The Sun’s Stability

How Steady Is the Sun?Using E = m c2, we know that the sun

hasenough fuel to last billions of years.

Will it do so at a steady, even pace? Or will

it undergo extreme variations, like aflickering campfire?

Why Does This Matter?A steady, even luminosity is helpful for

the long-term stability of our climate, and for life on Earth.

(Indeed, the persistence of life on Earth suggests that the sun has indeed been stable. But can we understand why?)

Consider a balloon…

...and a snowball

Compare Them

Consider squeezing and releasing:

The balloon springs back into its original shape

The snowball preserves any deformation

Which of these most closely resembles the sun?

The Sun Restores Itself Quickly

In response to any large distortion, like a ‘squeeze’, the sun’s equilibrium (and its original shape) would be restored in about 15 minutes.

So the sun is remarkably stable in structure!

(It is like the balloon, unlike the snowball.)

Let’s Think About Why

Here’s an important factor we’ll meet soon:

the rates of nuclear reactions depend on the temperature

(To be specific, if the temperature is raised, the reactions increase, producing lots more energy.)

Now Consider a Bicycle Pump

Compressionof the air leads to increasedtemperature.

(The pump can feel quite hot!)

Here is Why:

Likewise for the SunIf we compress the sun somewhat:

The central temperature rises (the particles move faster)

The nuclear reaction rates increase, producing more energy

Together, these effects provide extra pressure, quickly pushing the sun back out to its original configuration

ConverselyIf we expand the sun somewhat: The central temperature falls The nuclear reaction rate decreases, producing

less energy Together, these effects mean there is too little

central pressure to withstand the inward pull of gravity. The sun quickly shrinks back to its original configuration.

There is a Very Fine Balance…

The sun is said to be in ‘quasi-static equilibrium,’essentially unchanging on very long timescales -- by human standards, that is!

(Of course, it will slowly change in structure as it uses up its fuel.)

An AnalogyWhen a doctor does a bloodpressure test, she doesn’tworry about the slow buildupof plaque in your arteriesduring the time that it takesher to write down the results afterwards, or during the coming few days.

SimilarlyIf I can determine what the sun is like

‘right now,’ that should be a very good description for, say, at least the coming million years or so!

But: BewareCantankerous Old Age!

Nothing is Forever!Late in their lives, as the stars are running

out offuel, their structure can change quickly.

For instance, some stars wind up varying inbrightness, pulsating like a ‘beating heart.’

Thevariation may be as quick as once every day

or so,and stars can double in brightness.

(Imagine life on Earth, with that going on!)

Stellar DeathEventually, all stars die.

Indeed, the most massive ones do so explosively -- and quite spectacularly!