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extreme horizontal branch star (EHB star)
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| In this image of the globular cluster NGC 362 (located
30,000 light-years away), obtained by the Galaxy Evolution Explorer,
the light blue dots surrounding the core are extreme horizontal branch
stars. Blue dots scattered throughout the image are hot, young stars
in the Small Magellanic Cloud, a satellite galaxy of the Milky Way
located approximately 200,000 light-years away. The stars in this
galaxy are much brighter intrinsically than EHB stars, but they appear
just as bright because they are farther away. The blue stars in the
Small Magellanic Cloud are only about a few tens of millions of years
old, much younger than the approximately 10-million-year-old stars
in NGC 362. Because NGC 362 sits on the northern edge of the Small
Magellanic Cloud galaxy, the blue stars are denser toward the south,
or bottom, of the image. Image credit: NASA/JPL-Caltech/Univ. of Virginia
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A star like our sun spends most of its life fusing hydrogen atoms in its core into helium. When the star runs out of hydrogen in its core, its outer envelope will expand. The star then becomes a red giant, which burns hydrogen in a shell surrounding its inner core. Throughout its life as a red giant, the star loses a lot of mass, then begins to burn helium at its core. Some stars will have lost so much mass at the end of this process, up to 85 percent of their envelopes, that most of the envelope is gone. What is left is a very hot ultraviolet-bright core, or extreme horizontal branch star.
Subdwarf B (sdB) stars are low-mass evolved extreme horizontal branch stars in which core helium-burning has just started.
Related categories
• TYPES OF STAR• STELLAR ASTROPHYSICS
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