David or Goliath?

Contributed by
Mar 7, 2007

Which of these galaxies is farther away? Or are they at the same distance?

For a long time, astronomers thought they were at the same distance. The upper galaxy -- NGC 5011 B -- is your standard edge-on disk galaxy, about which a lot is known. It is obviously part of the Centaurus cluster of galaxies, located about 150 million light years away.

The lower one -- NGC 5011 C -- is, well, a smear. If it's also at that distance, that would make a giant galaxy, like our Milky Way, and we have never seen any big galaxy shaped like it before. Is this a new breed of galaxy?

Nope. It's a familiar one, and we're suffering from a cosmic coincidence. Astronomers were able to determine the distance to NGC 5011 C, and found it was at a mere 13 million light years-- a skip and jump away from us, relatively speaking. This makes it a dwarf galaxy, and we see lots of those. In fact, the brightness and size of the galaxy are just right for a dwarf at that distance.

Clearly, the two galaxies are not physically associated; they just happen to line up in the sky and be about the same apparent size. The spiral is far bigger in reality, more than 10 times as big. It's a bit like the Moon and Sun in the sky: The Sun is 400 times the physical size of the Moon, but it's also 400 times farther away, making them both about the same apparent size to the eye.

So astronomers have closed the book on this particular mystery, a book that we shouldn't judge too strictly by its cover. Sometimes it pays to read between the lines.

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