APOD Retrospective: April 16

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: April 16

Postby bystander » Mon Apr 16, 2012 3:23 am


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2015
While most spiral galaxies, including our own Milky Way, have two or more spiral arms, NGC 4725 has only one. In this sharp color composite image, the solo spira mirabilis seems to wind from a prominent ring of bluish, newborn star clusters and red tinted star forming regions. The odd galaxy also sports obscuring dust lanes a yellowish central bar structure composed of an older population of stars. NGC 4725 is over 100 thousand light-years across and lies 41 million light-years away in the well-groomed constellation Coma Berenices. Computer simulations of the formation of single spiral arms suggest that they can be either leading or trailing arms with respect to a galaxy's overall rotation. Also included in the frame, sporting a noticably more traditional spiral galaxy look, is a more distant background galaxy.



2014
A beautiful, reddened Moon slid through dark skies on April 15, completely immersed in Earth's shadow for well over an hour. It was the year's first total lunar eclipse and was widely enjoyed over the planet's Western Hemisphere. Seen from the Caribbean island of Barbados, the dimmed lunar disk is captured during totality in this colorful skyview. The dark Moon's red color contrasts nicely with bright bluish star Spica, alpha star of the constellation Virgo, posing only about two degrees away. Brighter than Spica and about 10 degrees from the Moon on the right, Mars is near opposition and closest approach to Earth. The Red Planet's own ruddy hue seems to echo the color of the eclipsed Moon.



2013
One of the brightest galaxies in planet Earth's sky is similar in size to our Milky Way Galaxy: big, beautiful M81. This grand spiral galaxy lies 11.8 million light-years away toward the northern constellation of the Great Bear (Ursa Major). The deep image of the region reveals details in the bright yellow core, but at the same time follows fainter features along the galaxy's gorgeous blue spiral arms and sweeping dust lanes. It also follows the expansive, arcing feature, known as Arp's loop, that seems to rise from the galaxy's disk at the upper right. Studied in the 1960s, Arp's loop has been thought to be a tidal tail, material pulled out of M81 by gravitational interaction with its large neighboring galaxy M82. But a subsequent investigation demonstrates that at least some of Arp's loop likely lies within our own galaxy. The loop's colors in visible and infrared light match the colors of pervasive clouds of dust, relatively unexplored galactic cirrus only a few hundred light-years above the plane of the Milky Way. Along with the Milky Way's stars, the dust clouds lie in the foreground of this remarkable view. M81's dwarf companion galaxy, Holmberg IX, can be seen just above the large spiral. On the sky, this image spans about 0.5 degrees, about the size of the Full Moon.



2012
From afar, the whole thing looks like an Eagle. A closer look at the Eagle Nebula, however, shows the bright region is actually a window into the center of a larger dark shell of dust. Through this window, a brightly-lit workshop appears where a whole open cluster of stars is being formed. In this cavity tall pillars and round globules of dark dust and cold molecular gas remain where stars are still forming. Already visible are several young bright blue stars whose light and winds are burning away and pushing back the remaining filaments and walls of gas and dust. The Eagle emission nebula, tagged M16, lies about 6500 light years away, spans about 20 light-years, and is visible with binoculars toward the constellation of the Serpent (Serpens). This picture combines three specific emitted colors and was taken with the 0.9-meter telescope on Kitt Peak, Arizona, USA.



2011
This telescopic close-up shows off the otherwise faint emission nebula IC 410 in striking false-colors. It also features two remarkable inhabitants of the cosmic pond of gas and dust above and left of center, the tadpoles of IC 410. The picture is a composite of images taken through both broad and narrow band filters. The narrow band data traces atoms in the nebula, with emission from sulfur atoms in red, hydrogen atoms in green, and oxygen in blue. Partly obscured by foreground dust, the nebula itself surrounds NGC 1893, a young galactic cluster of stars that energizes the glowing gas. Composed of denser cooler gas and dust the tadpoles are around 10 light-years long, potentially sites of ongoing star formation. Sculpted by wind and radiation from the cluster stars, their tails trail away from the cluster's central region. IC 410 lies some 12,000 light-years away, toward the constellation Auriga.



2010
Up close, the solar surface is a striking patch work of granules in this very high resolution picture of the quiet Sun. Caused by convection, the granules are hot, rising columns of plasma edged by dark lanes of cooler, descending plasma. But the high-resolution view reveals that the dark lanes are dotted with many small, contrasting bright points. Constantly present on the solar surface, the bright points do not seem to be related to sunspots that come and go with the magnetic solar cycle. Nonetheless, the bright points are regions of concentrated magnetic fields and are bright because the magnetic pressure opens a window to hotter deeper layers below the photosphere. For scale, the white bar at the lower left corresponds to 5,000 kilometers across the Sun's surface. The sharp, narrow-band image was recorded in September, 2007 using the Swedish Solar Telescope on the astronomical island of La Palma.



