Astronomy Picture of the Day Search Results for "nova"

APOD: 2024 April 3 – Unusual Nebula Pa 30
Explanation: What created this unusual celestial firework? The nebula, dubbed Pa 30, appears in the same sky direction now as a bright "guest star" did in the year 1181. Although Pa 30's filaments look similar to that created by a nova (for example GK Per), and a planetary nebula (for example NGC 6751), some astronomers now propose that it was created by a rare type of supernova: a thermonuclear Type Iax, and so is (also) named SN 1181. In this model, the supernova was not the result of the detonation of a single star, but rather a blast that occurred when two white dwarf stars spiraled together and merged. The blue dot in the center is hypothesized to be a zombie star, the remnant white dwarf that somehow survived this supernova-level explosion. The featured image combines images and data obtained with infrared (WISE), visible (MDM, Pan-STARRS), and X-ray (Chandra, XMM) telescopes. Future observations and analyses may tell us more.

APOD: 2024 March 2 - Odysseus on the Moon
Explanation: Methalox rocket engine firing, Odysseus' landing legs absorb first contact with the lunar surface in this wide-angle snapshot from a camera on board the robotic Intuitive Machines Nova-C moon lander. Following the landing on February 22, broken landing legs, visible in the image, ultimately left the lander at rest but tilted. Odysseus' gentle lean into a sloping lunar surface preserved the phone booth-sized lander's ability to operate, collect solar power, and return images and data to Earth. Its exact landing site in the Moon's far south polar region was imaged by NASA's Lunar Reconnaissance Orbiter. Donated by NASA, the American flag seen on the lander's central panel is 1970 Apollo program flight hardware.

APOD: 2022 April 2 - Nova Scotia Northern Lights
Explanation: This almost otherworldly display of northern lights was captured in clear skies during the early hours of March 31 from 44 degrees north latitude, planet Earth. In a five second exposure the scene looks north from Martinique Beach Provincial Park in Nova Scotia, Canada. Stars of the W-shaped constellation Cassiopeia shine well above the horizon, through the red tint of the higher altitude auroral glow. Auroral activity was anticipated by skywatchers alerted to the possibility of stormy space weather by Sun-staring spacecraft. The predicted geomagnetic storm was sparked as a coronal mass ejection, launched from prolific solar active region 2975, impacted our fair planet's magnetosphere.

APOD: 2021 August 22 - Explosions from White Dwarf Star RS Oph
Explanation: Spectacular explosions keep occurring in the binary star system named RS Ophiuchi. Every 20 years or so, the red giant star dumps enough hydrogen gas onto its companion white dwarf star to set off a brilliant thermonuclear explosion on the white dwarf's surface. At about 5,000 light years distant, the resulting nova explosions cause the RS Oph system to brighten up by a huge factor and become visible to the unaided eye. The red giant star is depicted on the right of the above drawing, while the white dwarf is at the center of the bright accretion disk on the left. As the stars orbit each other, a stream of gas moves from the giant star to the white dwarf. Astronomers speculate that at some time in the next 100,000 years, enough matter will have accumulated on the white dwarf to push it over the Chandrasekhar Limit, causing a much more powerful and final explosion known as a supernova. Starting early this month, RS Oph was again seen exploding in a bright nova.

APOD: 2021 August 16 - Perseid Meteor, Red Sprites, and Nova RS Oph
Explanation: This was an unusual sky. It wasn't unusual because of the central band the Milky Way Galaxy, visible along the image left. Most dark skies show part of the Milky Way. It wasn't unusual because of the bright meteor visible on the upper right. Many images taken during last week's Perseid Meteor Shower show meteors, although this Perseid was particularly bright. This sky wasn't unusual because of the red sprites, visible on the lower right. Although this type of lightning has only been noted in the past few decades, images of sprites are becoming more common. This sky wasn't unusual because of the nova, visible just above the image center. Novas bright enough to be seen with the unaided eye occur every few years, with pictured Nova RS Ophiuchus discovered about a week ago. What was most unusual, though, was to capture all these things together, in a single night, on a single sky. The unusual sky occurred above Zacatecas, Mexico.

