juliensimon/nasa-apod huggingface dataset:
Ice Halos at Yellowknife (2018-09-14)

You've probably seen a circle around the Sun before. More common than rainbows, ice halos, like a 22 degree circular halo for example, can be easy to spot, especially if you can shade your eyes from direct sunlight. Still it's rare to see such a diverse range of ice halos, including sundogs, tangent, infralateral, and Parry arcs, all found in this snapshot from planet Earth. The picture was quickly taken in the late morning of September 4 from Yellowknife, Northwest Territories, Canada. The beautiful patterns are generated as sunlight (or moonlight) is reflected and refracted in six-sided water ice crystals in Earth's atmosphere. Of course, atmospheric ice halos in the skies of other worlds are likely to be different.

Image credit: Stephen Bedingfield
Mont Blanc, Meteor, and Milky Way (2018-09-15)

Snowy Mont Blanc is near the center of this atmospheric night skyscape. But high, thin clouds fogged the skies at the photographer's location, looking south toward Europe's highest peak from the southern Swiss Alps. Still, the 13 second exposure finds the faint star fields and dark rifts of the Milky Way above the famous white mountain. Bloated by the mist, bright planet Saturn and Antares (right), alpha star of Scorpius, shine through the clouds to flank the galaxy's central bulge. The high-altitude scene is from the rewarding night of August 12/13, so it also includes the green trail of a Perseid meteor shooting along the galactic plane.

Image credit: Adrien Mauduit
A Solar Filament Erupts (2018-09-16)

What's happened to our Sun?  Nothing very unusual -- it just threw a filament. Toward the middle of 2012, a long standing solar filament suddenly erupted into space producing an energetic Coronal Mass Ejection (CME).  The filament had been held up for days by the Sun's ever changing magnetic field and the timing of the eruption was unexpected. Watched closely by the Sun-orbiting Solar Dynamics Observatory, the resulting explosion shot electrons and ions into the Solar System, some of which arrived at Earth three days later and impacted Earth's magnetosphere, causing visible aurorae. Loops of plasma surrounding an active region can be seen above the erupting filament in the featured ultraviolet image.  Although the Sun is now in a relatively inactive state of its 11-year cycle,  unexpected holes have opened in the Sun's corona allowing an excess of charged particles to stream into space. As before, these charged particles are creating auroras.
Cosmic Collision Forges Galactic Ring (2018-09-17)

How could a galaxy become shaped like a ring?  The rim of the blue galaxy pictured on the right is an immense ring-like structure 150,000 light years in diameter composed of newly formed, extremely bright, massive stars.  That galaxy, AM 0644-741, is known as a ring galaxy and was caused by an immense galaxy collision.  When galaxies collide, they pass through each other -- their individual stars rarely come into contact.  The ring-like shape is the result of the gravitational disruption caused by an entire small intruder galaxy passing through a large one.  When this happens, interstellar gas and dust become condensed, causing a wave of star formation to move out from the impact point like a ripple across the surface of a pond.  The likely intruder galaxy is on the left of this combined image from Hubble (visible) and Chandra (X-ray) space telescopes.  X-ray light is shown in pink and depicts places where energetic black holes or neutron stars, likely formed shortly after the galaxy collision, reside.   Open Science: Browse 1,800+ codes in the Astrophysics Source Code Library
Salt, Pepper, and Ice (2018-09-18)

There's a "camera" comet now moving across the sky. Just a bit too dim to see with the unaided eye, Comet 21P / Giacobini-Zinner has developed a long tail that makes it a good sight for binoculars and sensitive cameras. The movement of the Comet 21P on the sky was captured last week in the featured time-lapse video compressing 90 minutes into about 2.5 seconds. What might seem odd is that the 21P's tail is not following the comet's movement. This is because comet tails always point away from the Sun, and the comet was not moving toward the Sun during the period photographed. Visible far in the background on the upper left is the Salt & Pepper star cluster, M37, while the bright red star V440 Auriga is visible just about the frame's center. This 2-km ball of dust-shedding ice passed its nearest to the Sun and Earth only last week and is now fading as it crosses into southern skies.  Comet 21P should remain visible, however, and photogenic to stabilized cameras, for another month or so.

