Explanation: Most galaxies don't have any rings -- why does this galaxy have two? To begin, the bright band near NGC 1512's center is a nuclear ring, a ring that surrounds the galaxy center and glows brightly with recently formed stars. Most stars and accompanying gas and dust, however, orbit the galactic center in a ring much further out -- here seen near the image edge. This ring is called, counter-intuitively, the inner ring. If you look closely, you will see this the inner ring connects ends of a diffuse central bar that runs horizontally across the galaxy. These ring structures are thought to be caused by NGC 1512's own asymmetries in a drawn-out process called secular evolution. The gravity of these galaxy asymmetries, including the bar of stars, cause gas and dust to fall from the inner ring to the nuclear ring, enhancing this ring's rate of star formation. Some spiral galaxies also have a third ring -- an outer ring that circles the galaxy even further out.

Explanation: Gorgeous spiral galaxy NGC 3521 is a mere 35 million light-years away, toward the northern springtime constellation Leo. Relatively bright in planet Earth's sky, NGC 3521 is easily visible in small telescopes but often overlooked by amateur imagers in favor of other Leo spiral galaxies, like M66 and M65. It's hard to overlook in this colorful cosmic portrait though. Spanning some 50,000 light-years the galaxy sports characteristic patchy, irregular spiral arms laced with dust, pink star forming regions, and clusters of young, blue stars. This deep image also finds NGC 3521 embedded in fainter, gigantic, bubble-like shells. The shells are likely tidal debris, streams of stars torn from satellite galaxies that have undergone mergers with NGC 3521 in the distant past.

Explanation: The early morning planet parade continues. Visible the world over, the planets Jupiter, Venus, Mars and Saturn have been lining up in the pre-dawn sky since mid-April. In the featured image taken last month, these planets were captured over the Step Pyramid of Djoser, a UNESCO World Heritage Site. Located in the Saqqara necropolis of Egypt, the pyramid was constructed in the 27th century BC and is one of the oldest pyramids known. The two-image composite includes a foreground image taken during evening blue hour, and a background image captured from the same location the following morning. The morning planet line-up is slowly changing. At the end of last month, planets Jupiter and Venus switched places, while at the end of this month, Jupiter and Mars will switch after passing within one-degree of each other. Of course, this picturesque planetary angular alignment is a coincidence, as all of these worlds continue to orbit the Sun as they have for billions of years, well before even the ancient Pyramid of Djoser was buil

Explanation: That's no comet. Below the Pleiades star cluster is actually a planet: Mercury. Long exposures of our Solar System's innermost planet may reveal something unexpected: a tail. Mercury's thin atmosphere contains small amounts of sodium that glow when excited by light from the Sun. Sunlight also liberates these molecules from Mercury's surface and pushes them away. The yellow glow from sodium, in particular, is relatively bright. Pictured, Mercury and its sodium tail are visible in a deep image taken last week from La Palma, Spain through a filter that primarily transmits yellow light emitted by sodium. First predicted in the 1980s, Mercury's tail was first discovered in 2001. Many tail details were revealed in multiple observations by NASA's robotic MESSENGER spacecraft that orbited Mercury between 2011 and 2015. Tails, of course, are usually associated with comets.

Explanation: What's happened to the Sun? Two days ago, parts of South America were treated to a partial solar eclipse -- where the Moon blocked out part of the Sun. The featured image shows an image of the partially eclipsed Sun through clouds as it was setting over Patagonia, Argentina. In the tilted image, Earth is toward the right. During the eclipse, the Moon moved partly between Earth and the Sun. Although a visually impressive sight, the slight dimming of surroundings during this partial eclipse was less noticeable than dimming created by a thick cloud. In about two weeks, all of South America and part of North America will experience a total lunar eclipse -- where the Earth moves completely between the Moon and the Sun. In about two years, a total solar eclipse will cross North America.

