His work on nuclear reactions led Bethe to the discovery of the reactions which supply the energy in the stars. The most important nuclear reaction in the brilliant stars is the carbon-nitrogen cycle, while the sun and fainter stars use mostly the proton-proton reaction. Bethes main achievement in this connection was the exclusion of other possible nuclear reactions. The Nobel Prize was given for this work, as well as his work on nuclear reactions in general.
https://www.nobelprize.org/prizes/physics/1967/bethe/biographical/
Some of those reactions are described in wikipedia's Hans Bethe biography:
https://en.wikipedia.org/wiki/Hans_Bethe#United_States
I met Bethe in the early 'eighties. I was a troubled young man always in danger of dropping out of college permanently. I must have asked Bethe some questions he thought interesting because he invited me to a dinner he was hosting to raise money for a United Nations educational initiative. Mind you, I was a near-starving student, and very afraid I'd embarrass myself. And I was awestruck. But my self-confidence, which had been hoovering near zero at the time, got a tremendous boost. Bethe's small act of kindness and generosity lifted me out of a very dark place.
Barrack Obama has a very similar energy about him. These are people who have carried the weight of the world, Obama as President, Bethe with his work on the Manhattan Project, but this didn't make them bitter, mean, or less empathetic towards others.