For stars less than about 25 solar masses the end of their lives is to evolve to white dwarfs after substantial mass loss. Due to atomic structure limits, all white dwarfs must mass less than the Chandrasekhar limit. If their initial mass is more than the Chandrasekhar limit, then they must lose their envelopes during their planetary nebula phase till they are below this mass limit. An example of this is the Cat's Eye Nebula shown below:

At what stage a star leaves the AGB (Asymptotic Giant Branch) and becomes a white dwarf depends on how fast it runs out of fuel in its core. Higher mass stars will switch from helium to carbon burning and extend their lifetimes. Even higher mass stars will burn neon after carbon is used up. However, once iron is reached, fusion is halted since iron is so tightly bound that no energy can be extracted by fusion. Iron can fuse, but it absorbs energy in the process and the core temperature drops.

After evolving to white dwarfs, stars with original masses less than 25 solar masses slowly cool to become black dwarfs and suffer heat death.

Stars greater than 25 solar masses undergo a more violent end to their lives. Carbon core burning lasts for 600 years for a star of this size. Neon burning for 1 year, oxygen burning about 6 months (i.e. very fast on astronomical timescales). At 3 billion degrees, the core can fuse silicon nuclei into iron and the entire core supply is used up in one day.

An inert iron core builds up at this time where successive layers above the core consume the remaining fuel of lighter nuclei in the core. The core is about the size of the Earth, compressed to extreme densities and near the Chandrasekhar limit. The outer regions of the star have expanded to fill a volume as large as Jupiter's orbit from the Sun. Since iron does not act as a fuel, the burning stops.

The sudden stoppage of energy generation causes the core to collapse and the outer layers of the star to fall onto the core. The infalling layers collapse so fast that they `bounce' off the iron core at close to the speed of light. The rebound causes the star to explode as a supernova.

The energy released during this explosion is so immense that the star will out shine an entire galaxy for a few days. Supernova can be seen in nearby galaxies, about one every 100 years (therefore, if you survey 100 galaxies per year you expect to see at least one supernova a year). One such supernova (1991T) is shown below in the galaxy M51.

Supernova Core Explosion:

Once the silicon burning phase has produced an iron core the fate of the star is sealed. Since iron will not fuse to produce more energy, energy is lost by the productions of neutrinos through a variety of nuclear reactions. Neutrinos, which interact very weakly with matter, immediately leave the core taking energy with them. The core contracts and the star titers on the edge of oblivion.

As the core shrinks, it increases in density. Electrons are forced to combine with protons to make neutrons and more neutrinos, called neutronization. The core cools more, and becomes an extremely rigid form of matter. This entire process only takes 1/4 of a second.

With a loss of pressure from core, the unsupported regions surrounding the core plunge inward at velocities up to 100,000 km/s. The material crashes into the now-rigid core, enormous temperatures and pressures build up, and the layers bounce upward. A shock wave forms, which accelerates and, within a few hours, explodes from the surface of the star rushing outward at thousands of km/sec.

This entire process happens so fast that we can only follow it using supercomputer simulations. Maps of density and flow show the details in regions where observations can not be made.

As the outer layers are blasted into space, the luminosity of the dying star increases by a factor of 10⁸ or 20 magnitudes. In 1987, a supernova exploded in our nearest neighbor galaxy. That supernova, designated SN1987A (the first one discovered in 1987) was visible to the naked eye, rising to a maximum brightness 85 days after detonation with a slow decline over the next 2 years. The light curve for SN1987A is shown below:

Although a supernova is extremely bright, only 1% of its energy is released as optical light. The rest was released as neutrinos and kinetic energy to explode the star. Most of the initial luminosity is the shell of the star expanding outward and cooling. After a few hundred days, this shell of expanding gas has cooled to be almost invisible and the light we see at this point is due to the radioactive decay of nickel and cobalt produced by nucleosynthesis during the explosion.

Neutrinos and Gravity Waves:

Supernova are the most energetic events in the Universe and provide an opportunity to observe two very elusive phenomena, neutrinos and gravity waves.

The collapse of a supernova core produces a flood of those very strange particles, neutrinos. Neutrinos interact very weakly with matter. Under most conditions, matter is transparent to neutrinos. During the high densities of a supernova core collapse, some of the neutrinos provide the pulse to starts the outward moving shock wave. But most of the neutrinos zip out of the supernova core. Thus, when a supernova explodes, huge numbers of neutrinos pour into space, streaming across the Galaxy passing through dust, gas, nebula unhindered. Even if the supernova is obscured, the neutrinos will rain down on the Earth.

