Download Classifying Stars (pages 753–754) Key Concept: Characteristics

Classifying Stars (pages 753–754)
Key Concept: Characteristics used to classify
stars include color, temperature, size,
composition, and brightness.
A star’s color gives clues about the star’s
temperature. The coolest stars appear red. The
hottest stars appear blue.
Very large stars are called giant stars or supergiant
stars. Our sun is a medium-sized star. Most stars
are smaller than the sun.
Brightness of Stars (pages 754–755)
Key Concept: The brightness of a star depends
upon both its size and temperature.
Stars differ in how bright they are. A hot star
shines brighter than a cool star. A large star shines
brighter than a small star.
A star’s apparent brightness is the brightness you
see from Earth. A hot, large star that is very far
from Earth does not look very bright. But the sun
looks very bright because it is so close to Earth.
A star’s absolute brightness is the brightness the
star would have if all stars were the same distance
from Earth.
Measuring Distances to Stars (pages 756–757)
Key Concept: Astronomers use a unit called the
light- year to measure distances between the
stars.
A light-year is the distance that light travels in one
year. That distance is about 9.5 million-million
kilometers.
A light-year is a unit of distance, not time. You
could also measure distance on Earth in terms of
time. For example, if it takes you 1 hour to ride
your bike to the mall, you could say the mall is “1
bicycle-hour” away.
Key Concept: Astronomers often use parallax to
measure distances to nearby stars.
Parallax is the change in an object’s position you
seem to see when you change your own position.
The object does not really change position. It only
seems to change because you change your position.
Astronomers use parallax. They measure how far a
star seems to move when Earth moves from one
side of the sun to the other. The distance the star
seems to move tells an astronomer how far the star
is from Earth.
The Hertzsprung-Russell Diagram
(pages 758–759)
Key Concept: Astronomers use H-R diagrams to
classify stars and to understand how stars change
over time.
The Hertzsprung-Russell diagram, or the H-R
diagram, shows how the surface temperature of
stars is related to their absolute brightness.
The points on the H-R diagram form a pattern. Most
stars on the H-R diagram fall into a band that
spreads from the top left corner of the diagram to the
bottom right corner. This band is called the main
sequence. Stars in the main sequence are called
main-sequence stars. About 90 percent of all stars
are main-sequence stars.
Life Cycle of Stars
Video