The Milky Way – a galaxy with a bar

Two galaxies, Edwin Hubble

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.

Milky Way map, Atlas of the Universe

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.

Alan Plummer.

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)

5 responses to “The Milky Way – a galaxy with a bar

  • Seems the Spitzer telescope refine the above view? ie really only two spirals interrelated, one stemming from one side of the bar.. the other from the other side see article on Internet. But I am interested in the side on structure as should be revealed in the view that we get looking at it. The black strip along the middle is hard to miss. Have we been so fixated on what was twinkling that we failed to pay attention to what was not? – The aboriginal people of Northern South Australia seeing the gap as being more significant..

    Astronomers toast bar at centre of our galaxy
    By Richard Macey
    August 18, 2005

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    The Milky Way.

    Astronomers have produced the best image yet of what our home galaxy would look like to an intergalactic traveller far above the Milky Way.

    The new image, created by scientists using the Spitzer space telescope, an infra-red sister of the Hubble, shows that the Milky Way’s spiral arms are linked at the centre by a straight bar, 27,000 light years long – 7000 light years longer than previously thought.

    “This is the best evidence ever for this long central bar in our galaxy,” said Ed Churchwell, a University of Wisconsin-Madison astronomy professor and a senior author of a paper to be published in Astrophysical Journal Letters.

    A CSIRO galaxy expert, Naomi McClure-Griffiths, said yesterday working out what our galaxy looked like was no simple matter.

    “The Milky Way’s shape has always been incredibly uncertain. We are trying to figure out what the forest looks like while we are sitting in the middle.”

    About 50 years ago, said Dr McClure-Griffiths, “we thought it was a simple spiral with just a few arms”. In the past 15 years or so there had been growing evidence that the arms were joined at the centre by a mysterious bar.

    AdvertisementOur solar system was between two spiral arms and about 27,000 light years from the centre.

    • Hello Joanne. The dark gaps in the Milky Way are due to dark dust clouds obscuring the stars beyond. They are well known and studied and do not represent any easy way of looking at dark matter. Dark matter is only noticeable through its gravitational pull. The Emu constellation of the Australian Aboriginal people made up of these dark clouds is a wonderful constellation that becomes obvious once it is pointed out.

  • My aboriginal friends in south australia look up at the milky way and place major significance in the black gap that runs longditudinally down the middle of it.. What is that gap? Dark matter? Are we looking out at a galaxy that is really a light sandwich with black matter in between the bread? This would be one possible interpretation. You say our galaxy is between a spiral galaxy and a more concentric circle style galaxy.. and it has also been reported recently that it is thicker than they thought… Are my aboriginal friends right????? Cant find much in an internet search about the gap, only about the light parts on either side…. surely we havent failed to notice this basic feature?? but search so far cant reveal any mention of it

  • i’ve been wandered since i was a little kid, i’ve been dreaming of seeing the end of this indescribably good-created universe. if there is a way, i would take it, whatever it’s! so i can travel and enjoy the beauty of this unlimited,unimaginable, of the beauty.after all i concluded that, this universe is something that already exist in your own head. the more you can imagine, the more reality will imply in reality.

  • I would like to see our location in the universe, My understanding is that it is measured (currently) at 156 billion light years across.
    Of course we are not the center of the universe, the vergo cluster location is where?

    I have problems with “THAT BIG BANG THING” and the inflation theory.

    So to help me better understand I would like to see a map of the universe with our location.
    Can you send me such a map?

    Thank you John

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