Category: Astronomy

Image credit: NASA
The NASA-led Swift mission has opened its doors to a flurry of gamma-ray burst action.

Scientists were still calibrating the main instrument, the Burst Alert Telescope (BAT), when the first burst appeared on December 17. Three bursts on December 19, and one on December 20, followed.

Swift’s primary goal is to unravel the mystery of gamma ray bursts. The bursts are random and fleeting explosions, second only to the Big Bang in total energy output. Gamma rays are a type of light millions of times more energetic than light human eyes can detect. Gamma ray bursts last only from a few milliseconds to about one minute. Each burst likely signals the birth of a black hole.

“The optimists among us were hoping to detect two bursts a week, not three in one day just after turning the telescope on,” said Dr. Scott Barthelmy, the BAT lead scientist at NASA’s Goddard Space Flight Center, Greenbelt, Md. “Maybe we got lucky, or maybe we’ve underestimated the true rate of these bursts. Only time will tell,” he added.

Once the BAT, that covers about one-seventh of the sky at any time, detects a gamma ray burst, it quickly relays a location to the ground. Within about one minute, the satellite automatically turns toward the burst. The move brings the burst within view of Swift’s two other telescopes: the X-ray Telescope (XRT) and the Ultraviolet/Optical Telescope (UVOT).

Once all three instruments are turned on and calibrated, Swift will get down to the business of analyzing gamma ray bursts. “The universe kept up its side of the bargain, and we kept up ours,” said Dr. Neil Gehrels, Swift’s Principal Investigator at Goddard. “This is going to be an exciting mission,” he said.

The Swift team tested the BAT by observing Cygnus X-1, a well-known bright source that produces gamma rays in our galaxy. It is thought to be a black hole in orbit around a star. The team called this BAT’s “first light.”

The BAT is the most sensitive gamma ray detector ever flown. The BAT employs a novel technology to image and locate gamma ray bursts. Unlike visible light, gamma rays pass right through telescope mirrors and cannot be reflected onto a detector. The BAT uses a technique called “coded aperture mask” to create a gamma ray shadow on its detectors. The mask contains 52,000 randomly placed lead tiles that block some gamma rays from reaching the detectors. With each burst, some detectors light up while others remain dark, shaded by the lead tiles. The angle of the shadow points back to the gamma ray burst.

“The BAT coded aperture mask is about the size of a pool table, the largest and most intricate ever fabricated,” said Ed Fenimore of Los Alamos National Laboratory, N.M. Los Alamos created the BAT software. “BAT can accurately pinpoint a burst within seconds and detect bursts five times fainter than previous instruments,” he added.

Swift, a medium-class explorer mission managed by Goddard, was launched from Cape Canaveral on November 20, 2004. The mission is in participation with the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom.

Swift was built at Goddard in collaboration with General Dynamics, Ariz.; Penn State University, College Station, Pa.; Sonoma State University, Rohnert Park, Calif.; Los Alamos; Mullard Space Science Laboratory, Surrey, England; the University of Leicester, England; the Brera Observatory, Milan, Italy; and ASI Science Data Center, Rome.

Original Source: NASA News Release

Planetary nebulae are expanding gas shells that are ejected by Sun-like stars at the end of their lifetimes. Sun-like stars spend most of their lifetime burning hydrogen into helium. At the end of this hydrogen fusion phase, these stars increase their diameter by about a factor of 100 and become “red giant stars”. At the end of the red giant phase, the outer layers of the star are blown away. The ejected gas continues to expand out from the remaining central star, which later evolves into a “white dwarf” when all nuclear fusion has ceased. Astronomers believe that a planetary nebula forms when a fast stellar wind that comes from the central star catches up a slower wind produced earlier when the star ejected most of its outer layers. At the boundary between the two winds, a shock occurs that produces the visible dense shell characteristic of planetary nebulae. The gas shell is excited and lighted up by the light emitted by the hot central star. The light from the central star is able to light up the planetary nebula for some 10,000 years.

The observed shapes of planetary nebulae are very puzzling: most of them (about 80%) are bipolar or elliptical rather than spherically symmetric. This complexity has lead to beautiful and amazing images obtained with modern telescopes. The pictures below compare planetary nebulae with bipolar (left) and spherical (right) shapes.

