Transcript Document

The Interstellar Medium
No, it’s not a space psychic
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Goals
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What is the interstellar medium?
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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And allows us
to see dust
where we
wouldn’t
otherwise
expect it.
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Scattered light
• Q: But what happens
to the blue light that
is scattered?
• A: Reflection
nebulae.
The Witchhead Nebula
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The Trifid Nebula – copyright Jason Ware
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Interstellar Gas
• In lecture 2B we talked about Kirchhoff’s laws
and how they apply to hot and cool gases.
• Let’s look at some hot and cool gases in space.
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
• 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.
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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 3:
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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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:
– If a star’s worth of matter should clump
together in a denser region than the rest of the
cloud:
– Gravitational attraction will win out over
their combined pressure.
– The clump will begin to collapse.
– The cold cloud will fragment.
• Time: A few million years.
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Orion Nebula – copyright Robert Gendler
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2. Contracting Fragments
• A fragment about the same mass as the Sun slowly
contracts due to its gravity.
• Now a few hundredths of a parsec across or 100
times the size of the Solar System.
• Temperature still about the same.
• In the center some heat begins to be retained.
• Time: several tens of thousands of years.
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Eagle Nebula –
copyright J. Hester
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3. Continued Contraction
• Fragment now a gaseous sphere.
• Size of the Solar System
– (10,000 x the size of the Sun)
• Inner region is dense enough to be opaque to light
(remember the convection zone of Sun).
– Result: inner region heats up – 10,000 K.
– Outer edge still cool.
• The central opaque part is called a protostar.
• Mass increases as material rains down on it.
• Time: 100,000 years
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•Visible and IR image of the hot protostars in the Orion Nebula.
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4. Protostellar Evolution
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Gravity is still winning.
The protostar is still shrinking.
Size: Mercury’s orbit.
The protostar is also still warming.
– Core = 1,000,000 K (recall core of Sun 15,000,000 K)
– Still not hot enough for nuclear fusion
– Surface temp = 2000 – 3000 K (recall Sun = 5800 K)
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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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5. T Tauri Phase
• Protostar still shrinks: 10x the Sun.
• Smaller size  smaller surface area
 smaller luminosity.
• Luminosity = 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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6. A Star is Born
• Time: 6 million years since the protostar formed.
• 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
almost counter the contraction from gravity.
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7. The Main Sequence
• Time: 30 million years since
Stage 6.
• Central temp is finally
15,000,000 K
• Pressure from energy of
fusion balances gravity
– Contraction ends
• Surface temp is now 6000 K
• 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.
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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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