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?

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).

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.

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

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

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).

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.

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.

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.

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

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

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.

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.

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.

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.

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?

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.

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.