University of California, San Diego
Center for Astrophysics & Space Sciences

Supernovae

Following the Helium Burning Main Sequence in massive stars, a series of nuclear burning stages transforms the star into an onion-like shell structure, until Silicon and Sulfur burning create a core of iron (and other iron-peak elements. During this phase the star will criss-cross the upper regions of the H-R diagram from Red Supergiant to Blue Supergiant and back as different core and shell burning stages ignite. Each successive nuclear burning stage releases less energy than the previous stage, so the lifetime in each stage becomes progressively shorter. For a 20 M star:

• Main sequence lifetime ~ 10 million years
• Helium burning (3-) ~ 1 million years
• Carbon burning ~ 300 years
• Oxygen burning ~ 2/3 year
• Silicon burning ~ 2 days

The Structure of a Pre-Supernova Supergiant

Because iron is the most tightly bound nucleus (the "break-even point" between fusion and fission) the star is no longer able to produce energy in the core via further nuclear burning stages. Nuclear reactions will continue, however, because of the extremely high temperatures in the massive star's core. These further reactions have a devastating effect on the star, because they take energy out of the core. At such high temperatures and densities the gamma-ray photons present in the core have sufficient energy to destroy the heavy nuclei produced in the many stages of nuclear reactions, e.g.:

+ ⁵⁶Fe 13⁴He + 4n

p⁺ + e^- n +

Here is a NASA supernova animation of a supernova explosion and formation of a pulsar. The final death of a massive star is a Type II supernova. As described here there are two types of supernovae. A white dwarf star in a binary star system may accrete material from its companion star. If the white dwarf exceeds the 1.44M limit for support by electron degeneracy, it will collapse and produce a Type Ia supernova. Type I and II supernovae can be distinguished by their light curves and spectral emission lines. Here is some more supernova background and further links here.

A supernova explosion may for a short period of time shine as brightly as the hundreds of billions of stars in its galaxy. Here's an example - a time series of photos of SN 1998S in the galaxy NGC 3877 taken in March--June 1998.

SN 1987A in the Large Magellanic Cloud

SN 1987A

In February 1997 a supernova appeared near the Tarantula nebula in our satellite galaxy the Large Magellanic Cloud, about 169,000 light years away. As the first supernova discovered in 1987, it was called SN1987A following astronomical convention. SN1987A was the first "nearby" supernova of the modern era and the closest supernova since Kepler's supernova in 1604. Among the unique observations from SN1987A:

• For the first time astronomers have observations of the star that exploded, the supernova precursor, known as Sk -69 202 from a catalog of stars in the LMC by N. Sanduleak. Sk -69 202, shown in the photograph above was a B3I sopergiant with an atmospheric temperature, T = 16,000K, a luminosity of about 100,000L, and an estimated mass of about 20M. An unexpected aspect of this is that Sk -69 202 was a blue supergiant (note temperature and spectral type) rather than a red supergiant.

• At neutrino detectors in Japan (Kamiokande) and the US (IMB) neutrinos were detected from the collapse of the stellar core of Sk -69 202. Only about 20 neutrinos were detected of nearly 10⁵⁹ neutrinos emitted as the core collapsed. But, allowing for the weakness of neutrino interactions, this was just about the right number, strongly confirming our picture of supernova explosions and the formation of neutron stars.

• Gamma-ray observations have directly observed the nuclei produced in the supernova explosion whose radioactive decay powers the expanding supernova shell. e.g.:
  • ⁵⁶Ni ⁵⁶Co + e⁺ + +              (6 days)
  • ⁵⁶Co ⁵⁶Fe + e⁺ + +            (79 days)

• Continuing observations have provided a wealth of information about the supernova shockwave as it moves outward into the circumstellar medium, as described in this Scientific American article.

A supernova occurs about once per century in the Milky Way galaxy; most are obscured from our view by gas & dust, but there have been four well documented historical supernovae:

• SN 1006 in Centaurus in the southern sky.
• SN 1054 - The Crab Supernova in Taurus recorded by Chinese and Native American astronomers.
• SN 1572 - Tycho's Supernova, studied in detail by Tycho Brahe.
• SN 1604 - Kepler's Supernova, shown here in a historical illustration

plus other probable & possible Milky Way supernovae.

Pulsars

According to theory, the core of the star that remains after a supernova explosion is a tiny (R ~ 10km) remnant of extremely high density neutrons, supported by neutron degeneracy -- a neutron star. The existence of Neutron Stars was predicted by Baade & Zwicky (1934) and Oppenheimer (1939). But how to detect such a small remnant?

As with many things in astronomy, the discovery came in unexpected fashion with the discovery of Pulsars. In 1967, Cambridge graduate student Jocelyn Bell (now Burnell) and her advisor, Anthony Hewish, were using a special radio telescope to look for radio scintillation, fluctuations in the signals from distant radio sources producted by turbulence in the interplanetary and interstellar plasma similar to the twinkling of stars caused by atmospheric seeing. On November 28, Bell discovered a source with an exceptionally regular pattern of radio flashes. These radio flashes occurred every 1 1/3 seconds like clockwork. Puzzled by what sort of object could produce such a regular pattern, the source was initially dubbed "LGM" - standing for little green men, because the only source that they could imagine that could be so regular was some sort of extraterrestrial technical civilization. After a few weeks, however, three more rapidly pulsating sources were detected, all with different periods. They were dubbed "pulsars."

Radio pulses from a Pulsar

• Original chart record of Cambridge Pulsar 1919+21, the first pulsar.

