Information taken from Wikipedia and Books by Stephen and Lucy Hawking
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A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy release by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet. T Tauri Star A T Tauri star is stage in a star’s formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they’re about the same temperature but brighter because they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years. Supergiant Stars The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process. Goldilock’s Zone Our Milky Way contains at least 100 billion rocky planets. Our Sun has four : namely Mercury, Venus, Earth and Mars – but only Earth has life. What makes Earth special? The answer is water, especially in liquid form. Water is the great mixer for chemicals, breaking the apart, spreading them out and bringing the back together as new biological building blocks, such as proteins and DNA. Without water, life seems unlikely. To support life, a planet’s temperature must be between zero and 100 degrees Celsius to keep water in liquid form. A planet orbiting too close to its home star will receive so much light energy that it will heat up to scorching temperatures, boiling all the water into steam. Planets too far from their star will receive very little light energy, keeping the planet, keeping the planet so cold that any water will remain ice. Indeed, Mars has its water trapped as ice at the north and south poles. There is a certain distance from every star where a planet receives as much light as it emits heat. That energy balance serves as a thermostat, keeping the temperature lukewarm – just right to keep the water liquid in lakes and oceans. In this ‘Goldilocks Zone’ around a star, any planet would stay war and bathed in water for millions of years, allowing the chemistry to flourish. 
Alpha Centauri At just four light years away, Alpha Centauri is the closest star system to our Sun. In the night sky looks like just one star, but is in fact a triplet. Two Sun – like stars, Alpha Centauri A and Alpha Centauri B – separated but around 23 times the distance between the Earth and the Sun – orbit a common centre about once every 80 years. There is a third, fainter star in the system, Proxima Centauri, which orbits the other two but at a huge distance from them. Proxima is the nearest of the three of us. Alpha A is a yellow star and very similar to our Sun but brighter and slightly more massive. Alpha B is an orange star, slightly cooler than our Sun and a bit less massive. It is thought that the Alpha Centauri system formed around 1000 million years before our Solar System. Both Alpha A and Alpha B are stable stars, like our Sun, and like our Sun may have been born surrounded by dusty planet – forming discs. Alpha A and Alpha B are binary stars. This means that if you were standing on a planet orbiting one of the planets orbiting one of them, at certain times you can see two suns in the sky. In 2008 scientists suggested that planets ay have been formed around one or both of the stars. From a telescope in Chile they are now monitoring Alpha Centauri very carefully to see whether small wobbles in starlight will show us planets in orbit in our nearest star system. Astronomers are looking at Alpha Centauri B to see whether this bright, calm star will reveal Earth – like worlds around it. Alpha Centauri can be seen from Earth’s Southern Hemisphere, where it is one of the stars of the Centaurus constellation. Its proper name – Rigel Kentaurus – means ‘centaur’s foot’. Alpha Centauri is its Bayer designation (a system of star – naming introduced by astronomer Johann Bayer in 1603). 55 Cancri 55 Cancri is a star system 41 light years away from us in the direction of the Cancri constellation. It is a binary system : 55 Cancri A is a yellow star and 55 Cancri B is a smaller, red dwarf star. These two stars orbit each other at 1000 times the distance between the Earth and the Sun! On 6 November 2007 astronomers discovered a record – breaking fifth planet in orbit around Cancri B. This makes it the only star system other than our Sun known to have as many as five planet! The first planet around Cancri A was discovered in 1996. Named Cancri b, it is the size of Jupiter and orbits close to the star. In 2002 two more planets (Cancri c and d) were discovered; in 2004 a fourth planet, Cancri e, which is the size of Neptune and takes just three days to orbit Cancri A. This planet would be scorchingly hot, with surface temperature up to 1500 degrees Celsius! The fifth planet, Cancri f, is around half the mass of Saturn and lies in the habitable zone (Goldilocks Zone) zone of its star. This planet is a giant ball of gas – mostly made of Helium and Hydrogen, like Saturn in our solar system. But there may be moons in orbit around Cancri f or rocky planets within Cancri’s Goldilocks Zone where liquid water could exist on the surface! Cancri f orbits its star at a distance of 0.781 Astronomical Unit. An AU is the measure of the distance that astronomers use