Explanation: What does a black hole look like? To find out, radio telescopes from around the Earth coordinated observations of black holes with the largest known event horizons on the sky. Alone, black holes are just black, but these monster attractors are known to be surrounded by glowing gas. This first image resolves the area around the black hole at the center of galaxy M87 on a scale below that expected for its event horizon. Pictured, the dark central region is not the event horizon, but rather the black hole's shadow -- the central region of emitting gas darkened by the central black hole's gravity. The size and shape of the shadow is determined by bright gas near the event horizon, by strong gravitational lensing deflections, and by the black hole's spin. In resolving this black hole's shadow, the Event Horizon Telescope (EHT) bolstered evidence that Einstein's gravity works even in extreme regions, and gave clear evidence that M87 has a central spinning black hole of about 6 billion solar masses. Since releasing this featured image in 2019, the EHT has expanded to include more telescopes, observe more black holes, track polarized light,and is working to observe the immediately vicinity of the black hole in the center of our Milky Way Galaxy.

Explanation: Sharp telescopic views of NGC 3628 show a puffy galactic disk divided by dark dust lanes. Of course, this portrait of the magnificent, edge-on spiral galaxy puts some astronomers in mind of its popular moniker, the Hamburger Galaxy. It also reveals a small galaxy nearby (below), likely a satellite of NGC 3628, and a very faint but extensive tidal tail. The drawn out tail stretches for about 300,000 light-years, even beyond the upper left edge of the frame. NGC 3628 shares its neighborhood in the local universe with two other large spirals M65 and M66 in a grouping otherwise known as the Leo Triplet. Gravitational interactions with its cosmic neighbors are likely responsible for creating the tidal tail, as well as the extended flare and warp of this spiral's disk. The tantalizing island universe itself is about 100,000 light-years across and 35 million light-years away in the northern springtime constellation Leo.

Explanation: The world is waking up to a picturesque planet parade. Just before dawn, the eastern skies over much of planet Earth are decorated by a notable line of familiar planets. In much of Earth's northern hemisphere, this line of planets appears most nearly horizontal, but in much of Earth's southern hemisphere, the line appears more nearly vertical. Pictured over the Sydney Opera House in southern Australia, the planet line was captured nearly vertical about five days ago. From top to bottom, the morning planets are Saturn, Mars, Venus, and Jupiter. As April ends, the angular distance between Venus and Jupiter will gradually pass below a degree as they switch places. Then, as May ends, Jupiter will pass near Mars as those two planets switch places. In June, the parade will briefly expand to include Mercury.

Explanation: Just now, before you hit the button, two future universes are possible. After pressing the button, though, you will live in only one. A real-web version of the famous Schrödinger's cat experiment clicking the red button in the featured astronaut image should transform that image into a picture of the same astronaut holding one of two cats -- one living, or one dead. The timing of your click, combined with the wiring of your brain and the millisecond timing of your device, will all conspire together to create a result dominated, potentially, by the randomness of quantum mechanics. Some believe that your personally-initiated quantum decision will split the universe in two, and that both the live-cat and dead-cat universes exist in separate parts of a larger multiverse. Others believe that the result of your click will collapse the two possible universes into one -- in a way that could not have been predicted beforehand. Yet others believe that the universe is classically deterministic, so that by pressing the button you did not really split the universe, but just carried out an action predestined since time began. We at APOD believe that however silly you may feel clicking the red button, and regardless of the outcome, you should have a thought-provoking day. Or two.

Explanation: A gorgeous spiral galaxy, Messier 104 is famous for its nearly edge-on profile featuring a broad ring of obscuring dust lanes. Seen in silhouette against an extensive central bulge of stars, the swath of cosmic dust lends a broad brimmed hat-like appearance to the galaxy suggesting a more popular moniker, the Sombrero Galaxy. This sharp view of the well-known galaxy was made from over 10 hours of Hubble Space Telescope image data, processed to bring out faint details often lost in the overwhelming glare of M104's bright central bulge. Also known as NGC 4594, the Sombrero galaxy can be seen across the spectrum, and is host to a central supermassive black hole. About 50,000 light-years across and 28 million light-years away, M104 is one of the largest galaxies at the southern edge of the Virgo Galaxy Cluster. Still, the spiky foreground stars in this field of view lie well within our own Milky Way.

