juliensimon/nasa-apod huggingface dataset:
The Andromeda Galaxy in Ultraviolet (2021-07-18)

What does the Andromeda galaxy look like in ultraviolet light? Young blue stars circling the galactic center dominate.  A mere 2.5 million light-years away, the Andromeda Galaxy, also known as M31, really is just next door as large galaxies go. Spanning about 230,000 light-years, it took 11 different image fields from NASA's Galaxy Evolution Explorer (GALEX) satellite telescope to produce this gorgeous portrait of the spiral galaxy in ultraviolet light in 2003. While its spiral arms stand out in visible light images, Andromeda's arms look more like rings in ultraviolet. The rings are sites of intense star formation and have been interpreted as evidence that Andromeda collided with its smaller neighboring elliptical galaxy M32 more than 200 million years ago. The Andromeda galaxy and our own comparable Milky Way galaxy are the most massive members of the Local Group of galaxies and are projected to collide in several billion years -- perhaps around the time that our Sun's atmosphere will expand to engulf the Earth.
Framed by Trees: A Window to the Galaxy (2021-07-19)

The photographer had this shot in mind for some time. He knew that objects overhead are the brightest -- since their light is scattered the least by atmospheric air. He also that knew the core of our Milky Way Galaxy was just about straight up near midnight around this time of year in South Australia.  Chasing his mental picture, he ventured deep inside the Kuipto Forest where tall radiata pines blocked out much of the sky -- but not in this clearing.  There, through a window framed by trees, he captured his envisioned combination of local and distant nature. Sixteen exposures of both trees and the Milky Way Galaxy were recorded. Antares is the bright orange star to left of our Galaxy's central plane, while Alpha Centauri is the bright star just to the right of the image center.  The direction toward our Galaxy's center is below Antares.  Although in a few hours the Earth's rotation moved the Galactic plane up and to the left -- soon invisible behind the timber, his mental image was secured forever -- and is featured here.    Follow APOD on Instagram in: English, Farsi, Indonesian, Persian, Portuguese or Taiwanese

Image credit: Will Godward
Thor's Helmet (2021-07-20)

Thor not only has his own day (Thursday), but a helmet in the heavens.  Popularly called Thor's Helmet, NGC 2359 is a hat-shaped cosmic cloud with wing-like appendages. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. In fact, the cosmic head-covering is more like an interstellar bubble, blown with a fast wind from the bright, massive star near the bubble's center. Known as a Wolf-Rayet star, the central star is an extremely hot giant thought to be in a brief, pre-supernova stage of evolution. NGC 2359 is located about 15,000 light-years away toward the constellation of the Great Overdog. This remarkably sharp image is a mixed cocktail of data from broadband and narrowband filters, capturing not only natural looking stars but details of the nebula's filamentary structures. The star in the center of Thor's Helmet is expected to explode in a spectacular supernova sometime within the next few thousand years.    Almost Hyperspace: Random APOD Generator

Image credit: Bernard Miller
Colors: Ring Nebula versus Stars (2021-07-21)

What if you could see, separately, all the colors of the Ring? And of the surrounding stars? There's technology for that. The featured image shows the Ring Nebula (M57) and nearby stars through such technology: in this case, a prism-like diffraction grating.  The Ring Nebula is seen only a few times because it emits light, primarily, in only a few colors. The two brightest emitted colors are hydrogen (red) and oxygen (blue), appearing as nearly overlapping images to the left of the image center.  The image just to the right of center is the color-combined icon normally seen.  Stars, on the other hand, emit most of their light in colors all across the visible spectrum.  These colors, combined, make a nearly continuous streak -- which is why stars appear accompanied by multicolored bars.  Breaking object light up into colors is scientifically useful because it can reveal the elements that compose that object, how fast that object is moving, and how distant that object is.
NGC 7814: Little Sombrero with Supernova (2021-07-22)

Point your telescope toward the high flying constellation Pegasus and you can find this expanse of Milky Way stars and distant galaxies. NGC 7814 is centered in the pretty field of view that would almost be covered by a full moon. NGC 7814 is sometimes called the Little Sombrero for its resemblance to the brighter more famous M104, the Sombrero Galaxy. Both Sombrero and Little Sombrero are spiral galaxies seen edge-on, and both have extensive halos and central bulges cut by a thin disk with thinner dust lanes in silhouette. In fact, NGC 7814 is some 40 million light-years away and an estimated 60,000 light-years across. That actually makes the Little Sombrero about the same physical size as its better known namesake, appearing smaller and fainter only because it is farther away. In this telescopic view from July 17, NGC 7814 is hosting a newly discovered supernova, dominant immediately to the left of the galaxy's core. Cataloged as SN 2021rhu, the stellar explosion has been identified as a Type Ia supernova, useful toward calibrating the distance scale of the universe.