The reason why most planetary nebulae are not spherical is not well understood. Several hypotheses have been considered so far. One of them suggests that the strange shapes of planetary nebulae might be due to some centrifugal effect that results from the fast rotation of red giants. Another theory is that the symmetry of the star’s wind may be affected by a companion star. However, the most recent and convincing theories explaining the shapes of the nebulae involve magnetic fields.

The presence of magnetic fields would nicely explain the complicated shapes of planetary nebulae, as the ejected matter is trapped along magnetic field lines. This can be compared to iron filings trapped along the field lines of a bar magnet – a classic demonstration in high school physics classrooms. Since strong magnetic fields at the surface of the star also exert pressure on the gas, matter can more easily leave the star at the magnetic poles where the magnetic field is strongest.

There are several ways magnetic fields can be created in the vicinity of planetary nebulae. Magnetic fields can be produced by a stellar dynamo during the phase when the nebula is ejected. For a dynamo to exist, the core of the star must rotate faster than the envelope (as is the case in the Sun). It is also possible that the magnetic fields are fossil relics of previous stages of stellar evolution. Under most circumstances, the matter in stars is so highly electrically conductive that magnetic fields can survive for millions or billions of years. Both mechanisms, combined with the interaction of the ejected matter with the surrounding interstellar gas, would be able to shape the planetary nebulae.

Until recently, the idea that magnetic fields are an important ingredient in the shaping od planetary nebulae was a purely theoretical claim. In 2002, the first indications of the presence of such magnetic fields were found. Radio observations revealed magnetic fields in circumstellar envelopes of giant stars. These circumstellar envelopes are indeed progenitors of planetary nebulae. However, no such magnetic field has ever been observed in the nebulae themselves. To obtain direct clue of the presence of magnetic fields in planetary nebulae, astronomers decided to focus on the central stars, where the magnetic fields should have survived.

This first direct evidence has now been obtained. For the first time, Stefan Jordan and his team detected magnetic fields in several central stars of planetary nebulae. Using the FORS1 spectrograph of the 8-m class Very Large Telescope (VLT, European Southern Observatory, Chile), they measured the polarization of the light emitted by four of these stars. The polarization signatures in the spectral lines make it possible to determine the intensity of the magnetic fields in the observed stars. In the presence of a magnetic field, atoms change their energy in a characteristic way; this effect is called the Zeeman effect and was discovered in 1896 by Pieter Zeeman in Leiden (Netherlands). If these atoms absorb or emit light, the light becomes polarized. This makes it possible to determine the strength of the magnetic field by measuring the strength of the polarization. These polarization signatures are usually very weak. Such measurements require very high quality data that can only be obtained using 8-meter class telescopes such as the VLT.

Four central stars of planetary nebulae were observed by the team and magnetic fields were found in all of them. These four stars were chosen because their associated planetary nebulae (named NGC 1360, HBDS1, EGB 5, and Abell 36) are all non-spherical. Therefore, if the magnetic field hypothesis to explain the shapes of planetary nebulae is correct, these stars should have strong magnetic fields. These new results show that it is indeed the case: the strengths of the detected magnetic fields range from 1000 to 3000 Gauss, that is about one thousand times the intensity of the Sun’s global magnetic field.

These new observations published by Stefan Jordan and his colleagues support the hypothesis that magnetic fields play a major role in shaping planetary nebulae. The team now plans to search for magnetic fields in the central stars of spherical planetary nebulae. Such stars should have weaker magnetic fields than the ones just detected. These future observations will allow astronomers to better quantify the correlation between magnetic fields and the strange shapes of planetary nebulae.

In the few past years, polarimetric observations with the VLT have led to the discovery of magnetic fields in a large number of stellar objects in late evolutionary stages. In addition to improving our understanding of these beautiful planetary nebulae form, the detection of these magnetic fields allows science to take a step forward towards the clarification of the relationship between magnetic fields and stellar physics.

Original Source: NASA Astrobiology Story

Like the annual New Year?s fireworks display, astronomers at Gemini Observatory are ushering in 2005 with a striking image that dazzles the eye with stellar pyrotechnics.

