Transcript Lecture 12: Evolution of the Galaxy

ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
16. Evolution of the Galaxy
16.1 Star formation
16.2 Exchange of material between stars and ISM
16.3 Heavy element enrichment of the ISM
16.4 Collapse of the Galaxy and the formation
of the halo and disk
ASTR112 The Galaxy
Lecture 12
• Star formation takes place in dense molecular clouds
in galactic spiral arms
• Cloud mass ~ 1000 M⊙, enough to cause gravitational
contraction
• As density goes up, cloud fragments into a number of
collapsing sub-centres. This process continues,
eventually with a typical collapsing mass ~ 1 M⊙
• A star cluster forms, all stars having about the same age
Prof. John Hearnshaw
Star formation
ASTR112 The Galaxy
Lecture 12
As the collapse proceeds, the cloud fragments with progressively
more subcentres and sub-subcentres of collapse, each being
eventually of about stellar mass and hence a star cluster is formed
Prof. John Hearnshaw
Gravitational collapse of an interstellar dense molecular cloud
ASTR112 The Galaxy
Lecture 12
• Protostars can have high spin rates to conserve angular
momentum. This results in
(a) an accretion disk around equator
(b) strong bipolar magnetic field and hence mass loss
in bipolar outflow through poles (observed
through strong millimetre wave emission in lines
of CO)
• Typical mass loss rate of protostar ~ 10-4 to 10-6 M⊙/yr
• Protostars are often enshrouded in dust shells which are
warmed to radiate in the mid-infrared
Prof. John Hearnshaw
Protostars
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
Bipolar outflow from
a protostar observed
in L1551, in the IR
source IRS5.
A model for bipolar outflow from a protostar with
a magnetized accretion disk (here seen edge on)
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
Mass loss from stars
Other stars undergo sudden mass loss events at the end
of their lives:
• Planetary nebulae are mass lost from low mass stars,
possibly MPN ~ 0.5 M⊙, lost in a non-explosive event
• Supernovae: exploding massive stars, MSNR≥1 M⊙
Prof. John Hearnshaw
Many stars are continuously losing mass:
• protostars ~ 10–4 to 10–6 M⊙/yr
• red giants ~ 10–6 to 10–7 M⊙/yr
• Sun ~ 10–14 M⊙/yr (solar wind)
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
Planetary nebulae
ASTR112 The Galaxy
Lecture 12
The Crab Nebula
is the remnant of
a star that exploded
in 1054 AD. It was
observed by Chinese
astronomers
Prof. John Hearnshaw
The Crab nebula, M1
ASTR112 The Galaxy
Lecture 12
Interstellar
gas
clouds
New
star
formation
Stars grow
old and use
up H fuel
Low
mass
stars
Evolution of the Galaxy
Supernovae
Stars
losing
mass
Planetary
nebulae
Neutron
stars
Dead
stars
White
dwarfs
Prof. John Hearnshaw
High
mass
stars
ASTR112 The Galaxy
Lecture 12
Young blue stars in a star
forming region NGC3603,
like a maternity ward in
the Milky Way
Prof. John Hearnshaw
Star formation
Star formation in the Rosette nebula
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
Eagle nebula from ground
Eagle nebula from
space (Hubble)
Prof. John Hearnshaw
Star formation in the Eagle nebula
ASTR112 The Galaxy
Lecture 12
Dust columns in
the Eagle nebula
Prof. John Hearnshaw
Stars are forming in
giant dust clouds in the
centres of nebulae
ASTR112 The Galaxy
Lecture 12
Molecular clouds
are often associated
with dark dust clouds
and are where star
formation starts.
This example is
the dark cloud
Barnard 86
Prof. John Hearnshaw
Dark dust clouds
Protostars in Orion
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
The Vela SNR
Prof. John Hearnshaw
The Vela supernova
remnant, 10,000
years after the
explosion
The Pleiades - a young star cluster
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
The Cat’s Eye
Nebula ejected gas
from a dying
star
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
Supernova in
NGC 5253
ASTR112 The Galaxy
Lecture 12
Supernova remnants
Above
Crab nebula 1054 AD
Prof. John Hearnshaw
Below
Vela nebula ~ 8000 BC
The tiny pulsar
is the remains
of the exploding star
that created the Crab
nebula. It is a neutron
star whose diameter is
about 20 km across
and which is spinning
3 times a second
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
Tycho’s supernova of
1572 was one of three
very bright ones in the
Milky Way in the last
1000 years. This is a
radio map showing the
synchrotron emission
from a non-thermal source
with a magnetic field
Prof. John Hearnshaw
Tycho’s supernova
of 1572
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
The Cygnus Loop
supernova remnant
is estimated to be
about 20,000 years
old.
