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The Exotic Realm of Stellar Remnants “Do not go gentle into that good night, Old age should burn and rave at close of day; Rage, rage against the dying of the light” – Dylan Thomas White Dwarfs Sirius B is the closest white dwarf to us Sirius A + B in X-rays White Dwarfs • Degenerate matter obeys different laws of physics. • The more mass the star has, the smaller the star becomes! • increased gravity makes the star denser • greater density increases degeneracy pressure to balance gravity Limit on White Dwarf Mass • Chandra formulated the laws of degenerate matter. – for this he won the Nobel Prize in Physics • He also predicted that gravity will overcome the pressure of electron degeneracy if a white dwarf has a mass > 1.4 M – energetic electrons, which cause this pressure, reach the speed of light Chandrasekhar Limit Subrahmanyan Chandrasekhar (1910-1995) Novae • Term comes from the Latin Stella Nova for ‘new star’ • In reality the star is not new, it just gets much brighter in a matter of days (Greeks didn’t have telescopes) • Totally different event than a supernova Novae • They typically increase in brightness by 5 to 10 magnitudes for a few days, then fade. • An accretion disk is a rotating disk of gas orbiting a star. – formed by matter falling onto the star. • The hydrogen build-up on the surface of the white dwarf can ignite into an explosive fusion reaction that blows off a shell of gas. Artist’s impression of a nova Novae • Because so little mass is blown off during a nova, the explosion does not disrupt the binary system. • Ignition of the infalling hydrogen can recur again with periods ranging from months to thousands of years. recurrent nova T Pyxidis viewed by Hubble Space Telescope, past eruptions in 1890, 1902, 1920, 1944, 1966, 2011 Neutron Stars Chandra X-ray image of the neutron star created during a supernova observed in A.D. 386. The remnant is known as G11.20.3. Pulsars • In 1967, graduate student Jocelyn Bell and her advisor Anthony Hewish accidentally discovered a radio source in Vulpecula. • Sharp pulse recurred every 1.3 sec. • They determined it was 300 pc away. • They called it a pulsar, but what was it? Light Curve of Jocelyn Bell’s Pulsar Prof. Jocelyn Bell 1968: mystery solved when a pulsar was discovered in the heart of the Crab Nebula supernova remnant. The Crab pulsar also pulses in visual light (and gamma-rays). Pulsars • Discovered in radio, x-rays, gamma-rays • About 2000 known, expected population much higher in our galaxy. • Periods range from milliseconds to 10 seconds – younger pulsars have faster periods – fastest periods require very small rotators – rotator must be very dense to avoid breaking up • http://www.jb.man.ac.uk/pulsar/Education/Sounds/sounds.html Pulsars and Neutron Stars All pulsars are neutron stars, but all neutron stars are not pulsars!! Crab jet movie Pulsar Timing Pulsars are among the most accurate clocks known Testing general relativity PSR B1913+16 Binary pulsar Discovered 1974 Supernova • The neutron core collapses until abruptly stopped by neutron degeneracy – this takes less than a second • Core recoils and sends a shock wave outward – shock wave hits still infalling outer layers and accelerates them outward at 1/10th speed of light – Accelerated material heated above 109 K explosive nucleosynthesis – http://www.youtube.com/watch?v=2RxIwtxdE nQ Supernova The amount of energy released is so great, that most of the elements heavier than Fe are instantly created In the last millennium, four supernovae have been observed in our part of the Milky Way Galaxy: in 1006, 1054, 1572, & 1604 Likely occur every ~40 y in our Crab Nebula in Taurus galaxy, but they are obscured by supernova exploded in 1054 dust and gas Supernova Remnants Cas A: exploded 300 y ago but no one saw it! (obscured by dust) “superluminous supernovae” When outer layers reach R ~ 100 AU, opacity drops and optical photons escape - huge initial luminosity (109 Lsun) - slow decay due to radioactivity Type II : Hydrogen absorption lines, from massive star core collapse Type Ib : He but no H absorption lines, Massive star first loses outer layers via a wind, then core collapse occurs (Type II) (Type Ia) Type Ia : No H or He absorption lines, from white dwarf core collapse Type Ia (White Dwarf) Supernova • While a nova may reach an absolute magnitude of –8 (about 100,000 Suns)… • a white dwarf supernova attains an absolute magnitude of –19 (10 billion Suns). • Can use light curves of Type Ia supernovae to get true luminosity – they are more luminous than Cepheid variable stars – so they can be used to measure out to greater distances than Cepheid variables – led to discovery of dark energy in early 2000s Feb 23, 1987 A few hours later… Supernova 1987A Apparent Magnitude • Underluminous Type II supernova • Progenitor: massive blue giant star • Still no sign of the neutron star (Days) • SN1987A emitted 1058 neutrinos – and Kamiokande II detected… 11 of them SN1993J VLBA movie Stellar Evolution Summary Approx. mass range Fusion stages End product No deuterium fusion Planet 0.01 < M/Msun < 0.08 No H fusion Brown dwarf 0.08 < M/Msun < 0.5 No He fusion He white dwarf No C or O fusion C+O white dwarf No Si fusion Ne+Mg white dwarf Elements fused up to Fe Supernova, then neutron star or black hole M/Msun < 0.01 0.5 < M/Msun< 5 5 < M/Msun< 7 M/Msun > 7 Black Holes • After a massive star supernova, if the core has a mass > 3 M, the force of gravity will be too strong