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```The Life of Stars – the
Hertzsprung-Russell diagram
The HR diagram
Astronomers have observed thousands of stars and have figured out
that they are seeing a life cycle, they grow old and then die.
A good starting point is to plot a
graph of the luminosity of different
classes (essentially temperatures)
of star. By examining the light
coming from a star (its spectra) we
can determine what class of star it
is and where it fits on the diagram
shown.
You should be able to sketch the
main features of this very important
diagram including labelling the
axes. (notice the increase by
powers of 10)
HR diagram – what’s the point?
Are all stars the same? Not in the least!
Some stars are just beginning to form in
nebulae, others are enjoying middle age
along the main sequence, and some
have begun to die.
The Hertzsprung-Russell Diagram is a tool that shows relationships
and differences between stars.
It shows stars of different ages and in different stages, all at the
same time. But it is a great tool to check your understanding of the
star life cycle.
Notice the background colours shown. This relates to the colour the
star appears to us – this depends upon its temperature.
HR diagram – what’s the point?
In the Hertzsprung-Russell Diagram, each star is
represented by a dot.
There are lots of stars out there, so there are lots of
dots.
The position of each dot on the diagram tells us two
things about each star: its luminosity (or absolute
magnitude) and its temperature.
The vertical axis represents the star’s luminosity or absolute magnitude. Luminosity is
technically the amount of energy a star radiates in one second, but you can think of it as how
bright or how dim the star appears. The values are all relative to the sun
The horizontal axis represents the star’s surface temperature (usually labelled using the Kelvin
temperature scale), but notice the higher (hotter) temperatures are on the left, and the lower
(cooler) temperatures are on the right.
Reading the HR diagram
A star in the upper left corner of the diagram would be hot and bright.
A star in the upper right corner of the diagram would be cool and bright.
The Sun rests approximately in the middle of the diagram, and it is the star which we use for
comparison.
A star in the lower left corner of the diagram would be hot and dim.
A star in the lower right corner of the
diagram would be cold and dim.
Are there any stars that seem out of
place? For example, are there any stars
that are really hot but not very bright?
Are there any stars that are not very hot
but they shine very brightly? What do you
think could account for these differences
in stars that do not fit the pattern?
Accounting for some strange results
Stars that do fit the pattern are called main sequence stars. Most of the stars lie within this region.
There is a predictable
relationship between the
brightness and the
temperature - hotter
things are brighter.
There is a predictable
relationship between
the brightness and size
of a star – a bigger star
is brighter
Stars that do fit the pattern are called main
sequence stars. Most of the stars lie within this region.
There is a predictable relationship
between the brightness and the
temperature - hotter things are
brighter.
there is a predictable relationship
between the brightness and size of a
star – a bigger star is brighter
A family portrait
http://www.mhhe.com/physsci/astronomy/applets/Hr/frame.html
http://www.astro.ubc.ca/~scharein/a311/Sim/hr/HRdiagram.html
We can consider the HR diagram as a family portrait of stars
The HR diagram has stars on it at every phase in the life cycle.
Today, a star could
be residing on the
main sequence, but
when the star ages
by hundreds of
millions of years, it
destabilizes. The
next "family portrait"
may have that exact
same star off the
main sequence as a
red giant
Main sequence sample data activity
example star
Orionis C
Becrux
Spica
Achernar
Rigel
Sirius A
Fomalhaut
Altair
Procyon A
Alpha Centauri A .
The Sun
Mu Cassiopeiae
Tau Ceti
Pollux
Epsilon Eridani
Alpha Centauri B
Lalande 21185
Ross 128
Wolf 359
temperature K
33,000
30,000
22,000
15,000
12,500
9,500
9,000
8,700
6,400
5,900
5,800
5,600
5,300
5,100
4,830
4,370
3,400
3,200
3,000
luminosity
30,000
16,000
8,300
750
130
63
40
24
4.0
1.45
1.00
0.70
0.44
0.36
0.28
0.18
0.03
0.0005
0.0002
mass
18.0
16.0
10.5
5.40
3.50
2.60
2.20
1.90
1.35
1.08
1.00
0.95
0.85
0.83
0.78
0.68
0.33
0.20
0.10
5.90
5.70
5.10
3.70
2.70
2.30
2.00
1.80
1.20
1.05
1.00
0.91
0.87
0.83
0.79
0.74
0.36
0.21
0.12
Plot the following
relationships:
a. Temperature (y-axis)
against mass (x-axis)
b. Luminosity (y-axis)
against temperature
(decreasing x-axis)
Plot using scales as in the
previously seen examples
```
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