Slashdot Mirror
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.
It's a phone right? Most likely to be running Android
Galaxies are social constructs.
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).
Funny may not give karma, but +5 Informative never made anyone snort coffee out their nose.
I know a galaxy when I see one.
If Slashdot were chemistry it would look like this:Cadaverine
Pluto's not a planet, maybe it's a galaxy!
org.slashdot.post.SignatureNotFoundException: ewg
...or we might wind up living in the Milky Way Cluster
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.
Another non-functioning site was "uncertainty.microsoft.com."
The purpose of that site was not known.
I didn't RTFA but it seems like there should be a mathematical formula for mass & distance which could define a galaxy easily. There are obviously many well known cases which could be used as examples for the model.
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!
Keep passing the open windows...
Typically they are something far, far away and a long time ago. At least from our perspective that is.
Better known as 318230.
Galaxies are social constructs.
That is how I lost my beloved Pluto! Now you people want to take away my pretty, swirly Galaxies! Wait, this one could go my way if I get a consensus on "swirly." I can concede pretty, but I am standing firm on swirly.
Home of The Suki Series
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.
...that the universe pretty much covers everything.
No need for an analogy.
http://www.acetonestudio.com
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?
My God, it's Full of Source!
OUTSIDE_IP=$(dig +short my.ip @outsideip.net)
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 know, many other terms don't have hard and fast definitions. What exactly is a "point", mathematically speaking?
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...
Tsukasa: All I really want, is to be left alone...
In my opinion, w galaxy should be a group of at least 1,000 stars orbiting one or more black holes
excitingthingstodo.blogspot.com
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, the labels don't correspond directly to the categories they are supposed to describe, and the 'formal' language fails to form a commonly accepted means of communicating your ideas, which was probably the whole point of assigning labels in the first place. SO DON'T ASSIGN THE LABELS! They don't solve the problem they are supposed to solve, they make new problems, don't bother with them.
Instead, let the language evolve as the knowledge does. I'm sure all practicing astronomers interested in galaxy scale structures share a roughly isomorphic understanding of what a galaxy is, and would agree about how to classify a pretty large subset of galaxy-ish objects. The interesting stuff - they things they actively research - will often be in the grey areas, anyway, defying classification. Don't worry about it, just go write the paper describing 'weird new not quite galaxy thing I found', and describe it as best as possible. The knowledge base will grow, the language will evolve right along with it, and we won't have to undo some silly bit of formalism the next time someone finds something that defies description.
-V-
Who can decide a priori? Nobody.
-Sartre
Sounds like what I studied in Elementary Logic.
http://www.logicalparadoxes.info/heap/
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.
Finally, someone's found my marbles!
If all galaxies have them, then they are the requisite phenomenon for a galaxy. To my knowledge globular clusters and alike don't have these.
The Sloan Digital Sky Survey 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 of it.
He's been through two of them, I bet he knows what a Galaxy is by now.
What do I know, I'm just an idiot, right?
How do you divide space? In the vacuum of space, what constitutes a clear, measurable boundary?
Geographical boundaries are often based upon some physical feature, a body of water, river, mountain, etc. Some are arbitrary, such as a line of latitude or longitude, but it's something that's pretty easy to identify and measure. As far as we know, you don't have such neat boundaries in interstellar space.
Even the concept of "gravitationally bound" isn't sufficient, as stars and planets have objects gravitationally bound to them, and clusters of stars are gravitationally bound to each other, and the stars and clusters are bound to the "galaxy", and clusters of galaxies are gravitationally bound to one another. Where does one level end and the next level begin?
make imaginary.friends COUNT=100 VISIBLE=false
Is it a flip- or slide-open cell phone? No? Then it must be a candy bar cell phone.
Infuriate left and right
What is a galaxy? A miserable little pile of planets! But enough talk, have at you!
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 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.
