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.