juliensimon/nasa-apod huggingface dataset:
Cassiopeia A in a Million (2004-08-26)

One million seconds of x-ray image data were used to construct this view of supernova remnant Cassiopeia A, the expanding debris cloud from a stellar explosion. The stunningly detailed image from the Chandra Observatory will allow an unprecedented exploration of the catastrophic fate that awaits stars much more massive than the Sun. Seen in false-color, Cas A's outer green ring, 10 light-years or so in diameter, marks the location of the expanding shock from the original supernova explosion. At about 10 o'clock around the ring, a structure extends beyond it, evidence that the initial explosion may have also produced energetic jets. Still glowing in x-rays, the tiny point source near the center of Cas A is a neutron star, the collapsed remains of the stellar core. While Cas A is about 10,000 light-years away, light from the supernova explosion first reached Earth just over 300 years ago.
The Sedna Scenario (2004-08-27)

The discovery of Sedna (aka 2003 VB12), the most distant known object orbiting the Sun, presents a mystery. Pluto's orbit averages about 40 AU in radius, where an AU (Astronomical Unit) is the Earth-Sun distance. But the closest point in Sedna's eccentric orbit scarcely comes within 75 AU, while its farthest point extends to nearly 1,000 AU. So how did something as large as Sedna get so far out there? Exploring the problem with computer simulations, astronomers Alessandro Morbidelli and Harold Levison suggest that while Sedna was not formed in its current location, it was also not moved there by encounters with other solar system objects. Instead, they find it more likely that Sedna resides in its present orbit because of an encounter with another star. In one scenario, objects like Sedna are yanked out of closer orbits by the gravitational pull of a Sun-sized star passing near the solar system during its formative years. Alternatively Sedna could have formed of material from another system entirely, captured during an early encounter with a much smaller star. Both Sedna-forming stellar encounter scenarios are consistent with idea that the Sun itself was born in an ancient, dense, cluster of stars.
M17: A Hubble Close-Up (2004-08-28)

Sculpted by stellar winds and radiation, these fantastic, undulating shapes lie within the stellar nursery known as M17, the Omega Nebula, some 5,500 light-years away in the nebula-rich constellation Sagittarius. The lumpy features in the dense cold gas and dust are illuminated by stars off the upper left of the image and may themselves represent sites of future star formation. Colors in the fog of surrounding hotter material indicate M17's chemical make up. The predominately green glow corresponds to abundant hydrogen, with trace sulfur and oxygen atoms contributing red and blue hues. The picture spans about 3 light-years and was released in the thirteenth year of the Hubble Space Telescope's cosmic voyage of exploration.
Lunation (2004-08-29)

Our Moon's appearance changes nightly.  This time-lapse sequence shows what our Moon looks like during a lunation, a complete lunar cycle.  As the Moon orbits the Earth, the half illuminated by the Sun first becomes increasingly visible, then decreasingly visible.  The Moon always keeps the same face toward the Earth.  The Moon's apparent size changes slightly, though, and a slight wobble called a libration is discernable as it progresses along its elliptical orbit. During the cycle, sunlight reflects from the Moon at different angles, and so illuminates different features differently.  A full lunation takes about 29.5 days, just under a month (moon-th).

Image credit: Ant�nio Cidad�o
Announcing Comet C/2003 K4 (LINEAR) (2004-08-30)

A comet discovered last year has brightened unexpectedly and now may become visible to the unaided eye within the next month.  Designated Comet C/2003 K4 (LINEAR), the comet was discovered in 2003 May by project LINEAR.  Many reports already place the comet as brighter than magnitude 7, meaning that it can now be seen with binoculars.  Reports also indicate the comet already has a visible tail nearly the length of a full Moon.  Since predicting the future brightness of comets is a very tricky business, there remains the possibility that K4 might never become very bright.  Current predictions, however, estimate the comet may approach fifth magnitude in October.  K4 passes closest to the Sun on October 12 and then closest to the Earth on December 23.  Comet K4 was photographed above from Van Buren, Arkansas, USA on August 17.

