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

Download a movie of optical-to-IR transformation from the Spitzer Space Teelscope site Star-forming clouds emit infrared light because of the heat generated as stars form

Orion Nebula is one of the closest star-forming clouds Infrared light from Orion

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 perpendicular to 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.

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

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

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 occupying same state in same place at same time

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 MSun Gains thermal energy from gravitational contraction Radiates infrared 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?

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.

Clicker questions on activity #32, pages 109-112

1A: Does the force of gravity increase or decrease as you get closer to the star’s center – say halfway to the center? Increases Decreases Stays the same :00

What are the gravity and pressure at the exact center of the Sun What are the gravity and pressure at the exact center of the Sun? (Assume the Sun is perfectly symmetrical.) Zero gravity, high pressure High gravity, high pressure High gravity, zero pressure Zero gravity, zero pressure :00

2: In Figure 3, which plot shows the dependence of average particle speed on temperature? B Neither Both are possible :00

3: What would the pressure of a gas cloud in space do if the temperature of the gas was increased? Decrease Stay the same Increase :00

4: What should start to happen to the temperature and pressure in a star’s core when fusion stops in the core? Temp. decreases, pressure increases Temp. increases, pressure goes down Both T & P increase Both T & P decrease :00

5B: The force of gravity on the mass m is… Four times stronger at the surface of star A than at the surface of star B Four times stronger at the surface of star B than at the surface of star A Two times stronger at the surface of star A than at the surface of star B Two times stronger at the surface of star B than at the surface of star A :00

5D: As a star contracts, the force of gravity at its surface… Decreases Stays the same Increases It depends on other properties of the star :00

7: Some star cores stop collapsing by forming a gas where one or the other of two particle types are degenerate. What are those particle types? Electrons or Neutrinos Neutrons or Neutrinos Electrons or Neutrons Hydrogen or Neutrons Hydrogen or Helium :00

10. If the core of a star is sufficiently massive, neither kind of degeneracy pressure will stop its collapse, and the result is a… Black Dwarf Black Hole Brown Dwarf White Dwarf White Hole Yellow Submarine :00