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Classifying Stars
Brightness
Brightness
Some stars are so bright that you can
see them even in a lighted city, while
others are so dim that you can only
see them through a telescope.
 A star’s brightness depends on three
things:

how big the star is (size)
 how hot the star is (temperature)
 how far away from Earth the star is
(distance)

Brightness cont.
The bigger and hotter the star, the
more light it gives off.
 However, brightness also depends on
distance.
 A star that is nearer to Earth looks
brighter than one that gives off the
same amount of light, but is farther
away.

Magnitude




Absolute magnitude is the actual amount of light that
a star gives off.
Apparent magnitude is the amount of a star’s light
that is observed on Earth.
If two stars are the same distance from Earth, the
one with the greatest absolute magnitude will be the
brightest.
If one star is farther away than the other, the one
that is closer will appear the brightest even if it puts
off less light or has less absolute magnitude.
Magnitude cont.

For example:


Rigel has an absolute magnitude
greater than that of Sirius (it puts off
more light), but it looks dimmer from
Earth because it is 100 times farther
away.
Due to its apparent magnitude, Sirius
is the brightest star in the night sky.
Classifying Stars
Life Cycle
Nebula
A nebula is the earliest stage of a star’s
formation.
 It forms when the interstellar medium (clouds
of gas and dust) begins to collapse due to
gravity.
 Nuclear fusion of hydrogen occurs and more
substances or elements are created, fusing to
form a core and then a main sequence star.

Main Sequence Star
A star next becomes a main sequence
star.
 This is the stage that stars spend the
most time in.
 In fact, 90% of all stars are in this
stage.
 Our sun is a main sequence star.
 The cores of these stars shrink and
they become cooler and dimmer.

Giants
As a main sequence star begins to
die, it moves into the next stage.
 As a star cools and expands, helium
fuses to form carbon, becoming a red
giant.
 If it expands to a size that is more
than 700 times as large as the sun, it
is called a red super giant.

Red Giants
Red giants then form planetary nebula
as their outer layers drift into space.
 This is where planets form from.
 These then turn into white dwarfs
which are small, hot stars that consist
of a hot, dense core that contracts
under the force of gravity.
 White dwarfs cool becoming black
dwarfs.

Red Super Giants
The cores of red super giants collapse
producing an explosion and forming
supernovas.
 Stars about three times the size of our sun
then become neutron stars where only
neutrons exist in their cores.
 Stars larger than three times the size of our
sun become black holes.
 Black holes are so dense that nothing,
including light, can escape them.
