1. Section 15.2 CHARACTERISTICS OF STARS

2.  Constellation: imaginary patterns of stars  Orion: the hunter, was named after a Greek myth  Stars in a constellation look close together, but they actually are not INTRODUCTION

3.  Characteristics to classify stars:  Temperature  Size  Composition  brightness  Color and Temperature  Betelgeuse is reddish. Rigel, blue-white  The coldest stars are reddish. The hotter stars are bluish. CLASSIFYING STARS

4.  Size  Some stars are much bigger than the sun.  In fact, the Sun is just a medium-sized star  Most stars are smaller than the sun. White dwarves are about the size of earth.  Neutron stars are only around 20 km in diameter CLASSIFYING STARS

5.  Chemical Composition  Spectrograph: device that breaks light into colors and produces an image of the resulting spectrum  Each element absorbs light at a different wavelength  Scientists use a spectrograph to figure out what parts of the spectrum the star lights up. Then they can figure out what elements are in that star. CLASSIFYING STARS

7.  The brightness of a star depends upon both its size and temperature  Betelgeuse is fairly cool, so it is not as bright. Rigel is very hot, so it gives off a lot of light. BRIGHTNESS OF STARS

8.  Apparent brightness  Apparent brightness: brightness seen from earth  Think of a flashlight. When it is close, it is much brighter. Therefore, some stars that are closer to us appear brighter, even though stars that are farther away are actually brighter. BRIGHTNESS OF STARS

9.  Absolute brightness  Absolute brightness: how bright a star would be if it was they were all the same distance from earth BRIGHTNESS OF STARS

10.  The nearest star, Proxima Centauri, is 4.2 light-years away.  The Light-year  Astronomers typically use a unit called the light-year to measure distances between the stars.  Light-year: the distance that light travels in one year, about 9.5 million MILLION kilometers MEASURING DISTANCES TO STARS

11.  Parallax  Astronomers often use parallax to measure distances to nearby stars  Parallax is the apparent change in position of an object when you look at it from different places MEASURING DISTANCE TO STARS

12.  Parallax in astronomy  Because the earth is moving, the apparent position of the stars in the sky will change. They have to use <3 MATH <3 to figure out where exactly the stars are. MEASURING DISTANCE TO STARS

13.  These scientists were interested in figuring out if the brightness and the temperature of the star related.  Hertzsprung-Russell Diagram: shows the relationship between surface temperature and absolute brightness of stars THE HERTZSPRUNG-RUSSELL DIAGRAM

14. HERTZSPRUNG-RUSSELL DIAGRAM

15.  Astronomers use H-R diagrams to classify stars and to understand how stars change over time  Main sequence: most of the stars in the downward diagonal on the H-R diagram  Brightest stars on top, dimmest stars on bottom  Hottest stars on left, coolest stars on right THE HERTZSPRUNG-RUSSELL DIAGRAM