Sun Will Eventually Expand To Some 300 Times Its Current Size, Says Study

A recent observational study of some 191 giant stars at the end of their lives as hydrogen-burning, main sequence stars, places more precise parameters our own star’s expansionary endgame. The Sun will finish its life at perhaps two to three hundred times its current angular size.

The observations were made between 1996 and 2008 with the now defunct Palomar Testbed Interferometer atop Mount Palomar in southern California. They were coupled with data from the European Space Agency’s (ESA) Hipparcos and Gaia missions to more accurately calculate the stars’ distances and effective temperatures. This enabled a team of astronomers to obtain the angular diameters, the temperatures and true colors of these giant stars to a precision some two to four times better than previous surveys.

“This study has implications across the board, anywhere any astronomer needs to have supporting information on the size or temperature of a given giant star,” Gerard van Belle, an astronomer at Lowell Observatory in Flagstaff, Ariz. and the survey’s study lead, told me. “This survey is basically building a better ruler that you can then apply to other stars.”

In a presentation at the 241st meeting of the American Astronomical Society (AAS) this month in Seattle, van Belle noted that most of these stars would be close analogs of our own Sun in its old age.

Giant stars are stars that are no longer undergoing core hydrogen “burning” (fusion), says van Belle.

They have run out of nuclear fuel and have started hydrogen-shell burning, he says. The star’s surface shrinks down and there’s a shell around this cinder that is burning, says van Belle. But ultimately the core does get hot enough to then turn on the next phase of fusion, which is helium-burning, he says.

All of the stars we surveyed were either in this hydrogen shell burning or helium-core burning phase, says van Belle.

Even though giant stars and supergiant stars make up less than five percent of all stars, they represent about a third of all the stars one can see outside on a dark moonless night, says van Belle. That’s because they’re bright enough to be seen by the unaided eye out to much greater distances, he says. Familiar giants include Aldebaran, Arcturus, and Pollux; familiar supergiants include Betelgeuse, Rigel, and Antares, says van Belle.

A supergiant like Betelgeuse in the constellation of Orion Is a classic example, says Van Belle. Its core temperature gets much hotter than a smaller giant star, he says. Supergiants can fuse elements heavier than hydrogen and helium, says van Belle. But to support their tremendous mass, supergiants burn up the few much more quickly so that when all its fuels are exhausted, its core goes supernova, he says.

As for the importance of such stellar surveys?

Stars are the LEGOs of the universe, from which planetary systems to galaxies are built, and from which all the heavy elements are manufactured, says van Belle.

This study will aid astronomers anywhere and anytime they need supporting information on a given giant star’s size or temperature, says van Belle. They’ll be able to basically say, I now know the true color of this star, so I can determine its radius.

As for what the study has revealed about the Sun’s final stages?

Once the Sun begins its own hydrogen-shell burning, it will swell up to about 80 times its current size, says van Belle. But when the sun begins its helium-core burning phase, it will actually shrink back down a bit. Then after a couple of hundred million years, our Sun will swell back up and finish its life as a giant star some two to three hundred times its current size.

By giving us what van Belle terms “a fairly precise template for what these stars do as they age,” we now have a better picture of what the Sun will do when it runs out of hydrogen fuel at its core some five billion years from now. Mercury, Venus and most likely even Earth will be engulfed by the Sun’s expansion as a red giant.

In its very end stages, says van Belle, the Sun will expand so much —- that not unlike a smoke ring, it will just kind of evaporate, leaving only a hyper dense stellar white dwarf remnant at its former core.