The Legacy of Supernovae

Do supernovae leave anything behind? In 1939, Fritz Zwicky, a Swiss astronomer working in California, proposed that SN explosions could create and leave behind an incredibly dense star of neutrons He was widely ignored and sometimes even ridiculed The neutron had just been discovered the year before! Zwicky was known to be “erratic” – he made all kinds of weird proposals during his career

Fast forward to 1967: graduate student Jocelyn Bell, Cambridge University, UK Bright sources of astronomical radio emission were discovered The radio sources were not well localized, so it was not possible to compare them with visible images of the sky Not known if the sources might be tiny, like distant stars, or extended, like galaxies Jocelyn Bell’s PhD thesis topic: “Scintillation in Radio Sources”

“scintillation”: the twinkling of light When visible light passes through the Earth’s atmosphere, the motion of air molecules causes the brightness of light to flicker slightly This effect impacts tiny sources, like stars, but not extended sources, like planets

Radio waves can also scintillate, not due to our atmosphere, but due to passage through the interstellar medium on the way to our Solar System Bell and her thesis advisor, Prof. Anthony Hewish, reasoned that if they could detect scintillation in the mysterious radio signals, they would learn that the sources were extended (nebula or galaxies), not compact (stars) Radio telescopes were not previously designed to look for rapid flickering

Radio “clicks”, spaced every 1.3 seconds, over and over

What’s making the periodic radio clicks? The clicks turn out to keep absolutely perfect time, P = 1.337302088331 sec A more accurate clock than any clock! Bell and Hewish briefly considered “little green men”

More pulsars were rapidly found, each with a different but astonishingly accurate period Periods range from 4 seconds down to 1/1000th of a second More than a thousand now known

What can keep very accurate time? Radial pulsations Example: Pure tone of a bell But stars are not strong enough to hold together Spinning body Example: Lighthouse Almost no star strong enough to hold together, except if the material were pure neutron matter ⇒ A spinning neutron star might be the underlying clock in a pulsar

Mystery largely solved in 1968: a pulsar found in the center of the Crab Nebula, period 0.033 seconds Fritz Zwicky was right almost 30 years earlier: SN make neutron stars!!

The Crab pulsar is also seen to pulse in visible light and in X-rays with the same 0.033 second period as in radio (visible) (two panels are exposures separated by 0.017 secs, half of the pulsar period) (X-rays)

Characteristics of neutron stars For the star to squeeze its core so intensely that all the matter is pressed into neutrons, we can calculate that the radius of the star will be about 10 km! Our sun has a mass of 2 x 1030 kg. If all this mass is instead contained in a 10 km radius, the density of that (neutron) matter is huge

Density of typical rocks on Earth: 1 gm/cm3 Density of white dwarf material: 106 gm/cm3 Density of neutron stars: 1015 gm/cm3 1 thimble of neutron matter is 2,200 billion pounds If this building were made of neutron matter, it would weigh more than the entire Earth

Why should neutron stars spin? And spin so fast? Most stars rotate slowly. Our Sun rotates once every 25 days.

Conservation of Angular Momentum A large, slowly spinning body will spin faster if it gets smaller For a sphere, Angular Momentum = constant X radius X spin rate If the radius gets smaller, the spin rate has to get bigger

What makes pulsars pulse? Very small stars are expected to be very strongly magnetic If the magnetic field axis is not the same as the spin axis of the star, the magnetic beam sweeps past you twice per rotation

What makes the light in the pulses? Since some pulsars are known to make the pulses at radio, optical, and X-ray wavelengths, it is not line emission Clue: tiny stars are expected to be intensely magnetic: like angular momentum, magnetic strength is intensified when the magnet shrinks in size Our Sun is weakly magnetic

What makes the light in the pulses? When a star shrinks in radius from 700,000 km to 5 km, can calculate how much stronger the magnetic strength will be: 1015 stronger! Neutron stars are the strongest magnets known in the Universe! In very strong magnets, electrons are trapped, spin, and make light: “Synchrotron Radiation”

What makes the light in the pulses?

What’s the ultimate fate of neutron stars? We see periods as fast as 1/1000th sec, and as slow as 4 sec Most pulsars don’t have visible supernova remnants around them Interesting hint: younger supernova remnants have faster pulsars

Crab Nebula Pulsar: spins 30/sec, SN was 962 yr ago (1054 A.D.) Vela Pulsar: spins 11/sec, SN was about 10,000 yr ago Most other pulsars: spin more slowly, SN remnant not seen

What’s the ultimate fate of neutron stars? Apparently they gradually slow down as they age Makes sense: something has to be powering the synchrotron, and as the pulses leave, they carry away energy When the period slows to about 4 seconds, apparently the synchrotron shuts down Our Galaxy probably contains 106 – 107 “dead” pulsars

Are white dwarfs and neutron stars the final endpoints for all stars? Even neutron matter has a limit to how much mass it can support The calculation of this limit is still controversial and uncertain However, all calculations agree that no neutron star can be heavier than 3 Mʘ (and it’s probably less)