Astronomy Picture of the Day
Explanation: Unusual jets have been discovered emanating from Comet Hartley 2. The EPOXI spacecraft imaged the jets in unprecedented detail during itsflyby of the comet earlier this month. Pictured above, sun-illuminated jets shoot away from the two-kilometer long decaying iceberg that orbits the Sun between Earth and Jupiter. Comet Hartley 2 became active recently as it neared the Sun and sunlight warmed the comet. Preliminary analyses of images like that shown above indicate that the smooth regions around the middle are porous and leak frozen water vapor directly out into space. Unexpectedly, however, the rough regions at either end appear to shoot carbon dioxide jets that expel fluffy
2010 October 19
Explanation: What is that dark streak below Prometheus? Although it may look like a shadow or a trail blazed by sweeping up material, computer simulations indicate that the dark streak is better understood as an empty path pulled away by the gravity of Saturn’s small moon. The particles don’t follow Prometheus so much as glide sideways past where Prometheus used to be. One dark streamer is created during each pass of Prometheus through the F-ring that it shepherds. The streamers were unpredicted and first discovered in 2004 on high resolution images taken by the robotic Cassini spacecraft orbiting Saturn. Close inspection of the surface of Prometheus itself in the above image shows interesting structure and craters. The Cassini spacecraft arrived at Saturn in 2004 and, as it continues to function well, is now expected to continue to send back data and images from the distant ringed world until 2017.
2010 October 18 :
Explanation: What’s that coming over the edge of the Sun? What might appear at first glance to be some sort of Sun monster is actually a solar prominence. The above prominence, captured by the Sun-orbiting SOHO satellite earlier this year during an early stage of its eruption, rapidly became one of the largest everon record. Even as pictured, the prominence is huge — the Earth would easily fit inside. A solar prominence is a thin cloud of solar gas held just above the surface by the Sun’s magnetic field. A quiescent prominence typically lasts about a month, while an eruptive prominence like the one developing above may erupt within hours into a Coronal Mass Ejection (CME), expelling hot gas into the Solar System. Although very hot, prominences typically appear dark when viewed against the Sun, since they are slightly cooler than the surface. As our Sun evolves toward Solar maximum over the next three years, more large eruptive prominences are expected.
2010 October 17 :
Explanation: How and why are all these stars forming? Found among the Small Magellanic Cloud’s (SMC’s) clusters and nebulae NGC 346 is a star forming region about 200 light-years across, pictured above by the Hubble Space Telescope. A satellite galaxy of the Milky Way, the Small Magellanic Cloud (SMC) is a wonder of the southern sky, a mere 210,000 light-years distant in the constellation of the Toucan (Tucana). Exploring NGC 346, astronomers have identified a population of embryonic stars strung along the dark, intersecting dust lanes visible here on the right. Still collapsing within their natal clouds, the stellar infants’light is reddened by the intervening dust. A small, irregular galaxy, the SMC itself represents a type of galaxy more common in the early Universe. But these small galaxies are thought to be a building blocks for the larger galaxies present today. Within the SMC, stellar nurseries like NGC 346 are also thought to be similar to those found in the early Universe.
2010 October 16 :
Explanation: The 16th century Portuguese navigator Ferdinand Magellan and his crew had plenty of time to study the southern sky during the first circumnavigation of planet Earth. As a result, two fuzzy cloud-like objects easily visible to southern hemisphere skygazers are known as the Clouds of Magellan, now understood to be satellite galaxies of our much larger, spiral Milky Way galaxy. About 160,000 light-years distant in the constellation Dorado, the Large Magellanic Cloud (LMC) is seen here in a remarkably deep, colorful composite image, starlight from the central bluish bar contrasting with the telltale reddish glow of ionized atomic hydrogen gas. Spanning about 15,000 light-years or so, it is the most massive of the Milky Way’s satellite galaxies and is the home of theclosest supernova in modern times, SN 1987A. The prominent patch at top left is 30 Doradus, also known as the magnificent Tarantula Nebula. The giant star-forming region is about 1,000 light-years across.
