Thursday, November 4, 2010

How Far Can We See? (part one)

Astronomy enthusiast Matt, whose favorite celestial object is the three-star asterism (recognizable star pattern) Orion’s Belt, dropped quite a few questions into the Ask An Astronomer box. We’ll start with this one, since it’s perfectly timed for autumn stargazing:

“Is there a specific star that represents the farthest a person can see with the naked eye?”

I love this question, because it has two answers. First, the most distant object that a person with average eyesight can see with the naked eye is…
...not a star! It’s a galaxy, specifically, the Andromeda Galaxy.

Originally called the Great Andromeda Nebula, it became known as the Andromeda Galaxy after the famous astronomer Edwin Hubble confirmed its galactic nature in the 1920s. At an estimated 2.5 million light years away, it’s the nearest spiral galaxy to our Milky Way. The Andromeda Galaxy is more or less comparable to our home galaxy in size and mass, but astronomers believe it has a significantly higher star count.

Keep in mind that one light year is nearly six trillion miles, so the distance to the Andromeda Galaxy is pretty hard for us to wrap our heads around. But amazingly, with a little bit of effort, we can see it! Let’s gaze.

1) You’ll need a dark site, away from city lights, to see this “faint fuzzy.” Before beginning your hunt, be sure to dark adapt to maximize the acuity of your night vision. To dark adapt, simply avoid all white light for 20 minutes before stargazing. Then use a red flashlight during your stargazing session, to maintain your dark adaptation.

2) About one hour after sunset, face east. Look well above the eastern horizon for a large, four-star square. Each side of the square is about two fists wide, if you hold your fist at arm’s length against the sky and measure across the knuckles. This is the asterism called the Great Square of Pegasus.

Star maps created with Your Sky

3) Next, find the Chains of Andromeda, the two strands of stars that arc to the left (north) of the star that marks the Square’s lower left corner. Now look above the middle star of the upper chain for a faint, fuzzy patch. A Persian astronomer of the 10th century called it the “little cloud,” an apt description. If you spot it, you’re gazing upon a collection of about one trillion gravitationally-bound stars, what the farsighted philosopher Immanuel Kant called an island universe.

If you can’t see it, you may need to try again at a darker site with less light pollution. Additionally, your sky transparency, or atmospheric clarity, may be negatively impacted by the presence of clouds, haze, dust, or humidity. If this is the case, try again when conditions are improved.

When you look up at the night sky, every star you see is in the Milky Way. The exciting thing about spotting the Andromeda Galaxy—in addition to it being the farthest object you can see naked eye—is that you’re seeing a world beyond your galactic neighborhood. That’s right, it ain’t local.

For the second answer to Matt’s fine question, tune in next week. And until then, happy hunting.