What's bothering local galaxy Sextans A? A small dwarf irregular galaxy spanning 5 thousand light years across, Sextans A is located only 5 million light-years away. Named for its home constellation of Sextans, the "diamond in the rough" structure relates to an ancient unknown event. 100 million years ago, something mysterious started a new wave of star formation in Sextans A's center. Massive short-lived stars exploded in supernovae that caused more star formation and yet more supernovae, ultimately resulting in an expanding shell. Today, young blue stars highlight areas and shell edges high in current star formation, a shell that from our perspective appears roughly square. In the above picture, a bright orange star in our own Milky Way Galaxy appears superposed in the foreground.

Peering from the shadows, the Saturn-facing hemisphere of tantalizing inner moon Enceladus poses in this Cassini spacecraft image. North is up in the dramatic scene captured during November 2016 as Cassini's camera was pointed in a nearly sunward direction about 130,000 kilometers from the moon's bright crescent. In fact, the distant world reflects over 90 percent of the sunlight it receives, giving its surface about the same reflectivity as fresh snow. A mere 500 kilometers in diameter, Enceladus is a surprisingly active moon. Data collected during Cassini's flybys and years of images have revealed the presence of remarkable south polar geysers and a possible global ocean of liquid water beneath an icy crust.

Hurtling through a cosmic dust cloud a mere 400 light-years away, the lovely Pleiades or Seven Sisters open star cluster is well-known for its striking blue reflection nebulae. It lies in the night sky toward the constellation Taurus and the Orion Arm of our Milky Way Galaxy. The sister stars and cosmic dust cloud are not related though, they just happen to be passing through the same region of space. Known since antiquity as a compact grouping of stars, Galileo first sketched the star cluster viewed through his telescope with stars too faint to be seen by eye. Charles Messier recorded the position of the cluster as the 45th entry in his famous catalog of things which are not comets. In Greek myth, the Pleiades were seven daughters of the astronomical titan Atlas and sea-nymph Pleione. Their parents names are included in the cluster's nine brightest stars. This deep and wide telescopic image spans over 20 light-years across the Pleides star cluster.

The distance to the swirling grand design spiral M100 is causing quite a stir among astronomers. Many believe that the Hubble Space Telescope's recent distance measurement to this galaxy accurately calibrates the expansion rate of the universe. Others believe this distance measurement is misleading. The universe's expansion rate is usually given as a quantity called "Hubble's constant", a factor dividing well-measured recession velocity of a galaxy to give actual distance. Scientific debate over the value of Hubble's constant has been ongoing since it was first measured by Edwin Hubble in 1929. A real live debate involving the value of Hubble's constant titled "The Scale of the Universe" will occur in April 1996 in Washington, DC.

What mysteries might be solved by peering into this crystal ball? In this case, the ball is actually a moon of Jupiter, the crystals are ice, and the moon is not only dirty but cracked beyond repair. Nevertheless, speculation is rampant that oceans exist under Europa's fractured ice-plains that could support life. Europa, roughly the size of Earth's Moon, is pictured here in an image taken a few days ago when the Jupiter-orbiting robotic spacecraft Juno passed within 325 kilometers of its streaked and shifting surface. Underground oceans are thought likely because Europa undergoes global flexing due to its changing gravitational attraction with Jupiter during its slightly elliptical orbit, and this flexing heats the interior. Studying Juno's close-up images may further humanity's understanding not only of Europa and the early Solar System but also of the possibility that life exists elsewhere in the universe.

What's happening in the sky over Monument Valley? A meteor shower. Over the past weekend the Leonid meteor shower has been peaking. The image -- actually a composite of six exposures of about 30 seconds each -- was taken in 2001, a year when there was a much more active Leonids shower. At that time, Earth was moving through a particularly dense swarm of sand-sized debris from Comet Tempel-Tuttle, so that meteor rates approached one visible streak per second. The meteors appear parallel because they all fall to Earth from the meteor shower radiant -- a point on the sky towards the constellation of the Lion (Leo). Although the predicted peak of this year's Leonid meteor shower is over, another peak may be visible early tomorrow morning. By the way -- how many meteors can you identify in the above image?

