Summary
Highlights
The Sun, a giant nuclear reactor, is essential to our existence. Carl Sagan famously said, 'The cosmos is within us – we're made of star stuff.' Every carbon atom in our bodies was forged in stars, as the Big Bang only produced hydrogen, helium, and a tiny bit of lithium. These atoms come from various stars that lived and died over billions of years, creating a joyous and powerful connection to the universe.
Our lives are but a blink compared to the Sun's seemingly unchanging nature, but even the Sun has a life cycle. Stars are born from collapsing clouds of gas and dust called nebulae. Over millions of years, these protostars settle into 'main sequence stars,' like our Sun, where nuclear fusion takes place at their core. The Sun fuses 600 million tons of hydrogen into helium every second and has about four to five billion years left.
Initially, a star generates energy by fusing hydrogen. As hydrogen depletes and helium accumulates in the core, stars like the Sun begin to fuse hydrogen in a shell around the core, causing them to expand into a red giant. Our Sun will eventually engulf Earth. Once a star like the Sun exhausts its nuclear fuel, its core collapses into a dense white dwarf, expelling its outer layers as a planetary nebula. White dwarfs are incredibly dense, with the mass of the Sun compressed into the size of Earth.
More massive stars can lead to neutron stars or black holes. If a star is massive enough to overwhelm the electron degeneracy pressure, its electrons crush into protons to form neutrons, leading to an incredibly dense neutron star, which can be 1.5 times the Sun's mass but only 10 miles across. If the star is even more massive, nothing can stop the collapse, resulting in a black hole – an infinitely dense point from which nothing, not even light, can escape.
Stars with 10 or more times the Sun's mass can explode in a supernova. In extreme cases, a massive star (around 30 solar masses) collapses to form a rotating black hole, emitting energetic jets and an accretion disc in an event known as a hypernova. Hypernovae are one mechanism for long gamma-ray bursts. Pulsars, highly magnetized rotating neutron stars, emit beams of electromagnetic radiation. The first pulsar discovered was called LGM1, because its regular pulse led scientists to believe it could have been 'little green men,' later identified as a natural phenomenon.
Invisible rogue black holes exist in interstellar space, formed from the collapse of very massive stars. These can be caused by galaxy collisions. It's estimated there could be 12 rogue black holes in the Milky Way. In 2022, astronomers detected and measured an isolated stellar black hole using the Hubble Space Telescope. LIGO's detection of two black holes, each 30 times the Sun's mass, merging at two-thirds the speed of light, changed our understanding. This event released an unimaginable amount of energy, creating ripples in space and time that our detectors could observe.