juliensimon/nasa-apod huggingface dataset:
Neptune through Adaptive Optics (2000-02-18)

From the Earth's surface, Neptune usually appears as a fuzzy blotch.  The blurring effects of the Earth's atmosphere deny clearer images.  By distorting mirrors in the telescope itself in time with the changing atmosphere, however, these effects can be greatly reduced.  Many of the world's largest telescopes are now implementing these "rubber mirror" adaptive optics (AO) systems to bring out the finest details that these telescopes can resolve.  Recently the 10-meter Keck II telescope in Hawaii came on-line with AO capability.  The above image of Neptune in three infrared colors demonstrates the clarity of the new technique - as compared to an image of Neptune from Keck II without AO.
Young Suns (2000-02-19)

The star cataloged as NGC2264 IRS is normally hidden from the inquiring gaze of optical telescopes. It resides in the midst of the obscuring gas and dust of a nearby star forming region popularly known as the Cone Nebula. Imaged in penetrating infrared light by the Hubble Space Telescope's NICMOS instrument, this young and massive star was found to be surrounded by six "baby" sun-like stars - all within less than a tenth of a light-year of their "big brother". The diffraction spikes and rings surrounding big brother are image artifacts. Astronomers believe that the high speed winds generated by the massive star compressed nearby material causing the formation of the smaller stars in a text book example of triggered star formation. The young suns appear to lie along an otherwise invisible boundary where the high speed gas has collided with the wall of a denser molecular cloud. NGC2264 IRS also seems to be the source of the outflow which created the striking cone shape of the optical nebula.
The Virgo Cluster of Galaxies (2000-02-20)

Pictured are several galaxies of the Virgo Cluster, the closest cluster of galaxies to our Milky Way Galaxy.  The Virgo Cluster spans more than 5 degrees on the sky - about 10 times the angle made by a full Moon. It contains over 100 galaxies of many types - including spirals, ellipticals, and irregular galaxies.  The Virgo Cluster is so massive that it is noticeably pulling our Galaxy toward it.  The cluster contains not only galaxies filled with stars but also gas so hot it glows in X-rays.  Motions of galaxies in and around clusters indicate that they contain more dark matter than any visible matter we can see.  Notable bright galaxies in the Virgo Cluster include bright Messier objects such as M61, M87, M90, and M100.

Image credit: Digitized Sky Survey,
Palomar Observatory, 
STScI
A Giant Gouge on Asteroid Eros (2000-02-21)

Asteroid 433 Eros is posing several riddles. NASA's robot spacecraft NEAR began orbiting the 30-kilometer space rock last week, and new pictures are now being beamed back to Earth regularly.  As usual in science, when you arrive at a place you've never been before, you see things you don't immediately understand.  Scientists are contemplating, for example, the cause of the above-pictured giant gouge in the middle of Eros.  Intriguing internal features include groves oriented parallel to the asteroid's length and the relative paucity of craters.  These indicate that the gouge formed well after the asteroid itself.  One question answered by the shape and density is that, unlike asteroid 253 Mathilde, Eros is not a pile of rocks but one big rock.  Astronomers are hopeful that data taken over the coming year might indicate the nature and origin of the baffling bright patches.
Neighboring Galaxy: The Large Magellanic Cloud (2000-02-22)

The brightest galaxy visible from our own Milky Way Galaxy is the Large Magellanic Cloud (LMC).  Visible predominantly from Earth's Southern Hemisphere, the LMC is the second closest galaxy, neighbor to the Small Magellanic Cloud, and one of eleven known dwarf galaxies that orbit our Milky Way Galaxy.  The LMC is an irregular galaxy composed of a bar of older red stars, clouds of younger blue stars, and a bright red star forming region visible near the top of the above image called the Tarantula Nebula.  The brightest supernova of modern times, SN1987A, occurred in the LMC.

