ASTR 1040 October 17 Homework 3 due Thursday

ASTR 1040 – October 17 Homework 3 due Thursday. Homework 4 due next Thursday (26 th) Second mid-term exam next Thursday November 2 Next Observatory Opportunity Tonight at 8: 00 Website http: //casa. colorado. edu/~wcash/APS 1040. html

Mass Transfer Binary Accretion Disk Material Swirls In Friction allows the material to fall and heats while it falls. All the way to the surface

Energy Released Huge amounts of energy are released as the material swirls in. Material get hot. Really hot. Like a million degrees Kelvin. Emits ultraviolet and x-rays. We can see these accretion disks with x-ray telescopes!

Material Reaches Surface Layer of H build up on surface Carbon White Dwarf Pressure builds on the hydrogen. Material pouring in heats it.

Nova One day the hydrogen ignites in huge nuclear rush. Burns like a brush fire from one end of the star to the other. This is called a “Nova”. A new star appears in the sky. Often visible to the unaided eye. Lasts a few weeks to months.

Novae Hydrogen explodes into space to create a shell of expanding gas. Gas expands outward at 500 km/s The Sun can never go nova! It’s not a white dwarf in a close binary.

3 Kinds of Novae Classical Novae Recurrent Novae Dwarf Novae Only seen once Seen several times over last few hundred years Pop off every few weeks to months It’s just a matter of how fast material is transferring and how much needs to accumulate before the spark.

Supernovae Nature’s Biggest Explosion • • • 10, 000 BC 185 AD 396 1006 1054 1572 1604 1667 1987 m -3 -10 -6 -4. 1 -2. 2 >5 4. 0 1300 pc 1800 5000 7000 3400 55, 000 Crab Tycho’s Kepler’s Cas-A SN 1987 A We now see a dozen or so every year in distant galaxies. II I I II II

Supernovae Occur about once every hundred years per galaxy. Briefly outshines the other 100 Billion stars in the galaxy.

Type I Supernovae White dwarf is gaining mass. Over time, the mass will approach the Chandrasekhar Limit Remember, at 1. 4 M , electron degeneracy fails. What happens?

White Dwarf Collapse As WD starts to collapse, the material falls through the gravitational field of the star. It heats very rapidly. In just a few seconds it reaches >100, 000 K. Carbon and Oxygen ignite and burn by fusion to even heavier elements. The whole star explodes in a frenzy of nuclear burning. Blows completely apart. All that remains is an expanding shell of gas that used to be a white dwarf and the companion star slingshot into space.

Explosion Starts at Center where pressure is highest

Energy Released Nuclear Energy Generates 2 Me. V per atom in forming molecule (burning) 2 Me. V = 3 x 10 -13 Joules Number of Atoms in Star: Available Energy About 1044 J release in just a few seconds. That’s as much energy as the Sun emits during its entire lifetime. In a few seconds!!!! This is so titanic we can see it across the universe A billion trillion atomic bombs Gas returning to interstellar space has more CNO etc.

SN 1987 A – Before and After

The Crab Nebula Supernova Dominated Sky in 1054 AD Observed by Chinese (not in Europe) Recovered in 18 th Century by Messier Called a “Supernova Remnant” 1 pc in diameter Expanding Rapidly

Tycho’s Supernova Seen in X-ray Gas at 10, 000 K Expanding at 5000 km/s

Type II Supernovae High Mass Star --- M > 5 M In low mass star, envelope is blown off into space, creating planetary nebula, before Carbon in core can flash. High mass star has enough gravity to hold onto the gas. Get a Carbon flash just like the Helium Flash Carbon burns to Neon Then Neon flash Gets very complicated

Onion Skin Model

Nuclear Reactions 12 C + 12 C 20 Ne +4 He + g 16 O + 4 He oxygen shell + 16 O 28 Si + 4 He silicon shell + 28 Si 56 Fe iron core 20 Ne 16 O neon shell 28 Si Iron cannot nuclear burn at any temperature (On border between fusion and fission) Develops degenerate iron core than cannot flash Just gets hotter and heavier down in the middle of the star

Collapse When the degenerate iron core exceeds the Chandrasekhar limit, electron degeneracy can no longer support it. It will start to collapse. Electrons do not have individual quantum states left. They hide by merging with protons to form neutrons: P + e- n + n Every time this happens, a neutrino is also created. Neutrinos are free to escape to infinity and carry energy with them.

