Once every 15 years, Saturn flashes its paper-thin rings in edge-on formation relative to Earth.
Because the orbits of Saturn’s major satellites are in the ring plane, too, this alignment gives astronomers a rare opportunity to capture a spectacular parade of celestial bodies crossing Saturn’s surface.
Leading this moon train is Titan – larger than the planet Mercury. The frigid moon’s thick nitrogen atmosphere is tinted orange with the smoggy byproducts of sunlight interacting with methane and nitrogen. Several of the much smaller icy moons that are closer in to the planet line up along the upper edge of the rings.
In the image, snapped by the Hubble Space Telescope on February 24, the giant orange moon Titan casts a large shadow onto Saturn’s north polar hood. Below Titan, near the ring plane and to the left is the moon Mimas, casting a much smaller shadow onto Saturn’s equatorial cloud tops. Farther to the left, and off Saturn’s disk, are the bright moon Dione and the fainter moon Enceladus.
Hubble’s exquisite sharpness also reveals Saturn’s banded cloud structure, which is similar to Jupiter’s.
The top frame captures the giant moon Titan and its shadow near Saturn's northern polar hood. Dione, the brightest of the icy moons in this view – which are closer in to Saturn – can easily be traced crossing the disk from far left to image center. In the center frame, the smaller moon Enceladus can be seen near the western limb of Saturn. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
At the time, Saturn was at a distance of roughly 775 million miles (1.25 billion kilometers) from Earth. Hubble can see details as small as 190 miles (300 km) across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet.
Early 2009 was a favorable time for viewers with small telescopes to watch moon and shadow transits crossing the face of Saturn. Titan, Saturn’s largest moon, crossed Saturn on four separate occasions: January 24, February 9, February 24, and March 12, although not all events were visible from all locations on Earth.
This “ring plane crossing” occurs every 14-15 years. In 1995-96 Hubble witnessed the ring plane crossing event, as well as many moon transits, and even helped discover several new moons of Saturn.
This image of a pair of colliding galaxies called NGC 6240 shows them in a rare, short-lived phase of their evolution just before they merge into a single, larger galaxy. Image credit: NASA/JPL-Caltech/STScI-ESA
[/caption]
An imminent collision of biblical proportions has been captured by the Hubble and Spitzer Space Telescopes. The image here offers a rare view of a collision about to happen between the cores of two merging galaxies, each powered by a black hole with millions of times the mass of the sun. Already this union is considered to be one galaxy: NGC 6240, located 400-million light years away in the constellation Ophiuchus. Millions of years ago, each core was the dense center of its own galaxy before the two galaxies collided and ripped each other apart. Now, these cores are approaching each other at tremendous speeds and preparing for the final cataclysmic collision. They will crash into each other in a just a few million years.
“One of the most exciting things about the image is that this object is unique,” said Stephanie Bush of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a new paper describing the observation in an upcoming issue of the Astrophysical Journal. “Merging is a quick process, especially when you get to the train wreck that is happening. There just aren’t many galactic mergers at this stage in the nearby universe.”
Download and extra-large version of this image here.
It combines visible light from NASA’s Hubble Space Telescope and infrared light from Spitzer. It catches the two galaxies during a rare, short-lived phase of their evolution, when both cores of the interacting galaxies are still visible but closing in on each other fast.
NGC 6240 is already putting out huge amounts of infrared light, an indication that a burst of star formation is underway. The extra infrared radiation is common in interacting galaxies; as the two galaxies interact, dust and gas swept up by the collision form a burst of new stars that give off infrared light. Such galaxies are called luminous infrared galaxies. Spitzer’s infrared array camera can image the extra heat from newly formed stars, even though their visible light is obscured by thick dust clouds around them.
The blob-like shape of the galaxy is due to the sustained violence of the collision. Streams of millions of stars are being ripped off the galaxy, forming wispy “tidal tails” that lead off NGC 6240 in several directions. But things are about to get even more violent as the main event approaches and the two galactic cores meld into one.
In the center of NGC 6240, the two black holes in the cores will whip up a frenzy of radiation as they careen towards one another head-on, likely transforming the galaxy into a monster known as an ultra-luminous infrared galaxy, thousands of times as bright in infrared as our Milky Way.
