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Space Science

What Exactly Is a Galaxy? 225

Posted by Soulskill
from the will-smith-will-figure-it-out dept.
sciencehabit writes "Surprising as it may sound, astronomers don't have an answer to this basic question. There's no agreement on when a collection of stars stops being a cluster and starts being something more. Now, in an echo of the recent wrangling over Pluto's status as a planet, a pair of astrophysicists from Australia and Germany want to start a debate on the issue — and they have even set up a Web site for people to cast their votes." While we're on the subject of galaxies, reader mvar pointed out that astronomers using data from Hubble have spotted what could be a new record holder for the most distant known galaxy, located roughly 13.2 billion light years from Earth.
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What Exactly Is a Galaxy?

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  • Samsung (Score:2, Funny)

    by Anonymous Coward

    It's a phone right? Most likely to be running Android

  • Voting? (Score:5, Insightful)

    by SilverHatHacker (1381259) on Friday January 28, 2011 @03:26PM (#35036330)
    It doesn't seem like the definition of a scientific term is something that should be left to a democratic vote. Public opinion with regards to science is never a good thing to rely on (creation vs evolution, naturalistic healing, etc).
    • by Takichi (1053302)
      Maybe so, but it's a pretty good way to get people to read your paper. First question from the survey:

      1. Have you read the paper by Forbes and Kroupa accepted for publication in Publications of the Astronomical Society of Australia entitled "What is a galaxy? Cast your vote here..." (its available at http://arxiv.org/abs/1101.3309 [arxiv.org])

    • Re:Voting? (Score:5, Funny)

      by pushing-robot (1037830) on Friday January 28, 2011 @03:32PM (#35036426)

      A galaxy is a massive, gravitationally bound system consisting of at least one Stephen Colbert.

    • Re:Voting? (Score:5, Insightful)

      by Daniel_Staal (609844) <DStaal@usa.net> on Friday January 28, 2011 @03:34PM (#35036478)

      Creation vs evolution is a discussion of theories and facts, and facts aren't really subject to public opinion.

      However, this is just definitions. All we really need is some coherent way to draw the line between the two, and it doesn't really matter what the line is. The comparison to Pluto is apt: it didn't really matter whether Pluto was a planet or not, except that science works best with consistent definitions, and either we could use a definition that included Pluto and a couple dozen (at least) other objects in our solar system, or we could use the definition that excluded all of them, including Pluto. Basically the decision was that there would be less public outcry this way, and it made more 'sense'.

      But it'd have been better to have the discussion earlier, which is what these people are trying to do: Hold the discussion early enough that the results will get used before the public at large get emotionally attached to the one or two border cases.

    • Re:Voting? (Score:5, Insightful)

      by Fat Cow (13247) on Friday January 28, 2011 @03:38PM (#35036558)

      It doesn't seem like the definition of a scientific term is something that should be left to a democratic vote. Public opinion with regards to science is never a good thing to rely on (creation vs evolution, naturalistic healing, etc).

      It's not really a scientific term. No theories depend on the definition of a galaxy.

      • by jd (1658)

        No, but I would consider a scientific definition to require some very specific collection of theories and to not require non-theorized constructs of any kind. (Thus, requiring a planet to be a specific size or in a specific location is NOT a scientific definition; requiring it to have certain properties that a well-defined group of planetary-like objects will all share and all definitely non-planetary-objects will not possess is a scientific definition.)

        It's the same way we define fundamental properties lik

        • by Kjella (173770)

          Somehow I suspect a galaxy to be more a paradox of the heap [wikipedia.org] kind of problem. A huge bunch of stars is a galaxy, remove one by one and at some point it stops being a galaxy. The question is what star turns a galaxy into a non-galaxy, I don't think there's a "fundamental property" to that.

      • Just like a meter isn't a scientific term. No theories depend on the definition of a meter.
    • Re:Voting? (Score:5, Interesting)

      by icebike (68054) on Friday January 28, 2011 @03:49PM (#35036732)

      It doesn't seem like the definition of a scientific term is something that should be left to a democratic vote. Public opinion with regards to science is never a good thing to rely on (creation vs evolution, naturalistic healing, etc).

      But you miss the central point of the story.

      There is no formal definition, scientific or otherwise. Its just a term in common usage with no universally agreed upon definition.

      As such voting is as good a method of arriving at a definition as any other, and certainly a better method than was originally used (namely no method at all).

