Everything about Wolf-rayet Stars totally explained
Wolf-Rayet stars (often referred to as WR stars) are evolved, massive
stars (over 20
solar masses), and are losing their mass rapidly by means of a very strong
stellar wind, with speeds up to 2000 km/s. While our own
sun loses 10
−16 of its own mass every year, a Wolf-Rayet star loses 10
−5 solar masses a year. These stars are also very hot: their surface temperatures are in the range of 25,000 K to 50,000 K. It is believed that the star in the
galaxy NGC 2770 that exploded into a
supernova on
January 9, 2008 - the first supernova ever observed in the act of exploding - was a Wolf-Rayet star.
Observation history
In 1867, astronomers using the 40 cm Foucault telescope at the
Paris Observatory, discovered three stars in the constellation
Cygnus that displayed broad emission bands on an otherwise continuous spectrum. The astronomer's names were
Charles Wolf and
Georges Rayet, and thus this category of stars became named Wolf-Rayet (WR) stars. Most stars display
absorption bands in the spectrum, as a result of overlaying 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 theorized that the lines were caused by an unusual state of
hydrogen, and it was found that this "Pickering series" of lines followed a pattern similar to the
Balmer series, when half-integral quantum numbers were substituted. It was later shown that the lines resulted from the presence of
helium; a gas that was discovered in 1868.
By 1929, the width of the emission bands was being attributed to the
Doppler effect, 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.
In addition to helium, emission lines of
carbon,
oxygen and
nitrogen were identified in the spectra of Wolf-Rayet stars. In 1938, the
International Astronomical Union 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 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 230 Wolf-Rayets are known in our own
Milky Way Galaxy, about 100 are known in the
Large Magellanic Cloud, while only 12 have been identified in the
Small Magellanic Cloud.
Conti (1976) originally proposed that the WR stars as a class are descended from massive
O-stars in which the strong
stellar winds characteristic of extremely luminous stars have ejected the unprocessed outer
H-rich layers.
The characteristic emission lines are formed in the extended and dense high-velocity wind region enveloping the very hot stellar
photosphere, which produces a flood of
UV radiation that causes fluorescence in the line-forming wind region.
This ejection process uncovers in succession, first the nitrogen-rich products of
CNO cycle burning of hydrogen (WN stars), and later the carbon-rich layer due to
He burning (WC & WO stars).
Most of these stars are believed finally to progress to become
supernovae of Type Ib or Type Ic.
A few (roughly 10%) of the central stars of
planetary nebulae are, despite their much lower (typically ~0.6 solar) masses, also observationally of the WR-type; for example, they show emission line spectra with broad lines from helium, carbon and oxygen. Denoted [WR], they're much older objects descended from evolved low-mass stars and are closely related to
white dwarfs, rather than to the very young, very massive stars that comprise the bulk of the WR class.
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, 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 (γ² Vel), which is a bright star visible to those located south of 40 degrees northern
latitude. 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's dubbed the "Spectral Gem of Southern Skies".
Further Information
Get more info on 'Wolf-rayet Stars'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://wolf-rayet_star.totallyexplained.com">Wolf-Rayet star Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
