We know of a handful of very massive stars in the galaxy. And by massive, I mean whoppers: some with 100 times the Sun’s mass. These are enormous stars that are millions of times brighter than the Sun and live very short lives, ending in titanic supernova explosions.
The problem is, how do they form?
| Simulation of the formation of a 100 solar mass star from a cloud of gas, courtesy Mark Krumholz |
Stars form from clouds of dust and gas. These are generally pretty cold, just a few degrees above absolute zero (-450 Fahrenheit, -273 Celsius). The Sun formed from just such a cloud, we think. It’s not hard to form lots of stars like the Sun from a typical cloud, but massive stars are tougher. Cold gas tends to fragment into smaller clumps, and these clumps can’t make really big stars. So how do the monster ones get their start?
New work just published by Mark Krumholz at Princeton and Christopher McKee at Berkeley indicates that the big stars get a helping hand from small ones.
Imagine a giant cloud of gas, a light year or so across and containing a couple of hundred times the Sun’s mass. It’s cold inside, and the gas starts to collapse here and there under its own gravity. Small stars like the Sun start to form. When they do, they heat up the environment around them. This heat input suppresses the further fragmentation of the cloud, allowing it to collapse more than it would otherwise. So if a big star starts to form, there is a constant supply of gas to feed it, letting it grow larger. If the smaller stars weren’t there, the insides of the cloud would fragment and cut off the supply of mass to the nascent massive star.
This may sound a little contradictory: a warmed up gas expands, right? Well, usually, but we’re not talking about a major heating here. The sun-like stars raise the temperature of the cloud just a few degrees. That’s enough to stop the fragmentation, but not enough to counteract the cloud’s own gravity causing it to collapse. Note to creationists: when you talk about all gas expanding, and therefore stars cannot form, you look a little silly. You might want to look into some actual science once in a while.
Anyway, this new model explains something that has been seen but not well understood: massive stars only seem to form in clusters. The vast majority of truly huge stars we see are members of clusters, indicating that there is something in the cluster environment they need to get so big. It looks like Krumholz and McKee have found it: there have to be other stars around to help them grow.
Of course, this work is just beginning. There are lots and lots of things that can affect star formation, and they have only modeled a few. But this is an excellent start in solving one of the bigger — literally! — mysteries in star formation science today.
February 27th, 2008 at 2:38 pm
I haven’t read the paper, but by “small stars” they must mean cloud cores that will eventually become small stars. It is my understanding that once a cloud core shrinks enough to be optically think, it will only continue shrinking very gradually, and if it is to become a small star, it is very very gradual. So I don’t think we’d want the massive stars to have to wait that long to form– the effect must be caused more by low-mass “cloud cores” than by low-mass “stars”. Yes?
February 27th, 2008 at 2:49 pm
You know, when I read statements like this:
“3 – The birth of a star has never been observed. How then can anyone presume to tell us how it came about?”
I wonder if these folks ever think about what they are saying before they say them. I mean really. A statement like that can be turned around so easily…
“God has never been observed. How then can anyone presume to tell us that he exists?”
I get the feeling that God forgot to put brains inside these folks like he did the rest of the human race.
February 27th, 2008 at 2:49 pm
Wow, that creationist link you provided is dumbfounding. I had never heard that argument before. AGGHH!
February 27th, 2008 at 3:03 pm
Wow, dozy of a link there, BA. I’m trying to figure out how someone can be reputable and yet unknown. If no one knows you, then what kind of reputation could you have?
And that’s just the introduction…
February 27th, 2008 at 4:31 pm
I’m struggling to envisage the physical process that means that, as a cloud gets warmer, its clumpiness gets bigger.
I can understand that a cloud with random fluctuations in density will have local clumps that are in a sort of race to swallow up the gas and maybe each other, but I don’t understand the role of temperature. Does the warming smooth out the clumps and increase the time available for the cloud as a whole (or a large part of it) to collapse?
I confess I tried to read the linked paper (not the creationist blah!) and my mind just bounced – the author assumes a lot of prior knowledge (as you’d expect in a scientific paper).
February 27th, 2008 at 4:54 pm
This is the best kind of post.
February 27th, 2008 at 4:59 pm
Wow – the rate of brain-cell death from reading that site is measured in MegaCoulters!
In addition to everything else, they can’t count. Supposedly 16 reasons why stars can’t form, plus 20 more, but there are really only about half a dozen in the list due to redundant redundancy. Of course, all of those are either false or irrelevant. What the heck is “evolutionary red shift theory”, anyway? And stellar evolution theory doesn’t explain gravity? It doesn’t explain the infield fly rule, either, so it must be wrong!
Arggghhhh!!!!
Anyway, back to reality… It’s been 34 years since I read Larson’s papers on star formation, and I’m sure there’s been tons more work done on the subject since then, but as I recall the cloud collapse is a balancing act between gravity and gas pressure. The gravity depends on the mass and diameter of the cloud, and the gas pressure depends on the temperature. Initially, there is no source of heat (until fusion starts), so temperature depends on the initial temperature (a few degrees K, doesn’t matter much exactly what value you pick), and how much the cloud has collapsed so far, and how much energy it’s lost through radiation. While the cloud is still optically thin, any and all radiation escapes, but the optical thickness depends on wavelength. In the middle of the Lyman Alpha line, it is very thick! But at the wavelengths many metals emit (in the astronomical sense of “metal”, i.e. any element except H and He), it is thin until the cloud gets hot enough to ionize, but by that time, the core (for anything larger than a brown dwarf) is hot enough and dense enough to fuse Hydrogen.
