Beyond Our Galaxy
The LMC
Large Megellanic Cloud
This relatively small neighbor of ours is an irregular dwarf galaxy, and along with its cousin, the Small Magellanic Cloud (SMC), is visible to the naked eye in the Southern hemisphere. They are only about 180,000 and 200,000 light-years away, respectively, whereas the nearest major galaxy, Andromeda (M31), is over 2 million light-years away.
They are named for the explorer Magellan who brought them to the attention of the European world in 1519. In the early 1980's still another was discovered, dubbed the Mini Magellanic Cloud (MMC), hiding behind the SMC, 20,000 light-years beyond, and appears to have been a part of the SMC until it was torn loose by a near collision with the LMC some 200 million years ago.
Photo © Anglo-Australian Observatory/Royal Observatory, Edinburgh; from UK Schmidt plates by David Malin
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Supernova 1987A A star, located in the Large Magellanic Cloud, goes supernova in 1987. Before and after photos by the 3.9m Anglo-Australian Telescope, 1987 |
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A Closer Look, 7 to 9 years later: Credit: Chun Shing Jason Pun (NASA/GSFC), Robert P. Kirshner (Harvard-Smithsonian Center for Astrophysics), and NASA This Hubble Space Telescope picture shows Supernova 1987A and its neighborhood. The series of four panels shows the evolution of the SN 1987A debris from February 1994 to February 1996. Material from the stellar interior was ejected into space during the supernova explosion in February 1987. The explosion debris is expanding at nearly 6 million miles per hour. Ten years now after the explosion, this cosmic fireball is large enough --- about one-sixth of a light-year in diameter --- to be resolved from the Earth's orbit with the Hubble Space Telescope. The debris is resolved into two opposing blobs and is dim in the center. The apparent direction of ejection is the same as the short axis of the bright inner ring that surrounds the supernova. This suggests that the explosion is directed out of the plane of the ring. The ring is probably composed of materials lost by the pre-supernova star in the last stages of its evolution. Supernova 1987A is located 167,000 light-years away from Earth in the Large Magellanic Cloud. The telescope captured the images with the Wide Field and Planetary Camera 2. The central image of the supernova and the ring system was taken in light emitted by nitrogen gas (658 nanometers) on Sept. 24, 1994. The series of debris images were taken using a visible light filter of wavelength around 550 nanometers taken (from left to right) on Feb. 4, 1994, Sept. 24, 1994, March 5, 1995, and Feb. 6, 1996. |
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Bright Knot Appears in Supernova 1987A Ring Credit: Peter Garnavich (Harvard-Smithsonian Center for Astrophysics), and NASA [LEFT] - This NASA Hubble Space Telescope Wide Field and Planetary Camera 2 image shows the glowing gas ring around supernova 1987A, as it appeared in 1994. The gas, excited by light from the explosion, has been fading for a decade. [RIGHT] - Recent Hubble telescope observations show a brightening knot on the upper right side of the ring. This is the site of a powerful collision between an outward moving blast wave and the innermost parts of the circumstellar ring. The collision heats the gas and has caused it to brighten in recent months. This is likely to be the first sign of a dramatic and violent collision that will take place over the next few years, rejuvenating SN1987A as a powerful source of X-ray and radio emissions. The white sickle-shaped material in the center is the visible part of the shredded star, rushing outward at 3,000 kilometers per second, which is heated by radioactive elements created in the supernova explosion. The bright dot in the lower left is a star, which is the same direction as SN1987A, but is not physically part of the system. Both images were made from separate images taken in blue light, visual light and the narrow emission from glowing hydrogen. Computer image processing techniques were used to enhance details in the ring. |
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Stellar Nursery in the Small Magellanic Cloud
Credit: NASA and The Hubble Heritage Team (STScI/AURA)
NASA's Hubble Space Telescope has peered deep into a neighboring galaxy to reveal details of the formation of new stars. Hubble's target was a newborn star cluster within the Small Magellanic Cloud, a small galaxy that is a satellite of our own Milky Way. The new images show young, brilliant stars cradled within a nebula, or glowing cloud of gas, cataloged as N 81.
