juliensimon/nasa-apod huggingface dataset:
Marius Hills and a Hole in the Moon (2017-10-25)

Could humans live beneath the surface of the Moon? This intriguing possibility was bolstered in 2009 when Japan's Moon-orbiting SELENE spacecraft imaged a curious hole beneath the Marius Hills region on the Moon, possibly a skylight to an underground lava tube. Follow-up observations by NASA's Lunar Reconnaissance Orbiter (LRO) indicated that the Marius Hills Hole (MHH) visually extends down nearly 100 meters and is several hundred meters wide.  Most recently, ground penetrating radar data from SELENE has been re-analyzed to reveal a series of intriguing second echoes -- indicators that the extensive lava tubes exist under Marius Hills might extend down even kilometers and be large enough to house cities.  Such tubes could shelter a future Moon colony from large temperature swings, micro-meteor impacts, and harmful solar radiation.  Potentially, underground lava tubes might even be sealed to contain breathable air.  These lava tubes likely formed when lunar volcanos were active billions of years ago. Pictured, the surface of Marius Hills region was captured in the 1960s by NASA's Lunar Orbiter 2 mission, while an inset image of the MHH is shown from NASA's continuing LRO.  Several volcanic domes are visible, while Marius Crater is visible on the upper right.
NGC 7635: Bubble in a Cosmic Sea (2017-10-26)

Adrift in a cosmic sea of stars and glowing gas the delicate, floating apparition left of center in this widefield view is cataloged as NGC 7635, the Bubble Nebula. A mere 10 light-years wide, the tiny Bubble Nebula was blown by the winds of a massive star. It lies within a larger complex of interstellar gas and dust clouds found about 11,000 light-years distant, straddling the boundary between the parental constellations Cepheus and Cassiopeia. Included in the breathtaking vista is open star cluster M52 (lower left), some 5,000 light-years away. Above and right of the Bubble Nebula is an emission region identified as Sh2-157, also known as the Claw Nebula. Constructed from 47 hours of narrow-band and broad-band exposures, this image spans about 3 degrees on the sky. That corresponds to a width of 500 light-years at the estimated distance of the Bubble Nebula.

Image credit: Rolf
Geissinger
Mirach's Ghost (2017-10-27)

As far as ghosts go, Mirach's Ghost isn't really that scary. Mirach's Ghost is just a faint, fuzzy galaxy, well known to astronomers, that happens to be seen nearly along the line-of-sight to Mirach, a bright star. Centered in this star field, Mirach is also called Beta Andromedae. About 200 light-years distant, Mirach is a red giant star, cooler than the Sun but much larger and so intrinsically much brighter than our parent star. In most telescopic views, glare and diffraction spikes tend to hide things that lie near Mirach and make the faint, fuzzy galaxy look like a ghostly internal reflection of the almost overwhelming starlight. Still, appearing in this sharp image just above and to the left of Mirach, Mirach's Ghost is cataloged as galaxy NGC 404 and is estimated to be some 10 million light-years away.

Image credit: Kent Wood
NGC 6369: The Little Ghost Nebula (2017-10-28)

Wraithlike NGC 6369 is a faint apparition in night skies popularly known as the Little Ghost Nebula. It was discovered by 18th century astronomer Sir William Herschel as he used a telescope to explore the medicinal constellation Ophiucus. Herschel historically classified the round and planet-shaped nebula as a Planetary Nebula. But planetary nebulae in general are not at all related to planets. Instead they are gaseous shrouds created at the end of a sun-like star's life, the dying star's outer layers expanding into space while its core shrinks to become a white dwarf. The transformed white dwarf star, seen near the center, radiates strongly at ultraviolet wavelengths and powers the expanding nebula's glow. Surprisingly complex details and structures of NGC 6369 are revealed in this tantalizing image composed from Hubble Space Telescope data. The nebula's main round structure is about a light-year across and the glow from ionized oxygen, hydrogen, and nitrogen atoms are colored blue, green, and red respectively. Over 2,000 light-years away, the Little Ghost Nebula offers a glimpse of the fate of our Sun, which could produce its own planetary nebula about 5 billion years from now.
Night on a Spooky Planet (2017-10-29)

