Classifying Stars• Color

• Temperature

• Size

• Composition/Mass

• Brightness

Size ComparisonSize

ClassificationApproximate

Size (km) Examples

Neutron Star 16 km

Less than the distance to

school

pulsar

White Dwarf Less than the distance across

Asia

Sirius B

Van Maanen’s star

Medium star 109 X size of Earth or

1,392,000 km

Sun

Giant star 100 X size of Sun

Aldebaran

Supergiant 1000 X size of Sun

Rigel, Betelgeuse, Arcturus

http://www.spacetelescope.org/images/screen/opo0505b.jpg

Color and TemperatureStar Color Surface Temp (oC)

Blue or Blue-White 35,000 oC

White 10,000 oC

Yellow 6,000 oC

Red-Orange 5,000 oC

Red 3,000 oC

Chemical Composition

• Most stars are made of – 73% Hydrogen– 25% Helium– 2% Other elements• Astronomers use spectrographs to determine

elements found in stars• Spectrograph is a device that breaks light into

colors and produces an image of the resulting spectrum

Brightness of Stars• The brightness of a star depends upon both its size

and temperature• How bright a star looks from Earth depends on

both its distance from Earth and how bright the star truly is

Brightness of Stars

• Apparent magnitude- The brightness of a star as it appears on Earth

• Absolute magnitude- The amount of light a star actually gives off (uses formulas)

Why do stars twinkle?

• The scientific name for the twinkling of stars is stellar scintillation

• Stars twinkle when we see them from the Earth's surface because we are viewing them through thick layers of turbulent (moving) air in the Earth's atmosphere.

Measuring Distances to Stars

• Astronomers use a unit called the light year to measure distances between the stars

• Light travels at a speed of 300,000 km/s

• Light year- distance that light travels in one year =9.5 trillion km

• Light year=unit of distance

Parallax• Astronomers use parallax to measure

distances to nearby stars

• Parallax= the apparent change in position of an object when viewed from two locations

Parallax is larger for closer objects