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
Supernova remnant
Shajn 147
ASTR112 The Galaxy
Lecture 12
SN1987A
Prof. John Hearnshaw
Supernova
SN1987A in
the LMC, the
only naked
eye supernova
in recent times
ASTR112 The Galaxy
Lecture 12
Heavy element enrichment of the ISM
into the ISM
• Further heavy elements may be formed during the
explosion in high temperature shock waves
• The ejecta intially travel out at about 10,000 km/s
and eventually become well mixed with the
surrounding ISM
• Ejecta sweep up neutral H in a snow-plough action
over some 105 years before the expansion dissipates.
Prof. John Hearnshaw
• Supernovae eject gas rich in heavy elements back
ASTR112 The Galaxy
Lecture 12
• Supernova rate is ~ 3 SN/century in entire Milky Way
• Observed rate is about 3 SN/thousand years (only 10%)
as a result of obscuration by dust
• About 120 supernova remnants are known in the Galaxy
• Most famous is the Crab nebula of 1054 AD
• Also Tycho’s SN (1572); Kepler’s SN (1604); S And
in M31 (1885); SN1987A in the LMC (1987)
Prof. John Hearnshaw
Supernovae
ASTR112 The Galaxy
Lecture 12
Number of SN/century
~ 3 SN
Age of Galaxy
1.5 × 1010 yr
Number of SN over lifetime of Galaxy
4.5 × 108 SN
Total mass of Galaxy (stars and ISM)
2 × 1011 M⊙
Mean mass fraction of this mass in heavy
elements
Z ≈ 0.01
Mass of heavy elements in Galaxy
2 × 109 M⊙
Mass of heavy elements produced 2 × 109 M⊙/ 4.5 × 108
per SN
≈ 4.5 M⊙
Average mass ejected per SN
≈ 5 M⊙
Prof. John Hearnshaw
Heavy element enrichment of ISM by supernovae
• As a result of SN, mean heavy element content of
ISM slowly increases.
• New stars which form therefore have higher values of
heavy element mass fraction, Z, at the time of their birth.
The youngest stars are therefore the most heavy-element
rich, and the oldest ones (Population II stars) are the
most deficient in heavy elements relative to the Sun.
• Halo Population II stars have Z ~ 10-3 to 10-1 Z⊙
• Disk Population I stars have Z > 0.1 Z⊙ (NB: Z⊙ ~ 0.03)
• Youngest Popn I stars have Z up to ~ 2 × Z⊙
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
The change in the metal
content of stars and the ISM
in the Galaxy with time can be
investigated by measuring the
composition of stars from their
spectral lines and measuring
the ages of stars in clusters
from their HR diagrams.
The metallicity increased rapidly
in the first few × 108 yr, but
only slowly thereafter.
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
• The Galaxy is presumed to have started from a huge
extended low density cloud of H and He
• This underwent rapid gravitational collapse over ~2 × 108
years (halo era) during which time the Population II stars
were formed, all with low metallicity
• As collapse proceeded, gas and dust clouds formed disk
of Galaxy, as a result of galactic rotation
• Metallicity increased rapidly at first, because of very high
initial star formation and hence supernova rate
Prof. John Hearnshaw
Collapse of the Galaxy; formation of the halo and disk
ASTR112 The Galaxy
Lecture 12
• As ISM is slowly used up to form stars, star formation
rate declines, and so does the supernova rate and hence
rate of metal enrichment of ISM by supernovae
• Stars retain the metallicity and kinematics conferred on
them at birth. However, gas clouds collide with each
other and settle into regular circular orbits in the galactic
disk.
• This means that disk stars have circular orbits, unlike the
Population II halo stars.
Prof. John Hearnshaw
Collapse of the Galaxy (continued)
Prof. John Hearnshaw
ASTR112 The Galaxy
Lecture 12
ASTR112 The Galaxy
Lecture 12
Prof. John Hearnshaw
End of lecture 12
and of the Galaxy lectures!