lecture 21
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How can the stars continue to burn, emitting huge amounts of energy?
fusion
What is fusion?
A process in which small atoms combine or fuse to form a larger atom, in the process, energy is created
The missing mass between the mass of the elements that started the reaction and the mass of the final nucleus, which is less than the initial mass, is?
nuclear energy that powers the Sun and eventually radiates out into space
What is the most common fusion reaction?
4 hydrogen = Helium nucleus
How many steps is nuclear fusion in the sun?
3 steps
In the Sun, what triggers the fusion?
High P and T in the star’s interior
What is the net effect of nuclear fusion?
4 protons are converted into a He-4 nucleus, and energy is released
The sunlight that you see is generated by?
The conversion of 600 million tons of H into He each second
When are stars born?
The moment when Hydrogen burning (nuclear fusion reaction) initiates in a cloud of gas and dust composed of mostly H and He
What is the Hertzsprung-Russell Diagram?
It shows the different evolutionary stages of stellar evolution corresponding to a change in Temperature and luminosity
What are main-sequence stars?
Stars in the H-burning phase
What are red giants?
Stars that emit a lot of energy, but whose surfaces are cool
What are white dwarfs?
Stars that have low energy emission, but high surface temperatures
The main sequence stars balance?
gravity (inward pressure) and gas pressure (outward pressure) to reach hydrostatic equilibrium
How long do stars spend in the main sequence?
They spend 90% of their lifetime in this phase, as long as there is H to burn, it remains in the main sequence
What happens if H runs out in the core?
The core becomes extinguished and H burning will begin in the shell
What does it mean for a core to be extinguished?
The core is made entirely of He, gas pressure is less than gravity causing it to contract, and the contractions makes the core denser and hotter
What does H-burning in the shell do?
Any remaining H begins to undergo fusion, causing the star to expand again and increase in luminosity
Once there is no more H to burn, what happens?
The star becomes a red giant as the core will collapse and He fusion takes place to form C and O
During the red giant phase why does the star significantly expand?
Due to H-fusion in the shell causing outward force
When does nuclear fusion reactions stop, and why?
Once the He is exhausted meaning there is not enough outward force causing the star to collapse
The gravitational collapse will cause the star to?
Heat up
How is a planetary nebula formed?
After the nuclear fuel is exhausted, stars shed their outer layers into a ring leaving behind only the hot core of the star
Why do stars become white dwarfs after the planetary nebula is formed?
Only the hot core of the star remains, and because there is no nuclear fuel there is no more fusion, so no more energy output