In the image, the face-on spiral galaxy NGC 6946 is ablaze with colorful galactic fireworks fueled by the births and deaths of multitudes of brilliant, massive stars. Astronomers suspect that massive stellar giants have been ending their lives in supernova explosions throughout NGC 6946 in rapid-fire fashion for tens of millions of years.

?In order to sustain this rate of supernova activity, massive, quickly evolving stars must form or be born at an equally rapid rate in NGC 6946,? said Gemini North Associate Director, Jean-Ren? Roy. ?Its stars are exploding like a string of firecrackers!?

Astronomers speculate that if just a million years of this galaxy?s history were compressed into a time-lapse movie lasting a few seconds, there would be nearly constant outbursts of light as new stars flare into view, while old ones expire in spectacular explosions. Over the past century, eight supernovae have exploded in the arms of this stellar metropolis, occurring in 1917, 1939, 1948, 1968, 1969, 1980, 2002, and 2004. This makes NGC 6946 the most prolific known galaxy for supernovae during the past 100 years.

By comparison, the average rate for such catastrophic stellar outbursts in the Milky Way is about one per century, and only four have been recorded over the last thousand years. The last known supernova went off in our galaxy in the constellation Ophiuchus in 1604.

Yet, it is the ubiquitous occurrence of starbirth throughout NGC 6946 and not its supernovae that lend this galaxy its blazingly colorful appearance. For reasons not completely understood, it experiences a much higher rate of star formation than all the large galaxies in our local neighborhood. The prodigious output of stellar nurseries in this galactic neighbor eventually leads to accelerated numbers of supernova explosions.

Starbirth regions exist in most galaxies, particularly in spirals, and are obvious as clouds of predominantly hydrogen gas called H II regions. These areas coalesce over millions of years to form stars. Young, hot, massive stars formed in these regions emit copious amounts of ultraviolet radiation, which strip the electrons from hydrogen atoms in which they are embedded. When these ionized hydrogen atoms re-associate with electrons they radiate in a deep red color (at a wavelength of 656.3 nanometers) as the electrons transition back to lower energy levels.

This Gemini image of NGC 6946 utilizes a selective filter specifically designed to detect the radiation emanating from the starbirth regions. Additional filters help to distinguish other details in the galaxy, including clusters of massive blue stars, dust lanes, and a yellowish core where older more evolved stars dominate.

NGC 6946 lies between 10 and 20 million light-years away on the border between the constellations of Cepheus and Cygnus, and was discovered by Sir William Herschel (1738-1822) on September 9, 1798. It continues to fascinate astronomers, who estimate that it contains about half as many stars as the Milky Way. They often use it to study and characterize the evolution of massive stars and the properties of interstellar gas. As viewed in the new Gemini optical image, we see only the ?tip of the iceberg? of this galaxy. Its optical angular diameter is about 13 arcminutes, but viewed at radio wavelength at the frequency of neutral hydrogen (1420 Mhz or 21-cm line), it extends considerably more than the angular diameter of the Moon.

Original Source: Gemini News Release

The Coronagraphic Imager with Adaptive Optics (CIAO) on the Subaru telescope captured this near-infrared (wavelengths of 1.25 – 2.2 microns) image of a star at the end of its life. BD +303639 is a planetary nebula, similar to the Ring Nebula in the constellation Lyra, the Harp. It is about five thousand light years from Earth in the direction of the constellation Cygnus, the Swan. The surface of the star in the center of the nebula sizzles at a temperature of forty two thousand degrees Kelvin, and shines fifty thousand times brighter than our Sun.

At the end of their lives, comparatively lightweight stars like our Sun shed dust and gas which pile around the star. BD +303639 rapidly puffed off its outer layers about nine hundred years ago. This material, weighing almost a quarter of the Sun, has now expanded into a shell one hundred times more extended than the Solar System. The central star illuminates the material which looks like a life preserver from our point of view.

With visible light we can only see the light from the central star scattering off the dust. In infrared light, we can also see light emitted by the dust itself. CIAO used a technique called adaptive optics, which removes the twinkle of light due to turbulence in Earth’s atmosphere, to obtain an extraordinarily sharp image of the dust surrounding the star. (Note 1)

Spectra of the central star from the Subaru telescope’s High Dispersion Sepctrogrtaph indicates that the sizzling at the star’s surface is generating large quantities of carbon. This carbon is a likely ingredient of the dust surrounding the star.