Make sure everyone's vote counts: Verified Voting
And here I thought we were talking about Android OS fragmentation and Samsung's product line being completely doomed.
it's that game where you control a spaceship at the bottom of the screen and you try to shoot all of the bug-looking aliens at the top of the screen.
Still smell as sweet?
FWIW, I always thought that Shakespeare's observation, though technically correct, was a strange sentiment for a poet.
Sheesh, evil *and* a jerk. -- Jade
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.
Slashdot's rate-of-post filter: Preventing you from posting too many great ideas at once.
The galaxy is here!
What do you mean ''here''?
Galaxy's millions of stars and planets.
How's it here?
You humans!
When will you learn size doesn't matter?
Just because something's important,
doesn't mean it's not very small.
I don't have an intelligent phone, so I need to be.
The IAU has some work ahead of it.
Having recently defined, what is a planet, which waited multiple millennia a formal definition; we now need additional IAU definitions. For in addition to what is a galaxy, there's also, what is a star?
For example, is a star restricted to a main sequence star or are degenerate objects like white dwarves and neutrons stars, stars? And if they are stars, then are black hole stars?
We wait with baited breath for further IAU amplification.
HRH The Duke of Windsor
The human brain gets fuzzy in matters of quantity. It must. If a "heap" had a very exact definition in terms of number of atoms or measurement of volume or mass, then we couldn't use the word in ordinary discourse. I wouldn't be able to say "there is a heap of salt on the table" without first measuring it using exacting scientific equipment.
If the definition of galaxy becomes to precise, we won't be able to determine whether much of what we see in the sky qualifies. The moment that happens, we will invent a new word with a convienent fuzzy definition to mean what "galaxy" used to mean.
Next we can debate the definition of an island and put it to a vote.
Or discuss what shades of turquoise are blue and which are green.
Language is imprecise.
And debating the meaning of words is not science, and even worse, it's not interesting.
He would know, of all people. It's that simple, really.
...the Arquillians WILL destroy the planet if they don't get it back.
Just doesn't have the same ring to it.
for this sort of thing. Experts in the field should decide because they are the ... experts.
The Kruger Dunning explains most post on
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."
!#@%*)anks for hanging up the phone, dear.
In 1947, we intercepted intergalactic mail carriers working for a private company, comparable to UPS, by the names of Bender B. Rodriguez and Doctor John A. Zoidberg.
The government does not want you to know this, but the first entity is a robot and the second is an employee of a company called Planet Express. They are registered with a United Nations sort of thing for the universe, the Democratic Order Of Planets. As a planet dominated by a violent species who tends to think all aliens are evil and must be killed if made contact with, at least that's how our broadcasts portray us, we are not yet invited into the DOOP.
Back on topic, this DOOP established a universal postal system in which the terms of a galaxy is defined. In order to be a galaxy, it must be assigned a galactic postal zip code in the universal postal system. In order to qualify for this definition of a postal region, it works much like the United States postal system. My interception of transmissions and espionage work in New Mexico have been a little poor so I can't quite explain all the details. But the point is clear, without the galactic zip code, it is not a galaxy.
I hope this knowledge can provide some guidance in the debate over defining what a galaxy is, so that there's no confusion for when we eventually join the DOOP in 2773.
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...
I've abandoned my search for truth; now I'm just looking for some useful delusions.
Then when two or more of them collide (as galaxies do), it will be known as a "cluster-fsck"!
I've abandoned my search for truth; now I'm just looking for some useful delusions.
A series of mobile devices from Samsung.
Aren't galaxies the cat's eyes in marbles? It's all a big flat hologram anyway. As Colbert said this week.
Our galaxy itself contains 100 billion stars ...
It's 100,000 light-years side-to-side
And our galaxy is only one of millions of billions
In this amazing and expanding universe
) Human Kind Vs Human Creation
) It'd be interesting to see how many humans would survive to serve us.
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.
Are you adequate?
I'm sure he'll find a way to tell us why not to call a galaxy a galaxy...