2010 October 15 :
Explanation: Drifting through the one-horned constellation Monoceros, these dusty streamers and new born stars are part of the active Monoceros R2 star-forming region, embedded in a giant molecular cloud. The cosmic scene was recorded by the VISTA survey telescope in near-infrared light. Visible light images show dusty NGC 2170, seen here just right of center, as a complex of bluish reflection nebulae. But this penetrating near-infrared view reveals telltale signs of ongoing star formation and massive young stars otherwise hidden by the dust. Energetic winds and radiation from the hot young stars reshape the natal interstellar clouds. Close on the sky to the star-forming Orion Nebula, the Monoceros R2 region is almost twice as far away, about 2700 light-years distant. At that distance, this vista spans about 80 light-years.
2010 October 14 :
Explanation: An alluring Comet Hartley 2 cruised through planet Earth’s night sky on October 8, passing within about a Full Moon’s width of the famous double star cluster in Perseus. The much anticipated celestial photo-op was recorded here in a 3 frame mosaic with greenish comet and the clusters h and Chi Persei placed at the left. The well-chosen, wide field of view spans about 7 degrees. It extends across the constellation boundary into Cassiopeia, all the way to the Heart Nebula (IC 1805) at the far right. To capture the cosmic moment, a relatively short 5 minute exposure was used to freeze the moving comet in place, but a longer exposure with a narrow-band filter was included in the central and right hand frames. The narrow-band exposure brings out the fainter reddish glowof the nebula’s atomic hydrogen gas, in contrast to the cometary coma’s kryptonite green. In the past few days, comet watchers have reported that Hartley 2 has become just visible to the unaided eye for experienced observers from dark, clear sites. On October 20, the comet will make its closest approach to Earth, passing within about 17 million kilometers. On November 4, a NASA spacecraft will fly by the comet’s small nucleus estimated to be only 1.5 kilometers in diameter.
2010 October 13 :
Explanation: Will the Hubble Space Telescope (HST) end up in a museum? Probably not, as when it finally goes bust, current plans call for it to be de-orbited into an ocean. But this won’t stop likenesses of the famous floating observatory from appearing in science museums around the globe, sometimes paired withamazing pictures it has taken. Pictured above, in a celebration of the 20th anniversary of the launching of Hubble, a replica of the telescope was given a picturesque setting in the Italian Istituto Veneto di Scienze, Lettere ed Arti in their beautiful and historic Palazzo Loredan. The scene there appears perhaps a bitsurreal as the deep space imager appears over a terrestrial tile floor, surrounded by the busts of famous thinkers, and under arches reminiscent of Escher. If you’re lucky, it may even be possible to find an exhibition of Hubble images near you. And if no HST model appears there, you could always build your own.
2010 October 12 :
Explanation: What’s creating those dark bands on Saturn? Sometimes it takes a little sleuthing to figure out the how and why of a picture taken by the Saturn-orbiting Cassini spacecraft. Let’s see. That large orb on the left must be Saturn itself. Those arcs on the right are surely the rings. The dark band running diagonally must be the shadow of Saturn on the rings. That leaves the unusual dark bands superposed on Saturn’s disk — are they the shadows of the rings? Apunctilious detective would conclude that they are not. If one looks carefully, the rings arc from behind the planet on the lower left, around to the right, and therefore must pass on the camera side of the planet on the upper left. So the rings themselves cause the dark streaks on Saturn. These rings segments appear dark because they are in the shadow of Saturn. The night part of Saturn shows a faint glow because of sunlight reflected from other parts of the rings. Got it?Unfortunately, if it weren’t for the tile floor, tomorrow’s picture would be even harder to understand.
2010 October 11 :
Explanation: Does spiral galaxy NGC 2683 have a bar across its center? Being so nearly like our own barred Milky Way Galaxy, one might guess it has. Being so nearly edge-on, however, it is hard to tell. Either way, this gorgeous island universe, cataloged as NGC 2683, lies a mere 20 million light-years distant in the northern constellation of the Cat (Lynx). NGC 2683 is seen nearly edge-on in this cosmic vista, with more distant galaxies scattered in the background. Blended light from a large population of old yellowish stars forms the remarkably bright galactic core. Starlight silhouettes the dust lanes along winding spiral arms, dotted with the telltale blue glow of young star clusters in this galaxy‘s star forming regions.
2010 October 10 :
Explanation: Why are there so many moonquakes? A recent reanalysis of seismometers left on the moon by the Apollo moon landings has revealed a surprising number of moonquakes occurring within 30 kilometers of the surface. In fact, 28 moonquakes were detected in data recorded between 1972 and 1977. These moonquakes were not only strong enough to move furniture but the stiff rock of the moon continued vibrating for many minutes, significantly longer than the soft rock earthquakes on Earth. The cause of the moonquakes remains unknown, with one hypothesis holding that landslides in craters cause the vibrations. Regardless of the source, future moon buildings need to be built to withstand the frequent shakings. Pictured above in 1969, Apollo 11 astronautBuzz Aldrin stands beside a recently deployed lunar seismometer, looking back toward the lunar landing module.
2010 October 9 :
Explanation: Globular star clusters roam the halo of our Milky Way Galaxy. Gravitationally bound, these spherical groupings of typically several hundred thousand stars are ancient, older than the stars of the galactic disk. In fact, measurements of globular cluster ages constrain the age of the Universe (it must be older than the stars in it!) and accurate cluster distance determinations provide a rung on the astronomical distance ladder. Globular star cluster NGC 6934 itself lies about 50,000 light-years away in the constellation Delphinus. At that distance, this sharp image from Hubble’s Advanced Camera for Surveys spans about 50 light-years. The cluster stars are estimated to be some 10 billion years old.
2010 October 8 :
Explanation: In late September, two planets were opposite the Sun in Earth’s sky, Jupiter and Uranus. Consequently closest to Earth, at a distance of only 33 light-minutes and 2.65 light-hours respectively, both were good targets for telescopic observers. Recorded on September 27, this well-planned composite of consecutive multiple exposures captured both gas giants in their remarkable celestial line-up accompanied by their brighter moons. The faint greenish disk of distant planet Uranus is near the upper left corner. Of the tilted planet’s 5 larger moons, two can be spotted just above and left of the planet’s disk. Both discovered by 18th century British astronomer Sir William Herschel and later named for characters in Shakespeare’s A Midsummer Night’s Dream, Oberon is farthest left, with Titania closer in. At the right side of the frame is ruling gas giant Jupiter, flanked along a line by all four of its Galilean satellites. Farthest from Jupiter is Callisto, with Europa and Io all left of the planet’s disk, while Ganymede stands alone at the right.
2010 October 7 :
Explanation: Touring the solar system with a 6 year orbital period, small comet Hartley 2 (103/P Hartley) will make its closest approach to planet Earth on October 20 and its closest approach to the Sun on October 28. It may become a naked-eye comet, just visible in clear, dark skies. Meanwhile the comet has been a tempting telescopic target, seen here with an alluring green coma as it shares the frame with emission nebula NGC 281 and stars of the constellation Cassiopeia on October 2. The nebula’s gaping profile defined by dust clouds against the red glow suggests its more playful moniker, the Pacman Nebula. An apparent short bright streak shows the comet’s motion against the background stars during the hour of accumulated exposure time. Over the next few days Comet Hartley 2’s motion will also carry it across a field of view featuring the famous double star cluster in Perseus. On November 4 a spacecraft from planet Earth will actually fly within about 700 kilometers of the comet’s nucleus. Now dubbed EPOXI, that spacecraft was formerly known as Deep Impact.
