juliensimon/nasa-apod huggingface dataset:
Crescent Neptune and Triton (2013-04-14)

Gliding silently through the outer Solar System, the Voyager 2 spacecraft camera captured Neptune and Triton together in crescent phase in 1989.  The elegant picture of the gas giant planet and its cloudy moon was taken from behind just after closest approach.  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.   New Fan-Run APOD Social Page: Horizon on Facebook
IC 1848: The Soul Nebula (2013-04-15)

Stars are forming in the Soul of the Queen of Aethopia. More specifically, a large star forming region called the Soul Nebula can be found in the direction of the constellation Cassiopeia, who Greek mythology credits as the vain wife of a King who long ago ruled lands surrounding the upper Nile river. The Soul Nebula houses several open clusters of stars, a large radio source known as W5, and huge evacuated bubbles formed by the winds of young massive stars. Located about 6,500 light years away, the Soul Nebula spans about 100 light years and is usually imaged next to its celestial neighbor the Heart Nebula (IC 1805). The above image appears mostly red due to the emission of a specific color of light emitted by excited hydrogen gas.   Follow APOD on: Facebook (Daily) (Sky) (Spanish) or Google Plus (Daily) (River)

Image credit: Bob Andersson
Grand Spiral Galaxy M81 and Arp's Loop (2013-04-16)

One of the brightest galaxies in planet Earth's sky is similar in size to our Milky Way Galaxy: big, beautiful M81. This grand spiral galaxy lies 11.8 million light-years away toward the northern constellation of the Great Bear (Ursa Major). The deep image of the region reveals details in the bright yellow core, but at the same time follows fainter features along the galaxy's gorgeous blue spiral arms and sweeping dust lanes. It also follows the expansive, arcing feature, known as Arp's loop, that seems to rise from the galaxy's disk at the upper right. Studied in the 1960s, Arp's loop has been thought to be a tidal tail, material pulled out of M81 by gravitational interaction with its large neighboring galaxy M82. But a subsequent investigation demonstrates that at least some of Arp's loop likely lies within our own galaxy. The loop's colors in visible and infrared light match the colors of pervasive clouds of dust, relatively unexplored galactic cirrus only a few hundred light-years above the plane of the Milky Way. Along with the Milky Way's stars, the dust clouds lie in the foreground of this remarkable view.  M81's dwarf companion galaxy, Holmberg IX, can be seen just above the large spiral. On the sky, this image spans about 0.5 degrees, about the size of the Full Moon.

Image credit: Bernard Miller
Mt. Hood and a Lenticular Cloud (2013-04-17)

What kind of cloud is next to that mountain? A lenticular. This type of cloud forms in air that passes over a mountain, rises up again, and cools past the dew point -- so what molecular water carried in the air condenses into droplets. The layered nature of some lenticular clouds may make them appear, to some, as large alien spaceships. In this case, the mountain pictured is Mt. Hood located in Oregon, USA. Lenticular clouds can only form when conditions are right -- for example this is first time this astrophotographer has seen a lenticular cloud at night near Mt. Hood. The above image was taken in mid-March about two hours before dawn.

Image credit: Ben Canales
Star Factory Messier 17 (2013-04-18)

Sculpted by stellar winds and radiation, the star factory known as Messier 17 lies some 5,500 light-years away in the nebula-rich constellation Sagittarius. At that distance, this degree wide field of view spans almost 100 light-years. The sharp, composite, color image utilizing data from space and ground based telescopes, follows faint details of the region's gas and dust clouds against a backdrop of central Milky Way stars. Stellar winds and energetic light from hot, massive stars formed from M17's stock of cosmic gas and dust have slowly carved away at the remaining interstellar material producing the cavernous appearance and undulating shapes. M17 is also known as the Omega Nebula or the Swan Nebula.
NGC 1788 and the Witch's Whiskers (2013-04-19)

This skyscape finds an esthetic balance of interstellar dust and gas residing in the suburbs of the nebula rich constellation of Orion. Reflecting the light of bright star Rigel, Beta Orionis, the jutting, bluish chin of the Witch Head Nebula is at the upper left. Whiskers tracing the red glow of hydrogen gas ionized by ultraviolet starlight seem to connect that infamous visage with smaller nebulae, like dusty reflection nebula NGC 1788 at the right. Strong winds from Orion's bright stars have also shaped NGC 1788, and likely triggered the formation of the young stars within. Appropriate for its location, NGC 1788 looks to some like a cosmic bat. The scene spans about 3 degrees on the sky or 6 full Moons.