Image credit: Maroun Habib (Moophz)
Cocoon Nebula Deep Field (2018-09-19)

Inside the Cocoon Nebula is a newly developing cluster of stars. The cosmic Cocoon on the upper right also punctuates a long trail of obscuring interstellar dust clouds to its left.  Cataloged as IC 5146, the beautiful nebula is nearly 15 light-years wide, located some 3,300 light years away toward the northern constellation of the Swan (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 a nearly 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 slowly 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.   Follow APOD on: Facebook,  Google Plus,  Instagram, or Twitter

Image credit: Marcel Drechsler 
(Baerenstein Obs.)
Stars and Dust in Corona Australis (2018-09-20)

Cosmic dust clouds and young, energetic stars inhabit this telescopic vista, less than 500 light-years away toward the northern boundary of Corona Australis, the Southern Crown. The dust clouds effectively block light from more distant background stars in the Milky Way. But the striking complex of reflection nebulae cataloged as NGC 6726, 6727, and IC 4812 produce a characteristic color as blue light from the region's young, hot stars is reflected by the cosmic dust. The dust also obscures from view stars still in the process of formation. At top right, smaller yellowish nebula NGC 6729 bends around young variable star R Coronae Australis. Near it, glowing arcs and loops shocked by outflows from embedded newborn stars are identified as Herbig-Haro objects. On the sky this field of view spans about 1 degree. That corresponds to almost 9 light-years at the estimated distance of the nearby star forming region.

Image credit: Josep Drudis
Irregular Galaxy NGC 55 (2018-09-21)

Irregular galaxy NGC 55 is thought to be similar to the Large Magellanic Cloud (LMC). But while the LMC is about 180,000 light-years away and is a well known satellite of our own Milky Way Galaxy, NGC 55 is more like 6 million light-years distant and is a member of the Sculptor Galaxy Group. Classified as an irregular galaxy, in deep exposures the LMC itself resembles a barred disk galaxy. Spanning about 50,000 light-years, NGC 55 is seen nearly edge-on though, presenting a flattened, narrow profile in contrast with our face-on view of the LMC. Just as large star forming regions create emission nebulae in the LMC, NGC 55 is also seen to be producing new stars. This highly detailed galaxy portrait highlights a bright core crossed with dust clouds, telltale pinkish star forming regions, and young blue star clusters in NGC 55.

Image credit: Martin Pugh
Window Seat over Hudson Bay (2018-09-22)

On the August 18 night flight from San Francisco to Zurich, a window seat offered this tantalizing view when curtains of light draped a colorful glow across the sky over Hudson Bay. Constructed by digitally stacking six short exposures made with a hand held camera, the scene records the shimmering aurora borealis or northern lights just as the approaching high altitude sunrise illuminated the northeastern horizon. It also caught the flash of a Perseid meteor streaking beneath the handle stars of the Big Dipper of the north. A few days past the meteor shower's peak, its trail still points across the sky toward Perseus. Beautiful aurorae and shower meteors both occur in Earth's upper atmosphere at altitudes of 100 kilometers or so, far above commercial airline flights. The aurora are caused by energetic charged particles from the magnetosphere, while meteors are trails of comet dust.

Image credit: Ralf Rohner
Equinox: Analemma over the Callanish Stones (2018-09-23)

Does the Sun return to the same spot on the sky every day at the same time?  No.  A more visual answer to that question is an analemma, a composite image taken from the same spot at the same time over the course of a year.  The featured analemma was composed from images taken every few days at noon near the village of Callanish in the Outer Hebrides in Scotland, UK.  In the foreground are the Callanish Stones, a stone circle built around 2700 BC during humanity's Bronze Age.  It is not known if the placement of the Callanish Stones has or had astronomical significance.  The ultimate causes for the figure-8 shape of this and all analemmas are the tilt of the Earth axis and the ellipticity of the Earth's orbit around the Sun.  At the solstices, the Sun will appear at the top or bottom of an analemma. Equinoxes, however, correspond to analemma middle points -- not the intersection point. Today at 1:54 am (UT) is the equinox ("equal night"), when day and night are equal over all of planet Earth. Many cultures celebrate a change of season at an equinox.

