JP © 1

JP © 2

JP © 3

• Stars are born, grow up, mature, and die.

• A star’s mass determines its lifepath.

Let MS = mass of the Sun = ONE SOLAR MASS

Stellar Evolution

• Low Mass ( INITIAL MASS < 8MS )

• High Mass ( INITIAL MASS > 8 MS )

JP © 4

Stellar Evolution

INITIAL MASS < 8MS INITIAL MASS > 8 MS

MAIN SEQUENCE

RED GIANT

PLANETARY NEBULA

WHITE DWARF

MAIN SEQUENCE

RED GIANT

SUPERNOVA

NEUTRON STAR BLACK HOLE

JP © 5

Stars are born deep in molecular clouds.

• Cold (10 – 30 K), dense

• Cold enough for H2 to exist

JP © 6

GRAVITY OVERCOMES THERMAL PRESSURE

THESE REGIONS (CORES) BECOME

MORE DENSE AND COMPACT

COLLAPSE RELEASE OF GRAVITATIONAL POTENTIAL ENERGY

THERMAL ENERGY

it radiates infrared from its surface

protostar

JP © 7

When protostar collapses, its angular momentum is conserved

– it rotates faster

– As matter falling into the protostar, it flattens into a (protostellar) disc

– a planetary system may form from this disc

Star Formation

JP © 8

The PROTOSTAR collapses until its core reaches 107 K in temperature

PROTON – PROTON CHAIN FUSION

REACTION BEGINS

“HYDROGEN BURNING HAS BEGUN”

THE STAR FINDS ITS POSITION ON THE MAIN SEQUENCE OF THE

“HERTZSPRUNG – RUSSELL DIAGRAM.”

JP © 9

BROWN DWARFS

Never get hot enough for fusion to begin

Never reach the main sequence

Brown dwarf

JP © 10

JP © 11

THE POSITION ASSUMED ON THE MAIN SEQUENCE DEPENDS UPON THE STARS MASS

10MS

MS

JP © 12

LARGER STARS HAVE SHORTER LIVES

60MS 1 million years

10MS 10 million years

MS 10 billion years

1 billion years3MS

0.1MS 1 million million years

JP © 13

END OF MAIN SEQUENCE

INERTHe

CORE

Hydrogen

Burning

Shell

Cool Extended

Hydrogen Envelope

JP © 14

LEAVING THE MAIN SEQUENCE

– Hydrogen fuel supply decreases

– Gravity gets the upper hand

– The core begins to collapse

– H shell heats up and He fusion begins there

– Energy released is greater than gravity from above

– The outer layers of the star expand and cool

– The star becomes a RED GIANT

M > 0.4 MS

JP © 15

HELIUM FUSION OCCURS AT 100 000 000 K

4He 4He4He

12CARBON

3 4 He ► 12 C + 2 γ

The “TRIPLE ALPHA” process

JP © 16

2500 K6000 K10000 K30000 K

SURFACE TEMPERATURE

AB

SO

LU

TE

MA

GN

ITU

DE

M

0

+5

+10

+15

-5

-10

REDGIANT

After 1 G year

JP © 17

RED GIANTS

THE STAR IS UP TO 10 000 TIMES BRIGHTER

HELIUM BURNING IS VERY TEMPERATURE SENSITIVE

SMALL CHANGES IN TEMPERATURE CAN LEAD TO LARGE CHANGES IN THE FUSION RATE, DESTABILISING THE OUTER LAYERS.

FLUCTUATIONS IN TEMPERATURE AND LUMINOSITY CAN OCCUR – UNTIL THE CORE HELIUM SUPPLY DWINDLES – AFTER 100 MILLION YEARS.

THE CORE IS NOW MAINLY CARBON AND OXYGEN

GRAVITAIONAL COLLAPSE OCCURS AGAIN, HEATING THE CORE SUFFICIENTLY FOR HELIUM FUSION TO BEGIN IN THE SHELL.

