juliensimon/nasa-apod huggingface dataset:
Snake in the Dark (2009-02-20)

Dark nebulae snake across a gorgeous expanse of stars in this telescopic view toward the pronounceable constellation Ophiuchus and the center of our Milky Way Galaxy. In fact, the twisting central shape seen here is well known as the Snake Nebula. It is also listed as Barnard 72 (B72), one of 182 dark markings of the sky cataloged in the early 20th century by astronomer E. E. Barnard. Unlike bright emission nebulae and star clusters, Barnard's nebulae are interstellar dark clouds of obscuring gas and dust. Their shapes are visible in cosmic silhouette because they lie in the foreground along the line of sight to rich star fields and glowing stellar nurseries near the plane of our Galaxy. Many of Barnard's dark nebulae are themselves likely sites of future star formation. Barnard 72 is about 650 light years away. With bluish star 44 Ophiuchi at bottom left, the intriguing star field spans nearly 2 degrees or almost 20 light-years at the estimated distance of the Snake Nebula.

Image credit: Stéphane Guisard
The Swift View of Comet Lulin (2009-02-21)

Now growing brighter, Comet Lulin is headed for its closest approach to planet Earth early next week. But the comet's greenish glow, familiar to earthbound skygazers, is replaced by false colors in this premier view from the orbiting Swift satellite. Image data from the Swift detectors, normally intended to follow cosmic gamma-ray bursts, were recorded on January 28. The data are combined here, along with a sky survey image of background stars, to show optical and ultraviolet light in green-blue hues and x-rays from the comet in red.  The result maps remarkable x-ray emission on the comet's sunward side as incoming solar wind ions interact with gases in the swollen coma. It also shows substantial ultraviolet emission opposite the Sun, in the direction of motion and the comet's tail. The ultraviolet emission is from the OH molecule derived from the breakup of water, an indicator of the copius amounts of water produced by this extremely active comet. In fact, astronomers estimate Lulin was releasing about 800 gallons of water each second, enough to fill an Olympic-size swimming pool in less than 15 minutes.
Orion Nebula: The Hubble View (2009-02-22)

Few cosmic vistas excite the imagination like the Orion Nebula. Also known as M42, the nebula's glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1,500 light-years away. The Orion Nebula offers one of the best opportunities to study how stars are born partly because it is the nearest large star-forming region, but also because the nebula's energetic stars have blown away obscuring gas and dust clouds that would otherwise block our view - providing an intimate look at a range of ongoing stages of starbirth and evolution.  This detailed image of the Orion Nebula is the sharpest ever, constructed using data from the Hubble Space Telescope's Advanced Camera for Surveys and the European Southern Observatory's La Silla 2.2 meter telescope. The mosaic contains a billion pixels at full resolution and reveals about 3,000 stars.
An Etruscan Vase Moon Rising (2009-02-23)

What's happened to the Moon?  Nothing, although from some locations, February's full moon, which occurred about two weeks ago, appeared strangely distorted as it rose.   Visible in particular was a curiously inverted image section pinched off near the horizon, an effect dubbed the Etruscan vase by the pioneering science fiction writer Jules Verne for its familiar shape.  This odd moon image piece was created by moonlight refracting through an atmospheric inversion layer on Earth where cold air was trapped near the surface. The photographer also reported that, as the moon rose, a red rim was faintly visible on the lower part of the moon, while a green rim appeared on the top.  Similar to the Sun's famous green flash, these effects arise when the Earth's atmosphere acts like a prism, sending different colors of light on slightly different paths.  The above image mosaic has been horizontally compressed by computer to fit a standard screen.

Image credit: John Stetson
Barnard's Loop around the Horsehead Nebula (2009-02-24)

Why is the Horsehead Nebula surrounded by a bubble?  Although hard to make out, the famous Horsehead Nebula is the slight dark indentation in the bright streak just to the left of the image center.  Glowing like an emission nebula, the origin of the bubble, known as Barnard's Loop, is currently unknown.  Progenitor hypotheses include the winds from bright Orion stars and the supernovas of stars long gone.  Barnard's Loop is too faint to be identified with the unaided eye.  The nebula was discovered only in 1895 by E. E. Barnard on long duration film exposures.  The above image was taken in a single  specific color emitted by hydrogen to bring out detail. To the left of the Horsehead Nebula, visible as a small dark indentation, is the photogenic Flame Nebula.

