The Orion Nebula is a small but dense stellar nursery credit: NASA, ESA, M. Robberto (STScI/ESA) and The Hubble Space Telescope Orion Treasury Project Team
Popular Mechanics has a great series of articles today on amateur astronomy, including Affordable Ways to Become an Amateur Astronomer, and How to Computerize Your Telescope. But my favorite is the Top Five Galactic Bodies Anyone Can See With a Cheap Telescope. Number one on the list is the Orion Nebula, above. Granted, with small telescopes, it won’t look like this Hubble Space Telescope image, but The Great Nebula is even visible with the naked eye in the northern hemisphere, and looks pretty impressive in small telescope, too. To find it, those in the northern hemisphere will have to wait until cooler weather approaches. But look for Orion’s belt, three bright stars in a row. Hanging south from the belt is Orion’s sword, composed of three bright dots; the center dot is the great nebula.
Andromeda_Galaxy Credit: Hubble
Number two is the Andromeda Galaxy. A.K.A M31, this beautiful galaxy is another naked eye object that shows up well in small telescopes. To find it, locate the North Star, then the constellation Cassiopeia, which looks like a giant “W” and is directly across the Big Dipper, with the North Star in between the two. Look at the right “V” shape within the larger “W” of Cassiopeia; 15 degrees down from the tip of the ‘V’ is M31. Popular Mechanics recommends using the lowest power on the telescope to get as much as the galaxy into the field of view as possible. Hercules
Number three is the Hercules Globular Cluster. It is relatively close, only about 25,000 light-years away and it pretty big –about 150 light-years wide, making it an easy target. Hercules is best viewed from the northern hemisphere in the summer months during a new moon. Locate Hercules by looking for the trademark trapezoidal keystone within the constellation. M13 is the brightest spot on the western side of the shape, about 20 degrees due west of the constellation Lyra. Crab Nebula. Credit: NASA/ESA
Number four on the list is the Crab Nebula. This is the left-overs from a supernova that occurred in the year 1054. Back then it was bright enough to see in the daytime, and now it makes for a great sight at night, but a telescope is required. M1 is located on the southern horn of Taurus, the bull shaped constellation southeast of Orion. The object is best seen using a 200x zoom from the northern hemisphere around midnight. Whirlpool Galaxy. Image credit: Hubble
Number five is the Whirlpool Galaxy. A.K.A. M51, this is one of the largest galaxies visible without using professional telescope. Millions of years ago two galaxies collided to create this colorful and dramatic object. To find it, look about 3.5 degrees southeast of the last star in the Big Dipper’s handle.
Scott Altman, in front of a portait presented to him by the Lakeview Museum in Peoria, IL and painted by artist Bill Hardin. Photo: N. Atkinson
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A spacesuit is a complicated conglomeration of switches, dials, hoses, tabs, and multiple layers of high-tech material. It serves various functions and is part miniature spacecraft, part atmospheric re-creation, and part medical necessity — with the number one priority of protecting those who wear it. But that doesn’t mean a spacesuit is always comfortable. “The whole suit is like a big bladder and it weighs about 80 lbs,” said astronaut Scott Altman, explaining the intricacies of the orange ACES launch and entry space suit to a group of children, “and it’s not always easy to move around in it.” But, undoubtedly today’s suit is more advanced and slightly more comfortable than the spacesuit Altman’s STS-125 crewmate, John Grunsfeld assembled as a child, concocted from vacuum cleaner parts and ice cream tins.
Altman was visiting the Lakeview Museum of Arts and Sciences in Peoria, IL, a facility he visited often while growing up. The museum presented him with a portrait painted by local artist and businessman Bill Hardin, a detailed depiction of Altman wearing the ACES suit, and Altman was asked to explain the various parts of the space suit to the children (and very interested adults) in attendance.
The Advanced Crew Escape Suit, or ACES, is currently worn by all space shuttle crews for the ascent and entry portions of flight. Scott Altman at the Lakeview Museum of Arts and Sciences in Peoria, IL. Photo: N. Atkinson
“It’s a full-pressure suit,” Altman explained, “and the idea is if you are in the space shuttle and the spacecraft loses pressure, the suit will inflate because your body needs pressure on it so you can keep breathing and it will provide you with oxygen to breathe as well.”
