NASA has turned off its Galaxy Evolution Explorer (GALEX) after a decade of operations in which the venerable space telescope used its ultraviolet vision to study hundreds of millions of galaxies across 10 billion years of cosmic time.
"GALEX is a remarkable accomplishment," said Jeff Hayes, NASA's GALEX program executive in Washington, D.C. "This small Explorer mission has mapped and studied galaxies in the ultraviolet, light we cannot see with our own eyes, across most of the sky."
Operators at Orbital Sciences Corporation in Dulles, Virginia, sent the signal to decommission GALEX at 3:09 p.m. EDT Friday, June 28. The spacecraft will remain in orbit for at least 65 years, then fall to Earth and burn up upon re-entering the atmosphere. GALEX met its prime objectives and the mission was extended three times before being cancelled.
Highlights from the mission's decade of sky scans include:
- Discovering a gargantuan, comet-like tail behind a speeding star called Mira
- Catching a black hole "red-handed" as it munched on a star
- Finding giant rings of new stars around old, dead galaxies
- Independently confirming the nature of dark energy
- Discovering a missing link in galaxy evolution — the teenage galaxies transitioning from young to old
The mission also captured a dazzling collection of snapshots, showing everything from ghostly nebulae to a spiral galaxy with huge, spidery arms.
In a first-of-a-kind move for NASA, the agency in May 2012 loaned GALEX to the California Institute of Technology in Pasadena, which used private funds to continue operating the satellite while NASA retained ownership. Since then, investigators from around the world have used GALEX to study everything from stars in our own Milky Way galaxy to hundreds of thousands of galaxies 5 billion light-years away.
In the space telescope's last year, it scanned across large patches of sky, including the bustling, bright center of our Milky Way. The telescope spent time staring at certain areas of the sky, finding exploded stars, called supernovae, and monitoring how objects, such as the centers of active galaxies, change over time. GALEX also scanned the sky for massive, feeding black holes and shock waves from early supernova explosions.
"In the last few years, GALEX studied objects we never thought we'd be able to observe, from the Magellanic Clouds to bright nebulae and supernova remnants in the galactic plane," said David Schiminovich of Columbia University, a longtime GALEX team member who led science operations over the past year. "Some of its most beautiful and scientifically compelling images are part of this last observation cycle."
Data from the last year of the mission will be made public in the coming year.
"GALEX, the mission, may be over, but its science discoveries will keep on going," said Kerry Erickson, the mission's project manager at NASA's Jet Propulsion Laboratory in Pasadena, California.
The Andromeda Galaxy (M31)
Hot stars burn brightly in this image from NASA's Galaxy Evolution Explorer, showing the ultraviolet side of a familiar face.
At approximately 2.5 million light-years away, the Andromeda Galaxy (M31) is our Milky Way's largest galactic neighbor. The entire galaxy spans 260,000 light-years across — a distance so large, it took 11 different image segments stitched together to produce this view of the galaxy next door.
The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-gray lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. Eventually, these dusty lanes will be blown away by strong stellar winds, as the forming stars ignite nuclear fusion in their cores. Meanwhile, the central orange-white ball reveals a congregation of cooler, old stars that formed long ago.
When observed in visible light, Andromeda’s rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope. Astronomers using Spitzer interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.
Andromeda is so bright and close to us that it is one of only ten galaxies that can be spotted from Earth with the naked eye. This view is two-color composite, where blue represents far-ultraviolet light, and orange is near-ultraviolet light.
Mira's comet-like tail
Ultraviolet images from NASA's Galaxy Evolution Explorer show a speeding star that is leaving an enormous trail of "seeds" for new solar systems. The star, named Mira after the Latin word for "wonderful," is shedding material that will be recycled into new stars, planets and possibly even life as it hurls through our galaxy.
The Galaxy Evolution Explorer discovered the strange tail during part of its routine survey of the entire sky at ultraviolet wavelengths. When astronomers first saw the picture, they were shocked because Mira has been studied for over 400 years yet nothing like this has ever been documented before.
Mira's comet-like tail stretches a startling 13 light-years across the sky. For comparison, the nearest star to our Sun, Proxima Centauri, is only about 4 light-years away. Mira's tail also tells a tale of its history — the material making it up has been slowly blown off over time, with the oldest material at the end of the tail being released about 30,000 years ago.