However, because neutrinos are weakly interacting, they are also just as difficult to detect. Our best neutrino `telescopes' are large tanks of water buried deep underground such as the Super Kamiokande in Japan. Water contains lots of protons in the form of hydrogen atoms. Neutrinos from a supernova explosion travel at or very near the speed of light and carry a lot of energy. On rare occasions, a neutrino will hit a proton in the tank of water (the more water, the greater the chance). This collision will produce a positron which recoils with such high speed that it emits a brief flash of light known as Cerenkov radiation. The detector tank of water is buried deep in the Earth to eliminate cosmic rays and other interactions that would distort the detection of the neutrinos. Only neutrinos can reach to such depths.

The supernova SN1987A was the first recorded neutrino detection of an astronomical event (most neutrinos detected are from the Sun). Twelve neutrinos were detected 3 hours after the supernova was seen in the optical. The neutrino detections also give us valuable information on the neutrino itself. Until recently, we did not know if the neutrino has zero mass (like the photon and, therefore, travels at the speed of light) or if it has a small mass and must travel less than the speed of light. If neutrinos are massless, then they would arrive at the Earth at the same time. The more massive the neutrino, the more spread out their arrival times. The results from these experiments showed that the neutrino has a very small mass, a surprise to the world of particle physics.

Another exotic technique to study supernovae is through the use of gravitational radiation. During the core collapse of the supernova, vast amounts of matter are moved about at enormous speeds. The dense mass is surrounded by a strong gravitational field. Einstein's general theory of relativity describes gravity as curves in the fabric of space. Vigorous changes in gravity will produce `ripples' in the geometry of space, and these ripples can propagate outward at the speed of light, called gravity waves.

Gravity waves can be detected by the effects they have on other masses. For example, two masses will vibrate when a gravity wave passes, so sensitive measurements of their motion with lasers will detect the motion. Currently our technology is unable to detect gravity waves, but a new system (LIGO) is currently under construction for use at the turn of the century.

Nucleosynthesis:

There are over 100 naturally occurring elements in the Universe and classification makes up the periodic table. One of the great successes of stellar evolution theory was the explanation of the origin of all these elements. Some of the elements were formed when the Universe was very young. The era immediately after the Big Bang was a time with matter was densely packed and temperatures were high (ten's of millions of degrees). Fusion in the early Universe produced hydrogen, helium, lithium, beryllium and boron, the first 5 elements in the periodic table.

Other elements, from carbon to iron, were formed by fusion reactions in the cores of stars. The fusion process produces energy, which keeps the temperature of a stellar core high to keep the reaction rates high. The fusing of new elements is balanced by the destruction of nuclei by high energy gamma-rays. Gamma-rays in a stellar core are capable of disrupting nuclei, emitting free protons and neutrons. If the reaction rates are high, then a net flux of energy is produced.

Fusion of elements with mass numbers (the number of protons and neutrons) greater than 60 uses up more energy than is produced by the reaction. Thus, elements heavier than iron cannot be fuel sources in stars. And, likewise, elements heavier than iron are not produced in stars, so what is their origin?

The construction of elements heavier than involves neutron capture. A nuclei can capture or fuse with a neutron because the neutron is electrically neutral and, therefore, not repulsed like the proton. In everyday life, free neutrons are rare because they have short half-life's before they radioactively decay. Each neutron capture produces an isotope, some are stable, some are unstable. Unstable isotopes will decay by emitting a positron and a neutrino to make a new element.

Neutron capture can happen by two methods, the s and r-processes, where s and r stand for slow and rapid. The s-process happens in the inert carbon core of a star, the slow capture of neutrons. The s-process works as long as the decay time for unstable isotopes is longer than the capture time. Up to the element bismuth (atomic number 83), the s-process works, but above this point the more massive nuclei that can be built from bismuth are unstable.

The second process, the r-process, is what is used to produce very heavy, neutron rich nuclei. Here the capture of neutrons happens in such a dense environment that the unstable isotopes do not have time to decay. The high density of neutrons needed is only found during a supernova explosion and, thus, all the heavy elements in the Universe (radium, uranium and plutonium) are produced this way. The supernova explosion also has the side benefit of propelling the new created elements into space to seed molecular clouds which will form new stars and solar systems.

Neutron Stars:

The idea of a neutron star was developed in 1939 when calculations were made of a star that was composed solely of degenerate neutrons. If the mass of a normal star were squeezed into a small enough volume, the protons and electrons would be forced to combine to form neutrons. For example, a star of 0.7 solar masses would produce a neutron star that was only 10 km in radius. Even if this object had a surface temperature of 50,000 K, it has such as small radius that its total luminosity would be a million times fainter than the Sun.

As with white dwarfs, neutron stars have an inverse relationship between mass and radius. As a neutron increases in mass, its radius gets smaller. Their extremely small size implies that they rotate quickly, according to the conservation of angular momentum.

The interior of a neutron star is hard to calculate since the physics covers a new realm not testable in our laboratories. Models suggest that neutrons packed into such a dense configuration becomes a superfluid sea. Normally superfluids, such as liquid helium, occur at very low temperatures. But that normal matter has an electric charge (positive for the protons, negative for the electrons). A dense mixture of neutrons (with zero electric charge) can become a friction-free superfluid at high temperatures.

