Last time: looked at proton-proton chain to convert Hydrogen into Helium, releases energy.

Transcription

1 Last time: looked at proton-proton chain to convert Hydrogen into Helium, releases energy.

2 Last time: looked at proton-proton chain to convert Hydrogen into Helium, releases energy. Fusion rate ~ Temperature i.e. the hotter it is, the more the core will fuse.

3 Last time: looked at proton-proton chain to convert Hydrogen into Helium, releases energy. Fusion rate ~ Temperature i.e. the hotter it is, the more the core will fuse. Even hotter temperatures: you can start fusing heavier elements. This is NOT happening in the sun now.

4 Last time: looked at proton-proton chain to convert Hydrogen into Helium, releases energy. Fusion rate ~ Temperature i.e. the hotter it is, the more the core will fuse. Even hotter temperatures: you can start fusing heavier elements. This is NOT happening in the sun now. E = mc 2

5 Stars exist in a state of hydrostatic equilibrium for most of their lives. This balances the inward force of gravity with the outward pressure of very hot gasses.

6 Thinking about hydrostatic equilibrium (gravity vs. pressure): Fusion rate ~ Temperature i.e. the hotter it is, the more the core will fuse. What would happen if the core temperature of the sun dropped a little bit? DISCUSS.

7 Stars exist in a state of hydrostatic equilibrium for most of their lives. This balances the inward force of gravity with the outward pressure of very hot gasses. Decline in core temperature causes fusion rate to drop, so core contracts and heats up. Rise in core temperature causes fusion rate to rise, so core expands and cools down.

8 What happens to the sun as it burns through its hydrogen? When the sun was born. 1 helium fractional composition hydrogen R Distance from the Sun s center. The sun started out with some Helium when it was born, ~10% of the sun by mass, and that helium was spread throughout the sun.

9 What happens to the sun as it burns through its hydrogen? After 5 billion years 1 helium fractional composition hydrogen R Distance from the Sun s center. The core of the sun is where fusion happens and suddenly most of the core is actually made up of helium, as the hydrogen is consumed, still most of the sun is still made of hydrogen, but towards the outter layers.

10 What happens to the sun as it burns through its hydrogen? After 10 billion years. 1 helium fractional composition hydrogen R Distance from the Sun s center. The core of the sun is where fusion happens and suddenly most of the core is actually made up of helium, as the hydrogen is consumed, still most of the sun is still made of hydrogen, but towards the outter layers.

11 When the sun is burning hydrogen via fusion as usual, it is a main sequence star. The temperature of the star determines the rate of nuclear fusion. hotter stars, much faster burners main sequence colder stars, slower burners If we have mass constraints on stars (giving their hydrogen mass = nuclear fusion fuel) and their luminosity, we can estimate their lifetimes.

12 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M How do we measure luminosity again? How do we measure mass again?

13 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M How do we measure luminosity again? How do we measure mass again? F = L 4 r 2

14 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M How do we measure luminosity again? How do we measure mass again? F = L 4 r 2

15 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M Life expectancy of the sun: 10 billion years. Life expectancy of blue, massive 10 M star: Life expectancy of 0.1 M red dwarf star:

16 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M Life expectancy of the sun: 10 billion years. Life expectancy of blue, massive 10 M star: Burning fuel 10 4 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun. Life expectancy of 0.1 M red dwarf star:

17 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M Life expectancy of the sun: 10 billion years. Life expectancy of blue, massive 10 M star: 10 million years Burning fuel 10 4 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun. Life expectancy of 0.1 M red dwarf star:

18 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M Life expectancy of the sun: 10 billion years. Life expectancy of blue, massive 10 M star: 10 million years Burning fuel 10 4 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun. Life expectancy of 0.1 M red dwarf star: Burning fuel 10 3 times slower, 1/10th the fuel, lifetime is 100 that of the sun.

19 Mass and Lifetimes of Main Sequence Stars L 10 M main sequence L 0.1 M Life expectancy of the sun: 10 billion years. Life expectancy of blue, massive 10 M star: 10 million years Burning fuel 10 4 times faster, 10 times more fuel, lifetime is 1/1000 that of the sun. Life expectancy of 0.1 M red dwarf star: 1 trillion years Burning fuel 10 3 times slower, 1/10th the fuel, lifetime is 100 that of the sun.

20 Eventually, the sun runs out of its fuel, like all stars. What happens then??

21 Eventually, the sun runs out of its fuel, like all stars. What happens then??

22 Stars Part 3: life & death.

23 Stars Part 3: life & death.

24 Where a star sits on the HR diagram is determined by - its mass, - its age. Stars on the main sequence are burning through their main supply of hydrogen via the p-p supergiants main sequence white dwarfs giants chain (and when really massive, the CNO cycle). We learned how to compute lifetimes of stars on the main sequence knowing their masses and luminosities.

25 How does a star get there? And what happens when that fuel runs out? Where a star sits on the HR diagram is determined by - its mass, - its age. Stars on the main sequence are burning through their main supply of hydrogen via the p-p supergiants main sequence white dwarfs giants chain (and when really massive, the CNO cycle). We learned how to compute lifetimes of stars on the main sequence knowing their masses and luminosities.

26 Early life! How are stars born?

27 Collapse of molecular gas clouds Conservation of Angular Momentum propels gas into fast rotating disks from which solar systems form.

28 Collapse of molecular gas clouds Conservation of Angular Momentum propels gas into fast rotating disks from which solar systems form.

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32 Along with the core which becomes a star, a disk forms

33 What do star-forming clouds look like?

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38 As a star is born, it accretes mass from its disk and heats up. Fusion has started, and the star is adding mass

39 Young stars which are still accreting material are called T-Tauri Stars. Because mass is piling on, they sometimes have explosive outbursts.

40 Young stars which are still accreting material are called T-Tauri Stars. Because mass is piling on, they sometimes have explosive outbursts.

41 Eventually the dust and debris in the protoplanetary disk is cleared out, potentially leaving planets & other debris behind. The star is no longer adding mass and has landed on the main sequence, where it will sit for most of its lifetime, until it runs out of fuel

42 The birth of stars. Why do young stars form disks? (a) once there is an overdense region of matter it has a run-away effect and everything collapses down into a disk due to gravity. (b) the angular momentum of the original star forming cloud builds with time forming a disk (c) the angular momentum of the star forming cloud is constant with time and so as things collapse they start to spin and through friction form a disk, (d) trick question; not all stars have disks when they form.

43 (a) (b) The birth of stars. Why do young stars form disks? once there is an overdense region of matter it has a run-away effect and everything collapses down into a disk due to gravity. the angular momentum of the original star forming cloud builds with time forming a disk (c) the angular momentum of the star forming cloud is constant with time and so as things collapse they start to spin and through friction form a disk, (d) trick question; not all stars have disks when they form.

44 The birth of stars.