Topics: Star, Stellar evolution, White dwarf Pages: 6 (938 words) Published: April 25, 2013
Al Nahda National Schools (Boys) Class: 12G, H & I Teacher: Nidal Al Halabi life cycle of a star

Physics The life cycle of a star

 The Life of a small Star, a star of mass up to eight times that of the sun(< 8M o) or has a core remnant of mass up to one and a half times that of the sun(< 1.5M o), can be summarized in the following stages: Stage 1- Stars are born in a region of high density Nebula. Stage 2 – Under its own gravitational force, interstellar clouds of dust and gases condenses into a huge globule of gas and dust to form a glowing Protostar. If there is not enough material in the protostar, one possible outcome is a large, notvery-luminous celestial body called brown dwarf. Stage 3–If a protostar contains enough matter the central temperature reaches 15 million ˚C. At this temperature, Hydrogen nuclei start to fuse together to form Helium and the star begins to release energy. This energy radiation production prevents further contraction of the star. It is now Main Sequence Star so it settles to an almost constant luminosity and temperature. The nearest main sequence star to Earth is the Sun. A star of one solar mass remains in main sequence for about 10 billion years, until all of the hydrogen has fused to form helium. Large main sequence stars have large gravitational forces acting on them. They therefore have a large inwards pull. They have higher temperatures so they burn hydrogen very quickly resulting in shorter stay as a main sequence star. Stage 4 - When the Hydrogen in the core runs out, the fusing of helium to form carbon starts in the core. This releases a huge amount of energy which causes the outer layers of star to expand and become a Red giant. Stage 5 - When the Helium in the core runs out, the core will collapse to form a white dwarf and the outer layers drift of away from the core as a gaseous shell; this gas that surrounds the core is called a Planetary Nebula. Stage 6 - As time continues, a white dwarf will eventually cool and dims. When it stops shining, the now dead star is called a Black Dwarf.

1 Astrophysics & Cosmology Done by: Nidal Al Halabi

 The Life of a massive Star, a star of mass more than eight times that of the sun(> 8M o) or has a core remnant of mass more than one and a half times that of the sun(> 1.5Mo): o A massive star begins life as a blue supergiant and the fusion process happens at high temperature, so the star can keep fusing elements until its core is made up of iron. o When the core runs out of fuel, the star undergoes a rapid collapse under enormous gravitational forces. This sudden increase in density produces a shock wave that blows outer layers away causing a supernova. The natural occurrence of elements above iron in the periodic table is the evidence that supernova must have occurred in the past because to fuse above iron-56 would involve a drop in binding energy per nucleon and so would need energy to put in and such energy is available in a supernova explosion. o After a supernova, the remnant core will either be a neutron star (if the core is up to 3Mo) or a black hole (if the core is more than 3Mo).Both are highly dense, small and aren’t easy to detect as they don’t emit light.

Nebula: large interstellar clouds of dust and gases mostly hydrogen. Protostar: a huge globule of Hydrogen and dust Brown dwarf: a "star" whose mass is too small to have nuclear fusion occur at its core. Main sequence star: a star fusing hydrogen to helium in the core. Red giant: a cool large surface area star. Planetary Nebula: a gaseous shell ejected from the explosion of a red giant star. White dwarf: a hot small surface area star that does not have nuclear reactions. Supernova: a star which suddenly becomes very bright. Astrophysics & Cosmology Done by: Nidal Al Halabi 2

Hertzsprung-Russell diagram: Astronomers have classified stars into groups according to their temperature:

Spectral Class

Effective or surface Temperature (K)


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