Image credit: CHART32 Team
Elephant, Bat, and Squid (2021-07-23)

Sprawling emission nebulae IC 1396 and Sh2-129 mix glowing interstellar gas and dark dust clouds in this 10 degree wide field of view toward the northern constellation Cepheus the King. Energized by its bluish central star IC 1396 (left) is hundreds of light-years across and some 3,000 light-years distant. The nebula's intriguing dark shapes include a winding dark cloud popularly known as the Elephant's Trunk below and right of center. Tens of light-years long, it holds the raw raw material for star formation and is known to hide protostars within. Located a similar distance from planet Earth, the bright knots and swept back ridges of emission of Sh2-129 on the right suggest its popular name, the Flying Bat Nebula. Within the Flying Bat, the most recently recognized addition to this royal cosmic zoo is the faint bluish emission from Ou4, the Giant Squid nebula.

Image credit: Patrick Hsieh
The Edge of Space (2021-07-24)

Where does space begin? For purposes of spaceflight some would say at the Karman line, currently defined as an altitude of 100 kilometers (60 miles). Others might place a line 80 kilometers (50 miles) above Earth's mean sea level. But there is no sharp physical boundary that marks the end of atmosphere and the beginning of space. In fact, the Karman line itself is near the transition between the upper mesosphere and lower thermosphere. Night shining or noctilucent clouds are high-latitude summer apparitions formed at altitudes near the top of the mesosphere, up to 80 kilometers or so, also known as polar mesospheric clouds. Auroral bands of the northern (and southern) lights caused by energetic particles exciting atoms in the thermosphere can extend above 80 kilometers to over 600 kilometers altitude. Taken from a cockpit while flying at an altitude of 10 kilometers (33,000 feet) in the realm of stratospheric aeronautics, this snapshot captures both noctilucent clouds and aurora borealis under a starry sky, looking toward planet Earth's horizon and the edge of space.

Image credit: Ralf Rohner
Crescent Neptune and Triton (2021-07-25)

Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase.  The elegant picture of the ice-giant planet and its cloudy moon was taken from behind just after closest approach in 1989.  It could not have been taken from Earth because Neptune never shows a crescent phase to sunward Earth. The unusual vantage point also robs Neptune of its familiar blue hue, as sunlight seen from here is scattered forward, and so is reddened like the setting Sun.  Neptune is smaller but more massive than Uranus, has several dark rings, and emits more light than it receives from the Sun.
CG4: A Ruptured Cometary Globule (2021-07-26)

Can a gas cloud grab a galaxy?  It's not even close.  The "claw" of this odd looking "creature" in the featured photo is a gas cloud known as a cometary globule.  This globule, however, has ruptured.  Cometary globules are typically characterized by dusty heads and elongated tails.  These features cause cometary globules to have visual similarities to comets, but in reality they are very much different.  Globules are frequently the birthplaces of stars, and many show very young stars in their heads. The reason for the rupture in the head of this object is not yet known. The galaxy to the left of the globule is huge, very far in the distance, and only placed near CG4 by chance superposition.

Image credit: Nicolas Rolland & 
Martin Pugh
Fleming's Triangular Wisp (2021-07-27)

Chaotic in appearance, these tangled filaments of shocked, glowing gas are spread across planet Earth's sky toward the constellation of Cygnus as part of the Veil Nebula. The Veil Nebula itself is a large supernova remnant, an expanding cloud born of the death explosion of a massive star. Light from the original supernova explosion likely reached Earth over 5,000 years ago. The glowing filaments are really more like long ripples in a sheet seen almost edge on, remarkably well separated into the glow of ionized hydrogen atoms shown in blue and oxygen in red hues. Also known as the Cygnus Loop and cataloged as NGC 6979, the Veil Nebula now spans about 6 times the diameter of the full Moon. The length of the wisp corresponds to about 30 light years, given its estimated distance of 2,400 light years.  Often identified as Pickering's Triangle for a director of Harvard College Observatory, it is also named for its discoverer, astronomer Williamina Fleming, as Fleming's Triangular Wisp.