The gloves and helmet are attached to the suit with locking metal rings. Altman said the neck can get a little uncomfortable because it has a “seal” that can get quite tight at the neck. “But it has some tabs we can pull on to bring the seal away from our necks when are walking around in the suit before you get on the space shuttle,” he said “which is nice because otherwise it is difficult to turn your head!”
Then Altman described the helmet. “It fits in and slides into the latches on the metal ring,” he explained. “The funny thing is that most helmets I’ve worn, when you turn your head the helmet turns with you. But in this helmet, you turn your head and you end up looking at the inside of the helmet. You actually have to turn the helmet manually with your hands by grabbing hold of the front of it and moving everything together.” Altman and his crewmates on the flight deck of the shuttle during the STS-125 mission. Note all the switches up above the heads of the astronauts. Credit: NASA
Another thing about the helmet is that wearing it makes it hard to see up above your head. What makes this interesting for Altman is that he is the shuttle commander, flying the spacecraft as the lead pilot. The shuttle has over 450 separate switches and buttons in the cockpit, not counting all the circuit breakers that can be pulled out. Some of them are located — you guessed it – up above the commander’s head.
“We are strapped in our seats very tight, and with the helmet on it is really hard to look up,” Altman said. “You can’t lean back very well in the seats, so to look up at all the switches up high, you kind of have to bend over and twist and turn your head, and turn the helmet. So it makes life a little more difficult.”
One child asked about the big zipper-like contraption on the front of the suit.
“When you’re standing up in the suit everything fits pretty well,” Altman said, “but imagine when you are sitting down the bottom of the suit rises up and everything else moves up, too. Then, when the suit starts to inflate the whole thing starts to rise up so pretty soon you find yourself looking at the bottom edge of the inside of the helmet and you can’t see. So this is a pulley system that allows you to tighten up the suit so it doesn’t go up over your head. These are all-important safety measures!” Scott Altman cuts a space shuttle cake at the Lakeview Museum in Peoria, IL. Photo: N. Atkinson
Altman used several acronyms to describe the different parts on the suit, saying NASA loves to make up new acronyms for everything. “We fly laptops in space to use but we don’t just call them laptops,” he said. “We call them PGSC’s and I don’t even know what that stands for!” (Payload and General Support Computer)
Later, Altman answered questions submitted by children about what he has seen on his space travels, how to eat and shower in space, and of course, how to go to the bathroom in space.
Space shuttle Endeavour and its crew of seven astronauts ended their 16-day mission by landing safely at Kennedy Space Center in Florida. If you missed seeing it live, watch the picture-perfect landing here. Good weather allowed the crew to come home on the first landing opportunity, after their orbital journey of more than 6.5 million miles. Endeavour touched down at 10:48 a.m. EDT, the 71st shuttle landing at KSC. It was the 23rd flight for Endeavour, the 127th space shuttle mission and 29th shuttle flight to the International Space Station. Continue reading “Endeavour Lands Safely (Video)”
Heads up for our friends in Southeast Europe, Northeast Africa, South America! In a matter of hours Antares is going to be occulted by the Moon! See the IOTA pages for times and locations and get out and watch! This weekend is a great time to do some lunar explorations and catch up on some double star work, too. Have you been watching for the impact site on Jupiter? Even if you don’t have a telescope, I’ve got another video in here to share with you that’s gonna’ blow your mind. Are you ready to do some observing? Then I’ll see you in the back yard…
Friday, July 31, 2009 – Heads up for our friends in Southeast Europe, Northeast Africa, South America! You don’t have long until Antares is going to be occulted by the Moon! See the IOTA pages for times and locations and get out and watch! For many of us the bright red “Rival of Mars” will simply be a close and appealing visitor tonight, so take this opportunity to view an occultation for yourself thanks to a little video magic from Joe Brimacombe!
Now let’s take an entirely different view of the Moon as we do some ‘‘mountain climbing.’’ Tonight the most outstanding feature on the Moon will be the emerging Copernicus, but since we’ve delved into the deepest areas of the lunar surface, why not climb to some of its peaks?
