Astronomy Picture of the Day Search Results for "shock wave"

APOD: 2010 July 20 - Lightning Over Athens
Explanation: Have you ever watched a lightning storm in awe? Join the crowd. Oddly, nobody knows exactly how lightning is produced. What is known is that charges slowly separate in some clouds causing rapid electrical discharges (lightning), but how electrical charges get separated in clouds remains a topic of much research. Lightning usually takes a jagged course, rapidly heating a thin column of air to about three times the surface temperature of the Sun. The resulting shock wave starts supersonically and decays into the loud sound known as thunder. Lightning bolts are common in clouds during rainstorms, and on average 6,000 lightning bolts occur between clouds and the Earth every minute. Pictured above, an active lightning storm was recorded over Athens, Greece earlier this month.

APOD: 2009 January 8 - NGC 2736: The Pencil Nebula
Explanation: This shock wave plows through space at over 500,000 kilometers per hour. Moving right to left in the beautifully detailed color composite, the thin, braided filaments are actually long ripples in a sheet of glowing gas seen almost edge on. Cataloged as NGC 2736, its narrow appearance suggests its popular name, the Pencil Nebula. About 5 light-years long and a mere 800 light-years away, the Pencil Nebula is only a small part of the Vela supernova remnant. The Vela remnant itself is around 100 light-years in diameter, the expanding debris cloud of a star that was seen to explode about 11,000 years ago. Initially, the shock wave was moving at millions of kilometers per hour but has slowed considerably, sweeping up surrounding interstellar gas.

APOD: 2007 August 19 - A Sonic Boom
Explanation: Is this what a sonic boom looks like? When an airplane travels at a speed faster than sound, density waves of sound emitted by the plane cannot precede the plane, and so accumulate in a cone behind the plane. When this shock wave passes, a listener hears all at once the sound emitted over a longer period: a sonic boom. As a plane accelerates to just break the sound barrier, however, an unusual cloud might form. The origin of this cloud is still debated. A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets. Above, an F/A-18 Hornet was photographed just as it broke the sound barrier. Large meteors and the space shuttle frequently produce audible sonic booms before they are slowed below sound speed by the Earth's atmosphere.

APOD: 2007 February 13 - Vela Supernova Remnant in Visible Light
Explanation: The explosion is over but the consequences continue. About eleven thousand years ago a star in the constellation of Vela could be seen to explode, creating a strange point of light briefly visible to humans living near the beginning of recorded history. The outer layers of the star crashed into the interstellar medium, driving a shock wave that is still visible today. A roughly spherical, expanding shock wave is visible in X-rays. The above image captures much of that filamentary and gigantic shock in visible light, spanning almost 100 light years and appearing twenty times the diameter of the full moon. As gas flies away from the detonated star, it decays and reacts with the interstellar medium, producing light in many different colors and energy bands. Remaining at the center of the Vela Supernova Remnant is a pulsar, a star as dense as nuclear matter that completely rotates more than ten times in a single second.

APOD: 2006 December 13 - A Large Tsunami Shock Wave on the Sun
Explanation: Tsunamis this large don't happen on Earth. One week ago, a large solar flare from an Earth-sized sunspot produced a tsunami-type shock wave that was spectacular even for the Sun. Pictured above, the tsunami wave was captured moving out from active region AR 10930 by the Optical Solar Patrol Network (OSPAN) telescope in New Mexico, USA. The resulting shock wave, known technically as a Moreton wave, compressed and heated up gasses including hydrogen in the photosphere of the Sun, causing a momentarily brighter glow. The above image was taken in a very specific red color emitted exclusively by hydrogen gas. The rampaging tsunami took out some active filaments on the Sun, although many re-established themselves later. The solar tsunami spread at nearly one million kilometers per hour, and circled the entire Sun in a matter of minutes.