Explanation: No sudden, sharp boundary marks the passage of day into night in this gorgeous view of ocean and clouds over our fair planet Earth. Instead, the shadow line or terminator is diffuse and shows the gradual transition to darkness we experience as twilight. With the Sun illuminating the scene from the right, the cloud tops reflect gently reddened sunlight filtered through the dusty troposphere, the lowest layer of the planet's nurturing atmosphere. A clear high altitude layer, visible along the dayside's upper edge, scatters blue sunlight and fades into the blackness of space. This picture was taken in June of 2001 from the International Space Station orbiting at an altitude of 211 nautical miles. Of course from home, you can check out the Earth Now.

Explanation: The eggs from this gigantic chicken may form into stars. The featured emission nebula, shown in scientifically assigned colors, is cataloged as IC 2944 but known as the Running Chicken Nebula for the shape of its greater appearance. Seen toward the bottom of the image are small, dark molecular clouds rich in obscuring cosmic dust. Called Thackeray's Globules for their discoverer, these "eggs" are potential sites for the gravitational condensation of new stars, although their fates are uncertain as they are also being rapidly eroded away by the intense radiation from nearby young stars. Together with patchy glowing gas and complex regions of reflecting dust, these massive and energetic stars form the open cluster Collinder 249. This gorgeous skyscape spans about 60 light-years at the nebula's estimated 6,500 light-year distance.

Explanation: The mission was to document night-flying birds -- but it ended up also documenting a beautiful sky. The featured wide-angle mosaic was taken over the steppe golden fields in Mértola, Portugal in 2020. From such a dark location, an immediately-evident breathtaking glow arched over the night sky: the central band of our Milky Way galaxy. But this sky had much more. Thin clouds crossed the sky like golden ribbons. The planet Mars appeared on the far left, while the planets Saturn and Jupiter were also simultaneously visible -- but on the opposite side of the sky, here seen on the far right. Near the top of the image the bright star Vega can be found, while the far-distant and faint Andromeda Galaxy can be seen toward the left, just below Milky Way's arch. As the current month progresses, several planets are lining up in the pre-dawn sky: Jupiter, Venus, Mars, and Saturn.

Explanation: Taken with a camera fixed to a tripod, many short exposures were aligned with the stars to unveil this beautiful, dark night sky. Captured near the rural village of Albany`a at the northeastern corner of Spain, the three stars of Orion's belt stretch across top center in the starry frame. Alnitak, the easternmost (left) of the belt stars is seen next to the more diffuse glow of the Flame Nebula and the dark notch of the famous Horsehead. Easily visible to the naked-eye The Great Nebula of Orion is below the belt stars. A mere 1,500 light-years distant, it is the closest large stellar nursery to our fair planet. Best seen in photographs, the broad and faint arc of Barnard's Loop seems to embrace Orion's brighter stars and nebulae though. In the northern spring the familiar northern winter constellation is setting. Near the western horizon toward lower right Orion's apparently bright blue supergiant Rigel just touches the branches of a pine tree.

Explanation: Spiral arms seem to swirl around the core of Messier 96 in this colorful, detailed portrait of a beautiful island universe. Of course M96 is a spiral galaxy, and counting the faint arms extending beyond the brighter central region it spans 100 thousand light-years or so. That's about the size of our own Milky Way. M96 is known to be 38 million light-years distant, a dominant member of the Leo I galaxy group. Background galaxies and smaller Leo I group members can be found by examining the picture. The most intriguing one is itself a spiral galaxy seen nearly edge on behind the outer spiral arm near the 1 o'clock position from center. Its bright central bulge cut by its own dark dust clouds, the edge-on background spiral appears to be about 1/5 the size of M96. If that background galaxy is similar in actual size to M96, then it would be about 5 times farther away.

