juliensimon/nasa-apod huggingface dataset:
NGC 2244: A Star Cluster in the Rosette Nebula (2000-08-22)

In the heart of the Rosette Nebula lies a bright open cluster of stars that lights up the nebula.  The stars of NGC 2244 formed from the surrounding gas only four million years ago and emit light and wind that define the nebula's appearance today.  High energy light from the bright young stars of NGC 2244 ionizes the surrounding hydrogen gas clouds to create the red emission nebula appearance.  The hot wind of particles that streams away from the cluster stars contributes to an already complex menagerie of gas and dust filaments while slowly evacuating the cluster center.  NGC 2244 measures about 50 light-years across, lies about 4500 light-years away, and is visible with binoculars towards the constellation of Monoceros.

Image credit: J. C. Cuillandre & G. Fahlman 
(CFHT Staff), 
CFHT
NGC 6960: The Witch's Broom Nebula (2000-08-23)

Ten thousand years ago, before the dawn of recorded human history, a new light must suddenly have appeared in the night sky and faded after a few weeks.  Today we know this light was an exploding star and record the colorful expanding cloud as the Veil Nebula.  Pictured above is the west end of the Veil Nebula known technically as NGC 6960 but less formally as the Witch's Broom Nebula.  The rampaging gas gains its colors by impacting and exciting existing nearby gas.  The supernova remnant lies about 1400 light-years away towards the constellation of Cygnus.  This Witch's Broom actually spans over three times the angular size of the full Moon.  The bright blue star 52 Cygnus is visible with the unaided eye from a dark location but unrelated to the ancient supernova.

Image credit: Robert Gendler
Eros At Sunset (2000-08-24)

Gleaming in the rays of the setting sun, boulders litter the rugged surface of asteroid 433 Eros. The brightest boulder, at the edge of the large, shadowy crater near this picture's bottom center, is about 30 meters (100 feet) across. In orbit around Eros since February 2000, the NEAR Shoemaker spacecraft's camera recorded the dramatic view earlier this month from an altitude of about 50 kilometers. Eros itself orbits the Sun with a perihelion of 1.13 Astronomical Units (AU) and aphelion of 1.78 AU. Part of a class of near-Earth asteroids, it spends much of its time between the orbits of Mars (at 1.5 AU) and Earth (at 1 AU) ... but it wasn't always that way. Eros and other near-Earth asteroids originally orbited in the main asteroid belt, between Jupiter and Mars. Over time, the gravitational influence of Jupiter and other planets perturbed their orbits sending them on trajectories closer to Earth.
Folding Europa (2000-08-25)

Astypalaea Linea on Jovian ice moon Europa is the broad smooth region running through these images recorded by the Galileo spacecraft in 1998. The pictures are different computer processed versions of the same mosaic -- on the left, small scale details have been enhanced while on the right, large scale features are emphasized. In both versions, the bold criss-crossing ridges believed to result from the upwelling of new material through cracks in the surface ice are apparent. But more easily seen on the right are recently recognized gentle rises and dips, about 15 kilometers across, which likely formed as the icy surface was compressed by the addition of the new material. Further evidence that stress is folding Europa's surface is offered by the presence of smaller cracks and wrinkles more easily seen on the left. These span the width of the broad swells suggestive of anticlines and synclines familiar to geologists on planet Earth. Though ice covered, the surface of Europa is thought to be geologically active, riding over a substantial ocean of liquid water.
Mir Dreams (2000-08-26)

This dream-like image of Mir was recorded by astronauts as the Space Shuttle Atlantis approached the Russian space station prior to docking during the STS-76 mission. Sporting spindly appendages and solar panels, Mir resembles a whimsical flying insect hovering about 350 kilometers above New Zealand's South Island and the city of Nelson, near Cook Strait. In late March 1996, Atlantis shuttled astronaut Shannon W. Lucid to Mir for a five month visit, increasing Mir's occupancy from 2 to 3. It returned to pick Lucid up and drop off astronaut John Blaha during the STS-79 mission in August of that year. Since becoming operational in 1986, Mir has been visited by over 100 spacefarers from the nations of planet Earth including, Russia, the United States, Great Britain, Germany, France, Japan, Austria, Kazakhstan and Slovakia.  After joint Shuttle-Mir training missions in support of the International Space Station, continuous occupation of Mir ended in August 1999. Mir is still in orbit and its operation is now being pursued by commercial interests.
Orion's Horsehead Nebula (2000-08-27)