Reversal of the Nuclear Reactions Every iron nucleus in the core was formed in nuclear burning. There is one electron for each proton. After electrons are absorbed, the nucleus consists of 56 neutrons. That’s unstable and the nucleus dissolves into free electrons. Millions of years of fierce nuclear burning is reversed in a few seconds! The star keeps shrinking. By the time it has shrunk from 6000 to 600 km, this process is complete. So it’s a ball of neutrons. Still nothing to stop its collapse. Keeps shrinking. Finally, when radius is about 7 km, it stops. Has at least 1. 4 M , but is a speck the size of Boulder

Neutron Degeneracy Neutrons, like electrons, must have individual quantum states. What stops the descent is “neutron degeneracy” Conceptually identical to electron degeneracy. Because a neutron is 1838 times more massive than an electron, the radius of the degenerate star is 1838 times smaller. This is called a Neutron Star. It is roughly 14 km in diameter and has 1. 4 times the mass of the Sun. They are formed in the middle of Type II Supernovae.

Energy of Collapse As neutron ball collapses it releases gravitational energy. Sun will only emit 1044 J in its entire life. This is about a thousand times greater than the energy released in at Type I supernova.

Where did the energy go? • Neutron Stars were found in supernova remnants in the 1960’s. • Type I and Type II Supernovae have comparable brightness. • Type II’s are NOT 1000 x brighter. • Where did 99. 9% of the energy go? ?

Neutrinos Ball of neutrons radiates thermal neutrinos the same way that a ball of electrons will radiate photons. These elusive particles carry away 99. 9% of the energy. So poorly coupled to regular matter that they travel unimpeded through the Universe at close to the speed of light. In fact, at this moment, each and every one of us has about 10 neutrinos per second passing through our bodies. They were generated in distant supernova in galaxies far, far away – long, long ago.

Neutrinos So what if we view this as a personal violation? Let’s go to the Department of Homeland Security and ask them to put up a shield that will protect us from these nasty neutrinos. We’ll make it out of one of the best materials for stopping them – lead. How thick will the shield have to be? Answer: About a parsec!! Looks like neutrinos have a mass about a million times lower than electrons.

Core Bounce When star reaches neutrons star size it is collapsing so fast that it overcompresses. It reverses direction and grows some. This outward shock wave couples into the rest of the star and drives the stellar envelope into space. That’s the 1044 J we see.

Explosive Nucleosynthesis Expanding shell starts out very hot. Nuclear reactions are taking place rapidly. As it expands it cools rapidly. Some very heavy elements can’t survive long at high temperature. A few get frozen in during cooling. That’s where all the elements heavier than iron come from. That’s why heavy elements are expensive. They’re made in supernovae! And they’re trace resultants at that.

SN 1987 A • First naked eye supernova since 1604. • Discovered by Ian Shelton. Feb 23, 1987 UT 23. 316 • Showed hydrogen escaping at 30, 000 km/s • In Large Magellanic Cloud • Tiny galaxy orbiting the Milky Way. • Huge international response of the astronomy community.

Progenitor Star that exploded was tracked down. Not terribly prominent. SK-69 202 B 3 Supergiant m=12. 4 M=-7. 8 T=16, 000 K, R= 40 R Distance 55, 000 pc – actually outside Milky Way.

Stellar History • Burned • • • POW!!!! H He C Ne O Si 10, 000 years 1, 000 300 5 months 6 2 days

Rings HST image of SN 1987 A a few years after the event. Center is dim 3 Bright rings Illuminated by the flash Hourglass shape again Means there was mass loss prior to the explosion

Impending Collision Blast wave is about to hit the ring.

Big Rings 30 Light Years With this geometry there is only a few light months delay.

Neutrino Astronomy Ground Level Fill mineshaft with water Put photo detectors around the inside

Neutrino Going Through Water n n n e+ e (scattering) n light produced by secondary interactions

Super Kamiokande

Detection of SN 1987 A • Kamiokande II • IMB Japan Ohio 12 events 15 s 8 events 5. 6 s Mont Blanc neutrino detector saw a marginal signal 4. 7 hours earlier. Real? ? ? In 1987 A we detected the formation of a new neutron star!

Pulsars • • • Neutrons Stars considered unobservable Forgot the effect of magnetic fields When a magnetic spins it creates electric fields Electric fields create accelerated electrons Accelerated electrons create strong radio signals Pulsar is a spinning, magnetized neutron star

Intense Magnetic Field not necessarily aligned with spin axis Particles get thrown out along the polar axes (cannot cross field lines) Beam radio signal along magnetic axis too.