Image credit: John Davis
Airglow, Gegenschein, and Milky Way (2013-04-20)

As far as the eye could see, it was a dark night at Las Campanas Observatory in the southern Atacama desert of Chile. But near local midnight on April 11, this mosaic of 3 minute long exposures revealed a green, unusually intense, atmospheric airglow stretching over thin clouds. Unlike aurorae powered by collisions with energetic charged particles and seen at high latitudes, the airglow is due to chemiluminescence, the production of light in a chemical reaction, and found around the globe. The chemical energy is provided by the Sun's extreme ultraviolet radiation. Like aurorae, the greenish hue of this airglow does originate at altitudes of 100 kilometers or so dominated by emission from excited oxygen atoms. The gegenschein, sunlight reflected by dust along the solar system's ecliptic plane was still visible on that night, a faint bluish cloud just right of picture center. At the far right, the Milky Way seems to rise from the mountain top perch of the Magellan telescopes. Left are the OGLE project and du Pont telescope domes.

Image credit: Yuri Beletsky
The Big Dipper (2013-04-21)

Do you see it?  This common question frequently precedes the rediscovery of one of the most commonly recognized configurations of stars on the northern sky: the Big Dipper.  This grouping of stars is one of the few things that has likely been seen, and will be seen, by every generation.   The Big Dipper is not by itself a constellation.  Although part of the constellation of the Great Bear (Ursa Major), the Big Dipper is an asterism that has been known by different names to different societies. Five of the Big Dipper stars are actually near each other in space and were likely formed at nearly the same time.  Connecting two stars in the far part of the Big Dipper will lead one to Polaris, the North Star, which is part of the Little Dipper.   Relative stellar motions will cause the Big Dipper to slowly change its apparent configuration over the next 100,000 years.

Image credit: Jerry Lodriguss
(Catching the Light)
The Horsehead Nebula in Infrared from Hubble (2013-04-22)

While drifting through the cosmos, a magnificent interstellar dust cloud became sculpted by stellar winds and radiation to assume a recognizable shape.  Fittingly named the Horsehead Nebula, it is embedded in the vast and complex Orion Nebula (M42).  A potentially rewarding but difficult object to view personally with a small telescope, the above gorgeously detailed image was recently taken in infrared light by the orbiting Hubble Space Telescope in honor of the 23rd anniversary of Hubble's launch. The dark molecular cloud, roughly 1,500 light years distant, is cataloged as Barnard 33 and is seen above primarily because it is backlit by the nearby massive star Sigma Orionis. The Horsehead Nebula will slowly shift its apparent shape over the next few million years and will eventually be destroyed by the high energy starlight.
X-rays from Supernova Remnant SN 1006 (2013-04-23)

What looks like a puff-ball is surely the remains of the brightest supernova in recorded human history. In 1006 AD, it was recorded as lighting up the nighttime skies above areas now known as China, Egypt, Iraq, Italy, Japan, and Switzerland. The expanding debris cloud from the stellar explosion, found in the southerly constellation the Wolf (Lupus), still puts on a cosmic light show across the electromagnetic spectrum. In fact, the above image results from three colors of X-rays taken by the orbiting Chandra X-ray Observatory. Now known as the SN 1006 supernova remnant, the debris cloud appears to be about 60 light-years across and is understood to represent the remains of a white dwarf star. Part of a binary star system, the compact white dwarf gradually captured material from its companion star. The buildup in mass finally triggered a thermonuclear explosion that destroyed the dwarf star. Because the distance to the supernova remnant is about 7,000 light-years, that explosion actually happened 7,000 years before the light reached Earth in 1006. Shockwaves in the remnant accelerate particles to extreme energies and are thought to be a source of the mysterious cosmic rays.   Follow APOD on: Facebook (Daily) (Sky) (Spanish) or Google Plus (Daily) (River)
Wringing a Wet Towel in Orbit (2013-04-24)

