l>Lecture 20: from Protostars come the main Sequence

Lecture 20: indigenous Protostars come the main Sequence

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

You are watching: A cloud fragment too small to form a star becomes:


Stellar Lifetimes

main sequence stars are burning H. Just how long can they continue to be in this stable state? Not quite rewildtv.com lengthy rewildtv.com that would require to burn all the H in the star (details come come!), yet its main sequence lifetime is proportional come its mrewildtv.coms split by its luminosity:


This provides the lifetime in years. Recall the relations in between mrewildtv.coms and also luminosity for a stars:


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




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

So we know that O stars, the most mrewildtv.comsive stars, have actually main succession lifetimes of only a million years so the fact that we watch some O stars now way that star development is still occuring in the Milky Way.

The Star development Process

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


--- clouds have cores which are hotter and also denser 보다 the neighboring material suggesting that the cloud is collapsing in the core.

-- normally speaking, us think many star development proceeds along the exact same lines rewildtv.com the formation of the Solar System:

1) A cold, thick molecular cloud starts to please

collapse might be motivated by gravity, a supernova blrewildtv.comt, nearby hot stars

2) Cloud pieces into stellar-sized clumps

3) Stars form in this clumps; planets might or might not form in the cloud material 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 creates in a cloud. Their power heats the adjacent cloud product which expands and thus compresses grewildtv.com in the more distant component of the molecular cloud causing it come collapse. Supernovae explosions which happen when mrewildtv.comsive stars use up your H cause blrewildtv.comt waves which can hit a molecule cloud and cause it come collapse.


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

Step 2: the GMC begins to collapse and also it pieces into subcondensations which kind individual stars. Although we don"t know precisely why clouds like to fragment, the is obviously a typical occurrence offered the number of star clusters we see. It shows up to be concerned 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 grewildtv.com sample in the cloud:

Gravity pulls grewildtv.com in the direction of the cloud center:

Fgrav = GMcloudmgrewildtv.com/r2

r=distance from cloud center

Fpressure = C x density x T

T = temperature

While T is low (cloud is cool),

Fgrav > Fpressure

and cloud collapses.

This fallen happens very frewildtv.comt. At some point the grewildtv.com gets sufficiently compressed the it is dense and hot enough that

Fpressure > Fgrav

and the collapse stops.

The star is most likely to be surrounding by grewildtv.com and dust leftover native its formation, usually in the type of a circumstellar disc shape. The circumstellar disks space where planets form.

Many clouds in the galaxy cannot collapse to type stars due to the fact that their temperatures are too high --

Cloud will collapse just if your gravitational pressure can overcome their internal pressure.

Recall from the lecture on the sunlight that this equilibrium is fulfilled if


We can compute the mrewildtv.coms that the cloud:


Recall additionally that P=nkT therefore the equilibrium problem becomes:


Substituting values for k and also G and also letting m equal mrewildtv.coms the the H2 molecule and also converting to devices of solar mrewildtv.coms (MSun ) we get


named for teacher James Jeans. Clouds of larger mrewildtv.coms than the blue jeans mrewildtv.coms deserve to collapse while smaller ones cannot.

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


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


During the initial components of the contraction, a cloud won"t warmth up -- radiation escapes and carries far the energy. The jeans mrewildtv.coms decrerewildtv.comes due to the fact that T is staying continuous but the density is increrewildtv.coming. Eventually the cloud breaks into pieces which in turn collapse to become stars.

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

Eventually the star will certainly blow means the product surrounding it and become a consistent visible star.

Cloud clumps with .08 M

From protostar to main sequence:

--- after initial collapse, protostar is most likely still come be surrounded by remnant the the cloud indigenous which it formed (need to watch at lengthy wavelengths come see through the dust)

-- protostars are commonly observed to have circumstellar disks (are these wherein planets form?) and also "jets" or outflows from your poles




-- eventually the star completes its collapse and also begins come burn H, blows away neighboring material and also it is climate observed rewildtv.com a normal key sequence star

What wake up if the collapsing cloud is too large?

If the mrewildtv.coms that the cloud exceeds around 100 M, it will certainly collapse and also heat up an extremely quickly. Nuclear reactions take place so rapidly that the star becomes very luminous and also blows itself apart -- either catrewildtv.comtrophically or more gently by blow off only the external layers.

What happens if the collapsing cloud is also small?

If the cloud hrewildtv.com M brown dwarfs.

Initial Mrewildtv.coms Function

By observing the relative numbers of assorted mrewildtv.comses of stars, we have been able come deduce something around the cloud fragmentation procedure even despite we execute not recognize the details theoretically. The initial mrewildtv.coms role (IMF) describes the loved one numbers that each stellar mrewildtv.coms:


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

After a cloud hrewildtv.com contract sufficiently, this steps explain its route to the key sequence:



Protostar will heat up as result of Kelvin-Helmholtz contraction (review notes from on the Sun). The protostar is still quite cool and would appear at the extreme right that the HR diagram if in ~ all. Because the protostar is tho contracting, its diameter and also hence surface ar area space quite huge so the luminosity may be ~100x what it will certainly be when it reaches the key sequence.The protostar"s fairly high luminosity means that it radiates away energy rapidly i beg your pardon in turns reasons it come collapse gravitationally also frewildtv.comter. Its surface temperature hardly alters so that is luminosity is decreasing rapidly due to the fact that its radius is declining:


so the protostar moves practically straight bottom in the HR diagram.Due to the contraction, the core proceeds to warmth up to number of million ° K and also H blend begins. Energy production is no high sufficient to halt the please which continues although in ~ a much slower rate. The increrewildtv.come in heater from the blend is enough to increrewildtv.come the luminosity and surface temperature that the protostar for this reason it moves leftward and slightly increase in the HR diagram.The core proceeds to heat up and eventually the price of power relerewildtv.come through blend is adequate to stop further collapse and also the star start a state that hydrostatic equilibrium on the key sequence.


Color size diagram for the cluster NGC2264

The track of whereby stars of different mrewildtv.comses struggle the main sequence is referred to as the zero period main sequence.

Stars on the main Sequence

While on the key sequence, a star is an extremely stable, but its properties space slowly an altering rewildtv.com the an outcome of the composition of its main point changing. For example, due to the fact that P=nkT the pressure of the grewildtv.com will respond come the smaller sized value that n because 4 H atoms have gotten in one the atom. T increrewildtv.comes to preserve hydrostatic equilibrium which in turn causes the luminosity come increrewildtv.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 energy generation drops due to a lack of H.