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Star Formation
A Star is Born
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Goals
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What is there between the stars?
What are dust clouds?
What are nebulae?
Where do 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.
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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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The IR Universe
Orion - visible
Orion – by IRAS
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The 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.
Ha emission nebulae
Copyright - Jason Ware
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Horsehead
Nebula –
copyright Arne
Henden
Dust obscuring Ha emission nebula
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Orion Nebula –
copyright Robert Gendler
HII Regions
• For light: atoms must be excited.
• Energy comes from very hot stars.
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Concept Test
•
An HII region appear red because
a. 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.
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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, gas
inside is very, very cold.
• Inside molecules 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:
HOTTER
COOLER
• Cold interstellar clouds:
No heat = no velocity = no outward pressure.
Gravity wins.
• Gas begins to contract.
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How to Make a Star
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2
3
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1. The Interstellar Cloud
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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.
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Orion Nebula – copyright Robert Gendler
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Eagle Nebula –
copyright J. Hester
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•Visible and IR image of protostars in the Orion Nebula.
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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.
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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 are
the building blocks
of the planets.
Orion Nebula – Copyright O’Dell and Wong
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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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Concept Test
• Which Cluster
is the oldest?
a.
b.
c.
d.
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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 #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
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