What happens if you wring out a wet towel while floating in space? The water shouldn't fall toward the floor because while orbiting the Earth, free falling objects will appear to float.  But will the water fly out from the towel, or what? The answer may surprise you. To find out and to further exhibit how strange being in orbit can be, Expedition 35 Commander Chris Hadfield did just this experiment last week in the microgravity of the Earth orbiting International Space Station. As demonstrated in the above video, although a few drops do go flying off, most of the water sticks together and forms a unusual-looking cylindrical sheath in and around the towel. The self-sticking surface tension of water is well known on Earth, for example being used to create artistic water cascades and, more generally, raindrops.    Astrophysicists: Browse 600+ codes in the Astrophysics Source Code Library
Lunar Eclipses (2013-04-25)

The dark, inner shadow of planet Earth is called the umbra. Shaped like a cone extending into space, it has a circular cross section and is most easily seen during a lunar eclipse. But the complete cross section is larger than the Moon's angular size in the stages of an eclipse. Still, this thoughtful composite illustrates the full extent of the circular shadow by utilizing images from both partial and total eclipses passing through different parts of the umbra. The images span the years 1997 to 2011, diligently captured with the same optics, from Voronezh, Russia. Along the bottom and top are stages of the partial lunar eclipses from September 2006 and August 2008 respectively. In the rightside bottom image, the Moon is entering the umbra for the total eclipse of September 1997. At left bottom, the Moon leaves the umbra after totality in May 2004. Middle right, center, and left, are stages of the total eclipse of June 2011, including the central, deep red total phase. During today's brief partial lunar eclipse seen only from the eastern hemisphere, the Moon will just slightly graze the umbra's lower edge.

Image credit: Igor Vin'yaminov
A Year on the Sun (2013-04-26)

Our solar system's miasma of incandescent plasma, the Sun may look a little scary here. The picture is a composite of 25 images recorded in extreme ultraviolet light by the orbiting Solar Dynamics Observatory between April 16, 2012 and April 15, 2013. The particular wavelength of light, 171 angstroms, shows emission from highly ionized iron atoms in the solar corona at a characteristic temperatures of about 600,000 kelvins (about 1 million degrees F). Girdling both sides of the equator during approach to maximum in the 11-year solar cycle, the solar active regions are laced with bright loops and arcs along magnetic field lines. Of course, a more familiar visible light view would show the bright active regions as groups of dark sunspots. Three years of Solar Dynamics Observatory images are compressed into this short video.
Sharp Stereo (2013-04-27)

Get out your red/blue glasses and gaze across the floor of Gale crater on Mars. From your vantage point on the deck of the Curiosity Rover Mount Sharp, the crater's 5 kilometer high central mountain looms over the southern horizon. Poised in the foreground is the rover's robotic arm with tool turret extended toward the flat veined patch of martian surface dubbed "John Klein". A complete version of the stereo view spans 360 degrees, digitally stitched together from the rover's left and right navigation camera frames taken in late January. The layered lower slopes of Mount Sharp, formally known as Aeolis Mons, are a future destination for Curiosity.
A Raging Storm System on Saturn (2013-04-28)

It was one of the largest and longest lived storms ever recorded in our Solar System. � First seen in late 2010, the above cloud formation in the northern hemisphere of Saturn started larger than the Earth and soon spread completely around the planet. The storm was tracked not only from Earth but from up close by the robotic Cassini spacecraft currently orbiting Saturn. Pictured above in false colored infrared in February, orange colors indicate clouds deep in the atmosphere, while light colors highlight clouds higher up. The rings of Saturn are seen nearly edge-on as the thin blue horizontal line. The warped dark bands are the shadows of the rings cast onto the cloud tops by the Sun to the upper left. A source of radio noise from lightning, the intense storm was thought to relate to seasonal changes when spring emerges in the north of  Saturn. After raging for over six months, the iconic storm circled the entire planet and then tried to absorb its own tail -- which surprisingly caused it to fade away.   Sweden: APOD editor to speak at Cosmonova in late May
Milky Way and Stone Tree (2013-04-29)

What's that next to the Milky Way? An unusual natural rock formation known as Roque Cinchado or Stone Tree found on the Spanish Canary Island of Tenerife.  A famous icon, Roque Cinchado is likely a dense plug of cooled volcanic magma that remains after softer surrounding rock eroded away.  Majestically, the central band of our Milky Way Galaxy is visible arcing across the right of the above seven image panoramic mosaic taken during the summer of 2010.  On the far right is the Teide volcano complete with a lenticular cloud hovering near its peak. �   Follow APOD on: Facebook (Daily) (Sky) (Spanish) or Google Plus (Daily) (River) (Portuguese)

