* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Introduction to Astronomy
International Ultraviolet Explorer wikipedia , lookup
Star of Bethlehem wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
Dyson sphere wikipedia , lookup
Dwarf planet wikipedia , lookup
Crab Nebula wikipedia , lookup
Perseus (constellation) wikipedia , lookup
Planetary habitability wikipedia , lookup
Stellar kinematics wikipedia , lookup
Cygnus (constellation) wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Brown dwarf wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Astrophysical X-ray source wikipedia , lookup
Corvus (constellation) wikipedia , lookup
Future of an expanding universe wikipedia , lookup
Star formation wikipedia , lookup
Introduction to Astronomy • Announcements White Dwarfs & Neutron Stars White Dwarfs Structure Observations • Background: the discovery of Sirius B – Companion star to large Sirius A – First time White Dwarf observed as part of binary system Sirius A: A-type main sequence star Sirius B: white dwarf Structure • Generally, – Hot, compact stars w/ mass comparable to the Sun & size comparable to the Earth – Shines from residual (left-over) heat produced in core during normal lifetime – 25,000 K < T < 4,500 K – Tavg = 10,000 K • A white dwarf is nothing more than the leftover core of a low-mass star (few x MSun) – During red giant phase, outer layers are blown off • Radiation pressure – These layers are mostly H & He • Fusion products of early star’s life • Blown off by Helium Flash – Not enough mass to ignite C/O fusion by contraction • Low mass • So the core just cools off, radiating heat into space – Over ~ 10 million years, cools down to 20,000 K • So still emits some observable light – Would take much longer than the age of the Universe to cool off to the point where it doesn’t emit any visible light • “BLACK DWARF” (still just theoretical) Interior of a White Dwarf • In hydrostatic equilibrium – But no fusion pressure, so how is this possible???? – Electron Degeneracy Pressure • Atoms squeezed incredibly close together • Electron orbits overlap – Electrons easily move around, from atom to atom – Like a “sea” of electrons flowing over atomic nuclei • The Pauli Exclusion Principle – A fundamental limit to the number of electrons that can be squeezed into a given volume – When this limit is reached, there appears a “pressure” that keeps any more electrons from entering the volume – This “electron pressure” supports the white dwarf against its own gravity • This leads to behavior that seems to defy common sense… – The more mass you pile onto a white dwarf, the smaller it gets! • In a normal gas, pressure depends on temperature and density • This “sea” of electrons is what is called a degenerate gas – In a degenerate gas, pressure does not depend on temperature, only density (of electrons) • A small chunk of “white dwarf material” would weigh ~ couple dozen tons on Earth! • Gravitational Redshift – Like Doppler shift due to star’s motion – As light escapes from the star’s surface, gravity pulls on it, “stretching” the light wave – So light from a highmass object appears redder than it actually is But photons have no mass, so how does Gravity work on it?! General Relativity! • White Dwarfs in binary systems – White dwarf – red giant system is potentially very dangerous – Outer layers of the red giant are not very bound to the star itself • White dwarf can “strip away” the outer layers (feeding) • Usually H-rich, so white dwarf gets a new fusion fuel source • Two options – A Nova – A “Type I” Supernova • Nova (or Nova Stella) – The white dwarf’s compression of H pulled from the red giant heats it to fusion temperatures – H explodes off the surface of the white dwarf • “Nova” – White dwarf may begin to strip Hydrogen from the red giant again • Repeated novae HST observation of a Nova Time, t Time, t + 7.5 months • Supernovae – Different from novae – When fusion in a high-mass star stops, core continues contracting, but no pressure to stop it – Temperature increases so much the Iron nuclei start to break apart – Collapse pushes electrons into protons, creating bulk of neutrons at nuclear density – Rapid implosion causes material to “bounce” off high-density core, creates massive shock wave that rips the star apart “Type II Supernova” • Type I Supernova – Only occurs if white dwarf strips off too much material – White dwarf passes the Chandrasekhar Limit • If mass of white dwarf grows beyond ~ 1.44MSun, it compresses violently • Squeezes Carbon & Oxygen together hard enough to fuse into 28Si • 2 28Si smashed together into an isotope of Nickel • Fusion releases tremendous amount of energy from white dwarf’s core, blows the dwarf apart (completely) • Leaves behind no remnant – Just an expanding cloud of heavy elements (C, O, Si, Ni, Co, Fe, etc) • Type II Supernova – Collapse of massive star (not white dwarf) White Dwarf Observations A team at the University of Arizona located a star cluster made almost completely of white dwarfs. This cluster had to be extremely old since every member had used up it’s nuclear fuel. From this, they figured the age of this particular cluster would give a lower bound to the age of the Universe. They found τuniverse > 13 billion