In 1926 Hubble published a classification of these extra-galactic nebulae, as galaxies were then known, and the two drawings reproduced above (I assume in Hubble’s own hand, as the article doesn’t say otherwise) are types Sb and Sc in the now-named Hubble Sequence of galaxy structure. The Milky Way is classified in my Norton’s atlas as type Sb/c, or half way in between the two drawings.
Variable star observer, astronomy writer and member of the Sydney City Skywatchers, Alan Plummer sends the following informative and easy-to-read article:
I recently saw a T-shirt with ‘You Are Here’ pointing into a picture of a galaxy. Clearly not the Milky Way as I know it, in what other galaxy, I wondered, was the T-shirt made? I was too polite to regale the innocent wearer with what our Galaxy is supposed to look like, but you, dear blogger, are fair game. (By convention, our home Galaxy has a capital ‘G’, to distinguish it from any other galaxy.) It turns out that a great deal of our Galaxy is not yet mapped at all, and other parts only poorly. Here’s a small sample of current thinking on the topic.
As a matter of simple perspective it is easier to see the structure of galaxies other than our own, and Edwin Hubble did a lot of the early work toward that end. Using the Mount Wilson 100” telescope in 1924, Hubble resolved individual stars 2,363,500 light years away in M31, the spiral galaxy in Andromeda. For the first time the true nature of these ‘nebulae’ was confirmed: They are large structures of gas, dust, and stars similar to, but out-side of, our own Galaxy.
A map of the Milky Way, courtesy of the Atlas of the Universe website
An example of the best maps available today is this one above from the Atlas of the Universe web site. Let’s have closer look: There is a small (~20,000 light years!) bar visible in the hub that was first deduced by Martin Weinberg in 1992. He examined populations of red giant stars in such a way as to determine their distances, and found this structure. It seems to be well confirmed, as does the discovery of a general warp to the disk as a whole. Warped galactic discs are observed to be caused by mergers or close encounters with neighboring systems, and this is assumed to be the case with the Milky Way, too.
Spiral arms in external galaxies are traced by young stellar populations, and by various types of gas and dust clouds, so we can find the same structures here and attempt to trace the arms of the Milky Way. Easier said than done! Notice the long Cygnus Arm (also called the Norma-Cygnus Arm) arching around from the bottom of the map to the top, and into the center; there are no observable tracer systems of this arm that I can find. It is a pure assumption from Hubble’s sequence, and may be confirmed, or modified in the future.
In fact, here’s how these maps have been made. The survey results of arm-tracing objects are first plotted on a chart. No spiral shape is obvious at all; although some arcs are visible, implying the Sagittarius Arm. Only afterwards are idealized spiral arms fitted in two, three, and four arm models. A two arm model doesn’t fit at all, but both the three and four arm models do. Current consensus is with the four arm model, as pictured above. Note that on that map parts of the same arm have different names. For instance, both the Cygnus and Norma Arms are sometimes called the Norma-Cygnus Arm.
That’s the gross structure, so what about the finer detail? It’s barely begun to be unraveled. For instance, the Sun seems to be in a ‘spur’, or separate small section of arm called the Local (or Orion) Arm, rather than in a major feature. That’s fine, but the idealized spiral models placed the next-arm-in hundreds of light years closer to the Sun than it is observed to be. So mapmakers just straightened that section of the Sagittarius Arm a little to fit, as can be seen on the map. Other galaxies show fragmentary arms, spurs, arches, warps, and bends, so why should the Milky Way be different? The mapping of these places has barely begun.
General references, and further reading:
Atlas of the Universe website
Edwin Hubble, Extra-Galactic Nebulae, 1926CMWCI.324….1H
Star-forming complexes and the spiral structure of our Galaxy. D. Russeil, A&A 397,133-146 (2003)