While most stars travel along together around the disk of our Milky Way, Mira is charging through it. Because Mira is not moving with the "pack," it is moving much faster relative to the ambient gas in our section of the Milky Way. It is zipping along at 291,000 mph (130 km/s) relative to this gas.
Mira's breakneck speed together with its outflow of material are responsible for its unique glowing tail. Images from the Galaxy Evolution Explorer show a large build-up of gas, or bow shock, in front of the star, similar to water piling up in front of a speeding boat. Scientists now know that hot gas in this bow shock mixes with the cooler, hydrogen gas being shed from Mira, causing it to heat up as it swirls back into a turbulent wake. As the hydrogen gas loses energy, it fluoresces with ultraviolet light, which the Galaxy Evolution Explorer can detect.
Runaway CW Leo
A runaway star, plowing through the depths of space and piling up interstellar material before it, can be seen in this ultraviolet image from NASA’s Galaxy Evolution Explorer. The star, called CW Leo, is hurtling through space at about 204,000 miles per hour (91 km/s), or roughly 265 times the speed of sound on Earth. It is shedding its own atmosphere to form a sooty shell of discarded material. This shell can be seen in the center of this image as a bright circular blob.
CW Leo is moving from right to left in this image. It is traveling so quickly through the surrounding material that it has formed a semicircular bow shock in front of itself, like a boat moving through water. This bow shock is made of superheated gas, which flows around the star and is left behind in its turbulent wake. This blown-out bubble is 2.7 light-years across, which is more than half the distance from our Sun to the nearest star, or 2,100 times the size of Pluto's orbit.
The size of the bubble (called the “astrosheath”) has allowed astronomers to estimate that CW Leo has been shedding its atmosphere for about 70,000 years. This is part of the star’s natural life cycle as it runs out of hydrogen fuel and gradually throws off its outer layers to expose its bare, dying core. This core is called a white dwarf, and is the end product of all low-mass stars like our Sun.
This image is the combination of near-ultraviolet data, shown in yellow, and far-ultraviolet data, shown in blue.
The Helix Nebula
This is the Helix Nebula (NGC 7293) as seen in ultraviolet light from NASA's Galaxy Evolution Exoplorer. It is a star like our Sun but at the very end of its life. The star is a small dot in the center, surrounded by billowy layers of expelled material.
The Cygnus Loop
Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop nebula, taken by NASA’s Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away and is a supernova remnant, left over from a massive stellar explosion that occurred between 5,000 to 8,000 years ago. The Cygnus Loop extends over three times the size of the Full Moon in the night sky and is tucked next to one of the "Swan’s wings" in the constellation of Cygnus.
The filaments of gas and dust visible here in ultraviolet light were heated by the shock wave from the supernova, which is still spreading outward from the original explosion. The original supernova would have been bright enough to be seen clearly from Earth with the naked eye.
The Cartwheel Galaxy
This false-color composite image shows the Cartwheel Galaxy as seen by the Galaxy Evolution Explorer's far-ultraviolet detector (blue); the Hubble Space Telescope's Wide Field and Planetary Camera 2 in B-band visible light (green); the Spitzer Space Telescope's infrared array camera at 8 microns (red); and the Chandra X-ray Observatory's advanced CCD imaging spectrometer-S array instrument (purple).
Approximately 100 million years ago, a smaller galaxy plunged through the heart of Cartwheel Galaxy, creating ripples of brief star formation. In this image, the first ripple appears as an ultraviolet-bright blue outer ring. The blue outer ring is so powerful in the Galaxy Evolution Explorer observations that it indicates the Cartwheel is one of the most powerful UV-emitting galaxies in the nearby universe. The blue color reveals to astronomers that associations of stars five to 20 times as massive as our Sun are forming in this region. The clumps of pink along the outer blue ring are regions where both X-rays and ultraviolet radiation are superimposed in the image. These X-ray point sources are very likely collections of binary star systems containing a black hole (called massive X-ray binary systems). The X-ray sources seem to cluster around optical/ultraviolet-bright supermassive star clusters.
The yellow-orange inner ring and nucleus at the center of the galaxy result from the combination of visible and infrared light, which is stronger toward the center. This region of the galaxy represents the second ripple, or ring wave, created in the collision, but has much less star formation activity than the first (outer) ring wave. The wisps of red spread throughout the interior of the galaxy are organic molecules that have been illuminated by nearby low-level star formation. Meanwhile, the tints of green are less massive, older visible-light stars.