Using Copernicus as our guide, to the north and northwest of this ancient crater lies the Carpathian Mountains, ringing the southern edge of Mare Imbrium. As you can see, they begin well east of the terminator, but look into the shadow! Extending some 40 kilometers beyond the line of daylight, you will continue to see bright peaks, some of which reach over 2,000 meters in height! When the area is fully revealed tomorrow, you will see the Carpathian Mountains eventually disappearing into the lava flow that once formed them. Continuing onward to Plato, which sits on the northern shore of Imbrium, we will look for the singular peak of Pico. It is between Plato and Mons Pico that you will find the scattered peaks of the Teneriffe Mountains. These may be the remnants of much taller summits of a once stronger range, but only about 1,890 meters still survives above the surface. Time to power up! To the west of the Teneriffes, and very near the terminator, you will see a narrow series of hills cutting through the region west-southwest of Plato. This is known as the Straight Range—Montes Recti—and some of its peaks reach up to 2,072 meters. Although this doesn’t sound particularly impressive, that’s over twice as tall as the Vosages Mountains in Central Europe, and on the average very comparable to the Appalachian Mountains in the eastern United States.
Saturday, August 1, 2009 – Let’s continue our lunar mountain climbing expedition and look at the ‘‘big picture’’ on the Moon’s surface. Tonight all of Mare Imbrium is bathed in sunlight, and we can truly see its shape. Let’s identify the mountain ranges again. Starting at Plato and moving east to south to west you will find the Alps, the Caucasus, and the Apennines (where Apollo 15 landed at the western end of Palus Putredinus), respectively. Next come the Carpathian Mountains just north of Copernicus.
Look at their form closely. Doesn’t it appear that once upon a time an enormous impact created the entire area? The Imbrium impact. . . Compare it to the younger Sinus Iridium. Ringed by the Juras Mountains, it may have also been formed by a much later and very similar impact.
And you thought they were just mountains. . .
Tonight let’s honor the 1891 birth on this date of Helen Sawyer Hogg, who cataloged distances to variable stars in globular clusters. Although it’s too bright to globular hunt tonight, we can start with our eyes on Delta Ophiuchi (RA 16 14 20 Dec +03 41 39), another undiscovered gem. Known as Yed Prior (the ‘‘Hand’’), look for its optical double Epsilon to the southeast, handily named Yed Posterior. Now have a look at this area in binoculars or a telescope, using absolutely minimum power. Delta Ophiuchi is 170 light-years from us, while Epsilon is 108. But look at the magnificent field they share. Stars of every spectral type are together in an area of sky that could easily be covered by a small coin held at arm’s length. Enjoy this fantastic field, from the hot blue youngsters to the old red giants!
Sunday, August 2, 2009 – Today we celebrate the official adoption of Greenwich Mean Time (GMT) in 1880. Tonight take time to head north of Sinus Iridum, across Mare Frigoris and northeast of the punctuation of Harpalus, and revisit the grand crater J. Herschel.
Although it looks small because it is seen on the curve, this wonderful old walled plain named for John Herschel contains some very tiny details. Its southeastern rim forms the edge of Mare Frigoris, and the small (24 kilometers) crater Horrebow dots its southwestern edge. The crater walls are so eroded with time that not much remains of the original structure. Look for many very small impact craters dotting J. Herschel’s uneven basin and exterior edges. Why return to a previous study? If you can spot the small central crater C, you are resolving a feature only 12 kilometers wide from some 385,000 kilometers away!
While we’re out, let’s have a look at another astounding system called 36 Ophiuchi, located about a thumb-width southeast of Theta (RA 17 15 20 Dec +26 36 10). Situated in space less than 20 light-years from Earth, even small telescopes can split this pair of 5th magnitude K-type giants—stars very similar to our own Sun. Larger telescopes can pick up the C component as well. Be sure to mark your lists with both of your observations tonight, because J. Herschel is a Lunar Club Challenge, and 36 Ophiuchi is on many doubles’ challenge lists.
If you haven’t taken the “time” to hunt down the impact site on Jupiter, then you’d better! Even if it isn’t visible while you’re out observing, you can always enjoy all the great features Jupiter has to offer. Who knows? You might catch a shadow transit… Or just enjoy the waltz of the Galiean moons as they shuttle around the scarred giant. Remember to use as much magnification as possible when looking for the impact site! While videos like Joe Brimacombe’s (seen here) make it very clear, viewing through a small telescope isn’t quite that crisp and easy. The details become much more pronounced in photographs than what can be seen visually, so extra magnification doesn’t harm… It actually helps to dim Jupiter and will pick up the contrast for you. But, take a look at what a Takahashi Mewlon can do!