2009
Clouds couldn't hide this bright Full Moon as it rose last week over the medieval castle of Tourrette-Levens near Nice, France. Exactly full on April 9 at 1456 UT, it followed the March equinox, making it the first Full Moon of spring in the north and autumn in the southern hemisphere. Known as the Easter Moon, it fixes the date for the celebration of Easter on the first Sunday after the first Full Moon of northern spring. Also called the Grass Moon or Egg Moon in the north, in the southern hemisphere, following the autumnal equinox, this Full Moon shines throughout the night as a Hunter's Moon.



2008
This sky is protected. Yesterday marked the 50 year anniversary of the first lighting ordinance ever enacted, which restricted searchlight advertisements from sweeping the night skies above Flagstaff, Arizona, USA. Flagstaff now enjoys the status of being the first International Dark Sky City, and maintains a lighting code that limits lights from polluting this majestic nighttime view. The current dark skies over Flagstaff not only enable local astronomers to decode the universe but allow local sky enthusiasts to see and enjoy a tapestry contemplated previously by every human generation. The above image, pointing just east of north, was taken two weeks ago at 3 am from Fort Valley, only 10 kilometers from central Flagstaff. Visible in the above spectacular panorama are the San Francisco Peaks caped by a lenticular cloud. Far in the distance, the plane of the Milky Way Galaxy arcs diagonally from the lower left to the upper right, highlighted by the constellations of Cassiopeia, Cepheus, and Cygnus. On the far right, the North America Nebula is visible just under the very bright star Deneb.



2007
What could cause a nebula to appear square? No one is quite sure. The hot star system known as MWC 922, however, appears to be imbedded in a nebula with just such a shape. The above image combines infrared exposures from the Hale Telescope on Mt. Palomar in California, and the Keck-2 Telescope on Mauna Kea in Hawaii. A leading progenitor hypothesis for the square nebula is that the central star or stars somehow expelled cones of gas during a late developmental stage. For MWC 922, these cones happen to incorporate nearly right angles and be visible from the sides. Supporting evidence for the cone hypothesis includes radial spokes in the image that might run along the cone walls. Researchers speculate that the cones viewed from another angle would appear similar to the gigantic rings of supernova 1987A, possibly indicating that a star in MWC 922 might one day itself explode in a similar supernova.



2006
How can gas float above the Sun? Twisted magnetic fields arching from the solar surface can trap ionized gas, suspending it in huge looping structures. These majestic plasma arches are seen as prominences above the solar limb. In September 1999, this dramatic and detailed image was recorded by the EIT experiment on board the space-based SOHO observatory in the light emitted by ionized Helium. It shows hot plasma escaping into space as a fiery prominence breaks free from magnetic confinement a hundred thousand kilometers above the Sun. These awesome events bear watching as they can affect communications and power systems over 100 million kilometers away on Planet Earth.



2005
Planetary nebulae can look simple, round, and planet-like in small telescopes. But images from the orbiting Hubble Space Telescope have become well known for showing these fluorescent gas shrouds of dying Sun-like stars to possess a staggering variety of detailed symmetries and shapes. This composite color Hubble image of NGC 6751 is a beautiful example of a classic planetary nebula with complex features. It was selected in April of 2000, to commemorate the tenth anniversary of Hubble in orbit. The colors were chosen to represent the relative temperature of the gas - blue, orange, and red indicating the hottest to coolest gas. Winds and radiation from the intensely hot central star (140,000 degrees Celsius) have apparently created the nebula's streamer-like features. The nebula's actual diameter is approximately 0.8 light-years or about 600 times the size of our solar system. NGC 6751 is 6,500 light-years distant in the high-flying constellation Aquila.



2004
Like grains of sand on a cosmic beach, individual stars of large spiral galaxy NGC 300 are resolved in this sharp image from the Hubble Space Telescope's Advanced Camera for Surveys (ACS). The inner region of the galaxy is pictured, spanning about 7,500 light-years. At its center is the bright, densely packed galactic core surrounded by a loose array of dark dust lanes mixed with the stars in the galactic plane. NGC 300 lies 6.5 million light-years away and is part of a group of galaxies named for the southern constellation Sculptor. Hubble's unique ability to distinguish so many stars in NGC 300 can be used to hone techniques for making distance measurements on extragalactic scales.



