Chapter 21, Section 2

Characteristics of Stars

Distance to Stars• Light-year- The distance that light travels in one

year, it is about 9.5 trillion kilometers ( that is 9.5 million million kilometers)

• Measuring distances with ParallaxParallax is the apparent change in position of an object when you look at it from different placesUsed only for nearby stars by looking at how stars move as the Earth goes around the sun.

Classifying Stars

• The main characteristics used to classify stars are

–Size

–Temperature

–Brightness

Sizes of Stars• Many stars are about the size of the sun

the sun is medium-sized• Very large stars are giant stars or

supergiant stars-The Supergiant Betelgeuse would

extend out to Jupiter if it was in sun’s position

• White dwarfs are Earth-sized• Neutron stars are only 20km in diameter

Color and Temperature of Stars

• A star’s color reveals its temperature

Cool stars burn RED at @ 3200°C

Hottest stars are bluer than white

and are blue-white burning at 10,000°C

Brightness of Stars• Brightness depends on size and

temperature• Apparent magnitude- is brightness as

seen from Earth

• Absolute magnitude- is brightness the star would have if at a standard distance from Earth

Hertzsprung-Russell Diagram

• H-R Diagram is a graph relating the temperature and brightness of stars

-Brightness increases moving up y-axis- Temperature decreases moving right on

x-axis• More than 90% of all stars are in a diagonal

band called the Main sequence- In the main sequence, temperature

increases as brightness increases- The sun is in the main sequence

Hertzsprung-Russell Diagram

• Upper right side of H-R diagram contains giant and supergiant stars

• White dwarfs are hot but not bright so the are at the bottom center of the H-R diagram

Hertzsprung-Russell Diagram

Image from http://austrinus.com/02/imagenes/diagrama_hr_2.jpg