To celebrate the 21st anniversary of the Hubble Space Telescope,
a spectacular image of the pair of colliding galaxies Arp 273 was
released. Click on the image to get a super-enormous 4000-pixel
wide version of the image (or this link for the full-size TIFF at roughly 8000
pixels across, and 120 MB in size):
Although the pair is known as Arp 273, the individual galaxies
are UGC 1810 (top) and UGC 1813 (bottom).
Most galaxies reside in clusters. The distances between
them may be typically tens to hundreds of galactic diameters.
Hence near-misses, collisions, and mergers are quite common as
galaxies orbit collectively around themselves in the cluster.
(This isn't the case for the stars inside the galaxies. The
stellar distances are millions of times larger than stellar
diameters, so stellar mergers and near-misses are far rarer for all
of the stars within a pair of colliding galaxies like above.)
The gravitational interplay between the pairs of galaxies lead
to the tidal tails of stars that seem to bridge the two. The
larger galaxy's outer spiral arm is wrapping around itself, almost
completing a ring around the galaxy. The close pass has also
disrupted the clouds of dark molecular gas and dust. Instead
of neatly following the spiral arms, they are spread out.
Some of it has been pushed and compressed, with clumps of clouds
collapsing to form clusters of young stars.
A periodic series of stellar clusters can be seen running
horizontally across the top of UGC 1810. Different telescope
filters were used to create the color composite from Hubble's Wide
Field Camera 3. Here the blue color represents ultraviolet
(UV) radiation from young, massive stars, which live fast, and die
young. The winds and outflows from these and other young
stars disrupt the molecular gas surrounding them in their natal
environment. Massive stars blowing up as supernovae can
finish off the clouds, leaving the remaining young stars in the
cluster -- now unbounded by the gravity of the parent cloud -- to
spread out and percolate in their orbits through the rest of the
In addition to a fascinating snapshot into the collision and
mergers of these galaxies (which can take hundreds of millions to
billions of years), a picture like this also reveals all of the
activity that goes on in galaxies. There are hints of star
birth, star death, and the cycling of matter as it goes from
molecular clouds to stars and back into the interstellar