Lectures #18 & 19: Plan •  Stellar Evolution:

– Low-mass stars – High-mass stars

Stellar Evolution

•  The most important factor determining a star’s fate is the mass

•  Stellar evolution is governed by competition between inward gravitational force and outward pressure (the need to maintain hydrostatic equilibrium)

Stellar Evolution: Early Stages •  Collapse of proto-stellar cloud →  Heating from contraction →  Accretion disk →  Jets

•  Ignite hydrogen fusion (�burning�) into helium in the core →  Enter main-sequence phase

Dense regions in molecular clouds

The birthplace of stars!

“Stellar Nursery” The Eagle Nebula

Protostars

Bipolar Flows

Post-Main Sequence Evolution: M < 8 Msun 1.  H core depleted → core contracts → envelope

expands → star leaves main sequence

Lifetime on main sequence: ~ 1010 / M2 years (reminder)

Post-Main Sequence Evolution: M < 8 Msun 2.  H shell burning + inert He core → red giant

phase (R ~ 30 Rsun)

Transition to Red Giants

Post-Main Sequence Evolution: M < 8 Msun 3.  Desperate attempt to maintain hydrostatic equilibrium:

Ignite He in core (�helium flash�) → contracts back to yellow giant phase (R ~ 10 Rsun)

He burning: “triple alpha reaction”

H burning: “p – p reaction”

A (temporary) new lease on life

He core burning

Post-Main Sequence Evolution: M < 8 Msun 4.  He core depleted → inert C core + H, He

burning shells → envelope expands → red supergiant phase (R ~ 300 Rsun ~ Mars’ orbit!)

5.  Shed outer envelope → planetary nebula 6.  Core remnant = white dwarf

(no more energy source – simply cooling)

white dwarf

The Hourglass Nebula

White Dwarfs •  M ~ Msun •  R ~ REarth •  Magnetic field ~ 108 x that of Earth •  M ≤ Chandrasekhar mass limit = 1.4 Msun

→ Density ~ 106 kg / liter = 106 g/cm3 !

B

A

Sirius

white dwarf

Evolution of M < 8 Msun stars

Evolution of M ~ Msun Stars

The Life-path of the Sun

What happens in a star of higher mass?

Reminder: Hydrostatic Equilibrium Outward pressure = Inward gravitational force

→ Pressure increases towards center of Sun

Reminder: Gas Pressure

Pressure = Constant × Temperature × Density

High-Mass Stars (M > 8 Msun) •  So high-mass stars must have higher core

temperatures and densities than low-mass stars •  Nuclear reactions other than hydrogen burning

(4 p ! He @ T > 107 K) and helium burning (3 He ! C @ T > 108 K) become possible: •  C + He ! O (oxygen) • O + He ! Ne (neon) •  Ne + He ! Mg (magnesium) •  And then silicon, sulfur and even up to iron!

Post-Main Sequence Evolution: M > 8 Msun

Nuclear reactions in massive stars

Post-Main Sequence Evolution: M > 8 Msun

Successive stages of shell and core burning produce ever heavier elements until it reaches iron …

Post-Main Sequence Evolution: M > 8 Msun

- It actually costs energy to build elements heavier than iron by fusion.

- Disastrous consequences: inward gravitational force > outward pressure…

Post-Main Sequence Evolution: M > 8 Msun

→  Catastropic collapse! →  Electron + proton → neutron (in core) →  Core bounce →  Kaboum! Supernova explosion! (accompanied by other nuclear reactions

that create atoms heavier than iron)

Before and After – a Supernova (SN 1987a in the Large Magellanic Cloud)

Supernova Remnant: SN 1987A

Mm-wave + visible + X-rays

Supernova Remnant

Supernova Remnant: Tycho Brahe (1572)

X-rays

Supernova Remnant: The Crab Nebula (1054)

Visible

Evolution of M > 8 Msun stars

The Life of a Massive Star

Low vs High-Mass Stellar Evolution

Stellar Remnants after Supernova

i.   If final core mass < 3 Msun → Neutron star

ii.   If final core mass > 3 Msun

→ Black hole iii.   No remnant!

Neutron Star

•  Giant ball of neutrons! •  M ~ 1.4 – 3 Msun •  R ~ 10 km

•  Very strong magnetic field –  1012 x that of Earth

•  Fast rotator

→ Density ~ 4 x 1014 g / cm3 ~ humanity / cm3 !

→ Pulsar

Pulsar: the Explanation

The Crab Nebula Pulsar

Pulsar

http://www.jb.man.ac.uk/~pulsar/Education/Sounds/

Black Hole

•  Gravity’s Ultimate Triumph! •  Vesc

2 = 2 G M / R

If Vesc = c (speed of light) RS = 2 G M / c2 = Schwarzschild radius = size of �event horizon� •  If M = 1 Msun →  RS ~ 3 km

Mass warps space!

Black Holes

Light Bending

May 29, 1919: Solar eclipse proved theory of general relativity (Einstein)

Sun

Light Bending

Light bending near a black hole

Light near a Black Hole Sitting back-to-back but seeing eye-to-eye

Photon Sphere (photon in orbit)

Event Horizon (photon unable to escape)

Rs

1.5 Rs

Photon Sphere

Event Horizon

Photons are orbiting the black hole at R = 1.5 Rs!

Strong Tides near Black Holes → �Spaghettification�!

How do we find black holes?

Here!

How do we find black holes?

1.  Motion of a visible companion star in orbit around the black hole

2.  Strong X-ray source due to mass accretion

Light curve from a black hole X-ray binary system

How do we find black holes? 3.  Gravitational waves!!!

Black holes merging and gravity waves

https://www.nytimes.com/video/science/100000004200661/what-are-gravitational-waves-ligo-black-holes.html

Black Hole •  LIGO: Laser Interferometer Gravitational-Wave Observatory -  It is actually two observatories: one in Louisiana, another in

the state of Washington (~10 milli-second apart at v = c !)

4-km baselines

Black Hole •  LIGO: Laser Interferometer Gravitational-Wave Observatory -  Needs to measure changes in space-time of <1 part in 1022 !!!

Over 4-km baselines, accuracy needed is less than 4 km x 10-22 = 4 x 10-19 m ~ 0.0005 proton radius!

-  Uses two laser beams at 90 degrees of each other to measure small displacements of test masses hung by pendulums

Black Hole

•  Results from LIGO: -  GW150914 event:

merger of a pair of black holes of 36 + 29 Msun ! 62 Msun -  3.0 Msunc2 is radiated in gravitational waves -  First detection of: gravitational waves, black hole binary,

black hole with mass ~25 Msun and above

-  Fast timeline: "  2015 September 14: LIGO detection "  2016 February 11: LIGO research announcement "  2017 October 3: Nobel Prize in Physics awarded

to Kip Thorne, Rainer Weiss, and Barry Barish

Black Hole

•  In principle, there is no upper limit to a black hole’s mass

–  MBH = 106 – 109 Msun in the centers of many galaxies ! –  We will discuss them later…

Neutron Star Merger!

(Troja+17)

•  Newest results from LIGO: -  GW170817 event:

A pair of neutron stars of ~1.3 Msun each merged into a ~ 2-3 Msun neutron star or black hole -  First detection of electromagnetic radiation from a

gravitational wave event -  Usher in a new era of “multi-messenger astronomy”!

Activity #9 What is the escape velocity at the event horizon of a black hole?