Another fascinating aspect of this rare object is that no two galactic mergers are the same. “Not only are there few objects at this stage, but each object is unique because it came from different progenitor galaxies,” said Bush. “These observations give us another layer of information about this galaxy, and galactic mergers in general.”
Infrared light taken by Spitzer’s infrared array camera at 3.6 and 8.8 microns (red) shows cold dust and radiation from star formation; visible light from Hubble (green and blue) shows hot gas and stars.
Perseus Cluster. Credit: NASA, ESA, and Z. Levay (STScI)
[/caption]
The Hubble Space Telescope has uncovered a strong new line of evidence that galaxies are embedded in halos of dark matter. By looking at the Perseus galaxy cluster, Hubble discovered a large number of small galaxies that have remained intact while larger galaxies around them are being ripped apart by the gravitational tug of other galaxies. “We were surprised to find so many dwarf galaxies in the core of this cluster that were so smooth and round and had no evidence at all of any kind of disturbance,” said astronomer Christopher Conselice of the University of Nottingham, UK, and leader of the team that made the Hubble observations. “These dwarfs are very old galaxies that have been in the cluster for a long time. So if something was going to disrupt them, it would have happened by now. They must be very, very dark-matter-dominated galaxies.”
Observations by Hubble’s Advanced Camera for Surveys spotted 29 dwarf elliptical galaxies in the Perseus Cluster, located 250 million light-years away and one of the closest galaxy clusters to Earth. Of these galaxies, 17 are new discoveries.
Cosmologists estimate that dark matter comprises 23 percent of all energy in the cosmos. An equally mysterious “dark energy,” which drives galaxies apart, is thought to take up another 73 percent or so. The ordinary matter that we can see is believed to represent only four percent of the total mass of the Universe.
Because dark matter cannot be seen, astronomers detected its presence through indirect evidence. The most common method is by measuring the velocities of individual stars or groups of stars as they move randomly in the galaxy or as they rotate around the galaxy. The Perseus Cluster is too far away for telescopes to resolve individual stars and measure their motions. So Conselice and his team derived a new technique for uncovering dark matter in these dwarf galaxies by determining the minimum additional mass contribution from dark matter that the dwarfs must have to protect them from being disrupted by the strong, tidal pull of gravity from larger galaxies. Galaxies in the Perseus Cluster. Credit: NASA, ESA, and Z. Levay (STScI)
The dwarf galaxies may have an even higher amount of dark matter than spiral galaxies. “With these results, we cannot say whether the dark matter content of the dwarfs is higher than in the Milky Way Galaxy,” Conselice said. “Although, the fact that spiral galaxies are destroyed in clusters, while the dwarfs are not, suggests that this is indeed the case.”
But these new images provide evidence that the undisturbed galaxies are enshrouded by a “cushion” of dark matter that protects them from being torn apart.
NASA/ESA Hubble Space Telescope images of the three galaxies studied by a team of astronomers who try to understand how galaxies formed when the Universe was half its current age (upper panels). The same galaxies were then studied with the FLAMES/GIRAFFE instrument on ESO’s Very Large Telescope (VLT) to probe the motions of gas in these objects (lower panels). P
[/caption]
Once upon a time, before our Sun and Earth existed, distant galaxies were being created. Because this happened so long ago, astronomers know very little about how these galaxies formed. But now, by combining the Hubble Space Telescope’s acute vision with the Very Large Telescope’s spectrograph, astronomers have obtained exceptional 3-D views of distant galaxies, seen when the Universe was half its current age. By looking at this unique “history book” of our Universe, scientists hope to solve the puzzle of how galaxies formed in the remote past.
Hubble allows fine details of galaxies to be seen, while the VLT’s FLAMES/GIRAFFE spectrograph can obtain simultaneous spectra from small areas of extended objects, and resolving the motions of the gas in these distant galaxies.
“This unique combination of Hubble and the VLT allows us to model distant galaxies almost as nicely as we can close ones,” said François Hammer, who led the team. “In effect, FLAMES/GIRAFFE now allows us to measure the velocity of the gas at various locations in these objects. This means that we can see how the gas is moving, which provides us with a three-dimensional view of galaxies halfway across the Universe.”