Explanation: Typically, the International Space Station is visible only at night. Slowly drifting across the night sky as it orbits the Earth, the International Space Station (ISS) can be seen as a bright spot several times a year from many locations. The ISS is then visible only just after sunset or just before sunrise because it shines by reflected sunlight -- once the ISS enters the Earth's shadow, it will drop out of sight. The only occasion when the ISS is visible during the day is when it passes right in front of the Sun. Then, it passes so quickly that only cameras taking short exposures can visually freeze the ISS's silhouette onto the background Sun. The featured picture did exactly that -- it is actually a series of images taken earlier this month from Beijing, China with perfect timing. This image series was later combined with separate images taken at nearly the same time but highlighting the texture and activity on the busy Sun. The solar activity included numerous gaseous prominences seen around the edge, highlighted in red, filaments seen against the Sun's face, and a dark sunspot.

Explanation: Was this image of the Moon's surface taken with a microscope? No -- it's a multi-temporal illumination map made with a wide-angle camera. To create it, the Lunar Reconnaissance Orbiter spacecraft collected 1,700 images over a period of 6 lunar days (6 Earth months), repeatedly covering an area centered on the Moon's south pole from different angles. The resulting images were stacked to produce the featured map -- representing the percentage of time each spot on the surface was illuminated by the Sun. Remaining convincingly in shadow, the floor of the 19-kilometer diameter Shackleton crater is seen near the map's center. The lunar south pole itself is at about 9 o'clock on the crater's rim. Crater floors near the lunar south and north poles can remain in permanent shadow, while mountain tops can remain in nearly continuous sunlight. Useful for future outposts, the shadowed crater floors could offer reservoirs of water-ice, while the sunlit mountain tops offer good locations to collect solar power.

Explanation: Only twenty-five years ago, Comet Hale-Bopp rounded the Sun and offered a dazzling spectacle in planet Earth's night skies. Digitized from the original astrophoto on 35mm color slide film, this classic image of the Great Comet of 1997 was recorded a few days after its perihelion passage on April 1, 1997. Made with a camera and telephoto lens piggy-backed on a small telescope, the 10 minute long, hand-guided exposure features the memorable tails of Hale-Bopp, a whitish dust tail and blue ion tail. Here, the ion tail extends well over ten degrees across the northern sky. In all, Hale-Bopp was reported as visible to the naked eye from late May 1996 through September 1997. Also known as C/1995 O1, Hale-Bopp is recognized as one of the most compositionally pristine comets to pass through the inner Solar System. A visitor from the distant Oort cloud, the comet's next perihelion passage should be around the year 4380 AD. Do you remember Hale-Bopp?

Explanation: Unlike most entries in Charles Messier's famous catalog of deep sky objects, M24 is not a bright galaxy, star cluster, or nebula. It's a gap in nearby, obscuring interstellar dust clouds that allows a view of the distant stars in the Sagittarius spiral arm of our Milky Way galaxy. When you gaze at the star cloud with binoculars or small telescope you are looking through a window over 300 light-years wide at stars some 10,000 light-years or more from Earth. Sometimes called the Small Sagittarius Star Cloud, M24's luminous stars fill this gorgeous starscape. Covering over 3 degrees or the width of 6 full moons in the constellation Sagittarius, the telescopic field of view includes dark markings B92 and B93 just above center, along with other clouds of dust and glowing nebulae toward the center of the Milky Way.

