The HR Diagram

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5/14/06

• The HR Diagram
• Or how we can tell how hot and bright a star
  really is!

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As I mentioned before in Class

• There are billions of stars in our galaxy (The
  Milky Way)

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And there are billions of galaxies in the
Universe
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That must mean there are billions upon billions
(maybe trillions) of stars in the Universe.
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Science uses The HR Diagram to keep track of the
life and times of star as well as its brightness
and temperature.
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The HR Diagram plots brightness (Y-Axis) to
temperature (X-Axis)
Brightness
Y-Axis
Temperature in Kelvin
X-Axis
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Brightness and Temperature
The Brightness is given in a range from 0.0001
to 10,000 solar units
Our Sun is designated as having 1 solar unit

• Our Sun is around 6000K.

The Temperature, given in Kelvin, has a range
from 3000K up to 40,000K
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Plotting Stars
10,000
Cool and bright
Very hot very bright
Brightness
Hot and bright
Solar Units
The Sun
1
Very Hot dim
Cool Dim
0.0001
10,000K
30,000K
6000K
3000K
40,000K
Temperature In Kelvin
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Classifying Stars

• Astronomers sometimes do not label items the easy
  way
• Instead of labeling stars, lets say, A through F
  class stars.
• They labeled them O,B,A,F,G,K,M
• Way to remember them Oh Big And
  Fearsome Gorilla Kill Me

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So, a simple HR Diagram may look like this!
10000
.
.
.
With stars plotted all over the chart. The dots
are stars!
.
.
.
100
.
.
.
.
1
.
.
.
.
.
.
.
0.01
.
0.0001
6000K
30000K
3000K
10000K
M
A
F
K
O
B
G
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Now most stars are not just randomly scattered on
The HR Diagram

• A relationship between brightness and temperature
  does exist
• Most stars are located in a band that stretches
  from the top left corner to the bottom right
  corner.

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Stars along the diagonal center line are known as
the MAIN SEQUENCE
A Sun sized star
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The color of a star!

• In addition to O,B,A,F,G,K,M. Stars emit a
  certain color.
• Stars that are way hot emit a kind of bright blue
  to even white color.
• Stars that are way cold (so to speak) emit a
  reddish color.
• Our Sun emits a yellowish/greenish color

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So the stars plotted along a HR Diagram should
look like this
.
10000
The Sun
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.
100
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.
.
Brightness Solar Units
.
.
.
.
.
.
.
.
.
1
.
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.
0.01
.
.
0.0001
6000K
60000K
3000K
20000K
TEMP
M
G
O
B
A
F
K
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Typically, but not always, stars that are off the
Main Sequence are either being born or dying off
.
10000
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.
.
Big and OLD (Red Giants)
.
.
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.
.
100
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Waiting for birth
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1
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0.01
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0.0001
OLD (White Dwarfs)
6000K
60000K
3000K
20000K
TEMP
M
G
O
B
A
F
K
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One last thought!

• Certain stars, the large ones, have very violent
  deaths. They become SUPERNOVAS and explode
  destroying any planets in that solar system.
  Possibly other solar systems as well.
• They may even become BLACK HOLES!
• Our Sun will not do this!