Download Chapter 27.1

Ch. 27 Stars and Galaxies
Ch. 27.1 Characteristics of Stars
Objectives
How is a star’s surface temperature and
composition determined?
 Explain why, to an observer on earth, stars
appear to move.
 Describe the ways distances from the
earth to the stars are measured.
 Explain the difference between absolute
and apparent magnitude.

Star—body of gas that gives off
tremendous radiant heat and light energy.
 The sun is a star.
 Stars may have different colors, masses
and diameters.
 Other differences include composition,
temperature, brightness, and distance
from earth.

Composition and Temperature
Spectrometers attached to optical
telescopes separate light into lines of
different colors, called a spectrum.
 Each chemical element has a
characteristic dark-line spectrum.
 The same elements found on earth can be
found in stars, but hydrogen and helium
are the two most common elements.

A stars surface temperature determines its
color.
 The hottest blue stars may be 50,000 C.
 Cool red stars average 3,000 C.
 Yellow stars (the sun) are about 5,500 C.

Motion
Due to Earth’s rotation, stars appear to
rotate around the motionless North Star.
 Because of Earth’s revolution around the
sun, some stars are not at visible all times
during the year.
 The stars that are visible year-round and
circle the North Star are the circumpolar
stars.

Due to the Doppler effect, stars moving
toward the earth have their spectrum
shifted to the blue wavelength (blue shift).
 Stars moving away from the earth
experience red shift.
 Most distant galaxies and star groups are
red-shifted…moving away from us.

Distance to the Stars
Very large distances are measured in light
years…the distance light travels in one
year.
 Alpha Centauri is 4.2 light years away.
 The North Star is 700 light years away.
 Parallax is used to measure the distance
to nearby stars.

Nearby stars apparent position in relation
to more distant stars changes as earth
moves in its orbit from one side of the sun
to the other.
 Limited to measuring the distance to stars
within 1000 light years of earth.

Distance to more distant stars can be
determined by comparing a stars true
brightness (deduced from its spectrum) to
its apparent brightness.
 Special Cepheid Variable stars can be
used to determine the distance to very
distant galaxies.

Stellar Magnitudes
Visibility of a star depends on two things;
its brightness, and its distance from earth.
 Apparent Magnitude—the brightness of a
star as it appears from earth. Assigned a
number on a scale. Small and negative
numbers are brighter. Larger numbers are
dimmer.

Absolute Magnitude
The true brightness of a star.
 How the star would appear if it was seen
from a distance of 32.6 light-years away.
 If the sun was 32.6 light years away, it
would be a +5 on the apparent magnitude
scale, so its absolute magnitude is +5.
 Most stars have an absolute magnitude of
between -5 and +15.

Classification of Stars
When the stars surface temperatures are
plotted against their absolute magnitude
on a graph, a pattern develops.
 This is the Hertzsprung-Russell Diagram
(H-R Diagram).
 Most stars (including the sun) fall along a
line from cool, dim stars on the lower right,
to hot, bright stars on the upper left.
These are the main sequence stars.

A group of cool, but large, and therefore
very bright stars are in the upper right side
of the H-R Diagram (the Giants and
Supergiants).
 A group of hot, but small, and therefore
very dim stars are in the lower left of the
diagram (White Dwarfs).
