Hubble Space Telescope's exquisite resolution has allowed astronomers to
resolve, for the first time, hot blue stars deep inside an elliptical
galaxy. The swarm of nearly 8,000 blue stars resembles a blizzard of
snowflakes near the core (lower right) of the neighboring galaxy M32,
located 2.5 million light-years away in the constellation Andromeda.
Hubble confirms that the ultraviolet light comes from a population of
extremely hot helium-burning stars at a late stage in their lives.
Unlike the Sun, which burns hydrogen into helium, these old stars
exhausted their central hydrogen long ago, and now burn helium into
heavier elements. The observations, taken in October 1998, were made
with the camera mode of the Space Telescope Imaging Spectrograph (STIS)
in ultraviolet light. The STIS field of view is only a small portion of
the entire galaxy, which is 20 times wider on the sky. For reference,
the full moon is 70 times wider than the STIS field-of-view. Thirty
years ago, the first ultraviolet observations of elliptical galaxies
showed that they were surprisingly bright when viewed in ultraviolet
light. Before those pioneering UV observations, old groups of stars were
assumed to be relatively cool and thus extremely faint in the
ultraviolet. Over the years since the initial discovery of this
unexpected ultraviolet light, indirect evidence has accumulated that it
originates in a population of old, but hot, helium-burning stars. Now
Hubble provides the first direct visual evidence.
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