juliensimon/nasa-apod huggingface dataset:
Mammatus Clouds over Saskatchewan (2021-08-11)

When do cloud bottoms appear like bubbles? Normally, cloud bottoms are flat. This is because moist warm air that rises and cools will condense into water droplets at a specific temperature, which usually corresponds to a very specific height.  As water droplets grow, an opaque cloud forms.  Under some conditions, however, cloud pockets can develop that contain large droplets of water or ice that fall into clear air as they evaporate.  Such pockets may occur in turbulent air near a thunderstorm.  Resulting mammatus clouds can appear especially dramatic if sunlit from the side.  The mammatus clouds pictured here, lasting only a few minutes, were photographed over Regina, Saskatchewan, Canada, just after a storm in 2012.    Meteor Shower Tonight: Peak of the Perseids

Image credit: Michael F Johnston
A Beautiful Trifid (2021-08-12)

The beautiful Trifid Nebula is a cosmic study in contrasts. Also known as M20, it lies about 5,000 light-years away toward the nebula rich constellation Sagittarius. A star forming region in the plane of our galaxy, the Trifid does illustrate three different types of astronomical nebulae; red emission nebulae dominated by light from hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark nebulae where dense dust clouds appear in silhouette. But the red emission region roughly separated into three parts by obscuring dust lanes is what lends the Trifid its popular name. Pillars and jets sculpted by newborn stars, below and left of the emission nebula's center, appear in famous Hubble Space Telescope close-up images of the region. The Trifid Nebula is about 40 light-years across. Just too faint to be seen by the unaided eye, it almost covers the area of a full moon in planet Earth's sky.

Image credit: Mike Selby
A Perfect Spiral (2021-08-13)

If not perfect then this spiral galaxy is at least one of the most photogenic. An island universe of about 100 billion stars, 32 million light-years away toward the constellation Pisces, M74 presents a gorgeous face-on view. Classified as an Sc galaxy, the grand design of M74's graceful spiral arms are traced by bright blue star clusters and dark cosmic dust lanes. This sharp composite was constructed from image data recorded by the Hubble Space Telescope's Advanced Camera for Surveys. Spanning about 30,000 light-years across the face of M74, it includes exposures recording emission from hydrogen atoms, highlighting the reddish glow of the galaxy's large star-forming regions. With a lower surface brightness than most galaxies in the Messier catalog, M74 is sometimes known as the Phantom Galaxy.   Notable APOD Image Submissions: Perseid Meteor Shower 2021
Island Universe, Cosmic Sand (2021-08-14)

Stars in our own Milky Way Galaxy are scattered through this eye-catching field of view. From the early hours after midnight on August 13, the 30 second exposure of the night sky over Busko-Zdroj, Poland records the colorful and bright trail of a Perseid meteor. Seen near the peak of the annual Perseid meteor shower it flashes from lower left to upper right. The hurtling grain of cosmic sand, a piece of dust from periodic comet Swift-Tuttle, vaporized as it passed through planet Earth's atmosphere at almost 60 kilometers per second. Just above and right of center, well beyond the stars of the Milky Way, lies the island universe known as M31 or the Andromeda Galaxy. The Andromeda Galaxy is the most distant object easily visible to the naked-eye, about 2.5 million light-years away. The visible meteor trail begins only about 100 kilometers above Earth's surface, though. It points back to the meteor shower radiant in the constellation Perseus off the lower left edge of the frame. Follow this bright perseid meteor trail below and left to the stars of NGC 869and NGC 884, the double star cluster in Perseus.   Notable APOD Image Submissions: Perseid Meteor Shower 2021

Image credit: Marzena Rogozinska
Perseid Rain (2021-08-15)