Shedding of material is an integral part of the life of stars. “Although astronomers have been studying the dust and gas surrounding stars of different ages and types, we are only beginning to be able to observe and understand detailed structures such those in BD +303639,” says Dr. Koji Murakawa, an astronomer at the Netherlands Foundation for Research in Astronomy. Murakawa adds that “images like these give us precious insight into the last moments in a stars life.”

Note 1: The coronagraph, a device that blocks the light from a bright central star, was not used to obtain this image.

Original Source: Subaru Telescope News Release

Image credit: NASA
To understand the Winter Solstice (and by contrast the Summer Solstice) we must first understand a fundamental fact about the earth. Earth?s axis of rotation is tilted approximately 23.5? from vertical. This means that as earth orbits the sun, it points first one hemisphere, then the other toward the sun. This tilt causes sunlight to strike the surface of earth at different angles at different times of year. In the summer, the sun is high overhead for the Northern hemisphere and the heat energy is concentrated over a smaller area. In the winter, when the angle of the sun is low, the energy covers a much larger area and therefore heats less efficiently.

Imagine pointing a flashlight directly at a piece of paper. It’ll create a bright circle of light concentrated in an area. Now tilt the flashlight so that it’s hitting the paper on an angle. The same amount of light is coming out of the flashlight, but it’s spread out over a much larger area of paper. It’s this changing of our angle towards the Sun that gives us seasons.

The season we call ?winter? begins on the Winter Solstice. The word Solstice means ?sun still?. But to understand the significance of the Winter Solstice, we must first go back in time to the Summer Solstice, or first day of summer. Starting on June 21st, the sun gradually loses altitude in the sky as seen at noon. By September 22nd, the sun?s noon time altitude is significantly lower in the sky. The process continues until December 21st. Around this date, the sun seems to hold its position in the sky and then slowly begins to climb northward again; hence the term ?sun still.? To ancient peoples, the Solstice was a significant point in the year.

Because ancient peoples knew nothing of the earth?s tilt, the southward march of the sun was a troubling time. There was fear that one day the sun might continue moving south until was lost entirely. Many cultures conducted rituals to encourage the sun to move north again and when it did there were great celebrations. These celebrations, regardless of culture, all had a common theme that of rekindled light.

Not surprising then that many of the traditions and customs of ancient Solstice celebrations have survived to the present day. Although we know that the sun will begin moving north without any encouragement from humans, we still use this time of cold and darkness to celebrate the theme of rekindled light. From the Hanukah Menorah, to the Scandinavian Yule log, to the lights of the Christmas tree, during this season we seek to push back the darkness with light. Although the forms have evolved over the centuries, we cans still see the spirit of many of the old ways in our present day Solstice celebrations.

Now here is an interesting question to ponder, if the earth were not tilted and we had no seasons, would we celebrate the holidays in the same way?

Written by Rod Kennedy

The Tarantula Nebula is one of the most impressive views in the Southern sky, cf. ESO Press Photos 14a-g/02. Visible to the unaided eye in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way that is located in the direction of the southern constellation Doradus at a distance of about 170,000 light-years, this huge nebula is the prototype of what astronomers refer to as a “Giant HII region”. In this complex of glowing gas and very hot and luminous stars, the gas is mainly composed of protons and electrons, which are kept apart by energetic photons emitted by the stars in this area.

The Tarantula Nebula (also designated 30 Doradus) owes its name to the arrangement of its brightest patches of nebulosity that somewhat resemble the legs of a spider. They extend from a central “body” where a cluster of hot stars (designated “R136”) resides that illuminate the nebula. This name, of the biggest spiders on the Earth, is also very fitting in view of the gigantic proportions of the celestial nebula – it measures nearly 1,000 light years across!

While the central regions of 30 Doradus may be compared to a tarantula, the entangled filaments in the outskirts of this nebula – some of which are seen in PR Photo 34a/04 – could well be likened with its cobweb. They testify to an ongoing history of very vigorous activity and make this spectacular sky region a showcase of dramatic effects caused by the tremendous output of energy from the most massive stars known.

