# Measuring the Stars Chapter 30.2. Grouping of Stars Groups of stars named after animals, mythological characters, or everyday objects are called constellations

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• Measuring the StarsChapter 30.2

• Grouping of StarsGroups of stars named after animals, mythological characters, or everyday objects are called constellations. Today there are 88 constellations that were named by ancient peoples.

• Grouping of StarsCircumpolar constellations can be seen all year long.In the Northern Hemisphere, there are five of these:

• Grouping of StarsUrsa Major (the big dipper, or the great bear)

• Grouping of StarsUrsa Minor (the little dipper)

• Grouping of StarsCepheus (the king)

• Grouping of StarsCassiopeia (the queen)

• Grouping of StarsDraco (the dragon)

• Star ClustersAlthough stars may look close together, or lie in the same constellation, they may not actually be close together.We cant tell because we cant see how far from earth the star is!

• Star ClustersA group of stars that actually lie close together and are held together by gravity is called a cluster. One that is closely packed is a globular cluster. One loosly packed is an open cluster.

• Star ClustersGlobular clusterOpen cluster

• BinariesTwo stars gravitationally bound together, orbiting a common center of mass are called Binary Stars. More than 50% of the stars in the sky are binary or a member of a cluster. The two stars often look like only one because they are too close together and one is often brighter than the other.

• Stellar PositionsTo measure distances to stars, scientists use:Light-year (ly) distance light travels in one year, or 9.461x1012 kmParsec (pc) 3.26 light-years

• Stellar PositionsThe apparent shift in position caused by the motion of the observer is called parallax.As the earth moves around the sun, stars appear to move in the sky.

• The main rule of parallax: The closer the star, the larger the shift.

• Basic Properties of StarsDiameter: 0.1 times the Suns diameter to hundreds of times largerMass: 0.01 to 20 times the Suns massMost massive stars can range from 50 to 100 sunsMagnitude: how bright it appears. The brightest stars have a magnitude of +1

• Apparent Magnitude: how bright it looks to youMagnitude +1 star is 100 times brighter than a magnitude +6 starObjects brighter than a magnitude +1 star are given negative numbers.Sun = -27Full moon = -13Venus = -4Sirius = -2

• Absolute magnitude: the brightness of an object if it were placed at a distance of 10 parsecsThis scale takes distance into account.This requires that the actual distance to the star be known. Sun =+5Sirius = +2

• Luminosity: the measure of energy output from the surface of a star per second.To calculate luminosity, you must know:Apparent magnitudeHow far away it isLuminosity is measured in Watts units of energy per secondThis varies greatly between stars

• Spectra of StarsStars are assigned spectra in these groups: O, B, A, F, G, K, and MEach class is subdivided again 0 to 9Examples: O7, A5The classes were first determined by similar spectral lines, but then temperatures were determined to correspond. O stars are the hottest; M are the coolest.

• All stars have the same general composition the difference in spectra is a result of varying temperatures73% hydrogen25% helium2% other stuff

• Doppler EffectSpectral lines become shifted in wavelength due to motion between the source and the observer.If a star is moving towards us, the wavelength becomes shorter, shifting it to the blue end.A star moving away has a longer wavelength and shifts towards the red end. Side to side motion cant be detected!

• Hertzsprung-Russel diagramPlots the basic properties of stars Absolute magnitude on the y axisSurface temperature on the x axis on topSpectral type on the x axis on bottom

90% of stars fall within the main sequence, running diagonally from the upper left to the lower right.

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