juliensimon/nasa-apod huggingface dataset:
Saturnian Aurora (2000-09-17)

Girdling the second largest planet in the Solar System, Saturn's Rings are one of the most spectacular sights for earthbound telescopes. This image from the orbiting Hubble Space Telescope's STIS instrument, offers a striking view of another kind of ring around Saturn - pole encircling rings of ultraviolet aurora. Towering more than 1,000 miles above the cloud tops, these Saturnian auroral displays are analogous to Earth's. Energetic charged particles in the Solar Wind are funneled by the planet's magnetic field into polar regions where they interact with atmospheric gases. Following the ebb and flow of Saturn's aurora, researchers can remotely explore the planet's atmosphere and magnetic field. In this false color image, the dramatic red aurora identify emission from atomic hydrogen, while the more concentrated white areas are due to hydrogen molecules. In 2004, NASA plans to begin making close-up studies of the Saturnian system with the Cassini Spacecraft.
Approaching the International Space Station (2000-09-18)

Last Monday the crew of Space Shuttle Atlantis took in this view as they approached the developing International Space Station (ISS).   From top to bottom, the astronauts saw a station currently consisting of the Progress supply module, the Zvezda service module, the Zarya cargo module, and the Unity connecting module.  Never before had astronauts seen the station since the remote-controlled additions of Progress and Zvezda.  Energy collecting flat solar panels can be seen extending from some of the modules.  Soon after this picture was taken, Atlantis docked with the ISS at the Unity end.  The astronauts have worked hard unloading supplies, installing and testing equipment, and even planning to reboost the floating space station to a higher orbit.  The Shuttle and its entire crew are scheduled to return to Earth Wednesday.   The Space Shuttle Discovery is then scheduled to visit the ISS in two weeks.
M17: Omega Nebula Star Factory (2000-09-19)

In the depths of the dark clouds of dust and molecular gas known as M17, stars continue to form.  Visible in the above recently released representative-color photograph of M17 by the New Technology Telescope are clouds so dark that they appear almost empty of near infrared light.  The darkness of these molecular clouds results from background starlight being absorbed by thick carbon-based smoke-sized dust.  As bright massive stars form, they produce intense and energetic light that slowly boils away the dark shroud.  M17's unusual appearance has garnered it such nicknames as the Omega Nebula, the Horseshoe Nebula, and the Swan Nebula.  M17, visible with binoculars towards the constellation of Sagittarius, lies 5000 light-years away and spans 20 light-years across.
Gangly Spiral Galaxy NGC 3184 (2000-09-20)

NGC 3184 is a large spiral galaxy with a small nucleus and long sprawling spiral arms.  Although NGC 3184 contains hundreds of billions of stars, the blue color of its spiral arms comes mostly from relatively few bright young blue stars. The galaxy is not empty of matter between these spiral arms -- the bright stars that highlight the arms were created in huge density waves that circle the center.  Visible with a small telescope towards the constellation of Ursa Major, light takes about 25 million years to reach us from NGC 3184, and about 50,000 years just to cross it.  NGC 3184 (Hubble type Sbc) is notable for its high abundance of heavy elements and a supernova that has occurred there recently.
XZ Tauri System Ejects Gas Bubble (2000-09-21)

Why is the binary star system XZ Tauri emitting a hot bubble of expanding gas? Although astronomers can only presently speculate, the Hubble Space Telescope clearly documents this unusual behavior in three dramatic photographs over the past five years.  Even without knowing why, the recently released sequence shows in unprecedented clarity the beginnings of a cooling zone -- a region where the expanding gas bubble cools off by emitting light as electrons and ions meet and recombine.   The XZ Tauri star system is known to reside in the Taurus star forming region located about 500 light-years away.  XZ Tau is composed of two very young stars separated by roughly the same distance as between our Sun and Pluto.  The bubble has been expanding over the past thirty years and now extends to nearly fifteen times the binary separation.
M55: Globular Star Cluster (2000-09-22)

