ESA Science & Technology - Hubble completes 8th year of exploration

To celebrate Hubble's eighth anniversary, we have "gift wrapped" Saturn in a colorful image taken by the telescope's new infrared camera. We also have assembled a gallery of the telescope's most compelling images and have prepared a summary of its top scientific accomplishments.

In its eight years of space exploration, the 12.5-ton orbiting observatory has set many milestones. Here are some of them:

• Circling Earth every 90 minutes, Hubble has traveled about 1.2 billion miles, which is more than a one-way trip to Saturn.
• Astronomers using Hubble data have published 1 700 scientific papers.
• The total amount of Hubble data placed in archives is 4.44 terabytes, which fills 710, 12-inch optical disks (6.66GB/disk).
• The telescope has taken about 120 000 exposures.
• Hubble has observed about 10 000 astronomical targets.

Hubble's Smash Hits

The Life Story of Galaxies

Astronomers have used Hubble to look back more than 10 billion years at infant galaxies, some of which date to nearly the beginning of the universe. What they found was astonishing: a bewildering assortment of about 2,000 galaxies at various stages of evolution.

This deepest, most detailed optical view of the universe is called the Hubble Deep Field. For 10 consecutive days in 1995, the telescope was pointed at a keyhole-sized piece of sky. Most of the galaxies the telescope uncovered are so faint (about 4 billion times fainter than can be seen by the human eye) that they have never been seen by even the largest telescopes. This observation has enabled astronomers to actually see the shapes of galaxies in the distant past. Astronomers have continued to analyze the Hubble Deep Field images to trace the evolution of stars and galaxies.

This analysis has led to intriguing evidence that the Big Bang may have been followed by a stellar "baby boom." The early universe may have had an active, dynamic youth where stars formed out of dust and gas at a ferocious rate. Consequently, most of the stars the universe will ever make may have already been formed, and the universe now contains largely "mid-life" stars.

How Old is the Universe?

For decades, astronomers have debated this question. Hubble is helping them determine the answer by studying distant supernovae and pulsating stars.

Peering halfway across the universe to analyze light from exploded stars that died long before the Sun was born, Hubble's crisp vision has allowed astronomers to determine that the universe and all its objects may have not slowed down since their creation and may continue to balloon outward. Based on preliminary observations of several distant supernovae - one of which erupted 7.7 billion years ago - the cosmos is not packed with enough material to halt its infinite expansion. If these early conclusions are true, then the universe could be 15 billion years old.

Other teams of astronomers are using different techniques to calculate the universe's age. They are using Hubble to accurately measure the distances to galaxies, an important prerequisite for calculating age. Hubble is measuring distances to neighboring galaxies by finding reliable "milepost markers," a special class of pulsating star called Cepheid variables. These, in turn, are being used to calibrate more remote milepost markers. By figuring out these distances, astronomers can determine the rate at which the universe is expanding, called the Hubble constant, and ultimately its age.

Quasars Live in a Variety of Homes

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. But a black hole needs enough food to "turn on" a quasar. 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.

After monitoring the visible afterglow that follows the gamma-ray explosion, Hubble's sensitive instruments have given astronomers some important information by pinpointing some gamma-ray bursts to faraway galaxies.

Hunting for Black Holes

Massive black holes cannot be seen because they are so dense and compact that nothing, not even light, escapes from their gravitational clutches. But black holes do leave a swirling trail of clues: a whirlpool-like stew of gas, dust, and stars orbiting them, like soap and water spiraling around a bathtub drain.

Hubble has provided convincing evidence of the existence of these powerhouses by measuring the speed of gas and stars in the cores of galaxies, where black holes reside. Hubble's efficient black hole hunter, the Space Telescope Imaging Spectrograph (STIS), has measured the increasing speed of a disk of gas orbiting a black hole in M84, located 50 million light-years away in the Virgo cluster of galaxies. The gas is swirling around the unseen black hole at 880,000 mph. Astronomers have calculated that the black hole contains at least 300 million solar masses.

Massive black holes such as the one in M84 apparently are a dime a dozen. The telescope helped prove that massive black holes are so common that nearly every large galaxy has one.

Galactic Smashup Creates Stellar Fireworks Show

Hubble images of a head-on wreck between two spiral galaxies have furnished astronomers with some surprising information about the birth of stars and star clusters.