These massive, recently formed stars inside N 81 are losing material at a high rate, sending out strong stellar winds and shock waves and hollowing out a cocoon within the surrounding nebula. The two most luminous stars, seen in the Hubble image as a very close pair near the center of N 81, emit copious ultraviolet radiation, causing the nebula to glow through fluorescence.
Outside the hot, glowing gas is cooler material consisting of hydrogen molecules and dust. Normally this material is invisible, but some of it can be seen in silhouette against the nebular background, as long dust lanes and a small, dark, elliptical-shaped knot. It is believed that the young stars have formed from this cold matter through gravitational contraction.
Few features can be seen in N 81 from ground-based telescopes, earning it the informal nick-name "The Blob." Astronomers were not sure if just one or a few hot stars were embedded in the cloud, or if it was a stellar nursery containing a large number of less massive stars. Hubble's high-resolution imaging shows the latter to be the case, revealing that numerous young, white-hot stars---easily visible in the color picture---are contained within N 81.
This crucial information bears strongly on theories of star formation, and N 81 offers a singular opportunity for a close-up look at the turbulent conditions accompanying the birth of massive stars. The brightest stars in the cluster have a luminosity equal to 300,000 stars like our own Sun. Astronomers are especially keen to study star formation in the Small Magellanic Cloud, because its chemical composition is different from that of the Milky Way. All of the chemical elements, other than hydrogen and helium, have only about one-tenth the abundances seen in our own galaxy.
The study of N81 thus provides an excellent template for studying the star formation that occurred long ago in very distant galaxies, before nuclear reactions inside stars had synthesized the elements heavier than helium.
The Small Magellanic Cloud, named after the explorer Ferdinand Magellan, lies 200,000 light-years away, and is visible only from the Earth's southern hemisphere. N 81 is the 81st nebula cataloged in a survey of the SMC carried out in the 1950's by astronomer Karl Henize, who later became an astronomer-astronaut who flew into space aboard NASA's space shuttle.
The Hubble Heritage image of N 81 is a color representation of data taken in September, 1997, with Hubble's Wide Field Planetary Camera 2. Color filters were used to sample light emitted by oxygen ([O III]) and hydrogen (H-alpha, H-beta).
N 81 is the target of investigations by European astronomers Mohammad Heydari-Malayeri from the Paris Observatory in France; Michael Rosa from the Space Telescope-European Coordinating Facility in Munich, Germany; Hans Zinnecker of the Astrophysical Institute in Potsdam, Germany; Lise Deharveng of Marseille Observatory, France; and Vassilis Charmadaris of Cornell University, USA (formerly at Paris Observatory). Members of this team are interested in understanding the formation of hot, massive stars, especially under conditions different from those in the Milky Way.
Celestial Composition
Credit: NASA, The Hubble Heritage Team and A. Riess (STScI)
Amid a backdrop of far-off galaxies, the majestic dusty spiral, NGC 3370, looms in the foreground in this NASA Hubble Space Telescope image. Recent observations taken with the Advanced Camera for Surveys show intricate spiral arm structure spotted with hot areas of new star formation. But this galaxy is more than just a pretty face. Nearly 10 years earlier NGC 3370, in the constellation Leo, hosted a bright exploding star.
In November 1994, the light of a supernova in nearby NGC 3370 reached Earth. This stellar outburst briefly outshone all of the tens of billions of other stars in its galaxy. Although supernovae are common, with one exploding every few seconds somewhere in the universe, this one was special. Designated SN 1994ae, this supernova was one of the nearest and best observed supernovae since the advent of modern, digital detectors. It resides 98 million light-years (30 megaparsecs) from Earth. The supernova was also a member of a special subclass of supernovae, the type Ia, the best tool astronomers have to chart the growth rate of the expanding universe.