Absolutely Tak sharp… For now? Wishing you clear and steady skies!
This week’s awesome images are (in order of appearance): Februrary 18, 2009 Antares Occultation Movie (credit – Joe Brimacombe), Montes Teneriffe and Montes Recti (credit—Wes Higgins), Gibbous Moon (credit—Greg Konkel), Delta Ophiuchi (credit—Palomar Observatory, courtesy of Caltech), Crater J. Herschel (credit—Alan Chu), 36 Ophiuchi (credit—Palomar Observatory, courtesy of Caltech) and Jupiter Impact Movie by Joe Brimacombe. We thank you so much!!
A billion years after the big bang, hydrogen atoms were mysteriously torn apart into a soup of ions. Credit: NASA/ESA/A. Felid (STScI)).
It’s no secret that the universe is an extremely vast place. That which we can observe (aka. “the known Universe”) is estimated to span roughly 93 billion light years. That’s a pretty impressive number, especially when you consider its only what we’ve observed so far. And given the sheer volume of that space, one would expect that the amount of matter contained within would be similarly impressive.
But interestingly enough, it is when you look at that matter on the smallest of scales that the numbers become the most mind-boggling. For example, it is believed that between 120 to 300 sextillion (that’s 1.2 x 10²³ to 3.0 x 10²³) stars exist within our observable universe. But looking closer, at the atomic scale, the numbers get even more inconceivable.
At this level, it is estimated that the there are between 1078 to 1082 atoms in the known, observable universe. In layman’s terms, that works out to between ten quadrillion vigintillion and one-hundred thousand quadrillion vigintillion atoms.
And yet, those numbers don’t accurately reflect how much matter the universe may truly house. As stated already, this estimate accounts only for the observable universe which reaches 46 billion light years in any direction, and is based on where the expansion of space has taken the most distant objects observed.
The history of the universe starting the with the Big Bang. Image credit: grandunificationtheory.com
While a German supercomputer recently ran a simulation and estimated that around 500 billion galaxies exist within range of observation, a more conservative estimate places the number at around 300 billion. Since the number of stars in a galaxy can run up to 400 billion, then the total number of stars may very well be around 1.2×1023 – or just over 100 sextillion.
On average, each star can weigh about 1035 grams. Thus, the total mass would be about 1058 grams (that’s 1.0 x 1052 metric tons). Since each gram of matter is known to have about 1024 protons, or about the same number of hydrogen atoms (since one hydrogen atom has only one proton), then the total number of hydrogen atoms would be roughly 1086 – aka. one-hundred thousand quadrillion vigintillion.
Within this observable universe, this matter is spread homogeneously throughout space, at least when averaged over distances longer than 300 million light-years. On smaller scales, however, matter is observed to form into the clumps of hierarchically-organized luminous matter that we are all familiar with.
In short, most atoms are condensed into stars, most stars are condensed into galaxies, most galaxies into clusters, most clusters into superclusters and, finally, into the largest-scale structures like the Great Wall of galaxies (aka. the Sloan Great Wall). On a smaller scale, these clumps are permeated by clouds of dust particles, gas clouds, asteroids, and other small clumps of stellar matter.
Representation of the timeline of the universe over 13.7 billion years, and the expansion in the universe that followed. Credit: NASA/WMAP Science Team.
The observable matter of the Universe is also spread isotropically; meaning that no direction of observation seems different from any other and each region of the sky has roughly the same content. The Universe is also bathed in a wave of highly isotropic microwave radiation that corresponds to a thermal equilibrium of roughly 2.725 kelvin (just above Absolute Zero).
The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle. This states that physical laws act uniformly throughout the universe and should, therefore, produce no observable irregularities in the large scale structure. This theory has been backed up by astronomical observations which have helped to chart the evolution of the structure of the universe since it was initially laid down by the Big Bang.
The current consensus amongst scientists is that the vast majority of matter was created in this event, and that the expansion of the Universe since has not added new matter to the equation. Rather, it is believed that what has been taking place for the past 13.7 billion years has simply been an expansion or dispersion of the masses that were initially created. That is, no amount of matter that wasn’t there in the beginning has been added during this expansion.