Although astronomers have not identified exactly which galaxy collided with the Cartwheel, two of three candidate galaxies can be seen in this image to the bottom left of the ring, one as a neon blob and the other as a green spiral.
Previously, scientists believed the ring marked the outermost edge of the galaxy, but the latest GALEX observations detect a faint disk, not visible in this image, that extends to twice the diameter of the ring.
NASA/JPL-Caltech/VLA/MPIA
The Southern Pinwheel Galaxy (M83)
The outlying regions around the Southern Pinwheel Galaxy (M83) are highlighted in this composite image from NASA's Galaxy Evolution Explorer and the National Science Foundation's Very Large Array in New Mexico. The blue and pink pinwheel in the center is the galaxy's main stellar disk, while the flapping, ribbon-like structures are its extended arms.
The Galaxy Evolution Explorer is an ultraviolet survey telescope. Its observations, shown here in blue and green, highlight the galaxy's farthest-flung clusters of young stars up to 140,000 light-years from its center. The Very Large Array observations show the radio emission in red. They highlight gaseous hydrogen atoms, or raw ingredients for stars, which make up the lengthy, extended arms.
Astronomers are excited that the clusters of baby stars match up with the extended arms because this helps them better understand how stars can be created out in the "backwoods" of a galaxy.
In this image, far-ultraviolet light is blue, near-ultraviolet light is green, and radio emission at a wavelength of 21 centimeters is red.
NGC 1512 and NGC 1510
In this image, the wide ultraviolet eyes of NASA's Galaxy Evolution Explorer show spiral galaxy NGC 1512 sitting slightly northwest of elliptical galaxy NGC 1510. The two galaxies are currently separated by a mere 68,000 light-years, leading many astronomers to suspect that a close encounter is currently in progress.
The overlapping of two tightly wound spiral arm segments makes up the light blue inner ring of NGC 1512. Meanwhile, the galaxy's outer spiral arm is being distorted by strong gravitational interactions with NGC 1510.
The Pinwheel Galaxy (M33)
NASA's Galaxy Evolution Explorer mission studies the shape, brightness, size and distance of distant galaxies across 10 billion years of cosmic history, giving scientists a wealth of data to help us better understand the origins of the universe. One such object is pictured here, the Pinwheel Galaxy (M33).
This image is a blend of the Galaxy Evolution Explorer's M33 image and another taken by NASA's Spitzer Space Telescope. M33, one of our closest galactic neighbors, is about 2.9 million light-years away in the constellation Triangulum, part of what's known as our Local Group of galaxies.
Together, the Galaxy Evolution Explorer and Spitzer can see a broad spectrum of sky. Spitzer, for example, can detect mid-infrared radiation from dust that has absorbed young stars' ultraviolet light. That's something the Galaxy Evolution Explorer cannot see. This combined image shows in amazing detail the beautiful and complicated interlacing of the heated dust and young stars. In some regions of M33, dust gathers where there is very little far-ultraviolet light, suggesting that the young stars are obscured or that stars farther away are heating the dust. In some of the outer regions of the galaxy, just the opposite is true: There are plenty of young stars and very little dust.
Far-ultraviolet light from young stars glimmers blue, near-ultraviolet light from intermediate age stars glows green, and dust rich in organic molecules burns red. This image is a 3-band composite including far infrared as red.
The Ghost of Mirach
The "Ghost of Mirach" galaxy is shown in visible light on the left, and in ultraviolet as seen by NASA's Galaxy Evolution Explorer on the right. The fields of view are identical in both pictures, with the Ghost of Mirach — a galaxy called NGC 404 — seen as the whitish spot in the center of the images. Mirach is a red giant star that looms large in visible light. Because NGC 404 is lost in the glare of this star, it was nicknamed the Ghost of Mirach.
But when the galaxy is viewed in ultraviolet light, it comes to "life," revealing a never-before-seen ring. This ring, seen in blue in the picture on the right, contains new stars — a surprise considering that the galaxy was previously thought to be, essentially, dead.
The field of view spans 55,000 light-years across. The Ghost of Mirach is located 11 million light-years from Earth. The star Mirach is very close in comparison — only 200 light-years away and is visible with the naked eye.
The visible data come from the Digitized Sky Survey of the Space Telescope Science Institute in Baltimore, Maryland.