Greatest Mysteries of the Universe

How many galaxies are in our Local Group? Our Milky Way Galaxy wheels within the Local Group of Galaxies in a relatively quiet corner of the cosmos. The Virgo cluster of galaxies, some 50 million light-years away, plays city center to our boondocks. The Virgo cluster holds an amazing 2,000 "island universes." Our little Local Group, by contrast, contains roughly three dozen galaxies, most of them unimpressive dwarfs. Many, perhaps most, galaxies exist in such small groups scattered throughout the cosmos. The Local Group spans a mere 6 million light-years, and only three large galaxies lie within it. The most significant, the Andromeda Galaxy (M31), is an expansive spiral whose magnificent disk extends 140,000 light-years. Next in size is our own Milky Way, with a disk spanning 100,000 light-years. The third spiral in the group, M33 (sometimes called the Pinwheel Galaxy), measures 55,000 light- years across. The Local Group's remaining members include irregular, elli...

Is Jupiter a failed star? The brilliant planet Jupiter dazzles anyone with a clear sky. Roman observers named Jupiter after the patron deity of the Roman state following Greek mythology, which associated it with the supreme god, Zeus. But when Galileo turned his telescope skyward in 1610, Jupiter took on new significance. Galileo discovered the planet's four principal moons — and witnessed the first clear observation of celestial motions centered on a body other than Earth. Astronomers recognized Jupiter as the largest planet in the solar system long before any spacecraft provided detailed exploration.The planet's mammoth size — 88,846 miles (142,984 kilometers) at the equator — holds 2.5 times the mass of all the other planets combined. This makes Jupiter the most dominant body in the solar system after the Sun. The planet's volume is so great that 1,321 Earths could fit inside it. Jupiter is a magnificent example of a gas-giant planet. It has no solid surface and is c...

hat happens at the cores of galaxy clusters? The centers of rich clusters of galaxies contain the densest concentrations of matter in the universe. They're also among the most violent places we know of. As time rolls on and large galaxies swarm around meeker ones, mergers take place. Big galaxies grow larger by eating small ones. As this happens, worlds are torn apart, stars shredded, and gas clouds compressed into reckless new throes of star formation. We live in a relatively quiet corner of the Milky Way Galaxy. By contrast, the centers of rich galaxy dusters are the universe's most chaotic locations, constantly bustling with activity. Until recently, astronomers thought they understood how galaxy clusters form. As matter collapses inward, pulled by gravity, groups of galaxies and clumps of matter crush together. The monsters of the scene, the big galaxies, fall toward the center, where the most mass resides. Hot gas in the cluster core loses energy and cools by emitting...

How many brown dwarfs exist? In 1975, Jill Tarter, then at NASA's Ames Research Center, coined the term "brown dwarf" Before that time, astronomers hypothesized the existence of so-called black dwarfs, dark objects that were free-floating and lacked the mass to "turn on"as stars. Back then, ideas about low-mass, star-like objects suggested those with masses less than 9 percent of the Sun's wouldn't undergo normal stellar evolution. Instead, they would become "stellar degenerates" heavily laden with dust and characterized by cool outer atmospheres. Various ideas about star formation suggested there should be many brown dwarfs in the galaxy. But being nearly dark, they'd be hard to find. The best strategy would be to look in the infrared part of the spectrum. Lack of success in identifying brown dwarfs, which certainly should have existed, stymied astronomers. They turned to various methods in vain attempts to find them. These included ca...

Galaxies in groups and clusters frequently pass close to each other. They sometimes collide and merge in spectacular fashion. The Milky Way is a dominant member of a tribe of galaxies called the Local Group. Astronomers recognize about three dozen members, most of which are quite small. Although there's a great deal of space between the galaxies in the Local Group, the question arises: Will the Milky Way merge with another galaxy? In fact, our galaxy formed from past mergers, and it will be the scene of many to come. The likeliest scenario of galaxy formation and evolution suggests that galaxies grew in the early universe by merging with many small protogalaxies. Scientists think our galaxy grew during its first few billion years by shredding and cannibalizing as many as 100 small protogalaxies. But the galaxy's merger-mania continues. Astronomers see evidence the Milky Way has gobbled up as few as five and perhaps as many as 11 small galaxies in the past few hundred millio...

How did quasars form? Quasars, short for quasi-stellar objects, were first identified in 1962 by Maarten Schmidt at the California Institute of Technology. They appear as star-like points, but they lie at enormous distances, which means they're emitting incredible amounts of energy. By the 1980s, quasars' prodigious X-ray and radio emissions led most astronomers to believe these objects contain black holes in their centers. In the 1990s, scientists increasingly viewed quasars as young galactic cores where gas, dust, and stars fed a central black hole. One byproduct of the infalling matter is a high-energy jet erupting from near the black hole and hurling material into space. Quasars became part of a spectrum of energetic galaxies called active galactic nuclei (AGN), which also includes Seyfert galaxies, BL Lacertae objects, and radio galaxies. Perhaps these diverse objects, astronomers thought, were similar creatures viewed from different angles. Slowly, the question of wh...