Image credit: Giuseppe Petricca
Rover 1A Hops on Asteroid Ryugu (2018-09-24)

Two small robots have begun hopping around the surface of asteroid Ryugu. The rovers, each the size of a small frying pan, move around the low gravity of kilometer-sized 162173 Ryugu by hopping, staying aloft for about 15 minutes and typically landing again several meters away.  On Saturday, Rover 1A returned an early picture of its new home world, on the left, during one of its first hops. On Friday, lander MINERVA-II-1 detached from its mothership Hayabusa2, dropped Rovers 1A and 1B, and then landed on Ryugu. Studying Ryugu could tell humanity not only about Ryugu's surface and interior, but about what materials were available in the early Solar System for the development of life.  Two more hopping rovers are planned for release, and Hayabusa2 itself is scheduled to collect a surface sample from Ryugu and return it to Earth for detailed analysis before 2021.

Image credit: ISAS, 
JAXA, 
Hayabusa2 Mission
Highlights of the North Autumn Sky (2018-09-25)

What can you see in the night sky this season?  The featured graphic gives a few highlights for Earth's northern hemisphere. Viewed as a clock face centered at the bottom, early (northern) autumn sky events fan out toward the left, while late autumn events are projected toward the right. Objects relatively close to Earth are illustrated, in general, as nearer to the cartoon figure with the telescope at the bottom center -- although almost everything pictured can be seen without a telescope.  As happens during any season, constellations appear the same year to year, and, as usual, the Leonids meteor shower will peak in mid-November.  Also as usual, the International Space Station (ISS) can be seen, at times, as a bright spot drifting across the sky after sunset.  Planets visible after sunset this autumn include Jupiter and Mars, and during late autumn, Saturn.

Image credit: Universe2go.com
The Sun's Spectrum with its Missing Colors (2018-09-26)

It is still not known why the Sun's light is missing some colors. Here are all the visible colors of the Sun, produced by passing the Sun's light through a prism-like device.  The spectrum was created at the McMath-Pierce Solar Observatory and shows, first off, that although our white-appearing Sun emits light of nearly every color, it does indeed appear brightest in yellow-green light.  The dark patches in the above spectrum arise from gas at or above the Sun's surface absorbing sunlight emitted below.  Since different types of gas absorb different colors of light, it is possible to determine what gasses compose the Sun.  Helium, for example, was first discovered in 1870 on a solar spectrum and only later found here on Earth.  Today, the majority of spectral absorption lines have been identified - but not all.
M33: Triangulum Galaxy (2018-09-27)

The small, northern constellation Triangulum harbors this magnificent face-on spiral galaxy, M33. Its popular names include the Pinwheel Galaxy or just the Triangulum Galaxy. M33 is over 50,000 light-years in diameter, third largest in the Local Group of galaxies after the Andromeda Galaxy (M31), and our own Milky Way. About 3 million light-years from the Milky Way, M33 is itself thought to be a satellite of the Andromeda Galaxy and astronomers in these two galaxies would likely have spectacular views of each other's grand spiral star systems. As for the view from planet Earth, this sharp image shows off M33's blue star clusters and pinkish star forming regions along the galaxy's loosely wound spiral arms. In fact, the cavernous NGC 604 is the brightest star forming region, seen here at about the 7 o'clock position from the galaxy center. Like M31, M33's population of well-measured variable stars have helped make this nearby spiral a cosmic yardstick for establishing the distance scale of the Universe.

Image credit: Christoph Kaltseis
The Light, the Dark, and the Dusty (2018-09-28)

This colorful skyscape spans about two full moons across nebula rich starfields along the plane of our Milky Way Galaxy in the royal northern constellation Cepheus. Near the edge of the region's massive molecular cloud some 2,400 light-years away, bright reddish emission region Sharpless (Sh) 155 is below and right of center, also known as the Cave Nebula. About 10 light-years across the cosmic cave's bright walls of gas are ionized by ultraviolet light from the hot young stars around it. Dusty blue reflection nebulae, like vdB 155 at upper left, and dense obscuring clouds of dust also abound on the interstellar canvas. Astronomical explorations have revealed other dramatic signs of star formation, including the bright red fleck of Herbig-Haro (HH) 168. Near top center in the frame, the Herbig-Haro object emission is generated by energetic jets from a newborn star.

