Download Replenishing the ISM - Stockton University

General Astronomy
The Interstellar Medium
Credits: Much of this slideset is modified from lectures by Dr. Peter Newbury (UBC)
Interstellar Dust
Detection:
Dark nebula blocks light
from background stars
Temperature:
20 --- 500 K
Density:
1000 particles per cubic Km
Content:
Small bits of carbon,
silicates, nanodiamonds,
iron. Some water, methane
and ammonia ices.
Interstellar Dust
Detection:
Reflection nebula reflects
the light of nearby stars
Temperature:
20 --- 500 K
Density:
1000 particles per cubic Km
Content:
Small bits of carbon,
silicates, nanodiamonds,
iron. Some water, methane
and ammonia ices.
Subaru is Japanese for "Star Cluster"
Interstellar Dust Grains
Detection:
Interstellar grains block and
scatter (redden) starlight
Temperature:
Frozen
Density:
100 to 1000 grains per cubic Km
Content:
Grains of 106 to 109 atoms of
carbon, silicates, nanodiamonds,
iron.
Interstellar Dust Grains
Is there dust that we cannot see?
Yes. Quite a bit in fact.
The space between the stars is filled with thin
clouds of dust. Its major effect is in what is
known as extinction.
When light from distant objects passes
through these clouds, it is reddened (just
like a sunrise or sunset)
Extinction
• Dimming of starlight at all wavelengths.
– Extinction caused by scattering of light out of the line-of-sight--less light
reaches us.
• In 1930, R.J. Trumpler plotted the angular diameter of
clusters vs. distance to cluster.
– Distance found from inverse square law of brightness.
– He found a systematic increase of the linear size of the clusters with
distance.
– Unreasonable! It would mean that nature had put the Sun at a special place
where the size of the clusters was the smallest.
– More reasonable: the Sun is in a typical spot. It's simply that more distant
clusters have more stuff between us and the clusters so that they appear
fainter (farther away) than they really are.
Reddening
• Extinction depends on wavelength
– Bluer wavelengths are scattered more than
redder wavelengths.
• Behavior says that the dust size must be about the
wavelength of light
– Less blue light reaches us so object appears
redder than it should.
• Trumpler showed that a given spectral type
of star becomes increasingly redder with
distance.
Interstellar Gas Clouds
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•
•
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Neutral Hydrogen (H I)
Ionized Hydrogen (H II)
Ultrahot gas
Giant molecular clouds
Neutral Hydrogen (H I)
Detection:
Radio telescopes. This is
the source of the 21cm
band.
Temperature:
100 to 6000 K
Density:
1 to 50 atoms per cc
Content:
Clouds of Hydrogen 3 to
30 lightyears in
diameter.
Also C, O, N, CO, CH,
CN, H2 Organic Chemicals!
Optical Milkyway
21cm Milkyway
21-Cm Radio Line
A hydrogen atom has a
proton and an electron.
In addition to mass
and charge, they also
have a property known
as spin. There are two
possible states for
spin: Up and Down.
That means there are 4 possible configurations. Up/Up, Up/Down,
Down/Up and Down/Down (Proton Spin/Electron Spin). The
less-energy states are where the spins are opposite. So the act of
"jumping" from a higher state (Up/Up or Down/Down) to the
lesser will emit a photon at radio wavelengths (21-cm)
Ionized Hydrogen (H II)
Detection:
Ionized hydrogen is
continually capturing and
emitting photons. It is mostly
the red H line of Hydrogen
Temperature:
Hot. 100,000 K
Density:
100 to 10,000 atoms per cc
Content:
Glowing clouds of ionized
Hydrogen near hot stars
H II Regions
•Fluorescence of hydrogen atoms.
•Ultraviolet light from hot O & B stars is absorbed by
the Hydrogen gas and re-emitted mostly at visible
wavelengths, primarily 6563 Å(red color).
•Each UV photon produces a visible photon. O & B
stars only found in regions of star formation (know
why?).
•H II region spectra much simpler than star spectraeasier to decipher.
•Distribution of H II regions is in spiral pattern. O & B
are spiral tracers also.
Ultrahot Interstellar Gas
Detection:
Ultrahot outflow heats up
the surrounding gas
Temperature:
Ultrahot
105 --- 106 K
Density:
1000 atoms per cubic meter
Content:
Particles, atoms and molecules
are fired out into space at
1000s of km/sec; usually from
Supernovae or other eruptives
Giant Molecular Clouds
Detection:
Complex brightline and
darkline spectra.
Temperature:
Varies from very cold 10K
to very warm 105 to 106 K
(depending on nearby stars)
Density:
100,000 atoms per cc
Content:
Large clouds, 50 to 200 light
years across. H, O, C, N, S,
C2H5OH (ethanol),
HC3N (cyanoacetylene),
CH3CHO (acetaldehyde)
Building blocks of DNA
Molecular Clouds
• Most of the molecules in the ISM are clumped together into clouds
with masses anywhere from just a few solar masses to over a million
solar masses with radii ranging from a few pc to over 100 pc.
• Milky Way has about 2.5 billion solar masses of molecular gas
with about 70% of it in a ring at 4-8 kpc distance from the center.
• Not much molecular gas at 1-3 kpc distance from center.
• About 15% of total molecular gas mass is located close to galactic
center within 1.5 kpc from the center.
• Most of the gas is clumped in the spiral arms within the disk