Death of a star: Part three tells the tale of stellar demise

When a star runs out of hydrogen, it enters those last phases of its life before finally collapsing under its own weight.

SW PHOTO FOR JUNE 17-19, 2022.jpg
The Crab Nebula
Contributed / Mike Lynch
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Welcome to part three of my trilogy on stars' birth, life, and death.

In last week’s Starwatch column about the life of stars, I told you how stars cook up energy in their cores with nuclear fusion. Hydrogen atoms fuse to form heavier helium atoms because of intense pressure resulting in astronomically high temperatures. In the process, a tiny bit of hydrogen is converted into energy, which then forces its way to the outer levels of a star.

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Hydrogen is the fuel of a star, and smaller and less massive stars like our sun sip their hydrogen supply slowly and live a long time. Our sun has been around for about 5 billion years and should have enough hydrogen fuel left in the tank to keep it going for another 5 billion years.

More massive stars are not around for nearly as long, and they’re literally what you could call hydrogen gas guzzlers. The big guys of the stellar world may only last a few billion years. Sooner or later, all stars begin to run out of hydrogen in their cores, and stellar death gets underway. Smaller stars like our sun certainly die a violent but prolonged death, but the really massive stars go out with a huge bang.

Death of our sun

Low-mass stars like our sun get really fat as they die and gradually flicker out. In the case of our sun, it will run out of hydrogen in about 5 billion years, with helium building up as the hydrogen dwindles.


When there is no longer enough hydrogen in the core, there’s no longer a balance between internal nuclear fusion energy and the never-ending external gravitational pressure. The core begins to contract, causing the temperature in the core to climb even higher. That causes the helium atoms to fuse into carbon and oxygen atoms. Some heat escapes beyond the core, reaching the cooler, outer hydrogen layers toward the edge of the sun.

In time, the temperature rises high enough in these layers to fire up hydrogen nuclear fusion. All of this added energy causes the sun to bloat out into a red giant star. The reason the sun will turn red is that outer layers will expand and cool off.

When this happens to our sun, our home star will swallow up the planets Mercury and Venus and touch Earth. At that point, we’ll be toast. Even though the sun will have a cooler surface temperature of about 3,000 to 4,000 degrees F, it will be right on top of us.

Also, toward the end of our sun’s red giant phase, excess energy “burps” in the outer layers will cause large clouds of gas to blow off and form large rings and shells of gas around what’s left of our star. Astronomers call these planetary nebulae. Even though they’re called planetary nebulae, they have nothing to do with planets. They got that name because back in the 1700s and 1800s telescopes weren’t quite up to the standards of the Hubble Telescope, and through those archaic scopes, these burping stars resembled giant planets.

Planetary nebulae don’t last long, though. Eventually, stars like our sun will totally run out of all fusion fuel and shrink into white dwarfs. With no more nuclear reactions inside the star to hold it up, gravity collapses the corpse of the once-proud star. In our sun’s case, it’s believed that whatever is left of the sun’s original mass will be squished into a ball about the size of Earth. When that happens, our sun will be considered a white dwarf or a “retired star.”

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The southwestern sky, June 21-23, 2022.

Celestial happening this week

On Tuesday, Wednesday, and Thursday morning, the last quarter moon will be passing by the bright planets Jupiter and Mars in the predawn and early morning twilight southeastern sky. Jupiter is by far the brighter of the two planets. Mars will have a distinct orange-red glow to it.

Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul and is author of the book, “Stars, a Month by Month Tour of the Constellations” published by Adventure Publications. Send questions to .


The Rochester Astronomy Club welcomes new members and puts on public star parties. Their website is .

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