The telescope uncovered more than 1,000 bright young star clusters bursting to life in a brief, intense "fireworks show" at the heart of the Antennae galaxies, the nearest and youngest example of a pair of colliding galaxies. (Antennae is so named because a pair of long tails of bright matter formed by the collision resembles an insect's antennae.) The images of so many young globular star clusters in the Antennae suggest, for example, that they are not necessarily relics of the earliest generations of stars formed in a galaxy, as was once commonly thought, but may be the fossils of more recent collisions.

The Hubble images also are helping astronomers understand how globular star clusters formed from giant hydrogen clouds in space. The ages of the resulting clusters provide a clock for estimating the age of a collision. By knowing when the galaxies collided, astronomers can assemble a chronological sequence of how colliding galaxies evolve, which may then explain why some galaxies become spirals, and others, ellipticals.

Caution: Cosmic Nursery

Hubble images have provided a dramatically clear look at the birth of stars, and how planets may form from the dust and gas around them.

Hubble has revealed pancake-shaped disks of dust and gas swirling around and feeding embryonic stars. These disks, dubbed protoplanetary disks, contain the raw material for planet formation.

Hubble images also show new details of blowtorch-like jets of hot gas streaming from deep within the disks. Jets are an "exhaust product" of star formation.

Going Out with a Bang

Hubble has provided ringside seats to the violence of star death, from the titanic blast of a supernova explosion to the fireworks displays produced from the demise of ordinary stars like our Sun.

Hubble has followed the expanding wave of material from the explosion of supernova 1987A. The massive star's self-destruction was first seen nearly 11 years ago by astronomers using ground-based telescopes. Hubble's Wide Field and Planetary Camera 2 (WFPC2) and the Space Telescope Imaging Spectrograph (STIS) have shown that debris from the supernova blast is slamming into a ring of material around the dying star. The collision has illuminated part of the ring, which was formed before the star exploded. The crash has allowed scientists to probe the structure around the supernova and uncover new clues about the final years of the progenitor star.

Although the final outbursts of ordinary stars are not as powerful as supernova explosions, they do create quite a stellar light show. Their colors and shapes look like works of art: reds, greens, yellows on canvases shaped like hourglasses, pinwheels, and goblets. Hubble images reveal very fine structural details of these dying stars, called planetary nebulae. The images show lawn-sprinkler style jets of gas ejected from the burned out star, material resembling exhaust from a rocket engine, bubbles of glowing gas, and shells of gas that have been flung into space.

By studying the stunning Hubble images of planetary nebulae, astronomers hope to better understand the processes that shape dying stars.

A Unique View Of Our Cosmic Back Yard

A comet smashup on Jupiter, the mottled surface of Pluto, a mammoth crater on the asteroid Vesta, and new moons orbiting Saturn. These are just some of the important events and discoveries recorded in our solar system by Hubble.

• Hubble offered a view of a rare event when 21 fragments of comet Shoemaker-Levy 9 collided with Jupiter. As each comet fragment slammed into Jupiter, Hubble caught mushroom-shaped plumes of dust along the edge of the giant planet, detailed views not possible using any other telescope. The largest fragment impact created an Earth-sized "bulls-eye" pattern on Jupiter.
• The Hubble telescope has provided a complete visible and ultraviolet light view of Jupiter's oval-shaped curtains of light, called auroras. Images taken by the Wide Field and Planetary Camera 2 (WFPC2) and Space Telescope Imaging Spectrograph (STIS) show auroras at Jupiter's north and south poles. The images record daily changes in the auroras' brightness and motion. Vapor trails of light left by Io, one of Jupiter's moons, also can be seen in both auroras.
• Ganymede, Jupiter's largest moon, is manufacturing ozone. This find, based on Hubble spectroscopic information, may mean that the satellite has a thin atmosphere of oxygen. Hubble has detected a thin oxygen atmosphere on the Jovian moon, Europa.
• Pluto, the tiny, outermost planet, is so far away that ground-based telescopes cannot pick up any detail on its surface. It took Hubble's sharp vision to provide the first view of this distant planet's surface. The pictures show that Pluto has a remarkably varied surface, mottled with bright and dark regions.
• Hubble has discovered a gigantic impact crater on the asteroid Vesta. The huge crater is 285 miles across, which is nearly equal to Vesta's 330-mile diameter. Astronomers believe that when the collision occurred, chunks of Vesta were blown off and some may have fallen to Earth.