Explanation: On the upper right, dressed in blue, is the Pleiades. Also known as the Seven Sisters and M45, the Pleiades is one of the brightest and most easily visible open clusters on the sky. The Pleiades contains over 3,000 stars, is about 400 light years away, and only 13 light years across. Surrounding the stars is a spectacular blue reflection nebula made of fine dust. A common legend is that one of the brighter stars faded since the cluster was named. On the lower left, shining in red, is the California Nebula. Named for its shape, the California Nebula is much dimmer and hence harder to see than the Pleiades. Also known as NGC 1499, this mass of red glowing hydrogen gas is about 1,500 light years away. Although about 25 full moons could fit between them, the featured wide angle, deep field image composite has captured them both. A careful inspection of the deep image will also reveal the star forming region IC 348 and the molecular cloud LBN 777 (the Baby Eagle Nebul

Explanation: Our Earth is not at rest. The Earth moves around the Sun. The Sun orbits the center of the Milky Way Galaxy. The Milky Way Galaxy orbits in the Local Group of Galaxies. The Local Group falls toward the Virgo Cluster of Galaxies. But these speeds are less than the speed that all of these objects together move relative to the cosmic microwave background radiation (CMBR). In the featured all-sky map from the COBE satellite in 1993, microwave light in the Earth's direction of motion appears blueshifted and hence hotter, while microwave light on the opposite side of the sky is redshifted and colder. The map indicates that the Local Group moves at about 600 kilometers per second relative to this primordial radiation. This high speed was initially unexpected and its magnitude is still unexplained. Why are we moving so fast? What is out there?

Explanation: This almost otherworldly display of northern lights was captured in clear skies during the early hours of March 31 from 44 degrees north latitude, planet Earth. In a five second exposure the scene looks north from Martinique Beach Provincial Park in Nova Scotia, Canada. Stars of the W-shaped constellation Cassiopeia shine well above the horizon, through the red tint of the higher altitude auroral glow. Auroral activity was anticipated by skywatchers alerted to the possibility of stormy space weather by Sun-staring spacecraft. The predicted geomagnetic storm was sparked as a coronal mass ejection, launched from prolific solar active region 2975, impacted our fair planet's magnetosphere.

Explanation: The natural filter of a hazy atmosphere offered this recognizable architecture and sunset view on March 27. Dark against the solar disk, large sunspots in solar active regions 2975 and 2976 are wedged between the Duomo of Pisa and its famous Leaning Tower. Only one day later, Sun-staring spacecraft watched active region 2975 unleash a frenzy of solar flares along with two coronal mass ejections. The largest impacted the magnetosphere on March 31 triggering a geomagnetic storm and aurorae in high-latitude night skies. On March 30, active region 2975 erupted again with a powerful X-class solar flare that caused a temporary radio blackout on planet Earth.

Explanation: Some 60 million light-years away in the southerly constellation Corvus, two large galaxies are colliding. Stars in the two galaxies, cataloged as NGC 4038 and NGC 4039, very rarely collide in the course of the ponderous cataclysm that lasts for hundreds of millions of years. But the galaxies' large clouds of molecular gas and dust often do, triggering furious episodes of star formation near the center of the cosmic wreckage. Spanning over 500 thousand light-years, this stunning view also reveals new star clusters and matter flung far from the scene of the accident by gravitational tidal forces. The remarkably sharp ground-based image, an accumulation of 88 hours of exposure captured during 2012-2021, follows the faint tidal tails and distant background galaxies in the field of view. The suggestive overall visual appearance of the extended arcing structures gives the galaxy pair, also known as Arp 244, its popular name - The Antennae.

Explanation: Closest to the Sun on March 1, and closest to planet Earth on April 23, this Comet ATLAS (C/2020 R4) shows a faint greenish coma and short tail in this pretty, telescopic field of view. Captured at its position on May 5, the comet was within the boundaries of northern constellation Canes Venatici (the Hunting Dogs), and near the line-of-sight to intriguing background galaxies popularly known as the Whale and the Hockey Stick. Cetacean in appearance but Milky Way sized, NGC 4631 is a spiral galaxy seen edge-on at the top right, some 25 million light-years away. NGC 4656/7 sports the bent-stick shape of interacting galaxies below and left of NGC 4631. In fact, the distortions and mingling trails of gas detected at other wavelengths suggest the cosmic Whale and Hockey Stick have had close encounters with each other in their distant past. Outbound and only about 7 light-minutes from Earth this Comet ATLAS should revisit the inner solar system in just under 1,000 years.

