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Star Formation
Interstellar Medium (ISM): Gas & dust between stars
(100 – 200 atoms/cm3)
Composition
Gas: 70% H (neutral H, H+, H2)
22 – 25% He
3 – 5% “metals”
Dust: Silicate grains (rock/sand)
Graphite (Carbon)
Basic organic material
Star Formation
Nebula (Large cloud of ISM)
- low density (200 atoms/cm3)
- T ~ 20 K
- D ~ 150 L.Y.
- M ~ 104 – 106 MSuns
Great Orion Nebula
Gravity causes:
- globule contraction
- material accumulation
- central region to heat up
Rotation causes:
- Densest region to become spherical (proto-star)
- Outer gases cast off into proto-planetary disk
Star Formation
A protostar’s mass will determine the star’s:
∴Position on
- temperature (Spectral Class)
Main Sequence
- luminosity (Absolute Magnitude)
- length of protostar stage (in fact, all stages)
A star’s evolution depends on how massive it is:
Low Mass (0.08 MSuns < M < 2 MSuns)
Intermediate Mass (2 MSuns < M < 8 MSuns)
High Mass (M > 8 MSuns)
We’ll trace Sun’s evolution then compare more massive stars’ to it
Star Formation
HR Diagram is used to map out each star’s evolutionary cycle.
1. Cool protostar collapses under gravity
2. Pressure inside builds up which increases temperature
3. Gas pressure increases, but gravity is too strong; collapse continues
4. Once Tcore ~ 15 million K, H → He fusion reactions start in core
Stage 1: Protostar
Protostar
Protostar’s mass determines R, T, L, & formation time.
Higher mass ⇒ more gravity
• larger in size
• hotter
• more luminous
• quicker formation
Vice versa for lower mass stars
Upper: 120 MSun ⇒ Prad too great
Lower: 0.08 MSun ⇒ too cool for H-fusion to begin (Brown Dwarf)
Stage 2: Main Sequence
A STAR IS BORN!!!
(Once fusion reactions in the core begin & the star stabilizes)
Hydrostatic Equilibrium:
Equilibrium Inward Gravity = Outward Pressure
Main Sequence
Longest stage (90%) of star’s life
Core Hydrogen gradually replaced with Helium
(Presently the is Sun’s core is 35% H, 64% He)
Main Sequence
Following Relations Hold True for MS Stars
Size:
Size
R= M
Lifespan:
Luminosity:
3.5
L=M
3
1
t= 2
M
Answers to each property are given in solar units
Main Sequence
Exercise:
Exercise Find R, L, & t for a star with a mass of 4 MSuns
8
R Sun = 6.96 x 10 m
LSun = 3.83 x 10
26
W
TSun = 5800 K
10
t Sun = 10 years
Physical Units
R
Solar Units
2 RSun
L
128 LSun
4.90 x 1028 W
Life
1/16 tSun
6.25 x 108 years
1.392 x 109 m
Solar Structure
Solar Structure
We cannot see beneath the surface, but we create a model based on
computer simulations & physical equations
Solar Core
Diameter = 280,000 km (0.2 DSun)
Density = 160 g/cm3
Tcore = 15 million Kelvin
Temperature is high enough to:
• Create a fully ionized plasma
• Generate energy through nuclear fusion
Nuclear Fusion
Nuclear Fusion
4 H → 1 He (+ 2 e+ + 2ν + 2γ)
Mass of 4 H nuclei > mass of 1 He nucleus
Difference in mass is converted into energy.
Δ m → energy according to E = mc2
Astronomers estimate that the Sun will run out of core Hydrogen
in a little under 10 Billion years.
Radiation Zone (350,000 km region that surrounds core)
Energy is transported by absorption/emission of photons by atoms:
• photons are emitted in random directions
Radiation Zone
Core
Atoms
How long does it take the average photon to make it out of the Sun?
~ 1 Million Years!!!
Convection Zone
(210,000 km above radiation zone)
• energy is transported mostly through convection currents
Hot gas (bright) rises and heats surroundings; cool gas (dark) sinks
Photospheric Granulation
Photosphere
• marks the Sun’s outer boundary (500 km)
• photons emitted into space (“light sphere”)
• Solar spectrum made from gases here
Chromosphere
• Gaseous region (2300 km) above photosphere
• Appears red, due to strong Hα emission
Can only view using proper filter (Hα) or during a Solar Eclipse
Note: Helium was first observed in here
Corona
Large (106 km), hot (106 K) region of diffuse gas surrounding Sun
• Heated to such high temperatures by the Sun’s magnetic field
• Visible during a total solar eclipse or with use of coronagraph
Solar Wind
Continual stream of high energy ions and electrons that travel
through interplanetary space at speeds of ~ 300 km/s.
Earth’s magnetic field deflects the solar wind
Sunspots
Sunspots: large, dark areas that appear on the photosphere
Central dark region (umbra)
umbra surrounded by flowing gases (penumbra)
penumbra
D ~ 10,000 – 50,000 km; T ~ 4000 – 5000 K
Spectrum of sunspot indicates magnetic influence
Solar Dynamo
Differential rotation causes magnetic field to wrap around Sun,
forming kinks that penetrate the photosphere
Solar Cycle
Continual wrapping ⇒ more “kinks” ⇒ more activity
Once max. occurs, magnetic field re-stabilizes (with magnetic poles
reversed) and the cycle begins again.
Solar Activity
Solar Flares – violent eruptions of solar material (106 H-bombs)
Coronal Mass Ejection
High energy explosions (1012 H-bombs) resulting from coronal gas getting
trapped within tangled magnetic kinks
Coronal Mass Ejections
If directed toward Earth, they can:
- Cause power outages
- Disrupt satellite function
- Harm astronauts
- Destroy atmosphere/ozone?
Main Sequence
Interior structure differs by mass
Fusion process differs as well;
- low mass: proton-proton chain
- high mass: CNO cycle
Post-Main Sequence
(Low Mass Stars)
Once the core Hydrogen supply runs out:
- Inert Helium core left behind starts to contract
- H-burning shell surrounds the core
- Excess heat causes the star to expand (Subgiant)
Post-Main Sequence
(Low Mass Stars)
Simultaneously, the star expands and cools while the He core
contracts and heats up
Once Tcore ~ 108 K,
He → C fusion begins
The star now stabilizes as a Red Giant
Stage 3: Red Giant
Helium (in core) begins to fuse into Carbon (basic element of life)
(H-burning shell adds more He to core)
Triple-α Process
• Size grows to ~ R = 2x 108 km
• Stellar winds increase
Red Giant
Once the core Helium supply runs out:
- Inert Carbon core is left behind and starts to contract
- Helium just above core fuses into Carbon (He-burning shell)
(H-burning shell remains above)
- Excess heat causes the star to expand (2nd Red Giant stage)
2nd Red Giant Stage
Low mass stars won’t get hot enough to start fusing Carbon
Post-Main Sequence
(Intermediate/High Mass Stars)
Same process occurs for higher mass stars when core Hydrogen
runs out but the stars become Supergiants (Stage 3)
Carbon fuses into heavier elements
Supergiants
Multiple layers of fusion shells form around the core
Iron (Fe) is the last element to be created inside stellar cores
Supergiants can become highly unstable due to imbalance between
Inward Gravity & Outward Pressure
Pulsating Variable Stars
Stars found in the Instability Strip of the HR Diagram experience
a cyclic change in size & temperature ⇒ changes in luminosity.
Period-Luminosity Relationship
(used as a way to determine distance)