Observations by the Hubble Space Telescope (HST) have greatly expanded our knowledge of the cosmos, from the discovery of dark energy and determination of the age of our universe to the detection of extrasolar planets and supermassive black holes.

So said Mario Livio, an astrophysicist at the Space Telescope Science Institute, naming the top 10 scientific achievements of the HST in his Terzian Lecture Nov. 9 in Rockefeller Hall. The HST was put into a low Earth orbit in 1990 and continues to operate today.

The HST has peered at distant galaxies, as far as 7 billion light years away, and has identified luminous stellar explosions called supernovae. "These explosions can outshine the entire galaxy ... we can basically see how the universe was expanding 7 billion years ago," said Livio.

In 1998 this led to the discovery that the universe is expanding at an accelerating rate some 14 billion years after the big bang. The work earned this year's Nobel Prize in physics for two astronomers from the U.S. -- one from the Space Telescope Science Institute -- and one from Australia.

Because astronomers had thought that the expansion should be slowing down due to gravity, the discovery was "as shocking as it would be if I threw my keys up and instead of coming back down they start to accelerate toward the ceiling," said Livio. This mysterious force pushing against gravity is called dark energy, and it makes up 72 percent of the energy in the universe.

The HST has also observed Cepheid variables, stars whose luminosity pulsates at a regular rate. "There is a very tight relation between the period of this pulsation and how bright the stars are intrinsically. ... By seeing how much light we get from them we can tell how distant they are," Livio said. From many such observations, the age of the universe was pinned down to 13.72 billion years.

The Hubble Ultra Deep Field is the deepest image of the universe taken in visible light, capturing about 10,000 galaxies as they were 13 billion years ago. Pictures like this help scientists study galaxy collisions and star formation rates. "Galaxies evolve essentially by mergers and acquisitions," Livio quipped.

Until 1992, astronomers didn't know of a single planet outside our solar system. Today, there are about 1,800 known extra-solar planets. The HST has been used to determine the composition of the atmosphere of some of these planets. "We've discovered carbon, oxygen, hydrogen ... even methane and water in some planets," said Livio.

Black holes are super-dense compact objects that warp space-time so dramatically that nothing can escape them, not even light. The HST has shown that there is a supermassive black hole at the center of every galaxy, said Livio. For example, there is a 4 million solar mass black hole at the center of the Milky Way.

In addition, the HST has been used to locate gamma ray bursts, the most energetic known form of radiation in the universe; to image individual stars in our neighboring galaxy Andromeda; to witness the birth and death of stars and planets; and study the impact of comets on Jupiter.

The Terzian Lecture honors Yervant Terzian, the Tisch Distinguished University Professor of Astronomy at Cornell and the chair of the astronomy department from 1979-1999.

Graduate student Vivek Venkataraman is a writer intern for the Cornell Chronicle.