The Horsehead Nebula is one of the most famous nebulae on the sky.  It is visible as the black indentation to the red emission nebula seen just to the right of center of the above photograph.  The bright star near the center is located in the belt of the familiar constellation of Orion. The horse-head feature is dark because it is really an opaque dust cloud which lies in front of the bright red emission nebula. Like clouds in Earth's atmosphere, this cosmic cloud has assumed a recognizable shape by chance. After many thousands of years, the internal motions of the cloud will alter its appearance.  The emission nebula's red color is caused by electrons recombining with protons to form hydrogen atoms.  Also visible in the picture are blue reflection nebulae, which preferentially reflect the blue light from nearby stars.

Image credit: (AAO),
AATB, 
ROE,
UKS Telescope
The Helix Nebula from CFHT (2000-08-28)

One day our Sun may look like this.  The Helix Nebula is the closest example of a planetary nebula created at the end of the life of a Sun-like star.  The outer gasses of the star expelled into space appear from our vantage point as if we are looking down a helix.  The remnant central stellar core, destined to become a white dwarf star, glows in light so energetic it causes the previously expelled gas to fluoresce.   The Helix Nebula, given a technical designation of NGC 7293, lies 450 light-years away towards the constellation of Aquarius and spans 1.5 light-years.  The above image was taken with the Canada-France-Hawaii Telescope (CFHT) located atop a dormant volcano in Hawaii, USA.  A close-up of the inner edge of the Helix Nebula shows unusual gas knots of unknown origin.

Image credit: J.-C. Cuillandre 
(CFHT Staff), 
CFH12K CCD Camera,
CFHT
The Regolith of Asteroid Eros (2000-08-29)

Tomorrow's picture: Seeing Through Orion  < | 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.
The Brown Dwarfs of Orion's Trapezium (2000-08-30)

The bright stars above are well known as heart of the Trapezium, an open cluster of stars in the center of the Orion Nebula.  The many dim objects, however, are not well known, and have come to attention only on recent images in infrared light.  These dim objects are thought to be brown dwarfs and free-floating planets.  Brown dwarfs are stars too puny to create energy in their core by fusing hydrogen into helium.  Although many more brown dwarfs than hot stars have now been found in Orion, their very low masses make them inadequate to compose much of the dark matter expected in galaxies and the Universe.  The above false-color mosaic combines infrared and visible light images of the Trapezium from the Hubble Space Telescope. Faint brown dwarfs with masses as small as about one percent the mass of the sun are seen in the infrared data. Also visible are complex lanes of hot gas (appearing in blue) and cooler fine dust that blocks, glows and reflects nearby starlight.
Full Throttle For Deep Space 1 (2000-08-31)

At full throttle the Deep Space 1 spacecraft's innovative ion drive produces about 1/50th of a pound of thrust ... a force so great that it would just about hold up a piece of paper on planet Earth! Still, powered by solar arrays ion propulsion systems can run continuously. For long duration space missions they ultimately win out over the powerful but brief blasts of less efficient chemical rockets. Deep Space 1 is seen here suspended in an assembly room, a folded solar array resting above the circular ion propulsion module. Already a successful technology demonstrator with experimental autonomous software, the spacecraft flew by asteroid 9969 Braille in July of 1999 but later that year, in November, the robot probe was nearly lost due to the failure of its wide-field star tracker camera. Now, the adventures of Deep Space 1 can continue. Engineers were able to reprogram the navigation system to utilize another on-board camera and on 28 June 2000 the ion drive was throttled up. Once again steering by the stars, Deep Space 1 is presently bound for a September 2001 rendezvous with periodic Comet Borrelly.
SOHO Sungrazer (2000-09-01)

