3. Interstellar medium
•This is a photography of Milky Way.
•You can find black regions in the Milky Way. These regions are called interstellar
medium(星际介质).
•The starlight is blocked by the interstellar medium. > We cannot study interstellar
medium by optical telescope easily!
4. Interstellar medium
•This is a photography of Milky Way.
•You can find black regions in the Milky Way. These regions are called interstellar
medium(星际介质).
•The starlight is blocked by the interstellar medium. > We cannot study interstellar
medium by optical telescope easily!
5. Interstellar medium
•This is a photography of Milky Way.
•You can find black regions in the Milky Way. These regions are called interstellar
medium(星际介质).
•The starlight is blocked by the interstellar medium. > We cannot study interstellar
medium by optical telescope easily!
6. Interstellar medium
•Interstellar medium is made up of two components, gas(⽓体) and dust（尘埃）
Gas : The gas is made up mainly of individual atoms or molecules.
The typical size of gas is roughly 10−10
m(0.1nm)
Dust: The dust consists of clumps of atoms and molecules.
Typical size is The dust is not very well
known.
10−7
m(0.1μm)
•The density of interstellar medium
is extremely low.
~ 0.01 − 1000/cm3
•For dust, it is even rarer
~10−6
/cm3
7. Why sunset is red??
•We know the sunset seems to be red.
•This is because shorter wavelength(bluer)
light from sun is scattered by dust in the air
and longer wavelength(red) of light can reach
us.
Same physics is happened in
the dust in the universe!
8. Extinction and Reddening
•The overall starlight is reduced by dust. This
phenomenon is called extinction（消光）
•As similar as the case of sunset, long-
wavelength(red) light from stars reaches us more
than short-wavelength(blue) light.This is called
reddening（红化）
•(Left) Because of extinction, the interstellar
region is dark by visible band.
•(Right) But, when we observe interstellar
medium by infrared band, it is bright due to
reddening.
9. Nebulae
•Astronomical objects which consists of cosmic dust and interstellar gas is called nebulae
（星云）
⻢头星云
•If cloud happens to obscure(遮挡) stars lying behind it, we
see the cloud as a dark patch on bright background. In this
case, we call it dark nebulae.
•But, if there are hot young stars within the cloud, they ionize
nebulae and we see bright emission (emission nebulae).
LHA120 -N55
10. Emission nebulae
Ionized gas
Dust lane
•At on near the center of nebula, there are hot O or
B-type stars. They emit ultraviolet photons and
ionize nebula. From emission lines, we can know
typical temperature of emission nebula is roughly
8000K.
Reflection
nebula
•Dust lane consists of a relatively dense region
where star light is obscured.
•Reflection nebula is caused by starlight scattered
from dust particles in interstellar clouds.
13. Dark dust clouds
Dark dust clouds are…
•even colder than their surroundings (~ a few tens of K)
•1’000- 1’000’000 times denser than surroundings ( ).
107
− 1012
atoms/m3
(*)~8000K for emission nebula
•Because dark dust clouds are dense,
they obscure visible light.
•But, we can observe dark dust
clouds by radio wavelength
because dark dust clouds emits
radio emission(e.g CO).
14. Dark dust clouds
Dark dust clouds are…
•even colder than their surroundings (~ a few tens of K)
•1’000- 1’000’000 times denser than surroundings ( ).
107
− 1012
atoms/m3
(*)~8000K for emission nebula
Low temperature and dense number density
•Because dark dust clouds are dense,
they obscure visible light.
•But, we can observe dark dust
clouds by radio wavelength
because dark dust clouds emits
radio emission(e.g CO).
15. Absorption spectra
•Since dark dust clouds are high density and low temperature, they generate (narrow) absorption
lines.
•Absorption lines tell us what kind of
elements are contained in the dark interstellar
clouds.
•We can distinguish absorption lines
generated by star itself or generated by cloud
because absorption lines generated by stars
are broad, while absorption lines generated by
clouds are narrow.
16. 21cm radiation
•Another probe to interstellar medium is 21cm emission line.
•Although it is difficult to observe interstellar medium by optical wavelength, we have
another probe to study interstellar medium.
•The 21cm emission line is radio wavelength radiation from
neutral hydrogen atom.
•21cm line emission is generated by energy transition in neutral
hydrogen atom
High energy
Low energy
17. Interstellar molecules（星际分⼦）
•In some particular cold (10-20K) interstellar regions,
densities can reach as high as . At
these regions, material exist in the form of molecule.
1012
particles/m3
•Molecules emit radiation by rotating or violating.
We can use molecules as tracer of interstellar
medium.
18. Summary
• The interstellar medium occupies the space among the
stars.
• A nebula is a fuzzy bright or dark patch on the sky.
• Dark dust clouds are cold and dense.
• The interstellar medium also contains many cold, dark
molecular clouds.
• One of the methods to observe interstellar medium is
absorption lines or 21cm emission line.
20. Where do stars form??
(Question ) We know there are many stars in the universe, but where do stars form??
21. Where do stars form??
(Question ) We know there are many stars in the universe, but where do stars form??
(Answer) Stars formed in the interstellar medium(星际介质)
22. Star forming regions
•Young blue stars are born in the interstellar
medium.
•In the interstellar medium, one cold gas
begins to collapse to form stars by its
gravity.
•Let’s see more details how stars form next.
23. F = G
Mm
r2
Newton’s gravity
1. In the interstellar medium, there are many gas particles such as hydrogen atom and
hydrogen molecules.
Gravity and heat
24. F = G
Mm
r2
Newton’s gravity
2. Gas particles come together by their own gravity.
Gravity and heat
25. F = G
Mm
r2
Newton’s gravity
2. Gas particles come together by their own gravity.
Gravity and heat
26. F = G
Mm
r2
Newton’s gravity
Gravity and heat
3. But, when gas particles come together, the gas is heated and the temperature
increases. This also causes the increase of gas pressure.
T
27. F = G
Mm
r2
Newton’s gravity
Gravity and heat
3. But, when gas particles come together, the gas is heated and the temperature
increases. This also causes the increase of gas pressure.
T
28. F = G
Mm
r2
Newton’s gravity
Gravity and heat
3. But, when gas particles come together, the gas is heated and the temperature
increases. This also causes the increase of gas pressure.
T
T
29. F = G
Mm
r2
Newton’s gravity
Gravity and heat
3. But, when gas particles come together, the gas is heated and the temperature
increases. This also causes the increase of gas pressure.
T
T Gas pressure
P ∝ T
30. F = G
Mm
r2
Newton’s gravity
Gravity and heat
3. But, when gas particles come together, the gas is heated and the temperature
increases. This also causes the increase of gas pressure.
T
T Gas pressure
P ∝ T
31. F = G
Mm
r2
Newton’s gravity
Gravity and heat
4. Gas pressure prevents gas particle from gravity collapsing
T Gas pressure
P ∝ T
32. Gravity and heat
•We have to consider both gas pressure and gravity
for star formation.
P
F
•After gas pressure and gravity are balanced ( ),
nuclear fusion starts in the core of star and stars begin to
shine on themselves. Star is born!
F = P