What spooky planet is this? Planet Earth of course, on a dark and stormy night in 2013 at Hverir, a geothermally active area along the volcanic landscape in northeastern Iceland. Geomagnetic storms produced the auroral display in the starry night sky while ghostly towers of steam and gas venting from fumaroles danced against the eerie greenish light. Tonight, there is also a chance for geomagnetic storms triggered by recent solar activity, so high-latitude skygazers should beware. Ghostly shapes may dance in your neighborhood pretty soon, too.

Image credit: Stéphane
Vetter
Orionid Meteors from Orion (2017-10-30)

Meteors have been shooting out from the constellation of Orion.  This was expected, as October is the time of year for the Orionids Meteor Shower.  Pictured here, over a dozen meteors were caught in successively added exposures last weekend over Wulan Hada volcano in Inner Mongolia, China. The featured image shows multiple meteor streaks that can all be connected to a single small region on the sky called the radiant, here visible just above and to the left of the belt of Orion,  The Orionids meteors started as sand sized bits expelled from Comet Halley during one of its trips to the inner Solar System.  Comet Halley is actually responsible for two known meteor showers, the other known as the Eta Aquarids and visible every May.  Next month, the Leonids Meteor Shower from Comet Tempel-Tuttle should also result in some bright meteor streaks.   Free Download: The 2018 APOD Calendar

Image credit: Lu Shupei
Dark Matter in a Simulated Universe (2017-10-31)

Is our universe haunted?  It might look that way on this dark matter map.  The gravity of unseen dark matter is the leading explanation for why galaxies rotate so fast, why galaxies orbit clusters so fast, why gravitational lenses so strongly deflect light, and why visible matter is distributed as it is both in the local universe and on the cosmic microwave background.  The featured image from the American Museum of Natural History�s Hayden Planetarium Space Show Dark Universe highlights one example of how pervasive dark matter might haunt our universe.  In this frame from a detailed computer simulation, complex filaments of dark matter, shown in black, are strewn about the universe like spider webs, while the relatively rare clumps of familiar baryonic matter are colored orange. These simulations are good statistical matches to astronomical observations.  In what is perhaps a scarier turn of events, dark matter -- although quite strange and in an unknown form -- is no longer thought to be the strangest source of gravity in the universe. That honor now falls to dark energy, a more uniform source of repulsive gravity that seems to now dominate the expansion of the entire universe.    Not only Halloween: Today is Dark Matter Day.

Image credit: Tom Abel & 
Ralf Kaehler 
(KIPAC, 
SLAC), 
AMNH
Thor's Helmet Emission Nebula (2017-11-01)

This helmet-shaped cosmic cloud with wing-like appendages is popularly called Thor's Helmet. Heroically sized even for a Norse god, Thor's Helmet spans about 30 light-years across. In fact, the helmet is more like an interstellar bubble, blown as a fast wind -- from the bright star near the center of the bubble's blue-hued region -- sweeps through a surrounding molecular cloud. This star, a Wolf-Rayet star, is a massive and extremely hot giant star thought to be in a brief, pre-supernova stage of evolution. Cataloged as NGC 2359, the emission nebula is located about 12,000 light-years away toward the constellation of the Big Dog (Canis Major). The sharp image, made using broadband and narrowband filters, captures striking details of the nebula's filamentary gas and dust structures. The blue color originates from strong emission from  oxygen atoms in the nebula.   Follow APOD on: Facebook,  Google Plus,  Instagram, or Twitter

Image credit: Adam Block
NGC 891 vs Abell 347 (2017-11-02)

Distant galaxies lie beyond a foreground of spiky Milky Way stars in this telescopic field of view. Centered on yellowish star HD 14771, the scene spans about 1 degree on the sky toward the northern constellation Andromeda. At top right is large spiral galaxy NGC 891, 100 thousand light-years across and seen almost exactly edge-on. About 30 million light-years distant, NGC 891 looks a lot like our own Milky Way with a flattened, thin, galactic disk. Its disk and central bulge are cut along the middle by dark, obscuring dust clouds. Scattered toward the lower left are members of galaxy cluster Abell 347. Nearly 240 million light-years away, Abell 347 shows off its own large galaxies in the sharp image. They are similar to NGC 891 in physical size but located almost 8 times farther away, so Abell 347 galaxies have roughly one eighth the apparent size of NGC 891.

