NGC
1512 is a barred spiral galaxy in the southern constella- tion of Horologium;
the Hubble telescope reveals a stunning 2,400 light-year-wide circle of
infant star clusters. Circumstellar star-forming rings are common in the
universe. Such rings within barred spiral galaxies may in fact comprise
the most numerous class of nearby starburst regions. Astronomers generally
believe that the giant bar funnels the gas to the inner ring, where stars
are formed within numerous star clusters. In NGC 1512 newly born star
clusters exist in both dusty and clean environments. The clean clusters
are readily seen in ultraviolet and visible light, appearing as bright,
blue clumps in the image. However, the dusty clusters are revealed only by
the glow of the gas clouds in which they are hidden, as detected in red
and infrared wavelengths by the Hubble cameras.
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NGC
4650A is one of only 100 known polar-ring galaxies. Their unusual
disk-ring structure is not yet understood fully. One possibility is that
polar rings are the remnants of colossal collisions between two galaxies
sometime in the distant past, probably at least 1 billion years ago. What
is left of one galaxy has become the rotating inner disk of old red stars
in the center. Meanwhile, another smaller galaxy which ventured too close
was probably severely damaged or destroyed. During the collision the gas
from the smaller galaxy would have been stripped off and captured by the
larger galaxy, forming a new ring of dust, gas, and stars, which orbit
around the inner galaxy almost at right angles to the old disk. This is
the polar ring which we see almost edge-on in this Hubble telescope view.
In NGC 4650A, both the old, rotating disk and the dark matter surrounding
this galaxy pull on its polar ring.
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