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Vela Supernova Remnant

Vela Supernova Remnant
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McNaught comet

McNaught comet

FRISCO is a private Observatory involved in the field of research in supernovae and cometary objects.

Research

Supernovae 

Supernovae (the plural of supernova) are extremely important for understanding our Galaxy and the entire Univers. They heat up the interstellar medium, distribute heavy elements throughout the Galaxy, and accelerate cosmic rays. But just what is a supernova? And is there more than one type?

Indeed, there seems to be two distinct types of supernovae -- those which occur for a single massive star and those which occur because of mass transfer onto a white dwarf in a binary system. As you will see, however, it is only what gets the process started toward the explosion which differs between the two types.

Supernovae history
Supernovae history, from 1934 to 1990
Supernovae light curves
Supernovae light curves

Supernovae from Single, Massive Stars

 Stars which are 8 times or more massive than our Sun end their lives in a most spectacular way; they go supernova. A supernova explosion will occur when there is no longer enough fuel for the fusion process in the core of the star to create an outward pressure which combats the inward gravitational pull of the star's great mass. First, the star will swell into a red supergiant...at least on the outside.

On the inside, the core yields to gravity and begins shrinking. As it shrinks, it grows hotter and denser. A new series of nuclear reactions begin to occur....temporarily halting the collapse of the core... but alas, it is only temporary. When the core contains essentially just iron, it has nothing left to fuse (because of iron's nuclear structure, it does not permit its atoms to fuse into heavier elements). Fusion in the core ceases. In less than a second, the star begins the final phase of gravitational collapse. The core temperature rises to over 100 billion degrees as the iron atoms are crushed together. The repulsive force between the nuclei overcomes the force of gravity. So the core compresses, but then recoils. 

The energy of the recoil is transferred to the envelope of the star, which then expodes and produces a shock wave. As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes. The shock then propels the matter out into space. The material that is exploded away from the star is now known as a supernova remnant.

All that remains of the original star is a small, super-dense core composed almost entirely of neutrons -- a neutron star. Or, if the original star was very massive indeed (say 15 or more times the mass of our Sun), even the neutrons cannot survive the core collapse...and a black hole forms.

The hot material given off by the supernova, the radioactive isotopes, and the free electrons moving in the strong magnetic field of the neutron star... all of these things produce X-rays and gamma rays. These high energy photons are used by astrophysicists studying the phenomena of neutron stars and supernovae.

A White Dwarf Goes Thermonuclear

Another type of supernova involves the sudden explosion of a white dwarf star in a binary star system. A white dwarf is the endpoint for stars of up about 5 times that of the Sun. The remaining white dwarf has a mass less than 1.4 times the mass of the Sun, and is about the size of the Earth.

A white dwarf star in a binary star system will draw material off its companion star if they are close to each other. This is due to the strong gravitational pull of an object as dense as a white dwarf.

Should the in-falling matter from the companion star cause the white dwarf to approach a mass of 1.4 times that of the Sun (a mass called the Chandrasekhar limit after the scientist who discovered it), the pressure at the center will exceed the threshold for the carbon and oxygen nuclei to start to fuse uncontrollably. This results in a thermonuclear detonation of the entire star. Nothing is left behind, except whatever elements were left over from the white dwarf or forged in the supernova blast. Among the new elements is radioactive nickel, which liberates huge amounts of energy, including visible light. The evolution of these supernovae tend to all be similar.


Comets

Comets are small bodies made out of dust and ices ("dirty snowballs"). The term "comet" derives from the Greek aster kometes, which means "long-haired star" as reference to the tail.

Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

Comets belt
Comets belt

Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.

As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.

When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.

As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.

The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun.