Characteristics of Stars
Analyze how stars are classified
based on their physical
characteristics.
Color
• Astronomers classify stars
according to their physical
characteristics which include;
color, temperature, size,
composition, and brightness.
• Looking up at the night sky you will
notice that some stars look
reddish, while others are yellow or
blue-white.
• Differences in the colors of stars
are due to differences in their
surface temperature. The same is
true of all objects that glow.
Color and Surface Temperatures
• A star’s color reveals its
surface temperature.
The coolest stars—with a
surface temperature of
about 3,200 °C—appear
red. Our yellow sun has
a surface temperature of
about 5,500 °C. The
hottest stars, with
surface temperatures of
over 20,000 °C, appear
bluish.
Color and Surface Temperatures of
Stars
Color
Surface
Temperature (°C)
Blue
Above 25,000
Blue-white
10,000-25,000
White
7,500-10,000
Yellow-white
6,000-7,500
Yellow
5,000-6,000
Orange
3,500-5,000
Red
Below 3,500
Size
• Looking at stars in the sky, they all appear to be
points of light of the same size.
• Many stars are actually about the size of the sun.
However, some stars are much larger than the
sun. Most stars are smaller than the sun.
• Astronomers use the size of the sun to describe
the size of other stars.
Using Solar Radii
• Astronomers have indirectly measured the
dimensions of the sun. The Sun’s radius is
approximately 695,000 km, or about 109
times the radius of Earth. So the sun would
equal 1 solar radius.
• In comparison white dwarfs are about the
same size as Earth and would equal 0.01 solar
radius. Supergiants can have sizes up to 1,000
solar radii.
Chemical Composition
• Stars very in their chemical composition.
• The chemical composition of most stars is
about 73 % hydrogen, 25 %, and 2 % other
elements by mass.
• Astronomers use spectrographs to determine
the elements found in stars.
• A spectrograph is a device that breaks light
into colors and produces an image of the
resulting spectrum.
• The gases in a star’s atmosphere absorb some
wavelengths of light produced within a star.
• When the star’s light is seen through a
spectrograph, each absorbed wavelength is
shown as a dark line on a spectrum.
Each chemical
element absorbs
light at particular
wavelengths.
Just like
fingerprints,
each element
has a unique set
of spectra lines.
Brightness of Stars
• Stars differ in brightness, the amount of light they
give off.
• The brightness of a star depends on both its size
and temperature. A larger star tends to be
brighter than a smaller star. A hotter star tends to
be brighter than a cooler star.
• How bright a star appears depends on both its
distance from Earth and how bright the star truly
is. Because of these two factors, the brightness
of a star is described in two ways: apparent
brightness and absolute brightness.
Apparent Brightness
• Apparent brightness or
apparent magnitude is a
star’s brightness as seen
from Earth.
• Astronomers cannot
determine how much light a
star gives off from it’s
apparent brightness.
• A star closer to Earth may
appear to give off more
light than others star’s, but
it looks so bright simply
because its closer.
Magnitude
• Using only their eyes, ancient astronomers
described star brightness by magnitude.
• The brightest stars they could see were called
first magnitude and the faintest stars they
could see were called sixth magnitude.
• Using telescopes, astronomers were able to
see new stars to dim to see with the naked
eye. Instead of replacing the old 1-6 scale,
they added to it.
• Today the brightest stars have a magnitude of
about -2 and the faintest stars +30.
• The magnitude scale may seem backwards.
Faint stars have positive (larger) numbers;
bright stars have a negative (smaller)
numbers.
Absolute Brightness
• Luminosity means actual
brightness of a star.
• To measure a star’s luminosity,
astronomers use an absolute
brightness scale called absolute
magnitude.
• Absolute magnitude is a measure
of how bright a star would be if
the star were located at a
measured distance.
• It is like comparing stars
luminosity if they were all lined
up equal distances from the
Earth.
• To understand the difference between
apparent magnitude and absolute magnitude
look at the table below:
Magnitudes of Selected Stars
Star
Distance from
Earth
Apparent
Magnitude
Absolute
Magnitude
Sun
8.3 light-minutes
-26.8
+4.8
Sirius
8.6 light-years
-1.46
+1.4
Betelgeuse
640 light-years
+0.45
-5.6
H-R Diagram
• The Hertzsprung-Russell
Diagram shows the
relationship between
surface temperature and
absolute brightness of
stars.
• The surface
temperatures of stars
are plotted on the x-axis
and their absolute
brightness on the y-axis.
• Astronomers use H-R
diagram to classify stars
and to understand how
stars change over time.
• Most of the stars in the HR diagram form a diagonal
area called the main
sequence. 90% of all stars
are main sequence stars.
• Within the main
sequence, the surface
temperature increases as
absolute brightness
increases.
• Hot-bluish stars are
located at the left and
cooler reddish stars on the
right. Bright stars near the
top and dim stars near the
bottom.