MadSci Network: Astronomy Query:

### Re: What if... the Earth orbited a 1.5 solar mass star ?

Date: Sat Sep 5 16:37:31 1998
Posted By: Jay H. Hartley, Post-doctoral physicist, Lawrence Livermore National Lab
Area of science: Astronomy
ID: 904379596.As
Message:

In the galactic scheme of things, Patrick, a change from 1 solar mass to 1.5 solar masses would be pretty insignificant. Known stars range in mass from around 5 percent of the sun's mass up to about 50 solar masses. Our immediate neighborhood is dominated by smaller stars - only a handful of the 50 closest start are bigger than the sun. Our new star would still be substantially smaller than most of the brightest stars we see in the sky - Sirius A, Procyon, Vega, Regulus, Betelgeuse, etc.

According to the table in my Physicist's Desk Reference (Herbert Anderson, Ed., AIP Press, 1989, p75), a 1.5 solar mass star (I'll call it Sol Version 1.5) during the main sequence of its life, with composition roughly like the Sun, would be more than 6 times brighter than the Sun. Because of the increased light output, Sol 1.5 would burn out its fuel substantially faster, spending only 2 billion years as a main-sequence star rather than the 10 billion years we expect for the Sun.

If you want to look more at the details of stellar evolution, the Illinois Math and Science Academy has some interesting Stellar Structure Resources that are part of the student projects on the Origin and Evolution of Stars. The formulas derived there for calculating the increase in luminosity don't quite jive with the factor of 6 I quoted above; they would indicate more like a factor of 4 increase and a lifetime of just under 4 billion years. Those formulas are derived in a fairly simplistic way, so I decided to go with the numbers from the scientific literature. I'd have to track down the original journal article to figure out just where the difference arises. It may just be a result of the variations among main sequence stars and a difference in what the various sources choose as "typical." It's always possible I punched a wrong number on my calculator, too...

Now, let's look at the consequences of this for dear old Earth. Over six times more sunlight (or so) would be hitting our planet. That would be about the same intensity as now hits Mercury, and three times the amount that hits Venus. This would be a very hot place, indeed - all the water and much if not all of the atmosphere would boil away, I'm sure.

In addition, the pull of the new Sun's gravity would be 50% stronger. For a planet at the same orbit as the Earth, the orbital velocity would have to be higher, and the year would be reduced to 298 days.

In order to get the same luminosity from Sol 1.5 that we now enjoy from the Sun, the Earth would have to move about 2.5 times farther away, in the middle of the current Asteroid Belt. Presumably, a cold gas giant like Jupiter would no longer be able to survive so close, so we wouldn't have to worry about its tidal pull ripping our little planet into a bunch of asteroids.

The new locale would allow our average temperature to be about the same as it is now; however, the Earth's orbit would be lengthened to almost 1200 days - over 3 years. If the Earth were still tilted at the same angle, that would mean the summers would probably get hotter, and the winters colder than they do now in the temperate regions. The higher latitudes such as northern Canada and southern Argentina would experience summer "white nights," when the sun wouldn't set for several months rather than days or weeks, and a similarly long stretch in the winter with no sun at all.

Because of the higher surface temperature of Sol 1.5, the color spectrum of light would also shift, with the peak moving from the Sun's yellow all the way into the violet-ultraviolet range, where the light would be somewhat more attenuated by an atmosphere like ours.

How exactly these changes would impact the evolution of life on Earth is an exercise for a fertile imagination, and perhaps could seed some ideas for a good sci-fi story! The discoveries in the past decade of life at the bottom of the ocean and hidden deep in ancient glaciers have shown that we should very careful when we think about where life could and couldn't survive.

Fun question, Patrick. Stay curious.

Jay

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