Yep, that's why currently many are being reclassified based on genetic comparison, although I'm not sure what the cutoff is and whether the same one is used by all biologists.
The fundamental idea of evolutionary theory is that any two organisms have a common ancestor. Think about that closely: it entails that there cannot be a principled line to be drawn between species, because no matter how you propose to draw such lines, organisms of any "species" must have bred with members of an ancestor species.
The only strictly factual claims that can be made are about which organisms were the ancestors of which. "Species" are just vague areas of interest in the tree of life; no lines need to be drawn. There's reclassification going on in light of genetics, but the only really meaningful cases are the ones that involve changing hypotheses about the shape of the evolutionary tree: "The most recent common ancestor of A and B is D, not C."
Are you adequate?
Not content with making us all look like total fuckwits with the whole debate over what makes a planet - as if it matters in the slightest - some dingus is going about kicking up a fuss about what identifies a galaxy. Seriously, who gives a toss? The important thing is the science, which doesn't change whether we call a particular virialised cluster of stars a "cluster", a "galaxy" or "peter". The taxonomy is beyond unimportant.
The only place I can see that this kind of thing has is in education (and, yes, perhaps talking with the general public). And even in that case... who cares? Who said that we have to have some serious rigorous definition for everything we say when communicating in a general manner? I can sit there and have a loose definition of the word "galaxy" that changes depending on context and not only will the journalist not notice any slight changes but *they won't care*, and neither will their readers.
Stirring up something like this, though, just makes us look like a pack of dithering idiots who don't know what we're talking about.
i thought that galaxy is defined as a group of starts, planets, etc. orbiting a central blackhole. but probably it's more of an observation than the definition though.
Live your life each day as if it was your last.
As we must also expect that the revolving Galactic Core Object will precess; then there will be times when such precession will throw out smaller energy and mass ejections, out of line of the main ejection, to form smaller globular clusters scattered around the eventual galaxies so formed.
The energy, (from the sometimes billions of stars that have formed the full mass of the Galactic Core Object), is stored in the balanced gravity band between the inner and outer event horizons inside, INSIDE, the structure of the Galactic Core Object. The breakup of the outer ring of mass, (and thus the process that releases the energy and mass), is described in great detail in chapter 42, The Whirlpool Galaxy in a new light, The Universe is a Cloud of Surplus Proton Energy http://www.lrsp.com/b2c.html
This is exactly the same process, but on a very much larger scale, that forms the energy ejections from Planetary Nebula and is the well known, but never publicised, "big (and often disorienting) leaps forward" that was written into the final paragraph of "The Extraordinary Deaths of Ordinary Stars", Bruce Balick et. al. Scientific American, July 2004, page 35.
A galaxy is thus an object formed from energy and mass ejected from an older galaxy where, over a very great period of time, the central mass of the newly ejected energy and mass has come together to form a galactic Core Object sufficiently massive to bring the rest of the energy and mass into an accretion disc. The stars within a galaxy cannot "bump" into each other because they are all gravitationally attached to each other and balanced gravity effects within such attachments; balance out the forces and stabilise the whole entity.
Disclosure: I was an astrophysics major in college.
Plot the mass of gravitationally bound objects vs number of objects from small groups up to the largest galaxies. If there are clumps in the mass distribution, then give names to the clumps (cluster, galaxy, etc). If it's a smooth distribution by mass, then call them all the same general kind of object, and distinguish them by mass classes. For example "10 million solar mass cluster".
If you can determine some other distinguishing characteristic like presence of a central black hole, or significant dark matter, then use that to determine what you call it, even if allows overlap in mass classes. For example:
10 million solar mass, no dark matter, no black hole = cluster
10 million solar mass, dark matter and black hole = galaxy
I don't know if my examples are correct examples, but the idea is look at the data, and see if there is some characteristic in common among the objects that makes it useful to describe that set with a unique name.