The fifty-fifth entry in Charles Messier's catalog, M55 is a large and lovely globular cluster of around 100,000 stars. Only 20,000 light-years away in the constellation Sagittarius, M55 appears to earth-bound observers to be nearly 2/3 the size of the full moon. Globular star clusters like M55 roam the halo of our Milky Way Galaxy as gravitationally bound populations of stars known to be much older than stellar groups found in the galactic disk. Astronomers who make detailed studies of globular cluster stars can accurately measure the cluster ages and distances. Their results ultimately constrain the age of the Universe (... it must be older than the stars in it! ), and provide a fundamental rung on the astronomical distance ladder. This stunning three-color image made with astronomical (BVI) filters spans about 100 light-years across the globular cluster M55.
The Equal Night (2000-09-23)

Yesterday the Sun crossed the celestial equator heading south, marking the Equinox -- the first day of Autumn in the northern hemisphere and Spring in the south. Equinox means equal night and with the Sun on the celestial equator, Earthlings will experience 12 hours of daylight and 12 hours of darkness. For those in the northern hemisphere, the days will continue to grow shorter with the Sun marching lower in the sky as winter approaches. A few weeks after the Autumnal Equinox of 1994, the Crew of the Shuttle Endeavor recorded this image of the Sun poised above the Earth's limb. Glare illuminates Endeavor's vertical tail (pointing toward the Earth) along with radar equipment in the payload bay.
M16: Stars from Eagle's EGGs (2000-09-24)

Newborn stars are forming in the Eagle Nebula.  This image, taken with the Hubble Space Telescope in 1995, shows evaporating gaseous globules (EGGs) emerging from pillars of molecular hydrogen gas and dust.  The giant pillars are light years in length and are so dense that interior gas contracts gravitationally to form stars.  At each pillars' end, the intense radiation of bright young stars causes low density material to boil away, leaving stellar nurseries of dense EGGs exposed.  The Eagle Nebula, associated with the open star cluster M16, lies about 7000 light years away.
AR 9169: A Large Sunspot (2000-09-25)

One of the largest sunspots in recent years is now crossing our Sun.  Dominating active region AR 9169, the sunspot is the large dark complex visible below (west) and right of center in the above photograph of our Sun taken last Thursday.  Although quiet so far, the regions in and around this large magnetic depression are under watch for explosive eruptions.  AR 9169 holds the largest sunspot yet produced during this Solar Maximum, the time of greatest sunspot activity during the Sun's 11-year magnetic cycle.  The sunspot in AR 9169 is over twice the diameter of a normal sunspot, but only about half the area of a sunspot once recorded in 1947.  Rotating with the Sun, AR 9169 will take about 30 days to make one complete circle, slowly breaking up during this time.

Image credit: Ray Gralak
Approaching Jupiter (2000-09-26)

In 1979 the Voyager 1 spacecraft compiled this view as it approached the gas giant Jupiter.  Snapping a picture every time the Great Red Spot was properly aligned, the above time-lapse sequence shows not only spot rotation but also the swirling of neighboring clouds.  Since Jupiter takes about 10 hours to rotate, this short sequence actually covers several days.  Voyager 1 shot past Jupiter rapidly taking pictures on which many discoveries would be made, including previously unknown cloud patterns, rings, moons, and active volcanoes on Jupiter's moon Io.  Voyager is moving so fast that it will one day leave our Solar System.
Yepun (2000-09-27)

Pictured above on September 3rd, the enclosure for the 8.2 meter telescope christened Yepun glints dramatically in the light of the setting sun. Later that evening, under dark skies at Paranal Observatory, Chile, astronomers and engineers successfully captured Yepun's first light images, making Yepun the fourth and final unit of the European Southern Observatory's Very Large Telescope (VLT) array to reach this milestone. Ultimately, the light from the three other 8.2 meter unit telescopes (Antu, Kueyen, and Melipal) will be combined with Yepun's to achieve an effective aperture of 16.4 meters -- creating the world's largest optical telescope. But the next major step will be to combine beams from two of the telescopes creating an interferometer. The upper part of the mostly subterranean interferometer lab is the building in front of the telescope enclosure. The VLT unit telescope names have been taken from the Mapuche language. Originally thought to refer to the bright star Sirius, the word Yepun is now believed by linguists to mean Venus or evening star.
Heating Coronal Loops (2000-09-28)

ending above the photosphere or visible surface of the Sun, the faint, tenuous solar corona can't be easily seen from Earth, but it is measured to be hundreds of times hotter than the photosphere itself. What makes the solar corona so hot? Astronomers have long sought the source of the corona's heat in magnetic fields which loft monstrous loops of solar plasma above the photosphere. Still, new and dramatically detailed observations of coronal loops from the orbiting TRACE satellite are now pointing more closely to the unidentified energy source. Recorded in extreme ultraviolet light, this and other TRACE images indicate that most of the heating occurs low in the corona, near the bases of the loops as they emerge from and return to the solar surface. The new results confound the conventional theory which relies on heating the loops uniformly. This tantalizing TRACE image shows clusters of the majestic, hot coronal loops which span 30 or more times the diameter of planet Earth.
September Sky (2000-09-29)

