Lesson 7: Star Birth, Low Mass Stars, High Mass Stars Flashcards
Where do stars form? What do you call the space in-between stars?
- Stars form in dark clouds of dusty gas in interstellar space
- Interstellar- space between stars
Discuss the interplay between gravity and pressure with respect to star formation
- Gravity can create stars only if it can overcome the force of thermal pressure in a cloud
- Gravity within a contracting gas cloud becomes stronger as the gas becomes denser
What is the name of the type of cloud stars form in? Normally, how much mass can be found in one of these?
molecular cloud
contain at least a few hundred solar masses for gravity to overcome pressure
How does a molecular cloud dissipate heat?
- As stars begin to form, dust grains that absorb visible light heat up and emit infrared light
What would happen to a contracting cloud fragment if it were not able to radiate away its thermal energy?
It would continue contracting, but its temperature would not change
Explain the generally agreed upon process that creates stars
- Dense cores form within a molecular cloud
- Protostar with surrounding disk of material
- Stellar wind is created, jets as well
- Newly formed star with disk of material
What is conservation of energy? How is it related to star formation?
Because of conservation of energy, the cloud heats up as its collapses inwards
What is conservation of angular momentum? How is it related to star formation?
Because of the conservation of angular momentum, the cloud spins faster as it contracts and flattens
How do you go from contracting gas cloud to protostar to main sequence star?
- from the cloud, Jets are observed coming from centres of disks around new formed protostars
- A protostar contracts and heats until the core temperature is sufficient for hydrogen fusion
- Contraction ends when energy released by hydrogen fusion balances energy radiated from the surface
Summarize the steps in the formation of stars
- 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 achieves long-lasting state of balance
When a large cloud of gas forms stars, compare the number of high mass to low mass stars that form there
how common are massive stars vs low-mass stars
- Very massive stars are rare
- low-mass stars are common
What is the lower limit to a star’s mass? Why?
What is the upper limit to a star’s mass? Why?
Stars more massive than 300MSun would blow apart
* Photons exert a slight amount of pressure when they strike matter
* Very massive stars are so luminous that the collective pressure of photons drives their matter into space
Stars less massive than 0.08MSun can’t sustain fusion
* Fusion will not begin in a contracting cloud if some sort of force stops contraction before the core temperature rises above 107 K
(the cloud isn’t contracting enough, core has to be a certain temperature)
What is degeneracy pressure?
Degeneracy Pressure: prohibit two electrons from occupying the same state in the same place
What is thermal pressure?
- The main form of pressure in most stars
(depends on heat content)
What are Brown Dwarfs?
Starlike objects not massive enough to start fusion are **brown dwarfs **
A brown dwarf emits infrared light because of heat left over from contraction
How long does it take for a protostar to become a main sequence star?
use the Sun
It takes 30 million years for a star like the Sun to become a main sequence star
Why do protostars rotate rather fast and end up surrounded by disks of material?
If a cloud spins even a little bit, the spin increases as it contracts. The conservation of angular momentum causes this to occur.
What are the life stages of a low-mass star?
- Main Sequence: H fuses to He in core
- Red Giant: H fuses to He in shell around He core
- Helium Core Fusion: He fuses to C in core, while H fuses to He in shell
- Double Shell Fusion: H and He both fuse in shells
Planetary Nebula: leaves white dwarf behind
What is a helium flash?
- Ignition of He core happens VERY quickly (flash)
The core temperature rises rapidly when helium fusion begins
What does a low-mass star eventually turn into?
white dwarf
How is a white dwarf supported if it is not fusing?
what is the Chandrasekhar limit?
Supported only by electron degeneracy pressure, causing it to be extremely dense
halt the gravitational collapse of a star if its mass is below the Chandrasekhar limit (1.44 solar masses)
What are the life stages of a high-mass star?
- Main Sequence: H fuses to He in core
- Red Supergiant: H fuses to He in shell around He core
- Helium Core Fusion: He fuses to C in core, while H fuses to He in shell
- Multiple Shell Fusion: many elements fuse in shells
Supernova leaves neutron star behind
What is the CNO cycle? Why does the CNO cycle NOT exist in low-mass stars?
- The CNO cycle (carbo- nitrogen-oxygen) is one of the two known sets of fusion reactions by which stars convert hydrogen to helium (the other is the proton- proton chain)
- The CNO cycle is dominant in stars that are more than ~1.3 times the mass of the Sun
You may have heard that we are all ‘made of star stuff’ before, why do we say this?
- Big Bang made 75% H, 25% He — stars make everything else
- Helium fusion can make carbon in low-mass stars
- The CNO cycle can change Carb. into Nitrog. and Oxy.
Carbon wouldn’t exist if it weren’t for stars
Why is iron the last element that can be made in the high-mass star life cycle?
- Iron is a dead end for fusion because nuclear reactions involving iron do not release energy
(Fe has lowest mass per nuclear particle)
Iron doesn’t create energy anymore and can’t fall apart
What is a supernova? How does it occur?
- Iron builds up in the core until degeneracy pressure can no longer resist gravity
The core then suddenly collapses, creating a supernova explosion
What does a high-mass star eventually turn into?
- Core degeneracy pressure goes away because electrons combine with protons, making neutrons and neutrinos
Neutrons collapse to the centre, forming a **neutron star **
What are the timescales of evolution for different mass stars?
what is the general rule for low vs high mass stars going thru stages
general principle: massive stars go through all stages of evolution faster than low-mass stars do.