years Sun-like stars White dwarfs M4 Globular Cluster (7,000 ly distant) Red dwarfs (very cool MS stars) • Old White Dwarfs – Recall interiors are C, O, Si – When cool enough, the Carbon atoms can lock in to a crystalline structure – What is crystalline carbon? • A diamond! – Old white dwarfs may be giant diamonds floating around in space!!! Neutron Stars Structure Observations • One step beyond white dwarfs – Requires a star of higher initial mass • More intense collapse converts white dwarf’s “sea of electrons” to something else entirely… History of Neutron Stars • 1934, Astronomers Walter Baade & Fritz Zwicky – Proposed that gravity could crush the core of a star so much that the electrons are pushed into the nucleus – Positive protons combine with negative electrons to form neutrons – This would occur when core compressed to diameter of ~ 6 miles • Smaller than many asteroids! – Maximum possible mass 2 – 3MSun Electrons pushed into nucleus electron Ultradense ball of neutrons neutron proton This ultrahigh density means a single cm3 of neutron star material would weigh billions of tons here on Earth! Size of a Neutron Star… Neutron Star Observations 1997, first visible image (HST) of a lone Neutron star. Mysterious X-ray source found here in 1992, but astronomers couldn’t see anything. HST determined temperature ~ 1.2 million F Diameter < 17 miles Therefore, must be Neutron star because nothing else can be this hot, small, and dim. X-Ray image of wandering Neutron Star Pulsars • The hunt for neutron stars was on! • But none found… • Until a student discovered an extraterrestrial radio signal that pulsed precisely once every 1.33 seconds – Initially thought to be a sign of civilization, dubbed LGM-1 (“little green men #1”) • Other similar discoveries soon followed – Thought maybe they were pulsating stars • Radius expands & contracts periodically, making the star alternately brighter & dimmer – But periods were too short (impossibly small for the size-change required to change the brightness that much) • Not pulsating, but spinning ! – Like a cosmic lighthouse, see a pulse when some “beam” of radiation points toward Earth • But why so fast?! – Pulsar in Crab Nebula rotates 30 times a second! OFF ON • Conservation of Angular Momentum – A.k.a. the Ice-Skater Effect – Bringing mass closer to the axis of rotation makes the rotation speed increase – Same principle during formation of solar system from slowly rotating IS cloud – If Sun shrank down to 10 km: • Current rotational period = 30 days • New rotational period = 0.5 millisecond ! Pulsar Emission: The Lighthouse Effect • Intense electric and magnetic fields strip charged particles off star, accelerate them along magnetic poles • Fact: accelerating charges emit EM radiation – Like a radio transmitter/antenna • Charges travel along field lines, which form a tight core at the poles – Creates a tight cone of emission coming from each pole = the lighthouse beam! Protons & Electrons accelerated quickly here So a lot of emission near poles Protons & Electrons accelerated slowly here Synchrotron Radiation: created by accelerating particles • Non-thermal radiation – Depends on the strength of the charge, the speed of the particle, and the strength of the magnetic field – DOES NOT depend on the temperature • Most “pulsar lighthouses” emit radio waves, but some emit more broadband… – Crab Nebula pulsar emits flashes of visible light 30x a second Pulsar Spin-Down • Pulsar is constantly losing energy – Magnetic field exerts force on charged particles, so particles exert equal & opposite force on magnetic field (“back-reaction”) – Magnetic “friction” or “drag” • Slows rotation very slightly • Very lengthy measurements of pulsar periods show that time between pulses is slowly increasing • If it slows down enough, strength of EM radiation decreases until the “lighthouse” beams become invisible Hand-crank generator • Structure (kind of like a balloon) – Thin, gaseous atmosphere ( ~ 1 mm thick ) • Source of particles accelerated by magnetic field – Solid iron crust ( ~ few hundred meters thick ) – Liquid sea of neutrons (bulk) Why don’t the surface layers become neutrons? Pulsar Curiosities • X-Ray Bursters – Infalling gas violently heated as it flows down magnetic field to surface – Creates thermonuclear explosion like a nova • X-Ray Pulsars – Infalling gas doesn’t explode, but still heated enough to create a “hot-spot” on the surface of the neutron star – Rotates in and out of view, creating pulses of X-rays X-Ray Pulsar What We See… • Millisecond Pulsars – Rotate 1000x per second – Most have companion stars – Gravity attracts companion material into an accretion disk around the neutron star • This rotating disk transfers its angular momentum (rotation) to the neutron star, speeding it up • But some millisecond pulsars have been observed without companions…where did they go? – Another star passed by, gravitationally pulled companion away? – Companion evaporated by intense heat from pulsar • The “black widow” pulsar theory • Neutron star cannibalizes the companion for it’s mass, which generates heat that “boils away” the remains of the companion NEXT TIME • Black Holes! – The most exotic/fascinating/misunderstood objects in the universe