Posted on by Fraser Cain

What’s Up This Week – January 9 – January 15, 2006

What's Up 2006

Download our free “What’s Up 2006” ebook, with entries like this for every day of the year.

Plato crater on the Moon. Image credit: Wes Higgins. Click to enlarge.
Monday, January 9 – Today in 1839, South African Thomas Henderson measured the distance to the closest bright star other than the Sun. Using geometrical parallax, Alpha Centauri was found to be 4.3 light-years away – this amounted to a distance of almost 41 trillion kilometers! Such a distance is the equivalent of over 270,000 earth-sun distances (astronomical units – AU).

Speaking of parallax, let’s take a look at a star with the precisely measured distance of 10.67 light-years from our Sun – Epsilon Eridani. Epsilon is the third closest of the visible stars and can be found tonight by starting at Rigel. About a hand span southwest, locate Gamma Eridani and head another fist-width northwest for a pair of easy stars. Epsilon is the westernmost.

At magnitude 3.7, Epsilon is not one of the brightest stars in the night sky because it has only 85% of the mass of our own Sun. It is also a young star, some 4 billion years younger than Sol and slightly variable. But, like our own “star,” Epsilon has no companion. On a curious note, science fiction chose Epsilon Eridani to be the home of the Vulcans!
Now let’s have a look at the Moon and a crater so prominent that it can be spotted unaided. To the lunar north, look for the dark ellipse of Plato. This mountain-walled plain with a dark floor is a Class V crater. Its slightly oval shape spans 64 by 67 miles in diameter, but appears far more elliptical due to its northern latitude. Plato’s floor is its most curious feature. Consisting of 2,700 square miles of unique lava, and only broken by a couple of very minor and supremely challenging craters, Plato is one of the very few areas on the lunar surface that seems to have changed in recent history.

Be sure to notice how close the Moon and Pleiades are tonight and check on the internet (IOTA) for grazing and occultation events visible from your area.

Tuesday, January 10 – Robert W. Wilson was born this day in 1936. Wilson is co-discoverer, along with Arno Penzias, of the cosmic microwave background and in 1978, won the Nobel Prize for Physics. On this day in 1946, the US Army’s Signal Corps became the first to successfully bounce radar waves off the Moon. Although this might sound like a minor achievement, let’s look just a bit further into what it really meant.

Known as “Project Diana,” scientists were hard at work to find a way to pierce the Earth’s ionosphere with radio waves – a feat believed impossible at the time. Headed by Lt. Col. John DeWitt, and working with only a handful of full-time researchers, a modified bedspring-type radar antenna was set up at Camp Evans, Georgia. Anxiously, the power was cranked up and the antenna aimed at the rising Moon. A series of radar signals were broadcast and echoes were picked up exactly 2.5 seconds later – the time it takes light to travel to the Moon and back. The significance of Project Diana cannot be underestimated. Because the ionosphere could be pierced, communications became possible between Earth and future space missions. Although it would be more than a decade before the first satellites and manned missions were launched into space, Project Diana had paved the way.

To commemorate Project Diana, let’s have a look at one of the most impressive craters on the Moon – Copernicus.
While Copernicus is not the oldest, deepest, largest, or brightest crater on the Moon, it certainly is one of the most detailed. Visible in binoculars toward Plato and near the terminator, this youthful crater gives a highly etched appearance. Its location in a fairly smooth plain near the center of the Moon’s disc, and prominent “splash” ray system, all combine to make Copernicus visually stunning in a small telescope.
Tonight let’s try our hand at splitting a double star – Gamma Arietis. Known as Mesarthim, Gamma is the third star in the line of bright stars – about a hand span west of the Pleiades – pointing in the direction of Eta Piscium. This orange and green pair gives the appearance of two glowing eyes in the night. Seeing two equal magnitude stars so close together can’t help but get you out observing – even when there’s Moon!