Image credit: Drew Sullivan 
(Ancient Starlight)
Two Tails of Comet Lulin (2009-02-25)

Go outside tonight and see Comet Lulin.  From a dark location, you should need only a good star map and admirable perseverance -- although wide-field binoculars might help.  Yesterday, Comet Lulin passed its closest to Earth, so that the comet will remain near its brightest over the next few days.  The comet is currently almost 180 degrees around from the Sun and  so visible nearly all night long, but will appear to move on the sky about 10 full moons a night.  In this image, Comet Lulin was captured in spectacular form two nights ago from New Mexico, USA. The central coma of the comet is appearing quite green, a color likely indicating glowing molecular carbon gasses. Bright stars and a distant spiral galaxy are clearly visible in the image background.  The yellow dust tail, reflecting sunlight, is visible sprawling to the coma's left trailing behind the comet, while the textured bluish-glowing ion tail is visible to the coma's right, pointing away from the Sun.  Over the past few weeks, from the current vantage point of Earth, these two tails appeared to point in opposite directions.  Comet Lulin is expected to slowly fade over the next few weeks.

Image credit: Richard Richins 
(NMSU)
Moon, Mercury, Jupiter, Mars (2009-02-26)

When the Moon rose in predawn skies on February 23rd, it sported a sunlit crescent. It also offered early morning risers a tantalizing view of earthshine, the dark portion of the lunar disk illuminated by sunlight reflected from the Earth. Of course, on that morning a remarkable conjunction with three wandering planets added an impressive touch to the celestial scene. Recorded just before sunrise, this serene skyview looks east toward a glowing horizon across Tuggerah Lake on the Central Coast of New South Wales, Australia. Along with the waning crescent Moon, the picture captures (top to bottom) bright Mercury, Jupiter, and Mars.

Image credit: Mike Salway
Lulin and Saturn near Opposition (2009-02-27)

Tracking through the constellation Leo on February 23rd, bright planet Saturn and Comet Lulin were both near opposition -- opposite the Sun in planet Earth's sky. They also passed within only 2 degrees of each other creating a dramatic celestial photo-op. Comet Lulin was near its closest approach to planet Earth at the time, at a distance of some 61 million kilometers, but was orbiting in the opposite direction. As a result it swept remarkably rapidly across the background of stars. This telephoto image captures both bright Saturn and greenish Lulin in the same field in a scene not too different from binocular views. Don't recognize ringed Saturn? The rings are presently tilted nearly edge-on to our view and the brighter planet is overexposed to record details of the fainter comet. At the upper right, Saturn is marked by multiple diffraction spikes created by the aperture blades in the telephoto lens.

Image credit: Catching the Light
NGC 2442: Galaxy in Volans (2009-02-28)

Distorted galaxy NGC 2442 can be found in the southern constellation of the flying fish, (Piscis) Volans. Located about 50 million light-years away, the galaxy's two spiral arms extending from a pronounced central bar give it a hook-shaped appearance. This deep color image also shows the arms' obscuring dust lanes, young blue star clusters and reddish star forming regions surrounding a core of yellowish light from an older population of stars. But the star forming regions seem more concentrated along the drawn-out (right side) spiral arm. The distorted structure is likely the result of an ancient close encounter with the smaller galaxy seen near the top left of this field of view. The two interacting galaxies are separated by about 150,000 light-years at the estimated distance of NGC 2442.