Image credit: Mike Holloway
The Dotted Dunes of Mars (2004-08-31)

What causes the black dots on dunes on Mars?  As spring dawns on the Northern Hemisphere of Mars, dunes of sand near the poles begin to defrost.  Thinner regions of ice typically thaw first revealing sand whose darkness soaks in sunlight and accelerates the thaw.  By summer, the spots will have expanded to encompass the entire dunes that will then be completely thawed and dark. The carbon dioxide and water ice actually sublime in the thin atmosphere directly to gas.  Taken in mid-July, the above image shows a field of spotted polar dunes spanning about 3 kilometers near the Martian North Pole.  Meanwhile, in the Southern Hemisphere of Mars, the Earth-sent robot Martian rovers will try to survive through Martian winter, which peaks in mid-September.

Image credit: Malin Space Science Systems,
MOC,
MGS,
JPL, 
NASA
An Inner Neptune for 55 Cancri (2004-09-01)

Is our Solar System unique?  The discovery of a Neptune-mass planet in an sub-Mercury orbit around nearby Sun-like star 55 Cancri, announced yesterday along with the discovery of other similar systems, gives a new indication that planetary systems as complex as our own  Solar System likely exist elsewhere.  The planet, discovered in data from the Hobby-Eberly telescope in Texas, the Lick Observatory in California, and the orbiting Hubble Space Telescope, is one of four planets now known to orbit  55 Cancri -- the others being similar in mass to Jupiter.  The finding involved noting subtle changes in the speed of the star caused by its orbiting planets.  The above drawing depicts what this planet might look like, assuming a mass similar to Neptune, but a composition similar to Earth.  The star 55 Cancri, only 40 light-years distant, is visible with binoculars towards the constellation of Cancer.
The Large Cloud of Magellan (2004-09-02)

Portuguese navigator Fernando de Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two fuzzy cloud-like objects easily visible for southern hemisphere skygazers are known as the Clouds of Magellan. Of course, these star clouds are now understood to be dwarf irregular galaxies, satellites of our larger spiral Milky Way galaxy. The Large Magellanic Cloud (LMC) pictured above is only about 180,000 light-years distant in the constellation Dorado. Spanning about 15,000 light-years or so, it is the most massive of the Milky Way's satellite galaxies and is the site of the closest supernova in modern times.  The prominent red knot on the right is 30 Doradus, or the Tarantula Nebula, a giant star-forming region in the Large Magellanic Cloud.

Image credit: StarryScapes
Hurricane Frances Approaches Florida (2004-09-03)

A major hurricane is heading for Florida.  Hurricane Frances, one of the stronger storm systems of modern times, may cross the eastern coast of Florida sometime tomorrow.  Those in the path of a hurricane should take precautions.  For example, NASA's Kennedy Space Center has completely shut down.  The orbiting GOES-12 satellite took the above image of Hurricane Frances early yesterday. Hurricanes are huge swirling storms with cloud systems typically larger than a state.  Tropical cyclones, called hurricanes in Earth's Western Hemisphere and typhoons in the Eastern Hemisphere, get their immense energy from warm evaporated ocean water.  As this water vapor cools and condenses, it heats the air, lowers pressure and hence causes cooler air to come swooshing in.  Winds can reach over 250 kilometers per hour and become very dangerous.  Much remains unknown about cyclones, including how they are formed and the exact path they will take.
Neutron Mars (2004-09-04)

Looking for water on Mars, researchers using detectors on board the orbiting Mars Odyssey spacecraft have created this false-color global map of energetic neutrons from the otherwise Red Planet. What do neutrons have to do with water? As cosmic rays from interplanetary space penetrate the thin martian atmosphere and reach the surface they interact with elements in the upper layer of soil, scattering neutrons back into space. But if the martian soil contains hydrogen, it seriously absorbs energetic scattered neutrons. Tracking variations in absorption, neutron detectors can map changes in surface hydrogen content from orbit. Hydrogen content is taken as a surrogate measure of frozen water (H20), the most likely form of hydrogen close to the martian surface. Thus, bluer shades in the above map correspond to larger presumed concentrations of near-surface water ice. Water ice at the martian poles came as no surprise, but significant concentrations also seem to be present at lower latitudes. The melting of such near-surface ice could be responsible for the formation of martian gullies.
M51: The Whirlpool Galaxy in Dust and Stars (2004-09-05)

