Intergalactic space is to fill withclouds the gas(mostly H + He) and dust recognized as molecule clouds.

These clouds space supported againstgravitational collapseby their thermal pressure, yet if the clouds get too huge massive,gravity wins and they can start come collapse. This is the very first steptowards star formation.

Once started, how does the please forma starand the disc of material surrounding it?

Gravity is a centralforce-- that draws product in radially. So whydoesn"tit all collapse into a ball?


Angular Momentum

All clouds rotate, at the very least alittle,due come gravitational shearing in the galaxy"s disk. And also if a cloudrotates,it has angular momentum: l ~ mrv.

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And remember the angularmomentumis conserved. Therefore if a rotating cloud collapses (r getssmaller)than it need to spin faster (v gets bigger). Even a little pre-collapsespingoes a lengthy way!

How does rotation impact collapse? the adds acentripetalforce term come the collapse. For a fragment of mass m ~ above the sheet of thecloud, the force on it relies whether the is on the rotation pole ortheequator:


So as the cloud collapses, product along the spinaxiscan collapse onto the star, yet material in the spin airplane has collapsehalted by centripetal force. A decaying is formed!


What about the central mass -- the protostaritself?

As a ball of gas collapses, its gravitationalpotentialenergy changes. Fifty percent of this readjust is radiated away, the other halfheatsup the collapsing cloud.

So the very main portions of the cloud aregettingdenser and also hotter. Ultimately the density and also temperature will becomehighenough the nuclear reaction will begin to take place. Astar is born!

The high main density and also temperature also createsufficientpressure (via the best gas law) within the star to halt gravitationalcollapse. The young star is currently in hydrostatic equilibrium.

Nice theory. Does it haveanything to execute with reality?

Yes!We check out these stars and young disks in surrounding starforming regions:

The Orion Nebula


Proplyds (ProtoPlanetaryDisks) in the Orion starforming cloud

The Solar NebulaOkay, so us made a star and had part leftover ingredient inadisk roughly it. This leftover ingredient is called the solarnebula, and it is the ingredient from i m sorry the planets formed.Whatmore can we say around it?

What is that made of?

It come from the samecollapsingcloud which developed the Sun, so the should have the exact same chemicalcompositionas the Sun. Mostly hydrogen and helium, with much smaller quantities ofheavierelements: carbon, nitrogen, silicon, sulfur, iron, etc.

Also, the chemicalcomposition ofmeteorites is comparable to the Sun, minus the volatile facets such ashydrogen,helium, and also other gases.

How warm was it?
Recall that the proto-Sun isundergoinggravitational collapse as it forms, and also that gravitational collapseproducesa most heat. As the fallen slows, less energy is released - rememberour calculation of Jupiter"s warm in the critical homework set.

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So the early on solar nebulawasvery hot, and also cooled rapidly.

The within solar mechanism wasprobablyat temperature > 2000 K. When collapse to be halted and the diskformed,the nebula cooled come ~ 800 K. After ~ the decaying was finally cleared ofmaterialand the planets formed, the solar mechanism temperatures eventuallydroppedto the current equilibrium values. Where go it every go?Eventually, the mainly wind and also radiationpressurefrom the central star blew much of the material out the the within solarsystem (remember the physics that comet tails?). But prior to thathappens,we should start make planets...