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3 Stars are born, grow up, mature, and die. A star’s mass determines its lifepath. Let M S = mass of the Sun = ONE SOLAR MASS Stellar Evolution Low Mass ( INITIAL MASS < 8M S ) High Mass ( INITIAL MASS > 8 M S )

JP ©4 Stellar Evolution INITIAL MASS < 8M S INITIAL MASS > 8 M S 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 H 2 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 10 7 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

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JP ©11 THE POSITION ASSUMED ON THE MAIN SEQUENCE DEPENDS UPON THE STARS MASS 60M S 10M S 3M S MSMS 0.1M S

JP ©12 LARGER STARS HAVE SHORTER LIVES 60M S 1 million years 10M S 10 million years MSMS 10 billion years 1 billion years 3M S 0.1M S 1 million million years

JP ©13 END OF MAIN SEQUENCE INERT He 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 M S

JP ©15 HELIUM FUSION OCCURS AT K 4 He 12 CARBON 3 4 He ► 12 C + 2 γ The “TRIPLE ALPHA” process

JP © K 6000 K K30000 K SURFACE TEMPERATURE ABSOLUTE MAGNITUDE M RED GIANT After 1 G year

JP ©17 RED GIANTS THE STAR IS UP TO 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 INERT C & O CORE Helium Burning Shell Cool Extended Hydrogen Envelope Hydrogen Burning Shell

JP ©19 RED GIANT

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 M S ARE UNKNOWN THE CHANDRASEKHAR LIMIT MASSIVE STARS SUFFER A 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 M S 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 Mass typically 1.4 M S Diameter 10 km g = N kg -1 Density kg m -3 Magnetic Field a million times Earth’s ? 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 AXIS MAGNETIC 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 SPACE WARP TIME BLACK HOLES REALLY ARE HOLES IN SPACE HOLES 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 R Sch 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 If the radius of the object is smaller than 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