Comet dust rained down on planet Earth last week, streaking through dark skies in the annual Perseid meteor shower. The featured picture is a composite of many images taken from the same location over the peak night of the Perseids. The umbrella was not needed as a shield from meteors, since they almost entirely evaporate high in the Earth's atmosphere. Many of the component images featured individual Perseids, while one image featured the foreground near Jiuquan City, Gansu Province, China. The stellar background includes the central band of our Milky Way Galaxy, appearing nearly vertical, as well as the planets Jupiter and Saturn on the left. Although the comet dust particles are traveling parallel to each other, the resulting shower meteors clearly seem to radiate from a single point on the sky -- the radiant in the eponymous constellation Perseus. The image captured so long an angular field that the curvature of the sky is visible in the trajectory of the Perseids.   Notable APOD Image Submissions: Perseid Meteor Shower 2021

Image credit: Luo Hongyang
Perseid Meteor, Red Sprites, and Nova RS Ophiuchus (2021-08-16)

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.   Notable APOD Image Submissions: Perseid Meteor Shower 2021

Image credit: Daniel Korona
M57: The Ring Nebula from Hubble (2021-08-17)

cept for the rings of Saturn, the Ring Nebula (M57) is probably the most famous celestial circle. Its classic appearance is understood to be due to our own perspective, though. The recent mapping of the expanding nebula's 3-D structure, based in part on this clear Hubble image,indicates that the nebula is a relatively dense, donut-like ring wrapped around the middle of a (American) football-shaped cloud of glowing gas. The view from planet Earth looks down the long axis of the football, face-on to the ring. Of course, in this well-studied example of a planetary nebula, the glowing material does not come from planets. Instead, the gaseous shroud represents outer layers expelled from the dying, once sun-like star, now a tiny pinprick of light seen at the nebula's center. Intense ultraviolet light from the hot central star ionizes atoms in the gas. The Ring Nebula is about one light-year across and 2,500 light-years away.   Share the Sky: NASA Open API for APOD
Rings Around the Ring Nebula (2021-08-18)

The Ring Nebula (M57), is more complicated than it appears through a small telescope.  The easily visible central ring is about one light-year across, but this remarkably deep exposure - a collaborative effort combining data from three different large telescopes - explores the looping filaments of glowing gas extending much farther from the nebula's central star. This composite image includes red light emitted by hydrogen as well as visible and infrared light. The Ring Nebula is an elongated planetary nebula, a type of nebula created when a Sun-like star evolves to throw off its outer atmosphere to become a white dwarf star.  The Ring Nebula is about 2,500 light-years away toward the musical constellation Lyra.   Amateur Astronomers: Please take the Night Sky Network's Survey

Image credit: Robert Gendler
Bright Meteor, Starry Sky (2021-08-19)

Plowing through Earth's atmosphere at 60 kilometers per second, this bright perseid meteor streaks along a starry Milky Way. Captured in dark Portugal skies on August 12, it moves right to left through the frame. Its colorful trail starts near Deneb (alpha Cygni) and ends near Altair (alpha Aquilae), stars of the northern summer triangle. In fact this perseid meteor very briefly outshines both, two of the brightest stars in planet Earth's night. The trail's initial greenish glow is typical of the bright perseid shower meteors. The grains of cosmic sand, swept up dust from periodic comet Swift-Tuttle, are moving fast enough to excite the characteristic green emission of atomic oxygen at altitudes of 100 kilometers or so before vaporizing in an incandescent flash.   Notable APOD Image Submissions: Perseid Meteor Shower 2021

Image credit: Miguel Claro
Three Perseid Nights (2021-08-20)

Frames from a camera that spent three moonless nights under the stars create this composite night skyscape. They were recorded during August 11-13 while planet Earth was sweeping through the dusty trail of comet Swift-Tuttle. One long exposure, untracked for the foreground, and the many star tracking captures of Perseid shower meteors were taken from the village of Magyaregres, Hungary. Each aligned against the background stars, the meteor trails all point back to the annual shower's radiant in the constellation Perseus heroically standing above this rural horizon. Of course the comet dust particles are traveling along trajectories parallel to each other. The radiant effect is due only to perspective, as the parallel tracks appear to converge in the distance against the starry sky.   Notable APOD Image Submissions: Perseid Meteor Shower 2021

