Download Distance to Stars

Characteristics
of Stars
Distance to Stars
• Earth is about 150 million km from the
Sun. (93million miles)
• That is equal to 1AU (astronomical unit)
Distance to Stars
• Since Stars are so far
away:
– A light year is used to
express great distances.
– Light travels at 300,000
km/s
– 1 light year = the
distance a ray of light
travels in 1 year. 9.5
trillion km.
Horsehead Nebula
is 1,500 light
years away)
Proxima Centauri
• The next closest star is
40 trillion km from the
sun. (more than
260,000AU)
• Light takes 4.2 years to
reach Earth.
• Most stars are much
further away.
Distance to Stars
• Stars appear to be the
same distance from Earth
in the night sky.
• How do we know that they
are not?
– Parallax
• Is the apparent change in
position of an object when
you look at it from different
positions.
Distance to Stars
• Only works for near by stars. (less
than 1,000 light years away.
• Astronomers look at the star when
Earth is on one side of the sun and
again six months later.
Distance to Stars
• They measure how much the star appears
to move against the background of stars.
• The less the star appears to move the
further away it is.
Classifying Stars
• Size (Mass)
• Color & Temperature
• Brightness
Mass (size)
• Mass is the total amount of
material in a body.
• Some stars are five, ten or
more times the size of the
sun. Others are less
massive.
Mass (size)
• 1. Super Giant – HUGE stars.
– Betelgeuse
• 2. Giant Stars- very large stars
– Rigel
• 3. Medium Size Star- Our Sun
• 4. White dwarf- smaller than the sun,
about the size of Earth.
• 5. Neutron Star- very small, 20km
Temperature
• How can the temperature of a star be
determined?
– By the color
• Red Stars=
–Cool
• Blue Stars=
–Hot
Temperatures & Colors
1. Red Stars - 3,000◦C (Betelgeuse)
2. Red-Orange Stars- 5,000 ◦C (Aldebaran)
3. Yellow Stars – 5,500◦C – 6,000 ◦C (Sun)
4. White Stars- 10,000◦C (Sirius)
5. Blue Stars- more than 15,000◦C (Rigel)
Brightness (Luminosity)
• Depends on size and
temperature.
• Betelgeuse is large but
is cool. It’s size makes
it appear bright.
• Rigel is smaller but is
very hot so it shines
brightly.
Brightness (Luminosity)
• Stars are far from Earth.
• As a result, what problem does this
cause?
– Cannot accurately determine the actual
brightness of a star from earth.
• How bright a star appears from Earth
depends on far away the star is.
Apparent Magnitude
• The measure of how bright a star appears
to be from earth.
– Less bright objects that are closer to you
could be seem brighter than objects that are
far and actually brighter.
• The lower the magnitude the brighter the
star.
Absolute Magnitude
• The brightness a star would appear if it
was set at a standard distance from
Earth.
– Astronomers calculate the stars apparent
magnitude and it’s distance from Earth.
– Then calculate the brightness if it were a
standard distance from Earth.
HR- Diagram
HR- Diagram
• Herzsprung and Russell
• A graph used to find out if temperature
and brightness are related.
• Plots absolute magnitude vs. surface
temperature.
HR- Diagram
• More than 90 % of stars are Main
Sequence, form a diagonal band.