Download NOVAE and SUPERNOVAE

Stellar Explosions
 Throughout history “new stars" have suddenly appeared in the night sky,
then faded away. These objects were termed novae. Today, astronomers
have determined that these new stars are actually stellar explosions. Nova
explosions occur on the  surface  of white dwarfs, while supernova
involve the  disruption  of entire stars.
Novae:
 Physically, novae are thermonuclear detonations on the surface of white
dwarfs (WD). Fusion reactions occur on the “outside” of the otherwise dead
white dwarf, instead of in the core, as in normal stars.
 Hydrogen fuses into helium within a thin shell, releasing enormous amounts
of energy.
 Novae are caused by the transfer of matter onto a white dwarf within a
binary system from a companion star, which has overfilled its Roche lobe.
 This transferred matter first orbits the WD in an accretion disk, a flattened,
rotating structure surrounding the white dwarf.
 Matter gradually spirals inward and falls onto the WD's surface. Fresh nuclear
“fuel” is dumped onto an extremely hot, extremely dense object, which, after a
period of time ignites.
 The nova explosion causes the WD to increase in brightness by 10,000 times
or more. From our perspective a new star shines in the sky.
 Novae are repeat phenomena; WDs may explode many times. The
explosion cycle is regulated by the rate at which matter is deposited onto the
WD. More violent (and luminous) novae occur less frequently.
 Novae can ONLY occur in close binary star systems, where the possibility for
mass transfer exists. Single white dwarfs do not become novae; they slowly
cool over billions of years, becoming a dead stellar “ember”.
 During a nova, some of the accreted matter is ejected into space from the
white dwarf. This material expands outward, becoming visible as a clumpy
nebula, for a period of years. Some of the newly fused helium remains on
the surface of the white dwarf.
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However, a WD cannot collect more than 1.4 M, the so-called
Chandrasekhar limit. This limit was discovered by Subraman
Chandrasekhar, one of the most distinguished theoretical astrophysicists of
the 20th century.
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If a WD exceeds this limit, gravity wins the battle against the pressure
produced by the degenerate electrons within the WD. The WD cannot
survive.
Supernovae:
 Supernovae define a fundamentally different event – here, the entire star is
disrupted in a vastly more energetic explosion.
 Two types of supernovae are have been identified by astronomers, based on
a completely different set of circumstances. Oddly, each type produces about
the same explosion energy.
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Type I supernova:
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 Occur when a white dwarf accretes enough mass to exceed the
Chandrasekhar limit.
 The entire star collapses, the interior temperature rises to hundreds of
millions of degrees, fusion reactions are ignited everywhere within the
star. Fusion occurs through all stages right up to the production of iron.
 Nothing remains behind. The remnants of the star are blasted into
space.
Type II supernovae:
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 Occur only in stars whose masses are greater than 8 M.
 At the end of its life, massive stars form an iron core by fusing silicon.
The iron core forms in a few days. Fusion ends at this point. The core has
a mass of about 2 M.
 The iron core cannot support itself and collapses, from a size of several
thousand kilometers to a few kilometers.
 The rest of the star, which contains most of the star’s mass, implodes
onto the core in a matter of seconds.
 The incredible energies produced by this collapse and subsequent
“bounce", explodes the rest of the star into space.
 The core remains as a tiny (10-20 km), extremely dense object known as
a neutron star.
Ejected layers of stars that explode as supernova, are termed supernova
remnants. These nebulae become visible as extremely hot wisps of gas
expanding outward from the explosion site. Some of the energy of the
explosion goes into heating the interstellar medium.
The expanding remains of Type I supernova should contain no hydrogen, a
prediction met by observations of the spectra of such remnants. These
remnants will contain about 1.4 solar masses of iron and nickel.
Type II supernova remnants contain a great deal of hydrogen – the ejected
outer layers of the recently deceased massive star. They also contain
processed gas – the inner layers of the star which experienced fusion
reactions. This expanding hot gas will be composed of newly created
elements; helium, oxygen, silicon, sulfur, neon, and iron.
Supernovae are studied via their light curves. A light curve measures the
decline in brightness with time as the remnant expands and cools.
Supernovae are extraordinarily luminous, and can be observed across
most of the universe. A supernova occurring 2 parsecs from Earth would
have an absolute magnitude of –21.5, which is about 5000X brighter than the
Moon, and about 100X fainter than the Sun.