Recently, astronomers have compared nearby type Ia supernovae to more distant ones, determining that the universe is now accelerating in its expansion and is filled with mysterious "dark energy." Such measurements are akin to measuring the size of your room by stepping it off with your feet. However, a careful measurement of the length of your foot (to convert your measurements into inches or centimeters) is still needed to know the true size of your room. Similarly, astronomers must calibrate the true brightness of type Ia supernovae to measure the true size and expansion rate of the universe.
The very nearest type Ia supernovae, such as SN 1994ae, can be used to calibrate distance measurements in the universe, because other, fainter stars of known brightness can be observed in the same galaxy. These stellar "standard candles" are the Cepheid variable stars, which vary regularly in brightness with periods that are directly related to their intrinsic brightness, and thus allow the distance to the galaxy—and the supernova—to be determined directly. However, only the Hubble Space Telescope, equipped with its new Advanced Camera for Surveys, has the capability to resolve these individual Cepheids.
Adam Riess, an astronomer at Space Telescope Science Institute in Baltimore, Md., observed NGC 3370 a dozen times over the course of a month and has seen many Cepheid variables. Already he and his colleagues can see that these Cepheids are the most distant yet observed with Hubble. Because of their need to observe this galaxy with great frequency to record the variation of the Cepheids, the total exposure time for this galaxy is extremely long (about one full day), and the combined image provides one of the deepest views taken by Hubble. As a result, thousands of distant galaxies in the background are easily discernable.
Dr. Riess imaged NGC 3370 with Hubble in early 2003. His science only required looking at NGC 3370 in two filters that covered the visual and infrared portions of the spectrum. By teaming up with the Hubble Heritage Project, a third blue filter was added to the data to produce the composite three-color image that is shown.
The Spiral Galaxy M100
Credit: J. Trauger, JPL and NASA
Ground-based photo by the 3.9m
Anglo-Australian Telescope, 1992
An image of the grand design of spiral galaxy M100 obtained with NASA's Hubble Space Telescope resolves individual stars within the majestic spiral arms. These stars typically appeared blurred together when viewed with ground-based telescopes. HST's image is chevron-shaped because it is a mosaic of the instrument's three wide-field cameras and one planetary camera.
Hubble has the ability to resolve individual stars in other galaxies and measure
accurately the light from very faint stars. This makes the space telescope invaluable for
identifying a rare class of pulsating stars, called Cepheid Variable stars embedded within
M100's spiral arms.
Cepheids are reliable cosmic distance mileposts. The interval it takes for the Cepheid to complete one pulsation is a direct indication of the star's intrinsic brightness. This value can be used to make a precise measurement of the galaxy's distance, which turns out to be 56 million light-years.
M100 (100th object in the Messier catalog of non-stellar objects) is a majestic face-on spiral galaxy. It is a rotating system of gas and stars, similar to our own galaxy, the Milky Way. Hubble routinely can view M100 with a level of clarity and sensitivity previously possible only for the very few nearby galaxies that compose our "Local Group.''
M100 is a member of the huge Virgo cluster of an estimated 2,500 galaxies. The galaxy can be seen by amateur astronomers as a faint, pinwheel-shaped object in the spring constellation Coma Berenices.
The Hubble Space Telescope image was taken on December 31, 1993 with the Wide Field Planetary Camera 2 (WFPC 2). This color picture is a composite of several images taken in different colors of light. Blue corresponds to regions containing hot newborn stars.
The Central Region of Spiral Galaxy NGC 253 Credit: Jay Gallagher (University of Wisconsin-Madison), Alan Watson (Lowell Observatory, Flagstaff, AZ), Carnegie Institution of Washington, and NASA
Hubble's high resolution allows astronomers to quantify complex structures in the starburst core of the galaxy for the first time, including luminous star clusters, dust lanes which trace regions of dense gas and filaments of glowing gas. Hubble identifies several regions of intense star formation, which include a bright, super-compact star cluster. These observations confirm that stars are often born in dense clusters within starbursts, and that dense gas coexists with and obscures the starburst core. This image was taken with Hubble's Wide Field Planetary Camera 2 (in PC mode). |
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A Bright Ring of Star Birth
Around a Galaxy's Core
Credit: G. Fritz Benedict, Andrew Howell, Inger Jorgensen, David Chapell (University of Texas), Jeffery Kenney (Yale University), and Beverly J. Smith (CASA, University of Colorado), and NASA.
