How will our sun end?

Monday, Aug 12, 2024 3:34 PM

Greetings stargazers.

Happy monsoons. Even if I haven’t seen many stars in the past week, at least the plants in the garden have been happy.

In about five billion years or so, our sun will run out of useable fuel. I use the term useable because even though most of the sun will still be hydrogen, only hydrogen in the core experiences temperatures and pressures high enough to fuse. The outer layers provide the weight to compress the core enough for fusion. Today the power of the sun comes from fusing hydrogen into helium. Once the core runs out of hydrogen it will shrink and heat until the temperature and pressure are enough to fuse helium into carbon.

If the sun was much more massive, the process of fusing heavier and heavier elements would continue to iron and result in a Type II supernova. But our sun, as with all mid-sized stars, will have an end-of-life state that is just the carbon core. The core will shrink and heat enough to drive off all the outer layers of hydrogen and helium, leaving what is called a white dwarf star. These stars have a mass comparable to the sun, but a volume that is more comparable to the Earth. White dwarfs are very hot and very dense, made almost entirely of carbon. Because this is the end-state of all sun-like stars, white dwarfs are fairly common in the Milky Way.

Another common occurrence is for stars to be found in multiple-star systems. If a white dwarf happens to have a companion star, then interesting things can happen. Because white dwarfs are so compact, their surface gravity is much higher than the surface gravity on a typical main sequence star. If the companion star is entering a later stage of its life and is close enough to the white dwarf, it is possible for the strong surface gravity of the white dwarf to steal some of the hydrogen from the outer layers of its companion.

If enough hydrogen is taken to the surface of the white dwarf, a threshold can be reached when it will all fuse into helium and release a large outburst of energy. When this happens, the brightness of the white dwarf suddenly increases by a factor of 10,000 times. This event is called a nova, because it looks like there is a new star in the sky. If the companion star has enough hydrogen to continue supplying surface fuel to the white dwarf, this process can repeat multiple times. If the rate of mass transfer is steady, then these novae will occur at regular intervals.

One such predicted event is with the star T-Coronae Borealis, or the Blaze Star. Normally a star that is barely visible through binoculars, the Blaze star will be an easy naked eye star this month if it goes nova.

This month:

The two brightest stars in the summer sky are Arcturus and Vega. Both stars are close to magnitude 0.0, and both are bright enough to see their colors. Vega is blue-white and Arcturus is orange-white, which indicates Vega is the hotter of the two. Just after dark, Vega will be near zenith, the point straight overhead, and Arcturus will be toward the west. Another way to locate Arcturus is that the handle of the Big Dipper “Arcs to Arcturus.”

On a straight line between Vega and Arcturus are the constellations Hercules and Corona Borealis. Corona Borealis, the Northern Crown, is in the shape of a C. None of the stars in Corona Borealis are particularly bright. Off to the side of the C is the Blaze Star. Keep watching this month to see if there is something “new” visible in the constellation.