juliensimon/nasa-apod huggingface dataset:
An Iridescent Cloud Over France (2004-04-13)

Why would a cloud appear to be different colors?  A relatively rare phenomenon known as iridescent clouds can show unusual colors vividly or a whole spectrum of colors simultaneously.  These clouds are formed of small water droplets of nearly uniform size.  When the Sun is in the right position and mostly hidden by thick clouds,  these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts.  Therefore, different colors will come to the observer from slightly different directions.  Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors. Pictured above, an iridescent cloud was photographed near Cannes,  France last month.

Image credit: Michael Koch
Massive Star Forming Region DR21 in Infrared (2004-04-14)

Deep in the normally hidden recesses of giant molecular cloud DR21, a stellar nursery has been found creating some of the most massive stars yet recorded.  The orbiting Spitzer Space Telescope's Infrared Array Camera opened the window into the cloud last year in mid- infrared light.  The cloud is opaque to visible light because of dense interstellar dust.  Noticeable in the above representative color infrared Spitzer image are huge bubbles, a complex tapestry of dust and gas, and very massive stars.  The infrared filaments actually glow because of organic compounds known as PAHs.  The intricate patterns are caused by complex interactions between interstellar winds, radiation pressures, magnetic fields, and gravity.  The pictured region spans about 75 light years and lies about 6,000 light years distant toward the constellation of Cygnus.
Venus and the Pleiades (2004-04-15)

Venus still rules the western skies after sunset as the brilliant evening star. While wandering the ecliptic with its fellow naked-eye planets earlier this month, it passed near the Pleiades star cluster, providing a striking photo opportunity for earthbound skygazers. Cataloged as M45, the Pleiades stars make for a lovely sight on their own, often shown in long exposure images immersed in hazy blue reflection nebulae. In this picture though, recorded on the evening of April 3rd, brilliant Venus closes with the Seven Sisters and overwhelms the light from the delicate cosmic clouds. The view offers a study in contrasts as Venus appears about 700 times brighter than Alcyone, the Pleiades brightest star. With Venus just over 5 light-minutes from Earth, Alcyone and the other Pleiades cluster stars are about 400 light-years distant. Formed out of the contracting nebula which gave birth to the Sun, Venus is also roughly 4.5 billion years old. The stars of the Pleiades are likely aged a mere hundred million years.

Image credit: David Cortner
The Stars of NGC 300 (2004-04-16)

Like grains of sand on a cosmic beach, individual stars of large spiral galaxy NGC 300 are resolved in this sharp image from the Hubble Space Telescope's Advanced Camera for Surveys (ACS). The inner region of the galaxy is pictured, spanning about 7,500 light-years. At its center is the bright, densely packed galactic core surrounded by a loose array of dark dust lanes mixed with the stars in the galactic plane. NGC 300 lies 6.5 million light-years away and is part of a group of galaxies named for the southern constellation Sculptor. Hubble's unique ability to distinguish so many stars in NGC 300 can be used to hone techniques for making distance measurements on extragalactic scales.
Lunar Dust and Duct Tape (2004-04-17)

Why is the Moon dusty? On Earth, rocks are weathered by wind and water, creating soil and sand. On the Moon, the long history of micrometeorite bombardment has blasted away at the rocky surface creating a layer of powdery lunar soil or regolith. This lunar regolith could be a scientific and industrial bonanza. But for the Apollo astronauts and their equipment, the pervasive, fine, gritty dust was definitely a problem. On the lunar surface in December 1972, Apollo 17 astronauts Harrison Schmitt and Eugene Cernan needed to repair one of their lunar rover's fenders in an effort to keep the "rooster tails" of dust away from themselves and their gear. This picture reveals the wheel and fender of their dust covered rover along with the ingenious application of spare maps, clamps, and a grey strip of "duct tape".
Stellar Spectral Types: OBAFGKM (2004-04-18)

Astronomers divide stars into different  spectral types. First started in the 1800s, the spectral type was originally meant to classify the strength of hydrogen absorption lines. A few types that best describe the temperature of the star remain in use today.  The seven main spectral types OBAFGKM are shown above with the spectrum of a single "O" star at the top followed by two spectra each from the progressively cooler designations, respectively.  Historically, these letters have been remembered with the mnemonic "Oh Be A Fine Girl/Guy Kiss Me."  Frequent classroom contests, however, have come up with other more/less politically correct mnemonics such as "Oven Baked Apples From Grandpa's/Grandma's Kitchen. Mmmm."  Our Sun has spectral type "G".