APOD: 2006 November 24 - Alpha Cam: Runaway Star
Explanation: Runaway stars are massive stars traveling rapidly through interstellar space. Like a ship plowing through cosmic seas, runaway star Alpha Cam has produced this graceful arcing bow wave or bow shock - moving at over 60 kilometers per second and compressing the interstellar material in its path. The bright star above and left of center in this wide (3x2 degree) view, Alpha Cam is about 25-30 times as massive as the Sun, 5 times hotter (30,000 kelvins), and over 500,000 times brighter. About 4,000 light-years away in the long-necked constellation Camelopardalis, the star also produces a strong wind. The bow shock stands off about 10 light-years from the star itself. What set this star in motion? Astronomers have long thought that Alpha Cam was flung out of a nearby cluster of young hot stars due to gravitational interactions with other cluster members or perhaps by the supernova explosion of a massive companion star.

APOD: 2007 February 17 - Supernova Remnant and Shock Wave
Explanation: A massive star ends life as a supernova, blasting its outer layers back to interstellar space. The spectacular death explosion is initiated by the collapse of what has become an impossibly dense stellar core. Pictured is the expanding supernova remnant Puppis A - one of the brightest sources in the x-ray sky. Now seen to be about 10 light-years in diameter, light from the initial stellar explosion first reached Earth a few thousand years ago. Recorded by the Chandra Observatory's x-ray cameras, the inset view shows striking details of the strong shock wave disrupting an interstellar cloud as the shock sweeps through preexisting material. The larger field ROSAT image also captures a pinpoint source of x-rays near the remnant's center. The source is a young neutron star, the remnant of the collapsed stellar core kicked out by the explosion and moving away at about 1,000 kilometers per second.

APOD: 2006 January 20 - LL Ori and the Orion Nebula
Explanation: This esthetic close-up of cosmic clouds and stellar winds features LL Orionis, interacting with the Orion Nebula flow. Adrift in Orion's stellar nursery and still in its formative years, variable star LL Orionis produces a wind more energetic than the wind from our own middle-aged Sun. As the fast stellar wind runs into slow moving gas a shock front is formed, analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed. The small, arcing, graceful structure just above and left of center is LL Ori's cosmic bow shock, measuring about half a light-year across. The slower gas is flowing away from the Orion Nebula's hot central star cluster, the Trapezium, located off the upper left corner of the picture. In three dimensions, LL Ori's wrap-around shock front is shaped like a bowl that appears brightest when viewed along the "bottom" edge. The beautiful picture is part of a large mosaic view of the complex stellar nursery in Orion, filled with a myriad of fluid shapes associated with star formation.

APOD: 2003 November 15 - LL Orionis: When Cosmic Winds Collide
Explanation: This arcing, graceful structure is actually a bow shock about half a light-year across, created as the wind from young star LL Orionis collides with the Orion Nebula flow. Adrift in Orion's stellar nursery and still in its formative years, variable star LL Orionis produces a wind more energetic than the wind from our own middle-aged sun. As the fast stellar wind runs into slow moving gas a shock front is formed, analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed. The slower gas is flowing away from the Orion Nebula's hot central star cluster, the Trapezium, located off the lower right hand edge of the picture. In three dimensions, LL Ori's wrap-around shock front is shaped like a bowl that appears brightest when viewed along the "bottom" edge. The complex stellar nursery in Orion shows a myriad of similar fluid shapes associated with star formation, including the bow shock surrounding a faint star at the upper right. Part of a mosaic covering the Great Nebula in Orion, this composite color image was recorded in 1995 by the Hubble Space Telescope.

APOD: 2003 May 4 - A Sonic Boom
Explanation: Many people have heard a sonic boom, but few have seen one. When an airplane travels at a speed faster than sound, density waves of sound emitted by the plane cannot precede the plane, and so accumulate in a cone behind the plane. When this shock wave passes, a listener hears all at once the sound emitted over a longer period: a sonic boom. As a plane accelerates to just break the sound barrier, however, an unusual cloud might form. The origin of this cloud is still debated. A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets. Above, an F/A-18 Hornet was photographed just as it broke the sound barrier. Large meteors and the space shuttle frequently produce audible sonic booms before they are slowed below sound speed by the Earth's atmosphere.

