Where did all the planets go?
We've been treated to a plethora of planets this summer: Jupiter and Venus were around during the early summer until our sister planet's transit in early June. Then Saturn and Mars were the stars of the show during their conjunction in the late portion of the summer. However, they too have now drifted down into the glow of the sunset as the ecliptic (the path the Sun traces out through the sky and the path that the planets closely follow) dips low at a shallow angle in the southern sky...and we are left with nothing for the time being.
So when will the planets return? Unfortunately, the remainder of September offers us nothing but an opportunity to exercise patience as October approaches. For those who don't mind staying up past midnight, Jupiter rises in the east with Taurus and Orion, and Venus rises shortly before the Sun, but for those of you who cherish your hours of sleep, there are no opportunities for several weeks yet.
What, then, is a person supposed to view at night if the planets are nowhere to be seen and the Moon is in its New Phase (which occurs in a couple days)? This might be an excellent time to re-familiarize yourself with the constellations of the early autumn sky. Ursa Major dips low in the north right now, so you may need to use Cassiopeia to locate the North Star.
High above your head lies the three bright stars that form the asterism called the Summer Triangle. Named Vega, Deneb, and Altair, these stars form an isosceles triangle that points generally in a southern direction for most of the night, with Altair at the southern tip. However, it is a constellation adjacent (in the east) to the Summer Triangle that leads us to our object of interest. Rising in the east before sunset is the "Great Square" of Pegasus, the flying horse. Flying upside-down, this horse rises head and front legs first, with its square body coming next. Along the northeastern horizon, two trails of stars that look like its rear legs rise with it.
However, these rear legs actually comprise the constellation of Andromeda. These two trails of stars represent the princess rescued by Perseus from the jaws of the great sea monster Cetus. Within this constellation lies the Andromeda Galaxy, M31.
This galaxy is one of our Milky Way Galaxy's closest neighbors, lying a scant 2.5 million light-years away. Resembling the Milky Way in both its spiral shape and size, the Andromeda Galaxy is currently hurtling toward the Milky Way at breakneck speed, destined for collision some 5 billion years from now. This NASA website about the upcoming collision contains some nice images and video simulating the night sky as the intersection approaches. While it is described as a collision, the large distances between stars in both galaxies mean that they will more-or-less pass through each other at first, with few to none actual stellar collisions. Gravity's ceaseless pull will then cause them to double back on each other and ultimately merge together to form a large elliptical galaxy. Gas clouds will light up with star formation. If anyone is alive to witness the event, it should be spectacular - though there is a nonzero chance of that witness' home planet being flung out of the galaxy altogether or traded from one to the other during the first couple passes through. How cool would it be to live in the Milky Way Galaxy at first and then get traded to the Andromeda Galaxy as they interact with each other?!
If you have binoculars or a telescope, turn them to the Andromeda Galaxy and ponder this future. Practice star-hopping from Beta Andromedae to Mu Andromedae to Nu Andromedae and ultimately to M31. Getting to know where these objects are is half the fun!
Happy viewing!