Wednesday, January 11 – Tonight in 1787, Sir William Herschel discovered Uranus’ largest moons – Oberon and Titania. Let’s have a look. Sixth magnitude Uranus is around two finder-widths south-southwest of Lambda Aquarii. Its small, pale blue disc will be distinguishable from neighboring stars. Under the right conditions, the planet can sometimes be seen unaided and was once given the designation “34 Tauri” by 17th century astronomer John Flamsteed. The two satellites – both 14th magnitude – can be seen with very large scopes with excellent seeing conditions.

The most outstanding feature on the northern lunar surface this evening is the “Bay of Rainbows” – Sinus Iridum. Take the time to power up and enjoy its many wonderful features including the bright Promontorium LaPlace to the northeast and Heraclides to the southwest. Ringed by the Juras Mountains, Sinus Iridum also includes crater Bianchini at center and Sharp to the west.

Thursday, January 12 – This date celebrates the 1830 founding of what – one year later – would become the Royal Astronomical Society. Conceived by John Herschel, Charles Babbage, James South and others, the RAS has continuously published its Monthly Notices since 1831.

Tonight our primary lunar study is crater Kepler. Look for it as a bright point, slightly lunar north of center near the terminator. Its home is the Oceanus Procellarum – a sprawling dark mare composed primarily of dark minerals of low reflectivity (albedo) such as iron and magnesium. Bright, young Kepler will display a wonderfully developed ray system. The crater rim is very bright, consisting mostly of a pale rock called anorthosite. The “lines” extending from Kepler are fragments that were splashed out and flung across the lunar surface when the impact occurred. The region is also home to features known as “domes” – seen between the crater and the Carpathian Mountains. So unique is Kepler’s geological formation that it became the first crater mapped by U.S. Geological Survey in 1962.

With the nearly full Moon in Gemini, go north to Cassiopeia and check out wide double star 35 Cassiopeia about two finger-widths west of Epsilon and an equal distance north of Gamma. This is an easy split for telescopes and can be resolved in steady binoculars.

Friday, January 13 – Tonight let’s give the Moon a rest and turn our scopes to Mars high overhead. With the exception of Sirius, Mars remains brighter than any star in the sky. To the eye, the planet’s ruddy glow makes it unmistakable. Through the telescope, observers can make out large-scale details such as the planets polar caps, Syrtis Major, Sinus Sabaeus, and the three major Mares – Cimmerium, Sirenum and Acidalium. Although good “seeing” makes high power and fine details possible, sometimes just “viewing” is half the fun!

Saturday, January 14 – Tonight’s Full Moon is known as the Wolf Moon. For the northern hemisphere in January, extreme cold and deep snows gave rise to the legend of wolf packs howling hungrily outside Indian villages. Sometimes the January Full Moon is also referred to as the Old Moon, or the Moon after Yule. No matter what it is called, it is still a lovely sight to watch rise and glide across the luminous night sky.

As a challenge this evening, try tracking down 5th magnitude double star Zeta Piscium. Located two finger-widths due east of Epsilon, this pair is easily resolved at low magnification due to its near matched brightness. Look for subtle shades of color displayed by the blue primary, and ivory-colored secondary.

Sunday, January 15 – With only short time before the Moon rises tonight, let’s start our evening by viewing a distant multiple star system – Sigma Orionis.

You’ll easily find 1400 light-years distant Sigma less than a finger-width below the left hand star in Orion’s “Belt” (Zeta or Alnitak). What won’t be easy is to distinguish the closest and brightest pair! 3.8 magnitude A and 6.6 magnitude B revolve around each other every 170 years and are separated by a close 0.3 arc seconds. Among the most massive binaries known, these two stars have extremely hot surfaces (approaching 50,000 degrees K) and both appear white in the eyepiece.

At a more comfortable separation, the white 8.8 magnitude C star resides 11.4 arc seconds southwest of the brighter pair. At a similar distance from AB to the east, look for red 6.7 magnitude D. Considerably further away at 41 arc seconds, the blue E star resides east-northeast of the AB primary. Unusual star E shares the same spectral qualities as the AB primary, yet is rich in helium light (emission lines) within its color spectrum. If five isn’t enough, then look 30 arc seconds southwest of E – because it, too, has a companion. All of these stars are part of the same physical system spanning about one-third of a light-year.