APOD: 2021 June 7 - A Bright Nova in Cassiopeia
Explanation: What’s that new spot of light in Cassiopeia? A nova. Although novas occur frequently throughout the universe, this nova, known as Nova Cas 2021 or V1405 Cas, became so unusually bright in the skies of Earth last month that it was visible to the unaided eye. Nova Cas 2021 first brightened in mid-March but then, unexpectedly, became even brighter in mid-May and remained quite bright for about a week. The nova then faded back to early-May levels, but now is slightly brightening again and remains visible through binoculars. Identified by the arrow, the nova occurred toward the constellation of Cassiopeia, not far from the Bubble Nebula. A nova is typically caused by a thermonuclear explosion on the surface of a white dwarf star that is accreting matter from a binary-star companion -- although details of this outburst are currently unknown. Novas don't destroy the underlying star, and are sometimes seen to recur. The featured image was created from 14 hours of imaging from Detroit, Michigan, USA. Both professional and amateur astronomers will likely continue to monitor Nova Cas 2021 and hypothesize about details of its cause.

APOD: 2021 April 6 - Mars and the Pleiades Beyond Vinegar Hill
Explanation: Is this just a lonely tree on an empty hill? To start, perhaps, but look beyond. There, a busy universe may wait to be discovered. First, physically, to the left of the tree, is the planet Mars. The red planet, which is the new home to NASA's Perseverance rover, remains visible this month at sunset above the western horizon. To the tree's right is the Pleiades, a bright cluster of stars dominated by several bright blue stars. The featured picture is a composite of several separate foreground and background images taken within a few hours of each other, early last month, from the same location on Vinegar Hill in Milford, Nova Scotia, Canada. At that time, Mars was passing slowly, night after night, nearly in front of the distant Seven Sisters star cluster. The next time Mars will pass angularly as close to the Pleiades as it did in March will be in 2038.

APOD: 2020 July 2 - The Galaxy, the Planet, and the Apple Tree
Explanation: The Old Astronomer's Milky Way arcs through this peaceful northern sky. Against faint, diffuse starlight you can follow dark rifts of interstellar dust clouds stretching from the galaxy's core. They lead toward bright star Antares at the right, almost due south above the horizon. The brightest beacon in the twilight is Jupiter, though. From the camera's perspective it seems to hang from the limb of a tree framing the foreground, an apple tree of course. The serene maritime nightscape was recorded in tracked and untracked exposures on June 16 from Dover, Nova Scotia, planet Earth.

APOD: 2018 August 6 - Live: Cosmic Rays from Minnesota
Explanation: Cosmic rays from outer space go through your body every second. Typically, they do you no harm. The featured image shows some of these fast moving particles as streaks going through Fermilab's NOvA Far Detector located in Ash River, Minnesota, USA. Although the image updates every 15 seconds, it only shows cosmic rays that occurred over a (changing) small fraction of that time, and mostly shows only one type of particle: muons. The NOvA Far Detector's main purpose is not to detect cosmic rays, though, but rather neutrinos from the NuMI beam shot through the Earth from Fermilab near Chicago, Illinois, USA, 810 kilometers away. Only a few neutrino events are expected in NOvA per week, though. The NuMI / NOvA experiment is allowing humanity to better explore the nature of neutrinos, for example how frequently they change type during their trip. Cosmic rays themselves were discovered only about 100 years ago and can not only alter computer memory, but may have helped to create DNA mutations that resulted in, eventually, humans.