THE STAR FOLLOWS THE GIANT TRAIL AGAIN.

JP © 18

END OF CORE HELIUM BURNING

INERTC & OCORE

Helium

Burning

Shell

Cool Extended

Hydrogen Envelope Hydrogen

Burning

Shell

JP © 19

REDGIANT

JP © 20

HELIUM FLASHES CONTINUE.

CORRESPONDING CHANGES IN LUMINOSITY AND TEMPERATURE.

EVENTUALLY THE ENVELOPE IS BLOWN AWAY IN A

PLANETARY NEBULA.

A WHITE DWARF REMAINS – NO MORE FUSION

COOLS TO A BLACK DWARF

JP © 21

ENVELOPE EJECTION

BARE CORE

WHITE DWARF

BLACK DWARF

JP © 22

WHITE DWARFS WITH

A MASS > 1.4 MS

ARE UNKNOWN

THE CHANDRASEKHAR LIMIT

MASSIVE STARS SUFFERA DIFFERENT FATE

JP © 23

WHEN THE CORE CONTAINS ESSENTIALLY JUST IRON, IT HAS NOTHING LEFT TO FUSE

(GREATEST NUCLEAR BINDING ENERGY).

TYPICAL CORE MASS = 1.4 MS

TYPICAL SIZE = 0.5 EARTH DIAMETER

CORE FUSION CEASES. IN LESS THAN A SECOND, THE STAR BEGINS THE FINAL PHASE OF

GRAVITATIONAL COLLAPSE.

THE REPULSIVE FORCE BETWEEN THE NUCLEI OVERCOMES THE FORCE OF GRAVITY, AND THE CORE RECOILS OUT FROM THE

HEART OF THE STAR IN AN EXPLOSIVE SHOCK WAVE

THE CORE TEMPERATURE RISES TO OVER 100 BILLION DEGREES AS THE IRON ATOMS ARE CRUSHED TOGETHER.

JP © 24

The stellar remnant is a NEUTRON STAR

or a BLACK HOLE.

supernova

JP © 25

NEUTRON STARS

?

Crystalline Iron crust

Neutron superfluid

Superconducting protons

Under immense pressure electrons become degenerate

?

JP © 26

PULSARS ARE RAPIDLY ROTATING NEUTRON STARS

TYPICAL PULSE RATE = 4 Hz

PULSAR

SPIN AXISMAGNETIC AXIS IN

EARTH’S DIRECTION

Pulsars have jets of particles moving [ at near the light speed]

out above their magnetic poles.

These jets produce very powerful beams of light, which sweep around as the pulsar rotates.

They appear as a “cosmic lighthouse”.

JP © 27

WHEN THE CORE LEFT AFTER A SUPERNOVA EXCEEDS THREE SOLAR

MASSES A BLACK HOLE REMAINS

A 10 SOLAR MASS BLACK HOLE WOULD HAVE A DIAMETER OF 30 KM

BLACK HOLES WARP SPACEWARP TIME

BLACK HOLES REALLY AREHOLES IN SPACEHOLES IN TIME

JP © 28

WITHIN THE EVENT HORIZON, THE ESCAPE VELOCITY IS GREATER THAN THE VELOCITY OF LIGHT

EVENT HORIZON

NO PARTICLE CAN ESCAPE, NOT EVEN LIGHT ITSELF

JP © 29

RSch is known as the Scharwzchild Radius and marks the boundary of the Event Horizon.

The Escape Velocity on the surface of a gravitational body depends only on its mass and size. For a gravitational body

with mass M and radius R, the escape velocity on its surface is given by

R

GMvescape

22

If the radius of the object is smaller than 2

2

c

GMRSch

Light will not escape.

JP © 30

THERE IS PROBABLY A SUPER MASSIVE BLACK HOLE WITH A MASS OF A MILLIONS SUNS AT THE CENTRE OF OUR MILKY WAY

GALAXY