Transcript The Stuff Between The Stars

Lecture 19
The Interstellar Medium
The Stuff Between The Stars
Announcements
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Read Units 49 and 61 for Wednesday.
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Homework 10 Due Now
Homework 11 – Due Monday, April 23
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Unit 49: TY2
Unit 60: RQ2, 4, P1, TY1, 2
Unit 61: RQ2, TY2, 3
Unit 72: P3, TY1, 2
We’re Going To Talk About Clouds In
Space Today…
But Why Should You Care?
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They’re pretty! And
neat to look at!
But there is a more
important reason…
The Nebula-Star Connection…
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The stars don’t last
forever.
New stars have to be
born to replace the
ones that die.
But where do new
stars come from?
How do they form?
The Nebula-Star Connection…
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We know stars form from
clouds of gas out in space,
but…
Space is full of many
different types of gas
clouds.
Can stars form from any
gas cloud, or only certain
types?
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If only certain types, which
types?
The Star-Nebula Connection…
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So understanding the basics of the stuff
between the stars will help us understand
where new stars come from.
We’ll talk about…
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How we know there are clouds of dust and gas
out in space.
What the different types of clouds are.
Which types stars can form from.
The Stuff Between The Stars
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MYTH: Space is a
vacuum.
REALITY: Nearly 10%
the mass of the galaxy
is in the form of gas
and dust between the
stars.
The Evidence?
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How do we know there is
stuff out there?
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Nebulae
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Some small portions of
interstellar gas and dust
clouds glow brightly.
Extinction and Reddening
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Even invisible dust clouds
make stars beyond look
dimmer and redder.
Nebulae
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There are three kinds of
nebulae.
Emission Nebulae
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Occur near hot, bright stars
(O and B).
Ultraviolet light from the stars
strips electrons from hydrogen
atoms in the gas cloud.
As the electrons recombine
with the hydrogen, they give
off pink light.
Called HII regions:
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HI = Neutral Hydrogen
HII = Ionized Hydrogen
Nebulae
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Reflection Nebulae
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Dust in space scatters light.
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“us”
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Scattered means a photon of
light moving straight away
from the star “bounces off”
something in some random
direction.
Short wavelength (blue) light
is scattered much more often
than long wavelength (red)
light.
Many more photons of blue
light than red are scattered
toward us.
Make the cloud seem to glow
blue.
SAME REASON THE SKY IS BLUE
Nebulae
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Dark Nebulae
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Thick cloud of dust and
gas
Either:
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No stars nearby
Stars deep inside cloud.
All light from stars inside
and behind cloud are
blocked.
Make cloud look black.
Related to Extinction and
Reddening.
Extinction and Reddening
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Extinction
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Dust sometimes absorbs photons.
More dust means more photons absorbed.
Dust clouds make stars inside or beyond look
fainter. Called extinction.
Thicker dust = more extinction
Long distance = more dust = more extinction.
Extinction and Reddening
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Reddening
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Sometimes dust scatters blue light coming directly from
the star away from the Earth.
Makes the star look redder – called reddening.
Thicker dust = more reddening
Long distance = more reddening
SO: extinction and reddening happen at the same
time!
The fact that both are observed suggest there is
stuff between the stars.
The Stuff Between The Stars
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The Interstellar
Medium (ISM) is what
astronomers call the
material between the
stars.
The ISM is mostly gas
with some dust mixed
in.
The Interstellar Medium
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The gas is mostly
hydrogen and helium
(about 99%).
Other major components
include:
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Oxygen
Nitrogen
Carbon
The gasses are the raw
materials that stars form
from.
The Interstellar Medium
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The dust is similar to very
fine sand.
Tiny silicate particles.
Major effect on
temperature.
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Affects how fast a hot gas
cools off.
Molecules form on the
surface of dust grains.
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Include organic molecules
– the stuff living things are
made of!
Interstellar Dust
Probably formed in the
atmospheres of cool
stars.
Mostly observable
through infrared
emission.
Infrared and radio
emissions from
molecules and dust are
efficiently cooling gas in
molecular clouds
IRAS (infrared) image
of infrared cirrus of
interstellar dust.
The Components of the ISM
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The ISM is not just evenly spread between
the stars.
Divided into four components based on:
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Temperature
How dense the gasses are
The Four Components Of The ISM
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NEUTRAL HYDROGEN (HI) CLOUDS
INTERCLOUD MEDIUM (HII)
CORONAL GAS
MOLECULAR CLOUDS
Neutral Hydrogen Clouds
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Galaxy filled with clouds of
neutral hydrogen.
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Cool: 100 K
“Relatively” Dense (for
ISM)
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1-1000 atoms of hydrogen
per cm3
Still better than any
vacuum created on Earth.
About 300 LY in diameter.
About 6 to 10 per 3,000 LY
in the galaxy.
The Intercloud Medium
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Inside the galaxy.
Fills the space
between the denser
clouds (HI and
molecular)
Hot: 5,000 K
Very Thin – over
10,000 times thinner
than the HI clouds.
The Coronal Gas
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Outside the galaxy – forms a halo that surrounds the
galaxy.
Absurdly Hot: 500,000 K
Extremely Thin - Hundreds of times thinner than the HII
gas.
Probably originates in supernova explosions and winds
from hot stars
Molecular (Hydrogen) Clouds
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Very Cold: 20-50 K
Very Dense: 10,000 atoms
of hydrogen per cm3
Large range in sizes:
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Smallest are a few tens of
LY across.
Largest are thousands of
LY across.
Typically dark – light can’t
make it through.
Hydrogen, Hydrogen, Everywhere
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What exactly IS the difference between
neutral hydrogen (HI), ionized hydrogen
(HII), and molecular hydrogen (H2)?
Neutral hydrogen is
one electron
orbiting one proton.
Ionized hydrogen is
when the electron is Molecular hydrogen is when
torn away from the two neutral hydrogen atoms
share their electrons to form a
proton.
hydrogen molecule.
What Does All Of This Have To Do
With Stars?
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Stars form when nebulae collapse under their own
gravity.
A nebulae is MORE likely to collapse when…
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It is VERY COLD
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Causes the “air pressure” inside the nebula to be very low.
It is VERY DENSE
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Very dense means more mass.
More mass means more gravity.
More gravity means more likely to collapse.
Where Stars Form…
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Only one part of the
ISM is a good place
for stars to form.
MOLECULAR
CLOUDS!
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Very cold
Very dense
Where Stars Form…
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So now we know where the stars form
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But we still need to know:
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They only form from certain types of clouds in
space: Molecular clouds.
HOW they form?
AND how are newborn stars related to emission
and reflection nebulae?
We’ll find that out next time!