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Space Science

The Milky Way Is Much Less Massive Than Previous Thought 119

Posted by Soulskill
from the galactic-atkins dept.
schwit1 writes: New research by astronomers suggests that the Milky Way is about half as massive as previously estimated. It was thought to be roughly the same mass as Andromeda, weighing in at approximately 1.26 x 10^12 solar masses (PDF). This new research indicates its mass is around half the mass of Andromeda. "Galaxies in the Local Group are bound together by their collective gravity. As a result, while most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity. For the first time, researchers were able to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda. ... Andromeda had twice as much mass as the Milky Way, and in both galaxies 90 percent of the mass was made up of dark matter."
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The Milky Way Is Much Less Massive Than Previous Thought

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  • by i kan reed (749298) on Wednesday July 30, 2014 @10:48AM (#47565947) Homepage Journal

    Try the milky way diet plan. You too can lose 1.3 quadrillion solar masses in just one month trying the Milky Way(TM) diet.

    Check out these before and after shots: you can't even see the dark matter anymore.

    • What if I like Three Musketeers better?
    • The astronomers probably just missed the blue car [youtube.com] speeding away with the rest of it.
  • by cruff (171569) on Wednesday July 30, 2014 @10:51AM (#47565987)
    The Milky Way Galaxy was seen running off while crying about its impending demotion to dwarf galaxy status.
  • by ArcadeMan (2766669) on Wednesday July 30, 2014 @10:56AM (#47566035)

    The Milky Way is much less massive than previously thought

    Good news for everyone who's lactose intolerant.

  • by Pro923 (1447307)
    lHow can they possibly tell how much of the matter is "Dark"? I can get the idea of what they're doing - using the relative speeds of each local galaxy to determine the masses contained within each, but how could they possible determine how much mass in each galaxy wouldn't be seen by using light within the bounds of the visible spectrum?
    • by Anonymous Coward

      They measure the rotation curve - the orbital velocity at various distances from the galaxy's centre - and use those points to calculate the mass binding the orbiting stars. That give the total of dark + visible matter. The mass of normal, stellar matter is estimated from the star counts and knowledge of stars in our own galaxy.

      • Now if the amount of normal steller mattter is known and the galaxy has only half the mass estimated earlier, that should make the amount of unexplained 'dark' matter a whole lot smaller....

        • Yes it would mean there is half as much dark matter in the milky way as we previously would have thought but then there's half as much normal matter as well so you get the same ratio. Of course this has no impact on the total dark matter budget of the universe so it doesn't really have any bearing on our understanding of dark matter.
    • Re:Dark? (Score:5, Informative)

      by gurps_npc (621217) on Wednesday July 30, 2014 @11:23AM (#47566333) Homepage
      It's not just the visible spectrum, it's all radiations levels.

      Different amounts of mass result in different star types which give up different types of light. non-star objects - dust, planets, etc. block light and radiate out the energy they absorb as heat.

      So by looking at any point, we can tell how much mass it has by the amount and type of light it gives off, including the non-visible spectrum, i.e. heat.

      There are a few assumptions made, but it makes a lot of sense, mathematically.

      None of it would have been possible before we understood the formulas behind fusion.

    • but how could they possible determine how much mass in each galaxy wouldn't be seen by using light within the bounds of the visible spectrum?

      Such "dark matter" would show up on Xrays [harvard.edu] infrared [caltech.edu] or radio [nrao.edu], so that's not a problem. If, however, the "dark matter" does not interact with electromagnetism, but only with gravity and the weak force, (which would be an extremely odd, and frankly, a not very believable aspect of cosmology) things would get a bit tricky.

      • Re:Dark? (Score:5, Informative)

        by DM9290 (797337) on Wednesday July 30, 2014 @12:04PM (#47566827) Journal

        If, however, the "dark matter" does not interact with electromagnetism, but only with gravity and the weak force, (which would be an extremely odd, and frankly, a not very believable aspect of cosmology) things would get a bit tricky.

        That is EXACTLY what most of the dark matter is suspected to be and that is what makes it tricky.

      • by geekoid (135745)

        "(which would be an extremely odd, and frankly, a not very believable aspect of cosmology"
        says someone, every time something new is found.

        • by dryeo (100693)

          Always reminds me of the neutrino, first postulated to balance out some equations and found some 30 years later. I'm sure back then on the slashdot of the day you had people doubting that such a small non-interacting particle could exist.

    • by Anonymous Coward

      They estimate the amount of dark matter from the rotational speed of the galaxy. Basically, a rotating body is in equilibrium between the centripetal pseudo-force that wants it to spread out into an infinitely wide disk, and the binding force (gravity in this case) that wants it to be a perfect sphere. When you look at the amount of visible matter and attempt predict its shape from its rotational speed and mass, you realise the galaxy is the wrong shape. There must be an extra force pulling the stars togeth

    • by PvtVoid (1252388)

      lHow can they possibly tell how much of the matter is "Dark"? I can get the idea of what they're doing - using the relative speeds of each local galaxy to determine the masses contained within each, but how could they possible determine how much mass in each galaxy wouldn't be seen by using light within the bounds of the visible spectrum?

