How small can a galaxy be?

[holocene]

So we know that a galaxy can be a thousands of light-years in diameter, but how <i>small</i> can a galaxy be?<br />

 

[qso1]

Good question. I guess the best way to arrive at some kind of estimate is to determine what galaxy is the current smallest one we know of.<br /><br />For now I'll venture to guess 25,000 light years diameter. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[holocene]

25,000 light years is still very big!<br /><br />Looking at the Hubble deep feild, it's amazing that every single point of light is an entire galaxy!<br /><br /><br /><br />

 

[qso1]

25,000 is big but just a guess on my part at this point. Those Hubble deep field images are awesome. It is hard to imagine so many galaxies, especially considering the size of most galaxies. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[search]

Discovery of an Unusual Dwarf Galaxy in the Outskirts of the Milky Way<br /><br />We announce the discovery of a new dwarf galaxy, Leo T, in the Local Group. It was found as a stellar overdensity in the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram of Leo T shows two well-defined features, which we interpret as a red giant branch and a sequence of young, massive stars. As judged from fits to the color-magnitude diagram, it lies at a distance of 420 kpc and has an intermediate-age stellar population with a metallicity of [Fe/H] = -1.6, together with a young population of blue stars of age 200 Myr. There is a compact cloud of neutral hydrogen with mass 105 M and radial velocity +35 km s-1 coincident with the object visible in the HIPASS channel maps. Leo T is the smallest, lowest luminosity galaxy found to date with recent star formation. It appears to be a transition object similar to, but much lower luminosity than, the Phoenix dwarf.<br /><br />Astronomers discover smallest galaxy ever<br /><br />"Astronomers have found the smallest galaxy yet recorded, about one-sixteenth the diameter of the Milky Way.<br /><br />"Dwarf spheroidal galaxies (dSphs) are the smallest stellar systems showing evidence of substantial dark matter. They are also vivid reminders of the discrepancy between the numerous surviving dark matter subhalos predicted by cold dark matter (CDM) and the relatively few dwarf satellites observed in the Local Group (the "missing satellite" problem). Between 1938 and 1994, nine dSph satellites were discovered around the Milky Way. However, in the past two years alone, Sloan Digital Sky Survey (SDSS) data have yielded eight new dwarf satellites around the Milky Way and two around M31, in a new regime of extremel

 

[search]

dwarf elliptical galaxies that are companions to the Milky Way<br /><br />Researchers from the Sloan Digital Sky Survey (SDSS-II) announced the discovery of eight new dwarf galaxies, seven of them satellites orbiting the

 

[qso1]

Smallest galaxy ever at 1/16th the diameter of the Milky Way would be 6 or 7 thousand Ly diameter. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[search]

Well I found this and they talk of sizes of 1000 light years when comparing to the smaller Globular Clusters which are just collection of stars. I do think it is not hat small but in the order you are saying of a few thousand light years.<br /><br />http://www.astronomy.com/asy/default.aspx?c=a&id=3962<br />What's the difference between globular clusters and dwarf spheroidal galaxies?<br />GREG COMEGYS, CALEDONIA, MICHIGAN<br /><br />In a word: size.<br /><br />Dwarf spheroidal galaxies are much bigger than globular clusters. The Pegasus dwarf spheroidal galaxy, which orbits the famous spiral galaxy M31 in Andromeda, spans 2,000 light-years. A newly discovered object, the Ursa Major dwarf spheroidal (see "New Milky Way satellite," in News, July 2005) is about half this size.<br /><br />

 

[vandivx]

"So we know that a galaxy can be a thousands of light-years in diameter, but how small can a galaxy be?"<br />-----<br />I don't think there is any natural limit to how small a galaxy can be, it is just a matter of how little matter there was around during a galaxy formation and also how we make our definition of what is a galaxy, not sure though if there is such definition or if it is precise enough, probably once you start discussing it you run into the similar debate we had on what is a planet and what is not...<br /><br />personally I find imminently interesting that which you just glossed over - "So we know that a galaxy can be a thousands of light-years in diameter" - because apparently there is a natural limit on how big a galaxy can get and potentially there are some new physics to be found in that<br /><br />galaxies can build up in mergers but even then they never grow that way so that we might see a cluster of galaxies amount of matter being merged through a gravitational colapse into one super giant galaxy which should have happened at least in some clusters out there but it doesn't or does it?<br /><br />I mean the galaxies are gravitationally bound in a cluster but not in some clean, ordered orbits (like for example starts within galaxies orbiting their centers in galactic planes) which should make gravitational collapse/merger not too unlikely given enough time and time there should have been enough for us to observe some supergalaxy that would blow our minds... but it just doesn't happen no matter how far and in what detail we look around <br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[search]

I believe 100 billion is approximate definition number...