Image credit: KPNO
Comet Bradfield Passes the Sun (2004-04-19)

Today, Comet Bradfield is passing the Sun.  The above image, taken yesterday in the direction of the Sun by the SOHO LASCO instrument, shows the comet and its dust tail as the elongated white streak.  The Sun would normally be seen in the very center but has been blocked from view.  Comet C/2004 F4 (Bradfield) was discovered just one month ago and has brightened dramatically as it neared the Sun.  Careful sky gazers can see Comet Bradfield with the unaided eye near the Sun, although NASA's sun-orbiting SOHO satellite has the best view.  During the day, Comet Bradfield will continually shift inside the LASCO frame as it rounds the Sun.  There is even the possibility that the comet will break up.  If not, the bright comet's trajectory will carry it outside the field of LASCO sometime tomorrow.  Along with T7 and Q4, Comet Bradfield is now the third comet that is currently visible on the sky with the unaided eye, the most ever of which we are aware and quite possibly the most in recorded history.
Comet Hale-Bopp Over Indian Cove (2004-04-20)

Comet Hale-Bopp, the Great Comet of 1997, was quite a sight.  No comets of comparable brightness have graced the skies of Earth since then.  During this next month, however, even besides the fleeting Comet Bradfield, two comets have a slight chance of rivaling Hale-Bopp and a good chance of putting on a memorable sky show.  Unfortunately, most of the show will be confined to sky gazers in Earth's southern hemisphere.  Both comets are already visible to the unaided eye from there.  The first, Comet C/2002 T7 (LINEAR), should be at its best before dawn during the first weeks of May from the south.  The second, Comet C/2001 Q4 (NEAT), should be visible in early May from all over the Earth.  Both comets appear to be approaching the inner Solar System for the first time and so it is very hard to predict how bright each will become.  In the above photograph taken 1997 April 6, Comet Hale-Bopp was imaged from the Indian Cove Campground in the Joshua Tree National Forest in California, USA.  A flashlight was used to momentarily illuminate foreground rocks during this six minute exposure.

Image credit: Wally Pacholka (Astropics)
Wally Pacholka
Nebulas Surrounding Wolf-Rayet Binary BAT99-49 (2004-04-21)

How could two young stars power these colorful interstellar gas clouds? Although hidden by thick dust, the stars spew forceful ions and energetic radiation that cause the clouds to fragment and light up. The above composite color image from the European Southern Observatory's Melipal VLT telescope resolves details in the nebula complex known as BAT99-49, with emission from helium atoms in blue hues, oxygen atoms in green, and hydrogen atoms in red.  Located in the Large Magellanic Cloud (LMC), the largest satellite galaxy to our own Milky Way Galaxy, one of the stars in the central binary is an enigmatic Wolf-Rayet star while the other is a massive O star.  Wolf-Rayet stars have some of the hottest surfaces in the universe, while O stars are the most massive and energetic of normal main sequence stars.

Image credit: Y. Naze, G. Rauw, J. Manfroid, J. Vreux
(Univ. Liege),
Y. Chu (Univ. Illinois),
ESO
Comet C/2002 T7 (LINEAR) (2004-04-22)

Discovered by the the Lincoln Near Earth Asteroid Research (LINEAR) project in October of 2002, comet C/2002 T7 is now visiting the inner solar system, making its closest approach (see animation by L. Koehn) to the Sun tomorrow, April 23rd. Emerging from the solar glare, the comet is now just visible to the unaided eye in the constellation Pisces, near the eastern horizon in morning twilight. In this gorgeous telescopic view recorded before dawn yesterday, the clearly active comet has developed an extensive, complex tail extending over 2 degrees in the anti-sunward direction, and a pronounced anti-tail or anomalous tail. Later next month this comet should appear brighter, making its closest approach to planet Earth on May 19th. In fact, it could share southern skies with another naked-eye comet, also anticipated to brighten in May, designated C/2001 Q4 (NEAT).
Comet C/2001 Q4 (NEAT) (2004-04-23)

