juliensimon/nasa-apod huggingface dataset:
Anemic Spiral NGC 4921 from Hubble (2019-05-15)

How far away is spiral galaxy NGC 4921? It's surpringly important to know.  Although presently estimated to be about 300 million light years distant, a more precise determination could be coupled with its known recession speed to help humanity better calibrate the expansion rate of the entire visible universe.  Toward this goal, several images were taken by the Hubble Space Telescope in order to help identify key stellar distance markers known as Cepheid variable stars.   Since NGC 4921 is a member of the Coma Cluster of Galaxies, refining its distance would also allow a better distance determination to one of the largest nearby clusters in the local universe. The magnificent spiral NGC 4921 has been informally dubbed anemic because of its low rate of star formation and low surface brightness. Visible in the featured image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy.    APOD in other languages: Arabic, Catalan, Chinese (Beijing), Chinese (Taiwan), Croatian, Czech, Dutch, German, French, French, Hebrew, Indonesian, Japanese, Korean, Montenegrin, Polish, Russian, Serbian, Slovenian,  Spanish and Ukrainian
Dark Skies: Turn on the Night (2019-05-16)

Have you ever experienced a really dark night sky? One common and amazing feature is the glowing band of our Milky Way galaxy stretching from horizon to horizon.  If you live in or near a big city, though, you might not know this because city lights reflecting off the Earth's atmosphere could only allow you to see the Moon and a few stars. Today, however, being UNESCO's International Day of Light, the International Astronomical Union is asking people to Turn on the Night by trying to better understand, and in the future better reduce, light pollution. You can practice even now by going to the main APOD website at NASA and hovering your cursor over the Before image. The After picture that comes up is a panorama of four exposures taken with the same camera and from the same location, showing what happened recently in China when people in Kaihua County decided to turn down many of their lights.  Visible in the Before picture are the stars Sirius (left of center) and Betelgeuse, while visible in the After picture are thousands of stars with the arching band of our Milky Way Galaxy. Humanity has lived for millennia under a dark night sky, and connecting to it has importance for both natural and cultural heritage.

Image credit: Jeff Dai
(TWAN)
RS Puppis (2019-05-17)

Pulsating RS Puppis, the brightest star in the image center, is some ten times more massive than our Sun and on average 15,000 times more luminous. In fact, RS Pup is a Cepheid variable star, a class of stars whose brightness is used to estimate distances to nearby galaxies as one of the first steps in establishing the cosmic distance scale. As RS Pup pulsates over a period of about 40 days, its regular changes in brightness are also seen along its surrounding nebula delayed in time, effectively a light echo. Using measurements of the time delay and angular size of the nebula, the known speed of light allows astronomers to geometrically determine the distance to RS Pup to be 6,500 light-years, with a remarkably small error of plus or minus 90 light-years. An impressive achievement for stellar astronomy, the echo-measured distance also more accurately establishes the true brightness of RS Pup, and by extension other Cepheid stars, improving the knowledge of distances to galaxies beyond the Milky Way.

Image credit: NASA
Atlas, Daphnis, and Pan (2019-05-18)

Atlas, Daphnis, and Pan are small, inner, ring moons of Saturn. They are shown at the same scale in this montage of images by the Cassini spacecraft that made its grand final orbit of the ringed planet in September 2017. In fact, Daphnis was discovered in Cassini images from 2005. Atlas and Pan were first sighted in images from the Voyager 1 and 2 spacecraft. Flying saucer-shaped Atlas orbits near the outer edge of Saturn's bright A Ring while Daphnis orbits inside the A Ring's narrow Keeler Gap and Pan within the A Ring's larger Encke Gap. The curious equatorial ridges of the small ring moons could be built up by the accumulation of ring material over time. Even diminutive Daphnis makes waves in the ring material as it glides along the edge of the Keeler Gap.
A Circumhorizontal Arc Over Ohio (2019-05-19)

Why would clouds appear to be different colors? The reason here is that ice crystals in distant cirrus clouds are acting like little floating prisms.  Sometimes known as a fire rainbow for its flame-like appearance, a circumhorizon arc lies 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 are present.  Furthermore, 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 quite unusual to see.  This circumhorizon display was photographed through a polarized lens above Dublin, Ohio in 2009.