Image credit: Martin Pugh
Omega Centauri: The Largest Nearby Globular Cluster (2009-03-01)

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: Thomas V. Davis
Earthgrazer: The Great Daylight Fireball of 1972 (2009-03-02)

What is that streaking across the sky? A bright earthgrazing meteor.  In 1972, an unusually bright meteor from space was witnessed bouncing off Earth's atmosphere, much like a skipping stone can bounce off of a calm lake.  The impressive event lasted several seconds, was visible in daylight, and reportedly visible all the way from Utah, USA to Alberta, Canada.  Pictured above, the fireball was photographed streaking above Teton mountains behind Jackson Lake, Wyoming, USA.  The Great Daylight Fireball of 1972 was possibly the size of a small truck, and would likely have created an impressive airburst were it to have struck Earth more directly.   Earthgrazing meteors are rare but are more commonly seen when the radiant of a meteor shower is just rising or setting.  At that time, meteors closer to the Earth than earthgrazers would more usually strike the Earth near the horizon, while meteors further than earthgrazers would miss the Earth entirely.

Image credit: James M. Baker
The Helix Nebula from La Silla Observatory (2009-03-03)

Will our Sun look like this one day?  The Helix Nebula is one of brightest and closest examples of a planetary nebula, a gas cloud created at the end of the life of a Sun-like star.  The outer gasses of the star expelled into space appear from our vantage point as if we are looking down a helix.  The remnant central stellar core, destined to become a white dwarf star, glows in light so energetic it causes the previously expelled gas to fluoresce.  The Helix Nebula, given a technical designation of NGC 7293, lies about 700 light-years away towards the constellation of Aquarius and spans about 2.5 light-years.  The above picture was taken by the Wide Field Imager on the 2.2-meter Telescope at the European Southern Observatory's La Silla Observatory. A close-up of the inner edge of the Helix Nebula shows complex gas knots of unknown origin.
Saturn in View (2009-03-04)

Very good telescopic views of Saturn can be expected in the coming days as the ringed planet nears opposition on March 8th, its closest approach to Earth in 2009. Of course, opposition means opposite the Sun in planet Earth's sky - an arrangement that occurs almost yearly for Saturn. But while Saturn itself grows larger in telescopic images, Saturn's rings seem to be vanishing as their tilt to our line-of-sight decreases. In fact, the rings will be nearly invisible, edge-on from our perspective, by September 4. Recorded on February 28, this sharp image was made with the 1 meter telescope at Pic Du Midi, a mountain top observatory in the French Pyrenees. The rings are seen to be tilted nearly edge-on, but remarkable details are visible in the gas giant's cloud bands. The icy moon Tethys appears just beyond the rings at the lower left.

Image credit: Jean-Luc Dauvergne
IC 5146: The Cocoon Nebula (2009-03-05)

Inside the Cocoon Nebula is a newly developing cluster of stars. Cataloged as IC 5146, the beautiful nebula is nearly 15 light-years wide, located some 4,000 light years away toward the northern constellation Cygnus. Like other star forming regions, it stands out in red, glowing, hydrogen gas excited by young, hot stars and blue, dust-reflected starlight at the edge of an otherwise invisible molecular cloud. In fact, the bright star near the center of this nebula is likely only a few hundred thousand years old, powering the nebular glow as it clears out a cavity in the molecular cloud's star forming dust and gas. This exceptionally deep color view of the Cocoon Nebula traces tantalizing features within and surrounding the dusty stellar nursery.

Image credit: Neil Fleming
Crescent Moon and Venus (2009-03-06)

Last Friday, the Moon and Venus shared the early evening sky in a beautiful conjunction. Separated by only about 2 degrees, they also were both in a crescent phase.  Just like our Moon, Venus can appear as a full disk or a thin crescent.  Frequently the brightest object in the post-sunset or pre-sunrise sky, Venus is so small that it usually requires binoculars or a small telescope to clearly see its phase.  This telescopic image of Friday's conjunction shows off the similar crescent phases, with the tiny crescent Venus at the upper right.