Image credit: Anthony Saab
Ring Galaxy AM 0644-741 (2021-07-28)

The rim of the large blue galaxy at the right is an immense ring-like structure 150,000 light years in diameter composed of newly formed, extremely bright, massive stars.  AM 0644-741 is known as a ring galaxy and was caused by an immense galaxy collision.  When galaxies collide, they pass through each other and their individual stars rarely come into contact.  The large galaxy's ring-like shape is the result of the gravitational disruption caused by a small intruder galaxy passing through it. When this happens, interstellar gas and dust become compressed, causing a wave of star formation to move out from the impact point like a ripple across the surface of a pond.  Other galaxies in the field of view are background galaxies, not interacting with AM 0644-741. Foreground spiky stars are within our own Milky Way. But the smaller intruder galaxy is caught above and right, near the top of the frame taken by the Hubble Space Telescope. Ring galaxy AM 0644-741 lies about 300 million light years away toward the southern constellation Volans.
The Tulip and Cygnus X-1 (2021-07-29)

This tall telescopic field of view looks out along the plane of our Milky Way Galaxy toward the nebula rich constellation Cygnus the Swan. Popularly called the Tulip Nebula, the brightest glowing cloud of interstellar gas and dust above center is also found in the 1959 catalog by astronomer Stewart Sharpless as Sh2-101. Nearly 70 light-years across the complex and beautiful Tulip Nebula blossoms about 8,000 light-years away, shown in a Hubble palette image that maps the glow of the nebula's sulfur, hydrogen, and oxygen ions into red, green, and blue colors. Ultraviolet radiation from young energetic stars at the edge of the Cygnus OB3 association, including O star HDE 227018, ionizes the atoms and powers the emission from the Tulip Nebula. Also in the field of view is microquasar Cygnus X-1, one of the strongest X-ray sources in planet Earth's sky. Driven by powerful jets from a black hole accretion disk, its fainter bluish curved shock front is only just visible though, directly above the cosmic Tulip's petals near the top of the frame.

Image credit: Carlos Uriarte
Mimas in Saturnlight (2021-07-30)

Peering from the shadows, the Saturn-facing hemisphere of Mimas lies in near darkness alongside a dramatic sunlit crescent. The mosaic was captured near the Cassini spacecraft's final close approach on January 30, 2017. Cassini's camera was pointed in a nearly sunward direction only 45,000 kilometers from Mimas. The result is one of the highest resolution views of the icy, crater-pocked, 400 kilometer diameter moon. An enhanced version better reveals the Saturn-facing hemisphere of the synchronously rotating moon lit by sunlight reflected from Saturn itself. To see it, slide your cursor over the image (or follow this link). Other Cassini images of Mimas include the small moon's large and ominous Herschel Crater.
Remembering NEOWISE (2021-07-31)

It was just last July. If you could see the stars of the Big Dipper, you could find Comet NEOWISE in your evening sky. After sunset denizens of the north could look for the naked-eye comet below the bowl of that famous celestial kitchen utensil and above the northwestern horizon. The comet looked like a fuzzy 'star' with a tail, though probably not so long a tail as in this memorable skyview recorded from the Czech Republic on July 23th, 2020, near the comet's closest approach to planet Earth. Photographs of C/2020 F3 (NEOWISE) often did show the comet's broad dust tail and fainter but separate bluish ion tail extending farther than the eye could follow. Skygazers around the world were delighted to witness Comet NEOWISE, surprise visitor from the outer Solar System.   Notable Comet NEOWISE Images 2020: July 31 30, 29, 28, 27, 26, 25, 24

Image credit: Petr Horalek
Pluto in Enhanced Color (2021-08-01)

