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The Interstellar Medium
• About 99% of the material between the stars is in
the form of a gas
• The remaining 1% exists as interstellar grains or
interstellar dust
• If all the interstellar gas were spread evenly, there
would be about 1 atom per cm3

Dust grains are even scarcer
• Although the density is low, the total amount of
interstellar matter is huge

5% of the matter in the Milky Way galaxy
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
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Interstellar Gas
• Some of the most beautiful sights in the sky are
created by interstellar gas heated by nearby stars

Can be heated to 10,000 K and glows with the
characteristic red of hydrogen gas (Balmer line)
• Interstellar hydrogen gas near very hot stars is
ionized by the radiated UV

H II region

A single I means neutral, two II means ionized
• Light is emitted when protons and electrons
recombine to form atomic hydrogen
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
UV to visible light
Fluorescence
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
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Papillon Nebula
• The Papillon nebula is located in the Large Magellanic
Cloud which is the site of young massive stars
• The red in this
true color
picture is from
the hydrogen
and the yellow
from high
excitation
ionized oxygen
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
3
Neutral Hydrogen Clouds
• Ionized hydrogen makes pretty pictures but most
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interstellar hydrogen is not ionized
We can study these clouds by absorption

First done using binary stars to help isolate the absorption lines
from the interstellar clouds
• Sodium and calcium (Z=11 and 20) have distinctive
•
absorption lines and are easily seen using visible light
Hydrogen, oxygen ,nitrogen absorb in the UV have been
seen with satellite based observations

Interstellar gases are depleted in elements that can easily
condense


Aluminum, calcium, titanium, iron, silicon, magnesium
These elements become dust rather than gas
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
4
Trifid Nebula
• Red glow comes
from excitation of
hydrogen
• Blue comes from
scattering of light by
interstellar dust
• Black regions are
thick clouds of dust
that absorb all the
light
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
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Radio Observation of Cold Clouds
• Most of the interstellar material is cold hydrogen
• Hydrogen atoms can make a transition from electron spin
up to electron spin down radiating photons with a
wavelength of 21 cm

Radio waves! (1400 MHz)
• Observations at 21 cm show that the neutral hydrogen in
•
our galaxy is confined to a flat layer less than 300 LY
thick that extends throughout the flat disk of the Milky
Way
Hydrogen is located in cold clouds with diameters
ranging from 3 to 30 LY

Masses range from 1 to 1000 times the mass of the Sun
• About 20% of the interstellar hydrogen exists as warm
clouds
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
6
Ultra-hot Interstellar Gas
• Regions of ultra-hot interstellar gas have been
observed with temperatures up to 1 million K
• The heat source is supernovae

Exploding stars
• A supernova occurs about every 25 years in our
galaxy
• The shock wave spreads out and heats the gas
between the cold hydrogen clouds

Any given point is heated once every 2 million years
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
7
Interstellar Molecules
• A number of molecules (not just atoms) have been
observed in the interstellar medium
• Many complex molecules have been observed,
including progenitors of the basic amino acids
required to build life
• These complex molecules can only survive in
space when they are shielded by dense, dark, giant
clouds containing dust
• These giant clouds are interesting structures that
provide the raw material for stellar birth
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
8
The Eagle Nebula
• The Eagle Nebula consists of clouds of molecular
•
hydrogen and dust that have survived the UV radiation
from nearby hot stars
As the pillars are eroded
by the UV light, small
globules of denser gas
buried within the pillars
are uncovered

EGGs

Evaporating Gaseous
Globules

Embryonic stars
• Picture taken by HST,
April 1, 1995
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
9
Structure and Distribution of Interstellar Clouds
• Models for interstellar gas clouds required that the pressure of the
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•
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clouds and the interstellar material must be the same
Pressure depends on density and temperature
These clouds are embedded in
a thin gas with a temperature
of 1 million K from exploding
stars
The outer layers can be heated
to a few 1000 K
If the could is large enough,
the inner core can stay cool
and dense
Stars form from collapsing,
dense clouds of gas and dust
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
10
Interstellar Matter around the Sun
• A region of where the density of interstellar
matter is low surrounds the Sun


Local Bubble
Extends to 300 LY
• We should have observed about 2000 interstellar
clouds in the Local Bubble but we see very few

The Sun itself seems to be inside a cloud


Local fluff
One sizable warm cloud is known 60 LY from us
toward the center of the galaxy
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
11
Dark Nebula
• Dark nebula absorb light and block the view of stars
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•
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behind them
We can only see them visually when they block out light
from behind
Dark nebula absorb in the
visible and UV
Dark nebula radiate in the
infrared
In the Milky Way there are
dark nebula throughout the
plane of the galaxy


Visible in infrared
Infrared cirrus
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
12
Dark Nebula at Different Wavelengths
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
13
Reflection Nebula
• Some dense clouds are close to luminous stars and scatter
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•
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enough light to become visible
This example comes from
stars in the Pleiades cluster
The bluish hue comes about
because the dust particles are
small and scatter blue most
efficiently
This cloud is moving through
the Pleiades system and small
dust particles are being
slowed down faster than large
particles

Streamers and wisps
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
14
Interstellar Reddening
• Dust grains absorb and scatter light and make
distance stars appear to be dimmer

Interstellar extinction
• Some stars appear to be redder than they are
because of interstellar dust