The timescale involved depends on the metallic content and the size of the initial cloud, but is of the order of a few million years.
However, early in the universe, when there was nothing but H and He
(and a tiny bit of Li and Be, IIRC), lots of the cooling (which takes place due to collisional excitement of atoms or molecules followed by re-emission of photons as they go back to their ground state) was by photons in the emission/absorbtion lines of those few elements, most of which promptly got absorbed again and took a very long time to leave the cloud. So the collapse time for the same mass cloud was much longer. A more massive cloud would collapse more quickly. I wrote a paper on this in college where I took Larson’s model and removed the cooling from metals, and increasing the mass to get the collapse timescale back to a few million years, and the result was the mass had to be 60,000 to 100,000 solar masses, IIRC. (Which I noted was the typical mass of a globular cluster, which I believe at the time were considered to be mostly or exclusively Population II stars…) This was followed by lots of handwaving about fragmentation, etc.
I hope I haven’t forgotten so much of the terminology that is completely unintelligible (not to mention completely forgetting the results I obtained or whose paper I was quoting, etc.) If I have bollixed it up, remember I just read that web page, so it’s all BA’s fault!
February 27th, 2008 at 5:16 pm
Wow, that creationist website… O.o MAH BRAIN HURTS! I’m reminded of the little dogs that pick fights with big dogs because they have no sense of size. They don’t have any concept of large numbers. If only their “laws” and conclusions were true for them. The sun wouldn’t exist, and the earth (which also could not exist) wouldn’t be able to hold an atmosphere since “Gas on earth would never push into itself; it would expand. In the vacuum of outer space, it would be even less possible.”
Reductio ad absurdum, indeed.
February 27th, 2008 at 9:22 pm
I get the feeling that God forgot to put brains inside these folks like he did the rest of the human race.
Old joke: when God was handing out brains, they thought he said “trains”, and they responded: I’ll take a toy one.
J/P=?
February 28th, 2008 at 6:51 am
[Sesame Street Music]One of these things is not like the other,
One of these things just doesn’t belong…[/Sesame Street Music]
“These are generally pretty cold, just a few degrees above absolute zero (-450 Fahrenheit, -273 Celsius).”
Neat article, Phil! The Universe becomes even more fascinating the more we learn about it.
February 28th, 2008 at 7:39 am
Logic and Science are anathema to creationism.
February 28th, 2008 at 8:07 am
Wow. I read that whole, multi-font/bold/italics website this a.m. – People still believe the funniest things. The passion is astounding.
Consider the scientist who wakes up trying to find out more, to expand their knowledge….meeting a creationist on the bus who is reading the same newspaper from 40 years ago trying to tell the scientist the weather report. This is how I consider this debate. Im tired of being polite, id rather rip the paper out of the creationists hands, throw it to the floor….and give them a brand new copy of todays paper. Point to the weather report….”there…this is what is really happening….we figured this stuff out”
Baffled. Utterly baffled…..
February 28th, 2008 at 8:13 am
“Megacoulters”
That’s a great term!!
I would guess that it refers to the general act of shutting down Scientific or rational evidence as a tool to support conservative position, right?
You could also create other units of measurement:
The Hovind (kilohovind, megahovind)- a unit of creationistic ‘thought’
Or even units of rationalsim:
The Sagan or the Hawking- a unit of Science
February 28th, 2008 at 11:08 am
Wow, that website is full of garbage! The parts on the origin of matter are particularly laughable. My favourite quote, regarding the expanding Universe: “Leaving its gravitational field. The center of gravity, by that time (assuming an expanding universe), would be on the outer perimeter of the universe, not at its center—so there would be nothing to draw it back to where it theoretically came from.”
That sound you hear is my mind, boggling.
February 28th, 2008 at 1:12 pm
my god, thank you for linking to that “pathlights” website. ive been on it for an hour, and ive never been so entertained.
you get the usual nonsense creationist arguments, like evolution violoates the 2nd law of thermodynamics, but they go step further and add little gems like these:
“evolution teaches an upward arrow—all the way from nothingness, in the past, to present organization and ON INTO A GLORIOUS FUTURUE WHEN MAN WILL BECOME GOD, RULING THE UNIVERSE.” (emphasis mine)
awesome. i knew my high school science class was lacking…. not ONCE was i told that humanity would someday become omnipotent rulers of the cosmos.
i guess first we just have to break the back of that damned light. how dare you keep your speed unattainable to us godly men?!
February 28th, 2008 at 6:39 pm
I must admit I stole the concept of coulters from someone else. Read it in a newsgroup or website or blog or something, it might have been here. Someone said “Wow, that was really stupid. I could feel my brain cells dying…” and someone else responded something like “No, it wasn’t that bad. I’d only rate it 100 coulters or so. I’ve seen things much stupider…” It wasn’t clear to me from context whether a coulter was a unit of stupidity or a measure of brain cell death. C = ln(cells/second),
or something like that.