Chapter 12 Star Stuff

1
We are star stuff because the elements
necessary for life were made in stars
2
How do stars form?
3
Stars are born in molecular clouds consisting
mostly of hydrogen molecules
4
Stars form in places where gravity can overcome
thermal pressure in a cloud
5
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

6
Star-forming clouds emit infrared light because
of the heat generated as stars form
7
Orion Nebula is one of the closest star-forming
clouds
8
Solar-system formation is a good example of star
birth
9
As gravity forces a cloud to become smaller, it
begins to spin faster and faster
10
As gravity forces a cloud to become smaller, it
begins to spin faster and faster Conservation
of angular momentum
11
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
12
Angular momentum leads to Rotation of
protostar Disk formation and sometimes
Jets from protostar Fragmentation into
binary
13
Disks and jets seen around young stars
14
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)

15
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
16
How massive are newborn stars?
17
A cluster of many stars can form out of a single
cloud.
18
Very massive stars are rare
Low-mass stars are common
19
Stars more massive than 150 MSun would blow
themselves apart
Stars less massive than 0.08 MSun cant sustain
fusion
20
Pressure Gravity

• If M gt 0.08 MSun, then gravitational contraction
  heats core until fusion begins
• If M lt 0.08 MSun, degeneracy pressure stops
  gravitational contraction before fusion can begin

21
Degeneracy Pressure Laws of quantum mechanics
prohibit more than one electron (or neutron) from
occupying same state in same place at same time
22
Thermal Pressure Depends on heat content The
main form of pressure in most stars
Degeneracy Pressure Particles cant be in same
state in same place Doesnt depend on heat
content
23
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?

24
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.

25
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.

26
Clicker questions on activity 44
27
1A Does the force of gravity increase or
decrease as you get closer to the stars center
say halfway to the center?

1. Increases
2. Decreases
3. Stays the same

28
What are the gravity and pressure at the exact
center of the Sun?

1. Zero gravity, high pressure
2. High gravity, high pressure
3. High gravity, zero pressure
4. Zero gravity, zero pressure

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4 What should start to happen to the temperature
and pressure in a stars core when fusion stops
in the core?

1. Temp. decreases, pressure increases
2. Temp. increases, pressure goes down
3. Both T P increase
4. Both T P decrease

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5B The force of gravity on the mass m is

1. Four times stronger at the surface of star A than
   at the surface of star B
2. Four times stronger at the surface of star B than
   at the surface of star A
3. Two times stronger at the surface of star A than
   at the surface of star B
4. Two times stronger at the surface of star B than
   at the surface of star A

31
Question 5D

• So, as a star contracts, the force of gravity at
  its surface does what? _________________

32
7 Some star cores stop collapsing by forming a
degenerate gas, of what kinds of particles?

1. Electrons or Neutrinos
2. Neutrons or Neutrinos
3. Electrons or Neutrons
4. Hydrogen or Neutrons
5. Hydrogen or Helium