The ultimate fate of a star depends on its initial mass.
A massive star ends with a violent explosion called a supernova.
The matter ejected in a supernova explosion becomes a glowing supernova remnant

All you need to know about the periodic table (for the purposes of this lecture):
H = hydrogen (a nucleus containing 1 proton)
He = helium (2 protons)
C = carbon (6 protons)
O = oxygen (8 protons)
Fe = iron (26 protons) [``Fe'' stands for ``ferrum'', the Latin word for iron.]

Hydrogen is the most abundant element in our galaxy. Helium comes in second. Carbon and oxygen are neck and neck for third place. (1) The ultimate fate of a star depends on its initial mass.

M < 0.4 Msun: Too cool to fuse He to C and 0. These very low mass stars will eventually end as white dwarfs made of helium.
0.4 Msun < M < 4 Msun: Too cool to fuse C and O to heavier elements. These fairly low mass stars end as white dwarfs made of C and O.
M > 4 Msun: Hot enough to fuse C and O to heavier elements. Very massive stars (massive enough to qualify as supergiants) are able to fuse all the way to iron, the `end of the line' as far as fusion is concerned

Toward the end of its life, a massive supergiant star has a central iron core, surrounded by a shell where silicon is being fused to iron, surrounded by a shell where oxygen is being fused, surrounded by a shell where carbon is being fused, surrounded by a shell where helium is being fused, surrounded by a shell where hydrogen is being fused.

The energy generated in the center of the supergiant, where the temperature is extremely high, is carried away primarily by neutrinos, which can zip freely through the star. Although neutrinos carry energy away at a tremendous rate, the iron core is unable to replenish the energy by fusion reactions. The iron core, its energy bleeding away, undergoes a rapid collapse. Ordinary pressure can't save it from collapse. Degenerate-electron pressure can't save it from collapse. (The iron core is more massive than 1.4 Msun, the upper mass limit for a white dwarf.)

A massive star ends with a violent explosion called a supernova.In the absence of effective pressure support, the iron core collapses in less than a second. When the core reaches the density of an atomic nucleus (an amazing 400 million tons per cubic centimeter), it resists further compression and bounces back. The rebounding core sends a shock wave through the outer layers of the star, heating them up. (The heating process is helped by neutrinos, a few of which are actually absorbed at these high densities, and by turbulent convection.)

The shock-heated gas starts to expand outward at high speed (roughly 5 percent the speed of light, at first). Thus, the implosion (or rapid collapse) of the core ultimately triggers the explosion (or rapid outflow) of the star's outer layers. This explosion is what astronomers call a supernova. [The word ``nova'' is Latin for ``new''. When a star explodes, its luminosity shoots up, and may cause a previously invisible star to look like a ``new star'' (or ``nova stella'') in the sky. As long as I'm giving a Latin lesson, I should note that the plural of ``supernova'' is ``supernovae''.]

Supernovae are rare, luminous, and relatively brief events.
Rare: it's estimated that supernovae occur (on average) about once per century in our galaxy.
Luminous: the maximum luminosity of a supernova is typically between 1 billion and 4 billion times the luminosity of the Sun (even though light is only a minor byproduct of the supernova explosion).
Brief: after reaching peak luminosity, the supernova luminosity falls steadily, dropping by a factor of 100 over the course of a few months.

On July 5, AD 1054, Chinese astronomers noted the appearance of a ``guest star'' (as they called it) in the constellation Taurus. The ``guest star'' was visible in broad daylight for three weeks, and was visible at night for two years before it faded into invisibility. Plausible hypothesis: What the Chinese astronomers saw 949 years ago was a giant explosion. What we see today is the expanding debris from that explosion.

The matter ejected in a supernova becomes a glowing supernova remnant. The Crab Nebula is an example of a supernova remnant. The gas ejected in the supernova sweeps up the gas that was earlier lost in the supergiant's stellar wind, as well as scooping up interstellar gas.
The emission lines of a supernova remnant, in addition to lines from H, He, C, O, and Fe, also show lines from elements heavier than Fe (that is, containing more than 26 protons in their nuclei).

Making elements heavier than iron is difficult because it requires the addition of energy. (Going from wood to ashes is easy, because burning wood releases energy; going from ashes to wood is difficult, because it requires the addition of energy.) In fact, it seems that the high-energy shockwaves in supernovae are the only places in the universe where heavy elements are made in bulk.
Thus, gold, lead, silver, copper, uranium, and other heavy elements are forged in supernova explosions, and spread by expanding supernova remnants throughout the galaxy.

The scarcity of supernova explosions is frustrating to astronomers. No supernova has been seen in our galaxy since AD 1604 (this supernova was seen by Johannes Brahe, and hence is generally called Brahe's Supernova). The most recent naked-eye supernova was SN1987A, which appeared in the Large Magellanic Cloud, a satellite galaxy which orbits our own galaxy.

