The 10 greatest hits of the Hubble era

Paul Soucy | Special for USA TODAY and National Geographic Channel

Show Caption Hide Caption Building Hubble Video from the National Geographic Channel on Building the Hubble.

The Hubble Space Telescope is just 350 miles or so closer to deep space than those of us on Earth, but its vantage point outside the atmosphere and away from light pollution allows it to see billions of light-years farther.

In its 25 years of service, it has fundamentally changed our understanding of astrophysics. Its cameras have found black holes, galactic collisions and the violent deaths of stars. Hubble has helped us put an age to the universe and confirmed that it is full of planets. It has been called the single most important scientific instrument in the history of humankind.

Hubble's operations are managed by the Space Telescope Science Institute, on the campus of Johns Hopkins University in Baltimore. Here are what the institute considers Hubble's 10 most important scientific discoveries.

COMET IMPACT (1994)

Comet Shoemaker-Levy 9 was something special. For one thing, it wasn't orbiting the sun, like all other comets astronomers had seen. Instead, it was orbiting Jupiter. And soon after its discovery in 1993, scientists realized that it was about to slam into the planet. The impact occurred on the side of Jupiter facing away from Earth, but the planet's rapid rotation quickly brought the damage into view of Hubble's cameras. The comet had broken apart, with multiple fragments crashing into the gas giant and leaving dark scars, some of them with a diameter as large as the Earth. The impact marks, including those seen here, persisted for months before Jupiter's swirling atmosphere erased them.

STAR FACTORY (1995)

About 1,500 light-years from Earth, near the center of the constellation Orion, is a celestial maternity ward, where clouds of gas are coalescing into new stars. Surrounding many of these stars are disks of matter that may someday become planetary systems. Hubble's survey of the Orion Nebula helped confirm that planets are common throughout the universe, and the activity there provides clues about the birth of our own solar system. Scientists estimate that some of the stars in the Orion Nebula are just a million years old. "Just" a million years? Compared with the 4.5-billion-year-old star that we on Earth depend on, they're the brand-new kids on the block.

GALACTIC EVOLUTION (1996)

These smudges aren't stars. They're entire galaxies. For 10 straight days, Hubble stared at a tiny sliver of the sky — equivalent to the width of a dime from 75 feet away. What it saw through that "keyhole" was a stunning array of more than 1,500 galaxies in various stages of evolution, stretching to the very edge of the visible universe. Some are fairly young, some date nearly to the beginning of time, and many are so faint they had never been seen by even the most powerful terrestrial telescopes. And what might be most remarkable about this view is just how unremarkable it really is: Hubble could have pointed anywhere in the sky and found the same galactic diversity.

GAMMA-RAY BURST (1998)

Billions of years ago, across unfathomable distances in space, a cataclysmic explosion sent out a blast of high-energy radiation — gamma rays. In late 1997, that radiation reached Earth. Gamma-ray bursts typically last just a second or two, so it's impossible to swing a telescope around to "watch" them, but they leave an afterglow in visible light, X-rays, radio waves and infrared light. Hubble followed those across time and space to the source — an incredibly faint galaxy 12 billion light-years away. Gamma-ray bursts are exceedingly rare but exceedingly powerful. A burst pointed at Earth from within the Milky Way might cause mass extinctions.

AGE OF THE UNIVERSE (1999)

Astronomers knew the universe was big and old. They just didn't know how big or how old. Hubble helped narrow down the answers. Scanning galaxies like this one (NGC 4921), the telescope found hundreds of "Cepheid variables," pulsating stars whose characteristics make them a reliable tool for measuring distances. Pinpointing how far galaxies are from us and how fast they're moving — the farther they are, the faster they go — is key to determining the size, age and fate of the universe. Before Hubble, the universe was estimated at anywhere from 10 billion to 20 billion years old. After Hubble, scientists peg it at 13.8 billion.

BLACK HOLE (2000)

Most large galaxies have a supermassive black hole at their center, a phenomenon exhibited to dramatic effect in images captured by Hubble. The elliptical galaxy M87 lies 50 million light-years from Earth in the constellation Virgo. Even to Hubble, M87 is visible mostly as a yellow splotch, save for one distinguishing and astonishing feature — a jet of particles being blasted out of the galactic core at near-light speed by a black hole with a mass equal to 2 billion suns. The jet stretches 5,000 light-years. Back in the 1940s, scientists already knew something odd was going on in Virgo: It was one of the strongest sources of radio-frequency waves in the sky. Hubble revealed why.

ALIEN ATMOSPHERE (2001)

Just a few decades ago, scientists couldn't be sure there were any planets outside our solar system. Now nearly 2,000 "exoplanets" have been confirmed, with more being found all the time. Hubble's contribution to the search includes the first direct observation of an exoplanetary atmosphere. It was found on a Jupiter- like planet orbiting HD 209458, a yellow star much like our own sun and a mere 150 light-years from Earth. When the planet, shown here in an artist's conception, moved in front of its star, Hubble was able to analyze the starlight filtered through the atmosphere. Hubble detected sodium; later observations found hydrogen, carbon and oxygen.

DYING STAR (2003)

We are literally stardust. Heavy elements that make up our world — carbon, oxygen, silicon, iron — were produced inside stars as they burned through their fuel and then were scattered as they died. Hubble's image of the Egg Nebula, 3,000 light-years from Earth, shows a star in the later stages of its life, on its way to becoming a white dwarf. The nebula is a shell of dust, mostly carbon, shed by the aging star and stretching hundreds of billions of miles into space. The shell has layers, like an onion, that make the star's light appear to ripple. The star itself is hidden by a thick band of dust.

DARK ENERGY (2003)

Hubble images of two exploding stars in galaxies billions of light-years from Earth — and thus billions of years in the past — helped us better understand the expansion of the universe. "Type Ia supernovae" glow at a known level of brightness, making them useful for gauging intergalactic distances. By studying their light, astronomers can tell how far away they are and how fast they're moving. What these ones revealed was astounding: Although the universe is believed to be expanding at an accelerating rate, at one point in the distant past, that expansion was slowing. What prompted the reversal? The prime suspect is the mysterious force known as dark energy. Above, one of the telltale supernovae, 8 billion light-years from Earth.

DARK MATTER (2007)

Most of the matter in the universe is invisible to us. It's the underlying structure that scientists call "dark matter." They don't know exactly what it is, but they can observe it indirectly through its gravitational effects. Using Hubble to examine the shapes of distant galaxies, astronomers were able to identify dark matter by the way its gravity distorted light from those galaxies. That allowed them to construct a 3-D map of dark matter distribution, which shows it as a network of filaments stretching across immense distances. Normal matter tends to accumulate along these filaments. Above is Hubble's image of a galaxy cluster dubbed Cl 0024+17, overlaid with a map of the local dark matter, which appears as blue.