Image credit: Daniel L�pez 
(El Cielo de Canarias)
Humanity Explores the Solar System (2013-04-30)

What spacecraft is humanity currently using to explore our Solar System? Presently, every inner planet has at least one robotic explorer, while several others are monitoring our Sun, some are mapping Earth's Moon, a few are chasing asteroids and comets, one is orbiting Saturn, and several are even heading out into deep space. The above illustration gives more details, with the inner Solar System depicted on the upper right and the outer Solar System on the lower left. Given the present armada, our current epoch might become known as the time when humanity first probed its own star system. Sometimes widely separated spacecraft act together as an InterPlanetary Network to determine the direction of distant explosions by noting when each probe detects high energy photons. Future spacecraft milestones, as indicated along the bottom of the graphic, include Dawn reaching Ceres, the largest object in the asteroid belt, and New Horizons reaching Pluto, both in 2015.    Follow APOD in: Arabic, Bahasa Indonesian, Catalan, Chinese, Chinese, Czech, Dutch, Farsi, Farsi, Galego, German, French, Hebrew, Japanese, Polish, Portuguese, Romanian, Russian, Slovenian, Spanish, Spanish, or Turkish,
Omega Centauri: The Brightest Globular Cluster (2013-05-01)

This huge ball of stars predates our Sun. Long before humankind evolved, before dinosaurs roamed, and even before our Earth existed, ancient globs of stars condensed and orbited a young Milky Way Galaxy. Of the 200 or so globular clusters that survive today, Omega Centauri is the largest, containing over ten million stars. Omega Centauri is also the brightest globular cluster, at apparent visual magnitude 3.9 it is visible to southern observers with the unaided eye. Cataloged as NGC 5139, Omega Centauri is about 18,000 light-years away and 150 light-years in diameter.  Unlike many other globular clusters, the stars in Omega Centauri show several different ages and trace chemical abundances, indicating that the globular star cluster has a complex history over its 12 billion year age.

Image credit: Joaquin Polleri & Ezequiel Etcheverry (Observatorio Paname�o en San Pedro de Atacama)
Saturn Hurricane (2013-05-02)

Acquiring its first sunlit views of far northern Saturn late last year, the Cassini spacecraft's narrow-angle camera recorded this stunning image of the vortex at the ringed planet's north pole. The false color, near-infrared image results in red hues for low clouds and green for high ones, causing the north-polar hurricane to take on the appearance of a rose. Enormous by terrestrial hurricane standards, this storm's eye is about 2,000 kilometers wide, with clouds at the outer edge traveling at over 500 kilometers per hour. The north pole Saturn hurricane swirls inside the large, six-sided weather pattern known as the hexagon. Of course, in 2006 Cassini also imaged the hurricane at Saturn's south pole.
Horsehead: A Wider View (2013-05-03)

Combined image data from the massive, ground-based VISTA telescope and the Hubble Space Telescope was used to create this wide perspective of the interstellar landscape surrounding the famous Horsehead Nebula. Captured at near-infrared wavelengths, the region's dusty molecular cloud sprawls across the scene that covers an angle about two-thirds the size of the Full Moon on the sky. Left to right the frame spans just over 10 light-years at the Horsehead's estimated distance of 1,600 light-years. Also known as Barnard 33, the still recognizable Horsehead Nebula stands at the upper right, the near-infrared glow of a dusty pillar topped with newborn stars. Below and left, the bright reflection nebula NGC 2023 is itself the illuminated environs of a hot young star. Obscuring clouds below the base of the Horsehead and on the outskirts of NGC 2023 show the tell-tale far red emission of energetic jets, known as Herbig-Haro objects, also associated with newborn stars.
Hungarian Spring Eclipse (2013-05-04)

Last week, as the Sun set a Full Moon rose over the springtime landscape of Tihany, Hungary on the northern shores of Lake Balaton. As it climbed into the clear sky, the Moon just grazed the dark, umbral shadow of planet Earth in the year's first partial lunar eclipse. The partial phase, seen near the top of this frame where the lunar disk is darkened along the upper limb, lasted for less than 27 minutes.  Composited from consecutive exposures, the picture presents the scene's range of natural colors and subtle shading apparent to the eye. At next week's New Moon, the season's celestial shadow play will continue with an annular solar eclipse, the path of annularity tracking through northern Australia and the central Pacific.