    • by Belial6 (794905)
      You are misunderstanding the issue. When defining a scientific term like galaxy, vote is perfectly reasonable, as long as the definition is consistent. That is how naming things work. By definition, names are the sounds that people agree apply to specific things. Thus if the population decides that the sounds that make up sky mean the area in the air above us, then that is what it is. Creation vs. Evolutions, are not things that change by what we believe. We are either correct or incorrect. naturalis
    • Re:Voting? (Score:4, Funny)

      by 56ker (566853) on Friday January 28, 2011 @04:45PM (#35037470) Homepage Journal
      And there was me thinking it was a chocolate bar. :P ;)
    • by shmlco (594907)

      "and they have even set up a Web site for people to cast their votes."

      Yep. Counting noses is the very essence of the scientific process.

    • by g2devi (898503)

      True, but it's the nature of the problem.

      Before the periodic table, the elements were grouped by non-essential properties like boiling points, conductivity, colour, etc. The problem with such groupings is that different people group things differently. With such groupings, some kind of voting or fiat declaration is the only way to define the things.

      When the periodic table was created, there was finally a grouping based on essential properties, so no such voting is required.

      If you want to avoid this problem,

  • by paiute (550198) on Friday January 28, 2011 @03:27PM (#35036344)

    I know a galaxy when I see one.

  • by ewg (158266) on Friday January 28, 2011 @03:28PM (#35036364)

    Pluto's not a planet, maybe it's a galaxy!

    • by Suki I (1546431)

      Pluto's not a planet, maybe it's a galaxy!

      Not swirly enough, but I like your thought :)

  • by sottitron (923868) on Friday January 28, 2011 @03:29PM (#35036376)
    ...or we might wind up living in the Milky Way Cluster
  • by Anonymous Freak (16973) <prius.driver@nOSPam.mac.com> on Friday January 28, 2011 @03:29PM (#35036384) Journal

    Lots of astronomical terms are very vague in their definition. Heck, "planet" was only officially defined a couple years ago.

    There is no "official" difference between "ocean" and "sea", either.

    • Lots of astronomical terms are very vague in their definition.

      True. Does the difference between a Cluster and Galaxy really amount to that much anyway? Pluto at least we were familiar with as a planet and a couple generations of kids, at least, were taught it was a planet. So chaning it actually did have some affect. If Cluster HG42 gets reclassified as Galaxy HG42, does that really change much?
    • by bunratty (545641) on Friday January 28, 2011 @03:42PM (#35036628)
      It's the same for other subjects. In biology, there's no clear definition of the term species. You can define a species as a group of animals that can reproduce sexually with one another, but as far as I know there's no good definition of species for organisms that reproduce in other ways. Sometimes definitions are completely arbitrary, such as the difference between a tropical storm or a hurricane.
      • by Tynin (634655) on Friday January 28, 2011 @06:20PM (#35038620)

        ...such as the difference between a tropical storm or a hurricane.

        Sorry, but this isn't so.

        Tropical Storm = Distinct rotary circulation, constant wind speed ranges 39-73 miles per hour (34-63 knots).

        Hurricane = Pronounced rotary circulation, constant wind speed of 74 miles per hours (64 knots) or more.

        I'm guessing you don't live in an area that regularly gets hit by these storms, as I really though this was common knowledge. Nothing arbitrary about it, unless we are using different definitions of arbitrary. Source: NOAA [noaa.gov].

        • It's a matter of scope.

          It's not arbitrary at a personal use level because you can't personally decide, by whimsy, that a 59 knot storm that was particularly devastating was in fact a Hurricane.

          But it is arbirary at a definition level because 64 knots has no particular meaning, and it could just as easily have been some other number without losing any real meaning.

        • by bunratty (545641)
          I suppose we are using different definitions of arbitrary. Why couldn't the cutoff be at 70 mph or 80 mph? If it could, then the 75 mph cutoff is arbitrary. Maybe you're saying it is not arbitrary whether a particular storm is a hurricane or a tropical storm, yes. But the clear line of distinction between them is arbitrary. Just like when there's a law that you must be 21 years old to buy alcohol. It isn't arbitrary whether you're allowed to buy alcohol, but the fact that the difference happens on your 21s
      • by jamesh (87723)

        You can define a species as a group of animals that can reproduce sexually with one another,

        It's not even that simple even for animals that reproduce sexually. Horses and Donkeys can breed together, as can certain other pairs of animals of close but definitely different 'species'. You might then decide to add a sub-clause that the offspring must also be able to reproduce sexually, but in some cases even that doesn't clear things up. And even worse, you can have groups A, B, and C where A and B can breed together, B and C can breed together, but A and C can't.

