M31, is a spiral galaxy about 150,000 light years across and, with the exception of satellite galaxies of the Milky Way, our closest galactic neighbor at an estimated 2.5 million light years distance.  It is located in, and named for the constellation, Andromeda.  Under dark sky, the brighter core of this galaxy is visible to the naked eye and appears about the size of a quarter held out at arms length.  Telescopically, the darker dust lanes and brighter areas of the spiral arms are visible from dark sky sites.

Recent discoveries indicate that the galaxy spans 5 degrees of sky or 10 full moon widths.  This view shows only about 2.5 degrees of that extent.  Its size, brightness and distance makes this object the most distant object visible to the naked eye and the galaxy's core is visible from even light polluted urban sky with a pair of binoculars.  Together, Andromeda, M33 and our Milky Way along with a few more lesser galaxies make up the "Local Group". 

Two additional Messier objects are captured in this image.  Up and left of Andromeda's core is M110 and down and right from the core, appearing like a bright, fuzzy star is M32.  Both are dwarf elliptical satellite galaxies of Andromeda similar to our own galaxy's Large and Small Magellanic Clouds.  Light blue and reddish areas in the spiral arms of the galaxy are areas of active star formation.  The blue areas get their color from massive, young and very hot blue giant stars while the red regions are H II regions, areas of primarily hydrogen gas nebulae from which stars are formed.

Being about 2.5 million light years distant from us means it also takes about 2.5 million years for the light from the galaxy to reach us.  Being nearly edge on to our point of view, the light from the distant arms of the galaxy takes an additional 150,000 years to reach us than that from the near side arms. 

Andromeda and the Milky Way are gravitationally bound and are approaching each other at about 3 km/sec.  Eventually, the two galaxies will collide and may combine to form one massive elliptical galaxy.  As the galaxies close the distance, Andromeda will fill more and more of the night sky and will become brighter most especially in the core area.  Eventually the core will be very prominent in the night sky and the spiral arms will take on more or less the brightness and appearance of our own galaxy as seen under dark skies.

An observer in Andromeda looking back to our galaxy would see a very similar view.  The Milky Way from Andromeda is also nearly edge on and the two galaxies are close to the same size; Andromeda is perhaps 20% larger.  The most striking difference would be that the Milky Way is a barred spiral galaxy and would display a bright bar of stars across the core.

Back to Messier Gallery