On rare occasions, the Earth and Moon are photographed together. One of most spectacular times this occurred was 25 years ago this month when the Jupiter-bound Galileo spacecraft zoomed past our home planetary system. Then, robotic Galileo watched from about 15-times the Earth-Moon separation as our only natural satellite glided past our home world. The featured video combines 52 historic color-enhanced images. Although our Moon may appear small next to the Earth, no other planet in our Solar System has a satellite so comparable in size . The Sun, far off to the right, illuminated about half of each sphere, and shows the spinning Earth's white clouds, blue oceans, and tan continents. Tonight, a nearly full Oak supermoon will be visible from all of Earth from sunset to sunrise. Gallery: Supermoon 2017 Free APOD Lectures: January 5 in NYC & January 9 in DC

Six worlds orbit Kepler-11, a sunlike star 2,000 light-years distant in the constellation Cygnus. The new discovery is based on data from NASA's planet hunting Kepler spacecraft. Compared to our Solar System in this illustration, five of Kepler-11's planets orbit closer to their parent star than the Mercury-Sun distance, with orbital periods ranging from 10 to 47 days. All six are larger than Earth and are likely composed of mixtures of rocky material and gas. Their presence, sizes, and masses have been determined by carefully watching the planets dim the light of Kepler-11 while transiting or crossing in front of the star itself. In fact, in August 2010, Kepler's telescope and camera recorded a simultaneous transit of three of the planets in the system. As announced yesterday, using the transit technique the Kepler mission has now identified over 1200 exoplanet candidates in a field of view that covers only about 1/400th of the sky. The tantalizing result suggests there are many undiscovered planets orbiting the stars in our galaxy.

On October 6th, a nearly full perigee Moon shone in Earth's night sky. The bright moonlight, accurate planning, and proper equipment resulted in this amazing composite featuring sharp silhouettes of the International Space Station (ISS) as it rapidly crossed (right to left) in front of the lunar disk. The picture was constructed using six video frames recorded from a site just outside Tracy, California, USA. Sporting newly deployed solar arrays, the ISS was at a range of about 260 miles from the telescope/video camera setup. In the background, about a thousand times more distant than the ISS, lies bright lunar ray crater Tycho.

Why is this moon of Saturn so smooth? This past weekend, humanity's Saturn-orbiting Cassini spacecraft passed as close to Saturn's small moon Calypso as it ever has, and imaged the small moon in unprecedented detail. Pictured above is an early return, raw, unprocessed image of the 20-km long irregularly shaped moon. Like its sister moon Telesto and the shepherd moon Pandora, Calypso has shown itself to be unusually smooth, much smoother than most of Saturn's larger moons. A leading hypothesis for Calypso's smoothness is that much of the moon's surface is actually a relatively loose jumble of rubble -- making Calypso a rubble-pile moon. The loose nature of the small ice pieces allows them to fill in many small craters and other surface features. Calypso orbits Saturn always behind Saturn's much larger moon Tethys, whereas Telesto's orbit always precedes Tethys. Calypso's extremely white surface -- not unlike fresh snow -- may result from the continuous accumulation of fresh ice particles falling in from Saturn's E ring.

This immense ball of half a million stars older than the Sun lies over 30,000 light-years away. Cataloged as M3 (and NGC 5272), it is one of about 150 globular star clusters that roam the halo of our Milky Way Galaxy. Even in this impressively sharp image, individual stars are difficult to distinguished in the densely packed core, but colors are apparent for the bright stars on the cluster's outskirts. M3's many cool "red" giant stars take on a yellowish cast, while hotter giants and pulsating variable stars look light blue. A closer look at the deep telescopic view also reveals a host of background galaxies. Itself about 200 light-years across, the giant star cluster is a relatively bright, easy target for binoculars in the northern constellation Canes Venatici, The Hunting Dogs, and not far from Arcturus.

The 1970s are sometimes ignored by astronomers, like this beautiful grouping of reflection nebulae in Orion - NGC 1977, NGC 1975, and NGC 1973 - usually overlooked in favor of the substantial glow from the nearby stellar nursery better known as the Orion Nebula. Found along Orion's sword just north of the bright Orion Nebula complex, these reflection nebulae are also associated with Orion's giant molecular cloud about 1,500 light-years away, but are dominated by the characteristic blue color of interstellar dust reflecting light from hot young stars. In this sharp color image a portion of the Orion Nebula appears along the bottom border with the cluster of reflection nebulae at picture center. NGC 1977 stretches across the field just below center, separated from NGC 1973 (above right) and NGC 1975 (above left) by dark regions laced with faint red emission from hydrogen atoms. Taken together, the dark regions suggest to many the shape of a running man.