Star Formation

Posted By Pamela on Jan 11, 2007 | 3 comments

pillar-m16.jpgA couple more posts on the AAS are coming, but for now I want to take a moment to answer a question asked by a reader yesterday. Paul asked “I have a question about how stars form. I’m confused about the answers I’ve read or heard about. Some people say that stars “condense” from molecular space gas. As they do, they heat up and ignite. Others say they “collapse” from gas. Still others say that the gas is “compressed” by a nearby supernova. … Can you help explain how stars are formed?”

As I understand it, there are a couple different models. In one, a molecular cloud will very slowly, over lots and lots of time collapse due to gravity (some clouds formed with our galaxy still haven’t collapsed all the way into stars!). Higher density regions will collapse faster, and lower density regions will either get sucked into higher density regions, or just collapse very very slowly. Pretty much everything in the galaxy has some angular momentum due to inherent rotation. As the densities within a giant molecular cloud collapse, they begin to spin and flatten. There is a period of time during which gravity is pulling material into the center of the density while the radiation pressure from the warm gas is ejecting the material in jets. Luckily, the system is able to not blow itself apart in the process, and gravity wins. When a star turns on – when nuclear reactions start up in the center, the light from the star creates so much pressure on surrounding material that the inflow of mass stops and the star clears out the area around it.

Now, if all the stars in the universe where formed simply through the very very slow gravitational collapse and fragmentation of molecular clouds, we would live in a very boring universe. Shocks (such as those from supernovae, spiral density waves, and collisions) can speed up the collapse of gas by pushing stuff together (condensing it). In this scenario, only the highest density regions survive to form stars, and the lower density gas dispersed. Here is a way to picture it: Imagine you have a rake with very flexible light wieght tines. Thanks to the help of a squirrel, you have one small patch of lawn with an over density of leaves. When you rack that one section of leaves, leaves that aren’t part of the original clump get pushed into it, and the force from the rake condences the pile. If the clump gets big enough, with a lot of large friction with the ground, the tines may bend and leave that clump behind. In a similar way, the shock wave can push together a large density of material, and if the material is dense enough gravity will hold it together and it will grow into a star.

In colliding galaxies, massive amounts of star formation will be triggered by shockwaves from the collapse, but the material that doesn’t get turned into stars may get strewn through space or pushed into the central black holes. No matter its fate, after the collision, the two galaxies will be dead, and star formation will have ceased.

image credit goes to HST.


  1. Thanks for your response on star formation. I have another question that has been puzzling me:

    When I look up at the sky, I see no shape of the spiral galaxy that we are supposed to be in. And I also can’t perceive where we are located in our galaxy. How can we tell what our galaxy looks like and where we are located in it when we are looking edgewise at it? And, assuming there is a lot of dark matter out there, how do we know what structure is being hidden from our telescopes?


  2. Nice Post.

    That was well said. Always appreciate your indepth views. Keep up the great work!


  3. – What is the Source of the Universe ?
    Where did Existence come from?
    Now we have three ( 3) sources of the Universe:
    Big bang , vacuum and God.
    Which of them is correct ?

    About big band and God my opinion is:
    the action, when the God compressed all Universe
    into his palm, physicists had named -a singular point
    And action, when the God opened his palm,
    physicists had named – the Big Bang
    And about vacuum Paul Dirac wrote:
    ‘ The problem of the exact description of vacuum, in my opinion,
    is the basic problem now before physics. Really, if you can’t correctly
    describe the vacuum, how it is possible to expect a correct description
    of something more complex? ‘
    Best wishes.
    Israel Sadovnik Socratus
    ‘ ENTROPY. Gravity, Particles and Star Formation.’

    Best wishes.
    Israel Sadovnik. Socratus.
    =========== .

Submit a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.