Hydrogen-deficient star

A hydrogen-deficient star is a type of star that has little or no hydrogen in its atmosphere. Hydrogen deficiency is unusual in a star, as hydrogen is typically the most common element in a stellar atmosphere. Despite being rare, there are a variety of star types that display a hydrogen deficiency.

Hydrogen-deficient stars had been noted prior to the discovery of their hydrogen deficiency. In 1797, Edward Pigott noted the profound variation in stellar magnitude of R Coronae Borealis (R CrB). In 1867, Charles Wolf and Georges Rayet discovered unusual emission line structure in Wolf-Rayet stars.

Hydrogen deficiency in a star was first discovered in 1891 by Williamina Fleming, where she stated “the spectrum of υ Sgr is remarkable since the hydrogen lines are very faint and of the same intensity as the additional dark lines”. In 1906, Hans Ludendorff found that Hγ Balmer spectral lines were absent in R CrB.

It was widely believed at the time that all stellar atmospheres contain hydrogen, so these observations were discounted. Not until quantitative spectral measurements became available in 1935-1940 did astronomers begin to accept that stars such as R CrB and υ Sgr were hydrogen deficient. As of 1970, relatively few of these stars were known. Large-scale stellar surveys since then have greatly increased the number and variety of known hydrogen-deficient stars. As of 2008, about 2,000 hydrogen-deficient stars were known.

Despite being relatively rare, there are many different types of hydrogen-deficient stars. They can be grouped into five general classes: massive or upper-main-sequence stars, low-mass supergiants, hot subdwarf stars, central stars of planetary nebulae, and white dwarfs. There have been other classification schemes, such as one based on carbon content.

Wolf-Rayet stars show bright bands in continuous spectra that come from ionized atoms such as helium. Although there was some controversy, these were accepted as hydrogen-deficient stars in the 1980s.Helium-rich B stars, such as σ Orionis E, are chemically unusual spectral B or OB main sequence stars that show strong neutral helium lines. Hydrogen-deficient binaries, such as υ Sgr, have helium lines on a metallic spectrum and show large radial velocities that are thought to result from Population I stars orbiting the galactic center. Type Ib and Ic supernovae show no hydrogen absorption lines and are associated with stars that have lost their hydrogen envelope through supernova core collapse.

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