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On a clear, moonless night, you might be able to see thousands of stars sparkling like jewels above. But a keen eye will notice that they don’t all look alike. Some glow brighter than others, and some display warm red hues.
Astronomers have identified several different types of stars in the universe, as diverse as small brown dwarfs and red supergiants. Stars in the prime of their lives, known as main sequence stars, are typically classified by how hot they are. Since most star temperatures can’t be directly measured, explains Natalie Gosnell, an assistant professor in physics at Colorado College, astronomers need to look at another signal: temperature. This is largely inferred by the color of the light a star emits, which is reflected in many names given to star types.
Each category, however, is connected. A star moves through various designations throughout its lifetime, an evolution shaped by its original mass and the reactions that occur within the roiling stellar body.
In the beginning…
All stars form from a cloud of dust and gas when turbulence pushes enough of that material together, pressed into one body by gravity. As that clump collapses in on itself, it starts to spin. The material in the middle heats up, forming a dense core known as a protostar. Gravity draws even more material toward the developing star as it spins, making it bigger and bigger. Some of that stuff may eventually form planets, asteroids, and comets in orbit around the new star.
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The Hubble Space Telescope captured this blue star shedding outer layers of gas and dust. NASA, ESA, STScI
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