APOD: 2018 March 25 - Announcing Nova Carinae 2018
Explanation: How bright will Nova Carinae 2018 become? The new nova was discovered only last week. Although novas occur frequently throughout the universe, this nova, cataloged as ASASSN-18fv, is so unusually bright in the skies of Earth that it is now easily visible through binoculars in the southern hemisphere. Identified by the arrow, the nova occurs near the direction of the picturesque Carina Nebula. A nova is typically caused by a thermonuclear explosion on the surface of a white dwarf star that is accreting matter from a binary companion, although details of this outburst are currently unknown. Both professional and amateur astronomers will be monitoring this unusual stellar outburst in the coming weeks, looking to see how Nova Carinae 2018 evolves, including whether it becomes bright enough to be visible to the unaided eye.

APOD: 2017 June 29 - Symbiotic R Aquarii
Explanation: A long recognized naked-eye variable star, R Aquarii is actually an interacting binary star system, two stars that seem to have a close, symbiotic relationship. About 710 light years away, it consists of a cool red giant star and hot, dense white dwarf star in mutual orbit around their common center of mass. The binary system's visible light is dominated by the red giant, itself a Mira-type long period variable star. But material in the cool giant star's extended envelope is pulled by gravity onto the surface of the smaller, denser white dwarf, eventually triggering a thermonuclear explosion and blasting material into space. Optical image data (red) shows the still expanding ring of debris originating from a blast that would have been seen in the early 1770s. The evolution of less understood energetic events producing high energy emission in the R Aquarii system has been monitored since 2000 using Chandra X-ray Observatory data (blue). The composite field of view is less that a light-year across at the estimated distance of R Aquarii.

APOD: 2016 November 21 - Nova over Thailand
Explanation: A nova in Sagittarius is bright enough to see with binoculars. Discovered last month by the All-Sky Automated Survey for Supernovae (ASAS-SN), the stellar explosion even approached the limit of naked-eye visibility last week. A classical nova results from a thermonuclear explosion on the surface of a white dwarf star -- a dense star having the size of our Earth but the mass of our Sun. In the featured image, the nova was captured last week above ancient Wat Mahathat in Sukhothai, Thailand. To see Nova Sagittarius 2016 yourself, just go out just after sunset and locate near the western horizon the constellation of the Archer (Sagittarius), popularly identified with an iconic teapot. Also visible near the nova is the very bright planet Venus. Don’t delay, though, because not only is the nova fading, but that part of the sky is setting continually closer to sunset.

APOD: 2015 March 25 - Naked Eye Nova Sagittarii 2015 No 2
Explanation: It quickly went from obscurity to one of the brighter stars in Sagittarius -- but it's fading. Named Nova Sagittarii 2015 No. 2, the stellar explosion is the brightest nova visible from Earth in over a year. The featured image was captured four days ago from Ranikhet in the Indian Himalayas. Several stars in western Sagittarius make an asterism known as the Teapot, and the nova, indicated by the arrow, now appears like a new emblem on the side of the pot. As of last night, Nova Sag has faded from brighter than visual magnitude 5 to the edge of unaided visibility. Even so, the nova should still be easily findable with binoculars in dark skies before sunrise over the next week.

APOD: 2015 January 11 - Cataclysmic Dawn
Explanation: Will this dawn bring another nova? Such dilemmas might be pondered one day by future humans living on a planet orbiting a cataclysmic variable binary star system. Cataclysmic variables involve gas falling from a large star onto an accretion disk surrounding a massive but compact white dwarf star. Explosive cataclysmic events such as a dwarf nova can occur when a clump of gas in the interior of the accretion disk heats up past a certain temperature. At that point, the clump will fall more quickly onto the white dwarf and land with a bright flash. Such dwarf novas will not destroy either star, and may occur irregularly on time scales from a few days to tens of years. Although a nova is much less energetic than a supernova, if recurrent novas are not violent enough to expel more gas than is falling in, mass will accumulate onto the white dwarf star until it passes its Chandrasekhar limit. At that point, a foreground cave may provide little protection, as the entire white dwarf star will explode in a tremendous supernova.

