juliensimon/nasa-apod huggingface dataset:
The Far 3kpc Arm (2008-07-11)

A major discovery was lurking in the data. By accident, while preparing a talk on the Galaxy's spiral arms for a meeting of the American Astronomical Society, Tom Dame (Harvard-Smithsonian CfA) found it - a new spiral arm in the Milky Way. The arm is labeled in this illustration as the Far 3kpc Arm, located at a distance of 3 kpc (kiloparsecs) or about 10,000 light-years from the galactic center, on the opposite side from the Sun. Along with the Near 3kpc Arm whose presence was known since the mid 1950s, the counterpart inner arms now establish that the galaxy has a simple symmetry. The arms are defined by shocked interstellar gas flowing along both sides of the Milky Way's central bar. Dame and his collaborator Patrick Thaddeus recorded the presence of both inner spiral arms in their radio data tracking emission from carbon monoxide molecules along the galactic plane. How much star formation goes on in the counterpart arms? Despite this depiction of stars and star forming regions along the arms, the last attempt to search for star formation in the Near 3kpc Arm was in 1980 and didn't turn up any. The discovery of the Far 3kpc Arm has renewed interest in this and other questions about the center of the Milky Way.
NGC 7331 and Beyond (2008-07-12)

Big, beautiful spiral galaxy NGC 7331 is often touted as an analog to our own Milky Way. About 50 million light-years distant in the northern constellation Pegasus, NGC 7331 was recognized early on as a spiral nebula and is actually one of the brighter galaxies not included in Charles Messier's famous 18th century catalog. Since the galaxy's disk is inclined to our line-of-sight, long telescopic exposures often result in an image that evokes a strong sense of depth. The effect is further enhanced in this well-framed view by the galaxies that lie beyond this gorgeous island universe. The background galaxies are about one tenth the apparent size of NGC 7331 and so lie roughly ten times farther away. Their strikingly close alignment on the sky with NGC 7331 occurs just by chance. The visual grouping of galaxies is also known as the Deer Lick Group.

Image credit: Don Goldman
A Dark Sky Over Death Valley (2008-07-13)

This eerie glow over Death Valley is in danger. Scrolling right will show a spectacular view from one of the darkest places left in the continental USA: Death Valley, California.  The above 360-degree full-sky panorama is a composite of 30 images taken two years ago in Racetrack Playa.  The image has been digitally processed and increasingly stretched at high altitudes to make it rectangular.   In the foreground on the image right is an unusually placed rock that was pushed by high winds onto Racetrack Playa after a slick rain.  In the background is a majestic night sky, featuring thousands of stars and many constellations.  The arch across the middle is the central band of our Milky Way Galaxy.  Light pollution is threatening dark skies like this all across the US and the world, and therefore the International Dark-Sky Association and the US National Parks Service are suggesting methods that can protect them.
Changes in Angular Mars (2008-07-14)

Does Mars always appear the same?  No.  As both Earth and Mars orbit the Sun, the apparent angular size of Mars changes as viewed from the Earth.  Pictured above from Enschede, Holland, Mars was captured in 2007 and 2008 with 30 separate images, all taken with the same magnification.  When Earth and Mars are on opposite sides of the Sun, Mars appears relatively small.  Conversely, when Earth and Mars are near each other, Mars looms large and bright. The largest Mars has appeared in recent history was the opposition of August 2003.  Since Mars is always more distant from the Sun than the Earth, Mars never shows a crescent phase to Earthlings.  Visible also in the above images are the north polar cap of Mars, dark and light soil, clouds, and, in the early images, a global dust storm. The next opposition, when Earth again passes near to Mars, will occur in early 2010.

Image credit: Richard Bosman
Gas and Dust of the Lagoon Nebula (2008-07-15)

This beautiful cosmic cloud is a popular stop on telescopic tours of the constellation Sagittarius. Eighteenth century cosmic tourist Charles Messier cataloged the bright nebula as M8, while modern day astronomers recognize the Lagoon Nebula as an active stellar nursery about 5,000 light-years distant, in the direction of the center of our Milky Way Galaxy. Striking details can be traced through this remarkable picture, processed to remove stars and hence better reveal the Lagoon's range of filaments of glowing hydrogen gas, dark dust clouds, and the bright, turbulent hourglass region near the image center. This color composite view was recorded under dark skies near Sydney, Australia. At the Lagoon's estimated distance, the picture spans about 50 light-years.

