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Astronomy 114
Lecture 17: How Stars Form
Martin D. Weinberg
[email protected]
UMass/Astronomy Department
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—1/17
Announcements
Quiz #1 redux due today
PS #4 due on Friday
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—2/17
Announcements
Quiz #1 redux due today
PS #4 due on Friday
Today:
How stars form, role of Angular Momentum
The Birth of Stars, Chap. 20
Stellar Evolution, Chap. 21
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—2/17
HR diagrams of star clusters
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—3/17
HR diagrams of star clusters
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—3/17
Stellar Evolution: preview
Stars are not permanent
Thermonuclear reactions (fusion) that energize the
star use up the hydrogen
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—4/17
Stellar Evolution: preview
Stars are not permanent
Thermonuclear reactions (fusion) that energize the
star use up the hydrogen
Life time of a star:
Proportional to the mass
Inversely proportional to the luminosity
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—4/17
Stellar Evolution: preview
Life time of a star:
Proportional to the mass
Inversely proportional to the luminosity
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—4/17
Birth of stars
Interstellar gas and dust are common in the disk of
the Galaxy
Interstellar medium (ISM)
Diffuse gas (atomic H and
molecular hydrogen H2 )
Dust (formed in the envelopes
of stars)
Clouds of denser than average accumulations of dust
and gas are seen as nebula
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—5/17
Type of nebula
Emission nebula
Reflection nebula
Dark nebula
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—6/17
Interstellar reddening and extinction (1/4)
Dust grains preferentially absorb and scatter short
wavelengths (blue)
Light appears to “redden”
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—7/17
Interstellar reddening and extinction (2/4)
Left cluster is more
distant
More dust between us
and this cluster
Emission is reddened
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—8/17
Interstellar reddening and extinction (3/4)
99% gas
74% hydrogen, 25% helium
1% dust
Density: approx 1 atom per cubic cm
Density of air: 3 × 1019 molecules per cubic cm
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—9/17
Interstellar reddening and extinction (4/4)
Blue wavelengths from Sun are
preferentially scattered but
molecules, dust and turbulence
Blue sky!
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—10/17
Interstellar reddening and extinction (4/4)
Blue wavelengths from Sun are
preferentially scattered but
molecules, dust and turbulence
Blue sky!
Red sunset!
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—10/17
The Galaxy in the Infrared (1/2)
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—11/17
The Galaxy in the Infrared (1/2)
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—11/17
The Galaxy in the Infrared (2/2)
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—12/17
Protostars
Basic steps in forming a star:
1. Fragmentation of a giant molecular cloud into clumps
2. Collapse of the clumps due to their own gravity
(free-fall)
3. Contraction of the star through conversion of the
gravitational energy through slow falling into heat
(Kelvin-Helmholtz contraction)
4. Fusion reaction begins in center
(ignition of core Hydrogen burning)
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—13/17
What causes the collapse? (1/4)
1. Accretion
Clumps collide
New mass overcomes internal pressure
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—14/17
What causes the collapse? (1/4)
1. Accretion
Clumps collide
New mass overcomes internal pressure
2. Gravity and radiation pressure
Pressure of star light and gravity overcomes
internal pressure
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—14/17
What causes the collapse? (2/4)
3. Supernova blast wave
Exploding star compresses cloud, increasing
density
Gravity overcomes pressure
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—15/17
What causes the collapse? (3/4)
Shell from supernova explosion plowing into ISM
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—16/17
What causes the collapse? (4/4)
Collapsed clumps of dense gas in the Carina nebula
A114: Lecture 17—14 Mar 2004
Read: Ch. 20, 21
Astronomy 114—17/17