1111

Star FormationStar Formation

A Star is Born

1111

GoalsGoals

• What is there between the stars?• What are dust clouds?• What are nebulae?• Where do stars come from?

1111

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.

1111

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.

1111

Dust• Space is dirty.• Dust blocks or

scatters some light.• Result: black

clouds and patterns against the background sky.

1111

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.

1111

The IR Universe

Orion - visible

Orion – by IRAS

1111The Trifid Nebula – copyright Jason Ware

1111

Interstellar Gas

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

Hemission nebulae

Copyright - Jason Ware

1111

Dust obscuring H emission nebula

Horsehead Nebula – copyright Arne Henden

1111

HII Regions

• For light: atoms must be excited.• Energy comes from very hot stars.

Orion Nebula – copyright Robert Gendler

1111

Concept Test

• An HII region appear red becausea. it is hot and things that are hot glow red.b. it is ionized hydrogen which appears red

because the brightest emission line is red.c. it is cold and things that are cold appear

red.d. it is full of red stars.e. dust between the observer and the region

blocks the blue light, but lets the red light through.

1111

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, gas inside is very, very cold.

• Inside molecules form.

1111

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.

1111

Star Formation• Remember:

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

• Gas begins to contract.

HOTTER COOLER

1111

How to Make a Star

1

2

3

1111

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.

1111Orion Nebula – copyright Robert Gendler

1111

Eagle Nebula – copyright J. Hester

1111•Visible and IR image of protostars in the Orion Nebula.

1111

1111

Concept Test

• A new star reaches the main sequence when inward gravitational collapse is:a. Halted by degeneracy pressure in the

core.b. Halted when the atoms are pushed up

against one another and contraction stops.c. Finally balanced by outward thermal

pressure from nuclear reactions.d. Finally balanced by radiation emitted in

the photosphere.e. none of the above.

1111

…and the Nebula?

• Cloud around the protostar spins faster.

• Flattens to a disk.– Pizza dough.

1111

Planetesimals• Dust and gas

condense onto dust grains.

• Small clumps grow bigger.

• Bigger clumps have more mass and attract more matter.

• Planetesimals are the building blocks of the planets.

Orion Nebula – Copyright O’Dell and Wong

1111

An H-R Life-Track

1111

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.

1111

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

1111

Concept Test

• Which Cluster is the oldest?a.b.c.d.

1111

Open Clusters

• These are the new stars.

• Small groups of young stars.

• Slowly drifting apart.

Jewel Box – copyright MichaelBessell

1111

Homework #11Homework #11

• For Oct 15: Read B17.3 – 17.5• For Oct 18: Read B18.1 – 18.3• Do:

– Ch17 : Problem 4, 25– Ch18: Problem 11