Not since Galileo invented the telescope,over 400 years ago, has our view of the universe been so transformed. In April 1990, astronauts stationed the HubbleSpace Telescope in orbit… above the blurring effects of Earth’s atmosphere.
It returned scenes of unprecedented beauty. As well as clear, sharp images of a dynamic,changing universe. Stars… Planets… Galaxies… each evolving in time, from birth…to dissipation… and death. This portrait of a Universe in Motion… isHubble’s enduring legacy. The Hubble Space Telescope is now regardedas one of most revolutionary scientific instruments ever built.
While not the only telescope launched intoorbit, it has surely been the most versatile. Spacewalking astronauts returned four timesto upgrade its instruments to newer and more powerful technologies. As a result, Hubble has been able to probethe life cycle of stars, from their birth in nurseries of dust-laden clouds of gas…
All the way to their final farewell: as delicatenebulae, slowly blown into space… or as titanic supernova explosions that outshinetheir host galaxies. Hubble has peered into the breeding groundsof new solar systems: dusty discs around newborn stars that may condense into planets. And it has transported us into the billionsof galaxies that spread out across the depths of time and space. One of the most photogenic galaxies is a grandspiral called M74, located about 32 million light years from Earth. Amateur astronomers have long known it asthe “phantom galaxy,” because of its low surface brightness. Hubble astronomers, on the other hand, sawspiral arms laced with delicate tendrils of dust silhouetted against bright ribbons ofstars. These spiral arms are not like spokes on awheel. They are density waves that move around the galaxy compressing gas… and stimulatingthe birth of vast waves of new stars. Using Hubble, astronomers are uncovering fascinatingdetails within galaxies they once considered featureless and bland. NGC 1132 is an immense ball of stars some320 million light years away.
Astronomers have concluded that this giantis the product of a gravitational feeding frenzy. Hubble showed that its surroundings are dottedwith dense clusters of stars. They are what’s left of galaxies that were swallowed by 1132. How galaxies grow and evolve over time isan enduring mystery that Hubble astronomers have sought to unravel. The first galaxies are thought to have formedout of clumps of gas in the early Universe. These proto-galaxies came together to formlarger and larger galaxies. Such galactic mergers may play out over hundredsof millions of years. Hubble has shown that it is an elegant waltz of stars and gas…choreographed by gravity on a grand cosmic stage.
As the galaxies pass each other, their gravitypulls stars and gas into the space between them, building vast luminous bridges stretchingtens of thousands of light years. As the galaxies fall together again, longstreams of gas and dust, known as tidal tails, wrap around their disrupted shapes. As the galaxy cores approach each other, thegas and dust clouds that envelop them can be dramatically accelerated. This resultsin shockwaves that ripple through interstellar clouds…. Setting off bursts of star formationthat appear as brilliant blue knots.
Gravity is not the only force that can teara galaxy apart. Hubble spotted a spiral galaxy plowing througha cluster of galaxies. There, it is has encountered a vast cloud of superheated gas. Drag from this cloud is stripping away gasfrom the galaxy, creating tattered threads and blue tendrils. It’s also pulling away streams of murkydust, as shown by the dark brown tangled region around the galaxy’s center. When Hubble observations are combined withX-ray images, a bright, extended fog can be seen enveloping the galaxy and streaming offinto space. In the end, this encounter will leave thegalaxy with very little gas, and almost no chance of forming any new stars.