Image credit: AURA/
NOAO/
NSF
Sunspot Seething (2000-02-23)

Our Sun's surface is continually changing.  This time-lapse movie shows in five seconds what happens in 20 minutes on the Sun's surface near a sunspot.  Visible is boiling granulation outside the sunspot, inward motion of bright grains in the outer penumbral region toward the sunspot, and the churning of small magnetic elements between  solar granules.  Sunspots themselves are relatively cool regions of the solar surface depressed by magnetic fields. The dark lanes surrounding the sunspot are called penumbral filaments, and recent computer simulations have shown that their behavior is also dominated by magnetic fields.  The above sequence was taken with the new Dutch Open Telescope last September and focused on a sunspot that measured about 25,000 kilometers across.
Stereo Eros (2000-02-24)

Get out your red/blue glasses and float next to asteroid 433 Eros, 260 million kilometers away! Orbiting the Sun once every 1.8 earth-years, asteroid Eros is a diminutive 40 x 14 x 14 kilometer world of undulating horizons, craters, boulders and valleys. Its unsettling scale and bizarre shape are emphasized in this picture - a mosaic of recent images from the NEAR spacecraft processed to yield a stereo anaglyphic view.  Along with dramatic chiaroscuro, NEAR's 3-D imaging provides important measurements of the asteroid's landforms and structures, and hopefully clues to the origin of this city-sized chunk of solar system. The smallest features visible here are about 30 meters across.
The Comets Of SOHO (2000-02-25)

After four years of successful sun-gazing, the space-based SOlar and Heliospheric Observatory (SOHO) has also become the most successful comet-hunter in history, racking up 102 new comets. Above are examples of SOHO's comet discoveries imaged by LASCO, an on-board coronagraph. LASCO was designed to monitor the solar corona out to a distance of about 12 million miles while blotting out the bright solar disk. Most of the comets discovered with SOHO belong to a special class known as sungrazers - thought to be returning fragments of a large comet which broke up during its historic passage near the sun.  Subject to intense solar heat and tidal forces, sungrazers (examples in the left column) are unlikely to survive their close passage. However, ten of SOHO's new comets, like those in the right column, are more "typical" comets and pass the sun at safe distances. In fact, on its outbound journey, the comet at the top right was bright enough to be seen with the unaided eye. Discovered this year on February 4th, the comet at the bottom right holds the distinction of being SOHO comet number 100.
Impact: 65 Million Years Ago (2000-02-26)

What killed the dinosaurs? Their sudden disappearance 65 million years ago, along with about 70 percent of all species then living on Earth, is known as the K-T event (Cretaceous-Tertiary Mass Extinction event). Geologists and paleontologists often entertain the idea that a large asteroid or comet impacting the Earth was the culprit. In such a cosmic catastrophe, the good(!) news would be that the impact would generate firestorms, tidal waves, earthquakes, and hurricane winds. As for the bad news ... debris thrown into the atmosphere would have a serious global environmental consequences, creating extended periods of darkness, low temperatures, and acid rains - resulting in a planet-wide extinction event. In 1990, dramatic support for this theory came from cosmochemist Alan Hildebrand's revelation of a 65 million year old, 112 mile wide ring structure still detectable under layers of sediment in the Yucatan Peninsula region of Mexico. The outlines of the structure, called the Chicxulub crater (named for a local village), are visible in the above representation of gravity and magnetic field data from the region. In addition to having the right age, the crater is consistent with the impact of an asteroid of sufficient size (6 to 12 miles wide) to cause the global disruptions. Regardless of the true cause of the K-T event, it is fortunate that such impacts are presently believed to happen only about once every 100 million years!
The Pleiades Star Cluster (2000-02-27)

It is the most famous star cluster on the sky. The Pleiades can be seen without binoculars from even the depths of a light-polluted city.  Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and closest open clusters.   The Pleiades contains over 3000 stars, is about 400 light years away, and only 13 light years across.  Quite evident in the above photograph are the blue reflection nebulae that surround the bright cluster stars.  Low mass, faint, brown dwarfs have recently been found in the Pleiades.

