Reminder: Reading Assignment Read Chapter 17 (“Star Birth”). Ensure that you: know the items listed in the “Summary Of Key Concepts”. Expect a quiz on Chapter 17 on Monday (Mar. 3)!!

Reading Assignment Read Chapter 18 (“The Bizarre Stellar Graveyard”). Ensure that you: know the items listed in the “Summary Of Key Concepts”. Expect a quiz on Chapter 18 on Monday (Mar. 5)!!

Birth Stages Stage 1: The protostar forms. Energy is transferred within the protostar by convection. Stage ends when photosphere reaches 3,000 K. Large surface area gives luminosity > 10 L Sun

Birth Stages Stage 2: Convective Contraction. Energy continues to be transferred via convection. Surface temperature remains 3,000 K. Since the surface temperature remains the same, but the surface area is decreasing, the luminosity decreases.

Birth Stages Why is 3000 K so special? H - ions form at temperatures above 3000 K. H - ions interact strongly with visible light. Convection transfers heat until it gets to a layer that is around 3000 K, after that photons can escape.

Birth Stages Stage 3: Radiative Contraction. Radiative transport replaces convection as the temperature gradient changes. Surface temperature increases. Luminosity increases (slightly). Fusion begins to occur. Core temperature increases (slowly).

Birth Stages Stage 4: Self-Sustaining Fusion. Power from fusion equals luminosity. Star settles into main sequence.

Minimum Mass Lower mass protostars are better at cooling than higher mass protostars. This means that lower mass protostars must contract more than higher mass protostars. Why? There’s a limit to closely electrons can be packed together. This limits how much a protostar can contract. A protostar must have a mass of at least 0.08 M Sun to become a star.

Minimum Mass What happens to protostars that are too small? They gradually cool These failed stars are called brown dwarfs. Brown dwarfs radiate mostly infrared light.

GL 105C: The Coolest Star? Credit: NASA, HST, WFPC 2, D. Golimowski (JHU)NASAHSTJHU 27 light years away. Surface temperature is approx. 2,600 K. If it was as far away as the sun it would be four times brighter than the moon.

LP : A Failed Star Flares Credit: R. Rutledge (Caltech) et al., NASACaltechNASA

Maximum Mass Electromagnetic waves exert pressure. High mass stars produce so much energy that radiation pressure can be more significant than thermal pressure. If a star has a mass greater than (approx.) 150 M Sun the radiation pressure will start to blow away some of the stars mass. The maximum mass for a star is approximately 150 M Sun.

May have started with a mass of 200 M Sun. The nebula is 4 light years across and was formed by material blown off from the star. This is a false color image taken with infrared light.

Mass Distribution Stars range in mass from 0.08 M Sun to 150 M Sun. More low-mass stars are produced than high-mass stars. Mass (MSun)Number 0.08 – – –

Eagle Nebula

Gravity and Pressure From birth to death, stars experience a constant battle between gravity and pressure. Stages of a star’s life are demarcated by changes in this battle. Examples so far: Protostar – gravity overcomes pressure Main-Sequence Star – gravity and pressure balanced.

Classify Stars by Mass The properties of Main-Sequence Stars are determined by their mass. Mass provides a way of classifying stars: Low-mass stars:M Star < 2M Sun Intermediate-mass stars:2M Sun < M Star < 8M Sun High-mass stars:8M Sun < M Star

Low-Mass Stars This group includes the Sun. Getting to the main-sequence takes 10’s to 100’s of millions of years. Energy is primarily produced via the proton-proton chain while on the main-sequence. All main-sequence low-mass stars have similar structures.

Radiative diffusion requires high temperatures. Temperature is determined by mass. Why? The lower the mass, the bigger the convection zone. Very-low-mass stars don’t have a radiation zone. Low-Mass Stars

Convection is important for solar activity. Type M stars that rotate rapidly are VERY active. These stars are called flare stars. Flares produced by these stars can have a greater x-ray luminosity than the star’s visible and infrared luminosity.

Low-Mass Stars What happens when the fuel runs out? Thermal pressure drops when fusion stops. For the first time in billions of years gravity will overcome pressure.

Low-Mass Stars What happens when the fuel runs out? The core will be almost entirely helium. Both the core and a surrounding shell of gas will start to contract. Hydrogen shell burning will start.

Low-Mass Stars The hydrogen shell and core become very hot. The rate of fusion is higher than when the star was on the main sequence. Thermal pressure increases, causes expansion of the outer layers of the star. For a while the luminosity stays fairly constant, but surface temperature goes down – this is the subgiant branch.

Increasing luminosity as hydrogen is converted to helium. (still on the main sequence) Hydrogen shell burning. Star is expanding but luminosity is fairly constant. (leaving the main sequence)

Low-Mass Stars Shell burning produces helium. Where does the helium go? The core becomes more massive. The core and hydrogen shell contract and get hotter. The rate of fusion increases. This cycle is continuously repeating itself. Meanwhile, the luminosity increases and the outer layers continue to expand. The star becomes a red giant.

Low-Mass Stars The surface of a Red Giant is weakly bound. Red Giants lose a large amount of mass in their stellar winds. All low-mass stars are expected to become red giants … … but stars with very low masses have such long lifetimes on the main sequence that none of them have become red giants yet.

Low-Mass Stars How long do the core and hydrogen-burning shell contract and heat up? Until they reach a temperature of around 10 8 K, then helium will start to fuse. Note that heat goes from hot to cold. This means that the core cannot be colder than the hydrogen-burning shell. Another thing to note – degeneracy pressure limits how much the core contracts.

Triple Alpha Reaction

Low-Mass Stars While a star is on the main sequence it has a thermostat. Does a low-mass star have a similar thermostat when it starts fusing helium? No! Because degeneracy pressure does not depend on temperature. Once helium burning starts it increases rapidly leading to the helium flash. The star loses mass, and settles into a stable, helium burning phase.

Mathematical Interlude Force from gravity. G is the gravitational constant. G= 6.67 x m 3 kg -1 s -2 g= 9.8 m s -2 M 1 and M 2 are the masses of the two objects. d is the distance between the centers of the two objects.

Mathematical Interlude Force from gravity. If I ask for the gravitational force that object 1 exerts on object 2, what formula should I use? If I ask for the gravitational force that object 2 exerts on object 1, what formula should I use? Is there a difference?

Mathematical Interlude What gravitational force does the Sun exert on the Earth? G= 6.67 x m 3 kg -1 s -2 M sun = 2 x kg M earth = 5.97 x kg d= 1.5 x 10 8 km = 1.5 x m

Mathematical Interlude What gravitational force does the Sun exert on the Earth? What gravitational force does the Earth exert on the Sun? The formula uses the same values! The answer must be the same. Also, remember Newton’s third law.

Mathematical Interlude How much matter does the Sun convert to energy each second? E= energy (Joules) m= mass (kg) c= speed of light (3 x 10 8 m s -1 ) When we talk about converting matter to energy, what formula should we think of?

Mathematical Interlude How much matter does the Sun convert to energy each second? To answer the question, we need to know how much energy the Sun produces in a second. Power = Energy per unit time Power times unit time = Energy Luminosity of the Sun = 3.8 x W Energy produced by Sun in 1 second = 3.8 x J m = 4.2 x 10 9 g