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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. While our planet is in some ways a mere speck in the vast cosmos, we have a lot of company out there.
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.
Globular clusters are close groups of stars that, from a distance, resemble the puff ball of a dandelion. These clusters have had a significant role in the modern history of astronomy, as academic arguments raged about their true distances.
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
Researchers have found hundreds of extrasolar planets, or exoplanets, that reside outside our solar system; there may be billions of exoplanets in the Milky Way Galaxy alone, and some may be habitable (have conditions favorable to life).
In most cases, the mass turns out to be 10 times the mass of the visible stars in the galaxy. From this it can be inferred that the galaxy is mostly dark matter.
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).
In collaboration with Hubble, Spitzer helped characterize the galaxy GN-z11 about 13.4 billion light-years away, whose light has been traveling since 400 million years after the big bang. It is the farthest galaxy known.
Galileo sparked the birth of modern astronomy with his observations of the Moon, phases of Venus, moons around Jupiter, sunspots, and the news that seemingly countless individual stars make up the Milky Way Galaxy.
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.