Image credit: Fred Vanderhaven
Makemake of the Outer Solar System (2008-07-16)

Recently discovered Makemake is one of the largest objects known in the outer Solar System. Pronounced MAH-kay MAH-kay, this Kuiper belt object is only slightly smaller than Pluto, orbits the Sun only slightly further out than Pluto, and appears only slightly dimmer than Pluto. Makemake, however, has an orbit much more tilted to the ecliptic plane of the planets than Pluto.  Designated 2005 FY9 soon after its discovery by a team led by Mike Brown (Caltech) in 2005, the outer Solar System orb was recently renamed Makemake for the creator of humanity in the Rapa Nui mythology of Easter Island. Additionally, Makemake has been recently classified as a dwarf planet under the new subcategory plutoid, making Makemake the third cataloged plutoid after Pluto and Eris.  Makemake is known to be a world somewhat red in appearance, with spectra indicating it is likely covered with frozen methane. Since no images of Makemake's surface yet exist, an artist's illustration originally meant to depict Sedna has been boldly co-opted above to now illustrate Makemake.  A hypothetical moon is visualized above nearly in the direction of our distant Sun.
Extra Galaxies (2008-07-17)

A careful look at the full field of view for this sharp image reveals a surprising number of galaxies both near and far toward the constellation Ursa Major. The most striking is clearly NGC 3718, the warped spiral galaxy right of center. NGC 3718's faint spiral arms look twisted and extended, its bright central region crossed by obscuring dust lanes. A mere 150 thousand light-years to the left is another large spiral galaxy, NGC 3729. The two are likely interacting gravitationally, accounting for the peculiar appearance of NGC 3718. While this galaxy pair lies about 52 million light-years away, the remarkable Hickson Group 56 can also be seen clustered just below NGC 3718. Hickson Group 56 consists of five interacting galaxies and lies over 400 million light-years away.

Image credit: Dietmar Hager
Jupiter over Ephesus (2008-07-18)

A brilliant Jupiter shares the sky with the Full Moon tonight. Since Jupiter is near opposition, literally opposite the Sun in planet Earth's sky, Jupiter will rise near sunset just like the Full Moon. Of course, opposition is also the point of closest approach, with Jupiter shining at its brightest and offering the best views for skygazers. Recorded late last month, this moving skyscape features Jupiter above the southeastern horizon and the marbled streets of the ancient port city of Ephesus, located in modern day Turkey. At the left is a temple dedicated to the Roman emperor Hadrian. The beautiful night sky also includes the arc of the northern summer Milky Way. Lights on the horizon are from the nearby town of Selçuk. Clicking on the image will download the scene as a panorama.

Image credit: Tunç Tezel
M16 and the Eagle Nebula (2008-07-19)

Young star cluster M16 is surrounded by natal clouds of cosmic dust and glowing gas also known as The Eagle Nebula. This beautifully detailed image of the region includes fantastic shapes made famous in well-known Hubble Space Telescope close-ups of the starforming complex. Described as elephant trunks or Pillars of Creation, dense, dusty columns rising near the center are light-years in length but are gravitationally contracting to form stars. Energetic radiation from the cluster stars erodes material near the tips, eventually exposing the embedded new stars. Extending from the upper left edge of the nebula is another dusty starforming column known as the Fairy of Eagle Nebula. M16 and the Eagle Nebula lie about 7,000 light-years away, an easy target for binoculars or small telescopes in a nebula rich part of the sky toward the split constellation Serpens Cauda (the tail of the snake).

