Transcript Gas and Dust in Space - Wayne State University

Between the Stars:
Gas & Dust in Space
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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
Some interstellar material is
concentrated into giant clouds,
called nebulae (the Latin for
“clouds”)
Interstellar gas and dust can
produce colorful displays when
lit by the light of nearby stars
Animation: flight thru nebula
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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
and helium
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 about one atom 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
exceedingly large
Consequently, its total mass is humongous
The total mass of interstellar matter in our Milky Way Galaxy
has been estimated to be about 20% of the total mass of its
stars
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Interstellar Gas
Depending on where it is located, interstellar gas has
temperatures ranging from a few kelvin (only a few
degrees above absolute zero) to more than a million kelvin
Since hydrogen (H) is the main constituent of interstellar
gas, astronomers often characterize a region of space
according to whether its hydrogen is neutral or ionized
A cloud of ionized hydrogen is usually called an H II region
In contrast, the hydrogen atoms in an H I region are not
ionized and hence are neutral
In an H II region, the hydrogen is ionized by ultraviolet
radiation from nearby stars
The proton, however, will not remain alone for long, but will
quickly recombine with one of the electrons near it
The resulting neutral atom can then absorb UV radiation
again, and the process is repeated
H II regions are not very common because they require
very hot stars, which are rare
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H II Regions
These regions have temperatures
~10,000 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
hydrogen atoms
One way to excite an atom
Excited states emit light
The reddish glow is
characteristic of H,
corresponding to the
red Balmer line in its
emission-line spectrum
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Example of H II Regions
Dusty Nebulae in
the 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 gas is cold and hence not ionized
It consists mostly of hydrogen and helium
Other atoms and molecules are also seen: Ca (calcium), Na
(sodium), CN, CH, etc.
The cool gas located between the Earth and the stars can
yield absorption-line spectra
Although cold hydrogen does not produce spectral lines in the
visible range, some of the other elements do produce strong
spectral lines when they are cold
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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
Strong visible radiation is produced by the some of the
other elements in the gas
The first evidence for absorption by interstellar clouds
in H I regions came from the analysis of spectroscopic
binary stars
The Doppler effect was
expected to cause the spectral
lines to move
But some of the lines did not
move
Explanation: the stationary
lines were absorption lines
produced by cold gas located
between the binary stars and
the Earth
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interstellar gas
X
X
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The Hydrogen 21-cm Line
A hydrogen atom consists of a proton (p) & an electron (e)
Both p and e have “spin”, which could be “up” or “down”
In the ground (lowest energy) state, p is up and e down
In the slightly excited state, both p and e are up
The electron can move between the spin states by emitting
or absorbing a photon
Such a 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 neutral hydrogen
was predicted to produce 21-cm-long
radio waves
The prediction was confirmed by
observation in 1951 using sensitive
radio telescopes
This indicates that neutral-hydrogen
clouds must be cold, having
temperatures of about 100 K
Most of the 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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top
side
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Stellar Question
What’s a
supernova?
Ultra-Hot Interstellar Gas
Astronomers were surprised to discover very-hot
interstellar gas in some regions of space, even though
there was no visible source of heat nearby
The hot temperatures are about 1 million K!
Theoretical calculations have now shown the source of
energy that can yield such extreme temperatures is a
supernova, the explosion
of a massive star
Some stars, nearing the
end of their lives, become
unstable and literally
explode (to be discussed
in more detail in Ch. 22)
Supernova remnant Cassiopeia A
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Cosmic Dust
There are dark regions on the
sky that are seemingly empty
of stars
They actually are not voids, but
are clouds of dark dust
The dust betrays its presence by
blocking the light from distant
stars
making distant stars look redder
and fainter than they really are
reflecting the light from nearby
stars
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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Interstellar Extinction
Interstellar dust particles
are very tiny, just slightly
smaller than the wavelength
of visible light
Consequently, they readily
interact with visible light
Since interstellar dust grains both absorb and scatter
the starlight that they intercept, they reduce the
amount of light from distant stars that can reach us,
making the stars look dimmer
This dimming effect is called interstellar extinction
The situation is similar to that of the dimming of light by
fog or smoke
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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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Interstellar Reddening
Like air particles in the Earth’s atmospheric, the grains of
interstellar dust interact with the different colors of visible
light differently
Consequently, interstellar dust particles also make the
distant stars look redder
This is called interstellar reddening
Strictly speaking, this process should more properly be called
“de-blueing” because the blue and related colors have been
removed (scattered) by the dust
Interstellar reddening can even make some stars that are
extremely hot (and
hence should look
bluish) appear
reddish
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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),
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illuminated by a star, which is
visible just to the left of center
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Trifid Nebula in Sagittarius Constellation
It is about 3000 LY
from the Sun and
about 30 LY in
diameter
The reddish H-II
region is surrounded
by a blue reflection
nebula
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Detecting Interstellar Dust in the Infrared
While interstellar dust clouds are too cold to
radiate measurable amount of visible light,
they emit heat radiation and hence glow
brightly in the infrared
Horsehead Nebula in Orion
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Visible
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Infrared
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Cosmic Rays
These are particles that travel through interstellar
space at a typical speed of 90% the speed of light
They have nearly the same composition as ordinary
interstellar gas
But they behave very differently from the gas
Most cosmic rays are hydrogen nuclei (protons)
About 9% of cosmic rays are nuclei of helium and
heavier elements
Positrons (anti-electrons) are also found
Many cosmic rays are probably produced in
supernova explosions
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Recycling of Cosmic Material
Much of interstellar matter may have been
ejected by old and dying stars
The most massive stars end their lives with the
giant explosions called supernovae
The ejected gas and dust will likely become
part of the raw material for the formation of
future stars
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The dust
filaments in
the Trifid
Nebula are
supernova
debris
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