        It's a bit like the idea of 'race'. Bef

      • Even basic stuff in celestial mechanics... What constitutes an "orbit?" Why do we say the Earth orbits the sun? A better approximation would be that the earth-moon system and the sun both orbit the barycenter of those three bodies -- though even this is an approximation.

        It may seem "obvious" in the context of the universe immediately around us, but how do you formally define it so that if, say, you're running an n-body simulation on a computer with all n(n-1)/2 interaction forces, the program can automati

    • by iammani (1392285)

      There is no "official" difference between "ocean" and "sea", either.

      Is there a difference (even unofficially)?

      • The best one I've heard of is that it's an ocean if it has ocean in its name. That's it.

      • by Belial6 (794905)
        Well, I have heard of inland seas, but never an inland ocean, so someone seems to have a line between ocean and sea. Not sure what it is, but it seems to be out there.
        • by nschubach (922175)

          So what about lakes and ponds?

          I think people are too quick to rush to naming and categorizing stuff (in Astronomy and in life)... can't all bodies floating in space be called something like Satellites? Bound and unbound, just like landlocked and non-locked bodies of water. Earth would be a Bound Satellite where "Bound" was a term used for something with a regular orbit.

  • The interesteing thing not mentioned about the Distant Galaxy in the article. eventhough it's position 13.2 billion years ago was that far away from our current position, it is currently probably more like 45 billion light years away!
    • by bunratty (545641)
      Doesn't it depend on your frame of reference? In our frame of reference, it really currently is 13.2 billion light years away, isn't it? It's funny how many comments that involve relativity seem to implicitly assume there is one preferred frame of reference. Talk about not getting the point!
      • It's not really about preferred frame of reference, it's the different meanings of distance that arise in general relativity. In this case there are two meanings being discussed:
        1) How long has that photon been traveling to get to Earth ("light travel time")? 13.2 billion years
        2) How far away is that galaxy right now ("proper distance"). I.e., if each galaxy had a clock that counted seconds since the Big Bang and could instantaneously extend a long ruler to the other galaxy and the ruler was sent and
        • by kenj0418 (230916)

          2) How far away is that galaxy right now ("proper distance"). I.e., if each galaxy had a clock that counted seconds since the Big Bang and could instantaneously extend a long ruler to the other galaxy and the ruler was sent and received at the same time has measured by those clocks, how long would that ruler be? 32 billion light years

          So, if there was no such thing as Relativity then. Didn't you just prove his point?

        • by cforciea (1926392)
          Methinks you do not grasp what general relativity has to say on the concept of simultaneity. You have to pick a frame of reference to talk about something happening "at the same time" because observing the same events at a different speed will cause them to occur at different times. To quote GP:

          It's funny how many comments that involve relativity seem to implicitly assume there is one preferred frame of reference. Talk about not getting the point!

          • My understanding of general relativity, special relativity, and cosmological expansion are perfectly fine. But my ability to explain things clearly and unambiguously are probably more suspect. I was trying to explain why there were perfectly reasonable reasons to talk about several different definitions of distance.
            "In our frame of reference, it really currently is 13.2 billion light years away"
            We have to precisely define "currently" and "away"? There are several reasonable and different definitions
      • by ceoyoyo (59147)

        No, it's more a matter of space expanding. The 13.2 billion light years figure is how far a photon arriving at our galaxy had to travel. The 45 billion light years figure is how far a photon leaving now would have to travel, if the universe weren't expanding. Another, even bigger value, would be how far a photon leaving now would have to travel with the expansion of the universe at it's current rate (i.e. if we wanted to go there, how far would we have to go?).

        All these measurements are in the frame of r

      • I'm not an expert on relativity, but I'd love to understand your point if it's accurate. As pointed out by michaelwv, we know that the photons we are seeing now left that galaxy 13.2 billion years ago, so where we see it now is it's position then relative to our current position at this time. We also know that the universe has expanded a great deal since then (especially since the age of the universe is 13.75 billion years old give or take). Hence, from our perspective the actual current physical positio
  • by Dan East (318230) on Friday January 28, 2011 @03:34PM (#35036472) Homepage Journal

    Typically they are something far, far away and a long time ago. At least from our perspective that is.

  • by boristdog (133725) on Friday January 28, 2011 @03:37PM (#35036554)

    Your older Galaxies had more limited trim packages and slightly smaller engines. After 1969 the engines became larger overall and were available in a wider array of trim.