Image credit: Todd Sladoje
Planets of the Solar System: Tilts and Spins (2019-05-20)

How does your favorite planet spin? Does it spin rapidly around a nearly vertical axis, or horizontally, or backwards?  The featured video animates NASA images of all eight planets in our Solar System to show them spinning side-by-side for an easy comparison. In the time-lapse video, a day on Earth -- one Earth rotation -- takes just a few seconds.  Jupiter rotates the fastest, while Venus spins not only the slowest (can you see it?), but backwards.  The inner rocky planets, across the top, most certainly underwent dramatic spin-altering collisions during the early days of the Solar System.  The reasons why planets spin and tilt as they do remains a topic of research with much insight gained from modern computer modeling and the recent discovery and analysis of hundreds of exoplanets: planets orbiting other stars.
Deep Field: Nebulae of Sagittarius (2019-05-21)

These three bright nebulae are often featured on telescopic tours of the constellation Sagittarius and the crowded starfields of the central Milky Way. In fact, 18th century cosmic tourist Charles Messier cataloged two of them; M8, the large nebula just left of center, and colorful M20 on the top left. The third emission region includes NGC 6559 and can be found to the right of M8. All three are stellar nurseries about five thousand light-years or so distant. Over a hundred light-years across, the expansive M8 is also known as the Lagoon Nebula. M20's popular moniker is the Trifid. Glowing hydrogen gas creates the dominant red color of the emission nebulae. In striking contrast, blue hues in the Trifid are due to dust reflected starlight. Recently formed bright blue stars are visible nearby.  The colorful composite skyscape was recorded in 2018 in Teide National Park in the Canary Islands, Spain.

Image credit: Emilio Rivero Padilla
Primordial Contact Binary 2014 MU69 (2019-05-22)

Primordial contact binary 2014 MU69, also known as Ultima Thule, really is very red. In fact, it's the reddest outer solar system object ever visited by a spacecraft from Earth. Its reddish hue is believed to be due to organic materials on its surface. Ruddy color and tantalizing surface details seen in this composite image are based on data from the New Horizons spacecraft recorded during the January 1 flyby of the farthest world yet explored. Embedded in the smaller lobe Thule (top), the 8 kilometer wide feature nicknamed Maryland crater is the largest depression known on the surface of Ultima Thule. Transmission of data collected from the flyby continues, and will go on until the late summer 2020 as New Horizons speeds deeper into the dim and distant Kuiper Belt.
Moons Near Jupiter (2019-05-23)

On May 20, a nearly Full Moon and Jupiter shared this telephoto field of view. Captured when a passing cloud bank dimmed the moonlight, the single exposure reveals the familiar face of our fair planet's own large natural satellite, along with bright Jupiter (lower right) and some of its Galilean moons. Lined up left to right the tiny pinpricks of light near Jupiter are Ganymede, Europa, [Jupiter] and Callisto. (That's not just dust on your screen ...) Closer and brighter, our own natural satellite appears to loom large. But Ganymede, and Callisto are physically larger than Earth's Moon, while water world Europa is only slightly smaller.  In fact, of the Solar System's six largest planetary satellites, Saturn's moon Titan is missing from the scene and a fourth Galilean moon, Io, is hidden by our ruling gas giant.

Image credit: Betul Turksoy
Boulders on Bennu (2019-05-24)

An abundance of boulders litters the surface asteroid 101955 Bennu in this dramatic close-up from the OSIRIS-REx spacecraft. Taken on March 28 from a distance of just 3.4 kilometers (2.1 miles) the field of view is about 50 meters across while the light colored boulder at top right is 4.8 meters tall. Likely a loose conglomerate rubble pile asteroid, Bennu itself spans less than 500 meters. That's about the height of the Empire State Building. Mapping the near Earth asteroid since the spacecraft's arrival in December of 2018, the OSIRIS-REx mission plans a TAG (Touch-and-Go) maneuver for July 2020 to sample Bennu's rugged surface, returning the sample to planet Earth in September 2023. Citizen scientists have been invited to help choose the sample collection site.
Planet of the Tajinastes (2019-05-25)

