This is what the Milky Way actually looks like as shown on the Astronomy Picture of the Day for October 20, 2007. Most of us have rarely never seen it like this because we live in light polluted areas where most stars have become invisible. We are seeing our galaxy from one of the spiral arms looking toward the central cluster of stars. Much of the center is obscured by dust clouds.
The model below shows what the Milky Way galaxy was supposed to look like when I was growing up [from Astronomical Adventures]. It has four large spiral arms and a large central blob of stars. The model is based on our view of other spiral galaxies and on observations that were made from Earth-based telescopes.
Our solar system is located in the Cruz arm. Other models were similar although the names of the arms were different. The basic plan was a spiral with four large arms (Norma, Scutum-Centaurus, Sagittarius and Perseus) projecting outward from a dense circular mass of stars at the center of the galaxy.
This view has been substantially altered over the past few decades. Astronomers now recognize that the Milky Way is a bar galaxy whose central mass of stars forms a bar shape with arms trailing off the ends of the bar. Typical bar galaxies have only two main arms and recent results indicate that our galaxy conforms to this pattern [Two Of The Milky Way's Spiral Arms Go Missing]. The most recent model is shown below. As you can see, two of the four major arms have now disappeared or have been greatly reduced in size and importance.
The galaxy looks very different from the one I learned about as a child. Does anyone know where our solar system is located on the new model?
UPDATE: I replaced the old image (below) with one from Astronomy Picture of the Day from June 6, 2008. It shows where we are in the galaxy.
The press release locates the Solar system near what they know call "a small, partial arm called the Orion Arm, or Orion Spur, located between the Sagittarius and Perseus arms".
ReplyDeleteThe old Wikipedia illustration shows the location of this partial arm. The bar and remaining arms are decently comparable with the new artist's illustration, albeit the attachment of remaining arms to the larger bar is now on opposite ends.
On the new illustration, the Orion spur is not really an arm, just some faintly visible streaks, to the north of the now much more minor Sagittarius arm. Galactic north, 2/3 out, that is our current home.
That's a beautiful illustration of the Milky Way.
ReplyDeleteI've seen the Milky Way only once in my life in a dark sky location, though not as distinct as in your photograph.
In a clear night sky while stargazing (lying on my back), I sometimes get a disorienting feeling of "falling into space". Not sure if it's some sort of spiritual experience, but it is certainly an acrophobic feeling.
The latest APOD shows the solar system location nicely
ReplyDeleteDuh! Nay, make it duh^3.
ReplyDeleteI assumed all Milky Way views had the same orientation. Worse, I was thinking 0 longitude and saying "north", all the while forgetting that the galactic coordinate system is by convention centered on our observational frame. I'm propagating a urban legend (well, scifi) by my confusion, thanks for straightening me out!
This raises a new question. The new illustration defines two bars. What is their definitions? If the "long bar" is defined by infrared astronomy, is the "galactic bar" defined by radio astronomy?