    The two-door convertible with a 400 cu. inch engine would be my choice.

    Should NOT be confused with an Impala.

    I hope that helps.

    • by mcmonkey (96054)

      I hope that helps.

      Danm. I blew all my mods points on jokes in the Columbia thread.

    • by jd2112 (1535857)
      I thought it was a line of Android based snartphones and tablets manufactured by Samsung.
  • by bill_mcgonigle (4333) * on Friday January 28, 2011 @03:45PM (#35036674) Homepage Journal

    I thought galaxies were determined by the presence of a supermassive black hole as its primary gravitational organizer ... but the paper doesn't even contain the word 'black'. Globular clusters sometimes have medium-mass black holes, but no supermassive ones.

    Is my knowledge rusty?

    • I thought galaxies were determined by the presence of a supermassive black hole as its primary gravitational organizer ... but the paper doesn't even contain the word 'black'. Globular clusters sometimes have medium-mass black holes, but no supermassive ones.

      Is my knowledge rusty?

      Not even rusty, this has never been the case. Only in the last decade it became known that black holes are in the center of most galaxies. Also, the black hole is pretty irrelevant* to the galaxy as a whole, except for the few surrounding stars, it is not the "primary gravitational organizer", it just happens that in the center, so much mass accumulates that photons can't escape. That's all.

      *milky way galaxy number of stars = 3e11, black hole mass = 3.7 million stellar masses

      • by scharkalvin (72228) on Friday January 28, 2011 @04:22PM (#35037162) Homepage

        Actually the formation of a black hole in the galaxy center may be the norm for galaxy formation. In fact, it may be a requirement to separate a true galaxy from just a cluster of stars. A true galaxy forms when a huge collection of gas condenses into groups of stars. A young galaxy forms a massive black hole at the center where the collection of gas is the densest. Then the galaxy goes through a Qusar phase where it emits two jets of energy formed by the accretion of matter into the black hole. Once all the nearby matter has been accreted into the black hole the Qusar shuts down.

    • by Locke2005 (849178)
      For a long time, galaxies were assumed to be rotating around their center of mass, held together by mutual gravitational attraction. Only lately have we been able to prove that most if not all galaxies have a supermassive black hole at their center.
  • Doesn't matter (Score:5, Interesting)

    by crow (16139) on Friday January 28, 2011 @03:45PM (#35036676) Homepage Journal

    How you define "planet" or "galaxy" is very much in the nineteenth century scientific mindset of categorizing everything. Haven't we moved beyond that? Names and categories are useful as a way of generalizing a set of characteristics, but if you don't like a given definition, make up a new term for the set of characteristics that you want to generalize about.

    Language is not scientific, and it never will be. We can have starfish that aren't fish and koala bears that aren't bears, and that's just fine. Scientists need to be concerned about how things work, not what they're called.

    • You forgot pineapple. Not pine, nor apple. It's actually a berry. Who's fart-brained idea was that anyway?

      And while we're on the subject, can we PLEASE rename September, October, November and December? Or just move New Year's day to March 1st. to get the name-prefixes back in sync.

      Jeez. No wonder our kids are so confused.

    • by BitZtream (692029)

      Language is only useful if we all mean the same thing when we use the same sounds.

      You don't get to randomly make up new words and still communicate effectively unless you communicate the definition of the word as well somehow.

      While a starfish may not be a fish, everyone still knows what you mean because we've established a definition elsewhere in our lives.

      It doesn't matter if you Abremarlereed. The word is useless if you don't know what it means. Names and categories are not useful unless they define a s

  • by jameskojiro (705701) on Friday January 28, 2011 @03:49PM (#35036740) Journal

    Trillions and Trillions of Stars = Super Galaxy

    Billions and Billions of Stars = Galaxy

    Million and Millions of Stars = Dwarf Galaxy

    Thousands and Thousands of Stars = Stellar Cluster

    Hundreds and Hundreds of Stars = Dwarf Stellar Cluster

    Tens and Tens of Stars = Who gives a shit...

    • by arth1 (260657)

      Pfft. By that measure, the universe is a galaxy.

    • by Grishnakh (216268)

      Tens and Tens of Stars = Who gives a shit...

      Does such a thing even exist? I mean, are there stars floating around out there that are not in a galaxy (or cluster), or even a group of only tens of stars? Or do stars only occur in larger groups (hundreds or more)?

