Explanation:
The small, northern constellation
Triangulum
harbors this magnificent face-on spiral galaxy, M33.
Its popular names include the Pinwheel Galaxy or just
the Triangulum
Galaxy.
M33 is over 50,000 light-years in diameter, third largest in the
Local
Group of galaxies after the Andromeda Galaxy (M31), and our
own Milky Way.
About 3 million light-years from the Milky Way,
M33 is itself thought to be a satellite of the
Andromeda Galaxy and
astronomers
in these two galaxies would likely have spectacular views of
each other's grand spiral star systems.
As for the view from planet Earth,
this
sharp composite image, a 25 panel mosaic,
nicely shows off M33's blue star clusters
and pinkish star forming regions that
trace the galaxy's loosely wound spiral arms.
In fact, the cavernous NGC 604 is the
brightest star forming region, seen here at about the 1 o'clock position
from the galaxy center.
Like M31, M33's population of well-measured variable stars
have helped make this nearby spiral a
cosmic
yardstick for
establishing
the distance scale of the Universe.