The supergiant Betelgeuse is only 160 parsecs away (only 1/12 the distance of the Crab Nebula). When it becomes a supernova, its apparent brightness will be (for a short time) brighter than the full Moon.

Types of Supernovae
There is more than one way to set off a supernova. Although supernovae are rare within our galaxy, they are sufficiently bright to be seen in very distant galaxies. These distant supernovae are classified according to their spectra. There are two basic types of supernova, called (boringly enough) ``Type I'' and ``Type II''.

Type I: supernovae WITHOUT hydrogen absorption lines in their spectrum
Type II: supernovae WITH hydrogen absorption lines in their spectrum.

Type II
The type II supernovae are massive stars whose iron cores collapse and then rebound, shock heating the outer layers of the star, which then explode outward.

Type I
The type I supernovae are subdivided into three subclasses, called (boringly enough), ``Type Ia'', ``Type Ib'', and ``Type Ic''.
Type Ia: no hydrogen lines, no helium lines, strong silicon lines
Type Ib: no hydrogen lines, strong helium lines
Type Ic: no hydrogen lines, no helium lines, no silicon lines

Type Ib and type Ic supernovae are massive stars which lost their outer layers in a stellar wind before core collapse.
Type Ib supernovae lost their hydrogen-rich outer layer, revealing the helium-rich layer immediately below.
Type Ic supernovae suffered more mass loss as supergiants, losing both the hydrogen-rich layer and the helium-rich layer (revealing the carbon-rich layer below).
Type Ib and type Ic supernovae are essentially the same as type II supernovae.
In all these types, the iron core of a massive star collapses and rebounds; the differences in the spectra of type Ib, type Ic, and type II supernovae are due to superficial differences in the exploding stars.

Type Ia
A Type Ia supernova is caused by the transfer of matter onto a white dwarf.
Type Ia supernovae, however, are a very different species of beast, arriving at their explosive end by a different life path.

Ordinarily, low mass stars (those with initial masses less than 4 Msun) don't explode. Instead, they lose enough mass when they are bloated giant stars to become stable white dwarfs with M < 1.4 Msun. The life of a star is not always ``ordinary''. Consider a white dwarf made of carbon and oxygen. (This is the end state for stars whose mass on the main sequence is between 0.4 Msun and 4 Msun.) When such a white dwarf is solitary, it leads a boringly stable existence. But what if a white dwarf were to have matter poured onto it, bringing its mass up to the Chandrasekhar limit of 1.4 Msun? (2) Matter can be transferred between stars in a close binary system. One way of pouring large quantities of matter onto a white dwarf is to place it in a close binary system. In most binary systems, the stars are well separated. (In the Sirius system, for instance, Sirius B and Sirius A are separated by 20 A.U., on average; that's about 4000 times the radius of the Sun.) However, in some systems, the stars are much closer together.

If a white dwarf is in a close binary system with a main sequence star, the main sequence star, as it expands into a giant or supergiant, will start to dump gas onto the white dwarf. When the mass of the white dwarf is nudged up to the Chandrasekhar limit, it is no longer stable against collapse.

radius decreases.
Density increases.
Temperature increases.
Fusion Bomb

At the new higher density and temperature, the fusion of carbon and oxygen into iron occurs in a runaway fashion. The white dwarf is converted into a fusion bomb, and is blown completely apart by the explosion. (This represents a triumph of the outward force of pressure over the inward force of gravity.) The amount of energy released in the explosion is about 1044 joules, as much energy as the Sun has radiated away during its entire lifetime. Called a novae

Spectrum of Ia
The spectrum of a type Ia supernova contains no hydrogen or helium lines because the white dwarf that is blown apart consists of carbon and oxygen. (The gas dumped onto it by its stellar companion is likely to be hydrogen and helium, but the strong gravity at the white dwarf's surface compresses it to densities and temperatures high enough to fuse it into carbon and oxygen.)

The spectrum of a type Ia supernova contains silicon lines because silicon is one of the products of fusing carbon and oxygen. However, the main product of the fusion is iron: a type Ia supernova ejects about 1 Msun of iron into the interstellar medium. The reason why iron is such a common metal (making up most of the Earth's core, for instance) is that type Ia supernovae keep dumping it into the interstellar gas.

Compare/Contrast
Type Ia: a white dwarf in a close binary system (the white dwarf might be very old -- up to 10 billion years)
Type II: a massive supergiant star (the supergiant must be very young -- as young as 1 million years (Remember, type Ib and type Ic supernovae are very similar to type II supernovae.) Progenitor of the supernova

Source of energy:
Type Ia: nuclear fusion (carbon and oxygen to iron)
Type II: gravity (collapse of the iron core)

What's left over :
Type Ia: a gaseous supernova remnant, very rich in iron
Type II: a gaseous supernova remnant, containing elements heavier than iron. In addition, a Type II supernova leaves behind a compressed stellar core, which is now a neutron star or black hole