The Sun (Our Star)

If the sun was generating its energy by chemical means the sun would last only about 10000 years at the present rate of energy generation. If the sun produced its energy by gravitational contraction, it would last only 25 million years. We know that the sun has been producing energy for a much longer period than above. Therefore the Sun produces its energy by means of Thermonuclear Fusion of Hydrogen in its core. We know that these reactions take place in the sun's core because, very high temperatures and pressures are required for Fusion reactions to take place, and the necessary conditions are only found in the sun's core. It is known that 600 million tons of Hydrogen gets converted to Helium in order to power the Sun. At this rate the Sun can continue to produce energy for more than 6 billion years.

The internal structure of the sun is composed of three parts,
1. The Energy Core
2. The Radioactive Zone
3. The Convective Zone.

The Energy core is where the hydrogen undergoes Fusion to produce Helium at enormous temperatures and pressures. The energy core extends to about 0.25 of the Solar radius from the centre of the sun.

The radioactive zone is the zone through which the  energy produced travels outward through  radiative diffusion. It can take a very long time( thousands or millions of years) for the energy to come out of this zone.

The convective zone is a opaque region with a relatively low temperature and energy travels mainly through convection.

*Moving outwards from the centre to the surface of the sun, The temperature decreases, the density decreases, the mass percentage increases and the luminosity increases.

A lot of information regarding the sun such as pressures, densities, chemical compositions and rotation rates can be inferred by studying the vibrations of the sun. The study of the vibrations of the sun is called Helioseismology.

The Sun's atmosphere
The sun's atmosphere has three main parts,
1 Photosphere (innermost)
2 Chromosphere (above photosphere)
3 Corona (outermost)
Everything below the atmosphere is called the Solar interior.

Photosphere
The Photosphere is the visible surface of the sun. Because of its opaqueness, photons emitted below the photosphere cannot escape. The temperature decreases outwards of the photosphere. Since when viewing the Sun, we see deeper through the photosphere than when looking at the centre of the sun, The edges of the sun appear dimmer. This is called the Limb Darkening effect. The convection currents in the photosphere gives rise to what is known as "Granules" These granules are formed in a cyclic manner and are roughly about 1000km in size.

Chrmosphere
Chromosphere means "Sphere of Colour" and it consists of gases less denser than that of the Photosphere. Since the light of the chromosphere mainly consists of the red emission line of Hydrogen, H-alpha, The chromosphere is clearly seen through a telescope equipped with a H-alpha filter. On observation of the Chromosphere, thin columns of hot gas jets can be seen. These gas jets are called "Spicules". Spicules rise from the boundaries of large granules.

Corona
The corona is the outermost of the Solar atmosphere, has a very low density and a very high temperature (millions of degrees). This low density and high temperature of the corona makes it almost invisible, however it is seen spectacularly as a bright ring in a full solar eclipse. The corona extends to several million km from the surface of the sun. Sometimes gases escape to outer space from the Sun through regions with cooler gases called "Coronal Holes" This escaping gases form what is known as the "Solar Wind".

The Sun's Magnetism
The most common type of Solar magnetic activity are Sunspots. Sunspots are relatively cooler regions in the Sun which is why they appear darker than the rest of the sun due to the lower amount of radiation emitted. Sunspots are areas of concentrated Magnetic field. This is proved using the Zeeman Effect. Occurrence of Solar Prominences and Flares are also related to Sunspots.
It has been observed that the average number of  Sunspots increase and decrease in a cycle of 11 years. with the polarity of the Sun changing every 11 years. i.e the Magnetic poles of the Sun Change every 11 years giving rise to a 22year magnetic cycle.
It is known that the differential rotation of the sun gives rise to its magnetic field and its properties. Thus the solar cycles are caused by the differential rotation of the Sun.

The Solar System

The Solar System comprises of the Sun, 8 planets, satellites of the planets and the asteroid belt. The Kuiper belt and the Oort Cloud are composed of rocks and ice and are situated beyond the Solar System. The solar System is situated in the Milkyway Galaxy on its Orion Spiral arm at a distance of about 30000 light years away from the Galactic centre. It has been found by meteorite radiometric measurements that the Solar System is about 4.6 billion years old

The planets

The planets of the Solar System are divided into two categories,

1. The Inner or Terrestrial Planets

2. The Outer or Jovian Planets

The Terrestrial Planets

The terrestrial planets are, Mercury, Venus, Earth and Mars. These planets have hard rocky surfaces and are composed of elements such as Fe Ni Al and rocky silicates.

During the formation of the Solar System the newborn sun was so hot that only materials with a high melting point could survive to form planets near the sun. The air and water seen on earth and the gases on Venus are believed to have arrived by means of asteroids bombarding into the terrestrial planets.

