S.4 NOTES GALAXIES.pdf

TOPIC: STARS AND GALAXIES 1 Assignment The sky is clear at night with some bright twinkling objects in it. These objects appear as fixed points of light due to their great distance from the earth, but in real sense they are large and are in motion. A galaxy is a collection of a large number of stars that are held together by the force of gravity. Unit Abbreviation What it is used to measure Light year Ly A light year is a unit of astronomical distance equivalent to the distance light travels in one year. 1 ly = 3.0x108365.25x24x3600 = 9.5x1015 m Astronomical year Au This is a unit of length roughly the distance from the Earth to the sun, about 150 million kilometres Solar mass - The mass of a star is measured in terms of the mass the sun, with one solar mass unit being equal to the mass of sun. eg a star whose mass is twice that of the sun is said to have two solar masses. Solar radius - The radius of stars is also measured with respect to the radius of the sun, with one solar radius being equal to the radius of the sun. eg a star whose radius is three times that of the sun is said to have a radius of three solar radii. Some of these bodies are called stars. Stars exist in groups, known as galaxies, held together by gravity. The sun is also a star and it produces a huge amount of energy daily. Unit | Abbreviation | What it is used to measure | Light year | Ly | A light year is a unit of astronomical distance equivalent to the distance light travels in one year. 1 ly = 3.0x108