The Lives and Deaths of Stars: How Mass Decides Everything

Hi, I'm Lumi. Let's talk about the single most powerful number in a star's whole life: the mass it is born with. That one number decides almost everything else. A star shines because gravity squeezes its core hot enough to fuse hydrogen into helium. The more mass a star has, the harder gravity squeezes, so the core burns hotter and brighter. On the H-R diagram, which plots luminosity against surface temperature, this lines stars up along a band called the main sequence. High-mass stars sit at the hot, bright, upper-left end; low-mass stars sit at the cool, dim, lower-right end. Our Sun sits comfortably in the middle. One detail that catches many people off guard: temperature runs right-to-left on the H-R diagram, so the hottest stars are on the LEFT side of the chart, not the right. Here is the surprise. Massive stars have more fuel, but they burn it so wildly fast that they live only a few million years. Low-mass stars sip their fuel and can shine for many billions of years. Brighter does not mean longer. When the core hydrogen runs out, the star leaves the main sequence and swells into a red giant. What happens next depends on mass. A low- or medium-mass star (like the Sun) gently puffs off its outer layers and leaves behind a hot, dense core called a white dwarf. A high-mass star explodes as a supernova; its crushed core becomes a neutron star, or, if the star was truly enormous, a black hole. If you ever feel stuck, just ask: how much mass did this star start with? That question always points you to the next step.