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Stellar Nurseries
Stages of Star Birth
The interstellar medium
The space between the
stars is not empty
Light from distant stars passing through dark clouds of gas and dust
has an absorption spectrum imprinted on it.
This tells us what is in the interstellar medium
It is made up of 70% hydrogen, 28% helium, and 2% heavier atoms and molecules.
The interstellar medium consists of 70% H, 28% He, and 2% heavier elements (by mass).
Many of the dust and gas clouds are hot – thermal pressure holds them up against gravity.
They have to cool down (10K – 30K) before they are able to collapse: molecular clouds.
Radio image of CO emission – it is too cold for molecular H to emit
These molecular clouds also contain dust
These dust grains are usually less than 0.5 micrometers
across
This makes them very good at scattering visible light:
“Interstellar reddening”
“Reddening” is different
from Doppler shift
How can we tell if a star’s light has been reddened by interstellar dust rather than the star just
being red (very cool)?
A) The lines in the spectrum won’t be Doppler shifted if it’s interstellar reddening.
B) The spectral type still can be determined to tell us the surface temperature.
C) The only way to tell is if there are other nearby stars that also look red.
D) There is no way to tell for sure.
We can use the fact that infrared light gets through the molecular clouds to see inside star forming
regions…
Gravity has to overcome several obstacles to collapse:
• Thermal pressure
• Turbulence
• Rotation
• Magnetic fields:
Observations of starlight passing through molecular clouds
show that the light is often polarized, indicating magnetic
fields imbedded in the cloud.
To overcome these obstacles, the molecular cloud
generally needs a mass that is a couple hundred times
that of our Sun…
This graphical display of a computer simulation shows how turbulent motion of the gas in
the cloud causes it to get lumpy: some regions are denser than others.
Each lump can go on to form one or more stars:
This is “clustered star formation”
Smaller clouds of gas and dust can form individual stars as long as they are unusually cold
and dense.
First generation stars were made of H and He only – there were no heavier elements yet.
This means the clouds of gas were not as cold as the molecular clouds of today:
molecular hydrogen cannot radiate thermal energy away below about 100 K.
To overcome these higher temperatures, the clouds had to be much more massive to collapse
 First generation stars:
• Were very massive
• Lived very brief lives (none exist today)
• Sent the heavier elements they created back into space
Why were the first stars that formed in our universe necessarily so large?
A) Because the universe was smaller then, the gas was much denser everywhere.
B) The universe was much hotter in the first billion years, so the gas clouds had to be more
massive to overcome the warmer temperature.
C) Hydrogen can’t radiate away heat below about 100 K, so the gas clouds had to be more
massive to overcome the warmer temperature.
D) The heavier elements available today weren’t available then, so the gas clouds had to be more
massive to generate enough gravity.
Stages of Star Birth
Protostellar disk
The disk slows down
the rotation of the
protostar due to the
protostar’s magnetic
field lines dragging
through the disk.
Protostellar jets
Are probably the magnetic
field transferring angular
momentum to material in the
disk, but we don’t know for
sure. But they do happen … !
A protostar goes through several stages in its development
“cocoon”
Protostellar cocoons?
A protostar goes through several stages in its development
“cocoon”
Remember that “surface” means the layer from which visible
photons are emitted
At temperatures above 3000K,
H– ions keep the plasma
opaque – heat escapes only
through convection
Higher
temperatures
Lower
temperatures
3000 K
When the rising plasma cools to
3000K, it becomes fully neutral and
photons can escape
(that’s what defines “surface”)
A protostar goes through several stages in its development
“cocoon”
When the interior gets hot enough, the plasma becomes fully ionized
Then radiation is the primary transport mechanism
That allows for a higher rate of energy transport,
and the protostar’s surface becomes hotter
When the fusion rate balances the energy radiated from the surface,
gravitational equilibrium will be achieved.
The star is on the main sequence.
“cocoon”
The rate at which protostars go
through this process depends on
their mass
High mass stars will be born,
live out their entire lives, and die
violently before low mass stars
are even through the protostar
process!
Astro-Cash Cab!
Isaiah
Zach
Katie
James
1) What is interstellar reddening?
A) Interstellar dust absorbs more red light than blue light, making
stars appear redder than their true color.
B) Interstellar dust absorbs more red light than blue light, making stars
appear bluer than their true color.
C) Interstellar dust absorbs more blue light than red light, making stars
appear redder than their true color.
D) Interstellar dust absorbs more blue light than red light, making
stars appear bluer than their true color.
2) What temperature does an interstellar cloud of gas and dust have to reach in
order to collapse and form stars?
a) 20 K
b) 0 F
c) 212 F
d) 5800 K
e) 10,000,000 K
3) What element makes up the majority of the material in interstellar space?
(And the whole universe, for that matter!)
4) True or False?
We can peer inside a protostar’s dust cocoon using ultraviolet light.