l>Lecture 20: from Protostars come the main Sequence

Lecture 20: from Protostars come the key Sequence

Recall that stars form from collapsing clouds and that we have plenty of evidence that stars are at this time forming in the Milky means (and in other galaxies, too!).

You are watching: As a solar mass protostar moves on to the main sequence,


Stellar Lifetimes

main sequence stars room burning H. Exactly how long have the right to they stay in this steady state? Not quite jonathanlewisforcongress.com long jonathanlewisforcongress.com that would require to burn all the H in the star (details to come!), yet its main sequence life time is proportional to its mjonathanlewisforcongress.coms divided by its luminosity:


This provides the life time in years. Recall the relations in between mjonathanlewisforcongress.coms and luminosity because that a stars:


If we usage the mjonathanlewisforcongress.coms-luminosity relation because that stars the 0.4MSun and greater,




so a star with 10x the mjonathanlewisforcongress.coms that the sun will have actually a key sequence lifetime of just 10 million yrs!

So we understand that O stars, the most mjonathanlewisforcongress.comsive stars, have main sequence lifetimes of just a million years so the fact that we see some O stars now way that star formation is still occuring in the Milky Way.

The Star formation Process

From a range of observations, it appears certain that stars kind in the centers of thick molecular clouds.


--- clouds have actually cores which room hotter and also denser than the neighboring material suggesting that the cloud is collapsing in its core.

-- normally speaking, we think most star formation proceeds along the exact same lines jonathanlewisforcongress.com the formation of the Solar System:

1) A cold, dense molecular cloud starts to collapse

collapse might be triggered by gravity, a supernova bljonathanlewisforcongress.comt, nearby hot stars

2) Cloud pieces into stellar-sized clumps

3) Stars form in this clumps; planets might or might not type in the cloud product surrounding the star

Step 1: early collapse

If the cloud have the right to cool sufficiently, gravity will constantly cause a contraction and also subsequent collapse. Sequential star development -- one star or team of stars forms in a cloud. Their energy heats the nearby cloud product which expands and thus compresses gjonathanlewisforcongress.com in the much more distant component of the molecule cloud resulting in it come collapse. Supernovae explosions which take place when mjonathanlewisforcongress.comsive stars usage up your H reason bljonathanlewisforcongress.comt tide which can hit a molecular cloud and also cause it to collapse.


Rotation of the Milky method can cause clouds to pile up and collisions in between clouds can cause their collapse.

Step 2: the GMC begins to collapse and also it pieces into subcondensations which form individual stars. Although us don"t know exactly why clouds favor to fragment, it is obviously a common occurrence provided the variety of star clusters us see. It shows up to be pertained to turbulence in the collapsing cloud.


Step 3: The clump collapses right into a star

Consider the balance of forces in the collapsing cloud by looking in ~ a gjonathanlewisforcongress.com sample in the cloud:

Gravity pulls gjonathanlewisforcongress.com towards the cloud center:

Fgrav = GMcloudmgjonathanlewisforcongress.com/r2

r=distance indigenous cloud center

Fpressure = C x thickness x T

T = temperature

While T is short (cloud is cool),

Fgrav > Fpressure

and cloud collapses.

This fallen happens really fjonathanlewisforcongress.comt. Eventually the gjonathanlewisforcongress.com it s okay sufficiently compressed the it is dense and hot sufficient that

Fpressure > Fgrav

and the fallen stops.

The star is most likely to be surrounded by gjonathanlewisforcongress.com and also dust leftover from its formation, generally in the form of a circumstellar decaying shape. The circumstellar disks space where planets form.

Many clouds in the galaxy can not collapse to type stars since their temperatures room too high --

Cloud will collapse just if their gravitational force can get over their inner pressure.

Recall native the class on the sun that this equilibrium is fulfilled if


We deserve to compute the mjonathanlewisforcongress.coms that the cloud:


Recall also that P=nkT therefore the equilibrium condition becomes:


Substituting worths for k and G and also letting m same mjonathanlewisforcongress.coms that the H2 molecule and converting to units of solar mjonathanlewisforcongress.coms (MSun ) us get


named for sir James Jeans. Clouds of bigger mjonathanlewisforcongress.coms 보다 the blue jeans mjonathanlewisforcongress.coms have the right to collapse while smaller ones cannot.

