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Star FormationStar Formation

A Star is Born

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GoalsGoals

• What is there between the stars?• What are dust clouds?• What are nebulae?• How do these lead to the formation of

star?– Where do baby stars come from?

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The Stuff Between Stars

• Space isn’t empty.• Interstellar Medium – The gas and dust

between the stars.

All the interstellar gas and dust in a volume the size of the Earth only yields enough matter to make a pair of dice.

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The Distribution• Picture the dust under your bed.

– Fairly uniform thin layer– Some small clumps– Occasional big complexes

• Interstellar dust and gas is the same.

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Dust• Space is dirty.• Dust blocks or

scatters some light.• Result: black clouds

and patterns against the background sky.

• But what light gets through, and what light doesn’t?

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Absorption and Scattering

• Q: Why are sunsets red?• Light is absorbed or scattered by

objects the same size or smaller than its wavelength.

• Dust grains = wavelength of blue light• Dust clouds:

– Opaque to blue light, UV, X-rays– Transparent to red light, IR, radio

• A: Whenever there is a lot of dust between you and the Sun, the blue light is absorbed or scattered leaving the only the red light.

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Interstellar Reddening• Same thing with

dust clouds in space.

• Since space is full of dust, the farther away stars are, the redder they look.

• Enough dust and eventually all visible light is scattered or absorbed.

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Dust and IR• In a dark dust cloud:

– Even though all visible light may be gone, we can still use IR.

– If dust is warm, IR will show its blackbody emission.

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The IR Universe

And allows us to see dust where we wouldn’t otherwise expect it.

Orion - visible

Orion – by IRAS

88The Trifid Nebula – copyright Jason Ware

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Interstellar Gas

• In Lab 2 we talked about spectral lines and how they apply to hot and cool gases.

• Let’s look at some hot and cool gases in space.

Hemission nebulae

Copyright - Jason Ware

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Dust obscuring H emission nebula

Horsehead Nebula – copyright Arne Henden

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• In order for the hydrogen to emit light, the atoms must be in the process of being excited.

• The energy for the excitation comes from very hot stars (O and B stars) within the cloud.

Orion Nebula – copyright Robert Gendler

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Cold Dark Clouds• If dust clouds

block light, then inside thick dust clouds there should be no light at all.

• Without light, there is little energy.

• With little energy, any gas inside is very, very cold.

• Inside molecules can form.

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Gravity vs. Pressure

• Stars and other interstellar material are in a perpetual battle between forces pulling in (gravity) and forces pushing out (pressure).

• Gravity comes from the mass of the cloud or star.

• Pressure comes from the motion of the atoms or molecules.– Think of hot air balloons.– The hotter the air, the bigger the

balloon.

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Star Formation• Remember lecture 4:

• Cold interstellar clouds:No heat = no velocity = no outward pressure.Gravity wins.

• Gas begins to contract.

HOTTER COOLER

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How to Make a Star

1

2

3

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1. The Interstellar Cloud

• Cold clouds can be tens of parsecs across.

• Thousands of times the mass of the Sun.

• Temperatures 10 – 100 K.• In such a cloud:

– Something makes a region denser than normal.

– Force of gravity draws material to denser region.

– Gravitational collapse begins.

88Orion Nebula – copyright Robert Gendler

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Contracting Fragments

• Cloud about the size of solar system.• In the center:

– Collapsing material continues to heat up.– Density causes heat to be retained.

• Higher density makes center opaque.

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Eagle Nebula – copyright J. Hester

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2. Protostar

• The central opaque part is called a protostar.

• Mass increases as material rains down on it.

88•Visible and IR image of the hot protostars in the Orion Nebula.

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…and the Nebula?

• Cloud around the protostar spins faster.

• Flattens to a disk.– Pizza dough.

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Planetesimals• Dust and gas

condense onto dust grains.

• Small clumps grow bigger.

• Bigger clumps have more mass and attract more matter.

• Planetesimals become the building blocks of the planets.

Orion Nebula – Copyright O’Dell and Wong

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3. T Tauri Phase• Protostar still shrinks: 10x

the Sun.• Still heats up: surface =

4000 K• Core temp = 5,000,000 K• Violent surface activity

creates strong winds that blow material away near the protostar’s surface.

• Clear away the dust and gas between planets.

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A Star is Born

• Time: 40- 50 million years since the collapse started.

• Radius: 1,000,000 km (Recall the Sun = 700,000 km)

• Core temp: 10,000,000 K (Sun = 15,000,000 K)– Surface temp = 4500 K

• Fusion begins in core.• Energy released creates the pressure needed

to counter the contraction from gravity.• Contraction ends!

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An H-R Life-Track

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The Main Sequence

• For the Sun:– While it took 40 – 50

million years to get here, the new star will spend the next 10 billion years as a main sequence star.

• Bigger Stars:– Everything goes

quicker.

• Smaller Stars:– Everything longer.

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Now what?• The mass of the

star that is formed will determine the rest of its life!

• Recall: the more massive the star, the more pressure in the core.

• The more pressure, the more fusion.

• More fusion:– More energy

produced– Hotter– Shorter life span

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Open Clusters

• These are the new stars.

• Small groups of young stars.

• Slowly drifting apart.

Jewel Box – copyright MichaelBessell

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Homework #7Homework #7

• For 10/16:• Read B16.3 – 16.5, B17.1 – 17.3• Do:

– Ch16 : Review Questions 9, 18, Problem 1– Ch17: Review Question 1, Problem 4