APOD: 2013 December 7 - Naked Eye Nova Centauri 2013
Explanation: Brightest stellar beacons of the constellation Centaurus, Alpha and Beta Centauri are easy to spot from the southern hemisphere. For now, so is new naked eye Nova Centauri 2013. In this night skyscape recorded near Las Campanas Observatory in the Chilean southern Atacama desert on December 5, the new star joins the old in the expansive constellation, seen at early morning hours through a greenish airglow. Caught by nova hunter John Seach from Australia on December 2 as it approached near naked eye brightness, Nova Cen 2013 has been spectroscopically identified as a classical nova, an interacting binary star system composed of a dense, hot white dwarf and cool, giant companion. Material from the companion star builds up as it falls onto the white dwarf's surface triggering a thermonuclear event. The cataclysmic blast results in a drastic increase in brightness and an expanding shell of debris. The stars are not destroyed, though. Classical novae are thought to recur when the flow of material onto the white dwarf eventually resumes and produces another outburst.

APOD: 2013 August 23 - A Spectrum of Nova Delphini
Explanation: When a new star appeared in the constellation Delphinus late last week, astronomers found its spectrum hinted at the apparition's true nature. Now known as Nova Delphini 2013, its visible light spectrum near maximum brightness is centered in this image of the nearby star field captured with prism and telescope on the night of August 16/17 at the Sternwarte Bülach, Switzerland. Strong absorption lines due to hydrogen atoms are seen as the darkest bands in the nova's spectrum, but the strong absorption lines are bordered along their redward edge by bright bands of emission. That pattern is the spectral signature of material blasted from a type of catalysmic binary system known as a classical nova. Other stars in field are fainter, identified by their Hipparcus catalog numbers, brightness in magnitudes, and spectral types. By chance, the faint emission line from planetary nebula NGC 6905 was also included, indicated at the lower right.

APOD: 2013 August 17 - M8: The Lagoon Nebula
Explanation: This beautiful cosmic cloud is a popular stop on telescopic tours of the constellation Sagittarius. Eighteenth century cosmic tourist Charles Messier cataloged the bright nebula as M8. Modern day astronomers recognize the Lagoon Nebula as an active stellar nursery about 5,000 light-years distant, in the direction of the center of our Milky Way Galaxy. Hot stars in the embedded open star cluster NGC 6530 power the nebular glow. Remarkable features can be traced through this sharp picture, showing off the Lagoon's filaments of glowing gas and dark dust clouds. Twisting near the center of the Lagoon, the small, bright hourglass shape is the turbulent result of extreme stellar winds and intense starlight. The alluring color view was captured with a telescope and digital camera while M8 was high in dark, rural Argentina skies. At the nebula's estimated distance, the picture spans over 60 light-years.

APOD: 2013 August 16 - Nova Delphini 2013
Explanation: Using a small telescope to scan the skies on August 14, Japanese amateur astronomer Koichi Itagaki discovered a "new" star within the boundaries of the constellation Delphinus. Indicated in this skyview captured on August 15 from Stagecoach, Colorado, it is now appropriately designated Nova Delphini 2013. Sagitta, the Arrow, points the way to the newcomer's location high in the evening sky, not far from bright star Altair and the asterism known to northern hemisphere skygazers as the Summer Triangle. The nova is reported to be easy to spot with binoculars, near the limit of naked-eye visibility under dark skies. In fact, previous deep sky charts do show a much fainter known star (about 17th magnitude) at the position of Nova Delphini, indicating this star's apparent brightness suddenly increased over 25,000 times. How does a star undergo such a cataclysmic change? The spectrum of Nova Delphini indicates it is a classical nova, an interacting binary star system in which one star is a dense, hot white dwarf. Material from a cool, giant companion star falls onto the surface of the white dwarf, building up until it triggers a thermonuclear event. The drastic increase in brightness and an expanding shell of debris is the result - but the stars are not destroyed! Classical novae are believed to recur when the flow of material onto the white dwarf resumes and produces another outburst.