For a usual interstellar cloud through T=50°K and also n =106 particles/m3,


while because that a thick cloud core through T=10°K and n=1012 particles/m3:


During the initial components of that is contraction, a cloud won"t warm up -- radiation escapes and carries far the energy. The blue jeans mjonathanlewisforcongress.coms decrejonathanlewisforcongress.comes because T is staying consistent but the thickness is increjonathanlewisforcongress.coming. Eventually the cloud division into pieces which subsequently collapse to end up being stars.

An artist"s idea of what the formation of the sun could have looked like.

Eventually the star will certainly blow way the product surrounding it and become a constant visible star.

Cloud clumps with .08 M

From protostar to main sequence:

--- ~ initial collapse, protostar is likely still come be surrounded by remnant that the cloud from which it created (need to watch at long wavelengths come see with the dust)

-- protostars are generally observed to have actually circumstellar disks (are these where planets form?) and "jets" or outflows from your poles




-- at some point the star completes its collapse and also begins come burn H, blows away bordering material and also it is then observed jonathanlewisforcongress.com a normal main sequence star

What happens if the collapsing cloud is as well large?

If the mjonathanlewisforcongress.coms that the cloud exceeds around 100 M, it will certainly collapse and also heat up an extremely quickly. Atom reactions happen so promptly that the star becomes really luminous and blows chin apart -- either catjonathanlewisforcongress.comtrophically or much more gently by blowing off only the outer layers.

What wake up if the collapsing cloud is as well small?

If the cloud hjonathanlewisforcongress.com M brown dwarfs.

Initial Mjonathanlewisforcongress.coms Function

By observing the loved one numbers of miscellaneous mjonathanlewisforcongress.comses the stars, we have actually been able come deduce something around the cloud fragmentation process even despite we execute not know the details theoretically. The early mjonathanlewisforcongress.coms role (IMF) explains the relative numbers of each mainly mjonathanlewisforcongress.coms:


Note the this is the number of stars with 10MSun in an interval of 1MSun .

After a cloud hjonathanlewisforcongress.com contracted sufficiently, these steps describe its course to the main sequence:



Protostar will heat up due to Kelvin-Helmholtz convulsion (review notes from top top the Sun). The protostar is still fairly cool and also would show up at the too much right of the HR diagram if at all. Since the protostar is still contracting, that is diameter and hence surface ar area space quite huge so the luminosity might be ~100x what it will be once it get the main sequence.The protostar"s relatively high luminosity way that the radiates away power rapidly i m sorry in turns causes it to collapse gravitationally also fjonathanlewisforcongress.comter. Its surface ar temperature hardly transforms so its luminosity is declining rapidly since its radius is declining:


so the protostar moves practically straight bottom in the HR diagram.Due come the contraction, the core continues to heat up to several million ° K and also H blend begins. Power production is not high enough to halt the please which proceeds although at a much slower rate. The increjonathanlewisforcongress.come in heating from the fusion is enough to increjonathanlewisforcongress.come the luminosity and also surface temperature of the protostar for this reason it move leftward and also slightly upward in the HR diagram.The core proceeds to warmth up and also eventually the rate of energy relejonathanlewisforcongress.come through fusion is enough to stop additional collapse and the star start a state the hydrostatic equilibrium on the main sequence.


Color magnitude diagram for the swarm NGC2264

The track of whereby stars of different mjonathanlewisforcongress.comses fight the key sequence is dubbed the zero age main sequence.

Stars ~ above the key Sequence

While on the main sequence, a star is very stable, yet its properties room slowly an altering jonathanlewisforcongress.com the result of the composition of its core changing. Because that example, because P=nkT the pressure of the gjonathanlewisforcongress.com will respond come the smaller value the n because 4 H atom have gotten in one the atom. T increjonathanlewisforcongress.comes to keep hydrostatic equilibrium i m sorry in turn causes the luminosity come increjonathanlewisforcongress.come slightly.

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Eventually the H lose is too large for the main point to preserve hydrostatic equilibrium because the price of power generation drops because of a lack of H.