Image credit: Drew Sullivan
(Ancient Starlight)
Comet Lulin and Distant Galaxies (2009-03-07)

Now fading in our night sky, Comet Lulin has provided some lovely cosmic vistas. Moving rapidly against the background of stars, Lulin briefly posed with the likes of Saturn, and Regulus (Alpha Leo). But here it is seen against a field of distant galaxies. To reveal the faint background galaxies and trace the comet's fading tail, the remarkable picture is a blended composite of telescopic exposures aligned with the both the stars and the speedy comet. The largest galaxies seen left of the comet's head or coma are cataloged as NGC 3016, NGC 3019, NGC 3020 and NGC 3024 and lie at a distance of 100 million light-years or so. When the exposures were made, on February 28, the comet was about 3.6 light-minutes from Earth.

Image credit: Panther Observatory
Gibbous Europa (2009-03-08)

Although the phase of this moon might appear familiar, the moon itself might not. In fact, this gibbous phase shows part of Jupiter's moon Europa. The robot spacecraft Galileo captured this image mosaic during its mission orbiting Jupiter from 1995 - 2003. Visible are plains of bright ice, cracks that run to the horizon, and dark patches that likely contain both ice and dirt. Raised terrain is particularly apparent near the terminator, where it casts shadows. Europa is nearly the same size as Earth's Moon, but much smoother, showing few highlands or large impact craters. Evidence and images from the Galileo spacecraft, indicated that liquid oceans might exist below the icy surface. To test speculation that these seas hold life, ESA and NASA have together started preliminary development of the Europa Jupiter System Mission , a spacecraft proposed to better study Europa. If the surface ice is thin enough, a future mission might drop hydrobots to burrow into the oceans and search for life.
Kepler's Streak (2009-03-09)

Streaking skyward, a Delta II rocket carries NASA's Kepler spacecraft aloft into the clear night of March 6. The dramatic scene was recorded in a time exposure from the crowded pier in Jetty Park at the northern end of Cocoa Beach, Florida, about 3 miles from the Cape Canaveral launch site. Kepler's mission is to search for Earth-like planets orbiting in the habitable zone of other stars. A planet orbiting within a star's habitable zone would have a surface temperature capable of supporting liquid water, an essential ingredient for life as we know it. To find Earth-like planets, Kepler's telescope and large, sensitive camera will examine a rich star field near the plane of our Milky Way Galaxy. Located in the constellation Cygnus, Kepler's field of view will allow it to monitor the brightness of many stars in the solar neighborhood and detect a slight dimming as a potential Earth-like planet crosses in front of the star.

Image credit: Ben Cooper
Horsehead and Orion Nebulae (2009-03-10)

Adrift 1,500 light-years away in one of the night sky's most recognizable constellations, the glowing Orion Nebula and the dark Horsehead Nebula are contrasting cosmic vistas. They appear in opposite corners of this stunning mosaic taken with a digital camera attached to a small telescope. The magnificent emission region, the Orion Nebula (aka M42), lies at the upper right of the picture. Immediately to its left is a prominent bluish reflection nebula sometimes called the Running Man. The Horsehead nebula appears as a dark cloud, a small silhouette notched against the long red glow at the lower left. Alnitak is the easternmost star in Orion's belt and is seen as the brightest star to the left of the Horsehead. Below Alnitak is the Flame Nebula, with clouds of bright emission and dramatic dark dust lanes.  Pervasive tendrils of glowing hydrogen gas are easily traced throughout the region in this deep field image of the same region.

Image credit: Dale J. Martin
(Massapoag Pond Obs.)
Lunar X (2009-03-11)

The striking X near the center of this lunarscape is easily visible in binoculars or a small telescope. Yet, not too many have seen it. The catch is, this lunar X is only apparent during a four hour period just before the Moon's first quarter phase. At the terminator, or shadow line between lunar day and night, the X illusion is produced by a configuration of the craters Blanchinus, La Caille and Purbach. Near the Moon's first quarter phase, an astronaut standing close to the craters' position would see the slowly rising Sun very near the horizon. Temporarily, the crater walls would be in sunlight while the crater floors were still in darkness. Seen from planet Earth, contrasting sections of bright walls against the dark floors by chance look remarkably like an X. This sharp image of the Lunar X was captured at approximately 11:59 UT on March 3, 2009. The Moon's first quarter phase was at 7:46 UT on March 4.

