DEATH OF STARS

Nova: a "new" star. Actually an old star that suddenly flares in brightness, become visible where it used to be invisible.

EVOLUTION OF BINARY STARS

Novae occur in a binary system with a white dwarf, when the other star expands beyond its Roche lobe, or region of "gravitational influence". Because of viscosity (friction), mass transferred to the white dwarf spirals in, forming an accretion disk, and heats up. When the gas becomes hot enough to undergo fusion, the resulting burst of light is a nova. It can last several months. Recurrent novae are possible as the process repeats

EVOLUTION OF STARS MORE MASSIVE THAN THE SUN

In stars of mass greater than the Sun, gravity is strong enough that contraction makes core sufficiently hot to begin fusion of carbon into oxygen, and then through a whole series of reactions. A star of 20 solar masses burns hydrogen for 10 million years, helium for a million years, carbon for a thousand years, oxygen for a year, and silicon for a week

Iron is the last stage--fusion of iron does not release energy. Instead it absorbs energy, cooling the core and making it less able to support the star against gravity. The star implodes. All the nuclei and electrons get crushed together. The protons in the nuclei combine with electrons, turning into neutrons and neutrinos. p + e^- - > n + . Ultimately collapse of the star is halted by neutron pressure. The resulting object is called a neutron star.

Size: a few km.
Density: 1 cm³ has 100 million kg mass

At maximum collapse, the star "bounces," making a shock travel through the star and blowing off the outer layers, including all the heavy elements. This explosion is a supernova. A supernova is a million times brighter than a nova. The actual explosion takes less than a second.

TYPE I VS. TYPE II SUPERNOVAE

The time dependence of the intensity of the light from a supernovae is called a light curve. Light curves with two characteristic shapes are observed.

Type I (carbon-detonation) due to the implosion of a white dwarf after having accreted 1.4 solar masses (Chandrasekhar limit). -- Spectrum shows little hydrogen or helium; the progenitor star is a carbon core of an older star

Type II (core-collapse) supernovae result implosion of a massive star, as described above. -- spectrum shows more hydrogen and helium from the outer layers of the star.

SUPERNOVAE AS DISTANCE INDICATORS

Because Type I supernovae have such similar initial conditions (one Chandrasekhar mass), their brightness curves tend to be very similar, and they can be used as standard candles, i.e. they are objects of known brightness that can be used to estimate the distance to galaxies where they occur.

FREQUENCY OF SUPERNOVAE

The theory of stellar evolution, and the observed number of massive stars, indicate that there should be a supernova in our galaxy about once every hundred years. There have been 6 in the last thousand years, but none in since 1604-- we may be overdue for one.

Many supernovae in other galaxies have been observed in the 20th century, most notably Supernova 1987 A, in the Large Magellanic cloud.

SUPERNOVA REMNANTS

Supernovae leave behind glowing remains.

Crab Nebula, a famous example:

• explosion of a star in this location in 1054 AD was recorded by the Chinese and Native Americans
• bright enough to be seen during the daytime
• The expansion of the nebula can be directly observed

THE FORMATION OF ELEMENTS

Most of the hydrogen and helium in the universe was produced in the big bang, but the heavier elements formed from nuclear fusion in stars. (Elements heavier than iron are formed during the supernova itself.)

Evidence:

• Theory of formation of heavy elements (by Fowler) predicts precisely the observed abundance in the universe
• spectra as well as light curve of Type I Supernovae show that the unstable isotope nickel-56 is formed in supernovae (nickel-56 - > cobalt-56 - > iron-56)

The heavy elements formed in stars are thrown into the interstellar medium by supernovae, where they condense into new stars, and planets.

Evidence:

• older globular clusters are deficient in heavy elements: they are first generation stars

The blast produced by supernovae not only disperse the heavy elements, but can also trigger condensation of new stars from the interstellar medium, starting the cycle anew.

NEUTRON STARS -- observational evidence

Pulsars, discovered by Jocelyn Bell, in 1967, give off rapid periodic pulses of radio emission. (There's a pulsar in the Crab Nebula.) Periods: fractions of a second to several seconds.

Explanation, by Anthony Hewish:

• such rapid, periodic pulsations must be coming from a small rotating object
• white dwarfs can't rotate that fast: they'd fly apart
• therefore pulsars must be neutron stars.

Neutron stars must rotate very fast, from conservation of angular momentum. They also have very strong magnetic fields.

The pulses are explained by the "lighthouse effect:"

• charged particles escape from the poles
• are accelerated by the magnetic field
• a "beam" sweeps towards Earth each rotation, as the magnetic pole turns towards us

We would only see those pulsars whose poles happen to turn towards Earth: but it is consistent to suppose that all neutron stars are pulsars.

BLACK HOLES

If a neutron star exceeds 3 solar masses, gravity overcomes neutron pressure, and star collapses further. No known force can prevent complete collapse from that point. When the object is small and dense enough, the escape velocity exceeds the speed of light; since nothing travels faster than light, nothing can escape, not even light.

• The resulting object is called a black hole.
• the limit from which nothing can escape is called the event horizon. What happens inside the event horizon, no one can tell.
• the radius of event horizon is called the Schwarzchild radius.

OBSERVATIONAL EVIDENCE FOR BLACK HOLES

Black holes in binary systems would form an accretion disk, and emit strongly in the X-ray region. There are a few possible candidates, such as Cygnus X-1

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