Inbound from the distant solar system, comet C/2001 Q4 will soon pass just inside planet Earth's orbit and should be one of two bright, naked-eye comets visible in southern skies in May. First picked up nearly three years ago by the Near Earth Asteroid Tracking (NEAT) project Q4 appears in both of these stunning telescopic views recorded only a few days ago, on April 18th (left) and 19th, from a site near Alcohuaz, Chile. Remarkable changes in the structure of the long, graceful tail can be seen by comparing the two photos, including the dramatic kink seen near the tail's midpoint on April 19th. The apparent motion of the comet sweeping across the sky is evident when you compare the position of the tail relative to background galaxy NGC 1313, visible as a smudge near the top of each image. Q4's closest approach to the Sun will be on May 15th while its closest encounter with planet Earth will be on May 7th (see animation by L. Koehn).

Image credit: StarryScapes
M27: Not A Comet (2004-04-24)

While searching the skies above 18th century France for comets, astronomer Charles Messier diligently recorded this object as number 27 on his list of things which are definitely not comets. So what is it? Well, 21st century astronomers would classify it as a Planetary Nebula ... but it's not a planet either, even though it may appear round and planet-like in a small telescope. Messier 27 (M27) is now known to be an excellent example of a gaseous emission nebula created as a sun-like star runs out of nuclear fuel in its core. The nebula forms as the star's outer layers are expelled into space. The visible glow is generated as atoms are excited by the dying star's intense but invisible ultraviolet light. Known by the popular name Dumbbell Nebula, the beautifully symmetric interstellar gas cloud is about 1,200 light-years away in the constellation Vulpecula. This gorgeous synthetic color picture of M27 was produced during testing of one of the European Southern Observatory's Very Large Telescopes.
D. rad Bacteria: Candidate Astronauts (2004-04-25)

These bacteria could survive on another planet.  In an Earth lab, Deinococcus radiodurans (D. rad) survive extreme levels of radiation, extreme temperatures, dehydration, and exposure to genotoxic chemicals.  Amazingly, they even have the ability to repair their own  DNA, usually with 48 hours.  Known as an extremophile, bacteria such as D. rad are of interest to NASA partly because they might be adaptable to help human astronauts survive on other worlds.  A recent map of D. rad's  DNA might allow biologists to augment their survival skills with the ability to produce medicine, clean water, and oxygen.  Already they have been genetically engineered to help clean up spills of toxic mercury.  Likely one of the oldest surviving life forms, D. rad was discovered by accident in the 1950s when scientists investigating food preservation techniques could not easily kill it.  Pictured above, Deinococcus radiodurans grow quietly in a dish.
Ring Galaxy AM 0644-741 from Hubble (2004-04-26)

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 intruder galaxy has since moved out of the frame taken by the Hubble Space Telescope and released to commemorate last Saturday's fourteenth anniversary of Hubble's launch.  Ring galaxy AM 0644-741 lies about 300 million light years away.
Comet Bradfield Rising (2004-04-27)

Comet Bradfield has become quite a sight just before sunrise -- for those with binoculars or cameras.  Although fading noticeably each day, a sky chart, a northern location, and some persistence will allow curious sky gazers to locate the cosmic snowball and its spectacular tail.  One might call Bradfield a "camera" comet as its extended tail is too long for most telescopes but caught nicely by normal cameras capable of long exposures and set to rotate with the sky.  Pictured above just yesterday, Comet C/2004 F4 (Bradfield) was caught as it rose on successive three-minute exposures above the Rocky Mountains near Yampa, Colorado, USA.  Visible on the upper left as a bright fuzzy smudge is the Andromeda Galaxy (M31), far in the distance.  Comet Bradfield was discovered only last month and was briefly visible to the unaided eye.  It was imaged in spectacular fashion by the SOHO spacecraft as it rounded the Sun early last week.