What were the pulsars? From the short pulse duration and the rapid pulse rate, astronomers concluded that the pulsars must be exceedingly small objects. The radio pulses must come either from radial pulsations of a star, or from a rotation of a beam of light, like a lighthouse beam. Normal stars or even white dwarfs are too big to pulsate that fast, and rotation rates of several times per second would cause even the most compact stars to fly apart.

The answer to the nature of the pulsars came with the discovery of a pulsar in the direction of the Crab nebula , with a pulse rate of 30 times per second -- then the most rapid pulsar known. Continued observation of the Crab Pulsar showed that it was slowing down -- its period was increasing by 38 nanoseconds per day. This first confirmed that the pulses are produced by rotation; a pulsating object pulses only at its natural frequency. More importantly, it simultaneously solved two mysteries: the nature of pulsars and why the Crab Nebula continues to shine so brightly 1000 years after the supernova explosion.

The Pulsar in the Crab Nebula

Spinning objects have rotational energy of motion (kinetic energy)just as moving objects have translational energy of motion. A simple calculation showed that a rapidly spinning neutron star slowing down by 38 nanoseconds per day releases almost exactly the energy which is being radiated by the Crab Nebula, just the p[lace where Baade & Zwicky would predict that a neutron star might have been formed. Although the details of how the pulsar's rotational energy is transformed into the luminous energy of the nebula, this agreement was too good to be coincidence --- astronomers were certain that the elusive neutron stars had been discovered! Anthony Hewish later shared in the Nobel Prize for the discovery of pulsars.

Two conservation laws in physics suggest that it is likely that neutron stars should rotate very rapidly and should have strong magnetic fields:

• Conservation of Angular Momentum - as a spinning object decreases its size, conservation of angular momentum dictates that its rate of spinning must increase. This is the physics that causes a spinning ice skater to "spin-up" as he or she pulls arms and legs inward. A star like the sun, rotating once per month would rotate about 1000 times per second when contracted down to 10 km in size.
  
  Conservation of Angular Momentum

• Conservation of Magnetic Flux - lines of magnetic force will be carried along with the collapse of the plasma core of the star. As the core collapses the magnetic field lines are pulled more closely together, intensifying the magnetic field to valuess of the order of 10¹² gauss.

The detailed mechanisms by which pulsars produce their emission from radio waves through x-rays (up even to gamma rays in some cases) is complex and not fully understood, but the basic idea --- that pulsars produce a Lighthouse Effect due to an intense magnetic field whose axis is misaligned with its rotational axis --- is generally accepted. (Remember that the earth's magnetic "North Pole" is not at the true nort pole, but in northern Canada.) One popular view is that the rapid rotation and intense magnetic field of the neutron star generate strong electric fields, which accelerate charged particles (principally electrons because they are less massive) near the magnetic poles where the magnetic field is most intense. The charged particles, accelerated along the curved magnetic field lines produce a type of light called curvature radiation. (Say in unison: "Whenever charged particles are accelerated electromagnetic radiation (light) is produced.")

Whatever the detailed mechanism, radiation near the neutron star poles produces strong, narrow beams of light which sweep around the sky like a tilted lighthouse, as shown below. If the earth lies in the path of the beam we see a pulsar. (This idea has the added attraction that it explains why we don't see pulsars in all supernova remnants.)

Energy is transported from the spinning neutron star into the nebula by the magnetic field. HST has provided dramatic evidence of the interaction of the Crab Pulsar and the supernova remnant. This Picture of the Day and this HST Press release show the ripples produced by the pulsar moving outward through the nebula, also shown in this MPEG movie.

Here is an excellent and detailed Introduction to Neutron Stars by Cole Miller at Chicago.

Listen to the sounds of pulsars from the Princeton Pulsar Group and also at England's Jodrell Bank Radio Observatory (includes Crab Pulsar).

Pulsars are frequently in the news because of their extreme nature and the wide variety of physical processes that may be studied in them. Here's a selection of recent news articles:

• Astronomy Pictures of the day of Neutron Stars
• New Pulsar discovery.
• Binary Pulsar which resulted in the award of the Nobel Prize in Physics to Joe Taylor and Russell Hulse, its discoverers. More about the binary pulsar next lecture.
• From extremely precise timing experiments, radio astronomers have detected shifts in pulsar periods caused by the orbital motion of the neutron star and accompanying planets.
• Bursting Pulsar discovered by RXTE.
• Recently the discovery of stars which produce bursts of gamma-rays with pulsar-like periodicities has suggested the existence of a new class of pulsar which are inferred to have exceptionally high magnetic fields, as high as 10¹⁵ gauss (1000 times the magnetic field of a typical pulsar). The discoverers have dubbed these pulsars magnetars. Here is a detailed account by one of the discoverers. (The existence of such high magnetic fields and of magnetars remains controversial).

Summary: End Points of Stellar Evolution
Remnant	Progenitor Mass	Remnant  Mass	Size	Density	Means of Support	Final Stage
White Dwarf	M* < 8M	MWD < 1.4M	RWD ~ Rearth	1 ton/cm3 (1 Volkswagen/cm3)	e- degeneracy	Planetary Nebula
Neutron Star	8M < M* < 20M	MNS < 3M	RNS ~ 10 km	200 million ton/cm3 (All Volkswagens/cm3)	n degeneracy	Supernova
Black Hole	M* > 20M	MBH > 3M	0 Rgrav = 2GM/c2		none	?

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