Posts Tagged ‘energetic’

Cosmic Rays

High speed particles that reach the Earth from outer space. Some low energy cosmic rays come from the Sun, but high energy cosmic rays are thought to originate from outside of the Solar System. These possibly originate from supernovae. The highest energy cosmic rays may originate in quasars.


The corona is a huge envelope of gases that form the outer portion of the Sun. It is seen as faint extensions of the Sun’s outer atmosphere during total eclipses and can be observed using a corongraph, a device that is used to create an artificial total solar eclipse.

The shape of the corona varies with the sunspot cycle. At maximum, it is much more even. At minimum, it is seen to be much more irregular with polar tufts, equatorial streamers and plumes being visible.

It has a temperature of between 1 and 2 million K and naturally at that temperature, is a plasma, for example, iron has been identified but with half of its electrons stripped away. It is indeed a very high energy environment. The corona of the Sun is the origin of the solar wind. An estimated 3 million tonnes of material is ejected from the Sun each second.

This material is experienced as the solar wind with ‘gusts’ of up to several hundred km per sec. The Earth is protected from this bombardment of charged particles by its magnetic field which shunts them into the Van Allen Belts. When these overflow at the poles, they create the aurora borealis and aurora australis.

The solar wind appears to be strongest where it flows from coronal holes. These are areas of lower temperature punched through the corona by the powerful magnetic fields of the Sun.

The Aurora

glow in the northern (aurora borealis) or the southern (aurora australis) sky caused by particles from the solar wind interacting with the atmosphere. The particles are held in the Van Allen belts. When there is a solar flare, there is a huge increase in the density of the Solar wind. This overloads the Van Allen belts and causes high energy particles to spill into the atmosphere. As these interact with the gases of the atmosphere, light of different wavelengths is emitted and we see this as an aurora.

Book: The nature of the Aurora was worked out by the Norwegian Scientist Kristian Berkeland. I was riveted by the book Northern Lights, an account of how the mysteries of the Aurora were unravelled and learn about the man who invented a weapon developed in the USA’s SDI (Star Wars) early in the twentieth century.

Accretion Disc

An accretion disc is a disc of material formed under the influence of gravity. It forms from smaller particles which are drawn towards the central body by the influence of Gravity.

The central body is generally a star although it can also be the nucleus of a galaxy or a black hole.

Also the rotating disc of material formed around a Black Hole.

The force of gravity is also reponsible for the emission of electromagnetic radiation from the disc. Gravity compresses the particles of the disc, heating them and giving rise to the radiation. The radiation wavelength depends on the strength of the gravitational force. Accretion discs around a black hole for instance can emit X-rays as the gravitational forces are immense. Accretion discs around forming or newly formed stars will only emit infrared as the gravitational forces are less, leading to a lower level of energy and therfore longer wavelengths of radiation. The Hubble Space Telescope is alleged to have seen and measured an accretion disc around a black hole using gravitational lensing to work out the colour profile.

The accretion discs around black holes and quasars are probably the most efficient way of generating energy from matter known, with about 10% matter to energy conversion. It is thought that accretion discs could be the source of the Gamma Ray Bursts seen from time to time in the universe. The Eddington Luminosity of Eddington Limit defines the point at which the outflow of energy from a star exactly balances the inward pull of gravity i.e. it is stable. Super eddington accretion discs are thought to be the source of the gamma ray bursters. Turbulence can cause material to fall inwards which is then converted into high energy radiation. These are very short lived events

Accretion discs round young stars supply the material for planets to form.

Accretion discs are also found in binary star systems. It happens when the two stars are of unequal ages and sizes and close together – the younger one will have lived out its life and become a white dwarf or even neutron star, then, when the older one reaches the end of its main sequence life, it expands. It it expands sufficiently, the outer envelope of gases can be more strongly attracted by the gravity of the smaller star and forms an accretion disc around the other.

The physics of an accretion disc is way beyond what we intend to explain here, however, if you want a detailed explanation, click here.

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