Image credit: Jimmy Westlake
(Colorado
Mountain College)
The Smooth Spheres of Gravity Probe B (2004-04-28)

Does gravity have a magnetic counterpart?  Spin any electric charge and you get a magnetic field.  Spin any mass and, according to Einstein, you should get a very slight effect that acts something like magnetism.  This effect is expected to be so small that it is beyond practical experience and even beyond laboratory measurement.  Until now.  In a bold attempt to directly measure gravitomagnetism, NASA launched last week the smoothest spheres ever manufactured into space to see how they spin.  These four spheres, each roughly the size of a ping-pong ball, are the key to the ultra-precise gyroscopes at the core of Gravity Probe B.  Will the gyroscopes feel gravitomagnetism and wobble at the rate Einstein would have predicted?  Stay tuned.  Better understanding space, time, and gravity can have untold long term benefits as well as likely shorter term benefits such as better clocks and global positioning trackers.
Titan's X-Ray (2004-04-29)

This June's rare and much heralded transit of Venus will feature our currently brilliant evening star in silhouette, as the inner planet glides across the face of the Sun. But on January 5, 2003 an even rarer transit took place. Titan, large moon of ringed gas giant Saturn, crossed in front of the Crab Nebula, a supernova remnant some 7,000 light-years away. During Titan's transit, the orbiting Chandra Observatory's x-ray detectors recorded the shadowing of cosmic x-rays generated by the Crab's amazing pulsar nebula, pictured above, in a situation analogous to a medical x-ray. The resulting image (inset at left) probes the extent of Titan's atmosphere. So, how rare was Titan's transit of the Crab? While Saturn itself passes within a few degrees of the Crab Nebula every 30 years, the next similar transit is reportedly due in 2267. And since the stellar explosion which gave birth to the Crab was seen in 1054, the 2003 Titan transit may have been the first to occur ... ever.
Eyeful of Saturn (2004-04-30)

Now a bright speck of light wandering through Earth's night sky, magnificent planet Saturn lies nearly 1.5 billion kilometers from the Sun. But after an interplanetary voyage of seven years the planet's stunning rings nearly fill the field of the Cassini spacecraft's narrow angle camera in this image recorded on March 27. Tip to tip, the ring system spans about 270,000 kilometers. Named for discoverers, the large, easily visible gap in the rings is known as the Cassini division, while the narrower outer gap is the Encke division. Illuminated from below and to the right, the rings cast a shadow on Saturn's upper hemisphere, interrupted where sunlight streams through the Cassini division and creates a light blue streak. At the left, Saturn also casts a stark shadow across the planet girdling rings. On July 1, the Cassini spacecraft is scheduled to fire its main engine and enter Saturn orbit.
A Western Sky at Twilight (2004-05-01)

On April 23rd, the Moon along with planets Saturn, Mars, and Venus (and planet Earth of course ...) were all visible in the west at twilight, captured here from a site near Saylorvillle Lake north of Des Moines, Iowa, USA.  Putting your cursor over the image will label our fellow solar system wanderers and also reveal the approximate trajectory of the ecliptic plane - defined by Earth's orbit around the Sun - angling above the western horizon. After sunset tonight, the western sky will present a similar arrangement of planets, although the Moon will have moved east out of the picture, passing bright Jupiter along the ecliptic and heading for May 4th's total lunar eclipse. May could also be a good month for comets.

Image credit: Stan Richard
Io in True Color (2004-05-02)

The strangest moon in the Solar System is bright yellow.  This picture, showing Io's true colors, was taken in 1999 July by the  Galileo spacecraft that orbited Jupiter from 1995 to 2003.  Io's colors derive from sulfur and molten silicate rock.  The unusual surface of Io is kept very young by its system of active volcanoes.  The intense tidal gravity of Jupiter stretches  Io and damps wobbles caused by Jupiter's other Galilean moons.  The resulting friction greatly heats Io's interior, causing molten rock to explode through the surface.  Io's volcanoes are so active that they are effectively turning the whole moon inside out.  Some of Io's volcanic lava is so hot it  glows in the dark.
Comets Bradfield and LINEAR Rising (2004-05-03)

Comet Bradfield is easy to see on the left, but can you find Comet LINEAR on the right?  Last week, just before sunrise from the northern hemisphere, two bright comets were visible in the same part of the sky at the same time.  The above long-exposure image was taken on the morning of April 25 from Joshua Tree National Park in California, USA.  Comet C/2004 F4 (Bradfield) is giving an unexpectedly good show as it recedes from the Sun and Earth and fades from view.  Its tail is estimated by some to be about 10 degrees long.  Having just rounded the Sun itself, Comet C/2002 T7 (LINEAR) is now moving toward the Earth.   Although intrinsically fading, T7 will appear to brighten until about mid-May and so continue to be visible to the unaided eye before sunrise to southern hemisphere observers into June.  Q4, the third coincidental naked eye comet, will become visible in mid-May to northern hemisphere observers.