An image from NASA's Hubble Space Telescope reveals clusters of infant stars that formed in a ring around the core of the barred-spiral galaxy NGC 4314. This stellar nursery, whose inhabitants were created within the past 5 million years, is the only place in the entire galaxy where new stars are being born. The Hubble image is being presented today (June 11) at the American Astronomical Society meeting in San Diego, Calif.
This close-up view by Hubble also shows other interesting details in the galaxy's core: dust lanes, a smaller bar of stars, dust and gas embedded in the stellar ring, and an extra pair of spiral arms packed with young stars. These details make the center resemble a miniature version of a spiral galaxy. While it is not unusual to have dust lanes and rings of gas in the centers of galaxies, it is uncommon to have spiral arms full of young stars in the cores. NGC 4314 is one of the nearest (only 40 million light-years away in the constellation Coma Berenices) examples of a galaxy with a ring of infant stars close to the core. This stellar ring - whose radius is 1,000 light-years - is a great laboratory to study star formation in galaxies.
The
image at left, taken in February 1996 by the 30-inch telescope Prime Focus Camera
at the McDonald Observatory in Texas, shows the entire galaxy, including the bar of stars
bisecting the core and the outer spiral arms, which begin near the ends of this bar. The
box around the galaxy's core pinpoints the focus of the Hubble image.
The Hubble image is a close-up view of the galaxy's core, taken in December 1995 by the Wide Field and Planetary Camera 2. The bluish-purple clumps that form the ring are the clusters of infant stars. Two dark, wispy lanes of dust and a pair of blue spiral arms are just outside the star-forming ring. The lanes of dust are being shepherded into the ring by the longer, primary stellar bar seen in the ground-based image. The gas is trapped inside the ring through the stars' gravitational attraction.
The two spiral arms outside the ring are probably unrelated to the dust lanes, and seem to contain very little dust or gas. The stars in these spiral arms are bluer than most of the galaxy, indicating that many of them are relatively young, less than 200 million years old. However, they are older than those in the ring. This information suggests that the neighborhood of star formation is moving closer to the galaxy's core. Another interpretation has the arms formed through the gravitational interaction of the embedded bar and ring of stars, causing them to spray outward.
This picture was created by combining images taken in ultraviolet, blue, visible, infrared, and H-alpha. The purple color represents hydrogen gas being excited by hot, young star clusters.
NGC 7742, a Seyfert 2 Active Galaxy
Credit: Hubble Heritage Team (AURA/STScI/NASA)
What may first appear as a sunny side up egg is actually NASA Hubble Space Telescope's face-on snapshot of the small spiral galaxy NGC 7742. But NGC 7742 is not a run-of-the-mill spiral galaxy. In fact, this spiral is known to be a Seyfert 2 active galaxy, a type of galaxy that is probably powered by a black hole residing in its core. The core of NGC 7742 is the large yellow "yolk" in the center of the image. The lumpy, thick ring around this core is an area of active starbirth. The ring is about 3,000 light-years from the core. Tightly wound spiral arms also are faintly visible. Surrounding the inner ring is a wispy band of material, which is probably the remains of a once very active stellar breeding ground.
Giant Starbirth Region in Galaxy M33
Credit: Hui Yang (University of Illinois), Jeff
J. Hester (University of Arizona) and NASA
Ground-based image courtesy of Palomar Observatory, Caltech and the STScI Digitized Sky
Survey (AURA)
This is a Hubble Space Telescope image (left) of a vast nebula called NGC 604, which
lies in the neighboring spiral galaxy M33, located 2.7 million light-years away in the
constellation Triangulum.
