Ask A Scientist Astronomy Archive

name         Cecilia
status       student
age          11

Question -   How do scientist know the sun's core temperature?
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Hi Cecilia,

This is a good question, after all, we have no direct access to the sun's
core, but if we apply some knowledge of physics, we can get to a reasonable
approximation quite readily.

First, we have to understand that the sun is mostly gas-like particles. If
the sun did not have gravity the gases would spread away since nothing would
hold them in place. However, if all the sun had was gravity, then that would
compress the gas into a smaller volume. Since the sun is giving off energy,
then the gases spread out and the sun is bigger than it would be if it was
not giving off energy. So there are two opposing forces: (1) the gravity of
the sun which pulls the gas-like particles inward, and (2) the energy output
of the sun which pushes the gas-like particles outward. The sun's size is
the result of the balancing of these two forces.

Next, we need to find the mass of the sun. We can measure the gravity of the
sun, by observing how the planets are moving around the sun. Since we know
that mass is related to gravity, we now have a value for the mass of the
sun.

BUT we know how much gravity this amount of mass should have. We also know
the effect this gravity would have on the size of the sun - IF it were not
giving off energy. Therefore, we can know just how much energy the sun is
giving off in order to maintain its bigger size.

So, we measure the volume of the sun. That's also easy to do. We simply look
at the sun (under a filter so that we do not burn out our measuring
instruments) and measure its diameter.

So, knowing the mass of the sun, and the size of the sun, can tell us how
much energy the sun is giving off. But we can relate energy to temperature.
And this is how we know the temperature of the core of the sun.

Greg (Roberto Gregorius)
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