The Whirlpool Galaxy is a classic spiral galaxy.  At only 30 million light years distant and fully 60 thousand light years across, M51, also known as  NGC 5194, is one of the brightest and most picturesque galaxies on the sky.  The above image is a digital combination of a ground-based image from the 0.9-meter telescope at Kitt Peak National Observatory and a space-based image from the Hubble Space Telescope highlighting sharp features normally too red to be seen. Anyone with a good pair of binoculars, however, can see this Whirlpool toward the constellation of Canes Venatici.  M51 is a spiral galaxy of type Sc and is the dominant member of a whole group of galaxies.  Astronomers speculate that M51's spiral structure is primarily due to its gravitational interaction with a smaller galaxy just off the top of this digitally sharpened image.
C153 Takes the Plunge (2004-09-06)

A comet-like tail of glowing gas, 200,000 light-years long, streams from galaxy C153 as it plunges through galaxy cluster Abell 2125 at nearly 8 million kilometers per hour. Itself a member of the giant cluster of galaxies, C153 may once have been a spiral galaxy like the Milky Way. But this remarkable series of images, false-color composites of x-ray and optical data, zooms in on the galaxy's fate. A headlong passage through the hot intracluster gas in the central regions of Abell 2125 is seen to be stripping C153 of its own star forming material and distorting its shape. As other galaxies in the cluster suffer a similar fate, the hot gas collecting in the cluster's core should become enriched in heavy elements. The violent spectacle was taking place about 3 billion light-years from Earth and is thought to illustrate a common process in the cosmic evolution of large clusters of galaxies.
A Supernova in Nearby Galaxy NGC 2403 (2004-09-07)

The closest and brightest supernova in over a decade was recorded just over a month ago in the outskirts of nearby galaxy NGC 2403.  Officially tagged SN 2004dj, the Type IIP explosion likely annihilated most of a blue supergiant star as central fusion could no longer hold it up.  The supernova can be seen as the bright object in the above image in the direction of the arrow.  The home galaxy to the supernova, spiral galaxy NGC 2403, is located only 11 million light years away and is visible with binoculars toward the northern constellation of Camelopardalis (the Giraffe).  The supernova is fading but still visible with a telescope, once peaking at just brighter than magnitude 12.  Supernovas of this type change brightness in a predictable way and may be searched for in the distant universe as distance indicators.
Molecular Torus Surrounds Black Hole (2004-09-08)

Why do some black hole surroundings appear brighter than others?  In the centers of active galaxies, supermassive black holes at least thousands of times the mass of our Sun dominate.  Many, called Seyfert Type I, are very bright in visible light.  Others, called Seyfert Type II, are rather dim.  The difference might be caused by some black holes accreting much more matter than others.  Alternatively, the black holes in the center of Seyfert Type II galaxies might be obscured by a surrounding torus.  To help choose between these competing hypotheses, the nearby Seyfert II galaxy NGC 4388 has been observed in X-ray light recently by many recent Earth-orbiting X-ray observatories, including CGRO, SIGMA, BeppoSAX, INTEGRAL, Chandra, and XMM-Newton.  Recent data from INTEGRAL and XMM-Newton have found that the X-ray flux in some X-ray colors varies rapidly, while flux in other X-ray colors is quite steady.  The constant flux and apparent absorption of very specific X-ray colors by cool iron together give evidence that the central black hole in NGC 4388 is seen through a thick torus composed of molecular gas and dust.
Sagittarius Triplet (2004-09-09)

These three bright nebulae are often featured in telescopic tours of the constellation Sagittarius and the view toward the center of our Milky Way galaxy. In fact, 18th century cosmic tourist Charles Messier cataloged two of them; M8, the nebula above and left of center, and colorful M20 at the lower left. The third, NGC 6559, is at the right of M8, separated from the the larger nebula by a dark dust lane. All three are stellar nurseries about five thousand light-years or so distant. The expansive M8, over a hundred light-years across, is also known as the Lagoon Nebula while M20's popular moniker is the Trifid. In this gorgeous digital composition, the dominant red color of the emission nebulae is due to glowing hydrogen gas energized by the radiation of hot, young stars. The contrasting blue hues, most striking in the Trifid as well as NGC 6559, are due to dust reflected starlight.

