Thursday, 19 June 2014

Competitive Examination Notes - 007

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 The Sun - 02
At this average distance of 150 million kilometers, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, nad the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th Centuary, prominent scientists had little knowledge of the Sun's physical composition and source of energy. This understanding is still developing.

While the temperature of the Sun drops from 15,000,000 K at the center to 5,800 K at the photosphere, a surprising reversal occurs above that point; the termperature drops to a minimum of 4,000 K, then begins to raise in the chromosphere, a layer about 7,000 kilometers high at a temperature of 8,000 K. During a total eclipse the chromosphere appears as a pink ring. Above the chromosphere is a dim, extended halo called the corona, which has a temperature of 1,000,000 K and reaches far past the planets.

The Core
The Sun's core has a tremendously high temperature and pressure. The temperature is roughly 15 million °C. At the temperature, nuclear fusion occurs, turning four hydrogen nuclei into a single helium nucleus plus a lot of energy. This 'hydrogen burning' releases gamma rays (high-energy photons) and neutrions (particles with no charge and almost no mass)

The Radiative Zone (or Radiation Zone)
It is the next layer which emits radiation. This radiation diffuses outwards. The temperature range from 15 million °C to 1 million °C. It may take photons of radiation millions of years to pass through the radiative zone, as they gradually make their way outwards.

The Convective Zone
In this layer, Photons continue to make their way outwards via convection (towards lower temperature and pressure). The temperature range from one million °C to 6,000 °C. 

The Photosphere
This is the lower atmosphere of the Sun and the part that we see(since it emits light at visible wavelengths). This layer is about 300 miles(500 Km) thick. The temperature is about 5,500 °C.

The Chromosphere
This reddish layer is an area of rising temperatures. The temperature range from 6,000 °C (at lower altitudes) to 5,000 °C (at higher altitudes). This layer is a few thousand miles thick. It appears red because hydrogen atoms are in an excited state and emit radiation near the red part of the visible spectrum. The Chromosphere is visible during solar eclipses.

The Corona
This is the outer layer of the Sun's atmosphere. The corona extends for millions of miles and the temperatures are tremendous, reaching one million °C.
Holes in the corona occur where the Sun's magnetic field loops out into space. These coronal holes may be the source of the solar wind, a stream of energetic particles that permeate the Solar System.

Detailed studies of the Sun's photosphere and the sunspots began with Galileo's telescopic camera obscura of the 17th centuary. Heinrich Schwabe, a german scientist in 1840 found out that the number and positions of sunspots vary over an 11-year period. In 1859 British astronomer Richard Carrington discovered solar flares. In 1908 American astronomer George Ellery Hale showed that sunspots contain magnetic fields that are thousands times stronger than Earth's magnetic field.
Next :- Space Missions to Sun

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