Download Prentice Hall Ch 21, Sec. 2

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 Parallax
Parallax is the apparent change in position of
an object when you look at it from different
places
Used 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 xaxis
• 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