Image credit: Balint Lengyel
Triple Transit and Mutual Events (2021-08-21)

These three panels feature the Solar System's ruling gas giant Jupiter on August 15 as seen from Cebu City, Philippines, planet Earth. On that date the well-timed telescopic views detail some remarkable performances, transits and mutual events, by Jupiter's Galilean moons. In the top panel, Io is just disappearing into Jupiter's shadow at the far right, but the three other large Jovian moons appear against the planet's banded disk. Brighter Europa and darker Ganymede are at the far left, also casting their two shadows on the gas giant's cloud tops. Callisto is below and right near the planet's edge, the three moons in a triple transit across the face of Jupiter. Moving to the middle panel, shadows of Europa and Ganymede are still visible near center but Ganymede has occulted or passed in front of Europa. The bottom panel captures a rare view of Jovian moons in eclipse while transiting Jupiter, Ganymede's shadow falling on Europa itself. From planet Earth's perspective, similar mutual events, when Galilean moons occult and eclipse each other, can be seen every six years or so when Jupiter is near its own equinox.

Image credit: Christopher Go
Explosions from White Dwarf Star RS Oph (2021-08-22)

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.

Image credit: David A. Hardy & 
PPARC
Abell 3827: Cannibal Cluster Gravitational Lens (2021-08-23)

Is that one galaxy or three? Toward the right of the featured Hubble image of the massive galaxy cluster Abell 3827 is what appears to be a most unusual galaxy -- curved and with three centers.  A detailed analysis, however, finds that these are three images of the same background galaxy -- and that there are at least four more images.  Light we see from the single background blue galaxy takes multiple paths through the complex gravity of the cluster, just like a single distant light can take multiple paths through the stem of a wine glass. Studying how clusters like Abell 3827 and their component galaxies deflect distant light gives information about how mass and dark matter are distributed. Abell 3827 is so distant, having a redshift of 0.1, that the light we see from it left about 1.3 billion years ago -- before dinosaurs roamed the Earth. Therefore, the cluster's central galaxies have now surely all coalesced -- in a feast of galactic cannibalism -- into one huge galaxy near the cluster's center.
PDS 70: Disk, Planets, and Moons (2021-08-24)

It's not the big disk that's attracting the most attention. Although the big planet-forming disk around the star PDS 70 is clearly imaged and itself quite interesting. It's also not the planet on the right, just inside the big disk, that’s being talked about the most.  Although the planet PDS 70c is a newly formed and, interestingly, similar in size and mass to Jupiter. It's the fuzzy patch around the planet PDS 70c that's causing the commotion. That fuzzy patch is thought to be itself a dusty disk that is now forming into moons -- and that has never been seen before. The featured image was taken by the Atacama Large Millimeter Array (ALMA) of 66 radio telescopes in the high Atacama Desert of northern Chile.  Based on ALMA data, astronomers infer that the moon-forming exoplanetary disk has a radius similar to our Earth's orbit, and may one day form three or so Luna-sized moons -- not very different from our Jupiter's four.
Solar System Ball Drop (2021-08-25)

Does a ball drop faster on Earth, Jupiter, or Uranus?  The featured animation shows a ball dropping from one kilometer high toward the surfaces of famous solar system bodies, assuming no air resistance.  The force of gravity depends on the mass of the attracting object, with higher masses pulling down with greater forces. But gravitational force also depends on distance from the center of gravity, with shorter distances causing the ball to drop faster.  Combining both mass and distance, it might be surprising to see that Uranus pulls the ball down slightly slower than Earth, despite containing over 14 times more mass. This happens because Uranus has a much lower density, which puts its cloud tops further away from its center of mass.  Although the falling ball always speeds up, if you were on the ball you would not feel this acceleration because you would be in free-fall. Of the three planets mentioned, the video demonstrates a ball drops even faster on Jupiter than either Earth and Uranus.