      • by owlstead (636356)

        As stars are sometimes flung out of galaxies during "collisions", I guess there must be lonely stars out there. And apparently they don't even always stay in the cluster out of themselves. It would not be a great surprise if a few went on a trip together.

        http://en.wikipedia.org/wiki/Star_system [wikipedia.org]

        I guess they do travel together :), of course these are stars that are considered part of our galaxy (it presume it would be a bit difficult to detect a single star travelling all by itself outside a galaxy).

        As for th

    • by StikyPad (445176)

      That should clear things up nicely.

      I've discovered a new..

      Who gives a shit!

      Exactly. How'd you know?

      No, I mean.. who gives a shit?

      Yes.

  • In my opinion, w galaxy should be a group of at least 1,000 stars orbiting one or more black holes
    • In my opinion, w galaxy should be a group of at least 1,000 stars orbiting one or more black holes

      I think your definition fits our current known facts [wikipedia.org] succinctly.

      A more important question is whether the definition should extend to say that the galaxy is the simply the accretion disk that forms around the black hole center.

      Furthermore, star count could play a part in naming in the range cluster->dwarf galaxy->galaxy.

  • IANAAP, but I was up late last night thinking about this one (coincidentally).

    My vote: SMBh and dark matter separates GCs from galaxies nicely. However, I think that large numbers of extant stars should not be required; ie, dark matter galaxies are galaxies. In this epoch, at least.

  • Still smell as sweet?

    Naming and labeling things in science is as old as science itself. Often, though, as our understanding changes, so to must the old naming scheme. Usually the knowledge change becomes obvious in the scientific community - the facts are the facts, after all.

    What causes all of the consternation is almost always semantics about the classification.

    If there is a clear-cut scientific definition, go ahead and assign names and classifications.

    Too often, though, there is no clear-cut definition

    • by Belial6 (794905)
      And there is the answer. Just ad '-ish' to the end of all definitions, and the problem is solved. Who can argue that Pluto is a 'planet-ish' object. Of course it is. Really, you are mostly right. Getting hung up on which noises we should grunt out to describe a particular shade of gray isn't really that productive.
    • Still smell as sweet?

      FWIW, I always thought that Shakespeare's observation, though technically correct, was a strange sentiment for a poet.

    • by Draek (916851)

      They don't solve the problem they are supposed to solve, they make new problems, don't bother with them.

      Yes, they do. You may notice "cluster" is a much shorter description than "weird new not quite galaxy thing I found", and *that* is the very purpose of language.

      Otherwise, we may as well call galaxies "bunch of bunches of fissioning hydrogen" instead. And God help you if you ever want to talk about sociology...

  • We have this ambiguity all the time in language, debating it might be "interesting" but is really useless.

    What's a "house"? How many rooms does it have? Is it a house if it has no bathroom/basement/attic? etc.

    Try the same thing with "chair".

    How do you know something is a "house"? You know when you see it. Just like teaching a child you point to it; ostensibly defined.

    • by geekoid (135745)

      except in this case, it need defining so we can catalog the finds appropriately.

      And house has a pretty good definition. It even has types based on size, attachment and location.
      In fact I wish are astronomical finds where as well defined and the different types of house.

  • The Sloan Digital Sky Survey [wikipedia.org] is a great scan of the visible universe.

    You can view it in Google Sky, NASA makes the raw data available, and you can even get a 3D crystal etching [bathsheba.com] of it.

  • And here I thought we were talking about Android OS fragmentation and Samsung's product line being completely doomed.
  • The universe is not so discriminating. Humans are just trying to categorize it, imposing artificial order on the chaos that results from natural order.

    That being said, a galaxy needs to be defined as a 1st order organization where the rotational mass is twice as wide as it is tall. Else, its a cluster.

    My orders are:
    -1 - multiverse
    0 - universe
    1 - galaxy / cluster
    2 - star
    3 - planet / belt
    4 - satellite (includes moon)
    5 - satellites of satellites.

  • He would know, of all people. It's that simple, really.
  • for this sort of thing. Experts in the field should decide because they are the ... experts.

  • The last time the definition of an astronomical term came up for debate, Pluto got thoroughly fscked. If that pattern holds, the Magellanic Clouds will be reclassified as "stellar dustballs."
  • How do we know that 13.2 billion year old light hasn't wrapped around the edge of the universe one or more times? That "distant galaxy" could be right next store, only a long time ago! With all the millions of galaxies we've discovered, has anybody checked to see if some of them look just like another galaxy, only viewed from a different angle and point in time? They checked to see if all snowflakes were different (they aren't), couldn't they do the same for galaxies?

    Ok, I'll go take my meds now...

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