 

[why06]

Yes. What is the definition of a galaxy? <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[qso1]

Interesting comparison, dwarf galaxies and globular clusters. I was looking to see what the upper size of a globular was. I can't recall it offhand except it is probably smaller than 2K ly.<br /><br />One day the International Astronomical Union will probably be debating what constitutes a galaxy just as they recently tried to hash out what constitutes a planet. <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[qso1]

Just offhand I'd say a galaxy is a collection of objects including stars, planets, nebula, dust concentrations, black holes, maybe a large black hole at the center. A galaxy does not orbit a larger object that we know of except in cases of satellite galaxies orbiting their primary host galaxies.<br /><br />Then theres always the wiki def.<br /><br />http://en.wikipedia.org/wiki/Galaxy <div class="Discussion_UserSignature"> <p><strong>My borrowed quote for the time being:</strong></p><p><em>There are three kinds of people in life. Those who make it happen, those who watch it happen...and those who do not know what happened.</em></p> </div>

 

[vandivx]

more sources on definition of galaxy<br /><br />http://www.answers.com/topic/galaxy<br /><br />vanDivX <div class="Discussion_UserSignature"> </div>

 

[dragon04]

I like the Wiki definition, with the absolute qualifier being <br /><br />"<i>n</i> stars all orbiting one center of mass" <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>

 

[MeteorWayne]

Yeah, but that definition would also apply to globular clusters within galaxies.<br /><br />And you can't really use the seperator of orbiting a center of mass, since satellite galaxies also orbit the center of mass of the galaxy. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[jdcardinal37]

I'm just curious. Can a galaxy or star be around the diameter of one's fingernail? Grain of sand? Earth? The Sun?<br /><br />/*Inappropriate solicitation for email removed*/

 

[stbean]

good question. Could there be galaxies within the microscopic world? Technology now only allow us to probe to a certain limit. I believe the smallest unit of measurement is the Planck(spelling?) unit. What beholds beyond this unit of space?

 

[logicize]

Have you looked closely at your finger nail? Perhaps there is a galaxy there.<br /><br />Please copy this and send it to yourself so you will have a copy in your email.

 

[brellis]

As everyone knows the galaxy on Orion's belt in Men In Black was quite small <img src="/images/icons/tongue.gif" /> <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

 

[logicize]

How about this:<br /><br />Are there any galaxies that are edible?

 

[stbean]

yeah, milky way bars are pretty tasty.

 

[alokmohan]

he NASA/ESA Hubble Space Telescope has found out the true nature of a dwarf galaxy that astronomers had for a long time identified as one of the youngest galaxies in the Universe. Astronomers using the NASA/ESA Hubble Space Telescope have made observations of the galaxy I Zwicky 18 which seem to indicate that it is in fact much older and much farther away than previously thought. <br /><br />Observations of I Zwicky 18 at the Palomar Observatory around 40 years ago seemed to show that it was one of the youngest galaxies in the nearby Universe. The studies suggested that the galaxy had erupted with star formation billions of years after its galactic neighbours, like our galaxy the Milky Way. Back then it was an important finding for astronomers, since this young galaxy was also nearby and could be studied in great detail; something that is not possible with observations made across great distances when the universe was much younger.<br /><br />But these new Hubble data have quashed that possibility. The telescope found fainter older red stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as 10 billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies. <br /><br />"Although the galaxy is not as youthful as was once believed, it is certainly developmentally challenged and unique in the nearby Universe," said astronomer Alessandra Aloisi from the European Space Agency/Space Telescope Science Institute, who led the new study.<br /><br />Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is mostly composed of hydrogen and helium, the main ingredients created in the Big Bang. In other http://www.astronomy.com/asy/default.aspx?c=a&id=6119

 

[jaxtraw]

This is basically an incidence of the paradox of the heap. How many grains of sand make a heap? Form a heap, and keeping taking grains away, one at a time. At what point does it cease to be a heap? 100, 50, 2, 1? Is one grain of sand a heap?<br /><br />The problem lies in the vague definition of the word. A heap is just a pile of many grains, without definition of what counts as many. Likewise, a galaxy is just a collection of many stars (which is not part of a larger collection) and, if you keep taking stars away, you end up with a galaxy of one star.<br /><br />As with definining planets rigorously, which was foolish in itself, you end up whatever definition you choose, with borderline cases. It's meaningless to define rigorously. Simply use the term colloquially. There's no need for a precise definition, and any precise definition will be contrived.<br /><br />Is it a galaxy? Much like pornography, you know it when you see it.<br /><br />Galaxies are big clusters. Clusters are small galaxies. Whatever.