By figuring out these distances, astronomers can determine the rate at which the universe is expanding, called the Hubble constant, and ultimately its age. Most quasars aren't choosy about where they live. Hubble's sharp vision has shown that most quasars lie at the cores of bright distant galaxies, both spiral and elliptical. Quasars are compact powerhouses of light that reside largely at the outer reaches of the universe. They are not much bigger than Earth's solar system but pour out 100 to 1,000 times as much light as an entire galaxy of 100 billion stars.
Most astronomers believe that massive black holes power quasars.
As a black hole gobbles up stars, gas, and dust, heat is generated, and intense radiation is emitted. Hubble data have shown that collisions or near encounters between galaxies may provide enough stellar material to make most quasars shine. The Universe's Biggest Blasts Astronomers have long pondered the origins of one of the universe's greatest enigmas, periodic bursts of gamma rays in deep space. Hubble has made an important contribution toward solving the source and nature of these fireballs, nature's most powerful explosions. The telescope has allowed astronomers to follow the fading visible-light counterpart of an invisible gamma ray burst, a cooling fireball associated with one of these massive outbursts.