A star is an approximately spherical body of plasma, which is held together by its own gravity and prevented from collapsing by the energy generated inside it by the fusion of hydrogen into helium. The Sun is a star.
Stars form from clouds of dust and gas. Like planets, stars accrete sufficient mass to assume a spherical shape, held together by their own gravity. Unlike planets, stars eventually generate sufficient pressure and temperature at their core for hydrogen fusion to begin. Hydrogen fusion is the process whereby hydrogen is transformed into helium, in a thermonuclear process that releases energy. The energy released from fusion prevents stars from collapsing under their own gravity, and so the stable lifetime of a star is bounded by how much hydrogen it has available for fusion. Stars have so much energy that their constituent elements are in the plasma phase — a phase of matter where atoms and molecules are stripped of at least one of their electrons, and the electrons can roam freely.
Stars vary hugely in size, longevity and temperature. The more massive a star is, the hotter it is and the more quickly it will burn through its hydrogen and die. For this reason, massive stars are said to ‘live fast and die young’. Stars are classified according to their temperature, which can be determined by scientists because a star’s apparent color depends on its surface temperature. The classification is: O, B, A, F, G, K and M. O-type stars are the hottest and the bluest, and M-type stars are the coolest and the reddest. Our Sun is a G-type star, and its apparent color is a yellowish white.
Most of the light and radiation we can observe in the Universe originates in stars — individual stars, clusters of stars, nebulae lit by stars and galaxies composed of billions of stars. Like human beings, stars are born, mature and eventually die, but their lifetimes span millions of years. Hubble has gone beyond what can be achieved by other observatories by observing the births, lives and deaths of individual stars and refining theories of stellar evolution. Continue reading from SciTechDaily
Naming Stars (International Astronomical Union)
Seeing in the Dark - Lives of Stars (PBS)
Stars: Facts about stellar formation, history and classification (Space.com)
Top 10 cool things about stars that you probably didn’t know (EarthSky)
Types of Stars | Stellar Classification, Lifecycle, and Charts (AstroBackyard)
Understanding the Life Cycles of Stars (HubbleSite)
Our solar system formed about 4.5 billion years ago from a dense cloud of interstellar gas and dust. The cloud collapsed, possibly due to the shockwave of a nearby exploding star, called a supernova. When this dust cloud collapsed, it formed a solar nebula—a spinning, swirling disk of material.
At the center, gravity pulled more and more material in. Eventually the pressure in the core was so great that hydrogen atoms began to combine and form helium, releasing a tremendous amount of energy. With that, our Sun was born, and it eventually amassed more than 99 percent of the available matter. Continue reading from NASA
