The Life Cycle of Stars

Stars begin life inside vast clouds of gas and dust called nebulae. Gravity draws the hydrogen together until temperatures reach millions of degrees, at which point nuclear fusion ignites and a star is born. After billions of years the star exhausts its nuclear fuel and collapses. Depending on its mass, it may end as a white dwarf, a neutron star, or a black hole. The heavy elements that make up our bodies — iron, calcium, oxygen — were forged in stellar cores or in supernova explosions. In a real sense, we are made of star dust.

Modern stellar astrophysics took shape in the 20th century. Edwin Hubble resolved the nature of galaxies (1929); Hans Bethe proved theoretically how stars burn via nuclear fusion (1939); Joseph Taylor and Russell Hulse discovered the first binary pulsar (1974); and the Event Horizon Telescope (EHT) collaboration captured the first image of a black hole in 2019. Our own Sun, a mid-sized star, is roughly 4.6 billion years old and is expected to become a red giant in about five billion years. Supernova 1987A, in the Large Magellanic Cloud, was the closest supernova observed in nearly 400 years — a massive star that collapsed in seconds, releasing in moments more energy than the Sun will emit across its entire lifetime.