Our Sun is roughly midway through its main sequence stage.Ī star’s gas provides its fuel, and its mass determines how rapidly it runs through its supply, with lower-mass stars burning longer, dimmer, and cooler than very massive stars. The star’s luminosity, size, and temperature will slowly change over millions or billions of years during this phase. This is the longest phase of a star’s life. Nuclear fusion releases energy, which heats the star and prevents it from further collapsing under the force of gravity.Īstronomers call stars that are stably undergoing nuclear fusion of hydrogen into helium main sequence stars. After millions of years, immense pressures and temperatures in the star’s core squeeze the nuclei of hydrogen atoms together to form helium, a process called nuclear fusion. Credit: NASA/SDOĪt first, most of the protostar’s energy comes from heat released by its initial collapse. Our Sun, a main sequence star, emits a strong solar flare flashes in this image captured by NASA's Solar Dynamics Observatory. Batches of stars that have recently formed from molecular clouds are often called stellar clusters, and molecular clouds full of stellar clusters are called stellar nurseries. When this happens, friction causes the material to heat up, which eventually leads to the development of a protostar – a baby star. Eventually, gravity causes some of these clumps to collapse. Some of these clumps can collide with each other or collect more matter, strengthening their gravitational force as their mass grows. Molecular clouds are cold which causes gas to clump, creating high-density pockets. Molecular clouds range from 1,000 to 10 million times the mass of the Sun and can span as much as hundreds of light-years. Stars form in large clouds of gas and dust called molecular clouds.
Every star has its own life cycle, ranging from a few million to trillions of years, and its properties change as it ages. Stars are giant balls of hot gas – mostly hydrogen, with some helium and small amounts of other elements. Our Milky Way alone contains more than 100 billion, including our most well-studied star, the Sun. Astronomers estimate that the universe could contain up to one septillion stars – which in numbers is 1,000,000,000,000,000,000,000,000.
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