Galileo's Pendulum

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

Day 22

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.