Image credit: Robert Gendler
Cat's Eye (2004-09-10)

Staring across interstellar space, the alluring Cat's Eye nebula lies three thousand light-years from Earth. A classic planetary nebula, the Cat's Eye (NGC 6543) represents a final, brief yet glorious phase in the life of a sun-like star. This nebula's dying central star may have produced the simple, outer pattern of dusty concentric shells by shrugging off outer layers in a series of regular convulsions. But the formation of the beautiful, more complex inner structures is not well understood. Seen so clearly in this sharp Hubble Space Telescope image, the truly cosmic eye is over half a light-year across.  Of course, gazing into the Cat's Eye, astronomers may well be seeing the fate of our sun, destined to enter its own planetary nebula phase of evolution ... in about 5 billion years.
The Star Trails of Kilimanjaro (2004-09-11)

The night had no moon, but the stars were out. And camped at 16,000 feet on Mt. Kilimanjaro, photographer Dan Heller recorded this marvelous 3 1/2 hour long exposure. Here the landscape is lit mostly by the stars. Flashlights give the tents an erie internal radiance while the greenish glow from the distant city lights of Moshi, Tanzania filter through the clouds below. The view from this famous equatorial African mountain is toward the south, putting the South Celestial Pole close to the horizon on the far left, near the center of the graceful concentric star trail arcs. In the thin air and clear dark skies, even the ghostly Milky Way left a faint triangular glow as it swept across the middle of the dreamlike scene.

Image credit: Dan Heller
Mercury: A Cratered Inferno (2004-09-12)

Mercury's surface looks similar to our Moon's.  Each is heavily cratered and made of rock.  Mercury's diameter is about 4800 km, while the Moon's is slightly less at about 3500 km (compared with about 12,700 km for the Earth).  But Mercury is unique in many ways.  Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit.  As Mercury slowly rotates, its surface temperature varies from an unbearably cold -180 degrees Celsius to an unbearably hot 400 degrees Celsius.  The place nearest the Sun in Mercury's orbit changes slightly each orbit - a fact used by Albert Einstein to help verify the correctness of his then newly discovered theory of gravity: General Relativity.   The above picture was taken by the only spacecraft ever to pass Mercury: Mariner 10 in 1974. A new mission, Messenger, launched for Mercury last month and is scheduled to enter orbit around the Solar System's innermost planet in in 2011.
Identify this Phenomenon (2004-09-13)

What caused this ring of colors?  At the time of this writing, MIT Physics Professor Walter Lewin had yet to find someone who can give the correct explanation.  Not students.  Not colleagues.  Not APOD editors.  He wonders how the astute readers of APOD will do.  Can you match wits with Professor Lewin?  Lewin took the above picture in a construction area in Massachusetts on June 20.  Your answer should be able to explain the color sequence and the bright area in the center.  Shortly after he gives the explanation on December 7 during a lecture in his course Vibrations and Waves at MIT, APOD will carry a link to it.  A discussion page for this image will be held in the APOD Forum on the Asterisk*.  Additionally, Professor Lewin will answer appropriate e-mail questions and guesses sent to asklewin@space.mit.edu.
Genesis Mission's Hard Impact (2004-09-14)

A flying saucer from outer space crash-landed in the Utah desert last week after being tracked by radar and chased by helicopters.  No space aliens were involved, however.  The saucer, pictured above, was the Genesis sample return capsule, part of a human-made robot Genesis spaceship launched three years ago by NASA itself to study the Sun.  The unexpectedly hard landing at over 300 kilometers per hour occurred because the parachutes did not open as planned.  The Genesis mission had been orbiting the Sun collecting solar wind particles that are usually deflected away by Earth's magnetic field.  A big question remains -- are the returned samples in good enough condition to recover information about the real composition of the Sun?  Genesis team scientists and engineers are working hard to find out.
Above the Eye of Hurricane Ivan (2004-09-15)

Ninety percent of the houses on Grenada were damaged.  Such is the destructive force of Hurricane Ivan, already one of the most powerful and destructive hurricanes on record.  And the storm will likely make landfall in southern USA tomorrow.  Ivan is the currently the third - and largest - hurricane set to strike the US this hurricane season.  The swirling eye of Hurricane Ivan was photographed above from the orbiting International Space Station (ISS) on Saturday as the storm's sustained 200 kilometer per hour winds wreaked havoc in the Caribbean.  The bad news is that hurricane season in the Atlantic typically lasts until November 30, still over two months away.  The more immediate bad news is that tropical storm Jeanne is next in line coming across the mid-Atlantic Ocean and could pass Puerto Rico sometime today.
Microquasar in Motion (2004-09-16)