Image credit: Panther Observatory
Crescent Rhea Occults Crescent Saturn (2008-07-20)

Soft hues, partially lit orbs, a thin trace of the ring, and slight shadows highlight this understated view of the majestic surroundings of the giant planet Saturn.  Looking nearly back toward the Sun, the robot Cassini spacecraft now orbiting Saturn captured crescent phases of Saturn and its moon Rhea in color a few years ago. As striking as the above image is, it is but a single frame from a recently released 60-frame silent movie where Rhea can be seen gliding in front of its parent world.   Since Cassini was nearly in the plane of Saturn's rings, the normally impressive rings are visible here only as a thin line across the image center. Although Cassini has now concluded its primary mission, its past successes and opportunistic location have prompted NASA to start a two-year Equinox Mission, further exploring not only Saturn's enigmatic moons Titan and Enceladus, but Saturn herself as her grand rings tilt right at the Sun in August 2009.
The Colliding Spiral Galaxies of Arp 271 (2008-07-21)

What will become of these galaxies? Spiral galaxies NGC 5426 and NGC 5427 are passing dangerously close to each other, but each is likely to survive this collision.  Most frequently when galaxies collide, a large galaxy eats a much smaller galaxy. In this case, however, the two galaxies are quite similar, each being a sprawling spiral with expansive arms and a compact core.   As the galaxies advance over the next tens of millions of years, their component stars are unlikely to collide, although new stars will form in the bunching of gas caused by gravitational tides.  Close inspection of the above image taken by the 8-meter Gemini-South Telescope in Chile shows a bridge of material momentarily connecting the two giants.  Known collectively as Arp 271, the interacting pair spans about 130,000 light years and lies about 90 million light-years away toward the constellation of Virgo. Quite possibly, our Milky Way Galaxy will undergo a similar collision with the neighboring Andromeda Galaxy in about five billion years.

Image credit: Gemini Observatory
Happy People Dancing on Planet Earth (2008-07-22)

What are these humans doing?  Dancing.  Many humans on Earth exhibit periods of happiness, and one method of displaying happiness is dancing.  Happiness and dancing transcend political boundaries and occur in practically every human society.  Above, Matt Harding traveled through many nations on Earth, started dancing, and filmed the result.  The video is perhaps a dramatic example that humans from all over planet Earth feel a common bond as part of a single species. Happiness is frequently contagious -- few people are able to watch the above video without smiling.
High Cliffs Surrounding Echus Chasma on Mars (2008-07-23)

What created this great cliff on Mars? Did giant waterfalls once plummet through its grooves?  With a four-kilometer drop, this high cliff surrounding Echus Chasma, near an impressive impact crater, was carved by either water or lava.  A leading hypothesis is that Echus Chasma, at 100-kilometers long and 10-kilometers wide, was once one of the largest water sources on Mars.   If true, water once held in Echus Chasma likely ran over the Martian surface to carve the impressive Kasei Valles, which extends over 3,000 kilometers to the north. Even if initially carved by water, lava appears to have later flowed in the valley, leaving an extraordinarily smooth floor.  Echus Chasma lies north of tremendous Valles Marineris, the largest canyon in the Solar System.  The above image was taken by the robotic Mars Express spacecraft currently orbiting Mars.
When Storms Collide (2008-07-24)

These detailed Hubble Space Telescope close-ups feature Jupiter's ancient swirling storm system known as the Great Red Spot. They also follow the progress of two newer storm systems that have grown to take on a similar reddish hue: the smaller "Red Spot Jr." (bottom), and smaller still, a "baby red spot". Red Spot Jr. was seen to form in 2006, while the smaller spot was just identified earlier this year. For scale, the Great Red Spot has almost twice the diameter of planet Earth. Moving horizontally from left to right past the Great Red Spot, Red Spot Jr. clearly went below the larger storm, but the smaller spot was pulled in. Emerging on the right, the baby spot's stretched and now paler shape is indicated by the arrow in the frame from July 8. It is expected that the baby red spot will be pulled back and merge, becoming part of the giant storm system.
Spitzer's M101 (2008-07-25)

Big, beautiful spiral galaxy M101 is one of the last entries in Charles Messier's famous catalog, but definitely not one of the least. About 170,000 light-years across, this galaxy is enormous, almost twice the size of our own Milky Way Galaxy. M101 was also one of the original spiral nebulae observed by Lord Rosse's large 19th century telescope, the Leviathan of Parsontown. Recorded at infrared wavelengths by the Spitzer Space telescope, this 21st century view shows starlight in blue hues while the galaxy's dust clouds are in red. Examining the dust features in the outer rim of the galaxy, astronomers have found that organic molecules present throughout the rest of M101 are lacking. The organic molecules tracked by Spitzer's instruments are called polycyclic aromatic hydrocarbons (PAHs). Of course, PAHs are common components of dust in the Milky Way and on planet Earth are found in soot. PAHs are likely destroyed near the outer edges of M101 by energetic radiation in intense star forming regions. Also known as the Pinwheel Galaxy, M101 lies within the boundaries of the northern constellation Ursa Major, about 25 million light-years away.
Central IC 1805 (2008-07-26)

