Download Between the Stars: Gas and Dust in Space

Between the Stars:
Gas & Dust in Space
29 March 2005
AST 2010: Chapter 19
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Gas and Dust in Space
To understand how stars form, we need to know the
raw material from which they are made
All the gas and dust material that lies in the region
between stars is referred to as interstellar matter
The entire collection of interstellar matter is called the
interstellar medium
The interstellar medium accounts for a large fraction
of the atoms in the universe (>50%)
and provides the raw material for new stars
Clouds of interstellar gas or dust that are seen to glow
with visible light or infrared radiation are usually
called nebulae (the Latin for "clouds”)
Interstellar gas and dust can produce colorful displays
when lit by the light of nearby stars
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Interstellar Medium
About 99% of the interstellar matter is in the form of
gas (individual atoms or molecules)
The most abundant elements in the interstellar gas are
hydrogen (75%) and helium (25%)
The remaining 1% of interstellar matter is in the form
of solid interstellar dust grains
The density of interstellar matter is very low
It has 103 atoms per cubic centimeter (cc)
Air has 1019 atoms per cc
The best vacuum created on
Earth has 107 atoms per cc
The volume of space occupied
by interstellar matter is huge
Consequently, the total mass of
interstellar matter is humongous
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Interstellar Gas
The color of a gas gives us clues about
its temperature and composition
The red color commonly seen in
interstellar gas comes from ionized
hydrogen, or H II
The proton recombines with an electron
which then moves down to the lowest-energy orbit by
emitting a red-wavelength photon
H I refers to a neutral
type of region
temperature (K)
hydrogen, and Fe III
HI: cold clouds
100
a doubly ionized iron
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HI: warm clouds
5000
hot gas
500,000
HII regions
10,000
giant molecular clouds
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H II Regions
These regions have temperatures near 104 K,
heated by nearby stars
The ultraviolet light from hot O and B stars ionizes
the surrounding hydrogen gas
The free electrons recombine with protons,
forming excited H atoms
Excited states emit light
The red glow is characteristic
of hydrogen (the red Balmer
line)
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H II Regions: Dusty Nebulae in Sagittarius Constellation
The red glow
that dominates
this image is
produced by the
red Balmer line
of hydrogen
This indicates
that there are
hot stars nearby
that ionize these
clouds of gas
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Absorption Lines
Most of the interstellar medium is cold and
hence not ionized
Mostly hydrogen and helium
Other atoms and molecules are also seen: Ca, Na,
CN, CH, H2, CO
The cool gas between the Earth and the stars
will cause an absorption spectrum
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Neutral-Hydrogen Clouds
Vast clouds of neutral-hydrogen (H I) gas are cold
and, therefore, do not emit strong (visible)
radiation
The first evidence for absorption by interstellar
clouds in H I regions came from the analysis of
spectroscopic binary stars
interstellar gas
binaries: doppler shift moves
spectral lines
some lines don't move
reason: absorption lines
in gas between binary
pair and Earth
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 spin-state: p up, e down
Excited spin-state: p up, e up
The electron can move between the spin states by
emitting or absorbing a photon
The photon has a wavelength of 21 cm, a radio wave
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21-cm Line From Cold H-I Regions
The “spin flip” in hydrogen was predicted
to produce 21-cm-long radio waves
The prediction was confirmed by
observation in 1951 using radio
telescopes
This indicates that neutral-hydrogen
clouds must be cold, having
temperatures of about 100 K
Most of cold hydrogen is confined to a
very flat layer (less than 300-LY thick)
that extends throughout the disk of the
Milky Way Galaxy
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side
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Ultra-Hot Interstellar Gas
Astronomers were surprised to discover
hot interstellar gas, even though there
was no visible source of heat nearby
The hot temperatures are about 1 million
degrees K!
We now understand that the gas is
heated by supernovae, the explosions of
massive stars
This topic will be discussed in Ch. 22
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Cosmic Dust
There are dark regions on the
sky that are seemingly empty of
stars
But they are not voids, but
clouds of dark dust
The dust betrays its presence by
blocking the light from distant
stars
reflecting the light from nearby
stars
making distant stars look redder
and fainter than they really are
Each dust particle has a rocky
core that is either sootlike
(carbon-rich) or sandlike
(containing silicates) and a
mantle made of icy material
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Blue Sky & Red Sunset
Blue light is scattered more
easily than red
because red wavelengths are
longer than blue
The blue colors in sunlight are
scattered repeatedly by
molecules in the air, and this
makes our sky look blue
Seen directly, the Sun looks
yellowish, as the light from it is
missing some of its blue
At sunrise or sunset, the Sun
appears redder than at noon
because the light from it
travels a longer path through
the air than at noon and
hence is missing more of its
blue
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Scattering of Light by Cosmic Dust
Interstellar dust particles are
very small, about the same
size as the wavelength of
visible light
The particles scatter blue light
more efficiently than red light,
thereby making distant stars
appear redder
and giving
clouds of dust
near stars a
bluish hue
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Reflection Nebulae
Some dense clouds of dust are close to luminous stars
and scatter enough starlight to become visible
Such a cloud is called a reflection nebula because the
light that we see from it is starlight reflected off grains
of dust
Since dust grains are
tiny, they scatter light
with blue wavelengths
better than light with
red wavelengths
As a result, a reflection
nebula usually appears
bluer than its illuminating
star
A reflection nebula (NGC 1999),
illuminated by a star, which is
visible just to the left of center
Trifid Nebula in Sagittarius Constellation
It is about 3000 LY
from the Sun, and
about 50 LY in
diameter
The reddish H-II
region is surrounded
by a blue reflection
nebula
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The Dust
Filaments in
the Trifid
Nebula are
due to debris
from
supernovae
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Dust Glows in the Infrared
infrared
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Visible and Infrared Images of
Horsehead Nebula in Orion
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Dust Pillar
very bright
star blowing
dust off of a
star near the
pillar's tip
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Cosmic Rays
These are particles that travel through
interstellar space at a typical speed of
90% the speed of light
The most abundant elements in cosmic
rays are the nuclei of hydrogen and
helium
Positrons (anti-electrons) are also found
Many cosmic rays are probably produced
in supernova explosions
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