Image credit: David Malin 
(AAO),
AATB, 
ROE,
UKS Telescope
The Sombrero Galaxy from VLT (2000-02-28)

Why does the Sombrero Galaxy look like a hat?  Reasons include the Sombrero's unusually large and extended central bulge of stars, and dark prominent dust lanes that appear in a disk that we see nearly edge-on.  Billions of old stars cause the diffuse glow of the extended central bulge.  Close inspection of the bulge in the above photograph shows many points of light that are actually globular clusters. M104's spectacular dust rings harbor many younger and brighter stars, and show intricate details astronomers don't yet fully understand.  The very center of the Sombrero glows across the electromagnetic spectrum, and is thought to house a large black hole.  Fifty million-year-old light from the Sombrero Galaxy can be seen with a small telescope towards the constellation of Virgo.
Julius Caesar and Leap Days (2000-02-29)

ven as leap days go, today is a remarkable one.  In 46 BC, Julius Caesar, pictured above in a self-decreed minted coin, created a calendar system that added one leap day every four years. Acting on advice by Alexandrian astronomer Sosigenes, Caesar did this to make up for the fact that the Earth's year is slightly more than 365 days.  In modern terms, the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun -- actually we now know this takes about 365.24219 rotations). So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years.  Eventually July (named posthumously for Julius Caesar himself) would occur during the northern hemisphere winter!  By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Julian Calendar system was used until the year 1582 when Pope Gregory XIII added that leap days should not occur in years ending in "00" except if divisible by 400, providing further fine-tuning.  This Gregorian Calendar system is the one in common use today.  Therefore, even though this year 2000 ends in "00", it remains a leap year, and today is the added leap day. That makes today the first leap day for a centurial year since year 1600 and the second such leap day of the Gregorian Calendar.

Image credit: Rune Rysstad
M13: The Great Globular Cluster in Hercules (2000-03-01)

M13 is one of the most prominent and best known globular clusters.  Visible with binoculars in the constellation of Hercules, M13 is frequently one of the first objects found by curious sky gazers seeking celestials wonders beyond normal human vision.   M13 is a colossal home to over 100,000 stars, spans over 150 light years across, lies over 20,000 light years distant, and is over 12 billion years old.  At the 1974 dedication of Arecibo Observatory, a radio message about Earth was sent in the direction of M13.  The reason for the low abundance of unusual blue straggler stars in M13 is currently unknown.

Image credit: Yuugi Kitahara
NGC 1999: Reflection Nebula in Orion (2000-03-02)

A dusty bright nebula contrasts dramatically with a dusty dark nebula in this Hubble Space Telescope image recorded shortly after December's orbital servicing mission. The nebula, cataloged as NGC 1999, is a reflection nebula, which shines by reflecting light from a nearby star. Unlike emission nebulae, whose reddish glow comes from excited atoms of gas, reflection nebulae have a bluish cast as their interstellar dust grains preferentially reflect blue starlight. While perhaps the most famous reflection nebulae surround the bright young stars of the Pleiades star cluster, NGC 1999's stellar illumination is provided by the embedded variable star V380 Orionis, seen here just left of center. Extending right of center, the ominous dark nebula is actually a condensation of cold molecular gas and dust so thick and dense that it blocks light. From our perspective it lies in front of the bright nebula, silhouetted against the ghostly nebular glow. New stars will likely form within the dark cloud, called a Bok globule, as self-gravity continues to compress its dense gas and dust.  Reflection nebula NGC 1999 lies about 1500 light-years away in the constellation Orion, just south of Orion's well known emission nebula, M42.
Dust Storm on Planet Earth (2000-03-03)

From low Earth orbit, NASA's SeaWIFS instrument records ocean color, tracking changes in our water world's climate and biosphere. But even an ocean planet can have dust storms. On February 26th, SeaWIFS returned this dramatic close-up view of a vast, developing cloud of Saharan desert dust blowing from northwest Africa (lower right) a thousand miles or more out over the Atlantic Ocean. While there are indications that the planet-spanning effects of the Saharan dust events include the decline of the ecologies of coral reefs in the Caribbean and an increased frequency of Atlantic hurricanes, there is also evidence that the dust provides nutrients to the Amazonian rain forests. From space-based vantage points, other satellite images have also revealed storms which transport massive quantities of fine sand and dust across Earth's oceans.
Saturn At Night (2000-03-04)