Image credit: Laszlo Bagi
A/2017 U1: An Interstellar Visitor (2017-11-03)

Traveling at high velocity along an extreme hyperbolic orbit and making a hairpin turn as it swung past the Sun, the now designated A/2017 U1 is the first known small body from interstellar space. A point of light centered in this 5 minute exposure recorded with the William Herschel Telescope in the Canary Islands on October 28, the interstellar visitor is asteroid-like with no signs of cometary activity. Faint background stars appear streaked because the massive 4.2 meter diameter telescope is tracking the rapidly moving A/2017 U1 in the field of view. Astronomer Rob Weryk (IfA) first recognized the moving object in nightly Pan-STARRS sky survey data on October 19. A/2017 is presently outbound, never to return to the Solar System, and already only visible from planet Earth in large optical telescopes. Though an interstellar origin has been established based on its orbit, it is still unknown how long the object could have drifted among the stars of the Milky Way. But its interstellar cruise speed would be about 26 kilometers per second. By comparison humanity's Voyager 1 spacecraft travels about 17 kilometers per second through interstellar space.
Hubble's Messier 5 (2017-11-04)

"Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for Earthbound telescopes. Of course, deployed in low Earth orbit on April 25, 1990, the Hubble Space Telescope has also captured its own stunning close-up view that spans about 20 light-years across the central region of M5. Even close to its dense core the cluster's aging red and blue giant stars and rejuvenated blue stragglers stand out in yellow and blue hues in the sharp color image.
A Year of Full Moons (2017-11-05)

Do all full moons look the same? No.  To see the slight differences, consider this grid of twelve full moons. From upper left to lower right, the images represent every lunation from 2016 November through 2017 October, as imaged from Pakistan.  The consecutive full moons are all shown at the same scale, so unlike the famous Moon Illusion, the change in apparent size seen here is real. The change is caused by the variation in lunar distance due to the Moon's significantly non-circular orbit. The dark notch at the bottom of the full moon of 2017 August is the shadow of the Earth -- making this a partial lunar eclipse.  Besides the sometimes exaggerated coloring, a subtler change in appearance can also be noticed on close examination, as the Moon seems to wobble slightly from one full moon to the next. This effect, known as libration, is more dramatic and easier to see in this lunation video highlighting all of the ways that the Moon appears to change over a month (moon-th).

Image credit: Talha Zia
A Dust Jet from the Surface of Comet 67P (2017-11-06)

Where do comet tails come from?  There are no obvious places on the nuclei of comets from which the jets that create comet tails emanate.  Last year, though, ESA's Rosetta spacecraft not only imaged a jet emerging from Comet 67P/Churyumov-Gerasimenko, but flew right through it.  Featured is a telling picture showing a bright plume emerging from a small circular dip bounded on one side by a 10-meter high wall.  Analyses of Rosetta data shows that the jet was composed of both dust and water-ice. The mundane terrain indicates that something likely happened far under the porous surface to create the plume.  This image was taken last July, about two months before Rosetta's mission ended with a controlled impact onto Comet 67P's surface.

