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Most of the hundreds of billions of stars in our galaxy are thought to have planets of their own, and the Milky Way is but one of perhaps 100 billion galaxies in the universe.
The Sun is a 4.5 billion-year-old yellow dwarf star – a hot glowing ball of hydrogen and helium – at the center of our solar system. It’s about 93 million miles (150 million kilometers) from Earth and it’s our solar system’s only star.
Astronomical units are a useful measure for distances in our solar system, while light years are more practical for distances to the stars. The nearest star system, Alpha Centauri, is seen from Saturn in this image from NASA’s Cassini spacecraft. Credit: NASA/JPL/Space Science Institute
Globular clusters are balls of thousands and hundreds of thousands of some of the oldest stars in our galaxy. They are fairly clean of interstellar gases. Many globular clusters have a core containing a group of unusually young and massive stars called blue stragglers.
The most recent definition of a planet was adopted by the International Astronomical Union in 2006. It says a planet must do three things: It must orbit a star (in our cosmic neighborhood, the Sun). It must be big enough to have enough gravity to force it into a spherical shape.
Imagine scientists’ surprise, however, when they found that there are individual stars and concentrations of gas in our galaxy at greater distance from the luminous nucleus than our sun that have rotational velocities the same as or greater than that of our sun!
Hubble's sharp vision uncovered a never-before-seen dwarf galaxy located far behind the cluster's crowded stellar population. The loner galaxy is in our own cosmic backyard, only 30 million light-years away (approximately 2,300 times farther than the foreground cluster).
Galileo and Cassini Provide Unique Double Perspective on the Largest Planet. Spacecraft Double-Team the King of Planets. Galileo Evidence Points to Possible Water World Under Europa's Icy Crust. Thunderstorms Found to be an Energy Source for Jupiter's Great Red Spot, Other Large Features.
At this point in development, the balls of gas began to shine. They became recognizable as stars. As the stars aged, the hydrogen, helium and other chemical elements used to create energy in their centers were used up. The stars began to cool, as though their nuclear furnaces had been turned off.
In dark corners of the galaxy are worlds fit for creatures of the night. Explore the homes of Frankenstein’s monster, Dracula’s lair, the place where zombies roam, and more. Take a journey to these dark and sinister worlds. Use Your Favorite Candy Bar to Describe Rocks like a NASA Scientist.