The most powerful optical telescopes on Earth could see only halfway across the universe. Even estimates for the age of the universe disagreed by a big margin. Supermassive black holes were only suspected to be the powerhouses behind a rare zoo of energetic phenomena seen at great distances. We didn't know if any planets orbited other stars.
Hubble, the first major optical observatory in space, gave us a rapid-fire series of discoveries and breakthroughs after its deployment into low Earth orbit April 25, 1990. The instrument quickly became the trailblazer of a golden age of space astronomy.
Today, Hubble's 2.4-meter mirror seems puny compared to the giant 8- to 10-meter giant monolithic and segmented mirrors at major mountaintop observatories. But the pristine view from above Earth's atmosphere makes a huge difference. Hubble consistently has a 10 times sharper view than ground-based telescopes. True, adaptive optics has narrowed this margin, but Hubble still yields razor-sharp images across a wide field of view. Add to that its unparalleled ability to see high-contrast objects because there is no sky background.
Hubble's optically stable view allows revisited targets a guaranteed same acuity and repeatability of data quality. Hubble can see across a wide swath of the spectrum of light, from ultraviolet all the way to near-infrared, giving it truly "panchromatic" vision.
Armed with these powerful capabilities, Hubble has made remarkable discoveries across astronomy, rewritten textbooks, been the source of more than 7,000 science papers, and reawakened the public to the wonders of the universe.
Here's a look at Hubble's top five top scientific achievements:
1 Galaxies evolved from smaller structures
In 1990, astronomers could detect only normal galaxies out to a redshift of 0.7 at best, corresponding to a distance of 7 billion light-years in a universe thought to be twice that big. For years, conjecture and theoretical models hypothesized how galaxies must evolve if the universe was a cooling fireball left over from the Big Bang. Ground-based observations couldn't establish which of several competing theories best described how galaxies formed and evolved in the early universe.
In 1985, a committee of top astronomers planning to use Hubble concluded that devoting 200 orbits to a "deep exposure" of the universe would be fruitless. Extrapolating from the known universe of the time, they assumed that the geometry of space at great distances would spread out the light of normal galaxies, making them too diffuse for Hubble to see.
Fortunately, nature cooperated. Early Hubble observations taken even before its optical repair in 1993 showed galaxies at a then record-breaking redshift of 1.5 (more than 9 billion light-years). They appeared more compact and therefore concentrated their light into a smaller area--making them detectable to Hubble. Astronomers noted many strange-shaped "pathological" galaxies dubbed "tadpoles" and "train wrecks." Even normal galaxies had bright knots of star birth that were easily visible.
These findings encouraged the director of the Space Telescope Science Institute (STScI) at that time, Robert Williams, to devote a large chunk of his director's observing time to carrying out a million-second exposure to make the deepest-ever view of the universe. It reached an unprecedented 28th magnitude.
With the installation of Hubble's Advanced Camera for Surveys in 2002, the next STScI director, Steve Beckwith, pushed further to make the Hubble Ultra Deep Field (HUDF). This ensured that astronomers weren't accidentally seeing only compact objects and missing larger galaxies. The HUDF reached 29th magnitude and still found only fragmentary developing galaxies.
Recent observations from Hubble's new Wide Field Camera 3, installed last May, pushed into near-infrared wavelengths (first pioneered by Hubble's Near Infrared Camera and Multi Object Spectrometer in 1997). This allowed it to identify objects at a staggering redshift of 9, corresponding to when the universe was only 600 million years old.
Like watching individual frames of a motion picture, the Hubble deep surveys reveal the emergence of structure in the infant universe and the subsequent dynamic stages of galaxy evolution. Before Hubble, nearby colliding galaxies were simply a curiosity. But these deep views showed that the galactic smashups were more the rule than the exception in the early days. This provided compelling, direct visual evidence that the universe truly changes as it ages.
2 Supermassive black holes are common in galaxies
When Hubble launched, scientists had only confirmed the existence of black holes in stellar binary systems, where a companion star explodes and its core collapses to a black hole of several solar masses. But astronomers suspected that far more massive black holes must be the powerful "gravitational engines" powering a wide range of extraordinarily energetic phenomena seen near and far: Seyfert galaxies, BL Lac objects, blazars, and, above all, the mysterious quasars (short for quasi-stellar radio sources, now widely thought to be the supermassive black holes at the center of distant galaxies).