Galaxy collisions are not always destructive.Take the case of Centaurus A, 32 million light years from Earth. Shockwaves produced by a collision have sparkedan intense round of star formation, as seen in the red patches visible here. There is something else about Centaurus Athat stands out. Using radio and x-ray telescopes, astronomers have spotted powerful jets blastingout of its center… and broad plumes of matter racing far beyond the galaxy. Where is all that energy coming from? Answeringthat question has become a major focus of Hubble observations since the day it was launched. Astronomers had long noticed that the centersof large galaxies are unusually bright. They speculated that there must be some kind ofmassive object lurking there. Could these objects be dense collections ofstars? Or are they a breed of supermassive black holes, millions or even billions oftimes the mass of our sun? Hubble’s search for the answer began inthe center of a giant nearby galaxy, M87. Astronomers saw that its color was not quitethe same on both sides. One side was shifted towards blue and the other towards red, ahint that it must be rotating very quickly. This is because the wavelength of light ischanged by the motion of whatever is emitting it. This is also known as the Doppler effect. Think about how the pitch of a train whistledrops as it races past. Similarly, if something in space is movingtowards you, the wavelength of its light gets squashed, making it appear bluer. If the objectis moving away, its light gets stretched, making it redder. By measuring how much the colors had shiftedfrom one side of the disk to the other, astronomers were able to determine its speed of rotation.It turned out that this disk was spinning at a rate of hundreds of kilometers per second.
Astronomers concluded that an object mustbe lurking in its center that’s at least 4 billion times the mass of our Sun – asupermassive black hole. This was a key piece of evidence in the discoverythat supermassive black holes occupy the centers of most, if not all, large galaxies, includingour own Milky Way. Back in the early 20th century, the youngastronomer Edwin Hubble joined a larger quest to understand the scales of time and distancethat define our universe. To make his measurements, he observed starsin the nearby Andromeda galaxy, just 2.5 million light years away. His namesake, the Hubble Space Telescope,has extended those measurements to the far limits of time and space. In its legendary Deep Field images, Hubblestared into seemingly blank regions of sky, revealing thousands of faint galaxies fromthe early days of the universe. These blotchy collections of stars are infantgalaxies. Over the 10 billion years their light has traveled to reach us, some may haveevolved into galaxies that resemble our own… With a supermassive black hole in its center…spiral arms… exploding stars… solar systems… planets… and perhaps even life. Hubble has shown that our Milky Way galaxyis a dynamic cosmic laboratory. Some of its most striking and beautiful imagesare giant structures known as nebulae. This one is nicknamed Horsehead, after itsclear and curiously familiar shape. Rising from a sea of gas and dust, this so-calleddark nebula is a cold, dark, dusty cloud set against a background of glowing gas. Then there’s the famed Eagle Nebula, nicknamedthe Pillars of Creation. A group of hot young stars is scouring these luminous towers withfierce winds of energetic particles. Dense pockets of gas resist these winds. Withinthem, are cocoons of gas and dust, where new stars are being born.
You can see the same process underway in theMonkeyhead Nebula, about 6400 light-years away in the constellation of Orion. The Monkeyhead is a stellar nursery with allthe ingredients needed for star formation. Its peaceful beauty masks the violent eventswithin it. In places where stars are able to form athigh rates, Hubble astronomers have zeroed in on the moment of birth. One team has been collecting high-resolutionHubble images of energetic jets of matter being shot from newborn stars. Unlike most astronomical phenomena, whichcan appear motionless over centuries of time, these jets visibly change on human timescales. Using Hubble, astronomers can see knots ofgas brightening and dimming. This shows that these jets are not being launched in a steadystream. Rather, they are racing out sporadically inclumps. The irregular structure of these jets is likely caused by material that periodicallyfalls onto an infant star. This image shows how violent the end stagesof star formation can be. In the constellation of Cygnus, a few thousandlight-years away, lies a compact star-forming region called S106. The beautiful colors of this nebula mask theviolent events taking place within.
