Chapter 12. Star Stuff (mostly different from book)I. Birth of Stars from Interstellar Clouds

•Young stars near clouds of gas and dust •Contraction and heating of clouds into protostars

• Hydrogen fusion stops collapse

II. Leaving the Main Sequence: Hydrogen fusion stops1. Low mass stars (M < 0.4 solar masses)

Not enough mass to ever fuse any element heavier than Hydrogen → white dwarf

2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)He fusion, red giant, ejects outer layers → white dwarf

3.High mass Stars (M > 4 solar masses)Fusion of He,C,O,…..but not Fe (Iron) fusion

Faster and faster → Core collapses → Supernova blows up and produces all elements heavier than Fe

How massive are newborn stars?

A cluster of many stars can form out of a single cloud.

Temperature

Lu

min

osi

ty•Very massive stars are rare

•Low-mass stars are common.

•Minimum mass needed to become a star: 0.08 solar masses

• How massive are newborn stars?

Low mass stars are more numerous than high mass stars

Newborn stars come in a range of masses, but cannot be less massive than 0.08MSun.

Below this mass, pressure in the core is not enough (10 million K) for hydrogen fusion, and the object becomes a “failed star” known as a brown dwarf.

Equilibrium inside M.S. stars

Question

What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools offB. Core shrinks and heats upC. Core stays at same temperatureD. Helium fusion immediately begins

Question

What happens when a star can no longer fuse hydrogen to helium in its core?

A. Core cools offB. Core shrinks and heats upC. Core stays at same temperatureD. Helium fusion immediately begins

1. Low mass stars (M < 0.4 solar masses)Not enough mass to ever fuse any element heavier than Hydrogen white dwarf

2.Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)He fusion, red giant, ejects outer layers white dwarf

3.High mass Stars (M > 4 solar masses)Fusion of He,C,O,…..but not Fe (Iron) fusionFaster and faster Core collapses Supernova Blows up and produces all elements heavier than Fe

Ch. 12 Part II (not like book). Leaving the

Main Sequence: Hydrogen fusion stops

I. Leaving the Main Sequence:BEWARE THAT THE BOOK DOES NOT USE THE SAME DEFINITIONS OF LOW, INTERMEDIATE AND HIGH MASS STARS.

AS MENTIONED, THE EXAM WILL BE BASED ON THE LECTURES AND NOT ON THE BOOK

Outline of Chapter 12 Part II Evolution and Death of Stars

Remember: Stellar Masses

Composition inside M.S. stars

Eventually the core

fills up with helium and hydrogen

fusion stops

1. Low mass stars (M < 0.4 solar masses)Not enough mass to ever fuse any element heavier than Hydrogen white dwarf

Leaving the Main Sequence: Hydrogen fusion stops

White Dwarfs

2. Intermediate mass stars (0.4 solar masses < M < 4 solar masses, including our Sun)He fusion, red giant, ejects outer layers white dwarf

I. Leaving the Main Sequence: Hydrogen fusion stops

Helium fusion requires much higher temperatures than hydrogen fusion because larger charge leads to greater repulsion

Stars like our Sun become Red Giants after they

leave the M.S. and eventually White Dwarfs

Most red giants stars eject their outer layers

A star like our sun dies by puffing off its outer layers, creating a planetary nebula.

Only a white dwarf is left behind

A star like our sun dies by puffing off its outer layers, creating a planetary nebula.

Only a white dwarf is left behind

A star like our sun dies by puffing off its outer layers, creating a planetary nebula.

Only a white dwarf is left behind

A star like our sun dies by puffing off its outer layers, creating a planetary nebula.

Only a white dwarf is left behind

3.High mass Stars (M > 4 solar masses)Fusion of He,C,O,…..but not Fe (Iron) fusionFaster and faster Core collapses Supernova Produces all elements heavier than Fe and blows up• •

II. Leaving the Main Sequence: Hydrogen fusion stops

3. High mass star (M > 4 solar masses)•Fusion of He,C,O,…..but not Fe (Iron) fusionFaster and faster Core collapses SupernovaProduces all elements heavier than Fe and blows envelope apart ejecting to interstellar space most of its mass• Supernova Remnants:Crab nebula and others

Supernovas

An evolved massive star (M > 4 Msun)

An evolved massive star (M > 4 Msun)

Supernova 1987A in a nearby galaxy is the nearest supernova observed in the last 400 years

before after

Crab Nebula: Remnant of a supernova observed in 1054 A.D.

Pulsar (a kind if neutron star) at center of Crab nebula

Older Supernova Remnant