Stars

Imaginary patterns of stars are called ________. Constellations Stars are hot, bright, burning spheres of ____. Gas

Characteristics Used to classify include: Color Temperature Size Composition Brightness

Color & Temperature Color reveals its surface temperature Hottest stars are blue followed by white, yellow, and red Think of a fire. The coolest part of the fire is at the top of the flame where it glows red, the hotter part in the middle glows yellow, and the hottest part near the fuel glows blue. Blue is about 60,000K That is about 1075400F

White about 9000K (157400F) Red about 4000K (67400F)

Size About the size of the sun (medium sized star) Some are larger than the sun (giants or supergiant stars) many of the stars you see at night are giant stars. This is because giant stars glow very brightly.

Order :dwarfs, medium sized stars, giant, or supergiants

Composition About 73% H 25% He 2% other elements Scientists use a spectrograph-a device that breaks light into colors and produces an image of the spectrum

Star brightness Depends on the size and temperature How bright a star looks from Earth depends on its distance from Earth and how bright the star really is Apparent brightness: brightness of a star from Earth Absolute brightness: a measure of the actual brightness determined as if were a specific distance away (more complex and can vary)

A scientists must know the star’s apparent brightness and its distance from Earth to calculate absolute brightness.

Why does the sun appear to the brightest star we see? It is the closest star to Earth

Picture This Image you are riding in a car at night, and two cars are approaching from the opposite direction. Which car appears to have brighter headlights? Does this mean that its headlights are actually brighter? A star may appear to be brighter than another star because it is closer to Earth, when in fact the brightness of the two stars could be the same.

Measuring Distances to Stars Scientists use a unit called a light year to measure the distances between the stars. Light travels at a speed of about 300,000 kilometers per second. Astronomers use parallax to measure distances to nearby stars. Parallax is the change in position of an object when you look at it from different places. (movies)

Hertzsprung-Russell diagram or H-R diagram page 758 A graph relating the surface temperatures and absolute brightness of stars. Most stars are part of what group? Main sequence Are the brightest stars always the hottest stars? No; giants and super giants are very bright but not very hot