Hubble has peered across space and time to study galaxies in an infant universe. The most famous of Hubble's faraway views is the Hubble Deep Field, a tiny speck of sky that revealed a zoo of about 3,000 galaxies, some as old as 10 billion years. The Hubble Deep Field, taken in 1995, has become one of the most studied regions of the sky and has been examined in a wide range of wavelengths, from radio to infrared.
Hubble's observations of deep space indicate that the young cosmos was filled with much smaller and more irregularly shaped galaxies than those that astronomers see in our nearby universe. These smaller structures, composed of gas and young stars, may be the building blocks from which the more familiar spiral and elliptical galaxies formed, possibly through processes such as multiple galaxy collisions and mergers.
A Speedy Universe
Gazing across space and time, the orbiting observatory identified the farthest stellar explosion to date, a supernova that erupted 10 billion years ago. By examining the glow from this dying star, a supernova called 1997ff, astronomers collected the first tantalizing observational evidence that gravity began slowing down the universe's expansion after the Big Bang. The finding, made in 2001, reinforces the startling idea that the universe only recently began speeding up, a discovery made in 1998 when the unusually dim light of several distant supernovas suggested that the universe is expanding more quickly than it has in the past.
What caused the universe's expansion to accelerate? Many scientists believe that a mysterious, repulsive force is at work in the cosmos, making galaxies rush ever faster away from each other.
Age of the Cosmos
The universe has been expanding since its creation in the Big Bang. Astronomer Edwin Hubble made that observation in the 1920s. Since then, astronomers have debated how fast the cosmos is expanding, a value called the Hubble constant. In May 1999 a team of astronomers announced that they had obtained a value for the Hubble constant, an essential ingredient needed to determine the age, size, and fate of the universe. They did it by measuring the distances to 18 galaxies, some as far as 65 million light-years from Earth. After obtaining a value for the Hubble constant, the team then determined that the universe is 12 to 14 billion years old. Measuring the Hubble constant was one of the three major goals for NASA's Hubble Space Telescope before it was launched in 1990.
In April 2002, another team of astronomers announced that they had used a different age-dating technique to reach a similar estimate for the universe's age: between 12 and 13 billion years. The team based their estimate on Hubble telescope observations of the oldest and faintest burned-out stars, called white dwarfs, in the Milky Way Galaxy. These extremely old, dim stars provide a completely independent reading on the age of the universe without relying on measurements of the expansion rate of the universe.
The Black Hole Hunter
Hubble also yielded clues to what is causing the flurry of activity in the hearts of many galaxies. These central regions are very crowded, with stars, dust, and gas competing for space. But Hubble managed to probe these dense regions, and in 1994 the telescope provided decisive spectroscopic evidence that supermassive black holes exist. Supermassive black holes are compact "monsters" that are millions or billions times more massive than our Sun and gobble up any material that ventures near them. These elusive "eating machines" cannot be observed directly, because nothing, not even light, escapes their stranglehold.
But the telescope did capture dramatic photographs of quasars, energetic light beacons that astronomers believe are powered by black holes. These photographs, released in 1996, revealed that quasars live in a variety of galaxies, from normal spiral galaxies to distorted colliding galaxies.
In 1997, a Hubble census of 27 nearby galaxies showed that supermassive black holes are common in large galaxies. The census also revealed a relationship between a black hole's mass and the mass of its home galaxy.
After proving that black holes are ubiquitous, the orbiting observatory then began further examining the relationship between supermassive black holes and their home galaxies. In 2000, a census of more than 30 galaxies showed that a galaxy's bulge determines the mass of its black hole.
The Hubble telescope provided visual proof that pancake-shaped dust disks around young stars are common, suggesting that the raw material for planet formation is in place. In 1994, the telescope revealed that these disks are swirling around at least half of the stars in the Orion Nebula, a cauldron of star formation. The finding reinforces the assumption that planetary systems are common in the universe. Scientists believe that the Earth and other planets of the solar system were formed out of similar disks about 4.5 billion years ago by the coalescing of matter caused by gravitational attraction.
In 2001, astronomers using the Hubble telescope made the first direct detection of the atmosphere of a planet orbiting a star outside our solar system and obtained the first information about the planet's chemical composition.
The planet, a gas giant like Jupiter, orbits a Sun-like star called HD 209458, located 150 light-years away in the constellation Pegasus. The orbiting observatory probed the planet's atmospheric composition by watching it pass in front of its parent star, allowing astronomers for the first time ever to see light from the star filtered through the planet's atmosphere. Astronomers then analyzed the light to determine the type of gases present in the planet's atmosphere.
Although the observation shows that the planet is too close to its parent star to support life, it demonstrates that astronomers can probe the atmospheres around other stars. Similar observations could potentially provide the first direct evidence for life beyond Earth by measuring unusual abundances of atmospheric gases caused by the presence of living organisms.