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Thread: What area of the Milky Way do we see?

  1. #1
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    What area of the Milky Way do we see?

    When we look at the band of the Milky Way, what portion of it are we looking at? Are we looking inward towards the galactic core and seeing the Sagittarius Arm? I've tried to figure this out for myself by looking at various images of the galaxy but haven't found a image/map that I'm sure of. This image was somewhat helpful: http://www.atlasoftheuniverse.com/5000lys.html But it seems like I need to know where some of the stars in the constellation Sagittarius are on a map like this to see the relationship and I haven't found one. Thanks.

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    The center of the Milky Way is between Scorpio and Sagittarius. You can still see the milky way somewhat dimly when you look at Cassiopeia, which is in the opposite direction. If you click Zoom out on that map you linked to, it helps explain the rest.
    Forming opinions as we speak

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    Thanks. I'm very new to this and didn't realize that, on this map from the same site http://www.atlasoftheuniverse.com/250lys.html, Sgr refers to Sagittarius. Got it!

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    The galactic center is in Sagittarius, to the upper right of the spout in the Teapot asterism and near the boundary junction with Scorpius and Ophiuchus. Here is a simple star chart that shows where the galactic center is located: https://earthsky.org/favorite-star-p...alactic-center (just scroll down a bit). The nearest Messier Object is the open cluster M6, which is about 3.5° south-ish of the galactic center. M6 is bright enough see with the unaided eye under dark skies, and even sometimes under suburban skies. Of course, you can't actually see the galactic center because it's well behind intervening dust, gas, stars, and candy (all Milky Way bars, of course ).

    To locate the galactic center point more precisely, you need to consult a detailed star atlas:
    Paper atlases -- The Galactic Center is labeled in Chart 18 in Wil Tirion's Cambridge Star Atlas, Chart 67 in Sky & Telescope's Pocket Sky Atlas, Chart 22 in Wil Tirion's SkyAtlas 2000.0, Chart 79 in Stoyan & Schurig's Interstellarum Deep Sky Atlas, and certainly others as well.
    Downloadable PDF atlases -- The galactic center is NOT shown in The AAVSO Variable Star Atlas, the three versions of TriAtlas (A, B, and C; with A being the smallest scale and least detailed and C being the largest scale and most detailed), the Toshimi Taki Star Atlas, the Mag7 Star Atlas, the EyesOnTheSky Star Atlas, and the DeepSkyWatch.com Deep Sky Atlas. However, DeepSkyWatch.com's more detailed Deep Sky Hunter Sky Atlas DOES show the galactic Center. Regardless, these are all good sky atlases and they are free to download.
    Software -- I did not see the galactic center in Cartes du Ciel or SkyTools 3, but it might be in Stellarium (which I don't have loaded on this computer) or other programs.
    iPhone apps -- I found the galactic center marked in Starmap Pro, but not SkySafari Pro.

    I hope this helps.

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    Thank you. This helps a lot. I've found the Milky Way in rather poor light conditions by first finding Sagittarius so I can visualize where this is from that chart. yay!

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    Quote Originally Posted by KatK View Post
    When we look at the band of the Milky Way, what portion of it are we looking at? Are we looking inward towards the galactic core and seeing the Sagittarius Arm?
    When we look at the night sky, the direction that we're looking into space varies throughout the night (because the Earth is rotating on its axis) and throughout the year (because the Earth is revolving around the sun). When you look at the Milky Way in the direction of Sagittarius, you're looking towards the center of the galaxy. But when you look at the Milky Way towards Auriga, you're looking in the exact opposite direction of the core of the Milky Way (sometimes this is referred to as the galactic anticenter).

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    Interesting, thanks. I'd never heard of Auriga and had to look it up.

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    Good question this.

    On the BBC's Sky At Night several years back, they told us that what you are seeing when you look at the milky way is simply the nearest spiral arm. Presumably that means looking with the unaided eye.

    But, by using image enhancing (low light) equipment you can take pictures of the central bulge.

    I'm not entirely clear what they mean by the nearest spiral arm. We live in the Orion Spur, a structure bridging between the (towards galaxy centre)) Sagittarius arm and the (anticentre) Perseus Arm. So are we simply seeing the in-plane density of stars looking through the Orion Spur, or are we seeing mainly the Sagittarius arm? A bit of both I would imagine, but where the balance is on surface brightness contribution I don't know.

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    It looks like the Sagittarius arm is between us and the galactic center. At least that's how I interpret this schematic. http://www.atlasoftheuniverse.com/galaxy.html

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    How far actually is the Large Sagittarius Star Cloud? And is it an arm or something else?
    https://en.wikipedia.org/wiki/Large_...rius_Annot.png

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    I don't know. I'm just beginning to try and understand this myself.

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    Quote Originally Posted by chornedsnorkack View Post
    How far actually is the Large Sagittarius Star Cloud? And is it an arm or something else?
    https://en.wikipedia.org/wiki/Large_...rius_Annot.png
    Wikipedia does say the Large Cloud is part of the central bulge, something I had always assumed. What I had not taken on board previously is that the obscuring dust clouds (The Great Rift) are actually not too far from us on galactic scales (2,600 to 3,300 light years distance) .

    That tells me that most of the surface brightness of the milky way, in the areas not obscured by the Great Rift, must originate from further away than that. Our own Orion Spur local arm must contribute very little, at least in the directions looking inwards. Not surprising really, because we are told we live close to the inner edge of the Orion Spur.

