New Look for the Home Galaxy
Once upon a time, everyone knew the world was flat. Then there was a span when everyone knew Earth was the center of the universe. And for quite a while now, we've known that our home galaxy, the Milky Way, looks like a big symmetrical pinwheel in space, with dusty, star-filled arms radiating from a spherical core also packed with stars...
Facts have a nasty habit of creeping up and catching us in the everyone-knows. However, gathering facts about the Milky Way has required passing time and advancing technology. For us planet-bound creatures to figure out the nature of our stellar neighborhood has been as tricky as it would be for a deep-sea clam to figure out the nature of weather. Clues filter down to where we live, but they aren't obvious.
Clams might do better if they had periscopes, and we've done well with telescopes. For someone surveying the night sky without any such aid, the Milky Way is only a well-named band of pale light studded with stars. Given some optical assistance, it's possible to find a whole sea of stars in that milky band.
With more powerful assistance, astronomers can observe distant galaxies and make some informed surmises about our own Milky Way. These big islands of stars seem to take on fairly few shapes. The ordinary spiral, which indeed does look like a glittering pinwheel, is one of the more common. Humankind had progressed in humility since the days when we assumed Earth lay at the center of everything; it seemed reasonable to think our galaxy was of a standard form.
Astronomers couldn't see anything to make them think differently. Because Earth lies inside the galactic disk, scientists looking edgewise through the disk have a hard time discerning features within it (think of trying to look edgewise through a fireworks pin- wheel). Careful plotting of stars and gas clouds has led most of them to conclude that three of the galaxy's arms have been identified, but no one knows how many more---if any---there may be.
It also has been impossible to tell for certain the shape of the center of this galaxy. Astronomers know there's a substantial bulge there, but that's all that can be told from our vantage point. In 1990, astronomers Leo Blitz of the University of Maryland and David Spergel of Princeton University suggested the conventional assumption that the galactic core is spherical is wrong. They said some infrared images captured by a Japanese balloon-borne experiment showed that the Milky Way's center looked more like a cigar than a sphere.
The elongated, roughly bar-like core is the mark of what is known as a barred-spiral galaxy. Barred spirals are about as common as ordinary spiral galaxies in the observed universe, so the odds are just as good for the Milky Way's being either type. Most astronomers were unwilling to let go of what they knew about the home galaxy, however, and Blitz and Spergel made few converts.
Now more facts have come creeping in. Blitz predicted that if the galactic center is a bar, one end probably slants away from Earth and one slants toward it. The end that slants toward us should appear brighter than the end that slants away, and that difference in brightness would be detected some day.
That day apparently arrived with infrared data gathered recently by the Cosmic Background Explorer satellite. The satellite's measurements showed that one end of the starry blob at the center of the galaxy was much brighter than the other. The data describe a galactic center that looks like a bar tilted about 45 degrees relative to Earth's line of sight.
Thus, the home galaxy doesn't really look like a pinwheel. Seen from above, the Milky Way probably looks more like a gigantic S drawn in starlight, with an extra arm or two. Or so we can think, until the next facts come in.