Image credit: James O'Donoghue 
(JAXA) & 
Rami Mandow 
(Space Australia);
Text: James O'Donoghue
A Blue Hour Full Moon (2021-08-26)

Nature photographers and other fans of planet Earth always look forward to the blue hour. That's the transition in twilight, just before sunrise or after sunset, when the Sun is below the horizon but land and sky are still suffused with a beautiful blue light. After sunset on August 21, this blue hour snapshot captured the nearly full Moon as it rose opposite the Sun, above the rugged Italian Alps from Cortina d'Ampezzo, Italy. Sharing bluish hues with the sky, the rocky pyramid of Monte Antelao, also known as the King of the Dolomites, is the region's prominent alpine peak. The moonlight is yellow, but even so this full Moon was known to some as a seasonal Blue Moon. That's because by one definition the third full Moon of a season with four full moons in it is called a Blue Moon. Recognizing a season as the time between a solstice and an equinox, this season's fourth full Moon will be rising in the blue hour of September 20, just before September's equinox.

Image credit: Giorgia Hofer
Elephant's Trunk and Caravan (2021-08-27)

Like an illustration in a galactic Just So Story, the Elephant's Trunk Nebula winds through the emission nebula and young star cluster complex IC 1396, in the high and far off constellation of Cepheus. Also known as vdB 142, seen on the left the cosmic elephant's trunk is over 20 light-years long. Removed by digital processing, no visible stars are in this detailed telescopic close-up view highlighting the bright swept-back ridges that outline pockets of cool interstellar dust and gas. But the dark, tendril-shaped clouds contain the raw material for star formation and hide protostars within. Nearly 3,000 light-years distant, the relatively faint IC 1396 complex covers a large region on the sky, spanning over 5 degrees. This starless rendition spans a 1 degree wide field of view though, about the angular size of 2 full moons. Of course the dark shapes below and right, marching toward the winding Elephant's Trunk, are known to some as The Caravan.

Image credit: Robert Eder
Mars Rock Rochette (2021-08-28)

Taken on mission sol 180 (August 22) this sharp image from a Hazard Camera on the Perseverance rover looks out across a rock strewn floor of Jezero crater on Mars. At 52.5 centimeters (21 inches) in diameter, one of the rover's steerable front wheels is at lower left in the frame. Near center is a large rock nicknamed Rochette. Mission planners don't want to avoid Rochette though. Instead Perseverance will be instructed to reach out with its 2 meter long robotic arm and abrade the rock's surface, to determine whether it has a consistency suitable for obtaining a sample, slightly thicker than a pencil, using the rover's coring bit. Samples collected by Perseverance would be returned to Earth by a future Mars mission.
Orbits of Potentially Hazardous Asteroids (2021-08-29)

Are asteroids dangerous? Some are, but the likelihood of a dangerous asteroid striking the Earth during any given year is low. Because some past mass extinction events have been linked to asteroid impacts, however, humanity has made it a priority to find and catalog those asteroids that may one day affect life on Earth. Pictured here are the orbits of the over 1,000 known Potentially Hazardous Asteroids (PHAs). These documented tumbling boulders of rock and ice are over 140 meters across and will pass within 7.5 million kilometers of Earth -- about 20 times the distance to the Moon. Although none of them will strike the Earth in the next 100 years -- not all PHAs have been discovered, and past 100 years, many orbits become hard to predict. Were an asteroid of this size to impact the Earth, it could raise dangerous tsunamis, for example. To investigate Earth-saving strategies, NASA's  Double Asteroid Redirection Test (DART) is planned for launch later this year. Of course rocks and ice bits of much smaller size strike the Earth every day, usually pose no danger, and sometimes creating memorable fireball and meteor displays.
A Fire Rainbow over West Virginia (2021-08-30)