The team has been reconstructing the history of about one hundred remote galaxies that have been observed with both Hubble and GIRAFFE on the VLT. The first results are coming in and have already provided useful insights for three galaxies. Combining the twin strengths of the NASA/ESA Hubble Space Telescope’s acute eye, and the capacity of ESO’s Very Large Telescope (VLT) to probe the motions of gas in tiny objects. Credit: ESO
In one galaxy, GIRAFFE revealed a region full of ionized gas, that is, hot gas composed of atoms that have been stripped of one or several electrons. This is normally due to the presence of very hot, young stars. However, even after staring at the region for more than 11 days, Hubble did not detect any stars! “Clearly this unusual galaxy has some hidden secrets,” said Mathieu Puech, lead author of one of the papers reporting this study. Comparisons with computer simulations suggest that the explanation lies in the collision of two very gas-rich spiral galaxies. The heat produced by the collision would ionise the gas, making it too hot for stars to form.
Another galaxy that the astronomers studied showed the opposite effect. There they discovered a bluish central region enshrouded in a reddish disc, almost completely hidden by dust. “The models indicate that gas and stars could be spiralling inwards rapidly,” said Hammer. This might be the first example of a disc rebuilt after a major merger.
Finally, in a third galaxy, the astronomers identified a very unusual, extremely blue, elongated structure — a bar — composed of young, massive stars, rarely observed in nearby galaxies. Comparisons with computer simulations showed the astronomers that the properties of this object are well reproduced by a collision between two galaxies of unequal mass.
“The unique combination of Hubble and FLAMES/GIRAFFE at the VLT makes it possible to model distant galaxies in great detail, and reach a consensus on the crucial role of galaxy collisions for the formation of stars in a remote past,” says Puech. “It is because we can now see how the gas is moving that we can trace back the mass and the orbits of the ancestral galaxies relatively accurately. Hubble and the VLT are real ‘time machines’ for probing the Universe’s history,” added Sébastien Peirani, lead author of another paper reporting on this study.
The astronomers are now extending their analysis to the whole sample of galaxies observed. “The next step will then be to compare this with closer galaxies, and so, piece together a picture of the evolution of galaxies over the past six to eight billion years, that is, over half the age of the Universe,” said Hammer.
Trio of galaxies. Image credit: NASA, ESA and R. Sharples (University of Durham, U.K.)
[/caption]
Is this an image of two galaxies? Actually, its three interacting galaxies that are locked in a gravitational tug-of-war. The give and take going on here may eventually tear at least one galaxy apart, and someday the three will likely merge into one super-large galaxy. This new image from the Hubble Space Telescope’s Advanced Camera for Surveys allows astronomers to view the movement of gases from galaxy to galaxy, and already, strong tidal interaction surging through the galaxies has dragged a significant number of stars away from their original homes.
The three galaxies are about 100 million light-years away, in the constellation of Piscis Austrinus (the Southern Fish). The three pictured galaxies — NGC 7173 (middle left), NCG 7174 (middle right) and NGC 7176 (lower right) — are part of the Hickson Compact Group 90, named after astronomer Paul Hickson, who first catalogued these small clusters of galaxies in the 1980s.
NGC 7173 and NGC 7176 appear to be smooth, normal elliptical galaxies without much gas and dust. In stark contrast, NGC 7174 is a mangled spiral galaxy, barely clinging to independent existence as it is ripped apart by its close neighbors. Stars are being moved away from their original locations, and are now spread out, forming a tenuous luminous component in the galaxy group.
Ultimately, astronomers believe that the stars in NGC 7174 will be redistributed into a giant ‘island universe’, tens to hundreds of times as massive as our own Milky Way.
[/caption]
NASA asked the public to vote on where they want the Hubble Space Telescope to be pointed in the “Hubble, You Decide” contest. Nearly 140,000 votes were cast online to help decide. And the winner is: a pair of interacting galaxies that look like they are hugging. Called Arp 274 (from the Arp Atlas of Peculiar Galaxies) these two galaxies won over five other celestial candidates. The Hubble observations will be taken during the International Year of Astronomy’s “100 Hours of Astronomy,” taking place April 2 – 5. The full-color galaxy image will be released publicly during that time.
