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 Prof. John Hearnshaw Star formation 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

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Gravitational collapse of an interstellar dense molecular cloud 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

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Protostars 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 ~ to M ⊙ / yr Protostars are often enshrouded in dust shells which are warmed to radiate in the mid-infrared

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Bipolar outflow from a protostar observed in L1551, in the IR source IRS5.

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw A model for bipolar outflow from a protostar with a magnetized accretion disk (here seen edge on)

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Mass loss from stars 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) Other stars undergo sudden mass loss events at the end of their lives: Planetary nebulae are mass lost from low mass stars, possibly M PN ~ 0.5 M ⊙, lost in a non-explosive event Supernovae: exploding massive stars, M SNR ≥1 M ⊙

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Ring nebula M57 Helix nebula Planetary nebulae

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw The Crab Nebula is the remnant of a star that exploded in 1054 AD. It was observed by Chinese astronomers The Crab nebula, M1

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Supernovae Planetary nebulae Neutron stars White dwarfs Interstellar gas clouds New star formation Stars grow old and use up H fuel High mass stars Low mass stars Dead stars Stars losing mass Evolution of the Galaxy

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Star formation Young blue stars in a star forming region NGC3603, like a maternity ward in the Milky Way

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Star formation in the Rosette nebula

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Star formation in the Eagle nebula Eagle nebula from ground Eagle nebula from space (Hubble)

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Dust columns in the Eagle nebula Stars are forming in giant dust clouds in the centres of nebulae

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Dark dust clouds Molecular clouds are often associated with dark dust clouds and are where star formation starts. This example is the dark cloud Barnard 86

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Protostars in Orion

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw The Vela SNR The Vela supernova remnant, 10,000 years after the explosion

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw The Pleiades - a young star cluster

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 Prof. John Hearnshaw Above Crab nebula 1054 AD Below Vela nebula ~ 8000 BC Supernova remnants

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw 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

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Tycho’s supernova of 1572 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

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 Prof. John Hearnshaw Supernova SN1987A in the LMC, the only naked eye supernova in recent times SN1987A

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Heavy element enrichment of the ISM Supernovae eject gas rich in heavy elements back 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 10 5 years before the expansion dissipates.

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Supernovae 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)

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Heavy element enrichment of ISM by supernovae Number of SN/century ~ 3 SN Age of Galaxy 1.5 × yr Number of SN over lifetime of Galaxy 4.5 × 10 8 SN Total mass of Galaxy (stars and ISM) 2 × M ⊙ Mean mass fraction of this mass in heavy elements Z ≈ 0.01 Mass of heavy elements in Galaxy 2 × 10 9 M ⊙ Mass of heavy elements produced 2 × 10 9 M ⊙ / 4.5 × 10 8 per SN ≈ 4.5 M ⊙ Average mass ejected per SN ≈ 5 M ⊙

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw 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 ~ to Z ⊙ Disk Population I stars have Z > 0.1 Z ⊙ (NB: Z ⊙ ~ 0.03) Youngest Pop n I stars have Z up to ~ 2 × Z ⊙

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw 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 × 10 8 yr, but only slowly thereafter.

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Collapse of the Galaxy; formation of the halo and disk 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 × 10 8 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

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw Collapse of the Galaxy (continued) 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.

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw

ASTR112 The Galaxy Lecture 12 Prof. John Hearnshaw End of lecture 12 and of the Galaxy lectures!