What bizarre planet are these alien creatures from? It's only planet Earth, of course. The planet's home galaxy the Milky Way stretches across a dark sky in the panoramic, fisheye all-sky projection composed with a wide lens. But the imposing forms gazing skyward probably look strange to many denizens of Earth. Found on the Canary Island of Tenerife in the Teide National Park, they are red tajinastes, flowering plants that grow to a height of up to 3 meters. Among the rocks of the volcanic terrain, tajinastes bloom in spring and early summer and then die after a week or so as their seeds mature. A species known as Echium wildpretii, the terrestrial life forms were individually lit by flashlight during the wide-angle exposures.

Image credit: El Cielo de Canarias
A Solar Prominence Eruption from SDO (2019-05-26)

One of the most spectacular solar sights is an erupting prominence. In 2011, NASA's Sun-orbiting Solar Dynamic Observatory spacecraft imaged an impressively large prominence erupting from the surface. The dramatic explosion was captured in ultraviolet light in the featured time lapse video covering 90 minutes, where a new frame was taken every 24 seconds.  The scale of the prominence is huge -- the entire Earth would easily fit under the flowing  curtain of hot gas.  A solar prominence is channeled and sometimes held above the Sun's surface by the Sun's magnetic field. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System.  The energy mechanism that creates a solar prominence is still a topic of research.  After our Sun passes the current Solar Minimum, solar activity like eruptive prominences are expected to become more common over the next few years.

Image credit: NASA/Goddard/SDO AIA Team
A Volcano of Fire under a Milky Way of Stars (2019-05-27)

Sometimes it's hard to decide which is more impressive -- the land or the sky.  On the land of the featured image, for example, the Volcano of Fire (Volc�n de Fuego) is seen erupting topped by red-hot, wind-blown ash and with streams of glowing lava running down its side. Lights from neighboring towns are seen through a thin haze below.  In the sky, though, the central plane of our Milky Way Galaxy runs diagonally from the upper left, with a fleeting meteor just below, and the trail of a satellite to the upper right.  The planet Jupiter also appears toward the upper left, with the bright star Antares just to its right.  Much of the land and the sky were captured together in a single, well-timed, 25-second exposure taken in mid-April from the side of Fuego's sister volcano Acatenango in Guatemala.  The image of the meteor, though, was captured in a similar frame taken about 30 minutes earlier -- when the volanic eruption was not as photogenic -- and added later digitally.   Follow APOD on: Instagram, Facebook,  Reddit, or Twitter

Image credit: Diego Rizzo
Stars, Dust, and Gas near NGC 3572 (2019-05-28)

Star formation can be colorful.  This chromatic cosmic portrait features glowing gas and dark dust near some recently formed stars of NGC 3572, a little-studied star cluster near the Carina Nebula.  Stars from NGC 3572 are visible near the bottom of the image, while the expansive gas cloud above is likely what remains of their formation nebula. The image's striking hues were created by featuring specific colors emitted by hydrogen, oxygen, and sulfur, and blending them with images recorded through broadband filters in red, green, and blue. This nebula near NGC 3572 spans about 100 light years and lies about 9,000 light years away toward the southern constellation of the Ship's Keel (Carina).  Within a few million years the pictured gas will likely disperse, while gravitational encounters will likely disperse the cluster stars over about a billion years.   Astrophysicists: Browse 1,900+ codes in the Astrophysics Source Code Library

Image credit: Andrew Campbell
M95: Spiral Galaxy with an Inner Ring (2019-05-29)