Drawn together by their gravity, the two galaxies are starting to interact. The spiral shapes of these galaxies are mostly intact, but evidence can be seen of the gravitational distortions they are creating within each other. When galaxies interact and merge together, the gas clouds inside them often form tremendous numbers of new stars.
According to NASA: “The new picture of Arp 274 promises to reveal intriguing never-before-seen details in the galactic grand slam.”
We’ll be sure to post the image when it is released.
[/caption]
The Hubble Space Telescope repair mission, STS-125 seemingly gets bad news after more bad news. The mission was already delayed due Hurricane Ike in 2008, and again when a data handling processor on the spacecraft failed. Now, the mission may be too risky for both spacecraft and astronauts following the collision of the Iridium satellite and a defunct Russian communications spacecraft last week. There may be too much debris floating around close to Hubble’s orbit, breaching the safety limits NASA has in place. Without a servicing mission by a space shuttle crew, currently targeted for launch in May, the telescope is not expected to last more than another year or two.
Astronauts on spacewalks are even more at risk than the shuttle or even Hubble, and there are five spacewalks planned during the Hubble servicing flight to replace the telescope’s batteries, install new science instruments (including a new camera) and re-apply radiation shielding.
Hubble orbits higher than the International Space Station, closer to the cloud of debris from the collision. Even before the collision, the probabilities of a debris strike for the Hubble mission were already close to NASA’s safety limit. NASA pegged the chance of a catastrophic impact to a shuttle in Hubble’s orbit at 1 in 185, just below its limit of 1 in 200.
Other debris in that orbit includes pieces of a satellite that China blew up in 2007 as part of a missile test, adding hundreds of pieces of potentially hazardous debris.
Mark Matney, an orbital-debris specialist at the Johnson Space Center in Houston, told Nature magazine that even before last week’s crash the risk of a debris impact for the shuttle already “uncomfortably close to unacceptable levels. This is only going to add on to that.”
A decision about whether to proceed with the Hubble repair mission could be made in the next week or two, Nature reports.
This deep image taken with the NASA/ESA Hubble Space Telescope shows the spiral galaxy NGC 4921 along with a spectacular backdrop of more distant galaxies. It was created from a total of 80 separate pictures taken with yellow and near-infrared filters. Credits: NASA, ESA and K. Cook (Lawrence Livermore National Laboratory, USA)
[/caption]
The Coma Galaxy cluster is home to a rich collection of galaxies in the nearby Universe. NGC 4921 is one of the rare spirals in Coma, and a rather unusual one. It looks “fluffy,” with lots of swirling dust. Astronomers say this galaxy is an “anemic spiral” where a small amount of star formation is taking place, and so less light is coming from the galaxy’s arms, as is usually seen in a spiral galaxy. This is an image from the Hubble Space Telescope, and with Hubble’s sharp vision, you can see a few bright young blue stars. But what’s really amazing, besides seeing the incredible detail of NGC 4921, is looking beyond the big fluffy galaxy and seeing how Hubble was able to pick up a marvelous collection of remote galaxies of all shapes, sizes and colors. Many have the spotty and ragged appearance of galaxies from the early Universe. Click here to get a bigger, better view.
This image was created from data obtained by Hubble’s Advanced Camera for Surveys. The Coma galaxy cluster, is in the northern constellation of Coma Berenices. The cluster, also known as Abell 1656, is about 320 million light-years from Earth and contains more than 1000 members. The brightest galaxies, including NGC 4921, were discovered back in the late 18th century by William Herschel. Annotated deep Hubble Space Telescope image of NGC 4921 indictating the locations of some of the more interesting features of the galaxy and its surroundings. Credits: NASA, ESA and K. Cook (Lawrence Livermore National Laboratory, USA)
The galaxies in rich clusters undergo many interactions and mergers that tend to gradually turn gas-rich spirals into elliptical systems without much active star formation. As a result, there are far more ellipticals and fewer spirals in the Coma Cluster than are found in quieter corners of the Universe.
