The Formation and Structure of Stars

1
The Formation and Structure of Stars

• Seeds chapter 9

2
Objective

• Examine the theory and evidence of how stars form
  from clouds of gas dust

3
Introduction

• Gravity forms a star
• Outward pressure stops the contraction
• Balance depends on nuclear energy
• 1s of solar energy could power civilisation for
  2Myr

4
The Birth of Stars
5
The Interstellar Medium

• Nebula gas/dust cloud
• Emission nebulae
• Reflection nebulae
• Interstellar reddening
• Infrared cirrus
• Our Sun lies inside a hot interstellar bubble

6
The Formation of Stars from the Interstellar
Medium

• Molecular clouds
• Stellar associations
• Our Sun probably formed in a cluster

7
The Formation of Protostars

• Protostar
• Evolutionary track
• The sun was much brighter as a protostar

8
Observations of Star Formation

• Visible at birth line
• T Tauri objects
• Herbig Haro objects
• Bipolar flow
• Bok globules
• Young stars interact with their environment

9
How Stars Generate Nuclear Energy
10
Nuclear Binding Energy

• 4 forces of nature
• Strong force binds atomic nuclei
• Nuclear fission separates
• Nuclear fusion combines

11
Hydrogen fusion

• Like charges must overcome the Coloumb barrier
• Proton-proton chain
• CNO (bi)cycle

12
The Solar Neutrino Problem

• Neutrinos almost never interact with atoms
• 1012 go through us each second could penetrate
  a lyr of lead
• Does the observed rate from the Sun indicate
  neutrinos have mass?

13
Heavy-Element Fusion

• Triple alpha process helium fuses to produce
  carbon
• He fusion 100MK, C600MK, even hotter for
  heavier elements

14
The Pressure-Temperature Thermostat

• Pressure balances gravity
• Expansion slows energy production
• Compression increases reaction rate
• Gravity always wins eventually

15
Stellar Structure
16
The Laws of Mass and Energy

• Conservation of mass
• Conservation of Energy

17
Hydrostatic Equilibrium

• Hydrostatic equilibrium pressure in each layer
  balances weight
• Gas (and radiation) pressure prevents the Sun
  from collapse

18
Energy Transport

• Energy transport is from hot to cool
• Opacity governs radiative transport

19
Stellar Models

• Stellar model calculates conditions in star from
  laws of
• mass conservation
• energy conservation
• hydrostatic equilibrium
• energy transport

20
Main-Sequence Stars
21
The Mass-Luminosity Relation

• Luminosity rises with mass
• More energy required to support weight
• Brown dwarfs too light for fusion

22
The Life of a Main-Sequence Star

• Gradual change
• Gets brighter, cooler
• In few billion years from now the Sun will
  brighten enough to boil Earths oceans

23
The Orion Nebula
24
Evidence of Young Stars

• Bright hot young stars
• T Tauri stars
• Orion nebula

25
The Molecular Cloud

• IR/radio studies
• Dense molecular clouds screen UV
• Warm, contracting centres

26
The Infrared Objects

• IR penetrates clouds
• Pre-main sequence stars in dust cocoons

27
Telling the Story of Star Formation in Orion

• Several generations of stars already formed
• The Orion Nebula may change in appearance in the
  next few millennia as new stars illuminate it