Would the Sun's luminosity decrease abruptly if the thermonuclear fusion stopped suddenly?

Question

Supposed that thermonuclear fusion stopped suddenly in the Sun. Would the Sun's luminosity also decrease as abruptly?

Answer 1

Actually, no. It takes (IIRC) somewhere around a half million years for photons from the Suns core to get to the surface, so it would get dimmer over a long period of time.

HTH ☺

Doug

Answer 2

That's a little hard to answer, as there is no known mechanism whereby a main-sequence star could abruptly shut off. I'd assume the end result would be a white dwarf, which would be considerably less luminous - even the hottest white dwarfs are about 5 magnitudes (100 times) dimmer than the Sun.

As to how long it would take, perhaps we could assume it would progress similarly to a supernova. Core collapse happens immediately, but it takes something on the order of a day for the shock wave from the collapse to reach the surface. At this point, the exploding star brightens dramatically in the space of a minute or two, and gradually fades over a period of about a year.

However, what brightens a supernova is the sudden fusion of degenerate material. Since you have turned fusion off, I guess it's up to you to decide how much energy is released in the collapse, which will determine how bright the Sun gets before it settles down, and how long it will take.

In any case, without the energy of fusion to keep it from collapsing, the Sun would shrink to about the size of the Earth. Even though it would be much hotter than it is now, its surface area would be something like one ten-thousandth of what it is now, so it would radiate much less energy.

Answer 3

What do you mean by "abrupt decrease"? If the luminosity was plotted as a function you would be able to look back and tell by the luminosity plot when the sun's reaction stopped.

Think of a super heated rock or piece of metal in a fire. It is as hot and bright as it will get and full of super excitied electrons that sometimes give off photos as they decrease on enegry levels. When you take the object out of the fire it continues to glow for some time as it cools, but it does start to cool and dim right away.

The light that comes to us from the sun is because the sun's surface is super heated in a similar way. At that point the laws of thermodynamics could tell you how the sun would cool, and based on the composition of the sun you could figure out at what temperture those elements stop giving off photons.

To help convince you of this, think of how starts have colors. A stars color is based (mostly) on it's surface temperature since stars are all (mostly) made of the same stuff and have the same fussion reaction at heart. So the light comes from the surface. (I say mostly because some older stars have heavier elements and differant elements give off differant color photons so that is another factor)

The surface temperature of the sun is pretty constant, but once the reaction went out the sun would start to cool off and dim right away, but not all that quickly.

Answer 4

Yes and no: Heat and light from the center of the sun, where density is high enough for fusion, takes _many_ years to reach the photosphere. However, "sound" waves propagate faster; the whole sun has been observed to quiver after some violent event. So, to the extent energy is carried by vibration, there might be a small drop in output in a day or two.
It's strange to think that in the sun, sound travels _faster_ than light.

Answer 5

The fusion reaction in the sun means the consumption of hydrogen. When we say it st oped,it means that ALL hydrogen gas has been consumed, and thus no reaction can be sustained or kept occurring. There will be no light produced, and no heat ejected. The whole mass - in that case would be helium gas. There will be a smaller size of the resulting planet and it will be called a "dwarf".

Answer 6

No, it has a huge mass. After some time I can't estimate, the size of the sun would start to contract with the pressure of thermonuclear reaction gone.