September: Colliding Galaxies?

September 2012

Colliding Galaxies?

By Calvin L. Chrisman

The Andromeda Galaxy is heading our way
The Andromeda Galaxy is heading our way

Bad news, good news: First the bad news: after extensive calculations involving the Hubble Space Telescope, astronomers have determined that the Andromeda Galaxy is on a direct collision course with our Milky Way Galaxy. Andromeda, which is roughly the same size as the Milky Way, is about 2.5 million light years away and heading toward us at 250,000 miles per hour. Now the good news: the collision will not occur for four billion years give or take a couple of years. You don’t need to start your end-of-days planning quite yet. Interestingly, for two objects that are so large (each is about 500,000 light years across) there actually won’t be a “collision” per se. Although each galaxy contains between 500,000 and one billion stars, there is a lot of empty space between them. The galaxies will merge with individual stars moving into new and different orbits. The shape of the resulting galaxy will change from two spiral galaxies to one elliptical galaxy.

The Andromeda Galaxy is visible to the naked eye as a fuzzy patch in the Andromeda constellation and just below the crooked “W” of Cassiopeia. It is most visible in the fall and early winter. Although we know that the universe is expanding and most galaxies are moving away from one another, Andromeda and the Milky Way are part of what is known as the Local Group of galaxies.  They are close enough together that they are gravitationally bound and it is this gravitational attraction that is bring our two galaxies together at 250,000 miles per hour. For observers in the very distant future, Andromeda will no longer be a fuzzy patch. It will dominate the night sky.

In mythology, Andromeda’s mother, Cassiopeia had boasted that she was more beautiful than the daughters of the sea god Nereus. Nereus then sent the monster Cetus to attack their land. Andromeda was left as a sacrifice to Cetus but was saved by Perseus riding astride the winged horse Pegasus and carrying the head of the Medusa to turn Cetus to stone. Perseus, Pegasus and Cetus all have their own constellations and Perseus was the radiant point for August’s Perseid’s meteor shower.

Autumn officially begins on Saturday, September 22 at 10:49 a.m. EDT. The sun will set due west on that day as it crosses the Earth’s equator on its way south for the winter. The full Moon following the autumn equinox is known as the Harvest Moon. It will occur the following Saturday, the 29th. If you are an early riser, you will want to look to the east to see extremely bright Venus as it passes through the constellation Leo in the early morning hours. It will have an extremely close conjunction with Regulus, the bright red heart of Leo, on October 3rd. This will be worth getting up to see!

Clear Skies!

Leave a Comment