Image credit: ESA, 
Rosetta, 
MPS, OSIRIS;
UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The Prague Astronomical Clock (2017-11-07)

In the center of Prague there's a clock the size of a building.  During the day, crowds gather to watch the show when it chimes in a new hour.  The Prague Astronomical Clock's face is impressively complex, giving not only the expected time with respect to the Sun (solar time), but the time relative to the stars (sidereal time), the times of sunrise and sunset at several latitudes including the equator, the phase of the Moon, and much more.   The clock began operation in 1410, and even though much of its inner workings have been modernized several times, original parts remain.  Below the clock is a nearly-equal sized solar calendar that rotates only once a year.  Pictured, the Prague Astronomical Clock was photographed alone during an early morning in 2009 March.  The Prague Astronomical Clock and the Old Town Tower behind it are currently being renovated once again, with the clock expected to be restarted in 2018 June.
NGC 2261: Hubble's Variable Nebula (2017-11-08)

What causes Hubble's Variable Nebula to vary?  The unusual nebula featured here changes its appearance noticeably in just a few weeks.  Discovered over 200 years ago and subsequently cataloged as NGC 2261, the remarkable nebula is named for Edwin Hubble, who studied it early last century.  Fitting, perhaps, the featured image was taken by another namesake of Hubble: the Space Telescope. Hubble's Variable Nebula is a reflection nebula made of gas and fine dust fanning out from the star R Monocerotis.  The faint nebula is about one light-year across and lies about 2500 light-years away towards the constellation of the Unicorn  (Monocerotis).  The leading variability explanation for Hubble's Variable Nebula holds that dense knots of opaque dust pass close to R Mon and cast moving shadows onto the reflecting dust seen in the rest of the nebula.   Open Science: Browse 1,500+ codes in the Astrophysics Source Code Library
NGC 1055 Close-up (2017-11-09)

Big, beautiful spiral galaxy NGC 1055 is a dominant member of a small galaxy group a mere 60 million light-years away toward the aquatically intimidating constellation Cetus. Seen edge-on, the island universe spans over 100,000 light-years, a little larger than our own Milky Way. The colorful stars in this cosmic close-up of NGC 1055 are in the foreground, well within the Milky Way. But the telltale pinkish star forming regions are scattered through winding dust lanes along the distant galaxy's thin disk. With a smattering of even more distant background galaxies, the deep image also reveals a boxy halo that extends far above and below the central bluge and disk of NGC 1055. The halo itself is laced with faint, narrow structures, and could represent the mixed and spread out debris from a satellite galaxy disrupted by the larger spiral some 10 billion years ago.

Image credit: Robert Gendler
Williamina Fleming's Triangular Wisp (2017-11-10)

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. Blasted out in the cataclysmic event, the interstellar shock waves plow through space sweeping up and exciting interstellar material.  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 red and oxygen in blue hues. Also known as the Cygnus Loop, the Veil Nebula now spans nearly 3 degrees or about 6 times the diameter of the full Moon. While that translates to over 70 light-years at its estimated distance of 1,500 light-years, this field of view spans less than one third that distance. Often identified as Pickering's Triangle for a director of Harvard College Observatory, the complex of filaments is cataloged as NGC 6979. It is also known for its discoverer, astronomer Williamina Fleming, as Fleming's Triangular Wisp.

Image credit: Sara Wager
A Colourful Moon (2017-11-11)

The Moon is normally seen in subtle shades of grey. But small, measurable color differences have been greatly exaggerated in this mosaic of high-resolution images captured near the Moon's full phase, to construct a multicolored, central moonscape. The different colors are recognized to correspond to real differences in the mineral makeup of the lunar surface. Blue hues reveal titanium rich areas while more orange and purple colors show regions relatively poor in titanium and iron. The intriguing Sea of Vapors, or Mare Vaporum, is below center in the frame with the sweeping arc of the lunar Montes Apenninus (Apennine Mountains) above it. The dark floor of 83 kilometer diameter Archimedes crater within the Sea of Rains, or Mare Imbrium, is toward the top left. Near the gap at the top of the Apennine's arc is the Apollo 15 landing site. Calibrated by rock samples returned by the Apollo missions, similar multicolor images from spacecraft have been used to explore the Moon's global surface composition.

