A subdwarf O star (sdO) is a type of hot, but low-mass star. O-type subdwarfs are much dimmer than regular O-type main-sequence stars, but with a brightness about 10 to 100 times that of the Sun, and have a mass approximately half that of the Sun. Their temperature ranges from 40,000 to 100,000 K. Ionized helium is prominent in their spectra. Gravity acceleration is expressed by log g between 4.0 and 6.5. Many sdO stars are moving at high velocity through the Milky Way and are found at high galactic latitudes.
In the early 1970s Greenstein and Sargent measured temperatures and gravity strengths and were able to plot their correct position on the Hertzsprung-Russell diagram. The Palomar-Green survey, Hamburg surveys, Sloan Digital Sky Survey and Supernova Ia Progenitor Survey (ESO-SPY) have documented many of these stars.
There is actually a variety of spectra from the sdO stars. They can be grouped into those with strong helium lines, termed He-sdO, and those with stronger hydrogen lines, H strong sdO. The He-sdO are fairly rare. Usually nitrogen is enriched and carbon depleted. However, there are variations with enhancement in concentration of even numbered elements such as carbon, oxygen, neon, silicon, magnesium or iron.
They can be plotted on the Hertzsprung–Russell diagram. They are from two stages in the stellar lifecycle, post–asymptotic giant branch (the luminous sdO), and post–extended horizontal branch compact sdO. The post-AGB stars are expected to be found in planetary nebulas, but only four of the sdO stars are known to be like this. The compact sdOs would be descendants of the B-type subdwarfs. However, statistics do not match sdB. An alternate theory is that sdOs have been formed by coalescing two white dwarfs. This could happen from a close binary that decays due to gravitational waves.
56 Pegasi (56 Peg) is a spectroscopic binary star in the constellation Pegasus. Its apparent magnitude is 4.74 and it is approximately 590 light years away based on parallax.The companion, a subdwarf O star, has a period of about 111 days.Abell 39
Abell 39 is a low surface brightness planetary nebula in the constellation of Hercules. It is the 39th entry in George Abell's 1966 Abell Catalog of Planetary Nebulae (and 27th in his 1955 catalog) of 86 old planetary nebulae which either Abell or Albert George Wilson discovered before August 1955 as part of the National Geographic Society - Palomar Observatory Sky Survey. It is estimated to be about 6,800 light-years from earth and 4,600 light-years above the Galactic plane. It is almost perfectly spherical and also one of the largest known spheres with a radius of about 2.5 light-years.Beta Canis Minoris
Beta Canis Minoris (β Canis Minoris, abbreviated Beta CMi, β CMi), also named Gomeisa , is a star in the constellation of Canis Minor. In the night sky it is notable for its proximity to the prominent star Procyon.O-Type
O-Type may refer to:
O type blood
O-type main sequence star
Subdwarf O star
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