The "rocky" terrestrial planets have got no rings like unlike the Jovian planets, and have relatively low speeds of rotation They also have very few satellites.

The Jovian Planets

The Jovian Planets are Jupiter, Saturn, Uranus and Neptune. During the early stages of the Solar System, at a considerable distance from the sun the gases could condense to form planets. Thus the terrestrial planets were born. It is known that Jupiter formed before Saturn and therefore accumulated more gas than Saturn could. Therefore Jupiter is Larger than Saturn. These planets actually have no Surface. the atmosphere gradually gets dense as depth increases and at some time liquefy. Then gradually it turns into solid. However these transitions are not sharply defined.

The Jovian Planets have a lot of satellites and have relatively high speeds of rotation. Because of the size and the speed of Uranus and Neptune, it is believed that they were formed between Jupiter and Saturn and later on swept away to where they are now.

*Jupiter and Saturn underwent orbital changes to achieve 2:1 orbital resonance, and this caused Neptune which was formed between Jupiter and Saturn to move further away from the Sun.

Pluto was sometime back considered a planet. But now is not taken as a planet, but as a member of the Kuiper belt that resides beyond the Solar System.

Formation of the Solar System

The current theory which explains the formation of the Solar System best is the Nebulae Theory, which originated in the 18th century and has been refined since then.

According to this theory, a nebula (i.e an interstellar cloud of dust and gases) spun fast causing it to collapse to form a disk like structure. It turned into a "Protoplanetary disk" in which the centre became a hot glowing cloud of gas-the Proto-Sun. Within 100 million years the temperature and pressure in the centre of the sun became so great that it started producing energy by hydrogen fusion. Thus this fusion energy countered the gravitational contraction of the sun and brought it to a state of Hydrostatic Equilibrium in which it attained stability.

Then the small particles in the nebula cloud underwent accretion(sticking together) to form the planets.

The nebulae theory has been strengthened by the discovery of extra solar planets.

History of Astronomy

Since ancient days people used to look at the sky with a sense of wonder. The tried to explain what the saw in the sky. From the rising and setting of the sun, the twinkling of the stars, shooting stars, comets, all these caught their imagination as the tried to device theories to explain their observations. The field of Astronomy thus began.

Many ancient civilizations are known to have observed and studied the heavens. They are, the ancient Indians, The Babylonians, the Greeks, the Chinese, the Persians and the Egyptians. Modern astronomy has it roots in ancient Greek Astronomy. However Greeks borrowed many of their ideas from other civilizations.

An ancient Chinese star chart:Modern Astronomy can be classified into three major periods

1. First Period: Ancient Greek (500BC to 150AD)

2. Second Period: Ptolemy to Copernicus and Islamic Astronomy (150AD to 1545AD)

3. Third Period: Copernicus to the 20th century

First Period

During this period, The physics of Aristotle and the assumptions of Plato were highly regarded. It was believed that we live in a Geo-centric universe, i.e The Earth lies in the centre of the universe and the heavenly bodies revolve around the Earth.

Using this system there were discrepancies between theory and observation. To solve this Ptolemy brought forward the theory of epicycle and deferent.

During this time Greek astronomer Aristarchus presented a Helio-centric model of the universe but was immediately rejected.

Important Achievements during this period include, Preparation of star catalogs and the compilation of Ptolemy's "Almagest" the great mathematical and astronomical treatise.

The Geo-centric model:

Second Period

During this period the Islamic civilization took a wide ranging interest in the Sciences and produced a vast range of Scientific and technological advancements. During this period, the Greek texts including the Almagest was translated into Arabic. The added onto the existing knowledge improving the star catalogs prepared by the Greeks.

Also for the first time in history the Islamic astronomers built Observatories in order to observe the skies better.

The Arabic texts were also translated to Latin Through which many sciences were communicated during this period.

Third Period

Nicolaus Copernicus presented the helio centric system but was met by serious rejection.

Galileo Galilei constructed the first telescope and used it to observe the moon and the planets. He was the first person to see the moons of Jupiter.

Galileo stood by and supported Copernicus's ideas and faced opposition by the Catholic Church.

Kepler, by using Brahe's data and by careful observation cemented the understanding of the universe and also devised three laws governing planetary motion.

Sir Isaac Newton with his laws of motion an theory of Gravity explained planetary motion. He also contributed to the development of optics and constructed the reflective telescope.

During the times of Herschel and Caroline telescopic astronomy boomed and it marked the discovery of Uranus and Neptune.

With the development of Spectroscopy it was used in astronomy to determine the composition of the universe.

With Albert Einstein's theory of Relativity and Quantum Theory Picture we have of the universe was further clarified while uncovering more horizons to explore.

With the development of technology, increasingly powerful telescopes, like the Hubble Space telescope which is placed in earths orbit., our understanding of the universe is still expanding.