but also one of the rarest, with just 20 known in the Milky Way and six of those in just a single cluster. They are sometimes referred to as cool hypergiants in comparison with O- and B-type stars, and sometimes as warm hypergiants in comparison with red supergiants.

<span class="mw-page-title-main">IK Pegasi</span> Star in the constellation Pegasus

IK Pegasi is a binary star system in the constellation Pegasus. It is just luminous enough to be seen with the unaided eye, at a distance of about 154 light years from the Solar System.

An AM Canum Venaticorum star, is a rare type of cataclysmic variable star named after their type star, AM Canum Venaticorum. In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.

<span class="mw-page-title-main">Hypergiant</span> Rare star with tremendous luminosity and high rates of mass loss by stellar winds

A hypergiant (luminosity class 0 or Ia+) is a very rare type of star that has an extremely high luminosity, mass, size and mass loss because of its extreme stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants are classed as Ia-0 or Ia+, but red supergiants are rarely assigned these spectral classifications. Astronomers are interested in these stars because they relate to understanding stellar evolution, especially star formation, stability, and their expected demise as supernovae.

<span class="mw-page-title-main">Red giant</span> Type of large cool star that has exhausted its core hydrogen

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around 5,000 K or lower. The appearance of the red giant is from yellow-white to reddish-orange, including the spectral types K and M, sometimes G, but also class S stars and most carbon stars.

<span class="mw-page-title-main">R136a1</span> Wolf–Rayet star with one of the highest mass and luminosity of any known star

R136a1 is one of the most massive and luminous stars known, at 196 M☉ and nearly 4.7 million L☉, and is also one of the hottest, at around 46,000 K. It is a Wolf–Rayet star at the center of R136, the central concentration of stars of the large NGC 2070 open cluster in the Tarantula Nebula in the Large Magellanic Cloud. The cluster can be seen in the far southern celestial hemisphere with binoculars or a small telescope, at magnitude 7.25. R136a1 itself is 100 times fainter than the cluster and can only be resolved using speckle interferometry.

<span class="mw-page-title-main">KPD 1930+2752</span>

KPD 1930+2752 is a binary star system including a subdwarf B star and a probable white dwarf with relatively high mass. Due to the nature of this astronomical system, it seems like a likely candidate for a potential type Ia supernova, a type of supernova which occurs when a white dwarf star takes on enough matter to approach the Chandrasekhar limit, the point at which electron degeneracy pressure would not be enough to support its mass. However, carbon fusion would occur before this limit was reached, releasing enough energy to overcome the force of gravity holding the star together and resulting in a supernova.

<span class="mw-page-title-main">O-type star</span> Stellar classification

An O-type star is a hot, blue-white star of spectral type O in the Yerkes classification system employed by astronomers. They have temperatures in excess of 30,000 kelvin (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.

WR 142 is a Wolf-Rayet star in the constellation Cygnus, an extremely rare star on the WO oxygen sequence. It is a luminous and very hot star, highly evolved and close to exploding as a supernova. It is suspected to be a binary star with a companion orbiting about 1 AU away.

<span class="mw-page-title-main">Melnick 34</span> Binary star in the Large Magellanic cloud

Melnick 34, also called BAT99-116, is a binary Wolf–Rayet star near R136 in the 30 Doradus complex in the Large Magellanic Cloud. Both components are amongst the most massive and most luminous stars known, and the system is the most massive known binary system.

References

  1. ↑ Yoon, S.-C.; Langer, N. (2004). "Helium accreting CO white dwarfs with rotation: Helium novae instead of double detonation". Astronomy and Astrophysics. 419 (2): 645–652. arXiv: astro-ph/0402288 . Bibcode:2004A&A...419..645Y. doi:10.1051/0004-6361:20035823. S2CID   7367981.
  2. ↑ Frost, E. B.; Barrett, S. B.; Struve, O. (1926). "Radial velocities of 368 helium stars". The Astrophysical Journal. 64: 1. Bibcode:1926ApJ....64....1F. doi:10.1086/142986. PMC   1084541 .
  3. ↑ McCully, Curtis (2014). "A luminous, blue progenitor system for the type Iax supernova 2012Z". Nature. 512 (7512): 54–56. arXiv: 1408.1089 . Bibcode:2014Natur.512...54M. doi:10.1038/nature13615. PMID   25100479. S2CID   4464556.
  4. ↑ Köhler, K.; Langer, N.; De Koter, A.; De Mink, S. E.; Crowther, P. A.; Evans, C. J.; Gräfener, G.; Sana, H.; Sanyal, D.; Schneider, F. R. N.; Vink, J. S. (2015). "The evolution of rotating very massive stars with LMC composition". Astronomy & Astrophysics. 573: A71. arXiv: 1501.03794 . Bibcode:2015A&A...573A..71K. doi:10.1051/0004-6361/201424356. S2CID   28962151.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.