for even neutron degeneracy to stop it. • The star will collapse into oblivion (gravity wins, finally) • The neutron star becomes infinitely small. – it creates a “black hole” in the Universe, from which nothing can escape, even light • Equations of general relativity break down at the infinitely dense ‘singularity’ point Black Holes • According to Einstein’s Theory of Relativity, gravity is really the warping of spacetime about an object with mass. • This means that even light is affected by gravity. Warping of Space by Gravity • Gravity imposes a curvature on space. – even though it has no mass, light will be affected by gravity – its path through space will be bent – within the event horizon, it can not climb out of the hole “Size” of a Black Hole • Spacetime is so highly warped around a black hole, even light can not escape. • Schwarzchild Radius – the distance from a black hole where the escape velocity equals the speed of light. • A sphere of radius Rs around the black hole is called the event horizon. Artist’s impression “Interstellar” Movie Black Hole http://dneg.com/dneg_vfx/blackhole/ Are Black Holes Suckers? • Far from the event horizon, a black hole exerts gravitational force according to Newton’s Law, just like any star of the same mass • Only at a distance of 3 Rs from the black hole will the gravity increase from what Newton’s Law predicts. – then one could eventually fall into the black hole, if you get rid of your angular momentum • Angular momentum is transferred from inner accretion disk outward via viscous torques How to have a ripping good time at a black hole • Gravity falls off as (1/distance)2, while tidal forces drop off as (1/distance)3 • Your feet are pulled much harder than your head, spagettification ensues… – occurs well outside the event horizon (160 Rs for a 1 Msun black hole) • Tidal forces are (non-intuitively) less strong for supermassive black holes Warping of Time by Gravity • In the vicinity of the black hole, time slows down. • If we launched a probe to it, as it approached the event horizon: – e.g., it takes 50 min of time on mother ship for 15 min to elapse on probe – from the mother ship’s view, the probe takes forever to reach event horizon – probe would eventually disappear as light from it is red-shifted beyond radio • From the probe’s view: – it heads straight into the black hole – light from the mother ship is blue-shifted Finding Black Holes If they are black, how do we know they exist? • Indirect evidence seen in: – – – – – x-ray binary stars (3 to 15 Msun) gamma-ray bursts (3 to 100? Msun) our galactic center (~106 Msun) centers of other (active) galaxies (106 to 109 Msun) centers of globular clusters? ( ~103 Msun?) No rock solid evidence yet for a black hole < 3 Msun or between 100 and 106 Msun Cygnus X-1 X-ray binary • First good candidate for a black hole – Strong x-ray source – In 1971 found to be a spectroscopic binary by Webster, Murdin and Bolton – Kepler’s 3rd Law gives a mass > 3 M for the unseen companion Supermassive Black Holes NGC 4258: Extragalactic Megamaser • Doppler (radial) velocities and proper motions of maser spots indicate Keplerian orbits • Central object is a 35 million Msun black hole 7.6 Mpc from Earth Gamma-Ray Bursts Discovered in late 1960s by Vela satellites CsI gamma-ray detectors to monitor atomic bomb test-ban treaty compliance found ~1 burst/day distributed over entire celestial sphere. discovery remained classified by the military until 1973 Origin was a mystery for 25 years Occur during massive supernova collapse or mergers of extremely compact objects NASA’s CGRO: 1991-2000 Compton Gamma Ray Observatory Eight NaI scintillators are oriented like the eight faces of an octahedron. Position of burst determined by relative counts in the 8 different detectors. Isotropy CGRO Findings • Each burst has unique light curve – durations 10-2 to 103 s • Many have sharp rise times and exponential falloff – ctrise ~ 30 km – led to suspicion that bursts might involve neutron stars GRB: galactic or extragalactic? - Bright bursts were localized by CGRO to an accuracy of 2°, dim ones to 10°. - prevented identifying X-ray or optical counterparts. - gamma-rays decayed too fast to point other telescopes 1 deg. GRB 970508 – Optical Counterpart BeppoSAX X-ray telescope found sky position of GRB Ground optical telescopes detected transient while still on the rise and obtained spectrum. Host galaxy located for GRB 970508 : 4 Gpc from Earth - implied 1045 J of energy released - comparable to a core-collapse supernova • If GRBs are extragalactic, how do you convert a stellar mass of material into gamma-rays in under 1 second? • Solution: gamma-rays from a very high velocity outflow are beamed into narrow cone – can reduce true energetics by a factor of 105 • But how do you create a high velocity outflow? Core-collapse / GRB connection • 1998: GRB 980425 detected in a galaxy 40 Mpc from Earth: – type Ib/c supernova SN1998bw detected at same location – although SN was particularly energetic (5 x 1045 J), GRB was not (8 x 1040 J) – energetics of the SN suggested a 3 Msun core collapsed into a black hole – Produced a very shortlived, fast jet Long vs Short GRBs Bursts come in ‘long’ and ‘short’ types (based on time to reach 90% of maximum flux) Long bursts: associated with core-collapse supernovae Short bursts: result of neutron star/neutron star or neutron star/black hole mergers. Collapsar model (long duration gamma ray bursts) Real-time GRB Sky Map • http://grb.sonoma.edu/