From Above Every time beam sweeps by we see a pulse

Lighthouse Analogy Some of the Cosmic Rays that reach Earth could have been created in Pulsars

Pulse Trains Listen to the Pulsars: PSR B 0329+54 1. 4 Hz Vela Pulsar 11 Hz

Center of the Crab Nebula Chandra (X-rays) and HST (Visible)

Pulsar Power • • Energy Source is the Rotation As Pulsar emits, rotation slows As pulsar slows, it becomes less luminous Rapidly fade out Period Glitches or Starquakes Time

The Binary Pulsar Pulses get closer on approaching side of orbit (Doppler) Can map out orbit Two neutron stars spiraling toward each other. Will merge into black hole in about 200, 000 years Friction that causes the inward spiral caused by Gravity Waves Period First confirmation of Einstein’s prediction Time

An X-ray Pulsar Hercules X-1 Discovered in 1971 Pulses every 1. 24 seconds in the x-ray Eclipses every 1. 7 days due to binary orbit Turns off every 35 days due to accretion disk wobble The Rosetta Stone of X-ray Astronomy Proved the bright x-ray sources were mass transfer binaries with a neutron star. There about 100 of these “Classical X-ray Binaries” in the Milky Way

X-ray Bursters Intensity Time Every few hours the x-ray source will “burst” for 30 seconds Explanation: Helium accumulates on surface of the neutron star in binary Periodically it explosively burns Nuclear release insufficient to blow it out into space Collapses back and cools A Neutron Star Nova

Black Holes • Are there any Black Holes? YES. Just 20 years ago, the answer was “probably”. The term didn’t exist until 1969

• What Happens when a neutron star exceeds its Chandrasekhar Limit? • Must Collapse • No known force powerful enough to halt the collapse • Result: Black Hole

Things that don’t happen • Collapse to Singularity • Winks out of universe • Explodes by some new force • Einstein’s prediction wasn’t a “sure thing”

Formation Neutron Star Collapses Now it’s inside event horizon and can never be seen again.

A Newtonian Black Hole Energy Falling To Surface Kinetic Energy OR if v=c

Schwarzschild Radius Two errors cancel. This is the radius of the “Event Horizon” The event horizon is a true singularity in space-time. It is a place where time and space cease to exist.

Geometry of Black Hole Can you boost the signal? No, that doesn’t help. Space Curves in on itself There’s no path out!

Curved Space

Rubber Sheet Analogy

Properties - Size The radius of a black hole is 3 km per solar mass There is no limit on size or mass. Note: Volume rises as the cube of the mass. Implies the larger the black hole gets, the lower is its density. A sphere of water the size of Saturn’s orbit would be a black hole!

Properties – Escape Velocity No way. Not even light can escape. No signal can escape No particle Nothing

Properties – Orbital Period Material orbiting a black hole will have milli-second periods

Properties - Basic 1. Mass (Schwarzschild Black Hole) 2. Electric Charge (doesn’t happen) 3. Angular Momentum (Kerr Black Hole) No “Surface” Features No Magnetic Fields No Pulsing “No Hair”

Jump on In You won’t make it, but it would be quite a ride!

Properties – Energy Emitted • Energy Released from accretion • about 0. 1 mc 2

Time Dilation Now it gets weird! Time does not run at the same rate everywhere in the universe. Twins are not always the same age. Clocks run a little bit faster in Colorado than in Washington DC. Folks in DC are actually a little slower than us. About one part in a trillion. That’s a millisecond over a lifetime.

Time Dilation Some event takes dt∞ out in free space. Same event takes place at radius r from center of a black hole. Now view it from free space. Takes dt instead. Longer. It looks like things are moving slower. If you are near the black hole, the rest of the universe appears to be moving faster.

Time Dilation As r approaches Rs dt gets longer and longer. When r reaches the event horizon, time stops. We know how to make a time machine with a forward switch only! Just fly to a black hole and orbit above the surface. But you can fall in really fast as viewed from outside.

A “frozen star” If time stops above the surface, it can’t go in. How does the hole form? Is the material still stuck in temporal limbo just above the surface? What’s inside? If nothing can get out, then how can gravity?

Hawking Radiation The vacuum makes pairs of electrons and positrons that pop into existence and then annihilate without any net effect. Above a black hole, one can get sucked in. The other annihilates above the surface to cause radiation. Since its close to the surface, the light gets redshifted escaping, but it carries energy with it!

Temperature Hawking derived that the temperature of a black hole is thermal. Energy for the radiation comes from the mass of the hole. The black hole shrinks with time. That’s a really long time unless the black hole is tiny. A 1 kg black hole would last only 10 -16 s.

Artist’s impression of Cyg X-1 (NASA) 6 M 0 BH orbiting an O 9 star. 2000 pc away in Cygnus