However, Einstein’s equivalence of mass and energy presents a slight complication to this theory. This is a consequence arising out of Special Relativity, in which the addition of energy to an object increases its mass incrementally. Between all the fusions and fissions, atoms are regularly converted from particles to energies and back again.
Atom density is greater at left (the beginning of the experiment) than 80 milliseconds after the simulated Big Bang. Credit: Chen-Lung Hung
Nevertheless, observed on a large-scale, the overall matter density of the universe remains the same over time. The present density of the observable universe is estimated to be very low – roughly 9.9 × 10-30 grams per cubic centimeter. This mass-energy appears to consist of 68.3% dark energy, 26.8% dark matter and just 4.9% ordinary (luminous) matter. Thus the density of atoms is on the order of a single hydrogen atom for every four cubic meters of volume.
The properties of dark energy and dark matter are largely unknown, and could be uniformly distributed or organized in clumps like normal matter. However, it is believed that dark matter gravitates as ordinary matter does, and thus works to slow the expansion of the Universe. By contrast, dark energy accelerates its expansion.
Once again, this number is just a rough estimate. When used to estimate the total mass of the Universe, it often falls short of what other estimates predict. And in the end, what we see is just a smaller fraction of the whole.
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If you haven’t had enough Apollo yet, this is like a firehose of image goodness. Gigapan and NASA Ames have collaborated to make huge, zoomable, panable images from two of the Apollo missions to the Moon. Apollo 16 and 17 are the only missions where the astronauts took panoramic images, so these are the only landing sites available in Gigapan. And if you really want to blow your socks off, look at these images in Google Moon. Click your icon for Google Earth (you DO have it downloaded already, don’t you?? If not go to Google Earth and download it,) choose Moon under the little Saturn-like icon on top, zoom in and find the flags for the Apollo 16 and 17 landing sites. Then look for the “camera” icons and click on one, and then choose the option to “fly” into the images. I’m still gasping from doing this with Apollo 17! Once you recover from flying in, you can then pan around and feel like you are walking alongside Gene Cernan and Harrison Schmitt on the Moon. It really is amazing!
Astronomers have used the comet record — including 2001 RX14 (Linear) at left, captured in 2002 by the Sloan Digital Sky Survey — to model a new route for incoming comets that sneaks past Jupiter’s gravity.
The pathway might even be the dominant one that delivers Oort Could comets on an Earth-bound trajectory, say the authors of a new study in Science this week — but if that’s true, comets only rarely cause extinctions on Earth.
(Image credit: Mike Solontoi/University of Washington)
Scientists have debated how many mass extinction events in Earth’s history were triggered by a space body crashing into the planet’s surface. Most agree that an asteroid collision 65 million years ago brought an end to the age of dinosaurs, but there is uncertainty about how many other extinctions might have resulted from asteroid or comet collisions with Earth.
In fact, astronomers know the inner solar system has been protected at least to some degree by Saturn and Jupiter, whose gravitational fields can eject comets into interstellar space or sometimes send them crashing into the giant planets. That point was reinforced last week (July 20) when a huge scar appeared on Jupiter’s surface, likely evidence of a comet impact.
There are about 3,200 known long-period comets, which can take anywhere from 200 to tens of millions of years to orbit the Sun. Among the best-remembered is Hale-Bopp, which was easily visible to the naked eye for much of 1996 and 1997 and was one of the brightest comets of the 20th century.
It has been believed that nearly all long-period comets that move inside Jupiter to Earth-crossing trajectories originated in the outer Oort Cloud, a remnant of the nebula from which the solar system formed 4.5 billion years ago. It begins about 93 billion miles from the sun (1,000 times Earth’s distance from the sun) and stretches to about three light years away (a light year is about 5.9 trillion miles). The Oort Cloud could contain billions of comets, most so small and distant as to never be observed.
The orbits of long-period comets can change when they are nudged by the gravity of a neighboring star as it passes close to the solar system, and it was thought such encounters only affect very distant outer Oort Cloud bodies.
It also was believed that inner Oort Cloud bodies could reach Earth-crossing orbits only during the rare close passage of a star, which would cause a comet shower. But it turns out that even without a star encounter, long-period comets from the inner Oort Cloud can slip past the protective barrier posed by the presence of Jupiter and Saturn and travel a path that crosses Earth’s orbit.