If you choose to look at the lunar surface, carefully check along the eastern edge where the terminator is now receding. In the north, look for the dark shades of Mare Humboldtianum and the equally dark floor of crater Endymion to its west. This lava filled area is around 70 miles in diameter.

I would personally like to thank all of you for your support and kind comments on the look at the year ahead. Be sure to stay tuned to the weekly column as breaking observing news is added. Until next week, ask for the Moon, but keep reaching for the stars! May all of your journeys be at light speed… ~Tammy Plotner

Posted on by Fraser Cain

Star Orbiting a Medium Sized Black Hole

An image of the central region of the starburst galaxy M82. Image credit: NASA Click to enlarge
Scientists using NASA’s Rossi X-ray Timing Explorer have found a doomed star orbiting what appears to be a medium-sized black hole ? a theorized “in-between” category of black hole that has eluded confirmation and frustrated scientists for more than a decade.

With the discovery of the star and its orbital period, scientists are now one step away from measuring the mass of such a black hole, a step which would help verify its existence. The star’s period and location already fit into the main theory of how these black holes could form.

A team led by Prof. Philip Kaaret of the University of Iowa, Iowa City, announced these results today in Science Express. The results will also appear in the Jan. 27 issue of Science.

“We caught this otherwise ordinary star in a unique stage in its evolution, toward the end of its life when it has bloated into a red giant phase,” said Kaaret. “As a result, gas from the star is spilling into the black hole, causing the whole region to light up. This is a well-studied region of the sky, and we spotted the star with a little luck and a lot of perseverance.”

A black hole is an object so dense and with a gravitational force so intense that nothing, not even light, can escape its pull once within its boundary. A black hole region becomes visible when matter falls toward it and heats to high temperatures. This light is emitted before the matter crosses the border, called the event horizon.

Our galaxy is filled with millions of stellar-mass black holes, each with the mass of a few suns. These form from the collapse of very massive stars. Most galaxies possess at their core a supermassive black hole, containing the mass of millions to billions of suns confined to a region no larger than our solar system. Scientists do not know how these form, but it likely entails the collapse of enormous quantities of primordial gas.

“In the past decade, several satellites have found evidence of a new class of black holes, which could be between 100 and 10,000 solar masses,” said Dr. Jean Swank, Rossi Explorer project scientist at NASA?s Goddard Space Flight Center, Greenbelt, Md. “There has been debate about the masses and how these black holes would form. Rossi has provided major new insight.”

These suspected mid-mass black holes are called ultra-luminous X-ray objects because they are bright sources of X-rays. In fact, most of these black hole mass estimates have been based solely on a calculation of how strong a gravitational pull is needed to produce light of a given intensity.

Kaaret’s group at the University of Iowa, which includes Prof. Cornelia Lang and Melanie Simet, an undergraduate, made a measurement that can be used in the equation to directly calculate mass. Using straightforward Newtonian physics, scientists can calculate an object’s mass once they know an orbital period and velocity of smaller objects rotating around it.

“We found a rise and fall in X-ray light every 62 days, likely caused by the orbit of the companion star around the black hole,” said Simet. ?The velocity will be hard to determine, however, because the star is located in such a dust-obscured area. This makes it hard for optical and infrared telescopes to observe the star and make velocity calculations. Yet for now, knowing just the orbital period is very revealing.?

The suspected mid-mass black hole, known as M82 X-1, is a well-studied ultra-luminous X-ray object in a nearby star cluster containing about a million stars packed into a region only about 100 light years across. A leading theory proposes that a multitude of star collisions over a short period in a crowded region will create a short-lived gigantic star that collapses into a 1,000-solar-mass black hole. The cluster near M82 X-1 has a high-enough density to form such a black hole. No normal companion could provide enough fuel to make M82 X-1 shine so brightly. But the 62-day orbital period implies that the companion must have a very low density. This fits the scenario of a bloated super-giant star losing mass at a rate high enough to fuel M82 X-1.