2010 October 6 :
Aurora Over Alaska
Credit & Copyright: Paul Alsop
Explanation: Are those green clouds or aurora? Photographed above two weeks ago, puffy green aurora help the Moon illuminate the serene Willow Lake and the snowy Wrangell and Saint Elias Mountains in eastern Alaska, USA. Although auroras might first appear to be moonlit clouds, they only add light to the sky and do not block background stars from view. Called northern lights in the northern hemisphere, auroras are caused by collisions between charged particlesfrom the magnetosphere and air molecules high in the Earth’s atmosphere. If viewed from space, auroras can be seen to glow in X-ray and ultraviolet light as well. Predictable auroras likely occur a few days after a powerful magnetic event has been seen on the Sun.
2010 October 5 :
Explanation: The dark Horsehead Nebula and the glowing Orion Nebula are contrasting cosmic vistas. Adrift 1,500 light-years away in one of the night sky’smost recognizable constellations, they appear in opposite corners of the above stunning mosaic. The familiar Horsehead nebula appears as a dark cloud, a small silhouette notched against the long red glow at the lower left. Alnitak is the easternmost star in Orion’s belt and is seen as the brightest star to the left of theHorsehead. Below Alnitak is the Flame Nebula, with clouds of bright emission and dramatic dark dust lanes. The magnificent emission region, the Orion Nebula(aka M42), lies at the upper right. Immediately to its left is a prominent bluish reflection nebula sometimes called the Running Man. Pervasive tendrils of glowing hydrogen gas are easily traced throughout the region.
2010 October 3 :
Explanation: The strangest moon in the Solar System is bright yellow. This picture, an attempt to show how Io would appear in the “true colors” perceptible to the average human eye, was taken in 1999 July by the Galileo spacecraft that orbited Jupiter from 1995 to 2003. Io’s colors derive from sulfur and moltensilicate rock. The unusual surface of Io is kept very young by its system of active volcanoes. The intense tidal gravity of Jupiter stretches Io and damps wobbles caused by Jupiter’s other Galilean moons. The resulting friction greatly heats Io‘s interior, causing molten rock to explode through the surface. Io’s volcanoes are so active that they are effectively turning the whole moon inside out. Some of Io‘s volcanic lava is so hot it glows in the dark.
2010 October 2 :
Explanation: Like brush strokes on a canvas, ridges of color seem to flow across this scene. But here, the canvas is nearly 3 light-years wide and the colors map emission from ionized gas in the Lagoon Nebula, recorded by the Hubble Space Telescope’s Advanced Camera for Surveys. Also known as M8, the nebula is a star forming region about 5,000 light-years distant in the constellation Sagittarius. Hubble’s remarkably sharp, close-up view reveals undulating shapes sculpted by the energetic light and winds from the region’s new born stars. Of course, the Lagoon nebula is a popular target for earthbound skygazers, too. It features a prominent dust lane and bright hourglass shape in small telescopes with wider fields of view.
2010 October 1 :
Explanation: A mere 20 light-years away in the constellation Libra, red dwarf star Gliese 581 has received much scrutiny by astronomers in recent years. Earthbound telescopes had detected the signatures of multiple planets orbiting the cool sun, two at least close to the system’s habitable zone — the region where an Earth-like planet can have liquid water on its surface. Now a team headed by Steven Vogt (UCO Lick), and Paul Butler (DTM Carnagie Inst.) hasannounced the detection of another planet, this one squarely in the system’s habitable zone. Based on 11 years of data, their work offers a very compelling case for the first potentially habitable planet found around a very nearby star. Shown in this artist’s illustration of the inner part of the exoplanetary system, the planet is designated Gliese 581g, but Vogt’s more personal name is Zarmina’s World, after his wife. The best fit to the data indicates the planet has a circular 37 day orbit, an orbital radius of only 0.15 AU, and a mass 3.1 times the Earth’s. Modeling includes estimates of a planet radius of 1.5, and gravity at the planet’s surface of 1.1 to 1.7 in Earth units. Finding a habitable planet so close by suggests there are many others in our Milky Way galaxy.