The Life History of Stars – Young Stars
The Importance of Mass
•The entire history of a star depends on its mass and almost
nothing else
•The more mass a star has, the faster it does everything
•The stages of a star differ based on what is happening in the
core of the star
•The properties of a star vary wildly as it passes through
different stages
•Qualitatively, stars have similar histories, with one big split:
•Low mass stars (< 8 MSun) have quiet deaths
•High mass stars (> 8 MSun) go out with a bang
Low Mass Stars (< 8 MSun) - Outline
•Molecular Cloud Mommy
•Protostar
Fetus
•Main Sequence
Adult
•Red Giant
•Core Helium-Burning Old Woman
•Double Shell-Burning
•Planetary nebula Cancer
•White Dwarf
Corpse
Which stage takes the
largest amount of time?
A) Main Sequence
B) Red Giant
C) Core Helium-Burning
D) Double Shell-Burning
E) Planetary Nebula
•The more massive the star, the faster it does everything
•From Main Sequence to Planetary Nebula, each stage goes
faster than the previous
Molecular Clouds
•Huge, cool, relatively dense clouds of gas
and dust
•Gravity causes them to begin to contract
•Clumps begin forming – destined to
become stellar systems
•Composition:
•75% hydrogen (H2), 23% helium (He),
< 2% other
•Molecular Cloud
•Protostar
•Main Sequence
•Red Giant
•Core HeliumBurning
•Double ShellBurning
•Planetary Nebula
•White Dwarf
Molecular Clouds – Eagle Nebula
Molecular Clouds – Keyhole and Orion
Formation of Protostars
•Cloud fragments to form multiple stars
•Stars usually form in clusters
•Often, two or more stars remain in
orbit
•The stars are a balance of pressure vs.
gravity
•Heat leaks out – they cool off
•Reduced pressure – gravity wins – it
contracts
•Molecular Cloud
•Protostar
•Main Sequence
•Red Giant
•Core HeliumBurning
•Double ShellBurning
•Planetary Nebula
•White Dwarf
Negative Heat Capacity
What happen as heat leaks out
•They cool off
•By P = knT, they have less pressure
•Gravity defeats pressure
•They contract
•Energy is converted
•Gravitational Energy  Kinetic energy
•Kinetic energy  Heat
•Net effect: When you remove heat, a star gets:
•Smaller
•Hotter (!)
H-R diagram: Protostar
Double ShellBurning
Core HeliumBurning
•Molecular Cloud
•Protostar
•Main Sequence
•Red Giant
•Core HeliumBurning
•Double ShellBurning
•Planetary Nebula
•White Dwarf
Stellar Winds
•Stars are still embedded in molecular clouds of gas and dust
•Stars begin blowing out gas - winds
•Wind blows away the dust – we see star
A Star is Born
•The interior of the star is getting
hotter and hotter
•At 10 million K, fusion starts
•This creates energy
•It replaces the lost heat – the
star stops getting dimmer
•The surface continues shrinking
for a while
•Left and a little up on the H-R
diagram
•It becomes a Main Sequence star
•Molecular Cloud
•Protostar
•Main Sequence
•Red Giant
•Core HeliumBurning
•Double ShellBurning
•Planetary Nebula
•White Dwarf
H-R diagram: To the Main Sequence
Double ShellBurning
Core HeliumBurning
•Molecular Cloud
•Protostar
•Main Sequence
•Red Giant
•Core HeliumBurning
•Double ShellBurning
•Planetary Nebula
•White Dwarf
Mass Distribution of Stars
•Stars Range from about 0.08 – 150 Msun
•Lighter than 0.08 – they don’t get hot enough for fusion
•Heavier than 150 – they burn so furiously they blow off
their outer layers
•Light stars much more common than heavy ones
•Objects lighter than 0.08 MSun are called
Brown
brown
Dwarf
dwarfs
Small
Star
High Mass Stars
Eta Carinae
About 150 MSun
HDE 269810
Peony Nebula Star
Life on the Main Sequence
•The star is now in a steady state – it is “burning” hydrogen
4H + 2e-  He + 2 + energy
•It burns at exactly the right rate to replace the energy lost
•For the Sun, there is enough fuel in the central part to keep
it burning steadily for 10 billion years
•All stars are in a balance of pressure vs. gravity
•To compensate for larger masses, they have to be bigger R  M
•They have lower density, which lets heat escape faster
3.5
•They have to burn fuel faster to compensate
LM
0.4
•To burn faster, they have to be a little hotter
T M
Structure of Main Sequence Stars
•All burn
hydrogen to
helium at their
cores
•Solar mass: Convection on the outside
•High mass: Convection on the inside
•Low mass: Convection everywhere
Announcements
Date
Today
Thursday
Friday
Monday
Read
Sec. 12.1, 12.2
Sec. 12.3
Sec. 13.2, 11.3, 13.1, 13.3
Study for Test
Lab Tonight
•Out-4, In-8
6/15
Evolution on the Main Sequence
4H + 2e-  He + 2 + energy
•Number of particles decreased:
•The neutrinos leave
•6 particles  1 particle
•Reduced pressure: P = knT
•Core shrinks slightly
•Temperature rises slightly
•Fuel burns a little faster
•Star gets a little more luminous
•Up slightly on H-R diagram
Evolution on the Main Sequence
Double ShellBurning
Core HeliumBurning
•Molecular Cloud
•Protostar
•Main Sequence
•Red Giant
•Core HeliumBurning
•Double ShellBurning
•Planetary Nebula
•White Dwarf
Lifetime on the Main Sequence
•The amount of fuel in a star is proportional to the mass
•How fast they burn fuel is proportional to the Luminosity
•Massive stars burn fuel much faster L  M 3.5
Cl
O5
B0
A0
A5
M
M
t
 3.5  M 2.5
G2
L
M
G5
Age of Universe
M7
•Stars lighter than Sun still main sequence
Which stars run out of fuel first?
A) Massive stars B) Light stars
C) Same time
D) Insufficient information
M
60
18
3
2
1
0.9
0.2
life
360 ky
10 My
400 My
1.1 Gy
10 Gy
15 Gy
500 Gy