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As a protostar gathers material from a spinning disk of gas and dust, not all of that material becomes part of the future star. Powerful, narrow jets can shoot outward from the regions near the protostar’s poles at very high speeds. These jets, along with wider flows called stellar winds, push gas away from the forming star. They can carve tunnels through the surrounding molecular cloud and create glowing knots where fast-moving material strikes colder gas. By removing some gas and carrying away angular momentum, jets and winds help the disk continue feeding the protostar without spinning too rapidly. They also limit how much mass the young star can collect.

A newly formed protostar shines mainly because gravity is squeezing it, not because it is yet powered by fusion. As gas falls inward, gravitational potential energy is changed into motion and heat. Collisions among particles slow the inward-moving gas, raising the temperature and pressure in the center. The protostar contracts because gravity pulls inward more strongly than pressure can push outward. Its center becomes hotter and denser over time, while the outer layers may remain hidden inside a dusty envelope. This heating phase can last millions of years. If the object gathers enough mass, its core eventually becomes hot enough for hydrogen nuclei to begin fusing, creating a stable new source of energy.

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After fusion begins, a star enters the main sequence, the longest and most stable stage of its life. During this stage, hydrogen in the core is steadily fused into helium. The energy released pushes outward, while gravity pulls inward, producing hydrostatic equilibrium. This balance gives the star a relatively steady size, brightness, and temperature for long periods. A star like the Sun will remain on the main sequence for about 10 billion years, though it is already about halfway through that stage. Stars are not all equally bright: hotter, more massive main-sequence stars shine blue-white, while cooler, lower-mass stars appear redder. Their color provides clues about surface temperature and energy output.

A star’s mass is the most important factor in determining its temperature, brightness, lifetime, and final fate. Massive stars have stronger gravity, so their cores become hotter and fusion happens much faster. Although they hold more fuel, they can use it up in only millions of years. Lower-mass stars burn fuel slowly and can survive for tens to hundreds of billions of years. When core hydrogen is exhausted, stars change as their internal balance shifts. Sun-like stars expand into red giants and eventually leave behind dense white dwarfs. The most massive stars can form red supergiants, explode as supernovae, and leave neutron stars or black holes. Mass therefore sets the pace and ending of stellar life.