Microquasars, bizarre binary star systems generating high-energy radiation and blasting out jets of particles at nearly the speed of light, live in our Milky Way galaxy. The energetic microquasar systems seem to consist of a very compact object, either a neutron star or a black hole, formed in a supernova explosion but still co-orbiting with an otherwise normal star. Using a very long array of radio telescopes, astronomers are reporting that at least one microquasar, LSI +61 303, can be traced back to its probable birthplace -- within a cluster of young stars in the constellation Cassiopeia. About 7,500 light-years from Earth, the star cluster and surrounding nebulosity, IC 1805, are shown in the deep sky image above. The cluster stars are identified by yellow boxes and circles. A yellow arrow indicates the common apparent motion of the cluster stars, the green arrow shows the deduced sky motion of the microquasar system, and the red arrow depicts the microquasar's motion relative to the star cluster itself. Seen nearly 130 light-years from the cluster it once called home, a powerful kick from the original supernova explosion likely set this microquasar in motion.
IC 1805: Light from the Heart (2004-09-17)

Sprawling across hundreds of light-years, emission nebula IC 1805 is a mix of glowing interstellar gas and dark dust clouds. Only about 7,500 light-years away, stars were born in this region whose nickname - the Heart Nebula - derives from its suggestive shape (seen here sideways). This gorgeous, deep telescopic image of the nebula is very colorful, but if you could travel there and gaze across these cosmic clouds with your own eyes, are those the colors you would really see? The short answer is no, even though the image was made with light visible to the human eye. Light from this and other glowing gas clouds surrounding hot, young stars comes in very narrow bands of emission characteristic of energized atoms within the clouds. In fact, the nebular glow is often dominated by hydrogen atoms emitting light in only a small fraction of that broad region of the spectrum that we see as the color red. Adopting an artificial color scheme commonly used for narrow band images of emission nebulae, this beautifully detailed view shows the light from sulfur atoms in red hues, with hydrogen in green, and oxygen atoms in blue.

Image credit: Richard Crisp
M55: Globular Star Cluster (2004-09-18)

The fifty-fifth entry in Charles Messier's catalog, M55 is a large and lovely globular cluster of around 100,000 stars. Only 20,000 light-years away in the constellation Sagittarius, M55 appears to earth-bound observers to be nearly 2/3 the size of the full moon. Globular star clusters like M55 roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars known to be much older than stellar groups found in the galactic disk. Astronomers who make detailed studies of globular cluster stars can accurately measure the cluster ages and distances. Their results ultimately constrain the age of the Universe (... it must be older than the stars in it! ), and provide a fundamental rung on the astronomical distance ladder. This stunning three-color image made with astronomical (BVI) filters spans about 100 light-years across the globular cluster M55.
Earth's North Magnetic Pole (2004-09-19)

A magnetic compass does not point toward the true North Pole of the Earth.  Rather, it more closely points toward the North Magnetic Pole of the Earth.  The North Magnetic Pole is currently located in northern Canada.  It wanders in an elliptical path each day, and moves, on the average, more than forty meters northward each day.  Evidence indicates that the North Magnetic Pole has wandered over much of the Earth's surface in the 4.5 billion years since the Earth formed.  The Earth's magnetic field is created by Earth's partially ionized outer core, which rotates more rapidly than the Earth's surface.  Indicated in the above picture is Ellef Ringnes Island, the location of Earth's North Magnetic Pole in 1994.
Seeing Through Saturn's C-Ring (2004-09-20)

Are Saturn's rings transparent?  The Cassini spacecraft that recently entered orbit around Saturn has confirmed that some of Saturn's rings are more transparent than others.  Pictured above, Saturn's main A, B, and C rings can be seen, top to bottom, superposed against the gas giant planet.  Although the B-ring across the top is opaque, Saturn's cloud tops can be clearly seen through the lower C-ring.  The translucent nature of the C-ring likely indicates that it is less densely populated with ring particles than the B-ring.  The above image was taken on July 30 while Cassini was over 7 million kilometers from Saturn.