As we have noted above, the region between the stars in a galaxy like the Milky Way
is far from empty. These regions have very low densities (they constitute a
better than can be produced artificially
on the surface of the Earth), but are filled with
gas, dust, magnetic fields, and charged particles. This is commonly termed the
Approximately 99% of the mass of the interstellar medium is in the form of gas with
the remainder primarily in dust. The total mass of the gas and dust in the
interstellar medium is about 15% of the total mass of visible matter in the Milky
Gas in the Interstellar Medium
Of the gas in the Milky Way, 90% by mass is hydrogen, with the remainder
mostly helium. The gas appears primarily in two forms
- Cold clouds of atomic or molecular hydrogen
- Hot ionized hydrogen near hot young stars
The clouds of cold molecular and atomic hydrogen represent the raw material from
which stars can be formed in the disk of the galaxy if they become gravitationally
unstable and collapse. Although such clouds do not emit visible radiation,
they can be detected by their radio frequency emission.
HI and HII Regions
Ionized hydrogen is produced when the ultraviolet radiation
emitted copiously by hot newly-formed stars ionizes surrounding clouds of gas. The
characteristic beautiful red colors of emission nebulae like the
Orion Nebula (M42) or the
(Ref) shown in the adjacent figures
visible light emitted when electrons recombine with the ionized hydrogen in these
regions. Such regions of ionized hydrogen are called HII regions, while cold
un-ionized hydrogen clouds are termed HI regions (with the I and II referring to the
ionization state of the hydrogen).
The disk and its spiral arms are heavily populated by both HI and HII regions.
The Orion Nebula is relatively nearby, about 1500 light years away in the same spiral arm of the galaxy as
our own Sun. The image shown above
is a mosaic of Hubble Space Telescope images showing the inner 2.5
light years of this large nebula (which is visible as the middle "star" in Orion's sword). As we have
already seen, M42 is the location of many stellar nurseries where stars are being born.
Dust in the Interstellar Medium
Interstellar dust grains are typically a fraction of
a micron across (approximately the wavelength of blue light),
irregularly shaped, and composed of carbon and/or silicates. Absorption of light
dark regions in our galaxy and in other
galaxies, as this picture of the
Milky Way looking toward the center indicates.
These dust clouds are visible if they absorb the light coming through them. We then
refer to these clouds as
dark nebulae such as the
adjacent Horsehead Nebula.
On the other hand, light can reflect from clouds of
dust and gas, giving rise to sometimes beautiful
Dust has two major effects on light passing through it:
Quantitative observations in astronomy must correct for both extinction and reddening
of light by the interstellar medium.
- The light is dimmed by the dust; this is called interstellar extinction.
- The light that does pass through the dust is depleted in blue wavelengths because
the size of the dust grains favors scattering blue light. This is called
interstellar reddening, because the resultant transmitted light is more red
than it would have been otherwise. This implies that transmitted light will be more
red, but reflected light will be more blue. (On Earth, the blueness of the sky is
due to similar effects in scattering of light from molecules in the atmosphere.)
The image adjacent left shows an example of a reflection nebula
called the Witch Head Nebula, because of its shape, but you can call it IC 2118 if you are
less poetic or an astronomer
(Ref). It is associated with the bright star Rigel in the constellation
Orion (Rigel is offstage to the right of the image
shown), and is about 900 light years away. The reflection nebula is blue for two reasons: the hot star
Rigel supplying the light being reflected is blue, and as noted above the fine dust in the nebula
is more efficient at scattering blue light than red light because of the size of the dust particles.
Another example of a reflection nebula is shown below. The Pleiades Cluster is a young cluster
of predominantly blue stars that is
visible to the naked eye. There is still some dust left from the nebula
in which they formed, and light reflecting from that dust causes the blue haze around each star of
The red part of the Trifid Nebula is an emission nebula powered by the hot star in its center.
The blue part of the Trifid Nebula is a reflection nebula, reflecting light from a hot star centered in
that part of the nebula. Thus, the Trifid Nebula is both an emission and reflection nebula.
Likewise, the Orion Nebula shown above contains both emission and reflection nebulae within it (as well as
The image below left
illustrates a portion of the sky near the star Antares (which is the brightest star in the
image). It contains many examples of all three kinds of nebulae that we have discussed:
red emission nebulae, blue reflection nebulae, and dark absorption nebulae
(Source). The image below right
is another example of a beautiful nebula. It is about 9000 light years away in the constellation Carina
and is called NGC 3372 or The Great Nebula in Carina
(Source). This nebula contains the massive
active star Eta Carina.
The Source of Interstellar Dust
The exact nature and
origin of interstellar dust grains is unknown, but they are presumably ejected
from stars. One likely source is from red giant stars late in their lives. In
particular, stars on the asymptotic giant branch of the HR diagram (AGB stars) are
known to eject much of their envelope into space and this could be a significant
source of interstellar dust grains.
The Distribution of Aluminum-26
One important probe of the interstellar medium is the distribution of the radioactive isotope
aluminum-26. It can be detected by gamma ray detectors on satellites (gamma rays are completely
absorbed in the atmosphere) because it emits gamma rays of a particular energy that are fingerprints for
just as emission lines in the optical spectrum at particular frequencies are fingerprints for atoms.
The distribution of Al-26 is important because (1) it can be produced in various processes in stellar
evolution, and (2) it decays radioactively to magnesium-26 with
a half-life that is short on astronomical scales (about a million years), so if it is
seen it must have been produced very recently. The following figure shows an all-sky map in galactic
coordinates of the observed distribution of Al-26, obtained using the COMPTEL instrument aboard the
orbiting Compton Gamma Ray Observatory
We don't know the distance to these Al-26 sources.
since the intensity of the gamma rays from a source decreases as the square of the distance to the source
and we do not expect enormous concentrations of Al-26 anywhere, this map presumably represents Al-26
concentrations within our own galaxy.
This distribution of Al-26 is not understood in any detail. It is concentrated in the plane of the galaxy,
and since it can't be far from where it was formed and the most likely process that produce it involve
massive stars (for example, supernova explosions and nucleosynthesis in aging massive stars),
it probably is correlated with star-forming regions in the galaxy.