The interior of a neutron star will consist of a large core of mostly neutrons with a small number of superconducting protons. Again, normally associated with low temperatures, superconducting protons, combined with the high rotation speeds of the neutron star, produce a dynamo effect similar to what creates the Earth's magnetic field. Surrounding the core is a neutron mantle, then a iron-rich crust.

Pulsars:

Every star has a magnetic field, usually a very weak one. However, when a stellar core is compressed into a neutron star during a supernova explosion, the weak magnetic field is also compressed. As the field lines squeeze together, the magnetic field becomes very powerful. A powerful magnetic field, combined with the rapid rotation, will produce strong electric currents on the surface of the neutron star.

Loose protons and electrons near the surface of the neutron star will be sweep up and stream along the magnetic field lines towards the north and south magnetic poles of the neutron star. The magnetic axis of the neutron star does not necessarily have to be aligned with the rotation axis (like the Earth), they can be inclined from each other as shown below.

The rotating neutron star has two sources of radiation: 1) non-thermal synchrotron radiation emitted from particles trapped in the magnetic field of the neutron star, and 2) thermal radiation from particles colliding with the neutron star surface at the magnetic poles. The thermal component contains x-rays, optical and radio radiation since the protons smashing into the surface of the neutron star at extremely high velocities. Given the geometry of the hotspots at the magnetic poles, the energy from the hotspots sweeps out into space like a lighthouse. Only when the Earth lies along the axis of the neutron star is the energy detected as a series of pulses, and the object is called a pulsar.

Pulsars were discovered by accident in 1967 during a search for distant sources of radio radiation. A special telescope had been constructed to look at short timescales of radio waves. One object displayed extremely evenly spaced pulses of radiation. The period was 1.337 seconds with an accuracy of 1 part in 10 million. A typical pulsar signature is shown below.

Notice that the shape of the pulses is similar from high energy photons down to the low energy radio photons. This indicates that the source of the radiation, over a range of wavelengths, is from the same region on the neutron star.

The fact that the pulses of radiation are so sharp and regular allows an astronomer to make very accurate measurements of the period of the pulses. When this is done, it is found that pulsars are slowing down with time. The rapidly changing magnetic field produces some of the energy that is beamed outward. Therefore, each pulse takes rotational energy from the neutron star and sends it into space, i.e. the neutron star loses rotational energy and slows down. Typical changes are about 10^-15 seconds per rotation. In other words, a neutron star with a rotation of 1 second will be slowed to 2 seconds in about 30 million years. Thus, the age of a pulsar is determined by its current rotation speed. Old pulsars are rotating slowly, young ones fast.

Pulsars also display sudden speed-up's in their rotation rates in sharp `glitchs' of their timing curves. The surface gravity of a neutron star is millions of times greater than the surface gravity of the Earth. The tremendous weight causes the crust to shift and contract suddenly, a starquake. The contraction, even though only a 1 mm in depth, causes a resulting starquake that is about a billion times more powerful than any earthquake on the Earth. This is visible in the rotation rate since it can be measure with a high degree of accuracy.

Accretion Disks:

If a supernova occurs in a binary system, the companion star will survive the blast (although it will lose some of its outer layers). A neutron star will be left in orbit around the secondary star. As the companion star evolves to become a red giant, its envelope will expand beyond the Roche limit and gas will spiral onto the neutron star.

The gas flowing towards the neutron star forms a thick disk of orbiting material called an accretion disk. Since the infalling gas retains the direction of orbital motion of the companion, the stream of material forms a rotating disk. Friction between the gas in neighboring orbits cause the gas to spiral inward until it hits the surface of the neutron star. As the spiraling gas moves inward, gravitational energy is released in the form of heat into the accretion disk.

The release of energy is greatest at the inner edge of the accretion disk where temperatures can reach millions of degrees. If the object at the center is very compact, then a highly energetic source is available with only a small accretion rate. This region will be the source of strong x-ray and UV radiation, the signature of an x-ray binary system such as seen in Puppisa shown below.

If the gas is dumped in vast amounts from the accretion disk to the neutron star, then the energy can not be released fast enough and tremendous pressures build up. The pressure can only be relieved if the gas is ejected. Since its easier for the plasmas to be ejected through the thinner poles, two powerful jets of high velocity hot gases form perpendicular to the accretion disk.

Microlensing :

When old neutrons stars have slowed down to the point where they no longer emit radio or x-ray radiation, they are invisible. However, we can detect dark neutron stars by their tremendous gravitational fields as they bend of light of stars behind them, called gravitational microlensing.

Surveys of microlensing will image a patch of the sky towards the center of the galaxy looking for sharp changes in light over a period of weeks. These changes in brightness mark the passage of a neutron star in front of the target star, and the lensing of the background star light by the neutron star. The period and shape of the microlensing event provides information on the mass of the neutron star.