SOHO, the space-based SOlar and Heliospheric Observatory, has become by far the reigning champion facility for discovering comets, its total having recently reached 200. As might be expected of a solar observatory, most of the SOHO discovered comets are sungrazers, destined to dive within a mere 50 thousand kilometers or so of the solar photosphere. At that range the intense heat and gravitational forces make it unlikely these primitive chunks of ice and dust will survive. Based on their similar orbits, as first worked out by 19th century German astronomer Heinrich Kreutz, all sungrazers are believed to originate from a single large parent comet which broke up during a perihelion passage perhaps 2,000 years ago. Over time, pieces have continued to split off producing a family of smaller comets which seem to travel in the same orbit. These frames from SOHO's coronograph were taken two hours apart on April 29 of this year. They show a sungrazer (SOHO comet discovery number 111) with a long, bright tail headed toward its fiery encounter. The sun itself is hidden behind the coronograph's occulting disk at each frame's upper right.
X-Ray Moon (2000-09-02)

This x-ray image of the Moon was made by the orbiting ROSAT (Röntgensatellit) Observatory in 1990. In this digital picture, pixel brightness corresponds to x-ray intensity. Consider the image in three parts: the bright hemisphere of the x-ray moon, the darker half of the moon, and the x-ray sky background. The bright lunar hemisphere shines in x-rays because it reflects x-rays emitted by the sun ... just as it shines at night by reflecting visible sunlight. The background sky has an x-ray glow in part due to the myriad of distant, powerful active galaxies, unresolved in the ROSAT picture but recently detected in Chandra Observatory x-ray images. But why isn't the dark half of the moon completely dark? It's true that the dark lunar face is in shadow and so is not reflecting solar x-rays. Still, the few x-ray photons which seem to come from the moon's dark half are currently thought to be caused by energetic particles in the solar wind bombarding the lunar surface.
Henrietta Leavitt Calibrates the Stars (2000-09-03)

Humanity's understanding of the relative brightness and variability of stars was revolutionized by the work of Henrietta Swan Leavitt (1868-1921).  Working at Harvard College Observatory, Leavitt precisely calibrated the photographic magnitudes of 47 stars to which all other stars could be compared.  Leavitt discovered and cataloged over 1500 variable stars in the nearby Magellanic Clouds.  From this catalog, Leavitt discovered that brighter Cepheid variable stars take longer to vary, a fact used today to calibrate the distance scale of our universe.
Aurora Persei (2000-09-04)

Last month, skywatchers were treated to an unexpected coincidence: bright aurorae occurred during the Perseid Meteor Shower.  The above picture was taken August 12 and captures eerie looking aurorae and a faint Perseid meteor above Cross Lake in Wisconsin, USA.  The near future holds promise for both more aurorae and a better meteor shower.  Aurorae are becoming increasingly common as their trigger -- our Sun -- nears its period of highest activity during its eleven-year magnetic cycle.  Coming up in mid-November is the quirky Leonids Meteor Shower.  Although one of the better studied meteor showers, the Leonids have surprised astronomers many times and so many an optimistic skywatcher promises to be outside this year hoping for a memorable show.

Image credit: Brent Price
CFHT Star Trails (2000-09-05)

High atop a dormant volcano in Hawaii, an eye 3.6-meters wide stares at a faint light on the night sky.  Unlike a human eye, which collects light for only a fraction of a second at a time, a telescope such as the Canada-France-Hawaii Telescope (CFHT) can collect light for hours. Faint sources become visible that were previously beyond human imagination.  These meticulous observations usually take so long, though, that the Earth's spin causes the telescope to move under the sky.  This spin is visible in the above photograph as star trails in the background.  The CFHT itself must counter-spin to keep on target.  The enormous size of the CFHT dome can be gauged by the car in the foreground.  The CFHT is one of the smaller telescopes atop Mauna Kea.
Emerging Planetary Nebula CRL 618 (2000-09-06)

Tomorrow's picture: Stellar Spirograph  < | 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.
IC 418: The Spirograph Nebula (2000-09-07)

