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what are supernova and planetary nebulae?

Supernova

A supernova is an explosion of a massive supergiant star. It may shine with the brightness of 10 billion suns! The total energy output may be 10⁴⁴ joules, as much as the total output of the sun during its 10 billion year lifetime. The likely scenario is that fusion proceeds to build up a core of iron. The “iron group”of elements around mass number A=60 are the most tightly bound nuclei, so no more energy can be gotten from nuclear fusion.

In fact, either the fission or fusion of iron group elements will absorb a dramatic amount of energy - like the film of a nuclear explosion run in reverse. If the temperature increase from gravitational collapse rises high enough to fuse iron, the almost instantaneous absorption of energy will cause a rapid collapse to reheat and restart the process. Out of control, the process can apparently occur on the order of seconds after a star lifetime of millions of years. Electrons and protons fuse into neutrons, sending out huge numbers of neutrinos. The outer layers will be opaque to neutrinos, so the neutrino shock wave will carry matter with it in a cataclysmic explosion.

On average, a supernova will occur about once every 50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe.

Exactly how a star dies depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova (though the news for Earth still isn't good, because once the sun runs out of its nuclear fuel, perhaps in a couple billion years, it will swell into a red giant that will likely vaporize our world, before gradually cooling into a white dwarf).

A star can go supernova in one of two ways:

• Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.

• Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.

Type I:

Type 1 supernovas lack a hydrogen signature in their light spectra.

Type Ia supernovae are generally thought to originate from white dwarf stars in a close binary system. As the gas of the companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst.

Astronomers use Type 1a supernovas as "standard candles" to measure cosmic distances because all are thought to blaze with equal brightness at their peaks.

Type 1b and 1c supernovas also undergo core-collapse just as Type II supernovas do, but they have lost most of their outer hydrogen envelopes.

Recent studies have found that supernovas vibrate like giant speakers and emit an audible hum before exploding.

Type II:

For a star to explode as a Type II supernova, it must be at several times more massive than the sun (estimates run from eight to 15 solar masses). Like the sun, it will eventually run out of hydrogen and then helium fuel at its core. However, it will have enough mass and pressure to fuse carbon. Here's what happens next:

• Gradually heavier elements build up at the center, and it becomes layered like an onion, with elements becoming lighter towards the outside of the star.

• Once the star's core surpasses a certain mass (the Chandrasekhar limit), the star begins to implode (for this reason, these supernovas are also known as core-collapse supernovas).

• The core heats up and becomes denser.

• Eventually the implosion bounces back off the core, expelling the stellar material into space ? the supernova.

What's left is an ultradense object called a neutron star.

There are sub-categories of Type II supernovas, classified based on their light curves. The light of Type II-L supernovas declines steadily after the explosion, while Type II-P's light stays steady for a time before diminishing. Both types have the signature of hydrogen in their spectra.

Stars much more massive than the sun (around 20 to 30 solar masses) might not explode as a supernova, astronomers think. Instead they collapse to form black holes.

A planetary nebula is a beautiful object created during the final stages of the life of a star whose birth mass was between 1 and 8 solar masses. The wispy, colorful halo of gas making up the nebula and surrounding the dying star is actually material that was originally part of the star itself but has been cast off and is expanding outward into interstellar space. It glows as the result of being heated by the ultraviolet radiation produced by the dying star. The word planetary is really misleading, as these objects have nothing to do with the planets in our solar system. Rather, they acquired the name because when they were first observed in the 19th century their extended appearance (versus the point-like image of a normal star) reminded astronomers of the way planets like Uranus and Neptune appear in a telescope. About 1,500 are known to exist in the Milky Way Galaxy. Most of them are concentrated toward the plane of the Milky Way's disk, but a few are also know to exist in the halo and a number have been identified in the bulge of the galaxy as well.

Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium).

In other galaxies, planetary nebulae may be the only objects observable enough to yield useful information about chemical abundances.