APOD: 2013 August 15 - The Magellanic Stream
Explanation: In an astronomical version of the search for the source of the Nile, astronomers now have strong evidence for the origin of the Magellanic Stream. This composite image shows the long ribbon of gas, discovered at radio wavelengths in the 1970s, in pinkish hues against an optical all-sky view across the plane of our Milky Way galaxy. Both Large and Small Magellanic Clouds, dwarf satellite galaxies of the the Milky Way, are seen near the head of the stream at the right. Data from Hubble's Cosmic Origins Spectrograph were used to explore abundances of elements along sightlines to quasars that intersect the stream. The results indicate that most of the stream's material comes from the Small Magellanic Cloud. The Magellanic Stream is likely the result of gravitational tidal interactions between the two dwarf galaxies some 2 billion years ago, the Small Magellanic Cloud losing more material in the encounter because of its lower mass.

APOD: 2011 November 5 - GK Per: Nova of 1901
Explanation: Early in the 20th century, GK Persei briefly became one of the brightest stars in planet Earth's sky, an event known as Nova Persei 1901. Documented in this modern day composite of two images from 2003 and 2011 the ejecta from the explosion, popularly called the Firework Nebula, continues to expand into space. These images are part of a time lapse video tracking the nebula's expansion over the last 17 years. About 1500 light-years away, the nebula is still just under a light-year in diameter. GK Per and similar cataclysmic variable stars known as classical novae are understood to be binary systems consisting of a compact white dwarf star and swollen cool giant star in a close orbit. The build up of mass transferred to the surface of the white dwarf from the giant star through an accretion disk eventually triggers a thermonuclear outburst, blasting the stellar material into space without destroying the white dwarf star. With a 2 day orbital period, the GK Per system has produced much smaller outbursts in recent years.

APOD: 2009 November 17 - Dawn Before Nova
Explanation: Will this dawn bring another nova? Such dilemmas might be pondered one day by future humans living on a planet orbiting a cataclysmic variable binary star system. Cataclysmic variables involve gas falling from a large star onto an accretion disk surrounding a massive but compact white dwarf star. Explosive cataclysmic events such as a dwarf nova can occur when a clump of gas in the interior of the accretion disk heats up past a certain temperature. At that point, the clump will fall more quickly onto the white dwarf and land with a bright flash. Such dwarf novas will not destroy either star, and may occur irregularly on time scales from a few days to tens of years. Although a nova is much less energetic than a supernova, if recurrent novas are not violent enough to expel more gas than is falling in, mass will accumulate onto the white dwarf star until it passes its Chandrasekhar limit. At that point, a foreground cave may provide little protection, as the entire white dwarf star will explode in a tremendous supernova.

APOD: 2007 February 19 - Nova Over Iran
Explanation: A bright new nova is being studied by astronomers. The officially dubbed Nova Scorpii 2007 has become so bright in recent days that it is now visible to the unaided eye. Adventurous early morning sky enthusiasts should look in dark skies toward the constellation of the Scorpion, just below Jupiter and Antares. The above image may help as a sky chart. A nova this bright occurs only every few years. Novas are caused by thermonuclear explosions casting off the outer layers of a white dwarf star. Pictured above on Friday, the nova was being studied through a small telescope as it appeared over the Varzaneh Desert in Isfahan, Iran. The nova will likely fade but remain visible with binoculars for at least a few more days.

APOD: 2006 July 26 - Explosions from White Dwarf Star RS Oph
Explanation: Spectacular explosions keep occurring in the binary star system named RS Ophiuchi. Every 20 years or so, the red giant star dumps enough hydrogen gas onto its companion white dwarf star to set off a brilliant thermonuclear explosion on the white dwarf's surface. At about 2,000 light years distant, the resulting nova explosions cause the RS Oph system to brighten up by a huge factor and become visible to the unaided eye. The red giant star is depicted on the right of the above drawing, while the white dwarf is at the center of the bright accretion disk on the left. As the stars orbit each other, a stream of gas moves from the giant star to the white dwarf. Astronomers speculate that at some time in the next 100,000 years, enough matter will have accumulated on the white dwarf to push it over the Chandrasekhar Limit, causing a much more powerful and final explosion known as a supernova.

