Transcript powerpoint - High Energy Physics at Wayne State

Chapter 19: Between the Stars:
Gas and Dust in Space
Gas and Dust in Space
To understand how stars are born, we begin by learning
about the raw material from which they are made.
 Interstellar matter: gas and dust that lies in the
regions between stars.
 Interstellar medium: the entire collection of interstellar
matter.
 The interstellar medium accounts for a large fraction of
the atoms in the universe (>50%).
 Provides the raw material for new stars.
 Nebulae: latin for "clouds”, aggregates of interstellar
matter that emits radio waves or light. Can produce
colorful displays when lit by the light of nearby stars.
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19.1: The Interstellar Medium
 75% hydrogen, 25% helium, and 1%
interstellar dust (by mass).
 The matter comes together in clouds.
 Density is low:
 103 atoms per cubic centimeter (cc).
 Air has 1019 atoms per cc.
 Best vacuum created on Earth has 107 atoms per cc.
 But very large in size
 Tens of LY’s.
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19.2 Interstellar Gas
 The color of a cloud tells us about its temperature and
composition.
 The common red color comes from hydrogen (H II).
 H II regions contain ionized hydrogen.
 H I indicates neutral hydrogen, H II is singly ionized
hydrogen, and Fe III is doubly ionized iron.
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type of region
temperature (K)
HI: cold clouds
100
HI: warm clouds
5000
hot gas
500,000
HII regions
10,000
giant molecular clouds
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H II Regions
 temperature near 104 K – heated by nearby
stars
 ultraviolet light from hot O and B stars
 ionizes the surrounding hydrogen gas
 free electrons recombine – form excited H
atoms
 excited states emit light
 red glow characteristic
of hydrogen
(red Balmer line)
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HII regions: Orion Nebula
•closest to us, 1,500 LY
•29x26 LY in size
•large star formation factory
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trapesizium
cluster:
•stars that
provide much
of the energy
which makes
the brilliant
Orion Nebula
visible
•other stars
obscured by
nebula
Absorption Lines
 Most of the interstellar medium is not ionized.
 Mostly hydrogen, and helium
 Other atoms and molecules seen: Ca, Na, CN, CH,
H2, CO
 Cool gas between stars and Earth will cause an
absorption spectrum.
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19.2.2 Neutral Hydrogen Clouds
Vast clouds of neutral
H I gas
Don't emit strong
(visible) radiation
Spectroscopic binaries
betray H I regions
interstellar gas
 binaries: doppler shift
moves spectral lines
 some lines don't move
 reason: absorption lines
in gas between binary pair
and Earth
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X
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The Hydrogen 21 cm Line
 Hydrogen: proton (p) plus electron (e)
 Both p and e have spin – "up" or "down"
 Ground state: p up, e down
 Excited state: p up, e up
 Can move between states by emitting/absorbing a
photon.
 The photon has a wavelength of 21 cm, a radio wave.
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21cm Line From Cold H I Regions
 This “spin flip” in hydrogen
produces the 21cm radio waves.
 Hydrogen clouds must be cold,
about 100K.
 First detected in 1951.
 Seen by a radio telescope.
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side
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19.2.3 Ultra-Hot Interstellar Gas
Astronomers were surprised to discover
hot interstellar gas.
Hot means about 1 million degrees K!
We now understand that the gas is
heated by supernovae explosions.
This topic will be discussed in Ch. 22.
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19.4 Cosmic Dust
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Cosmic Dust
 Dark regions seemingly
empty of stars
 Not voids – dark clouds
 Dark nebulae block light
from stars behind them
 Composed of dust grains
coated by ice.
 Visible only in infrared
 IR satellites IRAS, Hubble
 Thermal spectrum gives
the temperature, typically
10K to 100K.
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Dust Phenomena
 Extinction: light absorbed and scattered by dust 
black regions
• Reflection nebula:
scattered light
illuminates nearby gas
•
•
scattered light tends to
be more blue
the sky is blue
• Reddening: incomplete
absorption
•
•
direct light more red –
blue light scattered away
the sunset is red
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blue sky, red
sunset
blue light scattered more easily
than red
 blue light scattered out of
white rays from Sun
 lights atmosphere in blue
 reflection nebula work the
same way
red sunset – direct light
from the sun depleted of
blue – reddening
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reflection
nebula in Orion
reflection
nebula in
Orion
Trifid
Nebula
3000 LY distant
50 LY across
•H II region
(red)
high-energy UV
hits interstellar
gas
•dark dust
filaments –
extinction due
to debris from
supernovae
•blue reflection
nebula
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dust
filaments
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Dust Glows in the Infrared
infrared
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Dust
Pillar
very bright
star blowing
dust off of a
star near
the pillar's
tip
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Summary
The amount of gas and dust that exists
between stars was an important
discovery.
The gas and dust accounts for a large
fraction of the mass of the Milky Way
Galaxy.
The clouds are observed by a variety of
means, including the 21cm radio waves.
Cool clouds of gas and dust (giant
molecular clouds) are regions where new
stars can be created.
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