Pluto is more colorful than we can see. Color data and high-resolution images of our Solar System's most famous dwarf planet, taken by the robotic New Horizons spacecraft during its flyby in 2015 July, have been digitally combined to give an enhanced-color view of this ancient world sporting an unexpectedly young surface. The featured enhanced color image is not only esthetically pretty but scientifically useful, making surface regions of differing chemical composition visually distinct. For example, the light-colored heart-shaped Tombaugh Regio on the lower right is clearly shown here to be divisible into two regions that are geologically different, with the leftmost lobe Sputnik Planitia also appearing unusually smooth. After Pluto, New Horizons continued on, shooting  past asteroid Arrokoth in 2019 and has enough speed to escape our Solar System completely.    Pluto-Related Images with Brief Explanations: APOD Pluto Search
The Hubble Ultra Deep Field in Light and Sound (2021-08-02)

Have you heard about the Hubble Ultra-Deep Field?  Either way, you've likely not heard about it like this -- please run your cursor over the featured image and listen!  The Hubble Ultra-Deep Field (HUDF) was created in 2003-2004 with the Hubble Space Telescope staring for a long time toward near-empty space so that distant, faint galaxies would become visible.  One of the most famous images in astronomy, the HUDF is featured here in a vibrant way -- with sonified distances. Pointing to a galaxy will play a note that indicates its approximate redshift. Because redshifts shift light toward the red end of the spectrum of light, they are depicted here by a shift of tone toward the low end of the spectrum of sound.  The further the galaxy, the greater its cosmological redshift (even if it appears blue), and the lower the tone that will be played. The average galaxy in the HUDF is about 10.6 billion light years away and sounds like an F#. What's the most distant galaxy you can find?   Note: Sounds will only play on some browsers.  This week at NASA: Hubble #DeepFieldWeek
A Perseid Fireball and the Milky Way (2021-08-03)

It was bright and green and flashed as it moved quickly along the Milky Way. It left a trail that took 30 minutes to dissipate.  Given the day, August 12, and the direction, away from Perseus, it was likely a small bit from the nucleus of Comet Swift-Tuttle plowing through the Earth's atmosphere -- and therefore part of the annual Perseids meteor shower.  The astrophotographer captured the fireball as it shot across the sky in 2018 above a valley in Yichang, Hubei, China. The meteor's streak, also caught on video, ended near the direction of Mars on the lower left. Next week, the 2021 Perseids meteor shower will peak again.  This year the Moon will set shortly after the Sun, leaving a night sky ideal for seeing lots of Perseids from dark and clear locations across planet Earth.   Follow APOD in English on: Facebook,  Instagram, or Twitter

Image credit: Dandan Huang
EHT Resolves Central Jet from Black Hole in Cen A (2021-08-04)

How do supermassive black holes create powerful jets? To help find out, the Event Horizon Telescope (EHT) imaged the center of the nearby active galaxy Centaurus A.  The cascade of featured inset images shows Cen A from it largest, taking up more sky than many moons, to its now finest, taking up only as much sky as an golf ball on the moon. The new image shows what may look like two jets -- but is actually two sides of a single jet.  This newly discovered jet-edge brightening does not solve the jet-creation mystery, but does imply that the particle outflow is confined by a strong pressure -- possibly involving a magnetic field.  The EHT is a coordination of radio telescopes from around the Earth -- from the Caltech Submillimeter Observatory in Hawaii USA, to ALMA in Chile, to NOEMA in France, and more. The EHT will continue to observe massive, nearby black holes and their energetic surroundings.
Tycho and Clavius (2021-08-05)

South is up in this detailed telescopic view across the Moon's rugged southern highlands. Captured on July 20, the lunar landscape features the Moon's young and old, the large craters Tycho and Clavius. About 100 million years young, Tycho is the sharp-walled 85 kilometer diameter crater near center, its 2 kilometer tall central peak in bright sunlight and dark shadow. Debris ejected during the impact that created Tycho still make it the stand out lunar crater when the Moon is near full, producing a highly visible radiating system of light streaks, bright rays that extend across much of the lunar near side. In fact, some of the material collected at the Apollo 17 landing site, about 2,000 kilometers away, likely originated from the Tycho impact.  One of the oldest and largest craters on the Moon's near side, 225 kilometer diameter Clavius is due south (above) of Tycho. Clavius crater's own ray system resulting from its original impact event would have faded long ago. The old crater's worn walls and smooth floor are now overlayed by smaller craters from impacts that occurred after Clavius was formed. Observations by the Stratospheric Observatory for Infrared Astronomy (SOFIA) published in 2020 found water at Clavius. Of course both young Tycho and old Clavius craters are lunar locations in the science fiction epic 2001: A Space Odyssey.

