April 23, 1998
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.
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.
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 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.
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.
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.
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.
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.
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.
2. (Lagoon nebula). FUNNEL CLOUDS IN SPACE: Giant funnel-shaped clouds of gas (upper left) were captured in another stellar "maternity ward," the Lagoon nebula (M8).
3. (Cat's Eye). DYING IN COLOR: Rings of gas surround the dying star NGC 6543, nicknamed the Cat's Eye.
4. (Twin Jet nebula). STELLAR EXHAUST: The dying star M2-9 is nicknamed the Twin Jet nebula because the two tubes of gas streaming from it behave like exhaust from a jet engine.
5. (Abell 2218). A "ZOOM LENS" IN SPACE: The arc-like pattern spread across the picture like a spider web is an illusion caused by the gravitational field of a cluster of galaxies called Abell 2218. This process, called gravitational lensing, magnifies, brightens, and distorts images of objects that lie far beyond the cluster, providing a powerful "zoom lens" for viewing galaxies that are so far away they could not normally be observed with the largest available telescopes.
6. (Antennae galaxies). COSMIC COLLISION CREATES STARS: A collision between two spiral galaxies has spawned brilliant bursts of star birth.
7. (Eta Carinae). BALLOONS OF GAS: A pair of billowing dust and gas clouds have been cast off by the massive dying star Eta Carinae.
8. (Supernova 1987A). GASEOUS "HULA-HOOPS": A large pair of gaseous rings frame a glowing halo of gas surrounding the massive dying star Supernova 1987A.
9. (HDF). GALLERY OF GALAXIES: Hubble peers back more than 10 billion years to reveal at least 1,500 galaxies at various stages of development.
10. (Saturn's auroras). SATURN'S LIGHT SHOW: Saturn's north and south poles display spectacular oval-shaped curtains of light, called auroras, which were captured in ultraviolet light by Hubble's Space Telescope Imaging Spectrograph (STIS).
11. (Mars). HOW'S THE WEATHER?: Hubble has been used to monitor weather conditions on Mars.
12. (NGC 4261). FOOD FOR A BLACK HOLE: A spiral-shaped disk of dust is feeding a massive black hole in the galaxy, NGC 4261.
AN INFRARED VIEW OF SATURN
In honor of NASA Hubble Space Telescope's eighth anniversary, we have
gift wrapped Saturn in vivid colors. Actually, this image is courtesy of
the new Near Infrared Camera and Multi-Object Spectrometer (NICMOS),
which has taken its first peek at Saturn. The false-color image - taken
Jan. 4, 1998 - shows the planet's reflected infrared light. This view
provides detailed information on the clouds and hazes in Saturn's
The blue colors indicate a clear atmosphere down to a main cloud layer. Different shadings of blue indicate variations in the cloud particles, in size or chemical composition. The cloud particles are believed to be ammonia ice crystals. Most of the northern hemisphere that is visible above the rings is relatively clear. The dark region around the south pole at the bottom indicates a big hole in the main cloud layer.
The green and yellow colors indicate a haze above the main cloud layer. The haze is thin where the colors are green but thick where they are yellow. Most of the southern hemisphere (the lower part of Saturn) is quite hazy. These layers are aligned with latitude lines, due to Saturn's east-west winds.
The red and orange colors indicate clouds reaching up high into the atmosphere. Red clouds are even higher than orange clouds. The densest regions of two storms near Saturn's equator appear white. On Earth, the storms with the highest clouds are also found in tropical latitudes. The smaller storm on the left is about as large as the Earth, and larger storms have been recorded on Saturn in 1990 and 1994.
The rings, made up of chunks of ice, are as white as images of ice taken in visible light. However, in the infrared, water absorption causes various colorations. The most obvious is the brown color of the innermost ring. The rings cast their shadow onto Saturn. The bright line seen within this shadow is sunlight shining through the Cassini Division, the separation between the two bright rings. It is best observed on the left side, just above the rings. This view is possible due to a rare geometry during the observation. The next time this is observable from Earth will be in 2006. An accurate investigation of the ring's shadow also shows sunlight shining through the Encke Gap, a thin division very close to the outer edge of the ring system.
Two of Saturn's satellites were recorded, Dione on the lower left and Tethys on the upper right. Tethys is just ending its transit across the disk of Saturn. They appear in different colors, yellow and green, indicating different conditions on their icy surfaces.
Wavelengths: A color image consists of three exposures (or three film layers). For visible true-color images, the wavelengths of these three exposures are 0.4, 0.5, and 0.6 micrometers for blue, green, and red light, respectively. This Saturn image was taken at longer infrared wavelengths of 1.0, 1.8, and 2.1 micrometers, displayed as blue, green, and red. Reflected sunlight is seen at all these wavelengths, since Saturn's own heat glows only at wavelengths above 4 micrometers.
Image credit: Erich Karkoschka (University of Arizona), and NASA.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).