APOD: 2006 February 24 - Recurrent Nova RS Ophiuci
Explanation: This pretty star field in the constellation Ophiucus is centered on a star not often seen - RS Ophiuci. In fact, early last week RS Oph suddenly became visible to the naked eye for the first time since 1985. A type of cataclysmic variable star classified as a recurrent nova, RS Oph dramatically increased in brightness from 11th magnitude, too faint to appear on some star charts. Historically, RS Oph was seen to go through only four similar outbursts since 1898. Such stars are now modeled as interacting binary star systems, composed of a compact white dwarf star co-orbiting with a swollen red giant. As material falls away from the red giant it collects in a rotating accretion disk before ultimately falling on to the white dwarf. Disk instabilities, or a build up of material on the compact star result in the occasional but rapid release of energy through nuclear burning. At an estimated distance of 3,000 light-years, RS Ophiuci is now reported to be fading rapidly. This telescopic view spans about 2 degrees (4 full moons) and was captured on the morning of February 16 from the RAS Observatory under New Mexico skies.

APOD: 2005 November 27 - Light Echoes from V838 Mon
Explanation: What caused this outburst of V838 Mon? For reasons unknown, star V838 Mon's outer surface suddenly greatly expanded with the result that it became the brightest star in the entire Milky Way Galaxy in January 2002. Then, just as suddenly, it faded. A stellar flash like this has never been seen before -- supernovas and novas expel matter out into space. Although the V838 Mon flash appears to expel material into space, what is seen in the above image from the Hubble Space Telescope is actually an outwardly moving light echo of the bright flash. In a light echo, light from the flash is reflected by successively more distant rings in the complex array of ambient interstellar dust that already surrounded the star. V838 Mon lies about 20,000 light years away toward the constellation of the unicorn (Monoceros), while the light echo above spans about six light years in diameter.

APOD: 2005 January 16 - Nebula Nova Cygni Turns On
Explanation: Old photographs show no evidence of the above nebula. In 1992, a white dwarf star toward the constellation of Cygnus blew off its outer layers in a classical nova explosion: an event called Nova Cygni 1992. Light flooded the local interstellar neighborhood, illuminated this existing gas cloud, excited the existing hydrogen, and hence caused the red emission. The only gas actually expelled by the nova can be seen as a small red ball just above the photograph's center. Eventually, light from the nova shell will fade, and this nebula will again become invisible.

APOD: 2002 October 3 - V838 Mon: Mystery Star
Explanation: A leading candidate for the most mysterious star found in recent times is variable star V838 Monocerotis. At a distance of about 8,000 light-years, V838 Mon was discovered to be in outburst in January of this year. Initially thought to be a familiar type of classical nova, astronomers quickly realized that instead, V838 Mon may be a totally new addition to the astronomical zoo. Observations indicate that the erupting star transformed itself over a period of months from a small under-luminous star a little hotter than the Sun, to a highly-luminous, cool supergiant star undergoing rapid and complex brightness changes. The transformation defies the conventional understanding of stellar life cycles. A most notable feature of V838 Mon is the "expanding" nebula which now appears to surround it. Seen above in two separate images from the South African Astronomical Observatory's 1 meter telescope, the nebula is probably a light echo from shells of formerly unseen material lost by the star during its previous evolution. Light-years in diameter, the shells progressively reflect the light from V838 Mon's outbursts, providing an opportunity to look back at the history of this remarkable star's behaviour.

APOD: 2002 August 25 - Nebula Nova Cygni Turns On
Explanation: Old photographs show no evidence of the above nebula. In 1992, a white dwarf star toward the constellation of Cygnus blew off its outer layers in a classical nova explosion: an event called Nova Cygni 1992. Light flooded the local interstellar neighborhood, illuminated this existing gas cloud, excited the existing hydrogen, and hence caused the red emission. The only gas actually expelled by the nova can be seen as a small red ball just above the photograph's center. Eventually, light from the nova shell will fade, and this nebula will again become invisible!