Short wavelengths are absorbed and scattered more
strongly
Sunlight looks redder at sunset
 The sky looks blue

• Because long wavelengths penetrate better,
infrared astronomy can study stars that are more
than twice as far away
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
15
Portrait of Interstellar Reddening
• Red light passes through because

Dust tends to scatter blue light leaving more red light
to reach the observer
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
16
Interstellar Grains
• Interstellar gas is transparent

An enormous amount of interstellar gas would be
required to account for the absorption and scattering
we observe
• Small solid or liquid particles are much more
efficient at scattering light than gas molecules
• Interstellar grains are about the size of the
wavelength of light

10 to 100 nm
• There are many types of interstellar grains
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
Silicates, carbon
Probably formed by material ejected from stars
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
17
Portrait of an Interstellar Dust Grain
• Note that interstellar grains cannot be studies with
emission lines (they are solids)
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
18
Cosmic Rays
• High speed particles coming to Earth from space
are called cosmic rays
• Cosmic rays are high speed atomic nuclei,
electrons, and positrons

Most are protons
• The abundances of the elements in cosmic rays
are similar to those on earth except there is much
more lithium, beryllium, and boron (Z=3,4,5)

These elements are produced by fragmenting carbon,
nitrogen, and oxygen nuclei (Z=6,7,8)
• Cosmic rays that reach the surface of the Earth are
muons
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
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Origin of Cosmic Rays
• Cosmic rays are charged particles and their
motion is affected by magnetic fields

Difficult to pinpoint the origin of cosmic rays
• The galactic magnetic field is strong enough to
keep cosmic rays from leaving the galaxy
• From the abundance of Li,Be,B we can estimate
how far the cosmic rays have traveled

30 times around the galaxy, 10 million years
• The best candidates for the source of cosmic rays
are supernova explosions
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
20
Molecular Clouds
• Giant molecular clouds contain enough gas and dust to
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make 100 to 1,000,000 Suns
These clouds are 50 to 200 LY in diameter
• The cores of these clouds are
cool (10 K) and dense (104 to
105 atoms/cm3)
• Most of the gas exists as
molecules
• Perfect conditions for gravity
to compress the material and
produce densities and
temperatures high enough to
Giant columns of cool, dense
ignite a star
gas in the Eagle Nebula
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
21
The Orion Molecular Cloud
• The closest and best studied stellar nursery is in the constellation of
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Orion about 1500 LY away
The Orion nebula can be seen with binoculars along the sword of
Orion
In infrared light, the full extent of the nebula can be seen
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
22
Star Birth in the Orion Nebula
• A progression of star formation has been moving through the
molecular cloud
• On one end of the cloud, there are old stars (near the
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western shoulder of the hunter) about 12 million years
old
The stars in Orion’s belt are 8 million years old
The stars in the Trapezium cluster are 0.3 to 1 million
years old
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
23
Star Formation
• First step is the formation of cold cores in the cloud (a)
• A protostar forms with a surrounding disk of material (b)
• Stellar wind breaks out along the poles of the star (c)
• The solar wind sweeps away the cloud material and halts
the accumulation of more material and a newly formed
star is visible surrounded by a disk (d)
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
24
Winds and Jets
• Jets thought to form along the
poles of the protostar
• These jets of material collide
with existing material and
cause ionization

Herbig-Haro objects (HH)
• On the right is a very young
star (HH30, 100,000 years old)
obscured by a dust clouds
• The jets along the poles of the
star are clearly visible
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
HH30 photographed by HST
The disk of the flattened
cloud around the protostar is
seen edge-on
25
The H-R Diagram and Stellar Evolution
• A star forms at a
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particular size and
luminosity which places
is on the H-R diagram
As the star ages, it
“moves” on the H-R
diagram
When a protostar forms,
it contracts and heats up
until it reaches the main
sequence
Protostar
forming in
Orion nebula
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
26
Formation of Planets around Stars
• Planets outside our solar system are difficult to detect
• Planetary searches are done indirectly
• One method is to
•
study
protoplanetary
disks
About 50% of
known protostars
are surrounded by
by disks
Picture taken by HST of a developing
star called AB Aurigae
Clumps of dust and gas are visible that
may be leading to planet formation
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
27
Evidence for Planets
• Separated zones can form in protoplanetary disks if there
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is some large body like a planet that would stop the
inevitable fall of the material into the star
A visible dust ring
around a star is
evidence for an
unseen planet
In the HST picture
on the right, a very
young star HR
4796A is
surrounded by a
dust ring
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
28
Search for Planetary Orbital Motion
• One method would be to see the “wobble” of the
star as the planet orbited around it

No success so far
• Another method would be to study the Doppler
shift of the light of the star as it “wobbled” from
the effects of the planet
• This method has been
successful

More than 50 extrasolar planets are known
ISP 205 - Astronomy Gary D. Westfall
Lecture 19
29
Characteristics of Extra Solar Planets
• We are only able to detect very large, Jupiter size
planets
• Many of these planets are
very close to their stars

“Hot Jupiters”
• Planetary systems have
been found but no Earth
like planets have been
found
• The future is infrared
interferometry
ISP 205 - Astronomy Gary D. Westfall
Artist’s conception of giant planet
close to a Sun-like star
Lecture 19
30