2003
Mt. Etna erupted spectacularly in 2001 June. Pictured above, the volcano was photographed expelling bubbles of hot magma, some of which measured over one meter across. One reason planetary geologists study Earth's Mt. Etna is because of its likely similarity to volcanoes on Mars. Mt. Etna, a basalt volcano, is composed of material similar to Mars, and produces similar lava channels. Located in Sicily, Italy, Mt. Etna is not only one of the most active volcanoes on Earth, it is one of the largest, measuring over 50 kilometers at its base and rising nearly 3 kilometers high.



2002
Pictured above is the largest ball of stars in our Galaxy. About 10 million stars orbit the center of this globular cluster - named Omega Centauri - as this giant globular cluster orbits our Galactic center. Recent evidence indicates that Omega Centauri is by far the most massive of the about 150 known globular clusters in the Milky Way. Omega Centauri, cataloged as NGC 5139, spans about 150 light years across, lies about 15,000 light years away, and can be seen without visual aid toward the constellation of Centaurus. The stars in globular clusters are generally older, redder and less massive than our Sun. Studying globular clusters tells us not only about the history of our Galaxy but also limits the age of the universe.



2001
The Hydra Cluster of Galaxies contains well over 100 bright galaxies - but perhaps fewer galaxies than might be expected from its mass. Clusters of galaxies are the largest gravitationally bound objects in the universe. Most of a cluster's mass, however, appears to be in a form too dark to see, as analyses of the distribution of X-ray light, gravitational lensing, and internal motions indicate. Abell 1060, as the above cluster is also known, appears to have an even higher fraction of dark matter than seen in a similar cluster, a situation astronomers cannot easily reconcile with both clusters forming solely from gravitational attraction. The Hydra Cluster of Galaxies, named for its home constellation, spans about ten million light years.



2000
How can a round star make a square nebula? This conundrum came to light with the discovery of planetary nebulae like IC 4406. IC 4406 is most probably cylindrical, with its square appearance the result of our vantage point in viewing the cylinder. Hot gas is known to be flowing 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.



1999
Yesterday, astronomers announced the discovery of the first system of planets around a normal star other than our Sun. Previously, only single planet star systems had been found. Subtle changes in the wobble of Upsilon Andromedae, a Sun-like star in the constellation of Andromeda, allowed astronomers led by R. Paul Butler (AAO) and Geoffrey W. Marcy (SFSU /UCB) to make the breakthrough. This star system is quite different from our own Solar System, however. All three detected planets have masses near or above Jupiter. The discovery implies that multiple-planet systems are quite common, increasing speculation that life-bearing planets similar to Earth may one day be found. The drawing above is an artist's depiction of the Upsilon Andromedae system and its innermost planet. This planet orbits unexpectedly close to its parent star.



1998
The Mars Global Surveyor spacecraft has returned another close-up of the Cydonia region on Mars. Orbiting over clear Martian skies at a range of about 200 miles, the Mars Orbiter Camera looked down on features known as the "City" on Mars and produced a high resolution image covering a swath around 1.5 by 15 miles at a pixel size of about 8.2 feet. This cropped portion of the processed image shows an area approximately 1.5 miles wide. Heavily weathered hills and pocked surfaces suggest the erosion of layers of the ancient Martian crust.



1997
Comet Hale-Bopp continues to look impressive. The photograph above captured the comet on April 7th passing nearly in front of M34, a star cluster in the constellation of Perseus. Many of the stars in this open cluster can be seen through Comet Hale-Bopp's white dust tail. The bright blue ion tail now shows several streams. Now receding from both the Sun and the Earth, Comet Hale-Bopp should still remain an impressive sight for weeks to come as it slowly fades.



1996
Four hundred fifty light-years from Earth, the wind from a dying, sun-like star produced a planetary nebula popularly known as the Helix. While exploring the Helix's gaseous envelope with the Hubble Space Telescope (HST), astronomers discovered indications of 1,000s of striking "cometary knots" like those shown above. So called because of their resemblence to comets, they are actually much larger - their heads are several billion miles across (roughly twice the size of the our solar system itself) while their tails, pointing radially away from the central star, stretch over 100 billion miles. Previously known from ground based observations, the sheer number of cometary knots found in this single nebula is astonishing. What caused them to form? Hot, fast moving shells of nebular gas overrunning cooler, denser, slower shells ejected by the star during an earlier expansion may produce these droplet-like condensations as the two shells intermix and fragment. An intriguing possibility is that instead of dissipating over time, these objects, could collapse and form pluto-like bodies. If so, these icy worlds created near the end of a star's life, would be numerous in our galaxy.



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— Garrison Keillor

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