Explanation: When two planets pass on the night sky, they can usually be seen near each other for a week or more. In the case of this planetary conjunction, Venus and Mars passed within 4 degrees of each other earlier this month. The featured image was taken a few days prior, when Venus was slowing rising in the pre-dawn sky, night by night, while Mars was slowly setting. The image, a four-part mosaic, was captured in Brazil from the small town Teresópolis. Besides Venus and Mars, the morning sky now also includes the more distant planet Saturn. Of course, these conjunctions are only angular -- Venus, Mars, and Saturn continue to orbit the Sun in very different parts of our Solar System. Next week, the angle between Saturn and Mars will drop to below a quarter of a degree.

Explanation: The southernmost part of the Milky Way contains not only the stars of the Southern Cross, but the closest star system to our Sun -- Alpha Centauri. The Southern Cross itself is topped by the bright, yellowish star Gamma Crucis. A line from Gamma Crucis through the blue star at the bottom of the cross, Acrux, points toward the south celestial pole, located just above the small island in the featured picture -- taken in early March. That island is Madivaru of the Maldives in the Indian Ocean. Against faint Milky Way starlight, the dark Coal Sack Nebula lies just left of the cross, while farther left along the Milky Way are the bright stars Alpha Centauri (left) and Beta Centauri (Hadar). Alpha Centauri A, a Sun-like star anchoring a three-star system with exoplanets, is a mere 4.3 light-years distant. Seen from Alpha Centauri, our own Sun would be a bright yellowish star in the otherwise recognizable constellation Cassiopeia.

Explanation: Why would the surface of Titan light up with a blinding flash? The reason: a sunglint from liquid seas. Saturn's moon Titan has numerous smooth lakes of methane that, when the angle is right, reflect sunlight as if they were mirrors. Pictured here in false-color, the robotic Cassini spacecraft that orbited Saturn from 2004 to 2017 imaged the cloud-covered Titan in 2014 in different bands of cloud-piercing infrared light. This specular reflection was so bright it saturated one of Cassini's infrared cameras. Although the sunglint was annoying -- it was also useful. The reflecting regions confirm that northern Titan houses a wide and complex array of seas with a geometry that indicates periods of significant evaporation. During its numerous passes of our Solar System's most mysterious moon, Cassini has revealed Titan to be a world with active weather -- including times when it rains a liquefied version of natural gas.

Explanation: The night side of Pluto spans this shadowy scene. In the stunning spacebased perspective the Sun is 4.9 billion kilometers (almost 4.5 light-hours) behind the dim and distant world. It was captured by far flung New Horizons in July of 2015 when the spacecraft was at a range of some 21,000 kilometers from Pluto, about 19 minutes after its closest approach. A denizen of the Kuiper Belt in dramatic silhouette, the image also reveals Pluto's tenuous, surprisingly complex layers of hazy atmosphere. Near the top of the frame the crescent twilight landscape includes southern areas of nitrogen ice plains now formally known as Sputnik Planitia and rugged mountains of water-ice in the Norgay Montes.

Explanation: Braided and serpentine filaments of glowing gas suggest this nebula's popular name, The Medusa Nebula. Also known as Abell 21, this Medusa is an old planetary nebula some 1,500 light-years away in the constellation Gemini. Like its mythological namesake, the nebula is associated with a dramatic transformation. The planetary nebula phase represents a final stage in the evolution of low mass stars like the sun as they transform themselves from red giants to hot white dwarf stars and in the process shrug off their outer layers. Ultraviolet radiation from the hot star powers the nebular glow. The Medusa's transforming star is the faint one near the center of the overall bright crescent shape. In this deep telescopic view, fainter filaments clearly extend above and left of the bright crescent region. The Medusa Nebula is estimated to be over 4 light-years across.