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

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 Luminosity Very massive stars are rare Low-mass stars are common

Temperature Luminosity Stars more massive than 150 M Sun would blow themselves apart Stars less massive than 0.08 M Sun can’t sustain fusion

Pressure Gravity If M > 0.08 M Sun, then gravitational contraction heats core until fusion begins If M < 0.08 M Sun, 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 M Sun (= 80 M Jup ) but more massive than 13 M Jup (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