DEATH FROM THE SKIES!

Reviewed 5/15/2016

Death from the Skies!, by Philip Plait

DEATH FROM THE SKIES! These are the Ways the World Will End Philip Plait New York: Viking Penguin, October 2008				Rating: 5.0 High
ISBN-13 978-0-670-01997-7
ISBN 0-670-01997-6	326pp.	HC/BWI	$25.95

I base this page on Dr. Plait's Chapter 7: The Death of the Sun. This chapter presents a detailed account of our Sun's evolution from its current status, fusing hydrogen into helium at a nearly constant rate, through its far-future expansion into sub-giant and red-giant stages, and finally into a slowly cooling white dwarf.

The Sun's sub-giant phase will begin roughly 3.5 billion years from now. But life on Earth will be challenged by increasing output from the Sun long before then. In only 1.1 billion years, its output will have risen some 10 percent. This — assuming we do not intervene with advanced technology — will begin to strip away significant quantities of water from the top of the atmosphere.

It gets worse. By 3.5 billion years from now, the start of the sub-giant phase, the Sun will be 40 percent hotter. Earth's oceans — again, absent intervention — will have totally boiled away. Life forms might survive in heavily protected enclaves, but this would be merely a rear-guard action. As the Sun swells through its giant phases, it will vaporize Mercury and probably Venus. Earth might remain intact, depending on exactly how the scenario plays out; but its surface will certainly be lifeless.

As I say, Dr. Plait's Chapter 7 traces these astrophysical developments through the red-giant phase and well beyond. I've endeavored to summarize these changes in the table below.

The Evolution of Sol

Age of Sun (Bln Yrs)1	Time from now (Bln Yrs)1	Core		Solar		Notes
		Temp (Mln °K)2	Pressure (Bln Atm.)2	Radius (Mln km)	Brightness Increase
		Temp (Mln °K)2	Pressure (Bln Atm.)2				4.6	0	27	250	0.1	0%	Steadily fusing H into He
5.7 - 8.1	1.1 - 3.5	27	250			0.1	10% - 40%	Steadily fusing H into He, slowly brightening
10.9 - 11.6	6.3 - 7.0	27	250	0.15 - 0.23	80%	No H in core; He approaches fusion point
11.6 - 12.3	7.0 - 7.6	27	250	0.23 - 100	240,000%	Core degenerate; solar wind very dense
12.3 - 12.4	7.6 - 7.7	100	250	100	240,000%	At some point, He fusion flash occurs;3 Sun settles down afterward.
12.4 - 12.5	7.7 - 7.8	27	250	150	300,000%	Core of C & O again degenerate; He shell repeatedly flash-fuses.
12.5	7.8	27	250	0.1	0%	Sun a white dwarf surrounded by gas clouds, descending into darkness.

1 I've fudged these numbers a bit, to keep the number of significant figures constant.

2 He doesn't specify core temperature and pressure for every time interval.

3 This initial helium flash lasts only a few seconds, because the rate of Helium fusion has an extreme positive sensitivity to temperature.

Errata for Death from the Skies!