Lecture 10: A Star is Born- 7 Easy Steps to Thermonuclear Ignition

"The heavens call to you, and circle around you, displaying to you their eternal slendours, and your eye only gazes to earth."
Dante, Purgatorio

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Date: February 28, 1995
Reading Assignment: pp. 433-443

Description : theoretical overview of star formation

Objectives

be able to describe how heat, rotation, and magnetism hinder star formation

be able to explain why gravity eventually overcomes heat causing stars to form

be able to describe the concept of an evolutionary track

be able to describe conditions at the beginning of star formation

be able to describe the fragmentation inside an interstellar cloud and why it occurs

be able to describe the formation of a protostar

be able to explain Kelvin-Helmholtz contraction of a protostar and the changes that take place in the luminosity and temperature of the star

be able to describe the Hayashi track and the changes that take place in the luminosity and temperature of the star

be able to describe why nuclear burning begins in the star

be able to describe the final adjustments the star makes when it moves onto the main sequence

be able to describe how the stars final mass affects the speed in which it forms

Lecture Outline

Slide # 1: A Star is Born

7 Easy Steps to Thermonuclear Ignition

Slide # 2: The HR Diagram (GRAPHICS)

Why?

Slide # 3: Unanswered Questions

Why are 90% of stars on the Main Sequence?

Why is mass so important in determining luminosity?

How can a 1 solar mass star be a red giant, white dwarf, or on the main sequen

Why are red giants and white dwarfs different than MS stars?

Slide # 4: Star Modeling- Laws of Physics

hydrostatic equilibrium

energy transport

energy generation

mass continuity

Slide # 5: Evolutionary Tracks

every star corresponds to place on the HR diagram at any given time

stars may change luminosity or temperature over their lifetime

stars may follow tracks on the HR diagram

Slide # 6: The HR Diagram (GRAPHICS)

one solar mass star over 8 billion years

Slide # 7: Gravity Causes Collapse

gravity in a gas cloud may cause it to contract

Slide # 8: Forces the Oppose Contraction

gas pressure and heating

hydrostatic equilibrium

rotation

rotation flattens the system

opposes collapse

magnetic fields

rapidly strengthens with contraction

Slide # 9: Hydrostatic Equilibrium (GRAPHICS)

a balance between pressure and gravity

Slide # 10: Hydrostatic Equilibrium

most stars are in hydrostatic equilibrium

they are not expanding or contracting

gravity and pressure are balanced

forming stars ARE NOT in hydrostatic equilibrium

they are contracting

gravity is overcoming pressure

Slide # 11: Rotation (GRAPHICS)

angular momentum opposes collapse

rotation speeds up as the system collapses

Slide # 12: Magnetic Fields (GRAPHICS)

compression increases magnetic fields

stronger magnetic fields oppose compression

Slide # 13: A Simplification

the most important effect is pressure

gas pressure and heat

ignore rotation and magnetic fields

probably less important

Slide # 14: Pressure and Gravity

gas pressure is caused by moleculer motion

heating causes molecular motion

collapse causes heating

Slide # 15: How does collapse take place?

more compression means more heat

more heat means more pressure

so how does it collapse?

Slide # 16: Cooling of the Gas

higher density means more interactions between particles

more interactions mean more radiation

more radiation means more cooling

Slide # 17: Cooling (GRAPHICS)

cooling must occur before a star is formed

Slide # 18: Two Competing Effects

cooling - energy is radiated

heating - pressure increases

Slide # 19: Star Formation

7 basic steps to forming stars

examine formation of a ONE SOLAR MASS star

Slide # 20: The Steps

gas cloud

fragmentation

protostar

Helmholtz contraction

Hayashi track

ignition

adjustment to the Main Sequence

Slide # 21: Stage 1: Gas Clouds

high density HI or H2 region

1000 particles per cubic cm

10 parsecs in size

10 K temperature

often Molecular Clouds

a bit cooler and denser than most of the ISM

1 particle per cubic cm

100 K temperature

Slide # 22: Molecular Clouds

temperature = 20K

ISM is normally 100K

density = 106 atoms per cubic cm

ISM has a density of 1 atom per cubic cm

most gas is molecular hydrogen (H2)