Tomorrow's picture: Titania's Trenches  < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >  Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris. Specific rights apply. A service of: LHEA at NASA/GSFC & Michigan Tech. U.

Image credit: Stan Richard
Titania's Trenches (2000-09-30)

British astronomer Sir William Herschel discovered Titania and Oberon in January of 1787. He wasn't reading Shakespeare's A Midsummer Night's Dream though, he was making the first telescopic observations of moons of the planet Uranus (a planet which he himself discovered in 1781). In January of 1986, nearly 200 years later, NASA's robot explorer Voyager 2 became the only spacecraft to visit the remote Uranian system. Above is Voyager's highest resolution picture of Titania, Uranus' largest moon. The picture is a composite of two images recorded from a distance of 229,000 miles. The icy, rocky world is seen to be covered with impact craters. A prominent system of fault valleys, some nearly 1,000 miles long, is visible as trench-like features near the terminator (shadow line). Deposits of highly reflective material which may represent frost can be seen along the sun-facing valley walls. The large impact crater near the top, known as Gertrude, is about 180 miles across. At the bottom the 60 mile wide fault valley, Belmont Chasma, cuts into crater Ursula. Titania itself is 1,000 miles in diameter.
The Center of Centaurus A (2000-10-01)

A fantastic jumble of young blue star clusters, gigantic glowing gas clouds, and imposing dark dust lanes surrounds the central region of the active galaxy Centaurus A. This mosaic of Hubble Space Telescope images taken in blue, green, and red light has been processed to present a natural color picture of this cosmic maelstrom. Infrared images from the Hubble have also shown that hidden at the center of this activity are what seem to be disks of matter spiraling into a black hole with a billion times the mass of the Sun! Centaurus A itself is apparently the result of a collision of two galaxies and the left over debris is steadily being consumed by the black hole. Astronomers believe that such black hole central engines generate the radio, X-ray, and gamma-ray energy radiated by Centaurus A and other active galaxies. But for an active galaxy Centaurus A is close, a mere 10 million light-years away, and is a relatively convenient laboratory for exploring these powerful sources of energy.
Pangea Ultima: Earth in 250 Million Years? (2000-10-02)

Is this what will become of the Earth's surface?  The surface of the Earth is broken up into several large plates that are slowly shifting.   About 250 million years ago, the plates on which the present-day continents rest were positioned quite differently, so that all the landmasses were clustered together in one supercontinent now dubbed Pangea.  About 250 million years from now, the plates are again projected to reposition themselves so that a single landmass dominates.  The above simulation from the PALEAOMAP Project shows this giant landmass: Pangea Ultima.  At that time, the Atlantic Ocean will be just a distant memory, and whatever beings inhabit Earth will be able to walk from North America to Africa.

Image credit: C. R. Scotese 
(U. Texas at Arlington), 
PALEOMAP
Saturn Rotates (2000-10-03)

The dramatic rotation of the cloud-tops of Saturn every ten-hours is particularly evident from orbit around the gas giant planet.  With a good enough telescope, however, such rotation is visible even from Earth, as shown by this time-lapse image sequence from the Hubble Space Telescope taken in November 1990.  Particularly evident at that time was a light-colored giant storm cloud system that completely encircled the planet.  The storm was not evident twenty years ago during the flybys of the Voyager spacecraft -- a storm of this magnitude was last noted in 1933.  Studying the complex atmosphere of Saturn will be one objective of the Cassini spacecraft launched by NASA in 1997 and expected to arrive in 2004.
Barred Spiral Galaxy NGC 1300 (2000-10-04)

NGC 1300 is a large spiral galaxy that appears as a flattened figure eight.  A huge bar that spans over 150,000 light-years across the galaxy center dominates its appearance.  The picturesque galaxy lies about 75 million light-years distant, so that light that we see now left during the age of the dinosaurs. Although it is well known how fast different parts of NGC 1300 rotate, the specific orbits of many component stars -- including how they interact with the gigantic bar -- remains a topic of research.  Our own Milky Way Galaxy is a spiral galaxy with a less prominent bar.  NGC 1300 can be seen with a small telescope towards the constellation of Eridanus.