Image credit: Tasos Liampos
55 Nights with Saturn (2018-09-29)

For 55 consecutive nights Mediterranean skies were at least partly clear this summer, from the 1st of July to the 24th of August 2018. An exposure from each night was incorporated in this composited telephoto and telescopic image to follow bright planet Saturn as it wandered through the generous evening skies. Through August, the outer planet's seasonal apparent retrograde motion slowed and drifted to the right, framed by a starry background. That brought it near the line-of-sight to the central Milky Way, and the beautiful Lagoon (M8) and Trifid (M20) nebulae. Of course Saturn's largest moon Titan was also along for the ride. Swinging around the gas giant in a 16 day long orbit, Titan's resulting wave-like motion is easier to spot when the almost-too-bright Saturn is digitally edited from the scene.

Image credit: Tunc Tezel
The Lonely Neutron Star in Supernova Remnant E0102-72.3 (2018-09-30)

Why is this neutron star off-center? Recently a lone neutron star has been found within the debris left over from an old supernova explosion. The "lonely neutron star" in question is the blue dot at the center of the red nebula near the bottom left of E0102-72.3.  In the featured image composite, blue represents X-ray light captured by NASA's Chandra Observatory, while red and green represent optical light captured by ESO's Very Large Telescope in Chile and NASA's Hubble Space Telescope in orbit.  The displaced position of this neutron star is unexpected since the dense star is thought to be the core of the star that exploded in the supernova and created the outer nebula.  It could be that the neutron star in E0102 was pushed away from the nebula's center by the supernova itself, but then it seems odd that the smaller red ring remains centered on the neutron star.  Alternatively, the outer nebula could have been expelled during a different scenario -- perhaps even involving another star.  Future observations of the nebulas and neutron star appear likely to resolve the situation.
The First Rocket Launch from Cape Canaveral (2018-10-01)

A new chapter in space flight began in 1950 with the launch of the first rocket from Cape Canaveral, Florida: the Bumper V-2.  Featured here, the Bumper V-2 was an ambitious two-stage rocket program that topped a V-2 missile base with a WAC Corporal rocket.  The upper stage was able to reach then-record altitudes of almost 400 kilometers, about the height of the modern International Space Station.  Launched under the direction of the General Electric Company, the Bumper V-2 was used primarily for testing rocket systems and for research on the upper atmosphere.  Bumper V-2 rockets carried small payloads that allowed them to measure attributes including air temperature and cosmic ray impacts.  Seven years later, the Soviet Union launched Sputnik I and Sputnik II, the first satellites into Earth orbit.  In response in 1958, 60 years ago today, the USA created NASA.    Anniversary: NASA: 60 Years and Counting
Supernumerary Rainbows over New Jersey (2018-10-02)

Yes, but can your rainbow do this?  After the remnants of Hurricane Florence passed over the Jersey Shore, New Jersey, USA last month, the Sun came out in one direction but something quite unusual appeared in the opposite direction: a hall of rainbows.  Over the course of a next half hour, to the delight of the photographer and his daughter, vibrant supernumerary rainbows faded in and out, with at least five captured in this featured single shot.  Supernumerary rainbows only form when falling water droplets are all nearly the same size and typically less than a millimeter across.  Then, sunlight will not only reflect from inside the raindrops, but interfere, a wave phenomenon similar to ripples on a pond when a stone is thrown in. In fact, supernumerary rainbows can only be explained with waves, and their noted existence in the early 1800s was considered early evidence of light's wave nature.   Follow APOD on: Facebook,  Google Plus,  Instagram, or Twitter

Image credit: John Entwistle
NGC 1898: Globular Cluster in the Large Magellanic Cloud (2018-10-03)

Jewels don't shine this bright -- only stars do. And almost every spot in this glittering jewel-box of an image from the Hubble Space Telescope is a star. Now some stars are more red than our Sun, and some more blue -- but all of them are much farther away.  Although it takes light about 8 minutes to reach Earth from the Sun, NGC 1898 is so far away that it takes light about 160,000 years to get here. This huge ball of stars, NGC 1898, is called a globular cluster and resides in the central bar of the Large Magellanic Cloud (LMC) -- a satellite galaxy of our large Milky Way Galaxy. The featured multi-colored image includes light from the infrared to the ultraviolet and was taken to help determine if the stars of NGC 1898 all formed at the same time, or at different times. There are increasing indications that most globular clusters formed stars in stages, and that, in particular, stars from NGC 1898 formed shortly after ancient encounters with the Small Magellanic Cloud (SMC) and our Milky Way Galaxy.   Space Telescope Live: Where is Hubble looking right now?
Opportunity After the Storm (2018-10-04)

