25.1 – Properties of Stars – Part I

Do Now  What does a star's color tell us about a star?

Do Now  What does a star's color tell us about a star?  The color tells us the star’s temperature.

Key Word  Parallax

Vocab Words  Constellations  Binary Star  Light-year

Constellations  As early as 5000 years ago, people became fascinated with the star-studded skies and began to name the patterns they saw; called constellations.  Although the stars that make up a constellation all appear to be the same distance from Earth, some are many times farther away than others.  Today 88 constellations are recognized.  They are used to divide the sky into units, just as state boundaries divide the U.S.

Characteristics of Stars  Color is a clue to a star’s temperature.  Very hot stars with surface temperatures above 30,000 K emit most of their energy in short wavelengths and appear blue.  Red stars are much cooler, and have longer wavelengths.  Yellow stars, like the sun have temperatures from about 5,000 and 6,000 K.

 Binary stars are pairs of stars, pulled toward each other by gravity.  More than 50% of the stars in the universe may occur in pairs or multiples.  They are used to determine the star property most difficult to calculate – the mass.  It’s calculated by looking at where the center of mass is in a binary system. If one star is more massive than it’s partner the center of mass is closer to that said star. Binary Stars and Stellar Mass

Binary Star Systems

Parallax  Parallax is the most basic way to measure star distance.  Determined by photographing a nearby star against the background of other stars. Then, do this again six months later. The photos are compared, and the parallax angle is found.  The nearest stars have the largest parallax angles, while those of distant stars are too small to measure.  Proxima Centauri is the closest star besides the Sun.

Parallax Original Photo Photo taken 6 months later

Light-Year  Distances to the stars are so large that units such as kilometers or astronomical units are hard to use.  We use light-years to measure the distance of stars.  One light-year is 9.5 x 10^12 or 9.5 trillion kilometers.  Proxima Centauri is about 4.2 light-years away from the sun.

Group Challenge Question  The closest star to the sun, Proxima Centauri, is 4.3 light- years away. How many kilometers from the sun is Proxima Centauri?