This is a site where new stars are being born in a spiral arm of the galaxy. Though such nebulae are common in galaxies, this one is particularly large, nearly 1,500 light-years across. The nebula is so vast it is easily seen in ground-based telescopic images (right).
At the heart of NGC 604 are over 200 hot stars, much more massive than our Sun (15 to 60 solar masses). They heat the gaseous walls of the nebula making the gas fluoresce. Their light also highlights the nebula's three-dimensional shape,like a lantern in a cavern. By studying the physical structure of a giant nebula, astronomers may determine how clusters of massive stars affect the evolution of the interstellar medium of the galaxy. The nebula also yields clues to its star formation history and will improve understanding of the starburst process when a galaxy undergoes a "firestorm" of star formation.
The image was taken on January 17, 1995 with Hubble's Wide Field and Planetary Camera 2. Separate exposures were taken in different colors of light to study the physical properties of the hot gas (17,000 degrees Fahrenheit, 10,000 degrees Kelvin).
The Lure of the Rings
Image Credit: NASA, ESA, and The Hubble Heritage Team (AURA/STScI)
Acknowledgment: J. Higdon (Cornell U.) and I. Jordan (STScI)
Resembling a diamond-encrusted bracelet, a ring of brilliant blue star clusters wraps around the yellowish nucleus of what was once a normal spiral galaxy in this new image from NASA's Hubble Space Telescope (HST). This image is being released to commemorate the 14th anniversary of Hubble's launch on April 24, 1990 and its deployment from the space shuttle Discovery on April 25, 1990.
The sparkling blue ring is 150,000 light-years in diameter, making it larger than our entire home galaxy, the Milky Way. The galaxy, cataloged as AM 0644-741, is a member of the class of so- called "ring galaxies." It lies 300 million light-years away in the direction of the southern constellation Dorado.
Ring galaxies are an especially striking example of how collisions between galaxies can dramatically change their structure, while also triggering the formation of new stars. They arise from a particular type of collision, in which one galaxy (the "intruder") plunges directly through the disk of another one (the "target"). In the case of AM 0644-741, the galaxy that pierced through the ring galaxy is out of the image but visible in larger-field images. The soft spiral galaxy that is visible to the left of the ring galaxy in the image is a coincidental background galaxy that is not interacting with the ring.
The resulting gravitational shock imparted due to the collision drastically changes the orbits of stars and gas in the target galaxy's disk, causing them to rush outward, somewhat like ripples in a pond after a large rock has been thrown in. As the ring plows outward into its surroundings, gas clouds collide and are compressed. The clouds can then contract under their own gravity, collapse, and form an abundance of new stars.
The rampant star formation explains why the ring is so blue: It is continuously forming massive, young, hot stars, which are blue in color. Another sign of robust star formation is the pink regions along the ring. These are rarefied clouds of glowing hydrogen gas, fluorescing because of the strong ultraviolet light from the newly formed massive stars.
Anyone who lives on planets embedded in the ring would be treated to a view of a brilliant band of blue stars arching across the heavens. The view would be relatively short-lived because theoretical studies indicate that the blue ring will not continue to expand forever. After about 300 million years, it will reach a maximum radius, and then begin to disintegrate.
The Hubble Heritage Team used the Hubble Advanced Camera for Surveys to take this image in January 2004. The team used a combination of four separate filters that isolate blue, green, red, and near-infrared light to create the color image.
Fireworks of Star Formation Light Up a Galaxy
Credit: Hubble Heritage Team and NASA
Located some 13 million light-years from Earth, NGC 4214 is currently forming clusters of new stars from its interstellar gas and dust. In the Hubble image, we can see a sequence of steps in the formation and evolution of stars and star clusters. The picture was created from exposures taken in several color filters with Hubble's Wide Field Planetary Camera 2.
NGC 4214 contains a multitude of faint stars covering most of the frame, but the picture is dominated by filigreed clouds of glowing gas surrounding bright stellar clusters.
