Gas and Dust

ISM: properties, distribution

Reddening, extinction, scattering of blue light

How do we observe ISM

Absorption lines

21 cm emission

Emission nebulae

Molecular emission: vibrational, rotational

Absorption by IS Clouds

Fig. 18.14

Reddening

Dust scatters blue light more than red

This causes the observed spectrum to be much redder

Fig. 18.2

Neutral Hydrogen (21 cm line)

Image courtesy NRAO

Observing Atomic Hydrogen

21 cm radiation

Neutral Hydrogen

Alignment of electron spin relative to proton spin

Spin is quantized

Atomic collisions cause higher energy state to be populated

Observing Atomic Hydrogen

21 cm radiation

Line is red and blueshifted due to motion of gas

21cm radiation is not absorbed by ISM

Stellar Evolution

H-R Diagrams

Protostellar evolution

Star formation: High and low mass stars

Nucleosynthesis

Star deaths:

Formation of white dwarfs

Type I and II Supernovae

Neutron stars and black holes

Brief Summary of Stages of Stellar Evolution

Stage 1: interstellar cloud

Stage 2: Collapsing Fragment

Stage 3: Fragmentation Ceases

Stage 4: Protostar

Stage 5: Protostellar evolution

Stage 6: Newborn star

Stage 7: Main sequence star

Stability in Stars

Pressure = Force/area

Outward pressure (thermal and radiation)

Inward force is gravity

Fig. 20.1

Evolution of the Sun

Sun evolves from main sequence

Describe the various stages of the Sun's evolution

Fig. 20.12

High Mass Star Evolution

Higher mass stars fuse more massive atoms

Initial mass is most important property in determining the fate of a star

Properties of Stellar Evolution of the Sun

Evolution of stars on Main Sequence

Fig. 20.17

Open and Globular Clusters

Name the type of stars represented in each HR diagram

Which cluster is younger and why? Fig. 19.18 and 19.19

Type I Supernova

Material gradually accumulates on White Dwarf, as nuclear explosions do not eject all the new material from surface

Star reaches the Chandrasekhar limit (1.4 M_M), overcomes electron degenerate pressure and starts collapsing

Carbon starts fusing everywhere, thermonuclear detonation

Fig. 21.9a

Type II Supernovae

Type II Sne are massive stars that undergo core collapse

Fig. 21.9b

Fusion in Stars

End product depends on mass

Fig. 20.3 (Sun) and 21.5 (high mass star)

Nucleosynthesis

P-P chain

CNO cycle

Helium capture, alpha process

Conrast with deuterium formation in early Universe

P-P Chain vs. Deuterium Formation

P-P chain, Fig. 16.27 and Deuterium formation, 27.5

P-P chain: Energy results from He-4 having less mass than 4 p

Deuterium formation can only occur in Early Universe

CNO cycle

6 steps in CNO cycle

¹²C + ¹ H --> ¹³N + energy

¹³N --> ¹³C + positron + neutrino

¹³C + ¹ H --> ¹⁴N + energy

¹⁴C + ¹ H --> ¹⁵O + energy

¹⁵O --> ¹⁵N + positron + neutrino

¹⁵N + ¹ H --> ¹²C + ⁴He

Sum of the above: ¹²C + 4(¹ H) --> ¹²C + ⁴He

CNO cycle only dominates at high core temps

Fig. 16.27

Helium Capture

¹²C + ⁴ He

¹⁶O + ¹⁶ O

¹⁶O + ⁴ He

Compare (2) and (3)

Which fusion reaction is more likely

Alpha Process

High energy photons break apart heavy nuclei into He nuclei (at high temps)

He nucleus is also called an alpha particle

Then He capture occurs

Fig. 21-17

Elemental Abundance

Most H and He is primordial-made in the big bang

Fig. 21.13

Why Iron Core?

Iron-56 is one of the most stable elements

Fig. 21.6

Heaviest Nuclei

s-process (s-slow ~1 year)

Nuclei capture free neutrons and become a higher atomic number isotope

Eventually, isotope gets too many neutrons and becomes unstable and decays to another element with same atomic number

Zi, Pb, Cu, Ag

r-process (r-rapid), elements heavier than Fe

Occurs during SN explosion

So many free neutrons during explosion that neutron capture is easy and very fast, before decay can happen

Pulsars

Lighthouse model of emission

All pulsars are neutron stars

Not all neutron stars are pulsars

Depends on age and viewing angle

Fig. 22.3

Pulsars

Pulsed emission vs. time

Pulse intensities can be different

Regular intervals between pulses

First observed by Jocelyn Bell, 1967

Most pulsars have pulsed radio emission, some have radio through gamma ray

Fig 22.2

X-ray Bursters

Neutron star in binary pair, emission in X-rays from hot accretion disk

Analogous to novae and white dwarfs

Fig. 22.8

Possible Models of Gamma Ray Bursts

Inverse Squared Law places huge constraints on total energy

Emission is probably beamed in jets

Beaming reduces total energy needed

Fig. 22.13

Evidence for Black Holes

A black hole has never been directly observed

However one should see the accretion disk

Matter is very hot, emit X-rays

Black holes in binaries

Supermassive black holes in center of galaxies

Fig. 22.21

Black Hole Formation

If neutron core exceeds ~3 solar masses (M_M), core continues to collapse = black hole

Exact mass depends on rotation and magnetism

Black holes are defined by three properties

Mass, charge, angular momentum

Event horizon -the imaginary "surface" of a black hole

Event horizon occurs at the radius where the escape speed exceeds the speed of light

Schwarzschild radius

Gravitational Redshift

Photon gives up energy as it leaves gravitational "well"-photon's speed cannot change, but wavelength does

E=hc/l = hf

Fig. 22.18 and 22.17

Summary: Star Formation

How do heat, rotation, and magnetism compete with gravity in cloud collapse

Phases of star formation: sequence leading to our Sun's formation

Mass, time, shocks

Evolutionary track of protostar on HR diagram

Observational evidence for star formation theory: cloud fragments, T Tauri protostars, HH objects, brown dwarfs

Zero age main sequence

Star clusters (open, associations, and globular): formation of, ages, locations, and types of stars

Summary of Stellar Evolution

Evolution of Sun-like stars off main sequence

Sequence of fusion in stars, dependence on mass, change in composition

Properties of white dwarfs

Helium Flash

Contrast evolution of high and low mass stars

Binary modification of stars through mass transfer

Summary of Stellar Explosions

What is a nova

What is a Supernova

Core collapse

Core detonation

Difference between Type I and Type II SN

Stellar nucleosynthesis

Helium capture

Neutron capture

Stellar recycling

Summary of Neutron Stars and Black Holes

Formation of Neutron Stars and Black Holes

Properties of neutrons stars

Neutron degeneracy, pulsed emission, magnetic field, spin-up, spin down

Gamma ray bursts - models, observed emission

Properties of black holes

Schwarzschild radius, event horizon, mass, tidal stretching

Relativistic effects- gravitational radiation and redshift

Observable effects of relativity