APOD: 2003 March 25 - A Slow Explosion
Explanation: Why would a gamma ray burst fade so slowly? This behavior, recorded last October, is considered a new clue into the cause of gamma-ray bursts, the most powerful explosions known in the universe. The burst, first detected by the orbiting HETE satellite and later tracked by numerous ground-based telescopes, showed an unusually slow and tumultuous decay in visible light. Speculations on the cause of the unusual light curve include a blast wave striking a windy circumburst medium, a blast wave energetically refreshed by a faster outgoing shock, and non-uniformity in a fast moving jet. Pictured above is the massive Wolf-Rayet star WR124, a star itself undergoing a slow explosion by producing a very powerful but tumultuous wind. Popular candidate progenitor sources for GRBs include supernova or hypernova explosions from massive stars, possibly ones with similarities to Wolf-Rayet stars.

APOD: 2003 March 17 - SN 1006: History's Brightest Supernova
Explanation: Suddenly, in the year 1006 AD, a new star appeared in the sky. Over the course of just a few days, the rogue star became brighter than the planet Venus. The star, likely the talk of everyone who could see it, was recorded by people who lived in areas now known as China, Egypt, Iraq, Italy, Japan, and Switzerland. The celestial newcomer, now known to be a supernova, took months to fade. Modern observations have been used to measure the speed of the still-expanding shock wave, allowing a better estimate of its distance and hence a better estimate of the true brightness of the supernova. It turns out SN 1006 likely achieved an apparent visual magnitude of -7.5, making it the brightest supernova on record. The shock wave was imaged in 1998 from CTIO (left panel), and then subtracted from a similar image taken in 1986 (right panel), highlighting the relative expansion.

APOD: 2003 March 14 - DEM L71: When Small Stars Explode
Explanation: Large, massive stars end their furious lives in spectacular supernova explosions -- but small, low mass stars may encounter a similar fate. In fact, instead of simply cooling off and quietly fading away, some white dwarf stars in binary star systems are thought to draw enough mass from their companions to become unstable, triggering a nuclear detonation. The resulting standard candle stellar explosion is classified as a Type Ia supernova and perhaps the best example yet of the aftermath is this expanding cloud of shocked stellar debris, DEM L71, in the nearby Large Magellanic Cloud. The sharp false-color x-ray image from the orbiting Chandra Observatory shows the predicted bright edges of the outer blast wave shock region and the x-ray glow of an inner region of reverse shock heated gas. Based on the Chandra data, estimates for the composition and total mass of expanding gas strongly suggest that this is all that remains of a white dwarf star. Light from this small star's self-destructive explosion would have first reached Earth several thousand years ago.

APOD: 2003 February 1 - The Nebula And The Neutron Star
Explanation: The lonely RX J1856.5-3754 was formed from the collapsed core of an exploding star. At a distance of 180 light-years it is the closest known neutron star. More massive than the Sun but only 20 kilometers across, this tiny stellar juggernaut plows through the hydrogen gas and dust clouds of interstellar space at about 200 kilometers per second. The surface of the neutron star is fantastically hot, around 700,000 degrees Celsius, making it detectable with orbiting x-ray telescopes. But optical astronomers were surprised to discover that RX J1856.5-3754 is also surrounded by a cone-shaped nebula. Indicated in this deep image from the European Southern Observatory's Kueyen telescope, the nebula glows in the red light of ionized hydrogen atoms recombining with electrons. Its cone shape is analogous to the bow wave of a ship plowing through water. A faint blue dot near the tip of the cone is the neutron star itself. The nebula appears to have formed very near the surface of the neutron star and astronomers are trying to determine if the observed densities and temperatures can explain the nebula's appearance.

APOD: 2003 January 18 - Filaments in the Cygnus Loop
Explanation: Subtle and delicate in appearance, these are filaments of shocked interstellar gas -- part of the expanding blast wave from a violent stellar explosion. Recorded in November 1997 with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope, the picture is a closeup of a supernova remnant known as the Cygnus Loop. The nearly edge-on view shows a small portion of the immense shock front moving toward the top of the frame at about 170 kilometers per second while glowing in light emitted by atoms of excited hydrogen gas. Not just another pretty picture, this particular image has provided some dramatic scientific results. In 1999, researchers used it to substantially revise downward widely accepted estimates of distance and age for this classic supernova remnant. Now determined to lie only 1,440 light-years away, the Cygnus Loop is thought to have been expanding for 5 - 10 thousand years.

