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Characteristics of
Stars
Notes
Classifying Stars
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Characteristics used to classify stars
include color, temperature, size,
composition, and brightness.
Which color has the SHORTEST
wavelength? So which color has the MOST
energy? Shorter wavelength=More energy.
A star’s color gives clues about the star’s
temperature. The coolest stars appear red.
The hottest stars appear blue.
Classifying Stars
• Very large stars are called giant stars or
supergiants. Our sun is a medium-sized
star. MOST stars are small then the
sun.
• The majority of stars in the universe are
red dwarfs.
QuickTime™ and a
decompressor
are needed to see this picture.
If this video doesn’t play, click below:
The Biggest Stars In The Universe.mp4
Classifying Stars
• Stars differ in their chemical make-ups.
Yes, the majority of chemicals inside a
star include hydrogen and helium, but
there are other ingredients as well.
• Astronomers use spectrographs to find
out what elements are in a star. A
spectrograph is a device that breaks
light into colors (like a prism). Scientists
compare a star’s light with the light
produced by different elements to find
out what elements are in a star.
Spectrographs
Brightness of Stars
• The brightness of a star depends on
both its SIZE and TEMPERATURE.
• Stars differ in how bright they are. A hot
star shines brighter than a cool star. A
large star shines brighter than a small
star.
Brighter
Dimmer
Is it possible to make the
two lights the same
brightness without
modifying how much
light they produce?
If so, how?
Brightness of Stars
• A star’s brightness is described as its
magnitude.
• A star’s apparent magnitude is the
brightness you see from Earth, or what
it appears to look like from Earth. A hot,
large star that is very far from Earth
does not look very bright. But the sun
look very bright because it is so close to
Earth.
Brightness of Stars
• A star’s absolute magnitude is the
brightness the star would have if all
stars were the same distance from
Earth.
• If the spotlight was placed beside the
flashlight, the spotlight would have a
greater absolute magnitude. But if the
spotlight were placed 10 miles away,
then the flashlight would have a greater
apparent magnitude.
Increasing Distance from Earth
Both star A and star B appear to have the same
brightness from Earth.
Which star has the greater ABSOLUTE
MAGNITUDE?
Which star has the greater APPARENT
Measuring Distances
to
Stars
• A light-year is the distance that light travels in one
year. That’s 300,000 km/s for over 31.5 million
seconds. That gives us about 9.461 trillion kilometers.
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Our closest star (other than Sol) is Proxima Centauri. It
is over 4.5 light-years from us. That’s 9.461 trillion
kilometers times 4.5. That number won’t even fit in
your calculator without the use of Scientific Notation.
A light-year is a unit of DISTANCE, not time. You could
also measure distance on Earth in terms of time. For
example, if it takes you one hour to ride your bike to
the mall, you could say the mall is “one bicycle-hour”
Measuring Distances
to Stars
• Parallax is the change in an object’s position
you seem to see when you change your own
position. The object does not really move, it only
seems to change because you change your
position.
• To demonstrate this give a thumbs-up at eye
level at arms length. Close one eye. Quickly
switch eyes by closing the original eye, and
opening the other. What appeared to happen to
your thumb?
Parallax Demos
http://sci2.esa.int/interactive/media/flashes/2_1_1.htm
http://www.astronexus.com/node/84
Click “Animation” on the right.
Measuring Distances
to Stars
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Astronomers use parallax. They measure how
far a star seems to move when Earth moves
from one side of the sun to the other. The
distance the star seems to move tells an
astronomer how far the star is from Earth.
Notice that the farther away the star is, the
smaller the parallax angle. Stars can be too far
away to accurately measure their distance
using this method.
This uses simple geometric math.
The HertzprungRussell Diagram
• The Hertzsprung-Russell Diagram, or the H-R
Diagram, shows how the surface temperature
of stars is related to their ABSOLUTE
magnitude.
• The brightest stars are located near the top of
the H-R Diagram, and the dimmest stars are
located at the bottom.
• The hottest (blue) stars are at the left of the
diagram, whereas the coolest (red) stars are at
the right.
The HertzprungRussell Diagram
• The points on the H-R Diagram form a
pattern. Most stars on the H-R diagram
fall into a band that spreads from the
top-left corner to the bottom-right
corner. This band is called the main
sequence.
• Stars in the main sequence are called
main-sequence stars. About 90% of
all stars are main-sequence stars.
Betelgeuse is
a Red
Supergiant
located in
Orion
Rigel is a
Blue
Supergian
t located
in Orion