The Hubble images used to make this picture were originally obtained by a team led by Kem Cook (Lawrence Livermore National Laboratory, California). The team used Hubble to search for Cepheid variable stars in NGC 4921 that could be used to measure the distance to the Coma cluster and hence the expansion rate of the Universe.
Unfortunately the failure of the Advanced Camera for Surveys in early 2007 meant that they had insufficient data to complete their original program, although they hope to continue after the servicing mission. Very deep imaging data like this, which is available to anyone from the Hubble archives, may also be used for other interesting scientific exploration of this galaxy and its surroundings. A wide-field image of the region around the Coma galaxy cluster (Abell 1656) constructed from the images in the Digitized Sky Survey. NGC 4921 is the largest galaxy to the left, and slightly below, the pair of galaxies at the centre of the image. The field-of-view is approximately 2.7 x 2.85 degrees. Credits: NASA, ESA, and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble)
The top image was created from 50 separate exposures with a yellow filter and another 30 exposures with a near-infrared filter using the Wide Field Channel of the Advanced Camera for Surveys on Hubble. The total exposure times were approximately 17 hours and 10 hours respectively.
[/caption]
The folks who run the Hubble Space Telescope have decided to ask for some help in choosing where to next point the world’s most famous telescope. People from around the planet can vote to select the next object the Hubble Space Telescope will view. The choices are six objects Hubble has never observed before. You can also enter a drawing to win one of 100 new Hubble pictures of the object that is chosen. The winning image will be released between April 2 and 5, during the IYA’s 100 Hours of Astronomy, a global astronomy event geared toward encouraging as many people as possible to experience the night sky. You need to vote by March 1 to swing Hubble toward your favorite target. So get over to the HubbleSite and vote!
The choices are two planetary nebulae (NGC 40 and NGC 6072), an emission nebula (NGC 6634), an edge-on spiral (NGC 4289), a spiral galaxy (NGC 5172 – seen above from the Sloan Digital Sky Survey) and interacting galaxies (Arp 274). The choices (image stolen from the Bad Astronomer)
Here’s a screen shot of the HubbleSite page where the voting is taking place, with the rankings blurred out. (This image was stolen from Phil Plait’s Bad Astronomy site –thanks Phil!) But get over to the HubbleSite already and vote! If you need some more info to help you decide, there is a video with Dr. Frank Summers, a very fun astronomer-type guy who will explain each of the target objects. This event is part of the International Year of Astronomy (IYA), the celebration of the 400th anniversary of Galileo’s astronomical observations with his telescope.
Planetary nebula NGC 2818 is nested inside the open star cluster NGC 2818A. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
[/caption]
In another amazingly gorgeous image, Hubble has captured a unique planetary nebula nested inside an open star cluster. Both the cluster (NGC 2818A) and the nebula (NGC 2818) reside over 10,000 light-years away, in the southern constellation Pyxis (the Compass). This spectacular structure contains the outer layers of a sun-like star that were sent off into interstellar space during the star’s final stages of life. These glowing gaseous shrouds were shed by the star after it ran out of fuel to sustain the nuclear reactions in its core. Our own sun will undergo a similar process, but not for another 5 billion years or so. But what a beautiful way to go!
More about this image:
The image was taken in November 2008 with the Wide Field Planetary Camera 2. NGC 2818 is one of very few planetary nebulae in our galaxy located within an open cluster. The colors in the image represent a range of emissions coming from the clouds of the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen.
Open clusters, in general, are loosely bound and they disperse over hundreds of millions of years. Stars that form planetary nebulae typically live for billions of years. Hence, it is rare that an open cluster survives long enough for one of its members to form a planetary nebula. This open cluster is particularly ancient, estimated to be nearly one billion years old.
Planetary nebulae can have extremely varied structures. NGC 2818 has a complex shape that is difficult to interpret. However, because of its location within the cluster, astronomers have access to information about the nebula, such as its age and distance, that might not otherwise be known.
Planetary nebulae fade away gradually over tens of thousands of years. The hot, remnant stellar core of NGC 2818 will eventually cool off for billions of years as a white dwarf.
