ASHLAND Understanding the power of the Sun

DR. GEORGE SPAGNA

Special to The Local

The Keeble Observa- tory at RandolphMacon College will hold public viewing on Thursday evenings, 7 –9 p.m. For more information check their website (www.rmc.edu/Keeble), Instagram page (https:// www.instagram.com/ keebleobservatory) or call the information line (804752-3210).

Last month we discussed the “breakthrough” hydrogen fusion reaction achieved at Lawrence Livermore Laboratory, in which the fusion released more energy than the laser energy which triggered it. That reaction is not ready to produce energy for our use any time soon, since the inefficiency of lasers meant that far more energy went into producing the laser beams than the ultimate reaction produced.

We compared that experiment to the fusion powering the Sun, essentially converting four hydrogen nuclei into a single helium. This fusion takes place at the inner 10% of the Sun (it’s “core”) where the temperature is about 20 million kelvins. (One kelvin is the temperature equivalent of a Celsius degree, measured from “absolute zero” at minus 473 degrees Celsius.) Since only the outer 20% or so of the Sun is convective, there is no mixing of new “fuel” into the core nor helium mixing out to the surface. Which means that once the concentration of core hydrogen is too low, the “fire” will go out.

Five billion years ago, when the Sun first formed and the fusion started, the core was about 70% hydrogen by mass – the same as the current sur- face composition. Ninety percent if you’re counting nuclei. Modeling tells us that the concentration there is now closer to

35% and hydrogen fusion should continue for another 5 billion years before the concentration of hydrogen is so low that collisions between hydrogens become so rare that fusion essentially stops. What then? That energy release has led to a balance between the outward gas pressure and the inward force of gravity. When the fusion stops, gravity gains the upper hand and the core will begin to contract. That contraction releases gravitational potential energy which causes the core to get hotter! That will ignite hydrogen fusion in a thin shell around the core, which because it’s hotter will actually increase the total energy released causing the outer layers of the Sun to swell up and cool off. The Sun will become a red giant whose outer surface will approach or even overwhelm the size of Earth’s orbit. Our home world will be toast – or vapor. Time to sell real estate and move!

Next month I’ll continue the story – because it’s not finished yet!

Lunar phases for March: Full Moon on the 7th, at 7:40 a.m.; Last Quarter on the 14th, at 10:08 p.m.; New Moon on the 21st at 1:23 p.m.; and First Quarter on the 28th at 10:32 p.m. All times are Eastern Standard.

Mercury returns to evening dusk late in the month. Venus and Jupiter will be within a halfdegree of each other on the 1st – look west after sunset. They’ll be the

Please see UNIVERSE, Page 23