Goal: To understand the lifetime of a star and how the mass of a star determines its lifetime

1. Goal: To understand the lifetime of a star and how the mass of a star determines its lifetime Objectives: To learn what defines a Main sequence star To understand why Energy is important for a star To examine the Cores or stars To understand what determines the Lifetime of a star To see when the Beginning of the end is going to occur During break: Why does fusion create energy?

2. Why does fusion create energy? • 4 protons have more mass than 1 Helium atom. • So, when you fuse protons into helium, you loose mass. • Mass is a form of energy. • Once again, energy is always conserved! • So, you gain energy (in forms of photons and neutrinos).

3. Birth of a star • A star is born when its core can finally generate energy via nuclear fusion. • This energy prevents the core from collapsing. • This is done by radiation pressure and gas pressure (they counteract gravity). • But to keep this up requires the constant generation of energy in the core.

4. Main Sequence Star • Now that the star is born, what will it do?

5. Main Sequence Star • Now that the star is born, what will it do? • Well, not much.

6. Other than the stuff our sun does now • Stars on the main sequence slowly burn their fuel. • While the do get a little brighter with time (10-50% over their lifetime), their outer temperature, radius, and brightness all stay approximately the same (well within a small range anyway).

7. Core • Now lets examine different sizes of stars. • Stars come in all sizes from 200 times the mass of our sun to 1% the mass of our sun.

8. Smallest stars • The smallest stars are called Brown Dwarfs. • These stars are between 1-8% of the mass of our sun and about the size of Jupiter. • These stars are too small to fuse Hydrogen. • Instead they fuse Deuterium into Helium.

9. Red Dwarfs • Next up the stellar ladder are Red Dwarfs. • Red dwarfs are 8-40% the mass of the sun. • Unlike the sun, the Red Dwarfs do not have a Radiative Zone (a zone where matter does not move through). • In fact, the entire star is convective (like a boiling pan of water). • So, eventually, it will burn all the Hydrogen in the star to Helium.

10. continued • Red Dwarfs are very dim compared to the sun. • What does that tell you about the energy generated at the core of a Red Dwarf? • A) there is less of it • B) it takes longer to get to the surface • C) the energy has a harder time escaping from the star • D) tells you nothing

11. What will happen? • What happens when the red dwarf runs out of Hydrogen to burn?

12. What does this tell you about the expected lifetime of a Red Dwarf? • A) It is longer than our sun • B) It is the same as our sun • C) It is shorter than our sun • D) Tells us nothing about its expected lifetime.

13. Yellow/Orange Dwarfs • This is just a silly way of saying stars like our sun. • So, starts like our sun. • They have Radiative Zones which separate the core from the rest of the star (much like our Stratosphere keeps clouds in the Troposphere). • The core is about 10% of the mass of the sun.

14. Larger Main Sequence Stars • Here we have Blue stars. • Blue stars are always big. • They are very hot. • Their cores are very hot. • That means that even though they are bigger, they use up their fuel a lot faster. • So, they don’t live very long. • A star stays on the main sequence for about: 10 Billion years / (its Mass in solar masses)2 • So, a star 10 time the mass of our sun will only be on the main sequence for 100 million years – they don’t live long.

15. Properties of stars • Temperature: bigger star means higher temps both on surface and in the core. • Lifetimes: Bigger stars have shorter lives. • Color: Big main sequence stars are blue. Medium ones yellow/orange/white. Small ones are red. • Brightness: Bigger means much brighter (Mass cubed). • Size: More massive stars have bigger sizes (by factor of mass). • Density: Oddly, bigger stars have LOWER densities! The biggest stars have an average density of our air.

16. Concept question • If a star is fusing Helium into something else in its core then is it considered a Main Sequence Star? • Suppose a star uses up all its Hydrogen in its core so only does fusion of Hydrogen to Helium in a shell outside of the core. Is it considered a Main Sequence Star?

17. However • No matter what the size of star, with the exception of the Brown Dwarf, all fuse hydrogen into helium in the core (using either p-p chain or CNO cycle). • Eventually each of them will run out of fuel. • What happens next? Well, stay tuned. It all depends on the size of the star.

18. Conclusion • Stars don’t change much on the main sequence over the course of their lifetime. • Stars come in a wide range of masses (0.01 to 200 solar masses). • Different massed stars have slightly different attributes, but all do the same thing – fuse protons into Helium.