How are stars classified? What is an H-R
diagram and how do astronomers use it?
What does a star’s color indicate? Surface
temperature.
 What are some colors of stars? Yellow, blue - white or
reddish
 What color are the hottest stars? Bluish(question on
Winter’s interim test)
A star’s color reveals its surface temperature. Red has a
temperature of about 3200 Celsius. Yellow has a surface
temperature of about 5500 Celsius. Hottest stars with a
surface temperature of over 20,000 Celsius appear
bluish.
Why is it difficult to tell how large a star is? Stars appear
to be small because they’re very far away. Really, some
are much larger than our Sun.
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Astronomers use spectrographs to determine the elements found
in stars.
A spectrographs separates light into the individual colors of
which it is made. Then it produces an image of the resulting
spectrum. Most large telescopes have spectrographs to analyze
light.
How can the chemical elements be identified using spectrums?
Each element has its own signature spectrum, just like each
person has a unique set of fingerprints.
When the star’s light is seen through a spectrograph, each
absorbed wavelength is shown as a dark line on a spectrum.
So what are the dark lines in a spectrum? Wavelengths of light
absorbed by the element.
what happens to some of the light as it passes through the gases
in the outer layers of a star? The gases absorb certain
wavelengths of light, leaving dark lines in the spectrum.
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Brightness of a star depends on its size and also
its temperature.
What are the two ways to describe a star’s
brightness? Apparent brightness, and absolute
brightness.
Apparent brightness: how bright a star looks
from Earth.
Absolute brightness: how bright a star would
look if it were at the standard distance from
Earth.
What information is needed to determine a star’s
absolute brightness ?Apparent brightness and
distance from Earth.
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Example: imagine that you are riding in a car at
night, and two cars are approaching from the
opposite direction.
which car appears to have brighter headlights?
The car that is closer.
Does that mean the headlights are actually
brighter? Not necessarily, the headlights on the
other car may be the same brightness but appear
dimmer because they are farther away.
The brightness of the headlights was like
apparent brightness of stars.
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The scientists made graphs to see if the temperature and
the absolute brightness of stars were related.
They recorded information for many stars and developed a
diagram called an H-R diagram to classify stars and
understand how they change over their lifetimes.
Where do most stars fall on the H-R diagram? Most stars
are main sequence stars.
What kind of star is our sun? It is a main sequence star.
Do all main sequence stars look like our sun? No, some
can be blue or blue -white instead of yellow. Others are
orange or red.
What else do you need to know to classify a star as a main
sequence star? It’s absolute brightness.
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Students page 114 figure 3 –comment on
categories i.e. supergiant, color, temperature
etc. and him.
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What one can observe of figure 3 that we just saw is that
astronomers use HR diagrams to classify stars and to understand
how stars change over time.
Most of the stars in the diagram form a diagonal area called the
main sequence.
More than 90% of all stars, including the sun, are main sequence
stars.
Within the main sequence, the surface temperature increases as
absolute brightness increases.
Hot blue stars are located at the left of an HR diagram and cooler
red stars are located at the right of the diagram.
The brightest stars are located near the top of an H-R diagram
while the dimmest stars are located at the bottom.
Giant and supergiant stars are very bright. They can be found
near the top center and right of the diagram. White dwarfs are
hot but not very bright located at the bottom left or bottom
center of the diagram.
Homework:
Explore and explain the creation of an H-R
diagram. Draw a diagram and highlight in a
chart the key points.