Pulsars and Neutron Stars Lecture 29 October 2003 DISCOVERY OF THE FIRST PULSAR # 1967, Jocelyn Bell was a grad student, working with Tony Hewish # Hewish designed a 2000-dipole radio telescope (image a bunch of TV antennae in a field in England) # Bell and 5 others built the telescope; Bell analyzed all the data # Bell noticed a "SCRUFF" in the chart reader [see figure] # the scruff occurred at the same place on the sky every day they looked # the scruff is PULSED emission (on off on off on off) # There's a pulse every 1.33730115 seconds. exactly. [LISTEN] http://www.jb.man.ac.uk/%7Epulsar/Education/Sounds/sounds.html # THIS IS HOW EXTRA-TERRESTRIAL CIVILIZATIONS WOULD CONTACT EARTH! (For more info, read the great-fun novel "Contact" by Carl Sagan) # Hewish and Bell kept quiet about the discovery for months because they thought it might be ET calling. Was it? "I went home that evening very cross here was I trying to get a Ph.D. out of a new technique, and some silly lot of little green men had to choose my aerial and my frequency to communicate with us." - J. Bell # If this is coming from a planet, what would you predict? DOPPLER SHIFT They looked -- the only Doppler shift was from Earth going around the Sun. # Then, they discovered 4 more of these scruffs. How likely was it that 4 ET civilizations were all phoning Earth at the same time? Okay, probably something natural. But what? "In the paper to NATURE we mentioned that at one stage we had thought the signals might be from another civilization. When the paper was published the press descended, and when they discovered a woman was involved they descended even faster. I had my photograph taken standing on a bank, sitting on a bank, standing on a bank examining bogus records, sitting on a bank examining bogus records: one of them even had me running down the bank waving my arms in the air - Look happy dear, you've just made a Discovery! (Archimedes doesn't know what he missed!) Meanwhile the journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret (we have quaint units of measurement in Britain) and how many boyfriends did I have at a time?" -J Bell SO WHAT KIND OF OBJECTS WERE THESE PULSARS? # a white dwarf can spin about 1 time per second. so could make the pulses seen in the first pulsar (which spun every 1.3 seconds) In 1968, (1 year after Bell & Hewish), CRAB PULSAR discovered in radio * 30 pulses per second! * A white dwarf CANNOT rotate 30 times per second. It would be torn apart! 1969, CRAB PULSAR was discovered to be SLOWING DOWN * like a spinning top * time it takes to spin once INCREASES by 15x10^-6 seconds every year (by the way, this means we can estimate ages of pulsars by how fast they spin) also, CRAB NEBULA had been known to emit X-rays since 1964 * localized x-rays during lunar occulation w/ rocket detector (explain) * also, SN known in 1054 AD at crab's position. * in 1969, x-rays seen to be PULSED other wavelengths - OPTICAL * There's a big-old supernova remnant right on top of the crab pulsar! * Everything's expanding away from the position of the Crab pulsar. * So it looks like pulsars are associated with supernovae! * U. Arizona astronomers discovered optical light from the Crab pulsar in 1969 * pretty optical picture of the nebula, and optical movie of the pulsar: http://www.noao.edu/image_gallery/images/Crab1b.MPG LET'S LISTEN TO SOME PULSARS * http://www.jb.man.ac.uk/%7Epulsar/Education/Sounds/sounds.html * listen to the fast ones HOW DO YOU MAKE A NEUTRON STAR? # Recall from 2 lectures ago [See plot of FATE VS MASS] # 10 < mass < 30 solar mass : star becomes a neutron star after supernova # Mass > ~30 solar masses : star becomes a black hole. # There are many more 10-30 Msol mass stars than there are M>30 Msol stars. # So most core-collapse supernovae SHOULD leave behind a neutron star. ************************** reinforce: ***************************** NEUTRON STARS (AND BLACK HOLES) ARE MADE IN CORE-COLLAPSE SUPERNOVAE! Don't make ANYTHING in a Type Ia supernovae! DESTROY THE WHOLE STAR ******************************************************************* HOW DO YOU MAKE A NEUTRON STAR? (continued) # remember, as the iron core collapses in a core collapse supernova, the electrons and protons ---> neutrons # the core collapses until neutron degeneracy pressure provides support. # rest of star blows up, neutron star still there. BASIC PROPERTIES OF A NEUTRON STAR: (note that they're too small to every photograp unless we travel there.) Radii: 20 km (size of a small city) Temperatures: * 10^11 K when made * cool by neutrinos to 10^9 K in 1 DAY! * after ~100 years, T_surface is 10^6 K (makes L~Lsun) * stays at 10^6K