Search results
Results from the WOW.Com Content Network
The Pickering series (also known as the Pickering–Fowler series) consists of three lines of singly ionised helium found, usually in absorption, in the spectra of hot stars like Wolf–Rayet stars. The name comes from Edward Charles Pickering [1] and Alfred Fowler. [2]
The interval between outbursts is longer on average for stars with longer periods. The spectra show strong helium absorption lines during the outbursts, with many weaker emission lines of helium and iron near minimum. The spectral lines are typically doubled, producing broad flat-bottom absorption lines and sharp double-peaked emission lines.
Absorption lines for air, under indirect illumination, so that the gas is not directly between source and detector. Here, Fraunhofer lines in sunlight and Rayleigh scattering of this sunlight is the "source." This is the spectrum of a blue sky somewhat close to the horizon, looking east with the sun to the west at around 3–4 pm on a clear day.
The Fraunhofer lines are typical spectral absorption lines. Absorption lines are narrow regions of decreased intensity in a spectrum, which are the result of photons being absorbed as light passes from the source to the detector. In the Sun, Fraunhofer lines are a result of gas in the Sun's atmosphere and outer photosphere. These regions have ...
Helium has a diffuse series of doublet lines with wavelengths 5876, 4472 and 4026 Å. Helium when ionised is termed He II and has a spectrum very similar to hydrogen but shifted to shorter wavelengths. This has a diffuse series as well with wavelengths at 6678, 4922 and 4388 Å.
Arno Bergmann found a fourth series in infrared in 1907, and this became known as Bergmann Series or fundamental series. [14] In 1896 Edward C. Pickering found a new series of lines in the spectrum of ζ Puppis. This was believed to be the sharp series of hydrogen. In 1915 proof was given that it was actually ionised helium - helium II. [15] [16]
Spectrum of WR 137, a WC7 star [15] and one of the three original WR stars (horizontal axis : wavelength in Å) Wolf–Rayet stars were named on the basis of the strong broad emission lines in their spectra, identified with helium, nitrogen, carbon, silicon, and oxygen, but with hydrogen lines usually weak or absent.
Previously, a helium star was a synonym for a B-type star, but this use of for the term 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.