The Great Andromeda Galaxy (Messier 31)
This image was chosen by Astronomy Magazine as their Picture of the Day on October 31st, 2012 and won the Yahoo Digital Astro Galaxy Challenge for October 2012.
The Great Andromeda Galaxy is the most prominent member of our local group, which includes the large and small Magellanic Clouds, M33 (Pinwheel Galaxy), and the neighboring elliptical galaxies M110 and M32, seen above at 5 o'clock and 10 o'clock (just off center), respectively. This image represents a total 88 minutes of exposure. The Ha signal is particularly interesting, since it highlights the bright HII regions most prominent in the outer galactic arms. HII emission in galaxies is most often due to excitation of surrounding hydrogen gas by UV irradiation emitted by newly-formed, young stars. That's why we see so much HII emission in our own galaxy in regions such as the Orion Nebula and in Cygnus, for instance. In this image, notice the absence of prominent HII emission in the two elliptical galaxies, M110 and M32, which contain old stars and lack regions of new star formation (it is possible that HII regions exist in these galaxies as well but are simply below the level of resolution in this image).
M31, like most large spirals, is thought to contain a large, central black hole that consumes gas and nearby stars at a prodigious rate. The radiation emitted in the vicinity of black holes (due to extreme heating of matter as it flows into the region) compresses surrounding gas and most likely triggers a new wave of star formation in a disk surrounding the galactic center. In this regard, there is a rotating disk of about 400 blue stars that formed 200 million years ago and is rotating around the galactic center at an orbital velocity of 2.2 million miles per hour. Given the proximity of these star to the intense radiation emitted from the central black hole, it would be impossible for life as we know it to evolve around such stars. It is predictable that we exist in an outer arm of a spiral galaxy, where conditions are more favorable for life to evolve (i.e., the galactic habitable zone). Perhaps intelligent lifeforms are pondering the same things about us, as they gaze at the Milky Way from their vantage point in one of the outer arms of M31.
At a dark site, M31 is a naked eye object, although for city and suburban dwellers it's almost impossible to view without a telescope. The central portion glows slightly yellow as a result of older stars, whereas the periphery has a characteristic bluish hue due to a predominance of younger stars. Please check out the higher resolution links for more detail, especially in the HII regions.
(Description courtesy Steve Cannistra, 2007)
Object: Messier 31- The Andromeda Galaxy
Optics: 8" Boren-Simon Powernewt Astrograph
Mount: Orion Atlas EQ-G with EQMOD
Guiding: 50 mm Finder, Orion SSAG and PhD
Camera: Astrodon Filter Modified Canon Xsi (450D)
Exposure: Total 93.66 minutes from 22 x 4 minutes, 8 x 30 seconds and 10 x 10 seconds.
White Balance: Custom
Filter: Astronomik CLS EOS Clip-in
Conditions: Temp. 37 F
Date: October 11th, 2012
Location: Spruce Knob, WV
Calibration: ImagesPlus 4.50
Calibration Frames: Darks, flats, bias and flat darks
Processing: IP 4.50, Photoshop CS with HLVG and GradientXterminator Plug-ins