What's Up in Space
This weekend’s night sky (25 - 26 October 2014)
October is also a good time to look out for the zodiacal light, seen as a triangular glow in the west after sunset in a clear, dark sky. It is caused by light reflecting off dust along the plane of our solar system. This plane is marked by the ecliptic, the apparent path of the Sun across the sky, which runs through the constellations of the zodiac.
At this time of year the ecliptic makes a steep angle with the horizon, making the zodiacal light easier to observe. The new moon on Friday will leave us with dark skies this weekend, making it a particularly good time to spot this zodiacal glow. These dark skies also provide a great opportunity to observe some fainter objects in our skies.
The Great Galaxy in Andromeda, or M31, is the nearest large spiral galaxy to our own, and makes a rare appearance in our southern hemisphere skies at this time of year, but you’ll need a good, dark sky and a clear view of the northern horizon to spot it. The easiest way to find it is to star hop from the constellation of Pegasus in the north. The star at the bottom right of the Great Square of Pegasus is in fact Alpha Andromodae, or Alpheratz, the brightest star in the constellation of Andromeda. From Alpheratz look for two chains of stars extending out to the east.
Hop along the uppermost, and brightest, of these chains past Delta Andromedae to Mirach (Beta Andromedae), then turn sharp right and head down to Mu Andromedae before jumping on the same distance again to find the galaxy. The Andromeda galaxy covers an area around 6 times the diameter of the full moon, but only the brighter central region is easily visible to the naked eye, or with binoculars or a small telescope. At 2.5 million light years away, it is the most distant object easily visible with the naked eye and is thought to contain around one trillion stars, well over twice the number estimated in our own Milky Way. Some recent studies, however, have suggested that the Milky Way may contain more dark matter than Andromeda, giving the two galaxies a very similar mass. M31 is approaching the Milky Way at 110 km/s and is expected to collide and merge with our Galaxy in around 4 billion years.