What's happening to this cloud? Ice crystals in a distant cirrus cloud are acting like little floating prisms.  Known informally as a fire rainbow for its flame-like appearance, a circumhorizon arc appears parallel to the horizon. For a circumhorizontal arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds present below --  in this case cirrus fibrates.  The numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight in a collectively similar manner.  Therefore, circumhorizontal arcs are somewhat unusual to see.  The featured fire rainbow was photographed earlier this month near North Fork Mountain in West Virginia, USA.
A Blue Moon in Exaggerated Colors (2021-08-31)

The Moon is normally seen in subtle shades of grey or gold.  But small, measurable color differences have been greatly exaggerated to make this telescopic, multicolored, moonscape captured during the Moon's full phase.  The different colors are recognized to correspond to real differences in the chemical makeup of the lunar surface.  Blue hues reveal titanium rich areas while orange and purple colors show regions relatively poor in titanium and iron.  The familiar Sea of Tranquility, or Mare Tranquillitatis, is the blue area toward the upper right.  White lines radiate across the orange-hued southern lunar highlands from 85-kilometer wide ray-crater Tycho at bottom right.  The full moon that occurred earlier this month could be counted as a seasonal blue moon because it was, unusually, the third of four full moons to occur during northern summer (and hence southern winter).  The featured 272-image composite demonstrates that the full Moon is always blue, but usually not blue enough in hue to ooh.   Almost Hyperspace: Random APOD Generator

Image credit: Robert Fedez
Dancing Ghosts: Curved Jets from Active Galaxies (2021-09-01)

Why would galaxies emit jets that look like ghosts?  And furthermore, why do they appear to be dancing?  The curled and fluffy jets from the supermassive black holes at the centers of two host galaxies (top center and lower left) are unlike anything seen before.  They were found by astronomers using the Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope when creating maps tracing the evolution of galaxies.  Images preceding this Evolutionary Map of the Universe survey only showed amorphous blobs.  Eventually, comparisons of relative amounts of energy emitted revealed the glowing elongated structures were created by electrons streaming around magnetic field lines.  Overlaying the radio data on an optical view of the sky (Dark Energy Survey) confirmed that the electron streams originated from the centers of active galaxies. Usually such Active Galactic Nuclei (AGN) produce straight jets.  A leading hypothesis for the geometric origin of these unusually graceful shapes involves the flow of large-scale intergalactic winds.    Astrophysicists: Browse 2,500+ codes in the Astrophysics Source Code Library
M51: The Whirlpool Galaxy (2021-09-02)

Find the Big Dipper and follow the handle away from the dipper's bowl until you get to the last bright star. Then, just slide your telescope a little south and west and you'll come upon this stunning pair of interacting galaxies, the 51st entry in Charles Messier's famous catalog. Perhaps the original spiral nebula, the large galaxy with well defined spiral structure is also cataloged as NGC 5194. Its spiral arms and dust lanes clearly sweep in front of its companion galaxy (top), NGC 5195. The pair are about 31 million light-years distant and officially lie within the angular boundaries of the small constellation Canes Venatici. Though M51 looks faint and fuzzy to the eye, deep images like this one reveal its striking colors and galactic tidal debris.

Image credit: Josep Drudis
NGC 7023: The Iris Nebula (2021-09-03)

These cosmic clouds have blossomed 1,300 light-years away, in the fertile starfields of the constellation Cepheus. Called the Iris Nebula, NGC 7023 is not the only nebula to evoke the imagery of flowers. Still, this deep telescopic image shows off the Iris Nebula's range of colors and symmetries, embedded in surrounding fields of interstellar dust. Within the Iris itself, dusty nebular material surrounds a hot, young star. The dominant color of the brighter reflection nebula is blue, characteristic of dust grains reflecting starlight. Central filaments of the reflection nebula glow with a faint reddish photoluminesence as some dust grains effectively convert the star's invisible ultraviolet radiation to visible red light. Infrared observations indicate that this nebula contains complex carbon molecules known as PAHs. The dusty blue petals of the Iris Nebula span about six light-years.

Image credit: Satwant Kumar