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Thread: Which version of the Milky Way should I use?

  1. #1
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    Which version of the Milky Way should I use?

    I'm making a video about the structure of the Milky Way, but I haven't been able to settle on which version of the Milky Way is the right one. In the last few years, two competing images of the Milky Way have emerged, which I will politely refer to as male and female

    For decades it seemed male was in the ascendant, but a few years ago, female was settled on. But now it seems we're back to male again. I'm confused.
    "Occam" is the name of the alien race that will enslave us all eventually. And they've got razors for hands. I don't know if that's true but it seems like the simplest answer."

    Stephen Colbert.

  2. #2
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    I don't understand "male" and "female". Your link diagrams seem to refer to the model with four roughly equal arms, and the model with two major and two minor arms, both of which are more recent than the old long-standing model with two arms. Is that the distinction you're talking about?

    Grant Hutchison

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    I also don't get why they are male and female, or is the explanation too rude to put on here?

    Looking at the literature, I believe the four-arm model is in the ascendency.

    One of the diagrams refers to the Orion Spur, a name which I believe is now frowned upon.

  4. #4
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    Quote Originally Posted by grant hutchison View Post
    I don't understand "male" and "female". Your link diagrams seem to refer to the model with four roughly equal arms, and the model with two major and two minor arms, both of which are more recent than the old long-standing model with two arms. Is that the distinction you're talking about?

    Grant Hutchison
    Yes. I wasn't sure if "phallic" and "yonic" would get past the censor.
    "Occam" is the name of the alien race that will enslave us all eventually. And they've got razors for hands. I don't know if that's true but it seems like the simplest answer."

    Stephen Colbert.

  5. #5
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    Quote Originally Posted by parallaxicality View Post
    Yes. I wasn't sure if "phallic" and "yonic" would get past the censor.
    Different words, but still no idea what you're talking about.
    Anyway, you seem to be saying it's about four equal arms versus two major and two minor, right? That's a matter of degree, rather than absolute count. Why not just talk about the evolving view, rather than try to choose one "right" and one "wrong"?

    Grant Hutchison

  6. #6
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    Use the updated information. You can teach the conflict but make it clear which view is consistent with the latest evidence.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

  7. #7
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    Quote Originally Posted by Noclevername View Post
    Use the updated information. You can teach the conflict but make it clear which view is consistent with the latest evidence.
    Sometimes several views are consistent with the latest evidence. In this case, how many arms you find seems to depend on whether you use gas, young stars or old stars to map the arms. Which, in itself, is interesting.

    Grant Hutchison

  8. #8
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    Quote Originally Posted by kzb View Post
    One of the diagrams refers to the Orion Spur, a name which I believe is now frowned upon.
    That is correct. No one in the field uses that term any more. See the cover story in the November 2019 Sky & Telescope and in particular the part on page 18 entitled "The Local Arm: Spurning the Spur."

    That same article makes clear that there is not agreement as to whether the Scutum-Centaurus and Perseus Arms are more prominent than the Sagittarius and Outer Arms.

    Notwithstanding that uncertainty, the best current picture is on page 21 of the same article, with the caption on page 20 noting changes due to recent work. It's too bad the magazine didn't commission new artwork to reflect the updates.

  9. #9
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    Quote Originally Posted by SagittariusAStar View Post
    That is correct. No one in the field uses that term any more. See the cover story in the November 2019 Sky & Telescope and in particular the part on page 18 entitled "The Local Arm: Spurning the Spur."

    That same article makes clear that there is not agreement as to whether the Scutum-Centaurus and Perseus Arms are more prominent than the Sagittarius and Outer Arms.

    Notwithstanding that uncertainty, the best current picture is on page 21 of the same article, with the caption on page 20 noting changes due to recent work. It's too bad the magazine didn't commission new artwork to reflect the updates.
    There is a paywall though? The pictures on the free-to-view page are not the most up-to-date I think.

    https://skyandtelescope.org/sky-and-...er-2019-issue/

    Another interesting development in recent years is that the MW disk is at least 50kpc diameter (160,000 LY) not the traditional 30kpc (100,000 LY). Although the outer disc would be very faint to an external observer.

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