David or Goliath?

By Phil Plait | March 7, 2007 2:08 pm

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.

Comments (21)

  1. blizno

    I thought the Milky Way was an average galaxy. The first Google hit tells me it probably contains between 750 billion and a trillion solar masses and is indeed a giant galaxy.
    http://www.seds.org/messier/more/mw.html
    I was taught that we’re on an average planet orbiting an average star which is part of an average galaxy (admitted, grade school science was a long time ago for me). We’re part of a giant galaxy! Yay, us!

    I’m a little surprised by the picture. The lower “smear” looks like it has a hard, bright center and a diffuse outer region. My inexperienced eye would have guessed it be much smaller and much closer than the well-formed disk galaxy above it.

  2. kingnor

    so i’m sure this concept isn’t new to astronomers, i’m guessing they were thinking: “its either BIG and just as far away, or its small and realy close. or maybe its REALLY REALLY BIG and even farther away”

    What exactly happend that allowed them to narrow down the size?

  3. bswift

    The astronomers took spectra of the two which turned out to have very different redshifts. According to the linked article “coordinate confusion and wrong distance estimates in the literature for the last 23 years.”

    Doesn’t seem like it was that big of a controversy — someone just finally got around to getting spectra again.

  4. blizno

    I wonder how many old misinterpretations there are in the literature and how many of them will stay that way because there’s so much else to look at.

  5. Porto

    ” 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.”

    Of all the cosmics coincidences, thats is my favorite for sure. We are very luck to be on a planet that can witness a eclipse that matches the size of moon and the sun.

  6. Mark Martin

    Hey, sorry to get off-topic, but check out the full-page ad on the back cover of this week’s Newsweek. It has a list of “Things to do while you’re alive”. Check out the 8th one from the bottom. It’s not exactly bad astronomy, but including it in a list of things to accomplish (for those in the Northern Hemisphere) surely is symptomatic of a bad astronomy culture.

  7. It’s not luck, it’s simple coincidece. A billion years ago the Moon was so close it covered up way more than the Sun, and a billion years from now it’ll have moved far enough away that we’ll never get total eclipses any more. We’re basically in the middle of a long eclipse season.

  8. wright

    I love it:

    “We’re basically in the middle of a long eclipse season.”

    A great way of putting it, Phil. Another reminder that the universe is not made to suit us; we developed to suit the particular conditions of our little corner of the universe.

  9. The Sun is 400 times the physical size of the Moon,

    We’re talking diameter, right? I mean it has to be a way bigger difference in terms of volume, right?

  10. Ian Menzies

    “It’s not luck, it’s simple coincidece.”

    I would say that luck is coincidence.

  11. Crux Australis

    750 billion solar masses? That’s big. So what’s the latest count of stars in the MW?

  12. Crux Australis

    If the Sun has 400 times the diameter (and therefore radius) then it must have 400^3 or 64 million times the volume.

  13. Well now you went and made me look it up. Bugger.

    The diameter of the sun is 1.4 million km, the moon about 3500 km and that does indeed work out to about 400X.

    Now I know something I didn’t know yesterday.

    :)

  14. din

    “Another reminder that the universe is not made to suit us; we developed to suit the particular conditions of our little corner of the universe.”

    Or its just another reason why god made the universe just right for us. Long may the squid rule !

    its the sky being blue that interests me … from memory of Phil’s book, it wouldn’t be blue if things where slightly different, ie black if we looked up at the moon, and its red when we look at sunset.

  15. bassmanpete

    So what is the 8th from bottom of the List of things to do while you’re alive? The list of things to do after is probably quite short, I imagine :)

  16. Mark Martin

    The 8th one up is “Find the Big Dipper”. Quite a feat.

  17. Gary Ansorge

    Saw the big dipper last night. Clear skiis and low light polution are more common where I live now than they were in Los Angeles.

    So, when did the Milky Way get boosted from 100 billion or so stars to 1 trillion? I guess I missed that?

    Gary 7

  18. Gary 7:

    > Gary Ansorge Says:

    >> So, when did the Milky Way get boosted from 100 billion or so stars to 1 trillion? I guess I missed that?

    > Gary 7

    They did not. The Milky Way has about four hundred billion stars. The total stellar mas in the Milky Way is about an hundred billion solar masses (most stars are less massive than the sun. The total baryonic mass of the Milky Way is about two hundred billion solar masses. About three fourths of the milky Way is nonbaryonic mass (dark matter).

    Scientists hypothosize that the dark matter is neutrinoes, but no-one knows for sure. If you believe that dark matter is strange, than you should check out dark energy:

    * – http://wikipedia.org/wiki/dark_matter

    * – http://wikipedia.org/wiki/dark_energy

  19. Gary took my question, when did our galaxy increase from approx. 1 billion stars to trilllions of stars? I think what is meant is the whole shebang is trillions of stars and more. And I also agree with Din, We ‘adapted ‘to our part of the universe, not vice versa which makes it look as if the universe was made for us.

  20. Mark Martin

    Our galaxy never got a population boom. It’s still estimated to be in the neighborhood of between 100 & 200 billion stars. The “trillion” figure was just someone’s mistake.

  21. Magnum

    I’m surprised that astronomers are still fumbling around in the dark so much that they can’t tell if a galaxy is 13 million or 150 million light years away. But after reading the linked article I see it was (presumably) quite easy to determine the distances by observing the red-shifts, and there was also some good information about chemical composition (from spectography I guess). So much to learn!

NEW ON DISCOVER
OPEN
CITIZEN SCIENCE
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT

See More

ADVERTISEMENT
Collapse bottom bar
+

Login to your Account

X
E-mail address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »