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ngc6624.txt
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1996-01-12
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Photo Caption: STScI-PR93-20
This is a comparison of pictures of the core of the globular cluster
NGC 6624, as imaged with the European Space Agency's Faint Object
Camera aboard NASA's Hubble Space Telescope. This comparison image
demonstrates that Hubble's high resolution and ultraviolet sensitivity
allow astronomers to pick out the faint blue counterpart to an X-ray
burster buried in the globular cluster. (An X-ray burster is a class
of unusual double star that is a source of violent bursts of X-rays.).
HST clearly distinguishes the star from others crammed together in the
dense core of the cluster.
The image on the left shows that one star far outshines all others in
the cluster's core when viewed at UV wavelengths. Nearly all the
fainter stars in this UV image coincide with bright stars in the image
on the right, which was taken in visible (blue) light. They are all
cool red stars. When the two images are lined up, the UV-bright star
coincides with the inconspicuous star that is identified by tic-marks
in the right-hand image. It is the bluest (and therefore hottest) star
in the cluster and is at the position of an X-ray source known as 4U
1820-30 (first identified as a bright X-ray source in the 1970s during
an all sky survey carried out by the Uhuru satellite).
The X-ray source is known to be a binary star consisting of a neutron
star and white dwarf that complete an orbit about each other every
eleven minutes. The UV radiation comes from a disk of gas surrounding
the neutron star.
The double star lies only 1/10 of a light-year from the exact center of
the globular star cluster, which is identified by a "+". This closeness
follows from tying the HST observations to ground-based observations
that place the true center of the cluster in a different location from
that determined by earlier ground-based estimates.
The cluster is located approximately 28,000 light-years away in the
constellation Sagittarius.
Credit: Ivan King,University of California at Berkeley; and NASA/ESA