to talk about orbits and distance from stars. One Au = 93 million million miles, which is the average distance from the Earth to the Sun. Given that there is life on Earth and liquid water on the surface of our planet, we can say that one AU or 93 million million miles from our Sun is within the habitable zone of our Solar System. So for stars of roughly the mass, age and luminosity of our Sun, we can guess that a planet orbiting its star at around one AU might be in the Goldilocks Zone. Cancri A is an older and dimer star than our Sun, and astronomers calculate that its habitable zone lies between 0.5 AU and 2 AUs away from it, which puts Cancri f in a good position! It is very difficult to spot multiple planets around a star because each planet produces its own stellar wobble. To find more than one planet, astronomers need to be able to spot wobbles within wobbles! Astronomers in California have been monitoring 55 Cancri for over 20 years to make these discoveries! Constellations Quasars A quasar (or Quasi-Stellar Radio Source) occurs when gas near a supermassive black hole at the centre of a distant galaxy goes into the black hole (at very high speed), but electromagnetic forces cause it to swirl around above the hole and blast off into space in the form of huge jets of energy. When the gas gets close to the black hole, the gas heats up because of friction. Therefore, the gas glows very brightly, and this light is visible on the other side of the Universe. It is often brighter than the whole galaxy that quasar is in. The first quasars were discovered with radio telescopes in the late 1950s and are still actively studied by astronomers today. Astronomers now think that when a galaxy has a quasar, the quasar changes the galaxy. Gas and dust from the galaxy falls onto the quasar, and the bright quasar heats up gas in the galaxy. This stops new stars from forming in the galaxy, so many of the elliptical galaxies we see in the universe now may have once had a quasar in their centers. When the gas and dust stop falling onto the quasar and firîng out, it stops being so bright and the black hole becomes very hard to see. 
Redshifts Red shift is a way astronomers use to tell the distance of any object that is very far away in the Universe. The red shift is one example of the Doppler effect. The easiest way to experience the Doppler effect is to listen to a moving train. As the train moves towards a person, the sound it makes as it comes towards them sounds like it has a higher tone, since the frequency of the sound is squeezed together a little bit. As the train speeds away, the sound gets stretched out, and sounds lower in tone. The same happens with light when an object that emits light moves very fast. An object, like a star or a galaxy that is far away and moving toward us, will look more blue than it normally does. This is called blue shift. A star or galaxy moving away from us will look more red than it should, which is where red shift got its name, since the colors are shifted red. The reason astronomers can tell how far the light gets shifted is because certain chemical elements, like the calcium in bones or the oxygen people breathe has a unique fingerprint of light that no other chemical element has. They can see what colors of light are coming from a star, and see what it is made of. Once they know that, they check to see the difference between where the fingerprint, called spectral lines, are actually at, and then look at where they are supposed to be. When they see that, they can tell how far away the star is, whether it is moving toward us or away from us, and also how fast it is going, since the faster it goes, the farther the distance the spectral lines are from where they should be. Red shift is important because astronomers used it to figure out that the Universe is expanding. Pulsars Pulsars are neutron stars that turn quickly and produce electromagnetic radiation that can be received in the form of radio waves. The strength of radiation changes according to a regular period of time, which is thought to match to the period of time in which the star turns. Pulsars also show a so-called lighthouse effect, which occurs when the light and other radiation from a pulsar are only seen at certain periods of time and not all of the time. Werner Becker of the Max-Planck-Institut für extraterrestrische Physik recently said, "The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work.. There are many models but no accepted theory." In other words, scientists are still just beginning to understand pulsars and they do not all agree on how pulsars work. The first pulsar was discovered in 1967, by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge, UK. At first, they did not understand why pulsars have a regular change in the strength of radiation, they called their discovery LGM-1, for ":little green men"; their pulsar was later called CP 1919, and is now known by a number of names including PSR 1919+21. The word pulsar is short for "pulsating star", and was first seen written in 1968. 
