Physics – Topic D: Astrophysics – D.1 Stellar Quantities

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Comets
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Comets are irregular objects a few kilometres across comprising of frozen gases, rock and dust.
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Stars (Single & Binary)
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All stars initially form when gravity causes gas in a nebula to condense. As the atoms move towards one another, they lose gravitational potential energy that is converted into kinetic energy. This raises the temperature of the atoms which then form a protostar.
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Binary Stars
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Binary stars consist of two stars that rotate about a common centre of mass.
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Constellations
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A constellations is a pattern formed by stars that are in the same general direction when viewed from Earth.
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Stellar Clusters (Open)
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A stellar cluster is a group of stars that are positioned closely enough to be held together by gravity. Open clusters consist of up to several hundred stars that are younger than 10 billion years and may still contain some gas and dust.
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Stellar Clusters (Globular)
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A stellar cluster is a group of stars that are positioned closely enough to be held together by gravity. Globular clusters contain many more stars and are older than 11 billion years and, therefore, contain very little gas or dust.
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Nebulae
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Nebulae are regions of intergalactic clouds of dust and gas. All stars are “born” out of nebulae.
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Galaxies
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Galaxies are a creation of stars, gas, and dust held together by gravity and containing billions of stars.
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Galaxy Clusters
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Some galaxies exist in isolation but the majority of them exist in groups known as clusters. Regular clusters consist of a concentrated core and are spherical in shape. Irregular galaxies have no apparent shape and lower concentration of galaxies within them.
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Super Clusters of Galaxies
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Superclusters are even larger structures than galactic clusters. They form a network of sheets and filaments; approximately 90% of galaxies can be found within these.

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