Intricate colours
The marvellous richness of the filament colours is due to the varying conditions in the interstellar gas in this region, cf. PR Photo 34b/04. The red in these images is caused by emission of excited hydrogen atoms, the green shades correspond to emission from oxygen atoms from which two electrons (“doubly-ionized oxygen”) have been “knocked off” by the energetic radiation of hot stars in the R136 cluster, that is located beyond the lower right corner of this photo. The intensity of this emission increases towards R136, explaining the yellowish colour near the edge of the photo.

A blue colour is contributed by singly-ionized atoms of oxygen. Other atoms like nitrogen and sulfur at different levels of ionization also add to the emission of the nebula at specific wavelengths. The observed colours thus probe the physical condition of the emitting gas and the temperature of the star(s) that excite(s) it. The intricate appearance of the filaments is mostly a consequence of turbulence in the interstellar gas, of the magnetic fields, and of the energy input by the massive stars in the neighbourhood.
Supernovae blow interstellar “bubbles”

The large ring-shaped nebula slightly to the lower-left (South-East) of the centre of PR 34a/04 is known as DEM L 299 [1]. Detailed investigations show that it represents an “interstellar bubble” which was “blown” by supernovae explosions, most probably happening millions of years ago, as massive stars near the centre of this structure ended their comparatively short lives in glorious flashes.

A closer inspection shows that another supernova exploded somewhat later near the rim, forming a bright and more compact nebula known as SNR 0543-689 (PR 34c/04). Other supernovae in this general field exploded even more recently, such as the one that created the remnant B0544-6910 (PR 34d/04) only a few tens of thousands of years ago, a blink of an eye by all astronomical standards.

Nebulae with built-in powerhouses
Not all the nebulae seen in this region are caused by supernovae, however. The glow of N 164 [1], a bright, extended red-yellow nebula just below DEM L 299, is mostly due to its own “private” powerhouse, that consists of several massive stars deeply embedded in its interior (PR Photo 34e/04).

The same holds for DEM L 297, the somewhat smaller and fainter nebula to the right of DEM L 299 (PR Photo 34f/04). It is divided into two half-circle formed segments by a dark lane of interstellar dust in front of it. Indeed, within the Tarantula complex many such dark and dusty clouds are seen in silhouette as they obscure bright nebulosity behind them.

Many stellar clusters
The outskirts of the Tarantula Nebula are also rich in stellar clusters. One of them, NGC 2093 [1], cf. PR Photo 34g/04, has relatively few stars and is relatively young, just a few tens of millions of years. It appears that its stars have already excavated a sizeable cavity around them that is now relatively void of gas.

An older and much more compact cluster, NGC 2108, is seen near the bottom of PR Photo 34h/04 and reproduced in full in PR Photo 34a/04. It resembles the globular clusters in our own Galaxy, but it formed much more recently, about 600 million years ago. Still, NGC 2108 is much older than the Tarantula complex and it is quite possible that in its “youth” it was the core of another giant HII region that has since dissolved into interstellar space.

The images for this release were produced by two ESO astronomers who are impressed by this sky region. Nausicaa Delmotte did the observations for her thesis and notes that: “many of the nebulae and clusters seen in these photos would stand out prominently if they were located elsewhere in the sky and not this close to the core of the spectacular Tarantula complex.”. She is echoed by her colleague, Fernando Comeron: “This amazing concentration of clusters, HII regions, supernova remnants, and extremely hot and luminous stars in a single region makes the Tarantula in the LMC a unique celestial object, unrivalled in our own Galaxy and other nearby galaxies!”.
Note

[1]: The designation “DEM L 299” indicates that this object is no. 299 in the list of nebulae in the Large and Small Magellanic Clouds published in 1976 by astronomers R.D.Davies, K.H.Elliott and J.Meaburn. “N” refers to a list of bright nebulae in these galaxies that was compiled in 1956 by K.G.Henize. “NGC” stands for the “New General Catalogue” published in 1888 by J.L.E. Dreyer.