A Pulsar Nebula A nebula, which comes from the Latin word for mist or cloud, is an interstellar cloud of dust, hydrogen, helium, and other gases. An interstellar cloud is dust, plasma, or ionized gas in a galaxy. The Persian astronomer, Abd al-Rahman al-Sufi, mentioned a true nebula for the first time in his book, Book of Fixed Stars (964). He said that there was a "little cloud" near the Andromeda Galaxy A nebula is usually made up of hydrogen gas and plasma. It may be the first stage of a star's cycle, but it may also be one of the last stages. Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium or ISM. As the material collapses contracts, massive stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, making it visible at optical wavelengths. Examples of these types of nebulae are the Rosette Nebula and the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas. These are sites where star formation occurs. The formed stars are sometimes known as a young, loose cluster. Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labelled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and is a neutron star. Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, like Earth's Sun. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The nebula is 97% Hydrogen and 3% Helium with trace materials. In the past galaxies and star clusters were also called 'nebulae'. Types of nebulae Nebulae can be sorted by why we can see them. Emission nebulae Emission nebulae make their own light. Usually the gases in an emission nebula are ionized. This makes them glow. Emission nebulae are usually red because they usually produce red light. Reflection nebulae Reflection nebulae reflect light from nearby stars. Dark nebulae Dark nebulae do not emit light or reflect light. They block the light from stars that are far away. 
Unit 2 : Our Solar System Our Solar System Comets Asteroids Asteroids are small Solar System bodies or dwarf planets that are not comets. The term asteroids historically referred to objects inside the orbit of Jupiter. They have also been called planetoids, especially the larger ones. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets, and so were often distinguished from traditional asteroids. Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System within the orbit of Jupiter, which are usually rocky or metallic. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as minor planets, which is the term preferred in astronomical circles. In this article the term "asteroid" refers to the minor planets of the inner Solar System. There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich,stony, and metallic compositions, respectively. The first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer Sir William Herschel to propose the term "asteroid", coined in Greek as ἀστεροειδής asteroeidēs'star-like, star-shaped', from Ancient Greek ἀστήρ astēr 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably; for example, the Annual of Scientific Discovery for 1871, page 316, reads "Professor J. Watson has been awarded by the Paris Academy of Sciences, the astronomical prize, Lalande foundation, for the discovery of eight new asteroids in one year. The planet Lydia, discovered by M. Borelly at the Marseilles Observatory had previously discovered two planets bearing the numbers 91 and 99 in the system of asteroids revolving between Mars and Jupiter". Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross the Earth's orbital path are known as Earth-crossers. As of May 2010, 7,075 near-Earth asteroids are known and the number over one kilometre in diameter is estimated to be 500–1,000. 