What is creating the strange texture of IC 418?  Dubbed the Spirograph Nebula for its resemblance to drawings from a cyclical drawing tool, planetary nebula IC 418 shows patterns that are not well understood.  Perhaps they are related to chaotic winds from the variable central star, which changes brightness unpredictably in just a few hours.  By contrast, evidence indicates that only a few million years ago, IC 418 was probably a well-understood star similar to our Sun.  Only a few thousand years ago, IC 418 was probably a common red giant star.  Since running out of nuclear fuel, though, the outer envelope has begun expanding outward leaving a hot remnant core destined to become a white-dwarf star, visible in the image center.  The light from the central core excites surrounding atoms in the nebula causing them to glow.  IC 418 lies about 2000 light-years away and spans 0.3 light-years across.  This recently released false-color image taken from the Hubble Space Telescope reveals the unusual details.
Andromeda Island Universe (2000-09-08)

How far can you see? The most distant object easily visible to the unaided eye is M31, the great Andromeda Galaxy some two million light-years away. Without a telescope, even this immense spiral galaxy appears as an unremarkable, faint, nebulous cloud in the constellation Andromeda. But a bright yellow nucleus, dark winding dustlanes, gorgeous blue spiral arms and star clusters are recorded in this stunning telescopic digital mosaic of the nearby island universe. While even casual skygazers are now inspired by the knowledge that there are many distant galaxies like M31, astronomers seriously debated this fundamental concept only 80 years ago. Were these "spiral nebulae" simply outlying components of our own Milky Way Galaxy or were they instead "island universes" -- distant systems of stars comparable to the Milky Way itself? This question was central to the famous Shapley-Curtis debate of 1920, which was later resolved by observations of M31 in favor of Andromeda, island universe.

Image credit: Robert Gendler
X-Ray Moon and X-Ray Star (2000-09-09)

An x-ray star winks out behind the Moon in these before (left) and after views of a lunar occultation of the galactic x-ray source designated GX5-1. The false color images were made using data from the ROSAT (ROentgen SATellite), orbiting observatory. They show high energy x-rays in yellow (mostly from GX5-1), and lower energy x-rays in red (the Moon reflecting x-rays from the Sun). GX5-1 is a binary system consisting of a neutron star and a companion star in mutual orbit about the system's center of mass. The gas in the companion star's outer envelope falls toward the neutron star and accumulates in a disk around it. This disk material swirls deeper in to the neutron star's gravitational well, and is finally dumped onto its surface - in the process creating tremendous temperatures and generating the high energy x-rays.
White Dwarf Stars Cool (2000-09-10)

Diminutive by stellar standards, white dwarf stars are also intensely hot ... but they are cooling. No longer do their interior nuclear fires burn, so they will continue to cool until they fade away. This Hubble Space Telescope image covers a small region near the center of a globular cluster known as M4. Here, researchers have discovered a large concentration of white dwarf stars (circled above). This was expected - low mass stars, including the Sun, are believed to evolve to the white dwarf stage. Studying how these stars cool could lead to a better understanding of their ages, of the age of their parent globular cluster, and even the age of our universe.
Antarctic Ozone Hole Widens (2000-09-11)

It's back, and it's bigger than ever. The ozone hole that has been a cause of concern in recent years has again reformed over Earth's South Pole.  The seasonal recurrence of the ozone hole was expected, although the size of the hole has never been so large this early in the season.  Ozone is important because it shields us from damaging ultraviolet sunlight. Ozone is vulnerable, though, to CFCs and halons being released into the atmosphere.  The ozone hole's large size is probably related to unusually low temperatures, allowing CFC byproducts like chlorine to react with atmospheric ozone molecules with greater efficiency.  In the above false-color picture taken earlier this month, low ozone levels are shown in red and grey.
Slightly Above Mars Pathfinder (2000-09-12)