      You can see the light. So you do this: 1: Measure the mass of the galaxy. 2: Add up all the mass from the stuff you can see. Subtract (2) from (1).

    • Re: (Score:3, Informative)

      by Anonymous Coward
      They've been mapping Dark Matter based on gravitational lensing. "Nothing" doesn't cause gravitational lensing, so we know something is there, and whatever it is, there is almost 10x more of it than what we can see in the entire EM spectrum from radio to gamma.
  • Astrophysicist kept his foot on the scale again.
  • Does the new estimate for the mass of the Milky Way galaxy change the expected dynamics when the Milky Way and Andromeda galaxies collide in about 4 billion years [nasa.gov]?
  • by Anonymous Coward

    For your convenience.

  • I've been taught that Andromeda has approx. 2x as many stars as the Milky Way. I learned this years ago, as far as i can remember.

    Is it really surprising news that a galaxy with twice as many stars is twice as massive? Were these researchers just fact-checking?

    • by Bill, Shooter of Bul (629286) on Wednesday July 30, 2014 @11:41AM (#47566513) Journal
      Well, to be fair, most of the mass is *not* in the stars, but the dark matter. It might be a reasonable inference that twice the stars would also mean twice the dark matter, but that might not necissarily be true.
      • by buback (144189)

        So then really what they are confirming is that inference; that matter and dark matter are in a 1/9 ratio, and that if our galaxy has x stars and Andromeda has 2x stars, then Andromeda will also have 2y dark matter mass to our y matter mass.

        That seems to be a more interesting finding.

        • by buback (144189)

          *2y dark matter mass to our y dark matter mass*

          It would be 18y dark matter to Milky Way y matter.

  • by necro81 (917438) on Wednesday July 30, 2014 @11:35AM (#47566459) Journal
    Starting from the Earth getting kicked out from the center of the universe to the present hypothesis that visible matter is just a tiny fraction of all the stuff in the universe, having the mass of the Milky Way reduced is just another step in what Carl Sagan called The Great Demotions [google.com]. Hopefully by now humanity is getting used to it.
  • If they can be that wrong about something so fundamental, then how can they possibly claim to understand things or be right now?

    I read an article recently about scientists saying the speed of light is not constant. Has their new variable speed of light calculation been plugged into all these other cosmology equations? Maybe this dark matter fudge factor would disappear and we would stop being wrong by the 1/2 the mass of a galaxy.

    • If they can be that wrong about something so fundamental, then how can they possibly claim to understand things or be right now?

      It's not like they discovered that Andromeda is actually a 20-foot wide disco ball with funhouse mirrors making it look bigger than it really is. When you're talking about a branch of science that typically works in orders of magnitude, a factor of 2 is a pretty minor change.

    • by Anonymous Coward

      There is such a thing as degrees of wrong.

      Using Newton's equations and constant gravity on a particle-in-a-vaccum to calculate that I'd be in freefall for 1.8 seconds if I were climbing and fell at the end of a 25 foot runout is "wrong" because I'm not a particle, I'm not in a vaccum, gravity isn't position independent, and one really ought to be using GR instead of Newton. But 1.8 seconds is close enough for the purposes of knowing how long until the rope starts to catch me.

      Meanwhile you're acting as if kn

    • by geekoid (135745)

      "I read an article recently about scientists saying the speed of light is not constant. "
      scientist selling a book. Not scientists, and certainly not consensus.

      And what do you mean so fundamental?

      • But maybe its harder to measure MilkyWay than to measure the mass of other galaxies. Still, they cannot directly measure the mass of anything out there. So they are implying the mass by looking at the light coming from them and from neighboring objects. I would rather look forward to them being proven wrong about their assumptions because we would learn more. Maybe they might learn something that could help us out here on Earth.

        So I prefer to look at theories which challenge the accepted science in

        • We can look at a whole bunch of galaxies from a distance, and see the whole galaxy (except for the dark matter) somehow or another. There's precisely one galaxy that we can see from the inside, and we don't have a good global view of it. It's easier to categorize things we can observe in a similar way.

  • The article says that most of the galaxies are moving apart, but the Local Group is moving closer. Why would the local group be different than the other galaxies? Are there other groups of galaxies that are seeing the same effect, or is the Local Group an anomaly?

    • by PvtVoid (1252388) on Wednesday July 30, 2014 @12:25PM (#47567077)

      The article says that most of the galaxies are moving apart, but the Local Group is moving closer. Why would the local group be different than the other galaxies? Are there other groups of galaxies that are seeing the same effect, or is the Local Group an anomaly?

      The galaxies in the local group are close enough together to be a gravitationally bound system, and are therefore "decoupled" from the expansion. This is true of any cluster of galaxies, and there are many, many examples of such systems in the universe.

