Stars

Kate and Lilly

Astronomy is a natural science which is the study of celestial objects (such as stars. Galaxies, planets, moons, asteroids, comets and nebulae) the physics, chemistry, and evolution of such objects, and phenomena that originate outside the atmosphere of Earth, including supernova explosions, gamma ray burst, and cosmic microwave background radiation.

A star is luminous sphere of plasma held together by its own gravity. The nearest star to earth is the sun. other stars, mostley In the milky Way, are visible from earth during the night, appearing as a multitude of sixed luminouse points in the sky due to their immense distance from the Earth.

for at least a portion of its life, a star shines due to thermonucleaer fusions of hydrogen into helium in its core, releasing energy that traverses the stars interior and then radiates into outer space.

a stars life begins with the gravintatioal collaspe of a gasous nebula of material composed primerly of hydrogen, along with helium and trace amounts of heavier elements. once the steller core is sufficiently dence, hydrogen becomes steadily converted into helium through neucular fusion, realising energy in the process.

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Most stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.8 billion years old—the observed age of the universe. The oldest star yet discovered, HD 140283, nicknamed Methuselah star, is an estimated 14.46 ± 0.8 billion years old. (Due to the uncertainty in the value, this age for the star does not conflict with the age of the Universe, determined by the Planck satellite as 13.798 ± 0.037.)

The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly. The most massive stars last an average of a few million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and can last tens to hundreds of billions of years.

Historically, stars have been important to civilizations throughout the world. They have been part of religious practices and used for celestial navigation and orientation. Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used to regulate agricultural practices.