If you could have hovered above the Pathfinder mission to Mars in 1997, this is what you might have seen.  Directly below you is the control tower of Sagan Memorial Station.  Three dark solar arrays extend out to collect valuable energy, surrounded by light-colored deflated airbags that protected Pathfinder's instruments from directly colliding with the rocky Martian surface.  The left solar panel has ramps down which Pathfinder's rolling robot Sojourner started its adventure to nearby rocks.  Sojourner itself is visible inspecting a rock nicknamed Yogi at 11 o'clock.   Rocks cover the Martian surface, with Twin Peaks visible on the horizon at 9 0'clock.  The distant sky is mostly orange.  This image is a recently released digital combination of panoramic pictures taken by Pathfinder on Mars and a picture of a Lander scale model back on Earth.  The Mars Pathfinder Mission was able to collect data for three months, sending back information that has indicated a wet distant past for Mars.
Comet LINEAR: Fade To Black (2000-09-13)

Only last month the stage was set for Comet LINEAR (C/1999S4 LINEAR) to become the first "naked-eye" comet of Y2K. It didn't fill that role, of course, but it did turn in a very dramatic performance. Closely followed by astronomer Mark Kidger and colleagues with the Isaac Newton Group telescopes (La Palma, Canary Islands), comet LINEAR's nucleus apparently fragmented extensively on the night of July 25th. A faint fluorescent cloud fading against a background of stars is all that is still visible in this August 21st telescopic view from Loomberah, NSW Australia. Why did comet LINEAR break up? Comets are conglomerates of ice and rock. A very plausible scenario is that a substantial fraction of LINEAR's icy component was evaporated, leaving too little to hold the rocky material together. In any event, no bright telltale condensations remain. So, following its first tour through the inner Solar System, an encore from comet LINEAR seems unlikely!

Image credit: Gordon Garradd
M82's Middle Mass Black Hole (2000-09-14)

Black holes are probably the most bizarre creatures in the modern astronomical zoo. And after years of pondering black holes as either stellar mass objects seen in binary star systems or enormous supermassive black holes at the centers of galaxies, astronomers now have strong evidence for another exotic species -- middle mass black holes. The leading candidate for the ultradense middle ground is indicated in this false-color detail of a sharp x-ray picture from the space-based Chandra Observatory. A close-up of x-ray sources near the center of starburst galaxy M82, the cropped Chandra image spans about 4,000 light-years. M82 itself is around 11 million light-years distant. The arrowed source has recently been convincingly demonstrated to exhibit x-ray characteristics of an object whose gravitational field holds more than 500 times the mass of the sun within a volume the size of the moon! Astronomers also note that unlike the supermassive variety which are thought to lie at the centers of galaxies, this middle mass black hole is about 600 light-years from the center of M82. Theories for the formation of a middle mass black hole include the collapse of a "hyperstar" formed by the coalescence of many normal stars, or the direct merger of stellar mass black holes.
Aurora In West Texas Skies (2000-09-15)

The aurora borealis, or northern lights, are not a common sight in the southwestern United States. But a strong solar coronal mass ejection in early August triggered geomagnetic storms and aurora which were widely reported, even under west Texas skies. This striking view of the aurora was recorded from a site near El Paso, Texas and the Hueco Tanks State Historical Park at a latitude just shy of 32 degrees north. Polaris is the brightest star visible near the top and right of center while a Perseid meteor pierces the auroral glow left of picture center, below the bowl of the little dipper. Want to see an aurora? Dark skies and high latitudes (closer to the north or south poles) help. And you might keep an eye on the space weather report. The last big coronal mass ejection headed toward planet Earth was detected by space-based instruments on September 12. It may trigger geomagnetic storms and auroral activity beginning September 14th.

Image credit: Chris Grohusko
X-Ray Earth (2000-09-16)

Above is a picture of the Earth in x-rays, taken in March of 1996 from the orbiting Polar satellite. Most of the planet is dark with superposed continent and coordinate grids, while the bright x-ray emission near the north pole is shown in red. Why does the Earth have an x-ray glow? Actually, the Earth itself does not, but the aurora high in the Earth's atmosphere do glow with x-rays detectable by space-based instruments. Gusts of energetic ions from the Sun can distort the Earth's magnetosphere allowing high energy electrons spiraling along magnetic field lines to slam into the upper atmosphere above the magnetic poles. This activity causes shimmering visible aurora along with x-ray, ultraviolet, and radio emission. The x-rays are not dangerous to life on Earth because they are absorbed by the dense, lower atmosphere.