2018 June 12

Star Size Comparison 2
Video Credit: morn1415 (YouTube); Images Credit: NASA (typically); Music: Alpha (Vangelis)

How big is our Sun compared to other stars? In dramatic and popular videos featured on YouTube, the relative sizes of planets, stars, and even the universe are shown from smallest to largest. The featured video begins with Earth's Moon and progresses through increasingly larger moons and planets in our Solar System. Soon, the Sun is shown and compared to many of the brighter stars in our neighborhood of the Milky Way Galaxy. Finally, star sizes are shown in comparison with the Milky Way Galaxy, galaxies across the observable universe, and speculatively, regions of a potentially greater multiverse. Note that the true sizes of most stars outside of the Sun and Betelgeuse are not known by direct observation, but rather inferred by measurements of their perceived brightness, temperature, and distance. Although an inspiring learning tool that is mostly accurate, APOD readers are encouraged to complete the learning experience -- and possibly help make future versions more accurate -- by pointing out slight inaccuracies in the video.

2018 June 13

Red Cloudbow over Delaware
Image Credit & Copyright: Michael C. Neff (Neffworks Artography)

What kind of rainbow is this? In this case, no rain was involved -- what is pictured is actually a red cloudbow. The unusual sky arc was spotted last month during sunset in Rehoboth Beach, Delaware, USA. When the photographer realized that what he was seeing was extraordinary, he captured it with the only camera available -- a cell phone. Clouds are made of water droplets, and in a cloudbow a cloud-droplet group reflects back light from the bright Sun (or Moon) on the opposite side of the sky. Similar phenomena include fogbows and airplane glories. Here, the red color was caused by atmospheric air preferentially scattering away blue light -- which simultaneously makes most of the sky appear blue. A careful inspection reveals a supernumery bow just inside the outermost arc, a bow caused by quantum diffraction.

2018 June 15

Little Planet Soyuz
Image Credit & Copyright: Andrew Bodrov

Engines blazing, a large rocket bids farewell to this little planet. Of course, the little planet is really planet Earth and the large rocket is a Soyuz-FG rocket. Launched from the Baikonur Cosmodrome in Kazakhstan on June 6 it carried a Soyuz MS-09 spacecraft into orbit. On board were International Space Station Expedition 56-57 crew members Sergey Prokopyev of Roscosmos, Serena Aunon-Chancellor of NASA and Alexander Gerst of ESA. Their spacecraft successfully docked with humanity's orbiting outpost just two days later. The little planet projection is the digitally warped and stitched mosaic of images covering 360 by 180 degrees, captured during the 2018 Star Trek car expedition.

2018 June 16

Dusty With a Chance of Dust
Image Credit: NASA, JPL-Caltech, MSSS, Curiosity Mars Rover

It's storm season on Mars. Dusty with a chance of dust is the weather report for Gale crater as a recent planet-scale dust storm rages. On June 10 looking toward the east-northeast crater rim about 30 kilometers away, the Curiosity rover's Mastcam captured this image of its local conditions so far. Meanwhile over 2,000 kilometers away, the Opportunity rover ceased science operations as the storm grew thicker at its location on the west rim of Endeavour crater, and has stopped communicating, waiting out the storm for now. Curiosity is powered by a radioisotope thermoelectric generator, but the smaller Opportunity rover uses solar panels to charge its batteries. For Opportunity, the increasingly severe lack of sunlight has caused its batteries to run low.

2018 June 17

Mars Engulfed
Image Credit: J. Bell (ASU), M. Wolff (Space Science Inst.), Hubble Heritage Team (STScI / AURA), NASA

What's happened to Mars? In 2001, Mars underwent a tremendous planet-wide dust storm -- one of the largest ever recorded from Earth. To show the extent, these two Hubble Space Telescope storm watch images from late June and early September (2001) offer dramatically contrasting views of the martian surface. At left, the onset of smaller "seed" storms can be seen near the Hellas basin (lower right edge of Mars) and the northern polar cap. A similar surface view at right, taken over two months later, shows the fully developed extent of the obscuring global storm. Although this storm eventually waned, in recent days a new large dust storm has been taking hold of the red planet.

2018 June 18

An Active Prominence on the Sun
Video Credit: Chuck Ayoub (Chuck's Astrophotography)

Sometimes the Sun's surface becomes a whirlwind of activity. Pictured is a time-lapse video of the Sun's surface taken over a two hour period in early May, run both forwards and backwards. The Sun's surface was blocked out so that details over the edge could be imaged in greater detail. Hot plasma is seen swirling over the solar limb in an ongoing battle between changing magnetic fields and constant gravity. The featured prominence rises about one Earth-diameter over the Sun's surface. Energetic events like this are becoming less common as the Sun nears a minimum in its 11-year activity cycle.