Image credit: Eduardo Schaberger Poupeau
Stars and Dust Across Corona Australis (2021-08-06)

Cosmic dust clouds cross a rich field of stars in this telescopic vista near the northern boundary of Corona Australis, the Southern Crown. Less than 500 light-years away the dust clouds effectively block light from more distant background stars in the Milky Way. Top to bottom the frame spans about 2 degrees or over 15 light-years at the clouds' estimated distance. At top right is a group of lovely reflection nebulae cataloged as NGC 6726, 6727, 6729, and IC 4812. A characteristic blue color is produced as light from hot stars is reflected by the cosmic dust. The dust also obscures from view stars in the region still in the process of formation. Just above the bluish reflection nebulae a smaller NGC 6729 surrounds young variable star R Coronae Australis. To its right are telltale reddish arcs and loops identified as Herbig Haro objects associated with energetic newborn stars. Magnificent globular star cluster NGC 6723 is at bottom left in the frame. Though NGC 6723 appears to be part of the group, its ancient stars actually lie nearly 30,000 light-years away, far beyond the young stars of the Corona Australis dust clouds.

Image credit: Vikas Chander
Jezero Crater: Raised Ridges in 3D (2021-08-07)

Get out your red-blue glasses and hover over the surface of Mars. Taken on July 24, the 3D color view is from the Mars Ingenuity Helicopter's 10th flight above the Red Planet. Two images from Ingenuity's color camera, both captured at an altitude of 12 meters (40 feet), but a few meters apart to provide a stereo perspective, were used to construct the color anaglyph. Ingenuity's stereo images were made at the request of the Mars Perseverance rover science team. The team is considering a visit to these raised ridges on the floor of Jezero Crater during Perseverance's first science campaign.
A Perseid Below (2021-08-08)

rthlings typically watch meteor showers by looking up. But this remarkable view, captured on August 13, 2011 by astronaut Ron Garan, caught a Perseid meteor by looking down. From Garan's perspective onboard the International Space Station orbiting at an altitude of about 380 kilometers, the Perseid meteors streak below, swept up dust left from comet Swift-Tuttle heated to incandescence. The glowing comet dust grains are traveling at about 60 kilometers per second through the denser atmosphere around 100 kilometers above Earth's surface. In this case, the foreshortened meteor flash is right of frame center, below the curving limb of the Earth and a layer of greenish airglow, just below bright star Arcturus. Want to look up at a meteor shower?  You're in luck, as the 2021 Perseids meteor shower peaks this week. This year, even relatively faint meteors should be visible through clear skies from a dark location as the bright Moon will mostly absent.   Notable Perseids Submissions to APOD: 2018, 2019, 2020
Perseus and the Lost Meteors (2021-08-09)

What's the best way to watch a meteor shower?  This question might come up later this week when the annual Perseid Meteor Shower peaks. One thing that is helpful is a dark sky, as demonstrated in the featured composite image of last year's Perseids.  Many more faint meteors are visible on the left image, taken through a very dark sky in Slovakia, than on the right image, taken through a moderately dark sky in the Czech Republic.  The band of the Milky Way Galaxy bridges the two coordinated images, while the meteor shower radiant in the constellation of Perseus is clearly visible on the left. In sum, many faint meteors are lost through a bright sky. Light pollution is shrinking areas across our Earth with dark skies, although inexpensive ways to combat this might be implemented.    Notable Perseids Submissions to APOD: 2018, 2019, 2020

Image credit: Tomas Slovinsky 
(Slovakia) & 
Petr Horalek 
(Czech Republic;
Institute of Physics in Opava)
Fire in Space (2021-08-10)

What does fire look like in space? In the gravity on Earth, heated air rises and expands, causing flames to be teardrop shaped. In the microgravity of the air-filled International Space Station (ISS), however, flames are spheres. Fire is the rapid acquisition of oxygen, and space flames meet new oxygen molecules when they float by randomly from all directions -- creating the enveloping sphere.  In the featured image taken in the ISS's Combustion Integration Rack, a spherical flame envelopes clusters of hot glowing soot.  Without oxygen, say in the vacuum of empty space, a fire would go out immediately.  The many chemical reactions involved with fire are complex, and testing them in microgravity is helping humanity not only to better understand fire -- but how to put out fire, too.