But precision spectroscopy was necessary to "weigh" a black hole to see if the amount of hidden or "nonluminous" mass far exceeded what mass could be attributed to stars alone. When the Space Telescope Imaging Spectrograph (STIS) became operational in 1997, astronomers quickly aimed it at the nearest mini-quasar, the brilliant core of giant elliptical galaxy M87 in Virgo. Like far more distant quasars, the galaxy has a telltale jet of material ejected from the core at relativistic speeds, a behavior commonly associated with black holes.
Hubble measured a 3-billion-solarmass core in M87. This was possible because the STIS made velocity measurements of a never-before-seen spiral-shaped whirlpool of hot gas orbiting around the black hole. The disk's velocity indicates a concentration of mass vastly higher than what stars alone could contribute, confirming the presence of a black hole.
A 1997 census of 27 nearby galaxies found that they all had supermassive black holes at their centers. This led astronomers to conclude that supermassive black holes are so common that every major galaxy has one.
Even more profound, Hubble discovered that the mass of a central black hole is directly related to the mass of a galaxy's central bulge of stars: The bigger the bulge, the more massive the black hole. This links a galaxy's evolution to the growth of its black hole through some unknown feedback mechanism. Presently, six viable theories exist. This means that no one is absolutely sure what the galaxy-black hole connection is.
3 Dark energy exists
A key project for Hubble astronomers was to determine how fast the universe is decelerating. The reasoning was that gravity must be exerting drag on the expansion of space after the Big Bang, like a ball rolling up a gentle incline and eventually slowing down.
The question persisted for decades as to whether enough gravity exists to halt the universe's expansion altogether. Hubble's ability to see distant type la supernovae and precisely measure their brightness allowed astronomers to look further back into time to measure the expansion rate.
In 1998, Johns Hopkins University/ STScI astronomer Adam Riess wrote a computer program to calculate the universe's deceleration rate using the supernova survey data his team collected. Strangely, the program kept coming up with a negative mass for the universe. Riess at first thought this was simply a programming error. But then he realized that the computer program was trying to make sense out of the nonsensical: Empty space produces repulsive energy!
In California, another team led by Saul Perlmutter of Lawrence Berkeley National Laboratory independently discovered a similar acceleration to the universe. His team likewise found that distant supernovae were dimmer than predicted. This meant there was more space between them and us than if the universe were slowing down, or even "just coasting." Therefore, the universe must now be expanding at a faster rate than earlier in time.
Both groups had stumbled upon Einstein's prediction of a ghostly counterbalancing force to the universe that would keep it from imploding, called the cosmological constant. Because astrophysicists don't know if it precisely behaves like the cosmological constant, the phenomenon is now simply known as dark energy.
Hubble later bolstered the reality of dark energy when it observed a supernova 10 billion years into the past. It was anomalously bright, demonstrating that the universe was decelerating very long ago, but between then and now it sped up. This transition from "push me, pull you" happened about 7 billion years ago.
Since then, astronomers have pursued observations to better characterize dark energy and to see if it really does behave like Einstein's cosmological constant. Scientists have proposed several approaches for next-generation telescopes, including surveys to find more supernovae and measuring acoustic oscillations imprinted on the sky, triggered by forces in the primordial plasma of the Big Bang.
4 The universe's expansion rate nailed down
Scientists of the late 1800s imagined a very old Earth because of geologic evidence and Charles Darwin's concept of biological evolution. Even the great mind of Albert Einstein concluded that the universe must be static and perhaps therefore eternal. Otherwise it would have flown apart or collapsed according to his general theory of relativity.
In 1929, Edwin Hubble provided the first observational evidence that indicated the universe has a finite age. The Hubble constant, as it became known, showed that the farther a galaxy is, the faster it appears to be racing away from us. This means space expands in all directions. In fact, the reddening of light is not the result of velocity--as it would be if this were truly a Doppler effect--but rather the expansion of space stretching light into longer wavelengths.