“With this discovery of the orbital period, we now have a consistent picture of the whole evolution of a mid-mass black hole binary,” said Kaaret. “It was formed in a ‘super’ star cluster; the black hole then captured a companion star; the companion star evolved to the giant stage; and we now see it as an extremely luminous X-ray source because the companion star has expanded and is feeding the black hole.”

Original Source: NASA News Release

Posted on by Fraser Cain

Hazy View of Saturn

Haze layers in the atmosphere encircling Saturn. Image credit: NASA/JPL/SSI Click to enlarge
In this magnificent view, delicate haze layers high in the atmosphere encircle the oblate figure of Saturn. A special combination of spectral filters used for this image makes the high haze become visible. A methane-sensitive filter (centered at 889 nanometers) makes high altitude features stand out, while a polarizing filter makes small haze particles appear bright.

Methane in the atmosphere absorbs light with wavelengths around 889 nanometers as it travels deeper into the gas planet, thus bright areas in this image must represent reflective material at higher altitudes. Small particles or individual molecules scatter light quite effectively to a polarization of 90 degrees, which this polarizing filter is sensitive to. Thus, high altitude haze layers appear bright in this view.

The small blob of light at far right is Dione (1,126 kilometers, or 700 miles across).

The image was taken with the Cassini spacecraft wide-angle camera on Dec. 5, 2005, at a distance of approximately 2.9 million kilometers (1.8 million miles) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 100 degrees. The image scale is 169 kilometers (105 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov . The Cassini imaging team homepage is at http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Fraser Cain

10 Days Until Stardust Returns

An artist’s illustration of Stardust approaching Earth. Image credit: NASA/JPL Click to enlarge
Ten days before its historic return to Earth with the first-ever samples from a comet, NASA’s Stardust spacecraft successfully performed its 18th flight path adjustment. This second-to-last scheduled maneuver puts the spacecraft on the right path to rendezvous with Earth on Jan. 15 (Universal Time), when it will release its sample return capsule.

At 1800 Universal Time (10:00 am Pacific Time) on Thursday, Jan. 5, Stardust fired all eight of its 4.4 newton (1-pound) thrusters for a total of 107 seconds, changing the comet sampler’s speed by 2.4 meters per second (about 5.4 miles per hour). The maneuver required 385 grams (0.85 pounds) of hydrazine monopropellant to complete. A final trajectory correction maneuver is scheduled prior to release of the sample return capsule.

“It was a textbook maneuver,” said Ed Hirst, Stardust deputy mission manager at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “After sifting through all the post-burn data, I expect we will find ourselves right on the money.”

In the early morning hours of January 15, 2006, the Stardust mission returns to Earth after a 4.63 billion kilometer (2.88 billion mile) round-trip journey carrying a precious cargo of cometary and interstellar dust particles. Scientists believe Stardust’s cargo will help provide answers to fundamental questions about the origins of the solar system.

Scientists believe in-depth terrestrial analysis of cometary samples will reveal much not just about comets but about the earliest history of the solar system. Locked within the cometary particles is unique chemical and physical information that could be the record of the formation of the planets and the materials from which they were made.

Extensive information on the Stardust mission is available from the Stardust site at www.nasa.gov/stardust .

JPL manages the Stardust mission for NASA’s Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, developed and operates the spacecraft. JPL is a division of the California Institute of Technology. NASA’s Johnson Space Center contributed to Stardust payload development, and the Johnson Space Center will curate the sample and support analysis and sample allocation.

Original Source: NASA News Release

Posted on by Fraser Cain

Charon has no Atmosphere

An artist’s conception of Pluto and its moon Charon. Image credit: NASA Click to enlarge
If you want to learn something about a place that’s billions of miles away, it helps to be in the right place at the right time.