Image credit: S. Lee & 
D. F. Malin, 
AAO
N81: Star Cradle in the SMC (2000-10-05)

This dramatic Hubble Space Telescope image captures the birth of a cluster of massive stars. The newborn stars are seen just as they emerge from their natal nebula. Only 12 light-years across, the nebula is cataloged as N81 and lies some 200,000 light-years away within a neighboring galaxy known as the Small Magellanic Cloud (SMC). Other nebulae which surround massive star clusters can be a thousand or more light-years across.  But, prior to the Hubble observations, it was unknown whether N81 and similar, compact emission nebulae were cradles of single stars or star clusters. In the case of N81, the Hubble data clearly reveal multiple hot stars, some nearly 300,000 times as luminous as the Sun. The colorful image emphasizes graceful arcs of dark interstellar dust and fluorescing gas sculpted by the young stars' energetic winds and radiation.
X-Rays From Sirius B (2000-10-06)

In visible light Sirius A (Alpha Canis Majoris) is the brightest star in the night sky, a closely watched celestial beacon throughout recorded history. Part of a binary star system only 8 light-years away, it was known in modern times to have a small companion star, Sirius B. Sirius B is much dimmer and appears so close to the brilliant Sirius A that it was not actually sighted until 1862, during Alvan Clark's testing of a large, well made optical refracting telescope. For orbiting x-ray telescopes, the Sirius situation is exactly reversed, though. A smaller but hotter Sirius B appears as the overwhelmingly intense x-ray source in this Chandra Observatory x-ray image (lines radiating from Sirius B are image artifacts). The fainter source seen at the position of Sirius A may be largely due to ultraviolet light from the star leaking into the x-ray detector. With a surface temperature of 25,000 kelvins, the mass of the Sun, and a radius just less than Earth's, Sirius B is the closest known white dwarf star. Can you guess what makes Sirius B like Neptune, the Sun's most distant gas giant planet? While still unseen, the presence of both celestial bodies was detected based on their gravitational influence alone ... making them early examples of dark matter.
Sputnik: Traveling Companion (2000-10-07)

Sputnik means "traveling companion". Despite the innocuous sounding name, the launch of the Earth's first "artificial moon", Sputnik 1, by the Soviet Union on October 4, 1957 shocked the free world, setting in motion events which resulted in the creation of NASA and the race to the Moon. Sputnik 1 was a 184 pound, 22 inch diameter sphere with four whip antennas connected to battery powered transmitters. The transmitters broadcast a continuous "beeping" signal to an astounded earthbound audience for 23 days. A short month later, on November 3, the Soviet Union followed this success by launching a dog into orbit aboard Sputnik 2.
Earth's Richat Structure (2000-10-08)

What on Earth is that? The Richat Structure in the Sahara Desert of Mauritania is easily visible from space because it is nearly 50 kilometers across.  Once thought to be an impact crater, the Richat Structure's flat middle and lack of shock-altered rock indicates otherwise. The possibility that the Richat Structure was formed by a volcanic eruption also seems improbable because of the lack of a dome of igneous or volcanic rock.  Rather, the layered sedimentary rock of the Richat structure is now thought by many to have been caused by uplifted rock sculpted by erosion.  Why the Richat Structure is nearly circular remains a mystery.
A Polar Martian Dust Storm (2000-10-09)

On August 29, a large dust storm was photographed erupting out from the north polar cap of Mars.  Such dust storms are not uncommon as summer advances in the north.  In the above picture taken by the Mars Global Surveyor spacecraft currently orbiting Mars, the white material is frozen carbon dioxide that covers much of the extreme north.  As the north polar cap region begins to thaw, a temperature difference occurs between the cold frost region and recently thawed surface, resulting in swirling winds between the adjacent regions.  Visible in the storm is a strong central jet about 900 kilometers long that is creating symmetric swirling vortices.  Although winds can reach 100 km/hour, the thin atmosphere of Mars usually makes such storms less destructive than similar storms on Earth.
The Einstein Cross Gravitational Lens (2000-10-10)