APOD: 2002 March 13 - LL Orionis: When Cosmic Winds Collide
Explanation: This arcing, graceful structure is actually a bow shock about half a light-year across, created as the wind from young star LL Orionis collides with the Orion Nebula flow. Adrift in Orion's stellar nursery and still in its formative years, variable star LL Orionis produces a wind more energetic than the wind from our own middle-aged sun. As the fast stellar wind runs into slow moving gas a shock front is formed, analogous to the bow wave of a boat moving through water or a plane traveling at supersonic speed. The slower gas is flowing away from the Orion Nebula's hot central star cluster, the Trapezium, located off the lower right hand edge of the picture. In three dimensions, LL Ori's wrap-around shock front is shaped like a bowl that appears brightest when viewed along the "bottom" edge. The complex stellar nursery in Orion shows a myriad of similar fluid shapes associated with star formation, including the bow shock surrounding a faint star at the upper right. Part of a mosaic covering the Great Nebula in Orion, this composite color image was recorded in 1995 by the Hubble Space Telescope.

APOD: 2002 February 23 - Shocked by Supernova 1987A
Explanation: Fifteen years ago today, the brightest supernova of modern times was sighted. Over time, astronomers have watched and waited for the expanding debris from this tremendous stellar explosion to crash into previously expelled material. A clear result of such a collision is demonstrated above in two frames recorded by the Hubble Space Telescope in 1994 (left) and 1997(right). While the central concentration of stellar debris has clearly evolved over this period, the yellow spot on the ring in the righthand picture announces the collision of an outward moving blast wave with the pre-existing, light-year wide ring. The collision is occurring at speeds near 60 million kilometers per hour and shock-heats the ring material causing it to glow. Astronomers are hopeful that such collisions will illuminate the interesting past of SN 1987A, and perhaps provide more clues about the origin of the mysterious rings.

APOD: 2002 January 15 - Red Auroral Corona
Explanation: Few auroras show this level of detail. This unusual display of an auroral corona occurred on Earth three days after an unusual solar event -- the fifth most powerful explosion yet recorded on the Sun. An X14-class solar flare on April 15 sent a tremendous Coronal Mass Ejection (CME) into the Solar System. This CME did not directly impact the Earth. The Solar-System wide shock wave it created probably did, however, causing a G3-class geomagnetic storm and a night filled with colorful auroras across much of northern North America. The unusual red color of this Michigan aurora is caused by solar ions striking oxygen molecules 300 kilometers high in Earth's atmosphere. More typical green auroras are caused by oxygen recombining only 100 kilometers high.

APOD: 2001 February 21 - A Sonic Boom
Explanation: Many people have heard a sonic boom, but few have seen one. When an airplane travels at a speed faster than sound, density waves of sound emitted by the plane cannot precede the plane, and so accumulate in a cone behind the plane. When this shock wave passes, a listener hears all at once the sound emitted over a longer period: a sonic boom. As a plane accelerates to just break the sound barrier, however, an unusual cloud might form. The origin of this cloud is still debated. A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets. Above, an F/A-18 Hornet was photographed just as it broke the sound barrier. Large meteors and the space shuttle frequently produce audible sonic booms before they are slowed below sound speed by the Earth's atmosphere.

APOD: 2000 October 25 - The Nebula And The Neutron Star
Explanation: The lonely RX J1856.5-3754 was formed from the collapsed core of an exploding star. At a distance of 180 light-years it is the closest known neutron star. More massive than the Sun but only 20 kilometers across, this tiny stellar juggernaut plows through the hydrogen gas and dust clouds of interstellar space at about 200 kilometers per second. The surface of the neutron star is fantastically hot, around 700,000 degrees Celsius, making it detectable with orbiting x-ray telescopes. But optical astronomers were recently surprised to discover that RX J1856.5-3754 is also surrounded by a cone-shaped nebula. Indicated in this deep image from the European Southern Observatory's Kueyen telescope, the nebula glows in the red light of ionized hydrogen atoms recombining with electrons. Its cone shape is analogous to the bow wave of a ship plowing through water. A faint blue dot near the tip of the cone is the neutron star itself. The nebula appears to have formed very near the surface of the neutron star and astronomers are trying to determine if the observed densities and temperatures can indeed explain the nebula's appearance.

