Measuring the StarsChapter 30.2

Grouping of Stars Groups 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 Stars Circumpolar constellations can be seen all

year long. In the Northern Hemisphere, there are five

of these:

Grouping of Stars Ursa Major (the big dipper, or the great

bear)

Grouping of Stars Ursa Minor (the little dipper)

Grouping of Stars Cepheus (the king)

Grouping of Stars Cassiopeia (the queen)

Grouping of Stars Draco (the dragon)

Star Clusters Although stars may look close together, or

lie in the same constellation, they may not actually be close together.

We can’t tell because we can’t see how far from earth the star is!

Star Clusters A 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 Clusters Globular cluster Open cluster

Binaries Two 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 Positions To measure distances to stars, scientists

use: Light-year (ly) – distance light travels in one year,

or 9.461x1012 km Parsec (pc) – 3.26 light-years

Stellar Positions The 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 Stars Diameter: 0.1 times the Sun’s diameter to

hundreds of times larger Mass: 0.01 to 20 times the Sun’s mass

Most massive stars can range from 50 to 100 suns

Magnitude: how bright it appears. The brightest stars have a magnitude of +1

Apparent Magnitude: how bright it looks to you Magnitude +1 star is 100 times brighter than a

magnitude +6 star Objects brighter than a magnitude +1 star are

given negative numbers. Sun = -27 Full moon = -13 Venus = -4 Sirius = -2

Absolute magnitude: the brightness of an object if it were placed at a distance of 10 parsecs This scale takes distance into account. This requires that the actual distance to the star

be known. Sun =+5 Sirius = +2

Luminosity: the measure of energy output from the surface of a star per second. To calculate luminosity, you must know:

Apparent magnitude How far away it is

Luminosity is measured in Watts – units of energy per second

This varies greatly between stars

Spectra of Stars Stars are assigned spectra in these groups:

O, B, A, F, G, K, and M Each class is subdivided again – 0 to 9

Examples: O7, A5 The classes were first determined by similar

spectral lines, but then temperatures were determined to correspond.

O stars are the hottest; M are the coolest.

Blue stars

50000 K

Few spectra lines

Red Stars

2000 K

Many spectra lines

All stars have the same general composition – the difference in spectra is a result of varying temperatures 73% hydrogen 25% helium 2% other stuff

Doppler Effect Spectral 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 can’t be detected!

Hertzsprung-Russel diagram Plots the basic properties of stars

Absolute magnitude on the y axis Surface temperature on the x axis on top Spectral 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.

Hottest main sequence

Hot, dim stars

Coolest main sequence

Cool, bright stars

Bright main sequence

Dark main sequence

Smallest stars

Largest stars

SUN