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    Quote Originally Posted by KatK View Post
    It looks like the Sagittarius arm is between us and the galactic center. At least that's how I interpret this schematic. http://www.atlasoftheuniverse.com/galaxy.html
    Yes it is, and looking outwards from our position in the galaxy disk we see the Perseus arm.

    However, looking inwards again, we see a part of the galaxy's central bulge poking out from behind the dust clouds. This is the large Sagittarius star cloud. But the middle of the central bulge is obscured by the dust clouds.

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    See the cover story of the November 2019 issue of Sky & Telescope.

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    This article is hidden behind a paywall but it once showed an excellent view of the Milky Way, mapped out nicely. April 2020 issue of Sci Am.

    https://www.scientificamerican.com/a...the-milky-way/
    Do good work. —Virgil Ivan "Gus" Grissom

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    Quote Originally Posted by Roger E. Moore View Post
    This article is hidden behind a paywall but it once showed an excellent view of the Milky Way, mapped out nicely. April 2020 issue of Sci Am.

    https://www.scientificamerican.com/a...the-milky-way/
    The picture from that paywalled site is available on Wikipedia. Scroll down the page until you see the "new view of the Milky Way" picture on the right of the screen:

    https://en.wikipedia.org/wiki/Milky_Way

    BTW, is it just me, or has a lot of confusion and contradiction crept into that Wiki page ?

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    Quote Originally Posted by kzb View Post
    The picture from that paywalled site is available on Wikipedia. Scroll down the page until you see the "new view of the Milky Way" picture on the right of the screen:

    https://en.wikipedia.org/wiki/Milky_Way

    BTW, is it just me, or has a lot of confusion and contradiction crept into that Wiki page ?
    Isn't there an application that lets us look at our galaxy from a different perspective?
    I know that I know nothing, so I question everything. - Socrates/Descartes

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    Quote Originally Posted by DaCaptain View Post
    Isn't there an application that lets us look at our galaxy from a different perspective?
    There's a video on that same Wikipedia page that does this.

    It seems to use the two-spiral arm model rather than the four-arm model of the Scientific American referenced by Roger E. Moore upthread. So maybe it's outdated already.

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    How many novae a year do we see as opposed to how many we expect? I seem to remember one quarter?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    How many novae a year do we see as opposed to how many we expect? I seem to remember one quarter?
    What do you mean by "expect" in this context?

    Grant Hutchison

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    Quote Originally Posted by Tom Mazanec View Post
    How many novae a year do we see as opposed to how many we expect? I seem to remember one quarter?
    Please start your own thread on this; don't hijack other people's threads.
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  22. #22
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    Swift:
    This thread is titled What area of the Milky Way do we see?
    I am suggesting that if you would expect a galaxy of MW size to have, say, 40 novae a year and you only catalog 10, that suggests the answer to the question is one quarter.
    I thought I had read this somewhere, which is why I wasn't sure of the exact value and phrased it that way.
    I was trying to answer the OP question.
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    One thing that crosses my mind is that sometimes it is possible to see through dust clouds and dark nebulae using infra-red wavelengths.

    Would it be possible to see some of these hidden 75% of all novae events using infra-red?

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    Quote Originally Posted by eburacum45 View Post
    One thing that crosses my mind is that sometimes it is possible to see through dust clouds and dark nebulae using infra-red wavelengths.

    Would it be possible to see some of these hidden 75% of all novae events using infra-red?
    Well, the galactic centre is imaged in the infra-red, including the individual stars orbiting the central black hole (completely obscured in the optical, because of dust).

    If those central stars can be imaged, it seems highly likely to me that novae at similar or even further distances should be detected in the IR. I suppose the question is, are they looking in the right direction at the right time.

    Anyway the question is what are we actually seeing when we look at the Milky Way, and presumably that means in optical wavelengths. I think what Tom is getting at is, if we see a quarter of the novae we should see, that means we can see a quarter of the galaxy. I'm not sure that reasoning necessarily follows to be honest, but it is a idea worth looking into. A nova is so much brighter than a star that it can be seen from much further away, so I don't think it is a fair test of being able to see "milky way" from the same distance.

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    That is how I understood the question too, and it is a valid one. We don't see infra-red novas presumably because we haven't got all-sky infra-red coverage yet.

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    Quote Originally Posted by eburacum45 View Post
    That is how I understood the question too, and it is a valid one. We don't see infra-red novas presumably because we haven't got all-sky infra-red coverage yet.
    I was wondering about the probability of detecting novae in the IR. There is a terrific amount of IR astronomy going on and there's been quite a few all sky IR surveys. For example 2MASS and WISE. The UKIRT IR telescope spends time constantly surveying the sky, and maybe there are other instruments doing the same.

    So what is the likelyhood of missing a nova with all this going on?

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    Are these surveys looking for transients like novae, or just steady sources?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Are these surveys looking for transients like novae, or just steady sources?
    I think a sky survey is looking for anything a user of the data wants to look for. What I am trying to get at is, with the frequency of surveys and the lifetime of a nova (c. 100 days), what is the probability it would be detected?

  29. #29
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    As we thought, most MW novae have been missed because they are heavily obscured by dust in the optical.

    A population of heavily reddened, optically missed classical novae from Palomar Gattini-IR: Constraints on the Galactic nova rate

    The PGIR extinction distribution is inconsistent with that reported in previous optical searches (null hypothesis probability <0.01\%), suggesting that a large population of highly obscured classical novae have been missed in previous optical searches.

    https://arxiv.org/abs/2101.04045

  30. #30
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    So we only see a minority of the MW then.
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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