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.

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.

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: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).

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: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.