Why is stars bright

Most of the stars in our galaxy, including the sun, are categorized as main sequence stars. They exist in a stable state of nuclear fusion, converting hydrogen to helium and radiating x-rays.

This process emits an enormous amount of energy, keeping the star hot and shining brightly. Some stars shine more brightly than others. Their brightness is a factor of how much energy they put out—known as luminosity —and how far away from Earth they are. Color can also vary from star to star because their temperatures are not all the same.

Hot stars appear white or blue, whereas cooler stars appear to have orange or red hues. By plotting these and other variables on a graph called the Hertzsprung-Russell diagram, astronomers can classify stars into groups. Along with main sequence and white dwarf stars, other groups include dwarfs, giants, and supergiants.

Supergiants may have radii a thousand times larger than that of our own sun. Stars spend 90 percent of their lives in their main sequence phase. Now around 4. As stars move toward the ends of their lives, much of their hydrogen has been converted to helium. Helium sinks to the star's core and raises the star's temperature—causing its outer shell of hot gases to expand. These large, swelling stars are known as red giants.

The red giant phase is actually a prelude to a star shedding its outer layers and becoming a small, dense body called a white dwarf. White dwarfs cool for billions of years. Some, if they exist as part of a binary star system , may gather excess matter from their companion stars until their surfaces explode, triggering a bright nova.

Eventually all white dwarfs go dark and cease producing energy. At this point, which scientists have yet to observe, they become known as black dwarfs. Massive stars eschew this evolutionary path and instead go out with a bang—detonating as supernovae.

While they may appear to be swelling red giants on the outside, their cores are actually contracting, eventually becoming so dense that they collapse, causing the star to explode.

These catastrophic bursts leave behind a small core that may become a neutron star or even, if the remnant is massive enough, a black hole. The familiar eyeball shape of the Helix Nebula shows only two dimensions of this complex celestial body.

But new observations suggest it may actually be composed of two gaseous disks nearly perpendicular to each other. In cities and other densely populated areas, light pollution makes it nearly impossible to stargaze. By contrast, some parts of the world are so dark that looking up reveals the night sky in all its rich celestial glory.

Ancient cultures looked to the sky for all sorts of reasons. By identifying different configurations of stars—known as constellations—and tracking their movements, they could follow the seasons for farming as well as chart courses across the seas.

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By: Daniel Johnson December 24, Capella is the sixth-brightest star in the sky — and it's more than one star! The main two stars in the system are near-twins, bright yellow giants. By: Daniel Johnson November 23, Arcturus is one of the brightest stars in the sky — a cool red giant in Bootes, the Herdsman, that's often tied mythologically to Ursa Major, the Bear.

By: Daniel Johnson October 13, Antares is a red supergiant that — like Betelgeuse — will one day go supernova. By: Daniel Johnson September 3, Of the seven stars in the Big Dipper, Dubhe is an outlier. Its color, speed across the sky, and evolutionary age set it apart from its comrades.

By: Daniel Johnson April 17, The Pleiades are actually a star cluster of thousands of stars enshrouded in dust and gas, and they're easy to find if you know where to look.

By: Daniel Johnson February 26, Altair, centerpiece of Aqulia, the Eagle, is the twelfth-brightest star in the night sky and one of the closest stars to Earth. By: Daniel Johnson November 19, This faint red dwarf star is famous not because it's bright but because it's fast-moving — you can actually see it moving across the sky if you track it over several years.