Star-Forming Clouds Stars form in dark clouds of dusty gas in interstellar space. The gas between the stars is called the interstellar medium.
Gravity Versus Pressure Gravity can create stars only if it can overcome the force of thermal pressure in a cloud. Gravity within a contracting gas cloud becomes stronger as the gas becomes denser. Its called a molecular cloud because the Hydrogen is cool enough in the form H 2 molecules.
Mass of a Star-Forming Cloud A typical molecular cloud (T~ 30 K, n ~ 300 particles/cm 3 ) must contain at least a few hundred solar masses for gravity to overcome pressure. The cloud can prevent a pressure buildup as it collapses by converting thermal energy into infrared and radio photons that are radiated away to cool the cloud.
Fragmentation of a Cloud This simulation begins with a turbulent cloud containing 50 solar masses of gas.
Fragmentation of a Cloud The random motions of different sections of the cloud cause it to become lumpy.
Fragmentation of a Cloud Each lump of the cloud in which gravity can overcome pressure can go on to become on or more stars. A large cloud can make a whole cluster of stars containing thousands or millions of stars.
Glowing Dust Grains As stars begin to form, dust grains that absorb visible light heat up and emit infrared light. So we can find new stars by using and infrared telescope.
Glowing Dust Grains Long-wavelength infrared light is brightest from regions where many stars are currently forming.
Thought Question What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy? A. It would continue contracting, but its temperature would not change. B. Its mass would increase. C. Its internal pressure would increase.
Solar system formation is a good example of star birth. Recall the Nebula Model of the Solar System.
Cloud heats up as gravity causes it to contract due to conservation of energy. Contraction can continue if thermal energy is radiated away.
As gravity forces a cloud to become smaller, it begins to spin faster and faster, due to conservation of angular momentum.
As gravity forces a cloud to become smaller, it begins to spin faster and faster, due to conservation of angular momentum. Gas settles into a spinning disk because spin hampers collapse perpendicular to the spin axis.
Rotation of a contracting cloud speeds up for the same reason a skater speeds up as she pulls in her arms. Collapse of the Solar Nebula
Collisions between particles in the cloud cause it to flatten into a disk. Flattening
Collisions between gas particles also reduce up and down motions. Why Does the Disk Flatten?
Formation of Jets Rotation also causes jets of matter to shoot out along the rotation axis. These are probably due to magnetic fields in the new star. We usually only see these jets in new stars.
Jets are observed coming from the centers of disks around protostars. A protostar is the name we give a star that is just forming … i.e. it has not yet reached the main sequence. The initial disk that forms around the star is called a protostellar disk (also sometimes called a protoplanetary disk).
Thought Question What would happen to a protostar that formed without any rotation at all? A. Its jets would go in multiple directions. B. It would not have planets. C. It would be very bright in infrared light. D. It would not be round.
Protostar to Main Sequence A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion. Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface. It takes 30 million years for a star like the Sun (less time for more massive stars). Before it reaches the main sequence the cloud\protostar has a larger radius (it is collapsing) and a lower temperature (fusion has not started yet) than it will have on the main sequence. So it must approach the main sequence from the right hand side of the HR diagram.
Summary of Star Birth 1.Gravity causes gas cloud to shrink and fragment. 2.Cores of shrinking cloud fragments heat up. 3.Collapse only continues if the cloud cools by radiating away heat. 4.If the initial cloud was spinning a protostellar disk is formed. 5.Protostars approach the main sequence from the right hand side of the HR diagram. 6.Jets can be formed as the protostar collapses. 7.When core gets hot enough, fusion H to He begins and stops the collapse. 8.New star achieves long-lasting state of balance on the Main Sequence (i.e. the thermostat model that we discussed for the Sun where the rate of nuclear fusion produced sufficient thermal gas pressure to resist gravitational collapse).
A cluster of many stars can form out of a single cloud.
Very massive stars are rare. Low-mass stars are common. Only about 1 in 200 stars is an O type star, whereas 90% of all stars are either spectral type K or M.
Upper Limit on a Star’s Mass Photons of light exert a slight amount of pressure when they strike matter. Very massive stars are so luminous that the collective pressure of photons drives their matter into space. Hence very large stars are not stable and quickly fall apart do to photon pressure.
Upper Limit on a Star’s Mass Models of stars suggest that radiation pressure limits how massive a star can be without blowing itself apart. Observations have not found stars more massive than about 300M Sun.
Lower Limit on a Star’s Mass Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 10 7 K. Thermal pressure cannot stop contraction because the star is constantly losing thermal energy from its surface through radiation. Is there another form of pressure that can stop contraction?
Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying the same state in the same place.
Thermal Pressure: Depends on heat content The main form of pressure in most stars Degeneracy Pressure: Particles can’t be in same state in same place Doesn’t depend on heat content
Brown Dwarfs Degeneracy pressure halts the contraction of objects with <0.08M Sun before the core temperature becomes hot enough for fusion. Starlike objects not massive enough to start fusion are brown dwarfs.
Brown Dwarfs A brown dwarf emits infrared light because of heat left over from contraction. Its luminosity gradually declines with time as it loses thermal energy and cools.
Brown Dwarfs in Orion Infrared observations can reveal recently formed brown dwarfs because they are still relatively warm and luminous.
Stars more massive than 300M Sun would blow apart. Stars less massive than 0.08M Sun can’t sustain fusion.
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