Characteristics of Stars Chapter 21, Section 2 Characteristics of Stars

Classifying Stars 1. Stars are classified by temperature, size, and brightness.

Light Years Stars are so far apart that it is not practical to use kilometers. Light travels at 300,000 km/s, or 9.5 trillion km a year! So we use light years to measure distances in space. A light year is the distance light travels in one year which is about 6 trillion miles or 9.5 trillion km. Light from the closest star to us takes 4.2 years to reach us, so Proxima Centari is 4.2 light years away.

Our known universe is made up of billions of galaxies. . . and everything in between.

Galaxies are huge groups of single stars, star systems, star clusters, dust, and gas bound together by gravity.

The largest galaxies can contain TRILLIONS of stars! The Milky Way Galaxy has about 200 billion stars. You are looking at only 100,000 in this image. The largest galaxies can contain TRILLIONS of stars!

Sizes of Stars From Earth, all the stars appear to be the same size.

Sizes of Stars Neutron White Dwarf Medium Large Giant Supergiant Hyper-giant

Color and Temperature A star’s color indicates its temperature. BLUE-WHITE BLUE WHITE-YELLOW WHITE YELLOW RED ORANGE COLDEST

BRIGHTNESS (luminosity) Hertzsprung-Russell Diagram As temperature increases. . . absolute brightness increases. BRIGHTNESS (luminosity) TEMPERATURE

More than 90% of all stars are MAIN SEQUENCE stars.

brightness as seen from Earth Apparent magnitude brightness as seen from Earth You cannot tell how much light a star gives off by its apparent magnitude. A small dim star close by can look as bright as a really bright star much further away. If two stars are equally bright, one closer to us will appear brighter than one further away.

Absolute magnitude: brightness the star would have if at a standard distance from Earth

Which is more important when discussing a star’s characteristics, absolute or apparent magnitude?

How do astronomers determine the chemical composition of stars? SPECTROGRAPHS 2. The spectrograph breaks light into colors (frequencies). 3. Each element has a unique “fingerprint” within the colors. What do you think are the most common elements in stars? HYDROGEN and HELIUM

PARALLAX

MOTIONS OF STARS Stars do move, but we can not see this movement with our eyes. The APPARENT movement of the stars across the sky each night is caused by the rotation of the Earth.

North Star 2-hour time-exposure looking directly north http://www.youtube.com/watch?v=aa3wGt8B0VQ

What is one limitation of using parallax to measure distances to the stars? Parallax can’t be used to measure stars that are more than 200 light years from Earth.

Vega is 25. 3 light years from earth and Arcturus is 36 Vega is 25.3 light years from earth and Arcturus is 36.7 light years away. Which star would have a greater parallax? Explain why. Vega would have a greater parallax because it is closer to Earth; consequently, it appears to move more.