Image credit: Catching the Light
Thor's Helmet (NGC 2359) and Planetary Nebula (2009-03-12)

At the right, Thor's Helmet (NGC 2359) seems to gaze across a lovely star field. The broad skyscape itself covers about 1.5 degrees or 3 full moons toward the constellation Canis Major. A close look at the lower left corner of the image might identify the object of the cosmic stare as a faint, round nebula. Heroically sized even for a Norse god, Thor's Helmet is about 30 light-years across. The helmet is actually more like an interstellar bubble, blown as a fast wind from the bright, massive star near the bubble's center sweeps through a surrounding molecular cloud. 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. In contrast, the faint, round nebula is a planetary nebula, the gaseous shroud of a dying lower mass star. The distance to Thor's Helmet is estimated to be about 15,000 light-years.

Image credit: Rogelio Bernal Andreo
Hickson Compact Group 90 (2009-03-13)

Scanning the skies for galaxies, Canadian astronomer Paul Hickson and colleagues identified some 100 compact groups of galaxies, now appropriately called Hickson Compact Groups (HCGs). This sharp Hubble image shows one such galaxy group, HCG 90, in startling detail. Three galaxies are revealed to be strongly interacting: a dusty spiral galaxy stretched and distorted between a pair of large elliptical galaxies. The close encounter will trigger furious star formation. On a cosmic timescale, the gravitational tug of war will eventually result in the merger of the trio into a large single galaxy. The merger process is now understood to be a normal part of the evolution of galaxies, including our own Milky Way. HCG 90 lies about 100 million light-years away in the constellation Piscis Austrinus. This Hubble view spans about 80,000 light-years at that estimated distance. Of course, Hickson Compact Groups also make for rewarding viewing for Earth-bound astronomers with more modest sized telescopes.
Haute-Provence Star Trails (2009-03-14)

Fix your camera to a tripod and you can record the graceful trails traced by the stars as planet Earth rotates on its axis. For example, this dramatic 5 hour long exposure was made on February 24 from Haute-Provence Observatory (OHP) in southeastern France. Actually a composite of 300 consecutive 1-minute exposures, the image nicely shows stars near the celestial equator tracing nearly straight lines in projection, while stars north and south of the equator, respectively, appear to circle the north and south celestial poles. Domes at the bottom left and right house the OHP telescopes. Brilliant planet Venus makes the short bright trail at the lower right, while trails of stars in the constellation Orion end near the lower right observatory dome. Sirius, alpha star of Canis Major, traces the bright arc over the dome at the left. Astronomer Alexandre Santerne also briefly illuminated a foreground oak tree during the exposure sequence.

Image credit: Alexandre Santerne
A Prominent Solar Prominence from SOHO (2009-03-15)

What's happened to our Sun?  It was sporting a spectacular -- but not very unusual -- solar prominence. A solar prominence is a cloud of solar gas held above the Sun's surface by the Sun's magnetic field.  In 2004, NASA's Sun-orbiting SOHO spacecraft imaged an impressively large prominence hovering over the surface, pictured above.  The Earth would easily fit under the hovering curtain of hot gas.  A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the  Solar System.  Although somehow related to the Sun's changing magnetic field, the energy mechanism that creates and sustains a Solar prominence is still a topic of research.
Martian Moon Deimos from MRO (2009-03-16)

Mars has two tiny moons, Phobos and Deimos.  Pictured above, in a recently release image by HiRISE camera onboard the Mars-orbiting Mars Reconnaissance Orbiter (MRO), is Deimos, the smaller moon of Mars.  Deimos is one of the smallest known moons in the Solar System measuring only about 15 kilometers across.  The diminutive Martian moon was discovered in 1877 by Asaph Hall, an American astronomer working at the US Naval Observatory in Washington D.C.  The existence of two Martian moons was predicted around 1610 by Johannes Kepler, the astronomer who derived the laws of planetary motion.  In this case, Kepler's prediction was not based on scientific principles, but his writings and ideas were so influential that the two Martian moons are discussed in works of fiction such as Jonathan Swift's Gulliver's Travels, written in 1726, over 150 years before their actual discovery.