Astronomers from MIT and Williams College were lucky enough to watch as Pluto’s largest moon, Charon, passed in front of a star last summer. Based on their observations of the occultation, which lasted for less than a minute, the team reports new details about the moon in the Jan. 5 issue of Nature.

A second paper from another group, led by French astronomer Bruno Sicardy, also appears in this issue of Nature.

The MIT-Williams team was able to measure Charon’s size to an unprecedented accuracy and determine that it has no significant atmosphere. The atmosphere on Pluto, on the other hand, has been very well established.

“The results provide insight into the formation and evolution of bodies in the outer solar system,” said lead author Amanda Gulbis, a postdoctoral associate in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

Specifically, the team found that Charon has a radius of 606 kilometers, “plus or minus 8 kilometers to account for local topography or possible non-sphericity in Charon’s shape,” Gulbis said. That size, combined with mass measurements from Hubble Space Telescope data, show that the moon has a density roughly one-third that of the Earth. This reflects Charon’s rocky-icy composition.

The team also found that the density of any atmosphere on the moon must be less than a millionth of that of the Earth. This argues against the theory that Pluto and Charon were formed by the cooling and condensing of the gas and dust known as the solar nebula. Instead, Charon was likely created in a celestial collision between an object and a proto-Pluto.

“Our observations show that there is no substantial atmosphere on Charon, which is consistent with an impact formation scenario,” Gulbis said. Similar theories exist about the formation of the Earth-moon system.

The success of the MIT-Williams team in observing the Charon occultation bodes well for future adaptations of the technique the researchers used.

“We are eager to use (it) to probe for atmospheres around recently discovered Kuiper Belt objects that are Pluto-sized or even larger,” said James Elliot, co-author of the Nature paper and a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences and in the Department of Physics. Elliot has been observing stellar occultations by bodies in the solar system for more than three decades.

Jay Pasachoff, Williams College team leader and a professor in its Department of Astronomy, said, “It’s remarkable that our group could be in the right place at the right time to line up a tiny body 3 billion miles away. The successful observations are quite a reward for all of the people who helped predict the event, constructed and integrated the equipment and traveled to the telescopes.”

In addition to Elliot and Gulbis, members of the MIT team were Michael Person, Elisabeth Adams and Susan Kern, with support from undergraduate Emily Kramer. The Williams College team included Pasachoff, Bryce Babcock, Steven Souza and undergraduate Joseph Gangestad.

The work was supported by NASA.

Original Source: MIT News Release

Update: Pluto isn’t a planet. Why isn’t Pluto a planet?

Posted on by Fraser Cain

When a Meteor Slashed Mars

Color view of ‘butterfly’-shaped crater at Hesperia Planum. Image credit: ESA Click to enlarge
These images, taken by the High Resolution Stereo Camera (HRSC) on board ESA?s Mars Express spacecraft, show a large elliptical impact crater in the Hesperia Planum region of Mars.

The HRSC obtained these images during orbit 368 with a ground resolution of approximately 16.7 metres per pixel. The scenes show the region of Hesperia Planum, at approximately 35.3? South and 118.7? East. A large elliptical impact crater is visible within the scene, measuring approximately 24.4 km long, 11.2 km wide and reaching a maximum depth of approximately 650 metres below the surrounding plains.

Ejecta from this impact can be seen extending away from the crater, including two prominent lobes of material north-west and south-east of the crater.

The large circular feature, partly cut off by the border of the image, has a diameter of roughly 45 km.

This appears to be an impact crater that was subsequently resurfaced by lava flows, preserving the outline of the underlying crater. The curving features visible in the north of the image, known as ?wrinkle ridges?, are caused by compressional tectonics.

While the majority of impact craters are relatively circular, the elliptical shape of this impact crater suggests a very low impact angle (less than 10 degrees).

The long axis of the impact crater is viewed as the impacting direction of the projectile. Similar elliptical craters are observed elsewhere on Mars, as well as on our Moon.

The colour scenes have been derived from the three HRSC-colour channels and the nadir channel. The perspective views have been calculated from the digital terrain model derived from the stereo channels.