2018 June 19

Ancients of Sea and Sky
Image Credit & Copyright: Jingyi Zhang

They may look like round rocks, but they're alive. Moreover, they are modern versions of one of the oldest known forms of life: stromatolites. Fossils indicate that stromatolites appeared on Earth about 3.7 billion years ago -- even before many of the familiar stars in the modern night sky were formed. In the featured image taken in Western Australia, only the ancient central arch of our Milky Way Galaxy formed earlier. Even the Magellanic Clouds, satellite galaxies of our Milky Way and visible in the featured image below the Milky Way's arch, didn't exist in their current form when stromatolites first grew on Earth. Stromatolites are accreting biofilms of billions of microorganisms that can slowly move toward light. Using this light to liberate oxygen into the air, ancient stromatolites helped make Earth hospitable to other life forms including, eventually, humans.

2018 June 20

Pillars of the Eagle Nebula in Infrared
Image Credit: NASA, ESA, Hubble, HLA; Processing: Llu�s Romero

Newborn stars are forming in the Eagle Nebula. Gravitationally contracting in pillars of dense gas and dust, the intense radiation of these newly-formed bright stars is causing surrounding material to boil away. This image, taken with the Hubble Space Telescope in near infrared light, allows the viewer to see through much of the thick dust that makes the pillars opaque in visible light. The giant structures are light years in length and dubbed informally the Pillars of Creation. Associated with the open star cluster M16, the Eagle Nebula lies about 6,500 light years away. The Eagle Nebula is an easy target for small telescopes in a nebula-rich part of the sky toward the split constellation Serpens Cauda (the tail of the snake).

2018 June 21

Northern Lights and Noctilucent Clouds
Image Credit & Copyright: Adrien Mauduit

Luminous skies after the near-solstice sunset on June 17 are reflected in this calm lake. The tranquil twilight scene was captured near Bashaw, Alberta, Canada, northern planet Earth. Usually spotted at high latitudes in summer months, night shining or noctilucent clouds hang just above the horizon, transfusing light into a darker sky. Formed near the edge of space, the icy apparitions are condensations on meteoric dust or volcanic ash still in sunlight at extreme altitudes. Also near the edge of space on this short northern night, solar activity triggered the lovely apparition of aurora borealis or northern lights.

2018 June 22

Galaxy in a Crystal Ball
Image Credit & Copyright: Juan Carlos Munoz

A small crystal ball seems to hold a whole galaxy in this creative snapshot. Of course, the galaxy is our own Milky Way. Its luminous central bulge marked by rifts of interstellar dust spans thousands of light-years. On this long southern hemisphere night it filled dark Chilean skies over Paranal Observatory. The single exposure image did not require a Very Large Telescope, though. Experiments with a digital camera on a tripod and crystal ball perched on a handrail outside the Paranal Residencia produced the evocative, cosmic marble portrait of our home galaxy.

2018 June 23

Curiosity's Dusty Self
Image Credit: NASA, JPL-Caltech, MSSS, Curiosity Mars Rover

Winds on Mars can't actually blow spacecraft over. But in the low gravity, martian winds can loft fine dust particles in planet-wide storms, like the dust storm now raging on the Red Planet. From the martian surface on sol 2082 (June 15), this self-portrait from the Curiosity rover shows the effects of the dust storm, reducing sunlight and visibility at the rover's location in Gale crater. Made with the Mars Hand Lens Imager, its mechanical arm is edited out of the mosaicked images. Curiosity's recent drill site Duluth can be seen on the rock just in front of the rover on the left. The east-northeast Gale crater rim fading into the background is about 30 kilometers away. Curiosity is powered by a radioisotope thermoelectric generator and is expected to be unaffected by the increase in dust at Gale crater. On the other side of Mars, the solar-powered Opportunity rover has ceased its operations due to the even more severe lack of sunlight at its location on the west rim of Endeavour crater.

2018 June 24

Rocket Plume Shadow Points to the Moon
Image Credit: Pat McCracken, NASA

Why would the shadow of a space shuttle launch plume point toward the Moon? In early 2001 during a launch of Atlantis, the Sun, Earth, Moon, and rocket were all properly aligned for this photogenic coincidence. First, for the space shuttle's plume to cast a long shadow, the time of day must be either near sunrise or sunset. Only then will the shadow be its longest and extend all the way to the horizon. Finally, during a Full Moon, the Sun and Moon are on opposite sides of the sky. Just after sunset, for example, the Sun is slightly below the horizon, and, in the other direction, the Moon is slightly above the horizon. Therefore, as Atlantis blasted off, just after sunset, its shadow projected away from the Sun toward the opposite horizon, where the Full Moon happened to be.

2018 June 25

Hayabusa2 Approaches Asteroid Ryugu
Image Credit & Copyright: ISAS, JAXA, Hayabusa2 Team

It looks like a big space diamond -- but with craters. It's 162173 Ryugu (Dragon's Castle), and Japan's robotic Hayabusa2 mission is now arriving at this near-Earth asteroid. Ambitious Hayabusa2 is carrying an armada of separable probes, including two impactors, four small close-proximity hoverers, three small surface hoppers, and the Mobile Asteroid Surface Scout (MASCOT) which will land, study, and move around on Ryugu's surface. Most of these are equipped with cameras. Moreover, Hayabusa2 itself is scheduled to collect surface samples and return these samples to Earth for a detailed analysis near the end of 2020. Previously, what was known about asteroid Ryugu was its orbit, that it spans about one kilometer, and that it has a dark surface that reflects unusual colors. Studying Ryugu could tell humanity not only about Ryugu's surface and interior, but about what materials were available in the early Solar System for the development of life. Pictured, a series of approach images shows features suggestive of large boulders and craters.

