12.1 Star Birth Our Goals for Learning How do stars form? How massive are newborn stars?

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

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 hampers collapse perpendicular to spin axis

Angular momentum leads to: Rotation of protostar Disk formation Jets from protostar Fragmentation into binary

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 1.Gravity causes gas cloud to shrink and fragment 2.Core of shrinking cloud heats up 3.When core gets hot enough, fusion begins and stops the shrinking 4.New star achieves long- lasting state of balance

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 100 M Sun would blow 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 is due to packing of atoms, not to heat

Degeneracy Pressure: Laws of quantum mechanics prohibit two electrons from occupying same state in 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 Dwarf An object less massive than 0.08 M Sun Radiates infrared light Has thermal energy from gravitational contraction Cools off after degeneracy pressure stops contraction

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 0.08MSun. 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.