APOD: 2000 May 12 - X-Ray Ring Around SN1987A
Explanation: This false-color image from the Chandra X-ray Observatory reveals a one light-year diameter ring of hot, ten million degree plasma. It is one of the most detailed X-ray images of the expanding blast wave from supernova 1987A (SN1987A). At visible wavelengths SN1987A is famous for its evolving rings, and superposed on this image are white contour lines which outline the innermost optical ring as seen by the Hubble Space Telescope. The composite picture clearly shows that the X-ray emitting shocked material lies just inside the optical ring. In fact, the X-ray emission seems to peak (whitest color) close to where the optical emission peaks (closely spaced contours), a persuasive demonstration that the optical light is produced as the blast wave plows into surrounding material. What will SN1987A look like in the future? According to a popular model, in coming years the expanding supernova blast wave should hit and light up even more material while the violent impacts send reverse shocks back towards the site of the explosion and light up the ejected stellar debris. In any event, astronomers will watch eagerly from a ringside seat as a new supernova remnant emerges.

APOD: 2000 April 26 - Filaments In The Cygnus Loop
Explanation: Subtle and delicate in appearance, these are filaments of shocked interstellar gas -- part of the expanding blast wave from a violent stellar explosion. Recorded in November 1997 with the Wide Field and Planetary Camera 2 onboard the Hubble Space Telescope, the picture is a closeup of a supernova remnant known as the Cygnus Loop. The nearly edge-on view shows a small portion of the immense shock front moving toward the top of the frame at about 170 kilometers per second while glowing in light emitted by atoms of excited Hydrogen gas. Not just another pretty picture, this particular image has provided some dramatic scientific results. In 1999, researchers used it to substantially revise downward widely accepted estimates of distance and age for this classic supernova remnant. Now determined to lie only 1,440 light-years away, the Cygnus Loop is thought to have been expanding for 5 - 10 thousand years.

APOD: 2000 April 14 - Supernova Remnant E0102 72 from Radio to X-Ray
Explanation: Not all stars form a big Q after they explode. The shape of supernova remnant E0102-72, however, is giving astronomers a clue about how tremendous explosions disperse elements and interact with surrounded gas. The above image is a composite of three different photographs in three different types of light. Radio waves, shown in red, trace high-energy electrons spiraling around magnetic field lines in the shock wave expanding out from the detonated star. Optical light, shown in green, traces clumps of relatively cool gas that includes oxygen. X-rays, shown in blue, show relatively hot gas that has been heated to millions of degrees. This gas has been heated by an inward moving shock wave that has rebounded from a collision with existing or slower moving gas. This big Q currently measures 40 light-years across and was found in our neighboring SMC galaxy. Perhaps we would know even more if we could buy a vowel.

APOD: 2000 April 10 - Aurora in Red and Yellow
Explanation: The past week brought some spectacular aurora to northern skies. These aurorae were caused by a large interplanetary shock wave that exploded from the Sun on April 4. When the shock wave reached the Earth on April 6, the resulting aurora could be seen in clear skies as far south as North Carolina. As the aurorae occurred high in the Earth's atmosphere, they were accompanied by an unusual alignment of planets far in the background. Pictured above that night, an unusual multicolored auroral display graced the skies above the domes of the Brno Observatory in the Czech Republic.

APOD: 2000 February 17 - New Shocks For Supernova 1987A
Explanation: In February of 1987, astronomers witnessed the brightest supernova of modern times - supernova 1987A in the Large Magellanic Cloud. Mysterious rings of material surrounding the expanding stellar debris were soon emitting a visible glow excited by intense light from the explosion. After fading over the intervening years, the interior ring has just been seen to sprout four new hotspots, as illustrated in these two versions of a Hubble Space Telescope image recorded on February 2nd. The abrupt appearance of the new features suggests that matter from the stellar blast wave itself has begun to slam into the ring in earnest, shock-heating the gas and producing the bright hotspots. The left-hand picture shows the glowing ring, initially excited by light from the explosion, along with the shocked hotspots. The right-hand picture has been further computer enhanced to emphasize the hotspots. The brightest spot at the right was first observed in 1997, while the four spots on the left half of the ring are new. Astronomers now eagerly anticipate a dramatic rejuvenation of the glowing ring as the bulk of the blast wave material, traveling at about 60 million kilometers per hour, continues to plow into it.

