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Bell Ringer 10/14
• What are stars made of?
29.2 - Stars
• A star is a body of gases that gives off a
tremendous amount of radiant energy in the
form of light and heat
• Appear to be tiny specks of white light
• Most vary in color and are much larger than
• Stars move through the night sky towards
the west
• Stars rotate around the North Star, Polaris
– Circumpolar = stars that circle around Polaris
• Because of the earth’s rotation, some stars
are not visible during particular seasons
• Pattern of stars
• Location changes throughout the year due
to Earth’s orbit
– Classified by season
• 3000 years ago, people observed and
recorded shifting patterns
– Knew when to plant, harvest, and celebrate
rituals based on location of constellations
• Astronomers recognize 88 constellations
• Constellations are named for animals,
ancient gods, & legendary heroes
– Most do not look like what they are named for
Common Constellations
Ursa Major (Big Dipper)
Ursa Minor (Little Dipper)
Draco the Dragon
Zodiac Animals
Constellation stars
• Astronomers use constellations to locate particular
• Stars within a constellation are named according
to apparent magnitude
– Brightest star is labeled alpha
– Next brightest beta and so on
• Some constellation stars have individual names
– Polaris (North star) in the Little Dipper
Constellation Project
• Starting September 29, we will being a class
project on Constellations
• You will pair up and pick a constellation
• Do some research on the constellation
• Make a visual aid about the constellation
• More information tomorrow
Star Clusters
• Stars appear to be close to
each other because the
human eye cannot
distinguish the distance a
star is from earth
• Clusters are stars close to
each other due to
gravitational attraction
Binary Stars
• Multiple-star systems
– Pairs of stars that
revolve around one
– Gravitationally bound &
orbiting a common
center of mass
• Used to determine
stellar mass
• Appear as a single star
to the human eye
• Astronomers use different methods to
measure the distance between stars and
Distance to Stars
• Distance to stars from Earth is measured in
– Light-year = distance light travels in one year
– Light-year = 9.461 x 1015 m
• Light travels 9.5 trillion km in one year
– Speed of light = 300,000 km/s
– Sun is 8 light-minutes away because it takes 8
minutes for the sunlight to reach Earth
• Parsec = 3.26 light-year = 3.086 x1016 m
• Apparent shift in position caused by motion
of observer
• Change in position of Earth as it orbits
– Closer stars have larger change in parallax
– Farther stars have smaller change in parallax
• Distance determined by angle of change
Example of Parallax
• Extend your arm and hold up your left
• Close your right eye and note location of
thumb as compared to classroom wall
• Now close left eye and note location of
Example of Parallax
• You will see your thumb appears to move
• This movement is because your eyes view
everything at different angles, since they are
a couple centimeters apart on your face
• If you move your thumb closer to your face
you will see a bigger change in thumb
In-Class Assignment/Homework
• WKT 29.2
29.2 B Notes
Properties of Stars
Luminosity (Magnitude)
• Apparent Magnitude = how bright a star
• System established by Greeks
• Brightest stars = +1
• Absolute = how bright a star would appear
at 10 parsecs (30 light-years away)
• Allows for comparison based on how bright
stars would appear at equal distances
• Luminosity = Energy output from surface of
• Measured in watts
– Think light bulbs
• Stars are assigned spectral type then further
subdivided into numbers
– O stars have a temperature of ~50,000 K
– M stars have a temperature of ~2000 K
• Based on temperature and pattern of spectra lines
– Sun = G2, temperature = ~5800 K
• Temperature also related to luminosity and
absolute magnitude
All stars have nearly identical compositions
~73% Hydrogen
~25% Helium
2% - other elements
Classification of Stars
• H-R diagram = graph
showing pattern between
absolute magnitude and
surface temperature of a star
– Charts absolute magnitude,
temperature & spectral type
• Brightness of stars increases
as surface temperature
H-R Diagram
H-R Diagram
• Main-sequence stars = band of stars running
through middle of diagram
– Sun and most stars in our sky are mainsequence stars
• Band extends from cool, dim, red in the
lower right corner to hot, bright, blue stars
in upper left corner
Main Sequence
• Fusing Hydrogen in core
• As stars evolve they begin to fuse Helium
and burn hydrogen
– High mass stars burn Hydrogen faster than low
mass stars
H-R Diagram
• Other types of stars
– Giants= very large, cool, bright star
– Supergiants = extremely large, giant star
– White Dwarfs = small, hot, dim star
In-Class Assignment/Homework