A young star, named S106 IR, is being bornat the heart of the nebula. In the final stages of its formation, the star is ejecting materialat high speed, disrupting surrounding clouds of gas and dust. 3D visualizations show the extent to whichthe star has carved its surroundings into a complex shape, including hollow cavities. At the outer edges of these cavities, thegas has been compressed into shock fronts. The material spewing off the star not onlygives the cloud its hourglass shape, it is heating it up to temperatures of 10,000 degreesCelsius. The star’s radiation excites the gas, makingit glow like a fluorescent light bulb. A star is born when pressure and heat in itscore causes hydrogen gas to undergo nuclear fusion. The heat generated by this processpushes outward… countering the inward pull of gravity. From the violence of their birth, most starsspend their lives shining in relative peace, gradually using up the hydrogen fuel thatmakes up their cores. Smaller, cooler stars are incredibly efficient.A red dwarf, with 10% the mass of our sun, can burn for ten trillion years… almosta thousand times the current age of the universe. By comparison, larger, hotter stars like oursun burn more quickly. At about 5 billion years old, our own sun has gone through halfits expected lifespan. By observing stars similar to the Sun, scientistsnow have a good idea of what will happen to our Solar System in the distant future. The sun will grow steadily hotter… causingit to swell into a so-called red giant. When the Sun does this, it will destroy the innerplanets of the Solar System. Next, the outer layers will puff out, forminga dense cloud of gas and dust that will obscure the visible light from the star.
In this stage, it forms a proto-planetarynebula. Only dim infrared emissions from the dust cloud and reflected starlight let astronomerssee anything at all. Hubble images of this stage show a wide varietyof shapes, hinting at the complex dynamics at work inside. The spiral structure of this nebula is particularlyunusual, and is likely due to a second orbiting star that is producing swirling patterns inthe gas and dust. Over a period of a few thousand years, radiationfrom the hot remains of the star excites the gas in the nebula, causing it to glow.
The once faint nebula now becomes a brightand mysterious cloud called a planetary nebula. This type of nebula populates our galaxy…with luminous shapes that draw the gaze of many a sky watcher. Eventually, planetary nebulae fade to nothingas their gas and dust diffuse into space. All that remains is the tiny white dwarf — aform that our Sun will take billions of years from now. Planetary nebulae are more than just beautifulshapes that grace our galactic skies. They show important stages in the life cycleof stars… and how they interact with and even shape their surroundings. Hubble has given astronomers the sharpestviews yet of these ghostly, dynamic structures. Take the Ring Nebula, just over 2,000 lightyears away from Earth. From Earth’s perspective, it looks likea simple elliptical body with a fuzzy boundary. But Hubble observations show that the nebulais shaped more like a distorted doughnut. The doughnut hole may look empty, but it isfull of lower density gas that stretches toward and away from us, creating a shape a littlelike a rugby ball that’s been slotted into the doughnut’s hole. The space surrounding the nebula is turbulentand full of knotty structures that formed long ago. If we were able to rotate the Ring Nebulaby 90 degrees and view it side on, it would look more like the nebula M76, also knownas the “Little Dumbbell.
” In the act of dying, sun-like stars cast mostof their mass out into the galactic winds. In time, the atoms in our own sun may wellbe swept up into new suns, new solar systems. In the cycles of star birth and star death,the galaxy is dominated by a rare and extremely violent breed. Stars ten times the mass of our sun, and evenlarger, burn hot and fast.
Intense temperatures and pressure ignite nuclearfusion reactions in their cores. Hydrogen gas turns to helium, oxygen, carbon, calcium,silicon… all the way to iron. The outward pressure from heat radiating fromthe star’s core is no longer enough to hold it up under the crushing weight of these elements. Gravity wins the battle… and the star’score collapses inward. That produces a shock wave that races outthrough the star’s volume and obliterates it. Of the 200 million odd stars in our galaxy,one goes supernova about every century or so. The last one to be seen in the Milky Way wasobserved by the astronomer Johannes Kepler in 1604, just five years before the inventionof the telescope. The most famous supernova in recent yearsappeared in 1987 in the Large Magellanic Cloud, a dwarf galaxy just above the plane of theMilky Way. It was so bright it was visible to the nakedeye. Launched three years later, Hubble has been tracking the evolving spectacle for overa quarter of a century. Astronomers have marveled at the complexityof the explosion, including the patterns etched by its expanding shock wave. Even though a supernova is only bright fora short period of time, the dusty clouds it leaves behind can last for millennia. Theireffect on the surrounding interstellar gas lasts even longer.
