What Exactly Is a Galaxy?

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.

Comments

Samsung

[Anonymous Coward]

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

Voting?

[SilverHatHacker]

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).

Re:Voting?

[Takichi]

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)

Re:Voting?

[pushing-robot]

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

Re:Voting?

[Daniel_Staal]

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?

[Fat Cow]

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.

Re:Voting?

[icebike]

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).

Re:Voting?

[jd]

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 like distance in terms of fundamental constants. Distance is defined relative to the speed of light, for example.

Re:Voting?

[56ker]

And there was me thinking it was a chocolate bar. :P ;)

Re:Voting?

[empiricistrob]

Just like a meter isn't a scientific term. No theories depend on the definition of a meter.

Re:Voting?

[ThatMegathronDude]

Linux is strictly a kernel, but that doesn't stop the colloquial usage of "linux == operating system", when what is meant is "linux-based operating system".

Community standards

[paiute]

I know a galaxy when I see one.

Can Pluto be a galaxy?

[ewg]

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

Better Be Careful...

[sottitron]

...or we might wind up living in the Milky Way Cluster

Re:Better Be Careful...

[jason.sweet]

Wish I knew what you were looking for.
Might have known what you would find.

Standard for astronomy.

[Anonymous Freak]

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.

Re:Standard for astronomy.

[bunratty]

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.

Re:Standard for astronomy.

[Anonymous Freak]

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

Re:Standard for astronomy.

[Tynin]

...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.

Re:Standard for astronomy.

[bunratty]

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 21st birthday is arbitrary. I could happen at 19 years, 4 months, 18 days.

Generally speaking...

[Dan East]

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

Depends on model year

[boristdog]

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.

Supermassive Black Holes?

[bill_mcgonigle]

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?

Re:Supermassive Black Holes?

[buchner.johannes]

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

Re:Supermassive Black Holes?

[scharkalvin]

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.

Doesn't matter

[crow]

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.

Re:Doesn't matter

[BitZtream]

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 set of parameters that they match. Names are typically completely arbitrary parameters that can be applied to anything willy nilly but the name is useless until the list of what it applies too is well known between those who communicate. Do you know what Abremarlereed means? No? See what happens when you just make up words?

Scientists need to know what things are called so they can effectively communicate their ideas. What good is science to anyone if the knowledge can not be communicated effectively and clearly. To do so, we need to parameterize things so we can generally classify objects into useable groups. Without doing so, every scientific test would have to test everything in the known universe because we'd have no parameters to use to throw out things we know we don't need to test.

In order for a scientist to tell you how something works both of you need to know what the words mean.

Contrary to the popular opinion of random people on the Internet, Anarchy doesn't work no matter how many times you say it.

Use the Carl Sagan Scale

[jameskojiro]

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...

Re:Use the Carl Sagan Scale

[owlstead]

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

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 the "Who gives a shit", maybe alien life forms living on them, but certainly not me.

Re:Use the Carl Sagan Scale

[StikyPad]

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.

Ostensible Definition

[jimmerz28]

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.

You both win!

[A nonymous Coward]

Is it a flip- or slide-open cell phone? No? Then it must be a candy bar cell phone.

Re:Distant Galaxy Now even Further

[michaelwv]

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 received at the same time has measured by those clocks, how long would that ruler be? 32 billion light years

There are other definitions, and you can make this arbitrarily more complicated by considering moving reference frames.

Re:Distant Galaxy Now even Further

[cforciea]

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!

The general populace should not vote

[geekoid]

for this sort of thing. Experts in the field should decide because they are the ... experts.

Radial categories

[Estanislao Mart�nez]

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.

Do we, really? People keep assuming that this is the case for all sorts of cases when I don't see why it should be so. See also, e.g., whether Pluto is "really a planet."

My alternative guess: the laws of nature don't care what clumps of stuff are "really" galaxies and which are "really" clusters, and all sorts of intermediate cases are possible. In this case, "galaxy" and "cluster" are best seen as radial categories with fuzzy boundaries: they have central cases that are the "best" example of properties that often occur together, but also all sorts of non-central examples that depart from those central cases in a bunch of ways, until you hit a bunch of gray examples where there's just no principled criterion to decide whether they fit into the category or not.

Prime example of radial categories: colors. There's a "focal" shade that's the center of the category; shades close to a focal one will be judged as being definitely that color; shades that are far from any focal shade will be ambiguous. None of this gives rise to a dispute as to what shade we're talking about, because shades are completely described by parameters that don't need to settle what discrete color category they belong to.

These sorts of radial categories are useful as long as we don't confuse them for concepts of the actual scientific theory, like spacetime or matter/energy.