As long as we’re spending time with Comet Hartley this month and its host constellation Cassiopeia, let’s use Cassiopeia to point us to the Andromeda Galaxy, the farthest object most of us will ever see with the naked eye. Astronomers use the ‘light year’ as a standard when measuring distances to stars and galaxies because they’re all so far away miles and kilometers simply aren’t adequate. Light travels at 186,000 miles per second (300,000 km/sec), and since there’s about 31,536,000 seconds in a year, that adds up to some six trillion miles of travel in one year. That’s well beyond the most distant planets and asteroids but only about 1/4 the way to the nearest star system after the sun, Alpha Centauri. When we gaze at Alpha, the light we see left it 4.4 years ago and traveled across a chasm of space 25 trillion miles wide before finally reaching our eye.
The farthest star we can see with our naked eye is V762 Cas in Cassiopeia at 16,308 light-years away. Its brightness is magnitude 5.8 or just above the 6th magnitude limit. While that seems remote, we can delve much deeper, but we’ll need to leave the realm of stars and move outward to galaxies beyond the Milky Way to do so.
The Andromeda Galaxy is similar to our Milky Way in size and structure; both are spiral galaxies containing several hundreds of billions of stars. Every star you see in the sky on a clear night belongs to our galaxy. Picture Andromeda as another Milky Way 2.5 million light years from Earth. Amazingly, we can see this without optical aid from a moderately dark sky. Why? While that distance may seem enormous, it’s close as far as galaxies go, plus Andromeda is large and packs a bounty of stars.
If you lived on a planet revolving around a star in Andromeda, the sky would be filled with stars just like it is at home. They’d be strewn about in completely different patterns, and you’d be free to create your own constellations. Through a telescope, you’d be able to look at entirely different nebulas and star clusters scattered across the alien sky. Even better, if you were located in the thick disk of the galaxy as the sun and planets are in the Milky Way, you could look up and see a ribbon of hazy light cutting across the sky – the Andromeda Way. While its contours would vary, it would bear an uncanny resemblance to how the Milky Way looks from a dark sky on Earth.
Thank goodness for imagination. We need it, because with all that distance between us and Andromeda, the scenes described above are reduced to little more than a wisp of cloud below Cassiopeia. That’s what the galaxy looks like to the eye – a fuzzy patch with a brighter center where the majority of Andromeda’s stars are concentrated. It’s still one of the coolest things you’ll ever see; an entire galaxy right before your eyes. All those stars, planets and nebulas blend together to form an unresolved haze even in larger telescopes. At 2.5 million light years, its stars are not only too faint, but we can’t resolve them from one another, because they appear close together from such a great distance.
The Milky Way and Andromeda are the two largest galaxies in a ragtag assemblage of more than 30 called the Local Group. We’re part of a small cluster held together by our mutual gravities. On an even broader scale, the Local Group is a small piece of a vastly larger supercluster of galaxies centered in the constellation Virgo 60 million light years away.
We’ve come along way for one day. When you succeed in finding the Andromeda Galaxy, consider how deep you’re looking into space. It took the light you see 2.5 million years to travel the distance to your eye. Those are some old photons! With a little imagination, you can play time traveler and picture how Earth and life may have looked so long ago.