See Saturn’s very own Death Star, a round star in a square hole and those that guard the north in this week’s photos of the skies above.

IC 4406: A Seemingly Square Nebula

How can a round star make a square nebula? This conundrum comes to light when studying planetary nebulae like IC 4406. Evidence indicates that IC 4406 is likely a hollow cylinder, with its square appearance the result of our vantage point in viewing the cylinder from the side. Were IC 4406 viewed from the top, it would likely look similar to the Ring Nebula. This representative-color picture is a composite made by combining images taken by the Hubble Space Telescope in 2001 and 2002. Hot gas flows out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls. The star primarily responsible for this interstellar sculpture can be found in the planetary nebula's center. In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.

Photography: C. R. O'Dell (Vanderbilt U.) et al., Hubble Heritage Team, NASA

Stardust in the Perseus Molecular Cloud

Clouds of stardust drift through this deep skyscape. The cosmic scene spans nearly 2 degrees across the Perseus molecular cloud some 850 light-years away. A triangle of dusty nebulae reflecting light from embedded stars is captured in the telescopic field of view. With a characteristic bluish color reflection nebula NGC 1333 is at left, vdB13 at bottom right, and rare yellowish reflection nebula vdB12 lies at the top. Stars are forming in the Perseus molecular cloud, though most are obscured at visible wavelengths by the pervasive dust. Still, hints of contrasting red emission from Herbig-Haro objects, the jets and shocked glowing gas emanating from recently formed stars, are evident in NGC 1333. At the estimated distance of the molecular cloud, legs of the triangle formed by the reflection nebulae would be about 20 light-years long.

Photography: Lorand Fenyes

Mimas, Crater, and Mountain

Mimas is an icy, crater-pocked moon of Saturn a mere 400 kilometers (250 miles) in diameter. Its largest crater Herschel is nearly 140 kilometers wide. About a third the diameter of Mimas itself, Herschel crater gives the small moon an ominous appearance, especially for scifi fans of the Death Star battlestation of Star Wars fame. In fact, only a slightly bigger impact than the one that created such a large crater on a small moon could have destroyed Mimas entirely. In this Cassini image from October 2016, the anti-Saturn hemisphere of the synchronously rotating moon is bathed in sunlight, its large crater near the right limb. Casting a long shadow across the crater floor, Herschel's central mountain peak is nearly as tall as Mount Everest on planet Earth.

Photography: Cassini Imaging Team, SSI, JPL, ESA, NASA

Sentinels of a Northern Sky

Who guards the north? The featured picture was taken last March in Finnish Lapland where weather can include sub-freezing temperatures and driving snow. Surreal landscapes sometimes result, where white alien-looking sentinels seem to patrol the landscape. In actuality though, the aliens are snow-covered trees, and the red hut they seem to be guarding is an outhouse. Far in the distance, behind this uncommon Earthly vista, is a beautiful night sky which includes a green aurora, bright stars, and streaks of orbiting satellites. Of course, in the spring, the trees thaw and Lapland looks much different.

Photography: Pierre Destribats

Edge-On NGC 891

Large spiral galaxy NGC 891 spans about 100 thousand light-years and is seen almost exactly edge-on from our perspective. In fact, about 30 million light-years distant in the constellation Andromeda, NGC 891 looks a lot like our Milky Way. At first glance, it has a flat, thin, galactic disk of stars and a central bulge cut along the middle by regions of dark obscuring dust. But remarkably apparent in NGC 891's edge-on presentation are filaments of dust that extend hundreds of light-years above and below the center line. The dust has likely been blown out of the disk by supernova explosions or intense star formation activity. Fainter galaxies can also be seen near the edge-on disk in this deep portrait of NGC 891. (Editor's Note: The NGC 891 image used in today's APOD posting has been replaced and the credit corrected to indicate the author of the original work.)

Photography: Adam Block, Mt. Lemmon SkyCenter, U. Arizona