Color view of ‘butterfly’-shaped crater at Hesperia Planum. Image credit: ESA Click to enlarge

The 3D anaglyph image was calculated from the nadir and one stereo channel. Image resolution has been decreased for use on the internet.

Original Source: ESA Mars Express

Posted on by Fraser Cain

Superbubble Complex N44

Superbubble complex N44 as imaged with GMOS. Image credit: University of Alaska Anchorage. Click to enlarge
Known as the N44 superbubble complex, this cloudy tempest is dominated by a vast bubble about 325 by 250 light-years across. A cluster of massive stars inside the cavern has cleared away gas to form a distinctive mouth-shaped hollow shell. While astronomers do not agree on exactly how this bubble has evolved for up to the past 10 million years, they do know that the central cluster of massive stars is responsible for the cloud’s unusual appearance. It is likely that the explosive death of one or more of the cluster’s most massive and short-lived stars played a key role in the formation of the large bubble.

“This region is like a giant laboratory providing us with a glimpse into many unique phenomena,” said Sally Oey of the University of Michigan, who has studied this object extensively. “Observations from space have even revealed x-ray-emitting gas escaping from this superbubble, and while this is expected, this is the only object of its kind where we have actually seen it happening.”

One of the mysteries surrounding this object points to the role that supernova explosions (marking the destruction of the most massive of the central cluster’s stars) could have played in sculpting the cloud. Philip Massey of Lowell Observatory, who studied this region along with Oey, adds “When we look at the speed of the gases in this cloud we find inconsistencies in the size of the bubble and the expected velocities of the winds from the central cluster of massive stars. Supernovae, the ages of the central stars, or the orientation and shape of the cloud might explain this, but the bottom line is that there’s still lots of exciting science to be done here and these new images will undoubtedly help.”

The Gemini data used to produce this image are being released to the astronomical community for further research and follow-up analysis. Note to astronomers: Data can be found at the Gemini Science Archive by querying “NGC 1929”. The image provides one of the most detailed views ever obtained of this relatively large region in the Large Magellanic Cloud, a satellite galaxy to the Milky Way, located some 150,000 light-years away and visible from the Southern Hemisphere. The images captured light of specific colors that reveal the compression of material and the presence of gases (primarily excited hydrogen gas and lesser amounts of oxygen and “shocked” sulfur) in the cloud.

Multiple smaller bubbles appear in the image as bulbous growths clinging to the central superbubble. Most of these regions were probably formed as part of the same process that shaped the central cluster. Their formation could also have been “sparked” by compression as the central stars pushed the surrounding gas outward. Our view into this cavern could really be like looking through an elongated tube, which lends the object its monstrous mouth-like appearance.

The images used to produce the color composite were obtained with the Gemini Multi-object Spectrograph (GMOS) at the Gemini South Telescope on Cerro Pachon in Chile. The color image was produced by Travis Rector of the University of Alaska Anchorage and combines three single-color images to produce the image.

Original Source: Gemini Observatory

Posted on by Fraser Cain

A Supernova Every 50 Years

An artist’s illustartion of the sequence of radioactive decay that gives out gamma rays. Image credit: MPE Click to enlarge
Using ESA’s Integral observatory, an international team of researchers has been able to confirm the production of radioactive aluminium (Al 26) in massive stars and supernovae throughout our galaxy and determine the rate of supernovae – one of its key parameters.

The team, led by Roland Diehl of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, determined that gamma rays from the decay of Al 26 originate from the central regions of our galaxy, implying that production of new atomic nuclei is an ongoing process and occurs in star-forming regions galaxy-wide.

Our environment is composed of chemical elements formed long ago by nuclear fusion reactions in stellar interiors and supernovae. This process of ‘nucleosynthesis’ leads to the emission of gamma rays, which easily reach us from all regions of our galaxy. ESA’s Integral observatory has been measuring such gamma rays since October 2002.

Roland Diehl and his colleagues were able to measure the Al 26 gamma-ray emissions along the plane of the inner galaxy.

