Stars — Reading Comprehension Lesson

Reading Passage

Stars

Stars are enormous balls of plasma held together by gravity and powered by nuclear fusion in their cores. The universe contains an estimated 200 billion trillion stars — more than the number of grains of sand on all of Earth’s beaches combined. Our sun is a star — a perfectly ordinary, medium-sized star by cosmic standards, located about 93 million miles from Earth. It appears far larger and brighter than other stars only because of its relative proximity. The next nearest star to our solar system, Proxima Centauri, is about 4.2 light-years away — so far that light from it takes more than four years to reach us.

Stars are born in nebulae — vast clouds of gas (mostly hydrogen) and dust. When a region of a nebula becomes dense enough, gravity causes it to collapse inward. As the material compresses, it heats up. When the core temperature reaches about 27 million degrees Fahrenheit, nuclear fusion ignites: hydrogen nuclei fuse together to form helium, releasing enormous amounts of energy in the process. This energy pushes outward against gravity, and the balance between the two forces stabilizes the star. A star spends most of its life in this stable phase, called the main sequence.

What happens when a star exhausts the hydrogen fuel in its core depends on its mass. A star like the sun will expand into a red giant, then shed its outer layers to form a glowing shell of gas called a planetary nebula, leaving behind a dense, slowly cooling remnant called a white dwarf. More massive stars live faster and die more violently: they expand into red supergiants, then explode in spectacular supernovae that briefly outshine entire galaxies. The core left behind may become a neutron star — a city-sized object so dense that a teaspoon of its material would weigh a billion tons — or, if massive enough, a black hole.

Stars are classified by their temperature, luminosity, and color. The hottest stars burn blue-white; cooler stars are yellow, orange, or red. Our sun is a yellow dwarf star of spectral class G. Astronomers plot stars on a diagram called the Hertzsprung-Russell diagram, where most stars fall along the main sequence. Throughout history, humans have grouped stars into patterns called constellations to help navigate, tell time, and mark the seasons. Ancient cultures around the world developed rich mythologies around these star patterns. Today, astronomers recognize 88 official constellations that divide the entire sky into regions.

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