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Astronomy C10/L&S C70U
Stellar Life Cycles and Supernovae
Nicholas McConnell
Like many aspects of nature, star formation and destruction is a cycle. Stars form from clouds of gas,
remain stable for long periods of time, and then expel gas back into space, where it can later form new
stars. Below are four stages in the life of a massive star. Describe what happens in each stage, and
how energy is generated and used up.
Initial formation from a cloud of gas
Description:
Energy Source:
Energy Released As:
Main Sequence lifetime
Description:
Energy Source:
Energy Released As:
Red Giant/Supergiant Phase
Description:
Energy Source:
Energy Released As:
Supernova
Description:
Energy Source:
Energy Released As:
What happens in low-mass stars (less than about 10 solar masses) instead of a supernova?
Here's a quick list of qualities that should test your knowledge of two different supernova types. Mark
whether the qualities listed apply to Type Ia supernovae, Type II supernovae, or both. Try to keep in
mind how these relate to the ways in which the two types above are physically different from one
another.
Type Ia
Type II
Strong hydrogen lines in spectrum
No hydrogen lines in spectrum
Only occurs in a binary system
Requires a star more massive than 10*MSun
Completely destroys the exploding star
Leaves behind a neutron star or black hole
Creates heavy elements (including stuff humans are made of)
Almost always has a standard, predictable luminosity
X
We haven't learned the last one yet, but it will be important later. Alex's research team has used this
fact to show that the universe is expanding faster and faster, rather than slowing down as initially
expected.
Supernovae are extraordinarily energetic events. This next exercise will allow us to compare the
luminosity (energy output per second) of a light bulb, the Sun, all the stars in the Milky Way, and a
typical supernova.
Light bulb vs. Sun:
We know the formula for an object's brightness: b = L / 4d2 . This describes how much light we see,
but the same concept can also describe how much energy we feel as heat. By this token, we can
approximate that a light bulb is as "bright" as the Sun, when we stand close enough to it to feel the
same amount of heat as we would feel from sunlight.
If this is true, then bbulb = bSun, so Lbulb / 4dbulb2 = LSun / 4dSun2 .
Or, (LSun / Lbulb ) = (dSun / dbulb )2 .
There is a 100-Watt light bulb at the front of the classroom. 1 Watt is one Joule of energy released per
second, so Lbulb = 100 J/s.
dSun is 1 A.U., or 1.5 x 1013 cm. Our experiment will give us dbulb. Hold your hand out and approach
the light bulb until its warmth feels about the same as sunlight. Then measure the distance from the
bulb to your hand, in centimeters.
Now you have Lbulb, dbulb, and dSun. You can plug these values into the bold equation above and
calculate the Sun's luminosity in J/s. Do it!
Sun vs Milky Way:
Your answer above for LSun will depend on what distance you chose for dbulb. The actual value of LSun
is about 1026 Joules per second (!!!). Hopefully your answer was close (1024 - 1028 J/s might be
reasonable, given our method).
In any case, the Milky Way galaxy contains about 100 billion (1011) stars. Some are much more
luminous than the Sun, and many are less luminous, but an acceptable approximation is that the
average luminosity per star is 1 LSun . Therefore, the Milky Way has an approximate luminosity of
1011 * 1026 J/s.
So, LMW =
J/s (one easy step).
Milky Way vs Supernova:
Now, a typical Type II supernova explosion releases 1044 Joules of light (and hundreds or thousands of
times more energy in the form of neutrinos and high-speed gas!) Suppose this release occurs over a
single second. For that initial second, how many times brighter will the supernova be than the entire
Milky Way galaxy?
Wow!!!!!!!