# When did scientists discover that the Sun has a life cycle and that it is going to die?

I have been researching the history of scientific studies about the the Sun. However I have been unable to find out much information about how the scientific consensus started forming around stars' life cycles.

I know that it has been known since Galileo that some star have sudden changes in luminosity during their life. But I was unable to find out when scientists actually discovered that stars are born and die, and especially how this influenced the view on the future of the Solar System.

I suppose this happened somewhere at the start of the 20th century, when more information and research about supernovae was being found, but I couldn't find more precise information about it.

• Not an answer to your question but here's a recent question I asked about when it was discovered that our sun won't go supernova which may be of interest and gives some leads astronomy.stackexchange.com/questions/36802/… – Wiggo the Wookie Sep 11 '20 at 13:50
• I’m voting to close this question because I recommend migration to HSM.SE (history of science....) – Carl Witthoft Sep 11 '20 at 15:12
• @CarlWitthoft Unlike physics.SE, history questions are on topic in this corner of the StackExchange. There is even a tag for just that. – David Hammen Sep 12 '20 at 3:49
• @CarlWitthoft if after reviewing the 154 questions here tagged history you still feel that astronomy history questions should be off-topic and perhaps all of those questions closed, then you can advocate for that in a question in Astronomy Meta. – uhoh Sep 12 '20 at 4:28
• @CarlWitthoft - See also this question on Meta about whether history of astronomy questions are on-topic here. – user24157 Sep 14 '20 at 22:29

Although various astronomers have speculated that the Sun was a star (some were imprisoned or even burnt alive for such heresy), this was not known definitively until 1838 when Friedrich Bessel used parallax to calculated the distance to 61 Cygni.

In the late 19th century, Lord Kelvin provided rather small (less than 100 million years) for the ages of the Sun and of the Earth. He did not know about fusion. The nature of what powers the Sun was not known until the 1920s based on observations of the mass of a proton versus that of a helium-4 nucleus. The exact mechanism wasn't known until 1938 when Hans Bethe developed models by which hydrogen fuses to form helium.

This model provided the basis for knowing that the Sun will eventually die as it will eventually run out of fuel. Working from this, a number of scientists developed models of what happens inside stars of various masses based on the growing knowledge of stellar nucleosynthesis.

• It is important to note, I believe, that geological evidence for a longer age of the earth were already available to Kelvin, and his argument could be taken as a contradiction or challenge understanding that there was unknown physics. There was a period of time that the best known estimate for age of the earth based on geology exceeded the estimate for the lifetime of the sun based on thermodynamic principles without knowledge of fusion. – crasic Sep 11 '20 at 19:42
• The obsolete notion that stars are created hot and cool as they age survives in astronomical terminology, where "early-type" refers to hot stars and "late-type" refers to cooler stars, regardless of their actual stage of evolution. – John Doty Sep 11 '20 at 20:07
• @crasic - You are correct. Mostly. Geologists would have been happy with an age of a few hundred million years. (Kelvin later refined his estimates to about 20 million years, both for the Sun and the Earth, far too young for geologists tastes at the start of the 20th century.) It was the new science of evolutionary biology that balked at Kelvin's age estimates as far too short. Geologists however later objected to physicists who estimated that the Earth was much older than geologists thought it was. But that was because geologists did not yet know of plate tectonics. – David Hammen Sep 11 '20 at 21:36
• @JohnDoty - Of course it's an obsolete notion. Lord Kelvin did not know about quantum mechanics. No scientist of his time knew that. – David Hammen Sep 12 '20 at 3:56
• @DavidHammen And yet we confusingly cling to the terminology along with the even more obsolete spectral classification that assigns letters to temperatures out of order. But at least we don't still call x-ray AGNs "UHGLSs". – John Doty Sep 12 '20 at 12:54