ISM is mostly HI gas

Slide # 23: Molecular Cloud Composition

almost all gas is molecular Hydrogen (H2)

very little HI

interstellar dust grains

1 part in 1012

one part in a million or billion is other molecules

60 detected in interstellar space

Slide # 24: The ISM (GRAPHICS)

molecular clouds

Slide # 25: Stage 2: Fragmentation

cloud breaks into pieces

2 solar masses of material

10 6 particles per cubic cm

few one-hundreth's of a parsec

few hundred times the size of solar system

100 K temperature

Slide # 26: Fragmentation (GRAPHICS)

big cloud breaks into many little clouds

Slide # 27: Stage 3: Protostar

gas cloud heats up

radiation becomes trapped

size of the solar system

10,000 times the size of the Sun

temperature reaches 10,000 K at the core

density 1012 particles per cubic cm

age = 10,000 years

photosphere forms

gas becomes opaque

Slide # 28: Protostar (GRAPHICS)

photosphere forms

Slide # 29: Constellation Corner (GRAPHICS)

Constellation De Jour

Slide # 30: Gemini (GRAPHICS)

Feb 15 - 9pm - S - 4.0

Slide # 31: Gemini (GRAPHICS)

Feb 15 - 9pm - S - 4.0

Slide # 32: Gemini (GRAPHICS)

Feb 15 - 9pm - S - 5.0

Slide # 33: Stage 4: Kelvin-Helmholtz Contraction

protostar contracts and heats up

core temperature = 1,000,000 K

surface temperature = 3,000 K

size = 50 solar radii

no nuclear reactions yet

very luminous - more than 1,000 solar luminosity

age = 100,000 years

star can be plotted on HR diagram

appears in red giant area

Slide # 34: The HR Diagram (GRAPHICS)

HR diagram

Slide # 35: Stage 5: Hayashi Track

star contracts, surface temperature rise a small amount

10 times the size of the Sun

surface temperature 4000 K

luminosity = 10 solar luminosity

central temperature = 5,000,000 K

no nuclear reactions

age = 1 million years

luminosity decreases as star shrinks

Slide # 36: The HR Diagram (GRAPHICS)

HR diagram

Slide # 37: Stage 6: Nuclear Ignition

P-P Chain Begins in the Core

core temperature > 10,000,000 K

surface temperature 4,500 K

size = 1.25 solar radii

luminosity = 2/3 solar luminosity

age = 10 million years

star still not in hydrostatic equilibrium

internal structure slightly out of balance

Slide # 38: The HR Diagram (GRAPHICS)

HR diagram

Slide # 39: Stage 7: Adjustments to Main Sequence

star moves to main sequence

star is now a G2V

15,000,000 K core temperature

one solar luminosity

6,000 K surface temperature

central density = 100 gm/cm3

age = 40 million years

star in hydrostatic equilibrium

Slide # 40: The HR Diagram (GRAPHICS)

HR diagram

Slide # 41: Emission Nebula (GRAPHICS)

M42 - the Orion Nebula - an HII region

Slide # 42: Nebula (GRAPHICS)

M20 - the Trifid Nebula

emission, absorption, and reflection

Slide # 43: Effects Interstellar Dust

stars appear redder

absorbs blue light more than red

appear dimmer

absorbs visible light

star light is polarized

Slide # 44: How do we observe star formation?

found in molecular clouds

lots of dust

Slide # 45: Emission and Excitation

transition between orbitals

produces most visible line emission

spin-flip of electron

HI gas- 21cm line emission (radio)

rotational excitation

molecular emission - radio

thermal emission

peak of stars is in the visible range

peak of protostars is in the infrared