APOD: November 27, 1999 - Runaway Star
Explanation: Runaway stars are massive stars traveling rapidly through interstellar space. Like a ship plowing through the interstellar medium, runaway star HD 77581 has produced this graceful arcing bow wave or "bow shock" - compressing the gaseous material in its path. Located near the centre of this European Southern Observatory photograph, HD 77581 itself is so bright that it saturates the sensitive camera and produces the spiky cross shape. This star is over 6,000 light-years away in the constellation Vela, and appears to move at over 50 miles per second. What force could set this star in motion? A clue to the answer may lie in its optically invisible companion star, an X-ray bright pulsar known as Vela X-1. This pulsar is clearly the remnant of a supernova explosion ... which seems to have given this massive star and its companion a mighty kick!

APOD: August 3, 1999 - The Vela Supernova Remnant Expands
Explanation: The explosion is over but the consequences continue. About eleven thousand years ago a star in the constellation of Vela exploded, creating a strange point of light briefly visible to humans living near the beginning of recorded history. The outer layers of the star crashed into the interstellar medium, driving a shock wave that is still visible today. Different colors in the complex, right moving shock, pictured on the left, represent different energies of impact of the shock front. The star on the left appears by chance in the foreground, and the long diagonal line is also unrelated. Remaining at the center of the Vela Supernova Remnant is a pulsar, a star as dense as nuclear matter that completely rotates more than ten times in a single second.

APOD: April 30, 1999 - Solar Shock Wave
Explanation: On September 24, 1997 a shock wave blasted across the surface of the sun at speeds of 250 to 600 kilometers per second. On planet Earth, observer Barry Reynolds photographed the expanding shock front (left) in the light emitted by hydrogen atoms at the solar surface. His discovery image was nicely confirmed by a space-based extreme ultraviolet image (right) of the shock ramming through the sun's upper atmosphere as recorded by the SOHO satellite observatory. In both pictures a bright solar flare is seen near the center of a circular arc-like feature representing a shock front. The shock front is dark in the ground based photo and bright in the ultraviolet image. These shock fronts are believed to be tracers of a 3-dimensional disturbance caused by the flare but researchers are uncertain as to the exact physical mechanisms which produced it. Along with other violent events called coronal mass ejections, solar flares are known to generate streams of energetic particles which can affect the Earth's magnetosphere and Earth orbiting satellites.

APOD: June 18, 1998 - Cosmic Rays and Supernova Dust
Explanation: Cosmic Rays are celestial high energy particles traveling at nearly the speed of light, which constantly bombard the Earth. Discovered during high altitude balloon flights in 1912 their source has been a long standing mystery. But a recent theory suggests that cosmic ray particles are atomic nuclei blasted from dust grains formed in supernovae, the death explosions of massive stars. This artist's illustration shows a supernova explosion (at left) and a conical section of the expanding cloud of ejected material. Atoms are torn from the brownish bands of "dust" material by shock waves (represented by orange rings). The shocks in the expanding blast wave then accelerate the atoms to near light speeds firing them into interstellar space like cosmic bullets. The theory is supported by observations indicating that high velocity dust was formed in the nearby supernova 1987A, and that Beryllium, a light element created in Cosmic Ray collisions, is found equally in both old an young stars. NASA's Advanced Composition Explorer (ACE) satellite can also test details of the theory by directly measuring Cosmic Rays.

APOD: February 17, 1998 - Shocked by Supernova 1987a
Explanation: Eleven years ago the brightest supernova of modern times was recorded. Now the expanding debris from this tremendous stellar explosion is seen to be crashing into previously expelled material. The onset of this collision is shown by the arrow in the above picture as the yellow spot on the interior of the ring. Although the collision is occurring at speeds near 60 million km/hour, it will appear to take years due to the vast distances involved. As the supernova blast wave moves out, it shock-heats any gas it encounters, causing it to glow. Astronomers are thus hopeful that the blast wave will illuminate the interesting past of SN 1987a, and perhaps provide more clues about the origins of the mysterious rings.