Although no supernova in our galaxy has everbeen observed with a telescope, plenty of supernova remnants have been. Hubble’s sharpimages of their complex structures help explain the sequence of events… as well as the profoundimpact these explosions have on the galaxy. Take the Crab supernova, one of the most interesting,and most studied, objects in all of astronomy. Japanese and Chinese astronomers witnessedthe explosion in the year 1054. The filaments shown in these images are the tatteredremains of the star, consisting mostly of hydrogen. The collapsed core of the star embeddedin the center is barely visible in this Hubble image. Yet you can see its effects. The bluish glowcomes from electrons whirling at nearly the speed of light around magnetic field linesthat extend from the star’s collapsed core. Astronomers have been poring over the nebulaitself, still growing at a rate of a thousand kilometers a second.
What they’ve found is that the filamentsof matter that roared out of the blast contain large volumes of dust, an array of mostlycarbon or silicate compounds that absorb visible light. These solid particles are crucial for theformation of solar systems. Within the Crab nebula, there is enough dust to make 30-40,000Earths. Galaxies all around the universe bear witnessto the dusty legacy of countless supernovae. The bright central region of the famous pinwheelgalaxy, for example, is surrounded by dark, dusty lanes. In spiral galaxies, hot winds from explodingstars have helped push these clouds toward the periphery as well as above and below theirflat discs. You can see evidence of this in our view ofthe Milky Way galaxy. Dark dust lanes and ominous clouds dominate our view into thedisc, while tendrils of dust reach far above it. Some dust clouds are destined to light upwith new stars, as you can see in one of the Milky Way’s small companion galaxies: TheLarge Magellanic Cloud. Its most dramatic feature is the TarantulaNebula, a bright region of glowing gas and energetic star formation.
The Tarantula, shown in a these Hubble images,glows brightly because hydrogen gas within it is being excited by ultraviolet radiationfrom newborn stars. In a wider view, the luminous Tarantula Nebulastands out from its host galaxy. It is the brightest known star-forming region in thelocal Universe and one of the most attractive spots in the night sky. Thanks to Hubble, there is a place withinour own galaxy where you can see not only stars, but solar systems, being born. In the constellation of Orion the Hunter,just under the three stars that make up its belt, is the majestic Orion Nebula. It draws our attention for its beauty andmystery.
Ancient civilizations saw meaning as well, including the Maya in what is nowsouthern Mexico and northern Central America. In their story of creation, three of the brighteststars in the Orion constellation represented a hearth. The nebula was the fire that warmsit. At 1,500 light years distance, it’s oneof the best-known examples of a star-forming nebula – a swirling cloud of gas and dustwhere stars begin their journey of life. Within it, Hubble astronomers discovered isolatedpockets of gas called proplyds. These are protoplanetary discs that form around newbornstars in spinning mixtures of gas and dust. These discs are now thought to be planetarysystems in the making. The brightest star in the Trapezium star clusteraffects the nearby discs by heating up the gas within them, causing them to shine brightly.The excited material produces many glowing cusps that face the bright star. Other interesting features enhance the lookof these captivating objects, including jets and dramatic shock waves. They are formedwhen the stellar wind from the nearby massive star meets gas in the nebula.
The interaction produces shapes like boomerangsor arrows. In one case, the shock wave makes the proplyd look like a space jellyfish. The powerful radiation that allows us to seethese shapes also threatens their existence. Once heated up, the discs are more likelyto dissipate and dissolve, destroying their potential to spawn planets. Some of the bright proplyds are doomed tobe torn apart. The dimmer ones are the most likely to survive. Among those that do produce solar systems,Hubble has been documenting a wide diversity of planets. One of them, known as HD189733b, is a hugegas giant similar to Jupiter. It lies extremely close to its star, as shown in this animation. Proximity to the star makes its climate exceptionallyhot, with temperatures exceeding 1000oC.