Comets Kuiper Belt Before we start learning about the Kuiper Belt we need to know what is an A.U. An A.U. or an Astronomical Unit is the distance from the Sun to the Earth i.e. approximately 149,600,000 km. The Kuiper Belt is a region of the solar system beyond the planets, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While most asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. The classical belt is home to at least three dwarf planets: Pluto, Haumea, and Makemake. Some of the Solar System's moons, such as Neptune's Triton and Saturn's Phoebe, are also believed to have originated in the region. Since the belt was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 100,000 KBOs over 100 km (62 mi) in diameter are believed to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the classical belt is dynamically stable, and that comets' true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun. At its fullest extent, including its outlying regions, the Kuiper belt stretches from roughly 30 to 55 AU. However, the main body of the belt is generally accepted to extend from the 2:3 resonance (see below) at 39.5 AU to the 1:2 resonance at roughly 48 AU. The Kuiper belt is quite thick, with the main concentration extending as much as ten degrees outside the ecliptic plane and a more diffuse distribution of objects extending several times farther. Overall it more resembles a torus or doughnut than a belt. Between the 2:3 and 1:2 resonances with Neptune, at approximately 42–48 AU, the gravitational influence of Neptune is negligible, and objects can exist with their orbits essentially unmolested. This region is known as the classical Kuiper belt, and its members com Because the first modern KBO discovered, (15760) 1992 QB1, is considered the prototype of this group, classical KBOs are often referred to as cubewanos. When an object's orbital period is an exact ratio of Neptune's (a situation called a mean motion resonance), then it can become locked in a synchronised motion with Neptune and avoid being perturbed away if their relative alignments are appropriate. If, for instance, an object is in just the right kind of orbit so that it orbits the Sun two times for every three Neptune orbits, and if it reaches perihelion with Neptune a quarter of an orbit away from it, then whenever it returns to perihelion, Neptune will always be in about the same relative position as it began, since it will have completed 1½ orbits in the same time. This is known as the 2:3 (or 3:2) resonance, and it corresponds to a characteristic semi-major axis of about 39.4 AU. This 2:3 resonance is populated by about 200 known objects, including Pluto together with its moons. In recognition of this, the members of this family are known as plutinos. Many plutinos, including Pluto, have orbits which cross that of Neptune, though their resonance means they can never collide. Plutinos have high orbital eccentricities, suggesting that they are not native to their current positions but were instead thrown haphazardly into their orbits by the migrating Neptune. IAU guidelines dictate that all plutinos must, like Pluto, be named for underworld deities. The 1:2 resonance (whose objects complete half an orbit for each of Neptune's) corresponds to semi-major axes of ~47.7AU, and is sparsely populated. Its residents are sometimes referred to as twotinos. The 1:2 resonance appears to be an edge beyond which few objects are known. It is not clear whether it is actually the outer edge of the classical belt or just the beginning of a broad gap. Objects have been detected at the 2:5 resonance at roughly 55 AU, well outside the classical belt; however, predictions of a large number of bodies in classical orbits between these resonances have not been verified through observation. Earlier models of the Kuiper belt had suggested that the number of large objects would increase by a factor of two beyond 50 AU, so this sudden drastic falloff, known as the "Kuiper cliff", was completely unexpected, and its cause, to date, is unknown. In 2003, Bernstein and Trilling et al. found evidence that the rapid decline in objects of 100 km or more in radius beyond 50 AU is real, and not due to observational bias. Studies of the Kuiper belt since its discovery have generally indicated that its members are primarily composed of ices: a mixture of light hydrocarbons (such as methane), ammonia, and water ice, a composition they share with comets. The low densities observed in those KBOs whose diameter is known, (less than 1 g cm−3) is consistent with an icy makeup. The temperature of the belt is only about 50K, so many compounds that would be gaseous closer to the Sun remain solid.  
Meteors, Meteoroids and Meteorites Meteoroids are solid objects of a size considerably smaller than comets and asteroids and are made up of rocks and minerals. Meteoroids travel at a very high speed as they enter the Earth’s atmosphere. As the result of friction, they burn and can be seen as a streak of light. This streak of light looks like a shooting star (though it is not at all a star). We thus define this streak as a Meteor. Meteors generally occur in the Mesosphere from 75 to 100 km. Most meteoroids burn to ashes in a very short time, even before they reach the lower atmosphere. However, some large meteoroids do not fully burn up and fall on the Earth’s surface as solid pieces. These unburnt pieces of rocks that reach the Earth’s surface are called Meteorites. They are capable of forming craters on the surface of The Earth. Meteorites are considered to be very rare as they are made up of very rare minerals, many of which are not even found on the surface of the Earth.  
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