APOD: 2000 November 28 - BZ Cam Bow Shock
Explanation: BZ Cam is a binary star system that is not well understood. In most cataclysmic variables, matter from a normal star accumulates on the surface of the companion white dwarf star, eventually causing a nova-like flare as the material becomes hot enough to ignite nuclear fusion. In BZ Cam, however, light appears to flicker unpredictably, and an unusually large wind of particles is being expelled. Pictured above, BZ Cam's wind creates a large bow-shock as the system moves through surrounding interstellar gas. BZ Cam lies about 2500 light-years away toward the constellation of Camelopardalis.

APOD: 2000 August 16 - Unusual Giant Galaxy NGC 1316
Explanation: Can unusual giant galaxy NGC 1316 help calibrate the universe? Quite possibly -- if it turns out this atypical galaxy is composed of typical stars. NGC 1316, pictured above, is most obviously strange because it has a size and shape common for an elliptical galaxy but dust lanes and a disk more commonly found in a spiral galaxy. These attributes could be caused by interactions with another galaxy over the past billion years. Most recently, NGC 1316 has been monitored to find novae, explosions emanating from white dwarf stars that should have a standard brightness. Again, NGC 1316 was found atypical in that the nova rate was unexpectedly high. If, however, the stars and white dwarfs that compose NGC 1316 are typical, then the novae observed should be just as bright as novae in other galaxies so that astronomers can use them to compute an accurate distance. This distance can then be used to calibrate other distance indicators and result in a more accurate scale for distances throughout the universe.

APOD: December 15, 1999 - A Nova In Aquila
Explanation: On December 1st, experienced observers patroling the night sky with binoculars noticed what seemed to be a new star in the constellation of Aquila (The Eagle). It wasn't really a new star though. A comparison with detailed skymaps revealed the amazing truth, there was a known star at that position in the sky ... its brightness had simply increased by about 70,000 times. The star, now fondly known to variable star observers as Nova V1494 Aquilae, continued to grow brighter for several days, becoming easily visible to the unaided eye before starting to slowly fade away. Its position within the constellation is indicated on this wide-angle picture taken on December 4th, near the time it was brightest. What would cause a star to undergo such a cataclysmic change? This "new star" appears to be a classical nova. Classical novae are thought to be interacting binary star systems in which one of the pair is a dense, hot white dwarf. Material from the companion falls onto the surface of the white dwarf, building up until it triggers a thermonuclear blast. A stunning increase in brightness and an expanding shell of debris result - but the binary system is likely not destroyed! Classical novae are believed to recur as the flow of material resumes and produces another outburst in perhaps hundreds of years time.

APOD: May 24, 1999 - Introducing Nova Velorum 1999
Explanation: A bright nova was discovered Saturday that is currently visible to the unaided eye in southern skies. Nova Velorum 1999 was recorded near visual magnitude 3 independently by discoverers Peter Williams and Alan C. Gilmore (Mt. John U. Obs.), making it more luminous than many famous bright stars. The last nova this bright was Nova Cygni 1975, which peaked just brighter than magnitude 2. Nova Velorum 1999 is brighter now than the well-studied Nova Cygni 1992 ever appeared. A nova occurs when the surface of a white dwarf star undergoes a tremendous thermonuclear explosion, throwing off its outer layers. How the nova will appear over the next few weeks is uncertain, but the exploding debris will likely fade beyond detectability over the next few years. The above photograph of Nova Velorum 1999 was taken yesterday from Australia. The cross-hair like spikes that appear around it were caused by the photographing telescope and camera.