From a spectacular vantage point over 1.4 billion kilometers from the sun, the Voyager 1 spacecraft looked back toward the inner solar system to record this startling view of Saturn's nightside. The picture was taken on November 16, 1980, some four days after the robot spacecraft's closest approach to the gorgeous gas giant. The crescent planet casts a broad shadow across its bright rings while the translucent rings themselves can be seen to cast a shadow on Saturn's cloud tops. Since Earth is closer to the sun than Saturn, only Saturn's dayside is visible to Earth-bound telescopes which could never take a picture like this one. After this successful flyby two decades ago, Voyager 1 has continued outward bound and is presently humanity's most distant spacecraft. The next spacecraft to approach Saturn will be Cassini, on course to arrive in 2004.
The Pipe Dark Nebula (2000-03-05)

The dark nebula predominant at the lower left of the above photograph is known as the Pipe Nebula.  The dark clouds, suggestively shaped like smoke rising from a pipe, are caused by absorption of background starlight by dust.  These dust clouds can be traced all the way to the Rho Ophiuchi nebular clouds on the right.  The brightest star in the field is Antares. Many types of nebula are highlighted here: the red are emission nebula, the blue are reflection nebula, and the dark are absorption nebula. This picture has been digitally enhanced.

Image credit: Jerry Lodriguss
Abell 2142: Clash of the Galaxy Clusters (2000-03-06)

Over the course of billions of years, whole clusters of galaxies merge.  Above is an X-ray image of Abell 2142, the result of the collision of two huge clusters of galaxies, and one of the most massive objects known in the universe.  This false-color image shows a concentration of gas 50 million degrees hot near the center of the resulting cluster.  Oddly, it is the relative coldness of the gas that makes this situation particularly interesting.  The center of Abell 2142 is surrounded by gas fully twice as hot, a temperature thought to have been created by energy released during the colossal collision.  Still, since we can only see a snapshot in time, much remains unknown about how clusters of galaxies form and coalesce.
Zal Patera on Jupiter's Moon Io (2000-03-07)

Tomorrow's picture: Closer to Asteroid Eros  < | 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.
Nearer To Asteroid Eros (2000-03-08)

As the robot spacecraft NEAR lowers itself toward asteroid 433 Eros, more surface details are becoming visible.  Last week's maneuvers brought NEAR to within 204 kilometers of the floating mountain's surface.  With increased resolution, NEAR's camera then documented Eros' unusual shape, craters large and small, boulders, and mysterious grooves similar to asteroid Gaspra and Martian moon Phobos.  If you could stand on Eros, you would still be too small to be visible on this recent image, which shows features as small as 20 meters across.  However, you would feel gravity only 1/1000 that on Earth, so that you could easily jump over even this large 5 kilometer wide crater.
Sun Storm: A Coronal Mass Ejection (2000-03-09)

Late last month another erupting filament lifted off the active solar surface and blasted this enormous bubble of magnetic plasma into space. Direct light from the sun is blocked in this picture of the event with the sun's relative position and size indicated by a white half circle at bottom center. The field of view extends 2 million kilometers or more from the solar surface. While hints of these explosive events, called coronal mass ejections or CMEs, were discovered by spacecraft in the early 70s this dramatic image is part of a detailed record of this CME's development from the presently operating SOlar and Heliospheric Observatory (SOHO) spacecraft.  Near the minimum of the solar activity cycle CMEs occur about once a week, but as we approach solar maximum rates of two or more per day are anticipated. Though this CME was clearly not headed for Earth, strong CMEs are seen to profoundly influence space weather, and those directed toward our planet and can have serious effects.
Sky and Planets (2000-03-10)

On February 10th, an evocative evening sky above Rocklin, California, USA inspired astrophotographer Steve Sumner to record this remarkable sight - five planets and the Moon. Near its first quarter phase, the bright Moon was intentionally overexposed but Saturn, Jupiter, Mars, and Mercury (and, of course, planet Earth's horizon) are all clearly visible in the deepening twilight. Notably absent in this grouping of naked-eye planets is Venus which is still putting in an early appearance as the morning star. This month, Mercury has joined Venus in the dawn twilight while Saturn, Jupiter, and Mars still shine brightly in the western sky at nightfall making another gorgeous close grouping with the crescent Moon.