On Mars dust storms can't actually blow spacecraft over, but they can blot out the Sun. Over three months ago a planet-wide dust storm caused a severe lack of sunlight for the Mars rover Opportunity at its location near the west rim of Endeavour crater. The lack of sunlight sent the solar-powered Opportunity into hibernation and for over 115 sols controllers have not received any communication from the rover. The dust is clearing as the storm subsides though. On September 20th, when this image was taken by the Mars Reconnaissance Orbiter's HiRISE camera, about 25 percent of the sunlight was reaching the surface again. The white box marks a 47-meter-wide (154-foot-wide) area centered on a blip identified as the silent-for-now Opportunity rover.
The Last Days of Venus as the Evening Star (2018-10-05)

That's not a young crescent Moon poised above the hills along the western horizon at sunset. It's Venus in a crescent phase. About 54 million kilometers away and less than 20 percent illuminated, it was captured by telescope and camera on September 30 near Bacau, Romania. The bright celestial beacon is now languishing in the evening twilight, its days as the Evening Star in 2018 coming to a close. But it also grows larger in apparent size and becomes an ever thinner crescent in telescopic views. Heading toward an inferior conjunction (non-judgmental), the inner planet will be positioned between Earth and Sun on October 26 and lost from view in the solar glare. At month's end a crescent Venus will reappear in the east though, rising just before the Sun as the brilliant Morning Star.

Image credit: Radu-Mihai Anghel
Aurora: The Frog's View (2018-10-06)

What does an aurora look like to a frog? "Awesome!" is the likely answer, suggested by this imaginative snapshot taken on October 3rd from Kiruna, Sweden. Frequented by apparitions of the northern lights, Kiruna is located in Lapland north of the Arctic Circle, and often under the auroral oval surrounding planet Earth's geomagnetic north pole. To create a tantalizing view from a frog's perspective the photographer turned on the flashlight on her phone and placed it on the ground facing down, resting her camera's lens on top. The "diamonds" in the foreground are icy pebbles right in front of the lens, lit up by the flashlight. Reflecting the shimmering northern lights, the "lake" is a frozen puddle on the ground. Of course, in the distance is the Bengt Hultqvist Observatory.

Image credit: Mia Stalnacke
The Scale of the Universe - Interactive (2018-10-07)

What does the universe look like on small scales? On large scales?  Humanity is discovering that the universe is a very different place on every proportion that has been explored. For example, so far as we know, every tiny proton is exactly the same, but every huge galaxy is different. On more familiar scales, a small glass table top to a human is a vast plane of strange smoothness to a dust mite -- possibly speckled with cell boulders. Not all scale lengths are well explored -- what happens to the smallest mist droplets you sneeze, for example, is a topic of active research -- and possibly useful to know to help stop the spread of disease. The featured interactive flash animation, a modern version of the classic video Powers of Ten, is a new window to many of the known scales of our universe.  By moving the scroll bar across the bottom, you can explore a diversity of sizes, while clicking on different items will bring up descriptive information.

Image credit: Cary & Michael Huang
Comet 21P Between Rosette and Cone Nebulas (2018-10-08)

Small bits of this greenish-gray comet are expected to streak across Earth's atmosphere tonight. Specifically, debris from the eroding nucleus of Comet 21P / Giacobini-Zinner, pictured, causes the annual Draconids meteor shower, which peaks this evening.  Draconid meteors are easy to enjoy this year because meteor rates will likely peak soon after sunset with the Moon's glare nearly absent.  Patience may be needed, though, as last month's passing of 21P near the Earth's orbit is  not expected to increase the Draconids' normal meteor rate this year of (only) a few meteors per hour.  Then again, meteor rates are notoriously hard to predict, and the Draconids were quite impressive in 1933, 1946, and 2011.  Featured, Comet 21P gracefully posed between the Rosette (upper left) and Cone (lower right) nebulas two weeks ago before heading back out to near the orbit of Jupiter, to return again in about six and a half years.

Image credit: Fritz Helmut Hemmerich