A team of scientists used Hubble to observeit as it passed in front of its parent star. While backlit in this way, a planet’s atmosphereimprints its signature on the starlight, allowing astronomers to decode what is happening onscales far too small to image directly. They expected to confirm that the upper layersof the planet’s atmosphere are boiling off under the intense starlight. Hubble’s first observations showed no traceof this. Just before it could take a second look, theSwift satellite detected a huge flare coming from the surface of the star, with powerfulatmosphere-frying X-rays. When the planet slid into view a few hourslater, the changes were startling. Where astronomers had seen a slumbering planet before, now theysaw an atmosphere furiously boiling away. In a dramatic plume of gas, the planet waslosing at least 1000 tons of gas from its atmosphere every second. There’s no life on a planet that orbitsso close to its parent star. Such planets, however, are allowing Hubbleastronomers to hone their search for Earth-like planets further out. When the planet moves between the star andEarth, Hubble has been able to capture a small fraction of starlight passing through theplanet’s atmosphere.
Astronomers are looking for a hydrogen-carboncompound called Methane. On Earth, it’s produced by a combination of natural and manmadesources, including fossil fuel production. On this “hot jupiter,” methane is probablyproduced by a complex chemical process in its atmosphere. Astronomers plan to use data to identify prebioticmolecules in the atmospheres of planets in the “habitable zones” around other stars,where more moderate temperatures would allow liquid water to flow. The new measurements are an important steptoward the ultimate goal of identifying the conditions, such as temperature, pressure,winds, clouds, and chemistry on planets where life could exist. Astronomers have detected a wide range ofplanets around other stars by looking for clues, like the wobbling motion of a staras a planet orbits it, or a star getting dimmer as a planet passes in front of it.
Hubble was able to capture, for the firsttime, a direct image of a planet. Visible from the southern hemisphere, Fomalhautis relatively close, at around 25 light-years away. It is 15 times brighter than the sun, andmuch hotter. This star is blazing through its hydrogen fuel supply at such a furiousrate that it will burn out in only a billion years, 10% of the lifespan of our star. Its most interesting feature may be a largedisk of dust and gas that surrounds it. This strange ring is not exactly centeredon the star. Astronomers suspect that the gravity of another body — perhaps a planet— is pulling it out of shape. The suspected planet is a dim speck. To seeit, astronomers used an instrument called a coronagraph to block the star’s light. Then they gathered a host of clues to findout what it’s like. For one, the shape of the disk hints thatthe planet is at most three times the mass of Jupiter.
For another, the planet is much brighter thanexpected for an object of its size. That means it could have an enormous ring system thatreflects starlight in all directions. One day the material in these rings may even coalesceto form moons. Hubble is part of a larger quest to discoverand understand solar systems, including our own. Among the highlights, astronomers have usedHubble to track the changing climate of cloudy Venus. Dust storms that sweep across the planet Mars. The aftermath of comet Shoemaker-Levy’scollision with Jupiter.
Saturn’s stunning rings, and moons. Uranus’ rings. And Neptune’s intense, turbulent atmosphere. In our solar system, few Hubble images compareto its views of Saturn… And the fluttering aurorae that light up itspoles. Scientists created a movie from data collectedover several days during January and March 2009, when the rings appeared edge-on, andboth poles were visible to us. The Sun emits a wind of particles that reachesall parts of the Solar System. When this electrically charged stream gets close to a planet witha magnetic field, like Saturn or the Earth, the field traps these particles. The magnetic field is stronger at the poles,so the particles tend to concentrate there, where they interact with atoms in the upperlayers of the atmosphere. That’s what creates the familiar nighttime glow we know as thenorthern and southern lights. Saturn’s auroras are not only charming features,but they might teach us something about our own planet and its magnetic field. Beyond Saturn’s dancing lights… or thesudden explosion of a star… the universe appears unmoving against the ponderous marchof cosmic time. Among its greatest achievements, the HubbleSpace Telescope has been able to track the large-scale motions of the universe. Take an event close to home.