2018 June 26

Dark Nebulas across Taurus
Image Processing & Copyright: Oliver Czernetz - Data: Digitized Sky Survey (POSS-II)

Sometimes even the dark dust of interstellar space has a serene beauty. One such place occurs toward the constellation of Taurus. The filaments featured here can be found on the sky between the Pleiades star cluster and the California Nebula. This dust is not known not for its bright glow but for its absorption and opaqueness. Several bright stars are visible with their blue light seen reflecting off the brown dust. Other stars appear unusually red as their light barely peeks through a column of dark dust, with red the color that remains after the blue is scattered away. Yet other stars are behind dust pillars so thick they are not visible here. Although appearing serene, the scene is actually an ongoing loop of tumult and rebirth. This is because massive enough knots of gas and dust will gravitationally collapse to form new stars -- stars that both create new dust in their atmospheres and destroy old dust with their energetic light and winds.

2018 June 27

Highlights of the Summer Sky
Illustration Credit & Copyright: Universe2go.com

What can you see in the night sky this summer? The featured graphic gives a few highlights for Earth's northern hemisphere. Viewed as a clock face centered at the bottom, early (northern) summer sky events fan out toward the left, while late summer events are projected toward the right. Objects relatively close to Earth are illustrated, in general, as nearer to the cartoon figure with the telescope at the bottom center -- although almost everything pictured can be seen without a telescope. As happens during any season, constellations appear the same year to year, and meteor showers occur on or near the same dates. For example, like last year, the stars of the Summer Triangle will be nighttime icons for most the season, while the Perseids meteor shower will peak in mid-August, as usual. Highlights specific to this summer's sky include that Jupiter will be visible after sunset during June, and Venus will shine brightly in the evening sky during July and August. Saturn and Mars should be visible during much of this season's night, with Saturn appearing in the direction opposite the Sun in late June, and Mars at opposition in late July. Finally, a total lunar eclipse should be visible to anyone who can see the Moon in late July.

2018 June 28

Sigma Octantis and Friends
Image Credit & Copyright: Frank Sackenheim

South pole star Sigma Octantis (of the constellation Octans) is on the left of this starry expanse spanning over 40 degrees across far southern skies. You'll have to look hard to find it, though. The southern hemisphere's faint counterpart to the north star Polaris, Sigma Octantis is a little over one degree from the South Celestial Pole. Also known as Polaris Australis, Sigma Octantis is dimmer than 5th magnitude, some 25 times fainter than Polaris and not easy to see with the unaided eye. In fact, it may be the faintest star depicted on a national flag. The remarkable deep and wide-field view also covers faint, dusty galactic cirrus clouds, bounded at the right by the star clusters and nebulae along southern reaches of the plane of our Milky Way galaxy. Near the upper right corner is yellowish Gamma Crucis, the top of the Southern Cross. Easy to pick out above and right of center is the long Dark Doodad nebula in the southern constellation Musca, the Fly.

2018 June 29

Messier 24: Sagittarius Star Cloud
Image Credit & Copyright: Roberto Colombari

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 the left side of this gorgeous starscape. Covering about 4 degrees or the width of 8 full moons in the constellation Sagittarius, the telescopic field of view contains many small, dense clouds of dust and nebulae toward the center of the Milky Way, including reddish emission from IC 1284 near the top of the frame.

2018 June 30

The East 96th Street Moon
Image Credit & Copyright: Stan Honda

A very full Moon rose over Manhattan's Upper Eastside on June 28, known to some as the Strawberry Moon. Near the horizon, the warm yellow lunar disk was a bit ruffled and dimmed by a long sight-line through dense, hazy atmosphere. Still it fit well with traffic and lights along East 96th street in this urban astroimage. The telephoto shot was (safely) taken from elevated ground looking east-southeast from Central Park, planet Earth. Of course, the East 96th street moon was the closest Full Moon to this year's northern summer solstice.

2018 July 1

Fresh Tiger Stripes on Saturn's Enceladus
Image Credit: NASA, ESA, JPL, SSI, Cassini Imaging Team

Do underground oceans vent through the tiger stripes on Saturn's moon Enceladus? Long features dubbed tiger stripes are known to be spewing ice from the moon's icy interior into space, creating a cloud of fine ice particles over the moon's South Pole and creating Saturn's mysterious E-ring. Evidence for this has come from the robot Cassini spacecraft that orbited Saturn from 2004 to 2017. Pictured here, a high resolution image of Enceladus is shown from a close flyby. The unusual surface features dubbed tiger stripes are visible in false-color blue. Why Enceladus is active remains a mystery, as the neighboring moon Mimas,approximately the same size, appears quite dead. A recent analysis of ejected ice grains has yielded evidence that complex organic molecules exist inside Enceladus. These large carbon-rich molecules bolster -- but do not prove -- that oceans under Enceladus' surface could contain life.