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Stars
Chapter 11,
Lesson 4
Where do stars come from?
Stars form in a nebula, which is a large
cloud of gas and dust in space.
Gravity pulls some of the particles in the
cloud together. When the center of the
group has enough mass, pressure causes the
temperature to increase.
Stars
• Stars are balls of gas, mostly hydrogen,
that produce heat and light from fusion in
their core. Fusion joins the nuclei of
hydrogen atoms to form nuclei of helium
atoms.
• Scientists compare stars by their color,
brightness and surface temperature.
Star Classifications
• Two astronomers independently developed
diagrams of how absolute magnitude, or
brightness is related to the temperature of
stars.
• The diagram that plots temperature vs.
absolute magnitude is called the
Hertzsprung-Russell Diagram.
Star Classifications
The mass of a star determines what kind
of main sequence star it is. The more
massive the star, the hotter it is.
The brightest stars are at the top, with the
dimmer ones at the bottom. The hottest
stars are on the left, and the coolest ones
are on the right.
Star Classifications
• 90% of stars fall on a diagonal, curved line,
called the main sequence.
• The remaining stars fall into one of three
other groups.
- Red giants
- Supergiants
- White dwarfs
Star Types
• Stars have many
diameters, masses, and
surface temperatures.
• Our Sun is a mediumsized star with a surface
temperature of about
5800 K.
Star Temperature
• As metal gets hotter, it changes from red to
yellow to white.
• The color of stars also depends on
temperature.
Star Temperature
Star Temperature
Oh Boy, Another F's Gonna Kill Me.
The
temperature of
a star is
indicated by
the color it
glows.
(The Harvard
Stellar
Classification
System)
Type of Star
Color
Surface Temp (oC)
O
Blue
Above 25,000
B
Blue-White
10,000 – 25,000
A
White
7,500 – 10,000
F
Yellow-White
6,000 – 7,500
G
Yellow
5,000 – 6,000
K
Orange
3,500 – 5,000
M
Red
Below 3,500
Star Brightness
• The brightness of stars depends on two
things: energy and distance.
• Light looks brighter as you move closer
to the source.
How Stars End
• Eventually a star converts all its hydrogen
to helium.
• When the hydrogen is gone, the star is no
longer a main sequence star.
• Stars come to an end in different ways,
depending on its mass.
How Stars End
A high mass star may use up all its
hydrogen and swells into a supergiant.
Then the supergiant explodes into a
supernova. The core of the supernova
might form a small, dense neutron star.
The biggest supernovas collapse into
extremely dense black holes.