Image credit: Wally Pacholka (Astropics)
Missoula Crater on Mars (2004-05-04)

Scroll right to see the rocks, craters, and hills that were in view for the Spirit rover last week as it continued its trek across Mars.   Missoula crater, taking up much of the above frame, appeared from orbit to have ejecta from Bonneville crater inside it.  Upon closer inspection, however, Spirit finds only evidence for wind-blown drifts.  The rocks show numerous blisters and small cavities that may have occurred as ancient water vapor evaporated from hot cooling lava.  Columbia Hills in the distance is now planned as the ultimate destination for the Spirit rover.  Both of the Mars rovers have now successfully completed their original mission and are now exploring topical opportunities.  Lunar Eclipse Today: Visible from Europe, Asia, Africa. Times | Webcast | Photo Tips
NGC 6302: Big, Bright, Bug Nebula (2004-05-05)

The bright clusters and nebulae of planet Earth's night sky are often named for flowers or insects, and NGC 6302 is no exception. With an estimated surface temperature of about 250,000 degrees C, the central star of this particular planetary nebula is exceptionally hot though -- shining brightly in ultraviolet light but hidden from direct view by a dense torus of dust.  Above is a dramatically detailed close-up of the dying star's nebula recorded by the Hubble Space Telescope. Cutting across a bright cavity of ionized gas, the dust torus surrounding the central star is in the upper left corner of this view, nearly edge-on to the line-of-sight. Surprisingly, minerals including water ice, along with complex hydrocarbon molecules have been detected in this hot star's dusty cosmic shroud. NGC 6302 lies about 4,000 light-years away in the arachnologically correct constellation Scorpius.
A Lunar Eclipse Mosaic (2004-05-06)

From start to finish, this impressive digital mosaic covers May 4th's total eclipse of the Moon. Astronomer Anthony Ayiomamitis reports that the eclipse viewing was wonderful from Greece, where skies cleared shortly before the celestial show began. His mosaic includes images, recorded at five minute intervals and arranged sequentially in an arc, that trace the progress of the inner portion of the Earth's shadow or umbra across the lunar disk. A series depicting a surprisingly dark, totally eclipsed Moon is featured in the center. While May 4th's total lunar eclipse favored observers in Europe, Africa, and Asia, the Moon's next trip through Earth's shadow, scheduled for October 28, will give skygazers in the Americas a turn.

Image credit: Anthony Ayiomamitis
Look West for a NEAT Comet (2004-05-07)

On May 5th, while scanning western skies after sunset, astronomer Jimmy Westlake was glad to spot a visitor from the outer solar system, Comet NEAT, with his own eyes. Taken with a normal lens, the picture records his memorable view of comet, clouds, and Colorado Rocky Mountains against a backdrop of many faint stars (most not visible to the unaided eye) and one very bright one. In the three minute time exposure, the comet is seen as a fuzzy greenish smudge left of center, with brilliant Sirius, alpha star of the constellation Canis Major, just above the low cloud bank on the right. Comet NEAT (C/2001 Q4) is now near its closest aproach to planet Earth and tonight will lie well above bright Sirius. Look for the comet - the third naked-eye comet in as many weeks - after sunset in clear, dark, western skies.

Image credit: Jimmy Westlake
(Colorado
Mountain College)
Good Morning Sydney (2004-05-08)

Rising before dawn on May 5th, Stephen Thorley looked out across the skyline of Sydney, Australia. And while a leisurely lunar eclipse was clearly in progress, from his vantage point on planet Earth the Moon set as the total phase of the eclipse began. Still, before the setting Moon was hidden by the cityscape he captured this striking image of a nearly eclipsed lunar disk sliding past the beacon and lights of Centerpoint Tower, one of Sydney's familiar landmarks. So what's that star just visible above and to the right of the reddened Moon? That's Zubenelgenubi, of course.

Image credit: Stephen Thorley