Galaxies are where the action happens in space, islands of activity in a sea of emptiness. Hubble has helped identify the sources of the universe’s most powerful explosions, learned how galaxies came to be, and observed stars in the last stages of their lives.

Gone in a Flash

The galaxy in this image, 12 billion light-years from Earth, gave off a gamma-ray burst that was as bright as the entire rest of the universe for one or two seconds. Astronomers had never before witnessed such a swift and enormous release of energy.

In the 1960s, the United States Air Force launched a series of satellites to monitor gamma rays. The nuclear test ban treaty had just been signed, and the U.S. wanted to watch for the telltale radiation emitted by nuclear blasts.

The satellites quickly began picking up gamma ray events – but study rapidly ruled out bombs as a source. In fact, the bursts had no ties to Earth at all. As scientists investigated further, they slowly realized what they were seeing: explosions with the power of 10 million billion Suns, that took place on a daily basis, emanating from seemingly random points within the cosmos. They were witnessing the universe's most ferocious explosions, undetected until then.

Filaments of gas and dust surround a neutron star, all that’s left after a supernova explosion. Decades before this observation was taken, this neutron star gave off a tremendous gamma ray burst measured by numerous satellites.

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Observatories followed up on the mystery and came to the conclusion that the bursts came from supernovae, the explosions of massive stars that can end in the formation of a black hole or neutron star. But not all supernovae produced gamma ray bursts – what was the true cause?

Scientists now know there are at least two different kinds of gamma ray bursts. Hubble observations have helped discover that a large proportion of gamma ray bursts originate in the brightest star-forming regions that also have stars low in metal content. It appears that the process of creating a supernova from a star that contains lots of metal, and a star that contains little, may be different. One idea is that stars with low metal content tend to retain more mass upon their death, producing black holes, while stars with high metal content form neutron stars instead. If that’s true, it may be that the gamma ray burst is often the birth announcement of a black hole.

Tracing Galactic Histories


These visually stunning events stretch galactic shapes into taffy-like distortions and give rise to explosive star birth.

See the Interacting Galaxies Poster

Two galaxies cross paths, collide and merge in this scientific simulation of a galaxy collision. In this simulation, time passes at about 30 million years per second for a total of 1.5 billion years of galactic interaction.

Galaxies, drawn to one another by gravity, can often collide and merge. Such interactions happened more frequently in the early universe than they do today, and astronomers believe that these galactic crashes are an important way that galaxies grow and evolve.

Our own Milky Way galaxy, and its neighbor Andromeda, must have grown by absorbing smaller, nearby galaxies. We can test this hypothesis by studying the ages, arrangements, compositions and speeds of stars in a galaxy.

Hubble observed the Andromeda galaxy’s halo, that region of stars on the outskirts of the galaxy, beyond the main galactic disk. Andromeda is the closest galaxy to our Milky Way, and while it has been observed frequently from the ground, only Hubble has resolution powerful enough to see its individual stars.

Hubble observed the stars of the Andromeda galaxy, our closest neighbor.

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Astronomers expected that the stars in Andromeda’s halo would be old, since halos are thought to develop early during galaxy formation. But they found that the stars of the galaxy’s outer regions are a wide variety of ages, ranging from around 13 billion years old to six billion years old. These younger stars must have found their way into the Andromeda halo through collisions: Andromeda would have eaten the smaller galaxies, making their stars its own. Hubble’s observations provide strong support that galaxy interactions and mergers are a standard part of the history of all large galaxies.

Dying Stars

Planetary Nebulae Gallery

Planetary nebulae come in all shapes and sizes. See examples of this striking phenomenon.

Mid-sized stars like our Sun end their lives by ejecting their outer layers of gas into space over the course of about 10,000 years, leaving behind the star’s hot core – a white dwarf. Radiation from the white dwarf causes the gas to glow, creating a unique and beautiful formation called a planetary nebula. The name comes from the early days of astronomy, when observers thought the dim forms they saw through their telescopes might be related to planets.

Today, Hubble has observed many of these nebulae and found a wide range of complicated and extraordinary shapes, from tunnels to interlocking rings. The Cat’s Eye nebula, for example, consists of 11 bubbles of gas, each appearing from our perspective as a ring. Hubble’s observations show that planetary nebulae are formed in multiple outbursts, not just in one dying breath, since we can see the previously exuded material interacting with newly ejected material.

Though the dynamics that create such intricate structures in a relatively short time are still mysterious, each Hubble image helps us understand a little more about how Sun-like stars spend their final years.

A star eight to 25 times more massive than our Sun ends its life in another way – in a tremendous explosion, called a supernova, which may leave behind a neutron star or a black hole. When these massive stars exhaust their fuel, their cores collapse and explode, sending their outer layers speeding into space.

Planetary Nebulae Close-ups

Butterfly Nebula
Helix Nebula

A slow pan across the Butterfly Nebula, NGC 6302, shows the intricate detail in the glowing gas ejected by the central star.

The last time astronomers observed a supernova in our galaxy was in the 1600s, when Europe was still in the process of colonizing North America. But in 1987, the light from a supernova in one of the Milky Way’s satellite galaxies, the Large Magellanic Cloud, reached Earth. Three years later, upon its launch, Hubble began to monitor the explosion from the first-ever ringside seat for a supernova.

Hubble has observed Supernova 1987A repeatedly, witnessing rings and knots of gas brightening around the exploded star. Watching the supernova in progress for over two decades has led to a greater understanding of how these events play out over thousands of years.

A Star Explodes

Shock waves from the explosion of Supernova 1987A are illuminating a 6-trillion-mile ring of gas around the dying star.

Astronomers expect the unstable star, Eta Carinae, will someday explode in a supernova. An outburst 150 years ago produced the lobes of gas racing outward at 1.5 million miles (2.4 million km) per hour. Enlarge Image

Hubble has also monitored a supernova in the making. Eta Carinae is a supermassive star, more than 100 times the mass of the Sun. In 1843, it briefly became the second brightest star in our night sky. Hubble images revealed that the unstable star had blasted out two lobes of hot gas, a prelude to its ultimate explosive end. Eta Carinae is so massive that scientists think it could end its life as a “hypernova,” a kind of extra-powerful supernova that could outshine the entire galaxy.