for few 10^4 years. THIS IS the important Temp. magnetic fields: 10^12 (a trillion) times stronger than the Earth's magnetic fields get weaker with age. masses: 1 to 1.4 solar masses (only measured in a few binaries); maximum mass is probably 3-5 solar masses densities: up to 10^14 g/cc structure: [SHOW FIGURE from carrol & ostlie 15.11] PROPERTIES SPECIFIC TO SPINNING NEUTRON STARS (pulsars): Spin rate: most spin at 0.5 to 10 times per second; some 100-1000x per sec numbers: there are probably about a million active pulsars in the Galaxy ~700 pulsars have been found so far in the Milky Way ONLY *SOME* NEUTRON STARS ARE PULSARS. BUT ALL PULSARS ARE NEUTRON STARS. A BIT ABOUT NEUTRON STAR STRUCTURE: # regular iron on the surface, a thin "crust" a few football-fields thick # neutrons for almost all the rest of the neutron star # weirder state of matter in the core? core composition is uncertain THE THEORETICAL SIDE neutron stars were predicted 33 years before they were discovered! pretty good for the theorists! (who are usually scrambling to catch up) neutron stars predicted only 2 years after the neutron was discovered! (1932) "We advance the view that a supernova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density." -Baade & Zwicky, 1934 In 1939, Oppenheimer and Volkoff estimated the maximum mass that a neutron star can have. This is the maximum amount of mass that neutron degeneracy pressure can support, so it's like the Chandra mass for white dwarfs. The maximum mass for a neutron star is probably about 3 to 5 solar masses. It's hard to calculate exactly because we're uncertain how the core behaves. A FEW REASONS WHY NEUTRON STARS ARE IMPORTANT: # neutron stars are the fates of massive (10-30 Msol) stars # pulsars are some of the brightest sources in the radio and xray sky # neutron stars are some of the weirdest objects in the universe. # neutron stars have extreme composition, densites, magnetic fields, and gravity. WE CANNOT BUILD EXPERIMENTS ON EARTH that come CLOSE to these extremes. Therefore, neutron stars are laboratories that show us how physics works at conditions very different from Earth. NOW LET'S ADDRESS SOME "WHYS?" # already talked about why a neutron star forms: neutron stars are a natural outcome of core collapse SN WHY SHOULD NEUTRON STARS ROTATE QUICKLY? # angular momentum is conserved # (the make-you-puke spin-a-person angular momentum demo, again) # the massive star was rotating slowly to begin with # core shrinks by a factor of ~10^5, so it's like the ice-skater, instead of just pulling in her arms, getting skinnier by a factor of 10^5. slow rotation -->>> REALLY fast rotation # so the neutron star is born spinning rapidly HOW DO NEUTRON STARS MAKE LIGHT? # one way is that they emit x-rays from their million-degree surface # TRUE, and observed in several cases. but how could that pulse? (answer - it doesn't) WHY, OH WHY, SHOULD NEUTRON STARLIGHT PULSE??? # this isn't an easy question! # vibrations/quivering doesn't work # NEED a MODEL. # model in which there's a very strong MAGNETIC FIELD # and the rotation axis and magnetic axis are MISALIGNED # see the lovely figure, scanned in. # in class, jane played with a beach-ball model, where the light was coming from the north and south magnetic poles of the neutron star (whose magnetic field is like a huge bar magnet). Then she spun the beach ball around ANOTHER axis. So the rotation axis is not the magnetic axis. So the magnetic poles flash in and out of view, light a light-house beacon, as the NS rotates. The strong magnetic field, being dragged around as the star rotates, creates light above the north and south magnetic poles. As the rotation carries the magnetic poles into and out of your view, you see flashes of light. These are the radio pulses seen in pulsars. ########################################################################### THE SPIN OF THE NEUTRON STAR IS THE ENERGY SOURCE FOR THE LIGHT PULSES. THE PULSES ARE SPENDING ENERGY, SO THE PULSES ARE SLOWING DOWN THE PULSAR. So pulsars pulse because they're spinning. But the pulses slow down the spinning, until eventually the neutron spins too slowly to make pulses, and stpops being a pulsar. ########################################################################### WHY DO NEWLY-BORN NEUTRON STARS HAVE HONKING-STRONG MAGENTIC FIELDS? # This is like what happened to the spin # The star's original magnetic field collapses with the core, so it gets much stronger, too. So the magnetic field is something like 10^10 times stronger in the newly-born neutron star than in the massive star before collapse. # high B fields can be measured from the SPECTRA of pulsars