Image credit: Steve Sumner
Messier Marathon (2000-03-11)

Gripped by an astronomical spring fever, it's once again time for many amateur stargazers to embark on a Messier Marathon! The Vernal Equinox occurs March 20, marking the first day of Spring for the Northern Hemisphere. It also marks a favorable celestial situation for potentially viewing all the objects in 18th century French astronomer Charles Messier's catalog in one glorious dusk to dawn observing run. This year a bright full moon will interfere with dark skies near the actual equinox, so good nights near new moon for weekend marathoners are March 11/12 and April 1/2. (As an added bonus all the planets in the solar system can be viewed on these dates.) Astronomer Paul Gitto has created this masterful Messier Marathon grid with 11 rows and 10 columns of Messier catalog objects. In numerical order, the grid begins with M1, the Crab Nebula, at upper left and ends with M110, a small elliptical galaxy in Andromeda (lower right). Gitto's images were made with a digital camera and a 10-inch diameter reflecting telescope.

Image credit: P. Gitto
Supernova 1994D and the Unexpected Universe (2000-03-12)

Far away, long ago, a star exploded.  Supernova 1994D, visible as the bright spot on the lower left, occurred in the outskirts of disk galaxy NGC 4526. Supernova 1994D was not of interest for how different it was, but rather for how similar it was to other supernovae.  In fact, the light emitted during the weeks after its explosion caused it to be given the familiar designation of a Type Ia supernova.  If all Type 1a supernovae have the same intrinsic brightness, then the dimmer a supernova appears, the farther away it must be.  By calibrating a precise brightness-distance relation, astronomers are able to estimate not only the expansion rate of the universe (parameterized by the Hubble Constant), but also the geometry of the universe we live in (parameterized by Omega and Lambda). The large number and great distances to supernovae measured over the past few years have been interpreted as indicating that we live in a previously unexpected universe.
A Panorama of Oddities in Orion A (2000-03-13)

New stars, fast jets, and shocked gas clouds all occupy Orion A, a giant molecular cloud just south of the Orion Nebula.  The bright object visible below and slightly left of center of this recently released picture is the reflection nebula NGC 1999.  Wind from NGC 1999's central star, V380 Orionis, appears to have created the surrounding billows of red and brown gas.  Several bright young stars illuminate reflecting dust at the top right of the image.  Jets shoot from dozens of young stars creating glowing compressed shocked waves known Herbig-Haro objects.  One such shock is the unusual Waterfall, the bright streak on the upper right, which is a source of unusual radio waves.  The cone-shaped shock to the Waterfall's lower right may result from a jet emitted HH1 and HH2, located 10 light-years away below NGC 1999.  The unusual and energetic oddities that occur and interact in star forming regions are often as complex as they are beautiful.
A GRB 000301C Symphony (2000-03-14)

Telescopic instruments in Earth and space are still tracking a tremendous explosion that occurred across the universe.  A nearly unprecedented symphony of international observations began abruptly on March 1 when Earth-orbiting RXTE, Sun-orbiting Ulysses, and asteroid-orbiting NEAR all detected a 10-second burst of high-frequency gamma radiation.  Within 48 hours astronomers using the 2.5-meter Nordic Optical Telescope chimed in with the observation of a middle-frequency optical counterpart that was soon confirmed with the 3.5-meter Calar Alto Telescope in Spain.  By the next day the explosion was picked up in low-frequency radio waves by the by the European IRAM 30-meter dish in Spain, and then by the VLA telescopes in the US.   The Japanese 8-meter Subaru Telescope interrupted a maiden engineering test to trumpet in infrared observations.  Major telescopes across the globe soon began playing along as GRB 000301C came into view, detailing unusual behavior.  The Hubble Space Telescope captured the above image and was the first to obtain an accurate distance to the explosion, placing it near redshift 2, most of the way across the visible universe.  The Keck II Telescope in Hawaii quickly confirmed and refined the redshift.  Still, no one is sure what type of explosion this was.  The symphony is not over - oddly no host galaxy appears near the position of this explosion.  Will one appear as the din of the loud fireball fades?