Wolf-Rayet star - AbsoluteAstronomy.com

Wolf–Rayet stars are evolved, massive star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s (over 20 solar mass

Solar mass

The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...

es initially), which are losing mass rapidly by means of a very strong stellar wind

Stellar wind

A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...

, with speeds up to 2000 km/s. While our own Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

loses approximately 10⁻¹⁴ solar masses every year, Wolf–Rayet stars typically lose 10⁻⁵ solar masses a year.

Wolf–Rayet stars are very hot, with surface temperatures in the range of 25,000 K to 50,000 K.

Observation history

In 1867, astronomers using the 40 cm Foucault telescope at the Paris Observatory

Paris Observatory

The Paris Observatory is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world...

discovered three stars in the constellation Cygnus

Cygnus (constellation)

Cygnus is a northern constellation lying on the plane of the Milky Way. Its name is the Latinized Hellenic word for swan. One of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross...

(now designated HD191765, HD192103 and HD192641) that displayed broad emission bands on an otherwise continuous spectrum. The astronomers' names were Charles Wolf

Charles Wolf (astronomer)

Charles Joseph Étienne Wolf was a French astronomer.In 1862, Urbain Le Verrier offered him a post as assistant at the Paris Observatory.In 1867 he and Georges Rayet discovered Wolf-Rayet stars...

and Georges Rayet

Georges Rayet

Georges-Antoine-Pons Rayet was a French astronomer.He was born in Bordeaux, France. He began working at the Paris Observatory in 1863. He worked on meteorology in addition to astronomy...

, and thus this category of stars became named Wolf–Rayet (WR) stars. Most stars display absorption band

Absorption band

An absorption band is a range of wavelengths, frequencies or energies in the electromagnetic spectrum which are able to excite a particular transition in a substance...

s in the spectrum, as a result of overlying elements absorbing light energy at specific frequencies. The number of stars with emission lines is quite low, so these were clearly unusual objects.

The nature of the emission bands in the spectra of a Wolf–Rayet star remained a mystery for several decades. Edward C. Pickering

Edward Charles Pickering

Edward Charles Pickering was an American astronomer and physicist, brother of William Henry Pickering.Along with Carl Vogel, Pickering discovered the first spectroscopic binary stars. He wrote Elements of Physical Manipulations .Pickering attended Boston Latin School, and received his B.S. from...

theorized that the lines were caused by an unusual state of hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

, and it was found that this "Pickering series" of lines followed a pattern similar to the Balmer series

Balmer series

The Balmer series or Balmer lines in atomic physics, is the designation of one of a set of six different named series describing the spectral line emissions of the hydrogen atom....

, when half-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of helium

Helium

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

; a gas that was discovered in 1868.

By 1929, the width of the emission bands was being attributed to Doppler broadening

Doppler broadening

In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...

, and hence that the gas surrounding these stars must be moving with velocities of 300–2400 km/s
along the line of sight. The conclusion was that a Wolf–Rayet star is continually ejecting gas into space, producing an expanding envelope of nebulous gas. The force ejecting the gas at the high velocities observed is radiation pressure

Radiation pressure

Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...

.

In addition to helium, emission lines of carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

, oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

and nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

were identified in the spectra of Wolf–Rayet stars. In 1938, the International Astronomical Union

International Astronomical Union

The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...

classified the spectra of Wolf–Rayet stars into types WN and WC, depending on whether the spectrum was dominated by lines of nitrogen or carbon-oxygen respectively.

Description

Wolf–Rayet stars are a normal stage in the evolution of very massive stars, in which strong, broad emission lines

Spectral line

A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...

of helium and nitrogen ("WN" sequence) or helium, carbon, and oxygen ("WC" sequence) are visible. Due to their strong emission lines they can be identified in nearby galaxies.
About 300 Wolf–Rayets are catalogued in our own Milky Way Galaxy. This number has changed during the last years as the result of very deep photometric and spectroscopic surveys dedicated to discovering this kind of object in the Galactic plane

Galactic plane

The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...

. Additionally, about 100 are known in the Large Magellanic Cloud

Large Magellanic Cloud

The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...

, while only 12 have been identified in the Small Magellanic Cloud

Small Magellanic Cloud

The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....

, and a few more in galaxies in the Local Group

Local Group

The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...

, and nearby galaxies (M83, NGC300, etc.).