By precisely determining the expansion rate, scientists can rewind the cosmic clock and calculate the age of the universe. But the age estimate is only as reliable as the accuracy of the distance measurements. A precise value for the Hubble constant is a critical anchor point for calibrating other cosmological parameters for the universe (and, in hindsight, for characterizing dark energy--which no one even suspected in 1990).
This became a Key Project for Hubble early on because a space telescope could resolve Cepheid variable stars, a type of star that can serve as a local cosmic milepost, out to much greater distances from Earth than ground-based telescopes could.
When Hubble launched, the expansion rate of the universe had a huge degree of uncertainty. Estimates ranged from 50 kilometers per second per mega-parsec to 100 km/sec/Mpc. This meant that the universe could be as young as 8 billion years or as old as 16 billion years.
In 1994, Wendy Freedman of the Hubble Space Telescope Key Project on the Extragalactic Distance Scale announced a value of 80 km/sec/Mpc, implying a relatively young universe. The results were perplexing because they suggested a universe 8 billion to 12 billion years old, which is younger than the oldest stars. It looked like stellar evolution models were inaccurate.
By the late 1990s, the refined the value of the Hubble constant had an error of only about 10 percent. In 2009, Adam Riess and collaborators streamlined and strengthened the construction of a cosmic "distance ladder" by calibrating Cepheids in faraway galaxies. This allowed astronomers to measure precisely the expansion rate to be 74.3 km/ sec/Mpc, narrowing it to a value with an uncertainty of no more than 5 percent.
In hindsight, it was almost predictable that astronomers would simply end up splitting the difference between the earlier values of 50 km/sec/Mpc and 100 km/sec/Mpc. With dark energy factored in, this yields an age of 13.7 billion years--old enough to accommodate the measured ages of the oldest stars.
5 Sampling the atmospheres of extrasolar planets
Astronomers did not discover an extrasolar planet around a normal star until 5 years after Hubble's launch. The exoplanets were too dim for ground-based telescopes of the time to directly see them, but the rhythmic wobble of the parent star yielded telltale evidence for the tug of a planetary companion. The only information this provided was the orbital period and a rough mass estimate for exoplanets.
But by the late 1990s, astronomers had detected extrasolar planets transiting (passing in front of) their stars. This opened up the possibility for characterizing these worlds because a planet could be measured in silhouette against its parent star. Astronomers quickly applied Hubble's unique capabilities to the exoplanet sweepstakes.
Hubble made the first measurements of the atmosphere of an exoplanet. In a landmark observation, David Charbonneau of Harvard University spectroscopically measured a parent star's light as it filtered through an exoplanet's atmosphere. He found sodium present in the atmosphere of HD 209458b.
In subsequent observations, Hubble has found carbon dioxide, oxygen, and water vapor on transiting exoplanets. The hot Jupiter class planets are no doubt lifeless, but Hubble's capability to analyze their atmospheres is an important proof of concept in the search for extraterrestrial life by looking for atmospheric biotracers on earthlike planets.
Despite these strides, direct imaging of exoplanets proved elusive even for Hubble. That had to wait until 2008 when Hubble made the first-ever visible light image of a young gas giant orbiting the star Fomalhaut.
Thanks to the space shuttle program and its teams of astronauts, Hubble's cameras have seen upgrades with state-of-the-art instrumentation. Today, Hubble is 100 times more powerful and efficient than when it launched in 1990. It should continue operating well into the new decade.
Several key legacy programs are in place to extract the maximum science from the telescope's golden years. One program will study the distribution of dark matter in the universe by measuring gravitational lensing in 25 giant galaxy clusters. Hubble will also take multiple deep fields to catalog 250,000 remote galaxies. Another ambitious program will do a huge color photo mosaic of stars and nebula in a quadrant of the neighboring Andromeda Galaxy (M31).
Thus, Hubble's best discoveries may well still lie ahead.
Ray Villard is news director for the Space Telescope Science Institute in Baltimore, Maryland, which oversees Hubble's operations.
Villard, Ray. "Hubble's top 5 science discoveries: from dark energy to exoplanets to black holes, the Hubble Space Telescope helps astronomers understand some of the biggest mysteries of the universe." Astronomy July 2010: 30. Academic OneFile. Web. 10 June 2010.
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