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Luminosity (relative to Sun)
10,000
We start by drawing the axes:
•Luminosity up the vertical axis (measured relative to the Sun)
•Temperature along the horizontal axis (measured in Kelvin)
The stars Vega and Sirius are brighter than the
Sun, and also hotter. Where would you put
them? Where would you mark the Sun on the plot?
100
Vega
Sirius
•It has Luminosity of 1 relative to itself
•Its temperature is 5800 K
1
0.01
0.0001
Sun
In fact, most stars can be found
somewhere along a line in this graph.
Some stars are much cooler and less luminous, such
calledstar
the to
“Main
Sequence”.
asThis
the isclosest
the Sun,
Proxima Centauri.
Where would you plot these?
Proxima
Centauri
These stars are called red dwarfs.
40,000
20,000
10,000
5,000
Temperature (Kelvin)
3,000
The bright star Betelgeuse is even more
luminous than Aldebaran, but has a cooler
surface.
Rigel
Deneb
Luminosity (relative to Sun)
10,000
Betelgeuse
Aldebaran
This makes it a red supergiant.
Arcturus
100
Vega
Sirius
1
0.01
0.0001
Sun
Even brighter than Betelgeuse
are
likestars
Deneb
andthe
Rigel,
Butstars
not all
lie on
main sequence.
Sirius
B
which
much
hotter. and Aldebaran, are
Some,are
such
as Arcturus
much
brighter
than the
Sun,
but
cooler.
Some
of
the
hottest
stars
are
actually
much fainter than the
These
are
blue
supergiants.
Where
would
thesewould
lie onthey
the diagram?
Sun. Which
corner
be in?
These
giant stars.
These are
are orange
white dwarfs,
such as Sirius B which orbits Sirius.
40,000
20,000
10,000
5,000
Temperature (Kelvin)
3,000
Proxima
Centauri
Supergiants
Rigel
Luminosity (relative to Sun)
10,000
Betelgeuse
Deneb
Giants
100
Arcturus
Vega
Sirius
1
Almost all stars we see are in
one of these groups, but they
don’t stay in the same place.
Sun
Sirius B
Proxima
Centauri
As stars evolve they change in
luminosity and temperature.
0.01
This makes them move around the
Hertzprung-Russell diagram.
0.0001
40,000
20,000
10,000
5,000
Temperature (Kelvin)
3,000
Luminosity (relative to Sun)
10,000
100
1
0.01
Sun
The Sun has been on the Main Sequence
for billions of years, and will remain there
for billions more.
But eventually it will swell into a giant star,
becoming more luminous but cooler.
0.0001
40,000
20,000
10,000
5,000
Temperature (Kelvin)
3,000
Luminosity (relative to Sun)
10,000
100
Sun
1
At this point it is a red giant star.
0.01
It will get then hotter and slightly brighter,
briefly becoming a blue giant.
0.0001
40,000
20,000
10,000
5,000
Temperature (Kelvin)
3,000
Luminosity (relative to Sun)
10,000
Sun
100
1
Finally nuclear fusion in the core will cease.
0.01
The Sun will become a white dwarf, far less
luminous than before but with a hotter
surface temperature.
0.0001
40,000
20,000
10,000
5,000
Temperature (Kelvin)
3,000