APOD: January 19, 1998 - The Hubble 5 Planetary Nebula
Explanation: The Hubble Double Bubble Planetary Nebula is bubbling over with excitement. More mundanely known as Hubble 5, this bipolar planetary nebula is being created by a hot wind of particles streaming away from the central star system. The hot gas expands into the surrounding interstellar medium in a fashion similar to the inflation of hot air balloons. A supersonic shock-wave can form at the boundary, causing newly excited gas there to shine as electrons recombine with resident elements. In the above picture, colors are assigned according to the energy of the recombinant radiation. This star system lies about 2200 light-years from Earth, and likely includes a Sun-like star slowly transforming itself into a white dwarf.

APOD: December 3, 1997 - Runaway Star
Explanation: Runaway stars are massive stars traveling rapidly through interstellar space. Like a ship plowing through the interstellar medium, runaway star HD 77581 has produced this graceful arcing bow wave or "bow shock" - compressing the gaseous material in its path. Located near the centre of this European Southern Observatory photograph, HD 77581 itself is so bright that it saturates the sensitive camera and produces the spiky cross shape. This star is over 6,000 light-years away in the constellation Vela, and appears to move at over 50 miles per second. What force could set this star in motion? A clue to the answer may lie in its optically invisible companion star, an X-ray bright pulsar known as Vela X-1. This pulsar is clearly the remnant of a supernova explosion ... which seems to have given this massive star and its companion a mighty kick!

APOD: July 13, 1997 - Vela Supernova Remnant in Optical
Explanation: About 11,000 years ago a star in the constellation of Vela exploded. This bright supernova may have been visible to the first human farmers. Today the Vela supernova remnant marks the position of a relatively close and recent explosion in our Galaxy. A roughly spherical, expanding shock wave is visible in X-rays. In the above optical photograph, the upper left corner of the spherical blast wave is shown in detail. As gas flies away from the detonated star, it reacts with the interstellar medium, knocking away closely held electrons from even heavy elements. When the electrons recombine with these atoms, light in many different colors and energy bands is produced.

APOD: May 2, 1997 - X-Rays From IC 443
Explanation: The life-cycles of stars help drive the ecology of our Galaxy, churning, processing, and redistributing matter. Massive stars reach a spectacular evolutionary endpoint - supernovae explosions which blast off their outer layers, violently merging stellar material with the gas and dust of the Milky Way. The supernova remnant IC 443 is typical of the aftermath. Seen in this false color X-ray image are the shocked, expanding shells of gas from a star which exploded thousands of years ago. Known to be interacting with galactic molecular clouds, the expanding supernova remnant was also recently discovered to have regions of intense higher energy X-ray emission (coded blue in this map) near the molecular cloud boundaries. This X-ray emission may indicate that electrons are being accelerated within the remnant, gaining in energy as they surf back and forth across the expanding shock wave. If so, IC 443 could also be one source of our Galaxy's puzzling high energy cosmic-rays.

APOD: June 13, 1996 - Vela Supernova Remnant in Optical
Explanation: About 11,000 years ago a star in the constellation of Vela exploded. This bright supernova may have been visible to the first human farmers. Today the Vela supernova remnant marks the position of a relatively close and recent explosion in our Galaxy. A roughly spherical, expanding shock wave is visible in X-rays. In the above optical photograph, the upper left corner of the spherical blast wave is shown in detail. As gas flies away from the detonated star, it reacts with the interstellar medium, knocking away closely held electrons from even heavy elements. When the electrons recombine with these atoms, light in many different colors and energy bands is produced.

APOD: March 7, 1996 - Rampaging Fronts of the Veil Nebula
Explanation: A supernova explosion of a high-mass star results in fast moving blast waves. At the front of the waves shown above, ionized gas in the Veil Supernova Remnant rushes out from the explosion, sweeps up material, and breaks up many atoms into constituent ions and electrons. Observations with the Hubble Space Telescope in 1993 indicate that the blue shock wave was catapult away from the stellar explosion after the red shock wave and has yet to catch up to it in some regions. The Veil supernova remnant's has a very large angular size - six times the diameter of the full moon - and different parts of it are known as the "Cygnus Loop" and catalog numbers NGC 6960, NGC 6979, NGC 6992, and NGC 6995.