The youngest of these star clusters are located at the lower right of the picture, where they appear as about half a dozen bright clumps of glowing gas. Each cloud fluoresces because of the strong ultraviolet light emitted from the embedded young stars, which have formed within them due to gravitational collapse of the gas.
Young, hot stars have a whitish to bluish color in the Hubble image, because of their high surface temperatures, ranging from 10,000 up to about 50,000 degrees Celsius. In addition to pouring out ultraviolet light, these hot stars eject fast "stellar winds," moving at thousands of kilometers per second, which plow out into the surrounding gas. The radiation and wind forces from the young stars literally blow bubbles in the gas. Over millions of years, the bubbles increase in size as the stars inside them grow older.
Moving to the lower left from the youngest clusters, we find an older star cluster, around which a gas bubble has inflated to the point that there is an obvious cavity around the central cluster. The most spectacular feature in the Hubble picture lies near the center of NGC 4214. This object is a cluster of hundreds of massive blue stars, each of them more than 10,000 times brighter than our own Sun. A vast heart-shaped bubble, inflated by the combined stellar winds and radiation pressure, surrounds the cluster. The expansion of the bubble is augmented as the most massive stars in the center reach the ends of their lives and explode as supernovae.
Deprived of gas, the cluster at the center of NGC 4214 will be unable to form further new stars, and its luminous stars will continue to go supernova and disappear. Elsewhere in the galaxy, however, gas will start to collapse and form yet another new generation of stars, even as the clusters visible today gradually fade away.
The faint stars covering most of the picture are much older than the bright blue supergiants, and show us that episodes of star birth have been occurring in NGC 4214 for billions of years.
The principal investigators are: John MacKenty, Jesus Maiz Apellaniz (Space Telescope Science Institute), Colin Norman (Johns Hopkins University), Nolan Walborn (Space Telescope Science Institute), Richard Burg (Johns Hopkins University), Richard Griffiths (Carnegie Mellon University), and Rosemary Wyse (Johns Hopkins University).
Visible and Infrared Views of Nearby Galaxies Yield Clues to Early Universe
Credits: NASA, ESA, R. de Grijs (Institute of Astronomy, Cambridge, UK)
NASA's Hubble Space Telescope snapped these two views of the heart of the galaxy M82. The image at left was taken in visible light; the picture at right, in infrared light. In the infrared view, the telescope's Near Infrared Camera and Multi-Object Spectrometer peered through thick dust lanes to find some of the galaxy's more than 100 super star clusters. The clusters are the larger pink and yellow dots scattered throughout the picture. They were formed during a violent collision with the galaxy M81 about 600 million years ago. The galaxy is 12 million light-years from Earth in the constellation Ursa Major. The pictures were taken Sept. 15, 1997.
The Farthest Known Galaxy
Credit: ESA, NASA, J.-P. Kneib (Caltech/Observatoire Midi-Pyrénées) and R. Ellis (Caltech)
Galaxy cluster Abell 2218 is acting as a powerful lens, magnifying all galaxies lying behind the cluster core. The lensed galaxies are all stretched along the cluster's center and some of them are multiply imaged. Those multiple images usually appear as a pair of images with a third — generally fainter — counter image, as is the case for the very distant object.
The color of the lensed galaxies is a function of their distances and types. The orange arc is an elliptical galaxy at moderate redshift (z=0.7). The blue arcs are star-forming galaxies at intermediate redshift (z=1-2.5). The encircled very red pair is the newly discovered star-forming galaxy at about redshift 7.
The lensed galaxies are particularly numerous, as we are looking in between two mass clumps, in a saddle region where the magnification is quite large.
[Right] This NASA
Hubble Space Telescope image of the core of the nearest starburst spiral galaxy, NGC 253,
reveals violent star formation within a region 1,000 light-years across. A starburst
galaxy has an exceptionally high rate of star birth, first identified by its excess of
infrared radiation from warm dust. The galaxy is located about 8 million light-years away
in the constellation Sculptor.