Image credit: Tamas Ladanyi
A Supercell Thunderstorm Cloud Over Montana (2013-05-05)

Is that a spaceship or a cloud? Although it may seem like an alien mothership, it's actually a impressive thunderstorm cloud called a supercell. Such colossal storm systems center on mesocyclones -- rotating updrafts that can span several kilometers and deliver torrential rain and high winds including tornadoes.  Jagged sculptured clouds adorn the supercell's edge, while wind swept dust and rain dominate the center.  A tree waits patiently in the foreground.    The above supercell cloud was photographed in July west of Glasgow, Montana, USA, caused minor damage, and lasted several hours before moving on.   Follow APOD on: Facebook (Daily) (Sky) (Spanish) or Google Plus (Daily) (River)

Image credit: Sean R. Heavey
Tails of Comet Lemmon (2013-05-06)

What caused the interestingly intricate tails that Comet Lemmon displayed earlier this year? First of all, just about every comet that nears the Sun displays two tails: a dust tail and an ion tail. Comet Lemmon's dust tail, visible above and around the comet nucleus in off-white, is produced by sun-light reflecting dust shed by the comet's heated nucleus.  Flowing and more sculptured, however, is C/2012 F6 (Lemmon)'s blue ion tail, created by the solar wind pushing ions expelled by the nucleus away from the Sun. Also of note is the coma seen surrounding Comet Lemmon's nucleus, tinted green by diatomic C2 gas fluorescing in sunlight. The above image was taken from the dark skies of Namibia in mid-April.  Comet Lemmon is fading as it now heads back to the outer Solar System.   Cosmonova: APOD editor to speak in Sweden on May 28

Image credit: Gerald Rhemann
Galaxy Cove Vista (2013-05-07)

To see a vista like this takes patience, hiking, and a camera. Patience was needed in searching out just the right place and waiting for just the right time.  A short hike was needed to reach this rugged perch above a secluded cove in Julia Pfeiffer Burns State Park in California, USA. And a camera was needed for the long exposure required to bring out the faint light from stars and nebulae in the background Milky Way galaxy. Moonlight and a brief artificial flash illuminated the hidden beach and inlet behind nearby trees in the above composite image taken about two weeks ago. Usually obscured McWay Falls is visible just below the image center, while the Pacific Ocean is in view to its right.

Image credit: Rogelio Bernal Andreo
(Deep Sky Colors)
Earth's Major Telescopes Investigate GRB 130427A (2013-05-08)

A tremendous explosion has occurred in the nearby universe and major telescopes across Earth and space are investigating. Dubbed GRB 130427A, the gamma-ray burst was first detected by the Earth-orbiting Fermi and Swift satellites observing at high energies and quickly reported down to Earth.  Within three minutes, the half-meter ISON telescope in New Mexico found the blast in visible light, noted its extreme brightness, and relayed more exact coordinates.  Within the next few minutes, the bright optical counterpart was being tracked by several quickly re-pointable telescopes including the 2.0-meter P60 telescope in California, the 1.3-meter  PAIRITEL telescope in Arizona, and the 2.0-meter Faulkes Telescope North in Hawaii. Within two hours, the 8.2-meter Gemini North telescope in Hawaii noted a redshift of 0.34, placing the explosion about 5 billion light years away -- considered nearby in cosmological terms. Previously recorded images from the RAPTOR full-sky monitors were scanned and a very bright optical counterpart -- magnitude 7.4 -- was found 50 seconds before the Swift trigger. The brightest burst in recent years, a signal from GRB 130427A has also been found in low energy radio waves by the Very Large Array (VLA) and at the highest energies ever recorded by the Fermi satellite. Neutrino, gravitational wave, and telescopes designed to detect only extremely high energy photons are checking their data for a GRB 130427A signal. Pictured in the above animation, the entire gamma-ray sky is shown becoming momentarily dominated by the intense glow of GRB 130427A. Continued tracking the optical counterpart will surely be ongoing as there is a possibility that the glow of a classic supernova will soon emerge.