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A Distant Twin (Science Advent 22)

(Every day until Christmas, I’ll be posting a science-related image.)

Day 22

The large spiral galaxy UGC 12158 is remarkably similar to the Milky Way in many ways. Though it's a lot bigger, the galaxy shares many features with our galactic home: the strong spiral arms and long bar structure running through the center. [Credit: ESA/Hubble & NASA]

The large spiral galaxy UGC 12158 is remarkably similar to the Milky Way in many ways. Though it’s a lot bigger, the galaxy shares many features with our galactic home: the strong spiral arms and long bar structure running through the center. The bright blue point of light below the galactic center is supernova SN 2004ef. [Credit: ESA/Hubble & NASA]

A melancholy thought: we will probably never see our galaxy from the outside, as distant observers would see it. Traveling that far would take many thousands of years at close to light speed, and even if it were practical to plan such a trip, it would be a lower priority than exploring our galactic neighborhood. Nevertheless, thanks to detailed observations, we have a really good idea of what the Milky Way looks like: two major spiral arms that fragment into smaller arms and spurs, growing from a bar-like structure that runs through the galactic center.

The Hubble Space Telescope turned up a galaxy 400 million light-years away that looks remarkably like a larger version of the Milky Way. This beautiful galaxy, known as UGC 12158, is also has a bar structure (as do about two-thirds of all spiral galaxies) and two arms that break into smaller structures farther out from the center. One more detail of this photo stands out: the bright blue object below and to the left of the galactic center isn’t a star. It’s a supernova known as SN 2004ef. (Supernovas are named by the year of their discovery, followed by a sequence of letters marking how many were found that year. The first supernova of 2004 was SN 2004a, the second was SN 2004b, the 27th was SN 2004aa, and so forth. Your homework: work out where SN 2004ef falls in that sequence.) You can see that the supernova is remarkably bright, brighter than anything else in its galaxy. The other bright spiky objects in the image are stars in the Milky Way, 400 million light-years closer!

UGC 12158 is significantly bigger than the Milky Way: about 140,000 light-years across, compared to our galaxy’s 100,000 light-year diameter. However, if an alien astronomer in another galaxy could see the Milky Way face-on, it would look remarkably like UGC 12158. Our Sun would be too faint to see—the bright stars in the spiral arms of any galaxy are all far more massive and bluer than the Sun—but our alien astronomer colleagues would likely still be impressed by the Milky Way’s beauty.

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3 Responses to “A Distant Twin (Science Advent 22)”


  1. 1 Leticia December 23, 2012 at 11:13

    The milky way has a bright center too? (I always thought that there is a black hole in the center of our galaxy, and that black hole was the reason of the spiral arms).

    • 2 Matthew R. Francis December 24, 2012 at 09:46

      Yes, there is a black hole at the center of the Milky Way and nearly every other galaxy. However, that black hole is relatively small – smaller than our Solar System – so you can’t see it on the scale of the entire galaxy. It’s completely surrounded by the stars of the galactic center.

      The spiral arms are created by fluctuations in the density of stuff in the galaxy’s disk. (That’s at least the simple version: I could write a whole long post just on that!) The black hole doesn’t actually make a strong difference in the behavior of the galaxy on the biggest scales. If you removed the Milky Way’s black hole, for example, our Sun’s orbit around the galactic center wouldn’t change.


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