Cosmic clouds seem to form fantastic shapes in the central regions of emission nebula IC 1805. Of course, the clouds are sculpted by stellar winds and radiation from massive hot stars in the nebula's newborn star cluster (aka Melotte 15). About 1.5 million years young, the cluster stars appear on the right in this colorful skyscape, along with dark dust clouds silhouetted against glowing atomic gas. A composite of narrow and broad band telescopic images, the view spans about 15 light-years and includes emission from hydrogen in green, sulfur in red, and oxygen in blue hues. Wider field images reveal that IC 1805's simpler, overall outline suggests its popular name - The Heart Nebula. IC 1805 is located about 7,500 light years away toward the constellation Cassiopeia.

Image credit: Keith Quattrocchi
IC 4406: A Seemingly Square Nebula (2008-07-27)

How can a round star make a square nebula?  This conundrum comes to light when studying  planetary nebulae like IC 4406.  Evidence indicates that IC 4406 is likely a hollow cylinder, with its square appearance the result of our vantage point in viewing the cylinder from the side.  Were IC 4406 viewed from the top, it would likely look similar to the Ring Nebula.  This representative-color picture is a composite made by combining images taken by the Hubble Space Telescope in 2001 and 2002. Hot gas flows out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls.  The star primarily responsible for this interstellar sculpture can be found in the planetary nebula's center.  In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.
SDSSJ1430: A Galaxy Einstein Ring (2008-07-28)

What's large and blue and can wrap itself around an entire galaxy? A gravitational lens mirage. Pictured above on the left, the gravity of a normal white galaxy has gravitationally distorted the light from a much more distant blue galaxy. More normally, such light bending results in two discernable images of the distant galaxy, but here the lens alignment is so precise that the background galaxy is distorted into a nearly complete ring.  Since such a lensing effect was generally predicted in some detail by Albert Einstein over 70 years ago, such rings like SDSSJ1430 are now know as Einstein Rings.  SDSSJ1430 was discovered during the Sloan Lens Advanced Camera for Surveys (SLACS) campaign, an observation program that inspected lens candidates found by the Sloan Digital Sky Survey (SDSS) with the Hubble Space Telescope's ACS.  Strong gravitational lenses like SDSSJ1440 are more than oddities -- their multiple properties allow astronomers to determine the mass and dark matter content of the foreground galaxy lenses. Given these determinations, SLACS data has now been used, for example, to show that dark matter fraction increases with overall galaxy mass. The inset images on the right depict, from top to bottom, a computer reconstructed image of what the background blue galaxy really looks like, just the white foreground galaxy, and just the lensed blue background galaxy.
The Milky Way Over Ontario (2008-07-29)

Sometimes, after your eyes adapt to the dark, a spectacular sky appears.  Such was the case earlier this month over Ontario, Canada, when part of a spectacular sky also became visible in a reflection off a lake.  To start, the brightest objects visible are bright stars and the planet Jupiter, seen as the brightest spot on the upper left. A distant town appears as a diffuse glow over the horizon.  More faint still, the disk of the Milky Way Galaxy becomes apparent as a dramatic diffuse band across the sky that seems to crash into the horizon far in the distance.  In the foreground, a picturesque landscape includes trees, a lake, and a stone wall.  Finally, on this serene night in July when the lake water was unusually calm, reflections appear.  Visible in the lake are not only reflections of several bright stars, but part of the Milky Way band itself. Careful inspection of the image will reveal, however, that bright stars leave small trails in the lake reflections that do not appear in the sky above.  The reason for this is because the above image is actually a digital composite of time-consecutive exposures from the same camera.  In the first set of exposures, sky images were co-added with slight rotations to keep the stars in one place.

