Introduction to Astronomy
Transcript Introduction to Astronomy
• Wednesday night star parties begin this
week, 8:45 pm, weather permitting. Attend
one for 4 points extra credit! (Staff
• 10:00 office hour cut short today
• Majors Fest at Union Building Gallery
Nuclear Reactions in Stars
1 November 2006
• What makes the stars shine?
• How long do they last?
• Then what happens?
The Sun’s Interior
only take place in
the innermost 30%
of the sun’s radius.
The central density
is 150 times that of
water; the central
temperature is 15
The sun is a mass of incandescent gas,
A giant nuclear furnace,
Where hydrogen is built into helium
At a temperature of millions of degrees.
-- Zim and Baker, A Golden Guide to
Stars, 1951; borrowed by They Might
Be Giants, 1993
Can we test any of this theory?
Yes! Look for the neutrinos…
Fusion of Hydrogen into Helium
4 1H (protons)
This reaction powers all main-sequence stars.
The more massive the star, the more pressure
at its center and therefore the faster the reaction
Masses of Stars
Sizes of Main-Sequence Stars
Hottest stars are
Should be white,
Reds are greatly
Main Sequence Lifetimes
What happens when the core of a
star runs out of hydrogen?
• With no energy source, the core of the star resumes its
• As it collapses, gravitational energy is again converted
to thermal energy…
• This heat allows fusion to occur in a shell of material
surrounding the core…
• Due to the higher central temperature, the star’s
luminosity is greater than before…
• This increased energy production causes the outer part
of the star to expand and cool (counterintuitive!)…
• We now have a very large, cool, luminous star: a “red