ocr 5.5 Astro
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| Astronomical Unit | The average distance between the Earth and Sun (1.5x1011m)
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| Big Bang theory | The universe was created from a single point where all the universe current mass and energy was situated. At the time of creation the universe was much smaller, hotter, denser than it is now. Time and space were created at the instant of the big bang.
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| Black Hole | The last stage in the life cycle of a massive star that has a gravitational field so strong even light cannot escape.
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| Chandrasekhar limit | The maximum possible mass of a stable white dwarf star. It is equal to 1.4 x mass of our sun.Stars with masses above this will collapse further to become neutron stars or black holes
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| Closed Universe | A universe that has no spatial edges. Expansion will halt and the universe will contract to a point.
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| Comet | A relatively small astronomical body of dust and ice that orbits the sun in an eccentric elliptical orbit and has a characteristic tail
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| Continuous emission spectrum | A spectrum containing all the colours(photons) of visible light.Note this is produced by the surface of the star. The gases around the star then absorbed particular wavelengths that leads to us observing an absorption spectrum from distant stars.
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| Cosmic Microwave Background Radiation | A form of electromagnetic radiation that pervades the universe of wavelength 1.9mm and effective temperature 2.7K. This is considered a piece of evidence for the Big Bang, initially starting as gamma rays and as space has expanded it has been stretched to
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| Cosmological Principle | On a sufficiently large scale the universe is homogeneous (has a uniform composition) and isotropic (same in all directions)
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| Critical Density | The density of matter in the universe required to just bring the expansion of the universe to a halt resulting in a flat universe
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| Dark energy | A type of energy that permeates the whole universe and opposes the attractive force of gravitation between galaxies vis the exertion of negative pressure. It is not detected directly but we know it exits because we now know that the universe is accelerati
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| Dark matter | Matter that cannot be seen and that does not emit or absorb electromagnetic radiation. It is not detected directly but is detected indirectly based on its gravitational effects relating to either the rotation of galaxies or by gravitational lensing of sta
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| Flat Universe | A universe in which the expansion tends towards a finite limit but never reaches that limit
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| Galaxy | A gravitationally bound system of billions of stars and interstellar dust
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| Gravitational collapse | Inward movement of material in a star due to the gravitational force caused by its own mass. Start formation is due to the gradual gravitational collapse of a cloud of dust and gas. Gravitational collapse happens in mature stars when the internal gas and
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| Hubble’s Constant | The constant of proportionality linking recession velocity and distance of galaxies. It may be used to calculate the age of the universe and critical density
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| Hubble’s Law | The recession velocity of galaxies is directly proportional to the distance of the galaxies from a fixed point
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| Interstellar Medium | Low density dust and gas that pervades the space between stars
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| Light Year | The distance light travels in a vacuum over a year 9.46x1015m
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| Line absorption spectrum | a series of dark lines on a continuous spectrum. The dark lines represent the wavelengths that are absorbed.
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| Line emission spectrum | a series of bright coloured lines on a black background.
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| Line emission spectrum/Spectral line (fact) | Is evidence for discrete energy levels in atoms
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| Luminosity | Of a star is the total power or energy emitted per second
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| Main sequence star | A star in the main part of its life cycle, where it is fusing hydrogen into helium in its core.
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| Neutron Star | The core remnant of a massive star after a supernova. It has a high density and is composed entirely of neutrons
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| Open Universe | A universe in which the expansion is followed by contraction back to a single point (a big crunch)
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| Parsec | The distance to a star which will produce a parallax of 1 arc second over 6 months. The base of the triangle is 1AU long. (3.1x1016m)
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| Planet | An astronomical body made of rock or gas, massive enough to be rounded by it’s own gravity but not massive enough for fusion to start
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| Planetary Satellite | A body in orbit round a planet
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| Radiation pressure | Outward pressure due to the momentum of photons released in fusion reactions: acts in the direction of energy flow
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| Red Giant | A giant, relatively cool star formed when a main sequence star has exhausted hydrogen in its core and fusion continues in a shell
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| Red Shift | The apparent shift in wavelength of absorption lines towards the red end of the spectrum caused by the relative motion of light source and observer
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| Red super giant | A star that has exhausted all the hydrogen in its core and has a mass much greater than the Sun.
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| Star | A massive astronomical body composed of plasma and held together by gravitational forces. For most of their lives nuclear fusion generates energy
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| Stefan's Law | This relates the Luminosity L of a star with its absolute temperature T
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| Stellar Parallax | The apparent shifting a star viewed against a background a distance stars when viewed from different positions on the Earth e.g. different points on the Earth's orbit
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| Supernova | Extremely luminous explosions that remove the outer layers of stars leaving a core remnant
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| Wein's displacement law | States that λ_max∝ 1/T or λ_max T=constant (2.89x〖10〗^(−3) mK)Where T is the temperature on the absolute (kelvin) scale. It is used to estimate the peak surface temperature of a star from the wavelength at which the star's brightness is a maximum
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| White Dwarf | A late stage in the life of low mass stars. White dwarves have low mass but high density and temperature.
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