Astronomers havelong known that the Andromeda Galaxy, currently 2.5 million light-years away, is moving towardour Milky Way. A team of astronomers used the Hubble SpaceTelescope to find out how fast the two galaxies are moving, and whether there will be headon collision. They tracked the motion of stars in Andromeda…then projected their movement into the future. Based on these findings, they showed the courseof events over the next eight billion years, as the galaxies move closer… …then collide… and gradually merge intoa new larger galaxy. If you could wait a few billion years, ournight sky would change dramatically. As Andromeda approaches, it will loom largein the sky. Later, when the galaxies begin to merge, theywill twist and distort under the pull of their mutual gravity. In time, the new combined galaxy will becomean immense ball of stars… what’s known as an elliptical galaxy. Even though these two galaxies each have hundredsof billions of stars in them, the stars are all relatively far apart. The chance of anytwo colliding is extremely small. Our Sun, born in the Milky Way almost 5 billionyears ago, will follow a new path as it orbits a whole new galaxy. In the universe according to Hubble, galaxiesall around across the cosmos are circling each other… merging… and moving into ever-largerand denser groupings. Using Hubble to survey patterns of galaxies,scientists have been able to map a mysterious substance that envelops galaxies and clustersof galaxies. This so-called “dark matter” adds to thegravity of these structures and has been driving their collapse over time.
Because of the arrangement of galaxies, Astronomershave long known that dark matter stretches out across the cosmos in a vast web-like structure.Actually observing this web has been difficult. Now, a team of scientists has used Hubbleto make detailed observations of a dark matter filament, measuring its length, shape anddensity. Theories say galaxy clusters form where filamentsof the cosmic web meet. So the team focused Hubble on one such cluster with a stream ofgalaxies moving into it along several filaments. The astronomers used data from several groundtelescopes to measure distances to the galaxies within the filament mapped by Hubble, andto trace their motions. In so doing, they made the first ever three-dimensionalreconstruction of a filament. It extends across at least 60 million light-yearsof space. From our perspective, we see it gently curving towards us, then continuingalmost along our line of sight, before it plunges into the back of the galaxy cluster. Observing and reconstructing the cosmic webcan tell us how the universe has evolved to date.
Scientists wanted to know how it’sevolving on an even grander scale. If dark matter dominates the cosmos, willits gravity be enough to cause the universe itself to crash together in a heap at somepoint in the distant future? To find out, they searched for a type of explodingstar that’s visible across the cosmos. It is the product of a small burned out starcalled a white dwarf that orbits a larger star. The smaller star pulls matter from its neighbor,thereby gradually increasing its mass. Finally, when it reaches a critical mass,it undergoes a thermonuclear explosion. These so-called Type 1A explosions are thoughtto all have the same intrinsic brightness.
How bright they appear to us is a measureof how far away they are. What the scientists found is that the mostdistant of the explosions were much fainter than they expected. They deduced from thisdata that the space between Earth and those distant explosions had been expanding fasterand faster.
Scientists theorized that another unknownforce, dark energy, is actually pushing the universe apart at an accelerating rate. This means that the universe will not collapsein a heap. Rather, it will keep on expanding forever…. Until all matter and energy eventuallydissipate to nothingness. In our time, the light of the universe continuesto rain down on Earth in torrents, a measure of the energy emitted in a constant processof creation and destruction.
Hubble has led a broad effort to capture thislight in telescopes stationed both on mountaintops and in space. Through their lenses, we have seen a universethat is evolving on all time scales, from the very short to the very long.
In its own brief time in space, Hubble hasrevolutionized the science of astronomy… while inspiring untold legions of stargazers.