Most galaxies have a single nucleus -- does this galaxy have four?  The strange answer leads astronomers to conclude that the nucleus of the surrounding galaxy is not even visible in this image.  The central cloverleaf is rather light emitted from a background quasar.  The gravitational field of the visible foreground galaxy breaks light from this distant quasar into four distinct images.  The quasar must be properly aligned behind the center of a massive galaxy for a mirage like this to be evident.  The general effect is known as gravitational lensing, and this specific case is known as the Einstein Cross.  Stranger still, the images of the Einstein Cross vary in relative brightness, enhanced occasionally by the additional gravitational microlensing effect of specific stars in the foreground galaxy.

Image credit: J. Rhoads 
(STScI) et al., 
WIYN, 
AURA,
NOAO, 
NSF
Cassini Spacecraft Approaches Jupiter (2000-10-11)

A new spacecraft has entered the outer Solar System: Cassini.  Launched in 1997 and bound for Saturn in 2004, Cassini sent back the above image last week while approaching the giant planet Jupiter.  Cassini joins the Galileo spacecraft currently in orbit around Jupiter in studying the gas giant and its moons.  In fact, observations involving both spacecraft simultaneously are planned in the coming months.  This color picture was taken when Cassini was 81.3 million kilometers from Jupiter. The alternating dark and bright bands characteristic of Jupiter's cloud tops can be easily seen. Jupiter's moon Europa is also seen at the far right of the image casting a round shadow on the planet.
HETE-2 Rides Pegasus (2000-10-12)

The Stargazer, a modified Lockheed L-1011 aircraft, soared into the skies above Kwajalein Atoll in the pacific on October 9th. A small satellite observatory known as the High Energy Transient Explorer - 2 (HETE-2) was tucked into Stargazer's winged Pegasus rocket, slung beneath the large trimotor jet's fuselage. Dropped from its mother ship, the Pegasus then successfully flew HETE-2 into orbit. HETE-2's mission is to hunt gamma-ray bursts, brief, random flashes of high energy photons from the distant cosmos. Gamma-ray bursts are impressive, believed to be the most powerful explosions in the Universe, but so few have been well located and studied that the nature of the bursters themselves is still shrouded in mystery. HETE-2's x-ray and gamma-ray instruments will be able to rapidly alert ground-based observatories to point toward ongoing, bright gamma-ray bursts. Communications antennae and solar panels neatly folded, HETE-2 is seen here being carefully enclosed in the Pegasus nose fairing.
Eclipse Moon Trail (2000-10-13)

Tomorrow's picture: Ecliptic Plane  < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >  Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Technical Rep.: Jay Norris. Specific rights apply. A service of: LHEA at NASA/GSFC & Michigan Tech. U.

Image credit: Joe Orman
The Ecliptic Plane (2000-10-14)

The Plane of the Ecliptic is well illustrated in this picture from the 1994 lunar prospecting Clementine spacecraft. Clementine's star tracker camera image reveals (from right to left) the Moon lit by Earthshine, the Sun's corona rising over the Moon's dark limb, and the planets Saturn, Mars, and Mercury. The ecliptic plane is defined as the imaginary plane containing the Earth's orbit around the Sun. In the course of a year, the Sun's apparent path through the sky lies in this plane. The Solar System's planetary bodies all tend to lie near this plane, since they were formed from the Sun's spinning, flattened, proto-planetary disk. The snapshot above nicely captures a momentary line-up looking out along this fundamental plane of our Solar System.
Globular Cluster Omega Centauri (2000-10-15)

Does an old, red globular cluster have any hot, blue stars?  The rightmost picture, taken by the Ultraviolet Imaging Telescope in ultraviolet light, shows that indeed it does.  Pictured, Omega Centauri is the largest known globular cluster of over 200 in our Galaxy, containing well over a million stars.  Many of these stars are evident in the visible light photograph on the left.  When photographed in ultraviolet light, however, different and less numerous stars emerge, as evident on the photograph on the right.   Most of these stars are thought to have evolved past the current stage of our Sun.  These stars no longer fuse hydrogen to helium in their core but rather fuse helium into carbon.  These stars will soon shed their outer envelopes and end up as smoldering carbon embers known as white dwarf stars.