However, because the disc of the galaxy rotates about its central axis, with the inner regions orbiting faster, gamma rays from decaying Al 26 observed from these regions should be moderated by the Doppler effect in a characteristic way. It is this characteristic pattern that has been found by Integral.

From this measurement, the team found that Al 26 decay gamma rays do indeed reach us from the inner regions of the galaxy, rather than from foreground regions along the same line of sight possibly caused by local and peculiar Al 26 production. These regions would not have the observed high relative velocity.

From these new observations, it is possible to estimate the total amount of radioactive Al 26 in our galaxy as is equivalent to three solar masses. This is a lot, given that Al 26 is an extremely rare isotope; the fraction estimated for the early Solar System is 5/100 000 of Al 26, in proportion to its stable aluminium isotope (Al 27).

Because astrophysicists had inferred that the likely sources are mainly massive stars, which end their lives as supernovae, they could estimate the rate of such supernova events. They obtained a rate of one supernova every 50 years – consistent with what had been indirectly found from observations of other galaxies and their comparison to the Milky Way.

Integral’s study of gamma rays will continue to operate for several more years. Astrophysicists hope to increase the precision of such measurements. Project leader Roland Diehl said, “These gamma-ray observations provide insights about our home galaxy, which are difficult to obtain at other wavelengths due to interstellar absorption.”

Original Source: ESA Portal

Posted on by Fraser Cain

Shadows on the Moon

The full moon. Image credit: Robert Gendler. Click to enlarge
The moon is utterly familiar. We see it all the time, in the blue sky during the day, among the stars and planets at night. Every child knows the outlines of the moon’s lava seas: they trace the Man in the Moon or, sometimes, a Rabbit.

This familiarity goes beyond appearances. The moon is actually made of Earth. According to modern theories, the moon was born some 4.5 billion years ago when an oversized asteroid struck our planet. Material from Earth itself spun out into space and coalesced into our giant satellite.

Yet when Apollo astronauts stepped out onto this familiar piece of home, they discovered that it only seems familiar. From the electrically-charged dust at their feet to the inky-black skies above, the moon they explored was utterly alien.

Thirty years ago their strange experiences were as well-known to the public as the Man in the Moon. Not anymore. Many of the best tales of Apollo have faded with the passage of time. Even NASA personnel have forgotten some of them.

Now, with NASA going back to the moon in search of new tales and treasures, we revisit some of the old ones, with a series of Science@NASA stories called “Apollo Chronicles.” This one, the first, explores the simple matter of shadows.

On the next sunny day, step outdoors and look inside your shadow. It’s not very dark, is it? Grass, sidewalk, toes–whatever’s in there, you can see quite well.

Your shadow’s inner light comes from the sky. Molecules in Earth’s atmosphere scatter sunlight (blue more than red) in all directions, and some of that light lands in your shadow. Look at your shadowed footprints on fresh sunlit snow: they are blue!

Without the blue sky, your shadow would be eerily dark, like a piece of night following you around. Weird. Yet that’s exactly how it is on the Moon.

To visualize the experience of Apollo astronauts, imagine the sky turning completely and utterly black while the sun continues to glare. Your silhouette darkens, telling you “you’re not on Earth anymore.”

Shadows were one of the first things Apollo 11 astronaut Neil Armstrong mentioned when he stepped onto the surface of the moon. “It’s quite dark here in the shadow [of the lunar module] and a little hard for me to see that I have good footing,” he radioed to Earth.

The Eagle had touched down on the Sea of Tranquility with its external equipment locker, a stowage compartment called “MESA,” in the shadow of the spacecraft. Although the sun was blazing down around them, Armstrong and Buzz Aldrin had to work in the dark to deploy their TV camera and various geology tools.

“It is very easy to see in the shadows after you adapt for a while,” noted Armstrong. But, added Aldrin, “continually moving back and forth from sunlight to shadow should be avoided because it’s going to cost you some time in perception ability.”

