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Star Formation

Lecture 12

Stellar Birth

• Since stars don’t live forever, then they must be “born” somewhere and at some time in the past.

• How does this happen?

• And when stars are born, so are planets!

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Molecular clouds

• Stars form in giant clouds of gas and dust called molecular clouds.

• The term molecular cloud is used since molecules are present.

• The large amount of gas and dust in the cloud shields the molecules from UV radiation from stars in our galaxy.

Anatomy of a Stellar Factory

• Molecular cloud:

103 to 106 Msun of gas and dust in the cloud.

Collapsing Region

• Contains

‒ H, He, etc.

‒ H2, H20, OH, CO, H2CO, etc.

‒ dust of silicates, iron, ices, etc.

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M8 - Lagoon Neb.

Cloud fragmentation

• The molecular cloud does not collapse into a single star.

• It fragments into many clumps.

• These clumps can further collapse to form stars.

• 10 - 1000 stars can be formed from the cloud.

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Gravitational Collapse

• When a fragment of a molecular cloud reaches a critical mass - it collapses to form a star.

– Gas and dust pulled together by gravity until a star is formed.

• But to get this critical mass is not so easy.

Causing collapse: Method 1

• Accretion:

– Build up of small clouds of gas and dust into giant ones.

• Clouds “stick” together and grow.

• Very slow - due to low interstellar densities

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Causing collapse: Method 2

• Gravity and Radiation Pressure

Pressure of Starlight

High densities & Gravitational Collapse

Problem: But how do the first stars form!

Causing collapse: Method 3

• Compression by supernova blast waves

Exploding Star

Old Star Nearby

Cloud

Compressed cloud

Shock waves from Supernova

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M16 - Eagle

Nebula

NOAO Image

Pillars in M16

HST Image

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M16: Close-up

M16

10 ly

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The path to collapse

• Gravity makes the cloud collapse.

• Two hindrances to collapse

1. Internal heating

- Causes pressure build-up

2. Angular momentum

- Causes high speeds

(like a skater)

Internal Heating

• Cloud fragments collapse

• Potential energy => Kinetic Energy

– Gas particles speed up and collide.

• The temperature increases.

• This causes a pressure build-up which slows (or stops) the collapse.

• Energy is radiated away.

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Angular Momentum

• Angular momentum

A = mass vel. of rotation radius

= mvr

• Conservation of angular momentum.

– A = constant for a closed system.

• As the cloud fragment shrinks due to gravity, it spins faster.

Angular momentum

• Collapse occurs preferentially along path of least rotation.

• The cloud fragment collapses into a central core surrounded by a disk of material.

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Disk Formation

Rotating Disk

Rotating Central Core

Infall Material

OMC Proplyds 2

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Dust Disks around Stars

Planet formation

• The disk around the central core will fragment further, producing rings of material.

• The particles in these rings can accrete together to form planets!

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Protostars

• The central core is called a protostar.

• It is undergoing continuous gravitational contraction.

• Self-compression heats the central core.

• Surface ~ 300 K

• Energy emitted in the infrared. L = 4 R2 T4 , R is very large.

Overview of the build-up

• Collapse starts out in free fall controlled by gravity

• Central parts collapse more rapidly => central core becomes a protostar.

• Core accretes material from the surrounding envelope

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A Star is Born

• The protostar continues to collapse while the central core heats up to millions of degrees.

• Fusion reactions start => A star is born

What stops the collapse?

• Collapse is halted by the pressure of the heated gas which balances gravity.

An equilibrium

HOT

• Gas and Radiation Pressure balance Gravity

• No collapse or expansion.

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Entrance into the H-R diagram

Temperature

Lum

ino

sity

(Lsu

n)

O B A F G K M

Hayashi Contraction Phase

less massive

more massive

Time to form a star

• The time to reach the main-sequence varies with stellar mass.

Mass (Msun) Time (106 years)

15 0.16

5 0.7

2 8

1 30

0.5 100

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Making the stars visible

• After a star is born it heats the gas and dust around it.

• Eventually the gas and dust are pushed away.

• The star then becomes “visible.”

• Prior to this it could be seen only in the radio and the infrared.

30 Doradus (Opt/IR)

Massive newborn stars are indicated by the arrows. Note that some (2, 3, & 4) are hidden to visible light. Arrows 1 and 5 indicate a compact cluster of bright young stars. Sources 6 & 7 may be due to outflow jets from the cluster 5.

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NGC 3603 - Star Formation

Bok Globules – dark clouds that could form stars

Newborn stars emerging from their birth clouds

Young star cluster

Dying star showing outflows – may go supernova in a few thousand years

Evaporating disks around stars – planet nurseries?

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Stars Die!

• The fuel in stars is proportional to the mass, M.

• It is found that the luminosity of stars on the main-sequence varies with mass as:

5.3ML

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Stellar Lifetimes

• Assuming stars “consume” the same fraction of their mass (M), the lifetime, T, is given by:

T

T

Amount of Fuel

Rate of Consumption

M

M

1

M3.5 2.5

T

T

Amount of Fuel

Rate of Consumption

M

M

1

M3.5 2.5

where M is in solar masses and T is in solar lifetimes.

The Lifetime of Stars

• The mass of a star determines how long it will live.

• More massive stars evolve faster.

Mass Lifetime

1 Msun ~1010 yrs

5 ~108

10 ~107