Lecture 14Lecture 14
Star formation
Insterstellar dust and gasInsterstellar dust and gas
•Dust and gas is mostly found in galaxy disks, and blocks optical light
The interstellar mediumThe interstellar medium
•Stars are born from this gas and dust, collectively known as the interstellar medium.
•During their lifetime, stars may return some material to the ISM through surface winds or explosive events
Composition of the ISMComposition of the ISM
•Hydrogen is by far the most common element in the ISM
Molecular (H2) Neutral (HI) Ionized (HII)
• Also contains helium and other elements. The solid component is in the form of dust.
Neutral hydrogenNeutral hydrogen
•HI can emit radiation if the electron flips its spin angular momentum vector.
•This is a very small energy difference of only 5.9 eV, corresponding to a wavelength =21 cm.
- corresponds to radio frequencies, 1420 MHz
A map of neutral H in the Milky Way
The Milky Way in optical light
Neutral Hydrogen in the Milky WayNeutral Hydrogen in the Milky Way
HI gas in the Milky Way clearly reveals spiral structure
Properties of interstellar dustProperties of interstellar dust
•Grain sizes: 1nm-10 m (i.e. similar to visible light)
•Composition: graphite, SiC, silicates, H2, H2O
Interstellar dustInterstellar dust
•Interstellar dust is likely produced in the envelopes around red supergiant stars.
•Radiate in the infrared (cooling mechanism)•Are easily destroyed by collisions
Interstellar extinctionInterstellar extinction
Dust scatters starlight. Thus a star behind a dust cloud will appear fainter. The apparent magnitude of a star is therefore:
adMm 5log5 10
where d is measured in parsecs, and a is the number of magnitudes of extinction along the line of sight. How is this related to the optical depth?
086.1a
Molecular cloudsMolecular clouds
When hydrogen becomes dense enough, molecules of H2 form:
•H2 is nearly impossible to observe: there are no emission or absorption lines at visible or radio wavlengths
Thus we rely on tracer molecules, most commonly CO but also CH, OH, CS and C3H2.
Types of molecular cloudsTypes of molecular cloudsTranslucent cloudsT=15-50 Kn~5x108-5x109 m-3
M~3-100 MSun
R~ 1-10 pcaV~1-5
Giant molecular cloudsT~20 Kn~1x108-3x108 m-3
M~106 MSun
R~50 pc
Giant molecular cloud coresT~100-200 Kn~1x1013-3x1015 m-3
M~10 – 1000 MSun
R<1 pcaV~50-1000
The sites of star formationThe sites of star formation
The cores of molecular clouds are likely sites of new star formation
The formation of protostarsThe formation of protostars
There are many unanswered questions about the formation of protostars
Since they form in very dense, opaque clouds of dust and gas they are very difficult to observe in detail
BreakBreak
The Jeans massThe Jeans mass
A simple energetic argument can give a rough approximation for the conditions required for a molecular cloud to collapse and form stars.
The virial theorem relates (time-averaged) kinetic to potential energy, for a stable, gravitationally bound system: 02 UK
This indicates a stability criterion: if the kinetic energy is too low, the cloud will collapse under the force of gravity
This defines a critical mass, known as the Jeans mass:
2/12/1
2/1
3
32/1
33
3
4
15
4
375
T
Gm
kR
T
mG
kM
HJ
HJ
It can also be expressed as a radius, in terms of the Jeans length:
The two are related by: JH
J RmG
kTM
5
Example: molecular cloud coresExample: molecular cloud cores
What is the Jeans mass for a molecular cloud core?
314105
150
1000/10
mn
KT
MM Sun
SunJ MM 5.4
Thus these cores should be collapsing under the weight of their own gravity, consistent with their association with the sites of star formation.
Cloud collapseCloud collapse
A collapsing molecular cloud starts off simply: In free-fall, assuming the pressure gradients are too small to have much
effect The gas is approximately isothermal, if gas is optically thin so energy can be
efficiently radiated away.
1
3
8 0200
r
rrG
dt
dr
The time it takes for the shell containing mass Mr to collapse to r=0 is the free-fall time scale:
2/1
0
1
32
3
G
t ff
Example: cloud collapseExample: cloud collapse
Notice the collapse starts off slowly, but the density increases sharply during the final stages.