Image credit: Alain Paillou
A Happy Sky over Los Angeles (2017-11-12)

Sometimes, the sky may seem to smile over much of planet Earth.  On this day in 2008, visible the world over, was an unusual superposition of our Moon and the planets Venus and Jupiter.  Pictures taken at the right time show a crescent Moon that appears to be a smile when paired with the planetary conjunction of seemingly nearby Jupiter and Venus.  Pictured here is the scene as it appeared from Mt. Wilson Observatory overlooking Los Angeles, California, USA after sunset on 2008 November 30.  Highest in the sky and farthest in the distance is the planet Jupiter.  Significantly closer and visible to Jupiter's lower left is Venus, appearing through Earth's atmospheric clouds as unusually blue.  On the far right, above the horizon, is our Moon, in a waxing crescent phase.  Thin clouds illuminated by the Moon appear unusually orange.   Sprawling across the bottom of the image are the hills of Los Angeles, many covered by a thin haze, while LA skyscrapers are visible on the far left.  Hours after the taking of this image, the Moon approached the distant duo, briefly eclipsed Venus, and then moved on.  This week, another conjunction of Venus and Jupiter is occurring and is visible to much of planet Earth to the east just before sunrise.

Image credit: Dave Jurasevich
(Mt. Wilson Observatory)
Comet Machholz Approaches the Sun (2017-11-13)

Why is Comet Maccholz so depleted of carbon-containing chemicals?  Comet 96P/Machholz's original fame derives from its getting closer to the Sun than any other short period comet -- half as close as Mercury -- and doing so every five years. To better understand this unusual comet, NASA's Sun-monitoring SOHO spacecraft tracked the comet during its latest approach to the Sun in October.  The featured image composite shows the tail-enhanced comet swooping past the Sun. The Sun's bright surface is hidden from view behind a dark occulter, although parts of the Sun's extended corona are visible. Neighboring stars dot the background. One hypothesis holds that these close solar approaches somehow cause Comet Machholz to shed its carbon, while another hypothesis posits that the comet formed with this composition far away -- possibly even in another star system.    Free APOD Lectures: January 5 in NYC & January 9 in DC
The Pleiades Deep and Dusty (2017-11-14)

The well-known Pleiades star cluster is slowly destroying part of a passing cloud of gas and dust. The Pleiades is the brightest open cluster of stars on Earth's sky and can be seen from almost any northerly location with the unaided eye.  The passing young dust cloud is thought to be part of Gould's Belt, an unusual ring of young star formation surrounding the Sun in the local Milky Way Galaxy. Over the past 100,000 years, part of Gould's Belt is by chance moving right through the older Pleiades and is causing a strong reaction between stars and dust. Pressure from the stars' light significantly repels the dust in the surrounding blue reflection nebula, with smaller dust particles being repelled more strongly.  A short-term result is that parts of the dust cloud have become filamentary and stratified. The featured deep image also captured Comet C/2015 ER61 (PanSTARRS) on the lower left.

Image credit: Juan Carlos Casado
(TWAN,
Earth & Stars),
Miquel Serra-Ricart & Daniel Padron, 
FECYT
NGC 7789: Caroline's Rose (2017-11-15)

Found among the rich starfields of the Milky Way, star cluster NGC 7789 lies about 8,000 light-years away toward the constellation Cassiopeia. A late 18th century deep sky discovery of astronomer Caroline Lucretia Herschel, the cluster is also known as Caroline's Rose. Its flowery visual appearance in small telescopes is created by the cluster's nestled complex of stars and voids. Now estimated to be 1.6 billion years young, the galactic or open cluster of stars also shows its age. All the stars in the cluster were likely born at the same time, but the brighter and more massive ones have more rapidly exhausted the hydrogen fuel in their cores. These have evolved from main sequence stars like the Sun into the many red giant stars shown with a yellowish cast in this lovely color composite. Using measured color and brightness, astronomers can model the mass and hence the age of the cluster stars just starting to "turn off" the main sequence and become red giants. Over 50 light-years across, Caroline's Rose spans about half a degree (the angular size of the Moon) near the center of the wide-field telescopic image.

Image credit: Guillaume Seigneuret