In the new research, University of Washington astronomers Nathan Kaib and Thomas Quinn used computer models to simulate the evolution of comet clouds in the solar system for 1.2 billion years. They found that even outside the periods of comet showers, the inner Oort Cloud was a major source of long-period comets that eventually cross Earth’s path.
By assuming the inner Oort Cloud as the only source of long-period comets, they were able to estimate the highest possible number of comets in the inner Oort Cloud. The actual number is not known. But by using the maximum number possible, they determined that no more than two or three comets could have struck Earth during what is believed to be the most powerful comet shower of the last 500 million years.
“For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth,” Quinn said. “We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir.”
With three major impacts taking place nearly simultaneously, it had been proposed that the minor extinction event about 40 million years ago resulted from a comet shower. Kaib and Quinn’s research implies that if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began.
“That tells you that the most powerful comet showers caused minor extinctions and other showers should have been less severe, so comet showers are probably not likely causes of mass extinction events,” Kaib said.
He noted that the work assumes the area surrounding the solar system has remained relatively unchanged for the last 500 million years, but it is unclear whether that is really the case. It is clear, though, that Earth has benefited from having Jupiter and Saturn standing guard like giant catchers mitts, deflecting or absorbing comets that might otherwise strike Earth.
“We show that Jupiter and Saturn are not perfect and some of the comets from the inner Oort Cloud are able to leak through. But most don’t,” Kaib said.
A view of NASA's Deep Impact probe colliding with comet Tempel 1, captured by the Deep Impact flyby spacecraft's high-resolution instrument.
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A new study claims early comets contained vast interior oceans of liquid water that may have provided the ideal conditions for early life to form.
In a paper published in the International Journal of Astrobiology, Professor Chandra Wickramasinghe and his colleagues at the Cardiff Centre for Astrobiology suggest the watery environment of early comets, together with the vast quantity of organics already discovered in comets, would have provided ideal conditions for primitive bacteria to grow and multiply during the first 1 million years of a comet’s life.
The Cardiff team has calculated the thermal history of comets after they formed from interstellar and interplanetary dust approximately 4.5 billion years ago. The formation of the solar system itself is thought to have been triggered by shock waves that emanated from the explosion of a nearby supernova. The supernova injected radioactive material such as Aluminium-26 into the primordial solar system and some became incorporated in the comets. Professor Chandra Wickramasinghe together with Drs Janaki Wickramasinghe and Max Wallis claim that the heat emitted from radioactivity warms initially frozen material of comets to produce subsurface oceans that persist in a liquid condition for a million years.
Professor Wickramasinghe said: “These calculations, which are more exhaustive than any done before, leaves little doubt that a large fraction of the 100 billion comets in our solar system did indeed have liquid interiors in the past.
Comets in recent times could also liquefy just below their surfaces as they approach the inner solar system in their orbits. Evidence of recent melting has been discovered in recent pictures of comet Tempel 1 taken by the “Deep Impact” probe in 2005.”
The existence of liquid water in comets gives added support for a possible connection between life on Earth and comets. The theory, known as cometary panspermia, pioneered by Chandra Wickramasinghe and the late Sir Fred Hoyle argues the case that life was introduced to Earth by comets.
Simulated encounters between galaxies show how dwarf spheroidal galaxies loose much of their stars and gas. Credit: Harvard Smithsonian CfA
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What is small, mysterious, faint, in the process of losing mass, and can dance like crazy? Could it be Marie Osmond? Well, that might be the correct answer in this galaxy, but just on the outskirts of the Milky Way are small, mysterious galaxies called dwarf spheroidal galaxies, and a new study offers an explanation for the origin of these puzzling objects. But can they really dance? Yes, says lead author Elena D’Onghia of the Harvard-Smithsonian Center for Astrophysics.
These dwarf spheroidal galaxies are small and very faint, containing few stars relative to their total mass. They appear to be made mostly of dark matter – a mysterious substance detectable only by its gravitational influence, which outweighs normal matter by a factor of five to one in the universe as a whole.
Astronomers have found it difficult to explain the origin of dwarf spheroidal galaxies. Previous theories require that dwarf spheroidals orbit near large galaxies like the Milky Way, but this does not explain how dwarfs that have been observed in the outskirts of the “Local Group” of galaxies could have formed.