Why do some spiral galaxies have a ring around the center?  First and foremost, M95 is one of the closer examples of a big and beautiful barred spiral galaxy.  Visible in the featured combination of images from Hubble and several ground based telescopes are sprawling spiral arms delineated by open clusters of bright blue stars, lanes of dark dust, the diffuse glow of billions of faint stars, and a short bar across the galaxy center.  What intrigues many astronomers, however, is the circumnuclear ring around the galaxy center visible just outside the central bar.  Although the long term stability of this ring remains a topic of research, observations indicate its present brightness is at least enhanced by transient bursts of star formation.  M95, also known as NGC 3351, spans about 50,000 light-years, lies about 30 million light years away, and can be seen with a small telescope toward the constellation of the Lion (Leo).   Almost Hyperspace: Random APOD Generator
Sunrise at Copernicus Crater (2019-05-30)

A prominent impact site anchored in the lunar Oceanus Procellarum, Copernicus crater is at the center of this telescopic portrait in light and shadow. Caught in stacked and sharpened video frames recorded on April 14 at 3:30am UTC, the lunar terminator, or boundary between night and day, cuts across the middle of the 93 kilometer diameter crater. Sunlight is just beginning to strike its tall western walls but doesn't yet shine on lower terrain nearby, briefly extending the crater's outline into the lunar nightside. At that moment standing at Copernicus crater you could watch the sunrise, an event that happens at Copernicus every 29.5 days. Of course that corresponds to a lunar month or a lunation, the time between consecutive Full Moons as seen from planet Earth.

Image credit: Sage Gray
Lynds Dark Nebula 1251 (2019-05-31)

Stars are forming in Lynds Dark Nebula (LDN) 1251. About 1,000 light-years away and drifting above the plane of our Milky Way galaxy, the dusty molecular cloud is part of a complex of dark nebulae mapped toward the Cepheus flare region. Across the spectrum, astronomical explorations of the obscuring interstellar clouds reveal energetic shocks and outflows associated with newborn stars, including the telltale reddish glow from scattered Herbig-Haro objects seen in this sharp image. Distant background galaxies also lurk on the scene, visually buried behind the dusty expanse. The deep telescopic field of view imaged with broadband filters spans about two full moons on the sky, or 17 light-years at the estimated distance of LDN 1251.

Image credit: Francesco
Sferlazza
NICER at Night (2019-06-01)

A payload on board the International Space Station, the Neutron star Interior Composition Explorer (NICER) twists and turns to track cosmic sources of X-rays as the station orbits planet Earth every 93 minutes. During orbit nighttime, its X-ray detectors remain on. So as NICER slews from target to target bright arcs and loops are traced across this all-sky map made from 22 months of NICER data. The arcs tend to converge on prominent bright spots, pulsars in the X-ray sky that NICER regularly targets and monitors. The pulsars are spinning neutron stars that emit clock-like pulses of X-rays. Their timing is so precise it can be used for navigation, determining spacecraft speed and position. This NICER X-ray, all-sky, map is composed in coordinates with the celestial equator horizontally across the center.
A Live View from the International Space Station (2019-06-02)

If you were floating above the Earth right now, this is what you might see. In 2014, a robotic SpaceX Dragon capsule that delivered supplies to the Earth-orbiting International Space Station (ISS) also delivered High Definition Earth Viewing (HDEV) cameras that take and transmit live views of Earth. Pictured here, when working, is the live video feed that switches between four cameras, each pointed differently. Watch white clouds, tan land, and blue oceans drift by.  The featured live view will appear black when it is nighttime on the Earth below, but the space station's rapid 90-minute orbit compresses this dark time into only 45 minutes.  The present location of the ISS above the Earth can be found on the web. If the video appears gray, this indicates that the view is either being switched between cameras, or communications with the ISS is temporarily unavailable.  As the HDEV project continues, video quality will be monitored to assess the effects of high energy radiation, which types of cameras work best, and which Earth views are the most popular.
Stephan's Quintet from Hubble (2019-06-03)