Image credit: Kerry-Ann Lecky Hepburn
(Weather and Sky
Photography)
The International Space Station Transits the Sun (2008-07-30)

That's no sunspot. It's the International Space Station (ISS) caught by chance passing in front of the Sun. Sunspots, individually, have a dark central umbra, a lighter surrounding penumbra, and no solar panels.  By contrast, the ISS is a complex and multi-spired mechanism, one of the largest and most sophisticated machines ever created by humanity.  Also, sunspots occur on the Sun, whereas the ISS orbits the Earth.  Transiting the Sun is not very unusual for the ISS, which orbits the Earth about every 90 minutes, but getting one's timing and equipment just right for a great image is rare.  Strangely, besides that fake spot, the Sun, last week, lacked any real sunspots.  Sunspots have been rare on the Sun since the dawn of the current Solar Minimum, a period of low solar activity. Although fewer sunspots have been recorded during this Solar Minimum than for many previous decades, the low solar activity is not, as yet, very unusual.

Image credit: Martin Wagner
Galaxies on a String (2008-07-31)

Galaxies NGC 5216 (top right) and NGC 5218 really do look like they are connected by a string. Of course, that string is a cosmic trail of gas, dust, and stars about 22,000 light-years long. Also known as Keenan's system (for its discoverer) and Arp 104, the interacting galaxy pair is some 17 million light-years away in the constellation Ursa Major. The debris trail that joins them, along with NGC 5218's comma-shaped extension and the distorted arms of NGC 5216 are a consequence of mutual gravitational tides that disrupt the galaxies as they repeatedly swing close to one another. Drawn out over billions of years, the encounters will likely result in their merger into a single galaxy of stars. Such spectacular galactic mergers are now understood to be a normal part of the evolution of galaxies, including our own Milky Way.

Image credit: Warren Keller
Moon Games (2008-08-01)

The Moon's measured diameter is around 3,476 kilometers (2,160 miles). But apparent angular size, or the angle covered by an object, can also be important to Moon enthusiasts. Angular size depends on distance, the farther away an object is, the smaller an angle it covers. Since the Moon is 400,000 kilometers away, its angular size is only about 1/2 degree, a span easily covered by the tip of your finger held at arms length, or a measuring tape held in the distance by a friend. Of course the Sun is much larger than the Moon, 400 times larger in fact, but today the New Moon will just cover the Sun. The total solar eclipse can be seen along a track across northern Canada, the Arctic, Siberia, and northern China. (A partial eclipse is visible from a broader region). Solar eclipses illustrate the happy coincidence that while the Sun is 400 times the diameter of the Moon, it is also 400 times farther away giving the Sun and Moon exactly the same angular size.  Total Solar Eclipse: Webcast from China

Image credit: PixHeaven.net
Eclipse Shirt (2008-08-02)

Of course, everyone is concerned about what to wear to a solar eclipse. No need to worry though, nature often conspires to project images of the eclipse so that stylish and appropriate patterns adorn many visible surfaces - including clothing - at just the right time. Most commonly, small gaps between leaves on trees can act as pinhole cameras and generate multiple recognizable images of the eclipse. In Madrid to view the 2005 October 3rd annular eclipse of the Sun, astronomer Philippe Haake met a friend who had another inspiration. The result, a grid of small holes in a kitchen strainer produced this pattern of images on an 'eclipse shirt'. While Yesterday's solar eclipse was total only along a narrow path beginning in northern Canada, extending across the Arctic, and ending in China, a partial eclipse could be seen from much of Europe and Asia.

Image credit: Philippe Haake
Open Cluster NGC 290: A Stellar Jewel Box (2008-08-03)

Jewels don't shine this bright -- only stars do.  Like gems in a jewel box, though, the stars of open cluster NGC 290 glitter in a beautiful display of brightness and color.   The photogenic cluster, pictured above, was captured recently by the orbiting Hubble Space Telescope.  Open clusters of stars are younger, contain few stars, and contain a much higher fraction of blue stars than do globular clusters of stars.  NGC 290 lies about 200,000  light-years distant in a neighboring galaxy called the Small Cloud of Magellan (SMC).   The open cluster contains hundreds of stars and spans about 65 light years across.  NGC 290 and other open clusters are good laboratories for studying how stars of different masses evolve, since all the open cluster's stars were born at about the same time.