“These systems are ‘elves’ of the early universe, and understanding how they formed is a principal goal of modern cosmology,” said D’Onghia. This simulation demonstrates the resonant stripping process. Stars of a dwarf galaxy (bottom) orbiting a larger system are stripped off by gravity. Credit: CfA
D’Onghia and her colleagues used computer simulations to examine two scenarios for the formation of dwarf spheroidals: 1) an encounter between two dwarf galaxies far from giants like the Milky Way, with the dwarf spheroidal later accreted into the Milky Way, and 2) an encounter between a dwarf galaxy and the forming Milky Way in the early universe.
The team found that the galactic encounters excite a gravitational process which they term “resonant stripping,” leading to the removal of stars from the smaller dwarf over the course of the interaction and transforming it into a dwarf spheroidal.
“Like in a cosmic dance, the encounter triggers a gravitational resonance that strips stars and gas from the dwarf galaxy, producing long visible tails and bridges of stars,” explained D’Onghia.
“This mechanism explains the most important characteristic of dwarf spheroidals, which is that they are dark-matter dominated,” added co-author Gurtina Besla.
The long streams of stars pulled off by gravitational interactions should be detectable. For example, the recently discovered bridge of stars between Leo IV and Leo V, two nearby dwarf spheroidal galaxies, may have resulted from resonant stripping.
A new estimate of Saturn’s rotation rate reveals days on the gas giant are five minutes shorter than previously believed — and that Saturn’s atmosphere has much in common with that of its planetary neighbor, Jupiter.
The new results appear today in the journal Nature.
(Image caption: Saturn as photographed by Cassini-Huygens. Credit: NASA)
An image of Saturn from NASA's Cassini spacecraft, clearly showing the 'geographic' South Pole of the planet (at the center of the circle of clouds, lower left). The bulk rotation of the planet is around an axis passing through the South Pole and Saturn's clouds (of ammonia ice) are organised into dark 'belts' and light 'zones' that are generally aligned with lines of latitude, indicating the influence of the planet's rotation on its meteorology.
For planets with solid surfaces, the spin rate can simply be determined by tracking the motion of landforms as they rotate across the surface.
Like the rocky planets, gas giant planets such as Jupiter and Saturn spin on their axes with well defined rotation periods. But, with no solid surface features to track, measuring the rotation period of a gas giant is a challenge. The approach that has worked for Jupiter, Uranus and Neptune — using the rotation of the planet’s magnetic field to infer its bulk rotation — gives results for Saturn that change with time, and implies a pattern of atmospheric winds that is very different from that seen on Jupiter.
Peter Read, of the University of Oxford in the UK, and his colleagues used atmospheric dynamics on Saturn to derive a rotation rate that is slightly faster than those inferred from magnetic measurements. When Saturn’s atmospheric winds are viewed relative to this new interior reference frame, they show a pattern of alternating eastward and westward jets similar to the pattern seen on Jupiter.
“This shifted reference frame is consistent with a pattern of alternating jets on Saturn that is more symmetrical between eastward and westward flow,”Read and his co-authors write. “This suggests that Saturn’s winds are much more like those of Jupiter than hitherto believed.”
The authors propose a new rotation rate of 10 hours and 34 minutes, as opposed to the previous estimate of 10 hours 39 minutes. The new rate also sheds light on Saturn’s interior structure, including its density and the mass of a possible rocky core. And it bears on the latitudinal gradient of temperatures below the clouds.
In a related editorial, Adam Showman of the University of Arizona in Tucson writes that there remain key differences between the atmospheres of Saturn and Jupiter: “Saturn’s winds are stronger than Jupiter’s, its banded cloud patterns and populations of hurricane-like vortices differ considerably, and its magnetic field, which is almost symmetrical about its axis — a puzzle in its own right — contrasts with Jupiter’s tilted dipole,” he notes. “These contrasts indicate that the planets are cousins rather than twins, whose intriguing mix of similarities as well as differences will keep planetary scientists engaged for years to come.”
Second image caption: An image of Saturn from NASA’s Cassini spacecraft, clearly showing the ‘geographic’ South Pole of the planet (at the center of the circle of clouds, lower left). The bulk rotation of the planet is around an axis passing through the South Pole and Saturn’s clouds (of ammonia ice) are organized into dark ‘belts’ and light ‘zones’ that are generally aligned with lines of latitude, indicating the influence of the planet’s rotation on its meteorology.
