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We are “star stuff” because the elements necessary for life were made in stars
How do stars form?
Stars are born in molecular clouds consisting mostly of hydrogen molecules
Stars form in places
where gravity can overcome thermal
pressure in a cloud
HST Photo: Trifid Nebula
• Cloud heats up as gravity causes it to contract
• Conservation of energy
• Contraction can continue if thermal energy is radiated away
Star-forming clouds emit infrared light because of the heat generated as stars form
Infrared light from Orion
Orion Nebula is one of the closest star-forming clouds
Solar-system formation is a good example of star birth
As gravity forces a cloud to become smaller, it begins to spin faster and faster
As gravity forces a cloud to become smaller, it begins to spin faster and faster
Conservation of angular momentum
As gravity forces a cloud to become smaller, it begins to spin faster and faster
Conservation of angular momentum
Gas settles into a spinning disk because spin makes it hard to for gas cloud to collapse except along the spin axis
Angular momentum leads to:
Rotation of protostar Disk formation
… and sometimes …
Jets from protostar Fragmentation into binary
Disks and jets seen around young stars
Protostar to Main Sequence
• Protostar contracts and heats until core temperature is sufficient for hydrogen fusion.
• Contraction ends when energy released by hydrogen fusion balances energy radiated from surface.
• Takes 50 million years for star like Sun (less time for more massive stars)
Summary of Star Birth
Gravity causes gas cloud to shrink and fragment
Core of shrinking cloud heats up
When core gets hot enough, fusion begins and stops the shrinking
New star is now on the (long-lasting) main sequence
How massive are newborn stars?
A cluster of many stars can form out of a single cloud.
Temperature
Lu
min
osi
tyVery massive stars are rare
Low-mass stars are common
Temperature
Lu
min
osi
tyStars more massive than 150 MSun would blow themselvesapartStars less massive than 0.08 MSun can’t sustain fusion
Pressure Gravity
If M > 0.08 MSun, then gravitational contraction heats core until fusion begins
If M < 0.08 MSun, degeneracy pressure stops gravitational contraction before fusion can begin
Degeneracy Pressure: Laws of quantum mechanics prohibit more than one electron (or neutron) from having the same velocity in the same place at the same time. There are only so many different velocities a particle can have.
Degeneracy pressure: no two particles can have the same velocity and position
Degeneracy pressure means there’s a limit to how dense objects can get
Thermal Pressure:
Depends on temperature
The main form of pressure in most stars
Degeneracy Pressure:
Particles can’t be at same velocity in same place
Doesn’t depend on temperature, only density
Brown Dwarf• An object less massive than 0.08 MSun (= 80 MJup)
but more massive than 13 MJup (so, not a planet)• Gains thermal energy from gravitational contraction
& maybe deuterium fusion, but never from H fusion• Radiates infrared light, but almost no optical light• Cools off after degeneracy pressure stops
contraction … cools off forever!• VERY dim, VERY red, VERY hard to spot
…‘Nemesis’ in our own solar system?• Nearest known one is 12 light-years away, orbiting
the southern star Epsilon Indi
What have we learned?• How do stars form?• Stars are born in cold, relatively
dense molecular clouds. • As a cloud fragment collapses
under gravity, it becomes a protostar surrounded by a spinning disk of gas.
• The protostar may also fire jets of matter outward along its poles. Protostars rotate rapidly, and some may spin so fast that they split to form close binary star systems.
What have we learned?
• How massive are newborn stars?• Newborn stars come in a range of masses, but
cannot be less massive than 0.08 solar masses.• Below this mass, degeneracy pressure prevents
gravity from making the core hot enough for efficient hydrogen fusion, and the object becomes a “failed star” known as a brown dwarf.
Activity #32, pages 109-112
