Over the last 100 years or so, our view of the universe has changed dramatically. It was possible in the early 20th century to think that our galaxy (literally meaning Milky Way, same root as “lactose” and “lactate”) might be the entire universe. The famous “Great Debate” between astronomers Harlow Shapley and Heber Curtis was over whether the observed “spiral nebulae” were part of our galaxy, or separate galaxies in their own right. The issue wasn’t settled during the debate, but several years later by Edwin Hubble, who used techniques developed by Henrietta Swan Leavitt to measure the distance to the Andromeda galaxy. Needless to say, he found the distance to be much larger than the size of the Milky Way, thus showing the universe to be a very large place.
Edwin Hubble (1889-1953) and Henrietta Swan Leavitt (1868-1921)
All this is background for today’s story. Once the universe could be divided into parts—the part inside our galaxy, and the part outside—astronomers could start studying the structure of galaxies in earnest. Harlow Shapley himself located the center of our Milky Way and got a reasonably accurate measure of its diameter. The challenge then became to map out the complete structure, which is a difficult task: unlike (say) the Andromeda galaxy, we’re inside the Milky Way, so we can’t see the whole thing. If you go out to a place with a dark sky, you can see the milky streak of light that is the disc of our galaxy, but without special equipment, you can’t find the galactic center or discern that our galaxy is a spiral galaxy.
Andromeda galaxy (M31), the spiral galaxy closest to our own galaxy
Specifically, the special equipment we need includes radio telescopes and infrared telescopes. Radio telescopes look at the longest wavelengths of light, while infrared instruments pick out the kind of radiation we feel as heat. Radio waves help us find hydrogen, the most common chemical element in the universe, while infrared radiation highlights dust, which to astronomers means heavier molecules and aggregates of molecules, often containing carbon or oxygen atoms. As it turns out, dust traces the spiral arms of a galaxy very well, so if you have a good infrared telescope, you can map the spiral structure even while viewing from within the Milky Way.
Which leads us to the news of the day: the Spitzer space telescope, which is an infrared telescope, has done an extensive survey of the galaxy, and the results were announced at this week’s American Astronomical Society meeting in St. Louis:
This artist’s depiction of the structure of the Milky Way is based on the Spitzer observations, and it shows that our galaxy has two major spiral arms and a number of smaller structures. Previous observations had concluded our galaxy has four arms rather than two, which raised the question of why our galaxy seemed so much more complex than others we observed. (I guess I need to revise my lecture notes now.) With the Spitzer map of the Milky Way, the picture we can draw looks like many other two-armed spirals such as M51, the Whirlpool Galaxy:
To an alien astronomer in a distant galaxy, perhaps our Milky Way looks as beautiful.
On a personal note, I will be at the Great Lakes Cosmology Workshop at Carnegie-Mellon University next week. I will do my best to post anything exciting I find out during these proceedings.
Posted by Matthew R. Francis under Director's Notes & Science Ideas & Science News | No Comments »
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