Truly, moon shadows aren’t absolutely black. Sunlight reflected from the moon’s gently rounded terrain provides some feeble illumination, as does the Earth itself, which is a secondary source of light in lunar skies. Given plenty of time to adapt, an astronaut could see almost anywhere.

Almost. Consider the experience of Apollo 14 astronauts Al Shepard and Ed Mitchell:

They had just landed at Fra Mauro and were busily unloading the lunar module. Out came the ALSEP, a group of experiments bolted to a pallet. Items on the pallet were held down by “Boyd bolts,” each bolt recessed in a sleeve used to guide the Universal Handling Tool, a sort of astronaut’s wrench. Shepard would insert the tool and give it a twist to release the bolt–simple, except that the sleeves quickly filled with moondust. The tool wouldn’t go all the way in.

The sleeve made its own little shadow, so “Al was looking at it, trying to see inside. And he couldn’t get the tool in and couldn’t get it released–and he couldn’t see it,” recalls Mitchell.

“Remember,” adds Mitchell, “on the lunar surface there’s no air to refract light–so unless you’ve got direct sunlight, there’s no way in hell you can see anything. It was just pitch black. That’s an amazing phenomenon on an airless planet.”

(Eventually they solved the problem by turning the entire pallet upside down and shaking loose the moondust. Some of the Boyd bolts, loosened better than they thought, rained down as well.)

Tiny little shadows in unexpected places would vex astronauts throughout the Apollo program–a bolt here, a recessed oxygen gauge there. These were minor workaday nuisances, mostly, but astronauts were jealous of the minutes lost from their explorations.

Shadows could also be mischievous:

Apollo 12 astronauts Pete Conrad and Al Bean landed in the Ocean of Storms only about 600 yards from Surveyor 3, a robotic spacecraft sent by NASA to the moon three years earlier. A key goal of the Apollo 12 mission was to visit Surveyor 3, to retrieve its TV camera, and to see how well the craft had endured the harsh lunar environment. Surveyor 3 sat in a shallow crater where Conrad and Bean could easily get at it–or so mission planners thought.

The astronauts could see Surveyor 3 from their lunar module Intrepid. “I remember the first time I looked at it,” recalls Bean. “I thought it was on a slope of 40 degrees. How are we going to get down there? I remember us talking about it in the cabin, about having to use ropes.”

But “it turned out [the ground] was real flat,” rejoined Conrad.

What happened? When Conrad and Bean landed, the sun was low in the sky. The top of Surveyor 3 was sunlit, while the bottom was in deep darkness. “I was fooled,” says Bean, “because, on Earth, if something is sunny on one side and very dark on the other, it has to be on a tremendous slope.” In the end, they walked down a gentle 10 degree incline to Surveyor 3–no ropes required.

see captionA final twist: When astronauts looked at the shadows of their own heads, they saw a strange glow. Buzz Aldrin was the first to report “?[there’s] a halo around the shadow of my helmet.” Armstrong had one, too.

This is the “opposition effect.” Atmospheric optics expert Les Cowley explains: “Grains of moondust stick together to make fluffy tower-like structures, called ‘fairy castles,’ which cast deep shadows.” Some researchers believe that the lunar surface is studded with these microscopic towers. “Directly opposite the sun,” he continues,” each dust tower hides its own shadow and so that area looks brighter by contrast with the surroundings.”

Sounds simple? It’s not. Other factors add to the glare. The lunar surface is sprinkled with glassy spherules (think of them as lunar dew drops) and crystalline minerals, which can reflect sunlight backwards. And then there’s “coherent backscatter”–specks of moondust smaller than the wavelength of light diffract sunlight, scattering rays back toward the sun. “No one knows which factor is most important,” says Cowley.

We can experience the opposition effect here on Earth, for example, looking away from the sun into a field of tall dewy grass. The halo is there, but our bright blue sky tends to diminish the contrast. For full effect, you’ve got to go to the Moon.

Luminous halos; mind-bending shadows; fairy-castles made of moondust. Apollo astronauts discovered a strange world indeed.

Original Source: NASA News Release