      It's the same reason your body doesn't get bigger as the universe expands: the binding forces holding it together are stronger than the (tiny) force pulling it apart due to cosmological expansion.

      • your body ... gets bigger as the universe expands

        So THAT'S why I'm so fat -- and here I was afraid it was somehow my fault. Sure glad to hear it's not.

        Pass me that last piece of pie, would you?

    • I'd assume that all galaxy groups are Gravitationally bound, and when looking at the group you're in, the galaxies would appear to be closing, while the other groups would appear to be opening; this is an effect of Hubble's law, everything is moving away from any observer at 67.80±0.77 (km/s)/ Mpc, thus the farther away, the faster it is going away no matter where you are . Even at that, I've seen several Hubble images showing galaxies colliding just like we're about to do with Andromeda.

  • Therefore, the Milky Way's mass is less than zero. What a difference an "-ly" makes. They should have sent their words to Lolly's.

  • Is it possible that other stars are just hidden behind other stars and that contributes to a large portion of the missing mass?
    • by Anonymous Coward

      It's possible. The probability that a given star is behind any star is about 10^-15. To get a large portion of the Milky Way stars behind other stars, we're talking about a probability of about 10^-1500000000000.

    • Is it possible that other stars are just hidden behind other stars and that contributes to a large portion of the missing mass?

      No. If hidden by dust, we'd see more infrared heat in space as light emitted by stars has to go someplace. If hidden directly behind other solid objects (besides being so astronomically against the odds that things are only hidden from us), it wouldn't account for observations of galaxies rotations speeds that we see along the axis of rotation rather than against the edge. Even for the galaxies we see on edge, if they were all weighted with the mass on the other side of where we are, the rotations speeds, l

      • Dark matter is the only real solution left standing at this point and the astronomers and scientists of the world had to be drug to that conclusion, kicking and screaming, over the decades, long before the public started hearing about it.

        Maybe if the ones who started talking about it had used a diffrerent term than "dark matter", it would be easier to accept. We hear about quarks, leptons, muons, and things with spin and flavor, etc. I don't understand all that, since I am not a scientist, but I can believe it is serious. Calling it "dark matter" was a dumb move, because it makes it sound as believable as "pixie dust" or "magic beans". At least, they could have used the Japanese words for it like they did with "tsunami".

        • So you're complaining that it was named in the same spirit as black holes, quarks, gluons, and color (as applied to quarks)?

          • Actually, just the opposite. I even mention quarks as one of the 'weird' things that they made up a term for. "Black hole" isn't all that unusual of a term, and is quite easy to make a mental image of. Color goes with flavor and spin, which are simply properties of things, and the terms can be used creatively with little worry.

            But calling the stuff that we can't see, but know it must exist because the equations say so, as "dark matter" was just stupid. The name causes more confusion than it should, even in

  • New name (Score:5, Funny)

    by gmuslera (3436) on Wednesday July 30, 2014 @12:10PM (#47566909) Homepage Journal
    Marketing suggested that now it should be called the Skim Milky Way.
  • Did they add the mass of the supermassive black hole at the center of the galaxy?
    • by wierd_w (1375923)

      I am more interested in radical ideas, like anti-mass, being at work.

      Say you have a large aggregation of mass that is orbiting a large, semistationary singularity-- like, a galaxy does.

      Outside this rather bumpy gravity well, you have a diffuse cloud of antimass, which then pushes on, and chases the mass as it rotates around the central mass. This pushing cancels out the centripetal force.

      It's an interesting idea, as it was recently postulated that there is no real compelling reason for antimass to not exis

      • by geekoid (135745)

        Since E=mc2, have a negative mass would mean negative energy.

        How do you have negative energy content?

    • by tomhath (637240)
      Yes, but it turns out to be only a Mediocre Massive black hole
  • While it has been widely accepted in recent years that Dark Matter fixes the standard model. Increasing problems suggest that Dark Matter might not actually exist [dailygalaxy.com].

  • Helioshperes as well as our own Ionosphere (magnetic field) have shown that magnetism is a universal force to be reckoned with. Indeed gravity could be just a magnetic effect on an atomic scale. The more mass the more effect. Electrons pulsing in a atom synchronizing with identical atoms become a united energy. That's what holds things together. And what gravity is, basically. A united atomic force. Too simple? .
    • Okay, you've strung words together in an evocative way. (Honestly, I don't know of any way electrons "pulse", or any way atoms synchronize with identical atoms - not to mention that atoms with different numbers of neutrons behave very similarly in many way.) If you can make some sort of mathematical model or theoretical framework that agrees reasonably well with current observations, we can start considering it.

      In the meantime, remember that we have ways of detecting magnetic fields, and they aren't st

  • by Lumpy (12016) on Wednesday July 30, 2014 @03:04PM (#47568629) Homepage

    The commonwealth is a lot smaller than expected!

    Yes, I went there.... Deal with my vast knowledge of really bad SciFi!

    • by geekoid (135745)

      OTOH, Quark's garbage route will be shorter.

      I saw your bad SCI-F, and raised you :)

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