When did these big galaxies first begin to dance?  Really only four of the five of Stephan's Quintet are locked in a cosmic tango of repeated close encounters taking place some 300 million light-years away.  The odd galaxy out is easy to spot in this recently reprocessed image by the Hubble Space Telescope -- the interacting galaxies, NGC 7319, 7318B, 7318A, and 7317 (left to right), have a more dominant yellowish cast.  They also tend to have distorted loops and tails, grown under the influence of disruptive gravitational tides.  The mostly bluish galaxy, large NGC 7320 on the lower left, is in the foreground at about 40 million light-years distant, and so is not part of the interacting group. Data and modeling indicate that NGC 7318B is a relatively new intruder. A recently-discovered halo of old red stars surrounding Stephan's Quintet indicate that at least some of these galaxies started tangling over a billion years. Stephan's Quintet is visible with a moderate sized-telescope toward the constellation of Winged Horse (Pegasus).
SEIS: Listening for Marsquakes (2019-06-04)

If you put your ear to Mars, what would you hear? To find out, and to explore the unknown interior of Mars, NASA's Insight Lander deployed SEIS late last year, a sensitive seismometer that can detect marsquakes. In early April, after hearing the wind and motions initiated by the lander itself, SEIS recorded an unprecedented event that matches what was expected for a marsquake.  This event can be heard on this YouTube video.  Although Mars is not thought to have tectonic plates like the Earth, numerous faults are visible on the Martian surface which likely occurred as the hot interior of Mars cooled -- and continues to cool.  Were strong enough marsquakes to occur, SEIS could hear their rumbles reflected from large structures internal to Mars, like a liquid core, if one exists.  Pictured last week, SEIS sits quietly on the Martian surface, taking in some Sun while light clouds are visible over the horizon.   Create a Distant Legacy: Send your name to Mars
The Interstellar Clouds of Orion (2019-06-05)

The constellation of Orion is much more than three stars in a row. It is a direction in space that is rich with impressive nebulas. To better appreciate this well-known swath of sky, a new  long exposure image was taken over several clear nights in January, February and March. After 23 hours of camera time and untold hours of image processing, the featured collage in the light of hydrogen, oxygen, and sulfur was produced spanning over 40 times the angular diameter of the Moon.  Of the many interesting details that have become visible, one that particularly draws the eye is Barnard's Loop, the bright red orange arc just to the right of the image center.  The Rosette Nebula is not the giant orange nebula just to the left of the image center -- that is larger but lesser known nebula known as the Meissa Ring.  The Rosette Nebula is visible, though: it is the bright orange, blue and white nebula near the image bottom.  The bright orange star just left of the frame center is Betelgeuse, while the bright blue star on the upper right is Rigel. About those famous three stars that cross the belt of Orion the Hunter -- in this busy frame they can be hard to locate, but a discerning eye will find them just to the right of the image center.

Image credit: Andrew Klinger
Messier 63: The Sunflower Galaxy (2019-06-06)

A bright spiral galaxy of the northern sky, Messier 63 is about 25 million light-years distant in the loyal constellation Canes Venatici. Also cataloged as NGC 5055, the majestic island universe is nearly 100,000 light-years across. That's about the size of our own Milky Way Galaxy. Known by the popular moniker, The Sunflower Galaxy, M63 sports a bright yellowish core in this sharp telescopic portrait. Its sweeping blue spiral arms are streaked with cosmic dust lanes and dotted with pink star forming regions. A dominant member of a known galaxy group, M63 has faint, extended features that are likely star streams from tidally disrupted satellite galaxies. M63 shines across the electromagnetic spectrum and is thought to have undergone bursts of intense star formation.

Image credit: Bernard Miller
The Planet and the Pipe (2019-06-07)

Now posing against our galaxy's rich starfields and nebulae, brilliant planet Jupiter shines in the night sky. Its almost overwhelming glow is near the top of the frame in this colorful telephoto portrait of the central Milky Way. Spanning about 20 degrees on the sky, the scene includes the silhouette of LDN 1773 against the starlight, also know by the popular moniker the Pipe Nebula for its apparent outline of stem and bowl. The Pipe Nebula is part of the galaxy's Ophiuchus dark cloud complex. Located at a distance of about 450 light-years, dense cores of gas and dust within are collapsing to form stars.  Approaching its opposition, opposite the Sun in the sky on June 10, Jupiter is only about 36 light-minutes from planet Earth. Fans of dark markings on the sky can probably spot the Snake Nebula below and left of Jupiter's glare.

Image credit: Alain Maury