Several astronomers, among them Rublev (1965) and Conti (1976) originally proposed that the WR stars as a class are descended from massive O-stars in which the strong stellar wind

Stellar wind

s characteristic of extremely luminous stars have ejected the unprocessed outer H

Hydrogen

-rich layers.

The characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar photosphere

Photosphere

The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...

, which produces a flood of UV

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

radiation that causes fluorescence in the line-forming wind region.
This ejection process uncovers in succession, first the nitrogen-rich products of CNO cycle

CNO cycle

The CNO cycle is one of two sets of fusion reactions by which stars convert hydrogen to helium, the other being the proton–proton chain. Unlike the proton–proton chain reaction, the CNO cycle is a catalytic cycle. Theoretical models show that the CNO cycle is the dominant source of energy in stars...

burning of hydrogen (WN stars), and later the carbon-rich layer due to He burning

Triple-alpha process

The triple alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei are transformed into carbon.Older stars start to accumulate helium produced by the proton–proton chain reaction and the carbon–nitrogen–oxygen cycle in their cores...

(WC and WO stars).
Most of these stars are believed finally to progress to become supernovae of Type Ib or Type Ic

Type Ib and Ic supernovae

Types Ib and Ic supernovae are categories of stellar explosions that are caused by the core collapse of massive stars. These stars have shed their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon...

.
A few (roughly 10%) of the central stars of planetary nebula

Planetary nebula

A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...

e are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; i.e., they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they are much older objects descended from evolved low-mass stars and are closely related to white dwarf

White dwarf

A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...

s, rather than to the very young, very massive stars that comprise the bulk of the WR class.

Evolution models

Evolution models of WR stars.

For stars of ~75 M

Mass

Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...

_{SSun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...}

O → WN(H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

-rich) → LBV
Luminous blue variable
Luminous blue variables, also known as S Doradus variables, are very bright, blue, hypergiant variable stars named after S Doradus, the brightest star of the Large Magellanic Cloud. They exhibit long, slow changes in brightness, punctuated by occasional outbursts in brightness during substantial...

→ WN(H-poor) → WC → SN
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...

Ic

For stars of ~40-75 M_S

O → LBV → WN(H-poor) → WC → SN Ic

For stars of 25-40 M_S

O → LBV → WN(H-poor) → SN Ib

O → RSG
Red supergiant
Red supergiants are supergiant stars 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...

→ WN(H-poor) → SN Ib

It is possible for a Wolf–Rayet star to progress to a "collapsar" stage in its death throes: This is when the core of the star collapses to form a black hole

Black hole

A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

, pulling in the surrounding material. This is thought to be the precursor of a long gamma-ray burst.

The best known (and most visible) example of a Wolf–Rayet star is Gamma 2 Velorum

Gamma Velorum

Gamma Velorum is a star system in the constellation Vela. At magnitude +1.7, it is one of the brightest stars in the night sky. It has the traditional names Suhail and Suhail al Muhlif, which confusingly also apply to Lambda Velorum...

(γ² Vel), which is a bright star visible to those located south of 40 degrees northern latitude

Latitude

In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

. One of the members of the star system (Gamma Velorum is actually at least six stars) is a Wolf–Rayet star. Due to the exotic nature of its spectrum (bright emission lines in lieu of dark absorption lines) it is dubbed the "Spectral Gem of the Southern Skies".

External links

physics.usyd.edu.au Some Wolf–Rayet stars in binaries are close enough that we can image a rotating "pinwheel nebula
Pinwheel nebula
-Spiral galaxies:The term 'Pinwheel nebula' is an antiquated misnomer used by observers before Edwin Hubble realized that these spiral shaped nebulae were actually 'island universes'.-Wolf-Rayet nebulae:...

" showing the dust generated by colliding winds in the binary system, from Aperture Masking Interferometry
Aperture masking interferometry
Aperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...

observations.
harvard.edu Wolf–Rayet Stars: Spectral Classifications
astro.lsa.umich.edu ApJ 525:L97-L100 Nov. 10, 1999. Monnier, Tuthill & Danchi: Pinwheel Nebula Around WR98a (PDF)
uk.arxiv.org ApJ Jan. 3,2005. Dougherty, et al.: High Resolution Radio Observations of the Colliding Wind Binary WR140 (PDF)
harvard.edu A catalog of northern Wolf–Rayet Stars and the Central Stars of Planetary Nebulae (Harvard)
nytimes.com Scientists See Supernova in Action
nasa.gov Big Old Stars Don't Die Alone (NASA)

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Observation history

Description

Evolution models

See also

External links