APOD: September 11, 1998 - Help Map The Moon
Explanation: You can help map the Moon. Early tomorrow morning (Saturday, September 12) the Moon will occult, or pass in front of, the bright star Aldebaran as viewed from some Southern and Eastern areas of the U.S. as well as regions in the Caribbean Sea, Nova Scotia, Newfoundland, Mexico, and Central America. Aldebaran will disappear behind the bright edge of the third quarter moon and reappear behind the darkened edge. Accurately timed home video camera recordings from different locations can be used to make improved maps of the height of the lunar terrain at these occultation points. Interested? Follow the instructions on the International Occultation Timing Association HomePage which detail how to tape a familiar TV channel, take your running camcorder outside to record the occultation, and then return to tape a few more minutes of the TV channel. (First, determine if the occultation will be visible from your location!) You can then donate your tape by mailing it to the address given. Leave yourself plenty of time for a practice run and be sure to check the weather before going to a lot of trouble!

This mosaic mapping the North polar region of the lunar surface was constructed from images recorded by the Galileo spacecraft in 1992.

APOD: July 4, 1998 - The Firework Nebula
Explanation: Imaged by the WIYN Telescope, the Firework Nebula is the result of a type of stellar explosion called a nova. In a nova, a nuclear detonation on the surface of a compact white dwarf star blasts away material that has been dumped on its surface by a companion star. Also known as GK Persei or Nova Persei, this nova became one of the brightest stars in the night sky in the year 1901. As it faded, astronomers could see an expanding shell of gas that eventually became this spectacular nebula. While not exactly predictable, GK Per undergoes minor outbursts every three or four years.

APOD: September 25, 1997 - T Pyxidis: Recurrent Nova
Explanation: What happens when a thermonuclear blast occurs on the surface of a white dwarf star? Over the years astronomers have watched (at a safe distance ...) as, 6,000 light years from Earth in the southern constellation Pyxis, a binary star known as T Pyxidis repeatedly produces these fearsome explosions. This Hubble Space Telescope image of nova T Pyx captures what appear to be blobs rather than the expected shells of material expanding from this interacting star system. Like other binary star systems which produce nova outbursts, T Pyx is composed of a dense white dwarf and a close companion star. An outburst occurs when the temperature and density of the sea of matter dumped from the companion onto the surface of the white dwarf reach the nuclear flash point for hydrogen. While material is violently blown off, the white dwarf itself is not disrupted and soon begins to accumulate more matter from its companion, repeating the cataclysm a few years later.

APOD: December 16, 1996 - Nebula Nova Cygni Turns On
Explanation: Old photographs show no evidence of the above nebula. In 1992, a white dwarf star in Cygnus blew off its outer layers in a classical nova explosion: an event called Nova Cygni 1992. Light flooded the local interstellar neighborhood, illuminated this existing gas cloud, excited the existing hydrogen, and hence caused the red emission. The only gas actually expelled by the nova can be seen as a small red ball just above the photograph's center. Eventually, light from the nova shell will fade, and this nebula will again become invisible!

APOD: December 27, 1995 - Nova Cygni 1992
Explanation: In 1992 a tremendous explosion occurred in the constellation of Cygnus. Dubbed Nova Cygni 1992, this event most probably occurred in an accretion disk binary system. Astronomers hypothesize that this system's white dwarf had so much gas dumped onto it's surface that conditions became ripe for nuclear fusion. The resulting thermonuclear detonation blasted much of the surrounding gas into an expanding shell. The Hubble Space Telescope photographed this expanding shell in 1994. Nova Cygni 1992 was the brightest nova in recent history - at its brightest it could be seen without a telescope. It was observed in every part of the electromagnetic spectrum.

APOD: July 4, 1995 - The Firework Nebula
Explanation: The Firework Nebula, known to astronomers as "GK Per", is the result of a type of stellar explosion called a nova. In a nova, a very compact star called a white dwarf blasts away gas that had accumulated on its surface. In this case the nova occurred in the year 1901 and is called Nova Persei 1901. This nova became as bright as one of the brighter stars we see in the night sky, but then faded until only a telescope could see it. Soon astronomers could see an expanding shell of gas that eventually became this spectacular nebula. The unusual "fireworks" type feature of this nebula is still a matter of research and discussion.