Download The Temperature of Stars

EARTH & SPACE SCIENCE
Chapter 30 Stars, Galaxies, and
the Universe
30.1 Characteristics of Stars
30.1 Characteristics of Stars
Objectives
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Describe how astronomers determine the
compositions and temperature of stars.
Explain why stars appear to move in the
sky.
Describe one way astronomers measure the
distances to stars.
Explain the difference between absolute
magnitude and apparent magnitude.
Analyzing Starlight
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A star is a large celestial body that is
composed of gas and that emits light.
Nuclear fusion is the combination of light
atomic nuclei to form heavier atomic nuclei.
Astronomers learn about stars by analyzing
the light that the stars emit.
Starlight passing through a spectrograph
produces a display of colors and lines called
a spectrum.
Analyzing Starlight
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All stars have dark-line spectra, which are
bands of color crossed by dark lines
where the color is diminished.
A star’s dark-line spectrum reveals the
star’s composition and temperature.
Stars are made up of different elements
in the form of gases.
Because different elements absorb
different wavelengths of light, scientists
can determine the elements that make
up a star by studying its spectrum.
The Composition of Stars
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Scientists have learned that stars are made
up of the same elements that compose
Earth.
The most common element in stars is
hydrogen.
Helium is the second most common element
in star.
Small quantities of carbon, oxygen, and
nitrogen are also found in stars.
The Temperature of Stars
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The temperature of most stars ranges from
2,800˚C to 24,000˚C.
Blue stars have average surface
temperatures of 35,000˚C.
Yellow stars, such as the sun, have surface
temperatures of about 5,500˚C.
Red stars have average surface
temperatures of 3,000˚C.
The Sizes and Masses of
Stars
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Stars vary in size and mass.
Stars such as the sun are considered
medium-sized stars. The sun has a diameter
of 1,390,000 km.
Most stars visible from Earth are mediumsized stars.
Many stars also have about the same mass
as the sun, however some stars may be
more or less massive.
Stellar Motion
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Apparent Motion
– The apparent motion of stars is the motion
visible to the unaided eye.
– Apparent motion is caused by the movement of
Earth.
– The rotation of Earth causes the apparent
motion of stars sees as though the stars are
moving counter-clockwise around the North Star.
– Earth’s revolution around the sun causes the
stars to appear to shift slightly to the west every
night.
Stellar Motion
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Circumpolar Stars
– Some stars are always visible in the night sky.
– These stars never pass below the horizon.
– In the Northern Hemisphere, the movement of
these stars makes them appear to circle the
North Star.
– These circling stars are called circumpolar stars.
Stellar Motion
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Actual Motion of Stars
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Most stars have several types of actual motion.
Stars rotate on an axis.
Some stars may revolve around another star.
Stars either move away from or toward our solar
system.
Stellar Motion
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Actual Motion of Stars (continued)
– Doppler effect an observed change in the
frequency of a wave when the source or
observer is moving
– The spectrum of a star that is moving toward or
away from Earth appears to shift, due to the
Doppler effect.
– Stars moving toward Earth are shifted slightly
toward blue, which is called blue shift.
– Stars moving away from Earth are shifted
slightly toward red, which is called red shift.
The spectrum of a star that is moving toward or
away from Earth appears to shift, as shown in the
diagram below.
Distances to Stars
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A light-year is the distance that light travels
in one year.
Distances between the stars and Earth are
measured in light-years.
Parallax is an apparent shift in the position
of an object when viewed from different
locations.
For relatively close stars, scientists
determine a star’s distance by measuring
parallax.
http://ircamera.as.arizona.edu/NatSci102/Nat
Sci102/text/parallax.htm
Stellar Brightness
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Apparent magnitude the brightness of a
star as seen from the Earth.
The apparent magnitude of a star depends
on both how much light the star emits and
how far the star is from Earth.
Absolute magnitude the brightness that a
star would have at a distance of 32.6 lightyears from Earth.
The brighter a star is, the lower the number
of its absolute magnitude.
The lower the number of the star on the
scale shown on the diagram below, the
brighter the star appears to observers.