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Canis Major dwarf galaxy *thought* to be our closest neighbor?

  1. Mar 23, 2010 #1
    I just read this in Wikipedia: "The Canis Major dwarf galaxy is classified as an irregular galaxy and is now thought to be the closest neighbouring galaxy to our location in the Milky Way" (emphasis mine). Umm, why thought? What is it that makes us uncertain? Or is this an error in Wikipedia?
  2. jcsd
  3. Mar 23, 2010 #2
    I thought the Andromeda galaxy was the closest galaxy to our Milky Way..
  4. Mar 23, 2010 #3
    You do know that wikipedia isn't a reliable source to get your information right? As far as I know, unless somethings has changed in the last 3 months or so, andromeda is the closest galaxy to us
  5. Mar 23, 2010 #4


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    Only if you exclude dwarf galaxies from contention. Both the Magellanic clouds are dwarf galaxies that are closer than Andromeda. The Canis Major dwarf galaxy joins them. (due to the fact that it lays in the same direction as the Milky Way, it was not discovered until 2003.)

    However, there seems to be some recent controversy over whether or not it is a true dwarf galaxy orbiting our own or an over-concentration of stars formed by some other process.
  6. Mar 23, 2010 #5
    Janus, thanks for posting an answer that seems to be something other than "I thought..." or other kinds of hearsay. Any ideas on why the word "thought" is used? Surely it's not just because the word "galaxy" is ill defined?
  7. Mar 24, 2010 #6
    If you read the entire article it states that this dwarf galaxy was only discovered in 2003, so some of the data may be under scrutiny. The previous “closest dwarf galaxy” may be closer to the galactic center (or not), but is not closer to our solar system than Canis Major.
  8. Mar 24, 2010 #7
    'Thought' was probably used because they cannot confirm that there arn't smaller Galaxies close than it. Also probably because they would have needed to use parallax to find out how far away it was, and that parallax can sometimes need double checking as at long distances it becomes rather inaccurate.
  9. Mar 24, 2010 #8
    It's an open debate whether this overdensity is actually a dwarf galaxy or not.
    See this article for example: http://goo.gl/igfM

    (This is my PhD topic :)
  10. Mar 24, 2010 #9
    Very cool, thanks a lot. Want to give me a clearer expression of the facts than what's in Wikipedia right now? I'd be glad to make the corrections there if you have a better way of saying it.
  11. Mar 24, 2010 #10
    You're welcome!

    Check the 'Discussion' section in Wikipedia (http://goo.gl/o0pX), there's plenty of material there. The thing is that is not yet FULLY accepted (or proven) that this is in fact just an overdensity and NOT a dwarf galaxy being accreted by the Milky Way... so you can't just 'correct' the article.
    What you can do is add an article to Wikipedia, maybe something like 'Canis Major debate' and put in there all the links to scientific articles and stuff rebutting the 'dwarf galaxy' theory.

  12. Mar 24, 2010 #11
    I mean correct it only in the sense that this is confusing: "...is now thought to be the closest neighbouring galaxy to our location..."

    It sounds like we're unsure of its distance from us. I would want to re-word it so it doesn't imply uncertainty about the distance, just lack of consensus on the definition of "galaxy". I assume that we are certain about the distance.
  13. Mar 25, 2010 #12


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    GBB, it is not a question about the definition of what a galaxy is, but just whether that bunch of stars is a galaxy or something else. So it's not like say the 'is pluto a planet' kind of debate.

    Basically we think bigger galaxies like the Milky Way form by the mergers of many many small dwarf galaxies. This means a lot (most?) of the stars in the Milky Way disk were originally in smaller galaxies that crashed into the Milky Way and got gobbled up by it. This means that the disk is a mess of old dwarf galaxy cores, tidal tales that get thrown out during mergers and spiral arms. So, when we see an overdensity of stars it can be quite tricky actually working out whether that is the core of a dwarf galaxy that was accreted at some point or part of a tidal tale of a dwarf that is somewhere else, or a lump caused by the spiral arm or warping of the disk itself.

    When you really try and unpick the structure of the Milky Way it gets very tricky because its just a giant mess.

    I'd advise that if you are not an expert on this topic that you don't edit the Wiki page. I would advise Gaba_p to do so, since you clearly do have the relevant knowledge. Just remember to keep the neutral point of view.
  14. Apr 9, 2010 #13
    Actually Sirius, the binary star and dwarf galaxy in Canis Major is closer at 8.651352785 light years. Sirius is the brightest object in the sky at Mv=1.42 and luminocity=23.5. The Andromeda galaxy is 2380939ly away but closing fast.

    (Adapted from Sky Catalogue 2000.0, Vol. 2, Sky Publishing Corp., 1985.)
    Last edited: Apr 9, 2010
  15. Apr 9, 2010 #14


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    ??? Sirius is in the Canis Major constellation, but is not related to the Canis Major dwarf galaxy, other than the fact that they are in the same direction from Earth. Remember, a constellation is just a group of stars that form a pattern as seen from Earth. They are not a group in any physical sense.

    For example, the Andromeda galaxy is in the Andromeda constellation along with stars that are in our own galaxy even though it lies far beyond the limits of our galaxy.
  16. Apr 9, 2010 #15
    Canis Major dwarf galaxy is a separate entity in itself. It is 25,000 light years away from Earth.

    Sirius is a Binary Star Pair Sirius A ,white main sequence star of spectral type A1V, and Sirius B , a faint white dwarf companion of spectral type DA2 (not to be confused with the Canis Major dwarf galaxy).
  17. Apr 9, 2010 #16


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    Exactly. Which is why I wondered why you even brought up Sirius in a discussion about the Canis Major Dwarf Galaxy.
  18. Apr 9, 2010 #17


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    Wow, that's pretty impressive accuracy. You have measurements of the distance to Sirius down to +/- 6000 miles.

    Is that to the centre of Sirius' 1.7 million mile radius, or to the surface?
    And what time of year is that measurement taken? Spring or autumn?

    Last edited: Apr 9, 2010
  19. Apr 9, 2010 #18


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    How are you comparing a binary star system to a dwarf galaxy? Dwarf galaxies need to hold at least a couple million to a few hundred million or few billion stars...I'm completely not getting this statement. D=

    Also what is luminosity=23.5? This figure makes no sense. Even if you meant absolute magnitude is -23.5 (which would be the figure closest to being realistic), that would make the dwarf galaxy something like 10 times brighter than the Milky way (viewed face on)!

    I'm so confused =(
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