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Evolution of High-Mass Stars: Red Supergiants

•  Very massive stars burn up their H fuel quickly.

•  They also expand dramatically.

– 1000 times larger than the Sun!

Now called red supergiants.

Betelgeuse, a red supergiant star

Evolution of High-Mass Stars

A massive star is mostly unfused Hydrogen & Helium In its core, Helium is fusing into Carbon & Oxygen Around the core a shell of Hydrogen can fuse to Helium.

Death of Massive Stars •  Massive stars can fuse

Helium into Carbon and Oxygen after their Hydrogen fuel has run out.

•  They can also create heavier and heavier elements: Neon, Magnesium, … and even Iron.

•  However this process stops with Iron. –  Fusing Iron will not produce

additional energy.

Close-up of Core

Fe Si O C He H

Core-Collapse Supernovae

•  Once fusion stops, the core begins to collapse quickly

• Outer core layers rebound off the iron center

• Rebound causes an enormous explosion, called a supernova

The Crab Supernova Remnant

•  Today in that same part of the sky we find the Crab Nebula

•  It has a neutron star inside.

•  In 1054 AD observers in China, Japan, and Korea recorded a “Guest Star”

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What’s left after a supernova?

•  Depends on the mass of the original star! •  A star massive enough to supernova usually

is too massive to leave behind a white dwarf. •  8-20 MSun: Core collapses to a neutron star

– Spinning & Magnetic field? -> Pulsar •  More than 20 MSun: Black Hole!

Low-mass Stars: 0.4 MSun or less

•  Only fuse Hydrogen •  Remain a star (no planetary nebula) •  Stay on Main Sequence

Medium-mass Stars: 0.4 MSun - 8 MSun

•  Fuse H & He, some Carbon •  Become cool red giants •  Expel outer layers -> planetary nebulae

& White Dwarf

High-mass Stars: more than 8 MSun

•  Fuse heavy elements up to Iron •  Become supergiants •  End as Supernova & Neutron Star or BH

Different types of supernovae:

•  White Dwarf: white dwarfs in binaries gaining mass & exploding – Type Ia, no H lines –  Important for measuring large distances

•  Core-collapse: deaths of massive stars – Type II, Hydrogen lines present

Fig. 10-12, p. 212

Mass Transfer

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Observing Supernovae •  We observe supernovae in other galaxies. •  A few supernovae per century, per galaxy

Stellar Evolution Lecture-Tutorial: Page 133-134

•  Work with a partner or two •  Read directions and answer all questions

carefully. Take time to understand it now! •  Come to a consensus answer you all agree

on before moving on to the next question. •  If you get stuck, ask another group for help. •  If you get really stuck, raise your hand and I

will come around.

Chapter 14: Neutron Stars and Black Holes

Neutron Stars •  Form from a 8-20 MSun star

•  Leftover 1.4-3 MSun core after supernova

•  Neutron Stars consist entirely of neutrons (no protons) -> held up by neutron degeneracy pressure (similar to electron degeneracy pressure)

Neutron Star (tennis ball) and Washington D.C.

Pulsars: Stellar Beacons •  Pulsars are spinning neutron

stars

•  Their strong magnetic fields emit a beam radio waves along the magnetic poles

•  These are not aligned with the axis of rotation.

•  The beam of radio waves sweeps through the sky as the neutron star spins.

Model of a Pulsar(a rotating Neutron Star)

The Lighthouse Model of Pulsars

If the beam shines on Earth, then we see a pulse of energy (radio

waves)

Neutron star’s magnetic field

A pulsar’s beam is like a lighthouse

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The Crab Pulsar

The Crab supernova remnant contains a pulsar!

A massive star dies in a Supernova explosion.

Most of the star is blasted into space.

The core that remains can be a neutron star. However…

Neutron stars can not exist with masses M > 3 Msun

If the core has more than 3 solar masses…

It will collapse completely to single point –

=> A black hole!

Black Holes: Overview

• A total victory for gravity.

• Collapsed down to a single point.

• This would mean that they have infinite density (but only at one point)

• Their gravity is so strong, not even light can escape… from inside a certain distance.

Escape Velocity Escape Velocity (vesc) is the speed required to escape gravity’s pull.

On Earth vesc ≈ 11.6 km/s.

vesc

If you launch a spaceship at v= 11.6 km/s or faster, it

will escape the Earth

But vesc depends on the mass of the planet or star…

Why Are Black Holes Black? On planets with more gravity than Earth, Vesc would be larger.

On a small body like an asteroid, Vesc would be so small you could jump into space.

A Black Hole is so massive that Vesc = the speed of light. Not even light can escape it, so it gives off no light!

Black Holes & Relativity •  Einstein’s theory of General Relativity says space

is curved by mass •  So a star like the Sun should bend space, and

light traveling past it will get thrown off course •  This was confirmed during a solar eclipse in 1919

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Event Horizon

We have no way of finding out what’s happening inside!

Nothing can get out once it’s inside the

event horizon

The Schwarzschild Radius If Vescape > c, then nothing can leave the star, not light,

not matter.

Rs = 2GM ____ c2

Rs = Schwarzschild radius

If something is compressed smaller than Rs it will turn into a black hole!

G = gravitational constant M = mass

c = speed of light

We can calculate the radius of such a star:

Vesc = c

Black Holes: Don’t Jump Into One! If you fall into a Black Hole, you

will have a big problem:

Your feet will be pulled with more gravity than your head.

You would experience “tidal forces” pushing & pulling

Time is also distorted near a black hole

How do we know they’re real?

•  Black holes: – Kepler’s Laws, Newton’s Laws – Accretion disks – Gravitational Waves

•  Pulsars: – Observe radio jets – Strong magnetic fields

Evidence for Black Holes No light can escape a black hole, so

black holes can not be observed directly.

However, if a black hole is part of a binary star system, we can measure its mass.

If its mass > 3 Msun then it’s most likely a black

hole!

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Evidence for Black Holes: X-rays

Artists’ drawings of accretion disks

Matter falling into a black hole may form an accretion disk. As more matter falls on the disk, it heats up and emits X-rays. If X-rays are emitted outside the event horizon we can see them.

Evidence for Black Holes •  Cygnus X-1 is a source of X rays •  It is a binary star system, with an O type supergiant & a

compact object

Cygnus X-1: A black hole

The mass of the compact object is more than 20 Msun This is too massive to be a white dwarf or neutron star. This object must be a black hole, located about 6,000 light years away.

Supermassive Black Holes (SMBHs)

•  Stellar black holes come from the collapse of a single star and have masses of several Msun

•  If a black hole gains mass, its event horizon will expand

•  Most galaxies (including the Milky Way) have a central SMBH

A supermassive black hole devours a star, releasing X-rays