h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Galaxies - An Introduction - A893937
Author: Tango - Guru, Scout - U32077

This came from the FM (my first attempt at a FM entry...), it was originally at A565201 and part of a Uni Project, the FM thread was at F74125?thread=227834. There wasn't much to change, and i think it is now ready.

Tango

Tango, me old mate,
This has the germ of a good article. But UI think it's too short, despite the fact there is so much could be said about galaxies and it's difficult to find a balance at the best of times. I'd suggest you comment about (a) the presence of dark matter and (b) the difference between the elliptical and spiral galaxies, and their differing life histories.

Hope this helps
Toodle Pip
FM

Shapes of galaxies! I completely forgot about those!! I will certainly add that. How is dark matter specifically relevent to Galaxies? As it says in the title, this is only indended to be an intro to galaxies. There was going to be a whole project on them, maybe I will do some more entries on them later, there is really too much to include in one entry. I will go and add the shape info, thanks for reminding me.

Tango

Ok, i have added some info on shapes (how i forgot those in the first place i will never know) and also a few header, as it was getting a bit longer I though they might help. What do you think now?

Tango

Much better, I think. It might still be a bit too brief, though.

A few pertinent points:
* Galaxies collide, and quasars come about when they do so in such a way that the interstellar gas and dust of opne gets sucked into the central black hole of the other. How do we know this? Well, quasars can turn themselves on and off in a matter of about a week. No stellar object can turn itself on in less time than it takes for light to tavel across it, so this puts an *upper* limit of diameter the active region of the quasar to about a light-week at maximum. Something that small must be very powerful, and only one type of object fits the bill. Also, there were a lot of quasars at the beginning of the universe (which is why they are so far away) and colliding galaxies are likely to have been much more common when the universe was young, as they were so tightly crammed in together. Ou galaxy is dues for a big crunsh with the Andromeda spiral at some point.
* Elliptical galaxies contain old, reddish stars and little gas. Spiral galaxies contain younger stars and lots of gas. The former 'burned out' in an orgy of star formation very early on in their lives, and are now in their senescece. Look up the Hubble Deep field picture and you'll see what early galaxies looked like (see http://www.stsci.edu/ftp/science/hdf/hdf.html).
* We know that dark matter exists because of observations of Doppler shifting of starlight in distant galaxies. The speed at which the rim travels around the hub shows that there is a lot of mass holding them together, but the combined light output from the galaxy indicates that most of it (about 90%) is non radiating. So 'dark matter'was hence discovered.
* Diffuse galaxies are interesting: these are gigantic spiral galaxies which are very sparsely spread out. Malin II is an example of a diffuse galaxy.

However interesting all that might be, (oh, and are you sure Quasars are the result of collision? i've never heard that even proposed before, the cartwheel galaxy was a collision, it certainaly didn't make anything like a Quasar) this is intended to be an introduction (hence the title), I think that kind of detail can go in separate entries. Does anyone else have an opinion?
Tango

OK, fine. But do put in a link to the Hubble Deep Field; it's amazing.

Yeah, i'll put in the link. It is a great picture.

Tango

Still a bit thin I reckon...

How about some detail about the classification system (developed by Hubble?) or some more about the relative ages of different types of galaxies or even something about groupings (local group, Virgo group, etc.)?

There's a lot of detail missing for even a rudimentary introduction imo.



- Jimi X

Thanks, there is info on groups though, under the "Clusters" heading. I will go into some more detail when i get home about the rest.

Tango

Hi there,

Very interesting stuff but I agree that the article is rather thin as it stands.

What about a bit more info about how they are constructed (FM mentioned a 'rim' and a 'hub', terms that don't appear in the article). And also I felt that the Active Galaxies paragraph was missing a lot of information. It says some interesting things, e.g. radio galaxies have as much radio emission as light emission (or whatever) but that's it. Why should this be? What is the current theory?

A couple of other things:

>>held together by the attraction between each of them caused by gravity

is it right to say that the attraction is *caused* by gravity? Shouldn't it be 'gravitational attraction'?

>>Hubble made the breakthrough that put galaxies in their rightful place.

What breakthrough? What rightful place?

>>Others are, of course, completely irregular, although they are far fewer of these than one might expect.

Why of course?

Is there a difference between a group and a cluster? They seem to be interchangeable terms from this entry.

Last minor point: there is a great deal of grammatical errors in the entry at the moment.

> that includes out galaxy< our galaxy!


Alji the Magus

out -> our is typographical, not gramatical!

The rest I will look into when I get home from school.

Tango

Ok, i give up, i'm going to get rid of the "an introduction" bit and write a full entry, but it will take some time, so be patient. (but i'm not including the quasars being made by collisions though, because i have never heard it before, if someone can give me a link to a reputable astromony site, i might reconsider.) I'll get started when my mocks are finished, or maybe at the weekend, we'll see.

Tango

Hi Tango

If/when you consider my humble suggestions, please bear in mind that I know very little about the subject, which is probably why I think an *introduction* level entry would be a valuable addition to the guide

Although I have long been interested in (and fascinated by ) the subject, my knowledge of it is still rather minimal, but this entry (as is) has definitely whet my appetite



<< Seyfert galaxies have unusually bright centres sending out radiation from its centre >>

What type of *radiation*? When I read this word, I thought of the nuclear variety, then wondered if it was more innoccuous ~ like heat, light etc


<< (some are in both catalogues). >>

Perhaps a footnote at this point mentioning the names of the catalogues, and/or a link to a site detailing both of them


<< "Tuning Fork Diagram" >>

This prompted me to use google's image search, which provided many links [such as http://oposite.stsci.edu/pubinfo/pr/1999/34/illustrations.html and subsequently http://imgsrc.stsci.edu/op/pubinfo/pr/1999/34/content/TuningFork.jpg ]
Maybe the community artists could create something similar...

PS the link from *stars* goes to A403930, which is not edited

The Stars page is the project page for a finished uni project (*my* finished uni project ), so the link should be ok.

I hope the artists can make a pic for the tuning fork, it would be very helpful.

The cutoff point for what should be in a intro is too confusing for me, so i'll do a full entry, just read the parts of it you want to.

Tango

I have started updating the entry. The shapes and events sections will be finished later. What do you think so far?

Tango

Looking good, so far. I await the rest of it.

Cheers!

Arghh! I'd completely forgotten about this entry! Some scout i am! I will do some more on it now, i think.

Tango

There you go, that's the shape section done. I will add some examples once i have looked them up. I will get onto the events soon.

Tango