Original Source: ESO News Release

Image credit: NASA/JPL/UA
Astronomers who think they know how the very early universe came to have so much interstellar dust need to think again, according to new results from the Spitzer Space Telescope.

In the last few years, observers have discovered huge quantities of interstellar dust near the most distant quasars in the very young universe, only 700 million years after the cosmos was born in the Big Bang.

“And that becomes a big question,” said Oliver Krause of the University of Arizona Steward Observatory in Tucson and the Max Planck Institute for Astronomy in Heidelberg. “How could all of this dust have formed so quickly?”

Astronomers know two processes that form the dust, Krause said. One, old sun-like stars near death generate dust. Two, infrared space missions have revealed the dust is produced in supernovae explosions.

“The first process takes several billion years,” Krause noted. “Supernovae explosions, by contrast, produce dust in much less time, only about 10 million years.”

So when astronomers reported detecting submillimeter emission from massive amounts of cold interstellar dust in the supernova remnant Cassiopeia A last year, some considered the mystery solved. Type II supernovae like ‘Cas A’ likely produced the interstellar dust in the very early universe, they concluded. (Type II supernovae come from massive stars that blow apart in huge explosions after their cores collapse.)

Krause and colleagues from UA’s Steward Observatory and the Max Planck institute in Heidelberg have now discovered that the detected submillimeter emission comes not from the Cas A remnant itself but from the molecular cloud complex known to exist along the line of sight between Earth and Cas A. They report the work in the Dec. 2 issue of Nature.

Cas A is the youngest known supernova remnant in our Milky Way. It is about 11,000 light years away, behind the Perseus spiral arm clouds that are roughly 9,800 light years away. Krause suspects that the Perseus clouds explain why late 17th century astronomers didn’t report observing the brilliant Cas A outburst around A.D. 1680. Cas A is so close to Earth that the supernova should have been the brightest stellar object in the sky, but dust in the Perseus clouds eclipsed the view.

The Arizona and German team mapped Cas A at 160-micron wavelengths using the ultra-heat-sensitive Multiband Imaging Photometer (MIPS) aboard the Spitzer Space Telescope. These long wavelengths are the most sensitive to cold interstellar dust emission. They then compared the results with maps of interstellar gas previously made with radio telescopes. They found that the dust in these interstellar clouds account for virtually all the emission at 160 microns from the direction of Cas A.

Minus the emission from this dust, there is no evidence for large amounts of cold dust in Cas A, the team concludes.

“Astronomers will have to go on searching for the source of the dust in the early universe,” UA Steward Observatory astronomer and Regents’ Professor George Rieke said. Rieke is principal investigator for the Spitzer Space Telescope’s MIPS instrument and a co-author of the Nature paper.

“Solving this riddle will show astronomers where and how the first stars formed, or perhaps indicate there is some non-stellar process that can produce large amounts of dust,” Rieke said. “Either way, (finding the source of the dust) will reveal what went on at the formative stage for stars and galaxies, an epoch that is nearly unobserved in any other way.”

Authors of the Nature article, “No cold dust within the supernova remnant Cassiopeia A,” are Oliver Krause, Stephan M. Birkmann, George H. Rieke, Dietrich Lemke, Ulrich Klaas, Dean C. Hines and Karl D. Gordon.

Birkmann, Lemke and Klaas are with the Max Planck Institute for Astronomy in Heidelberg. Krause, Rieke, and Gordon are with the University of Arizona Steward Observatory. Hines is with the Space Science Institute in Boulder, Colo.

Original Source: UA News Release

A hit TV program like “Antiques Roadshow” lures viewers with its universal appeal. Who wouldn’t want to find secret riches in their attic or basement? But rare paintings and heirloom jewelry aren’t the only valuable items waiting to be discovered. Cosmic treasures also lay hidden in the vast realm of outer space. Among the most highly prized of those treasures are planets that formed around other stars.

Astronomers have just gained an important clue to guide their hunt for extrasolar worlds. And that clue points to the unlikeliest of places – our own backyard.

“It’s possible that some of the objects in our solar system actually formed around another star,” says astronomer Scott Kenyon (Smithsonian Astrophysical Observatory).

How did these adopted worlds join our solar family? They arrived through an interstellar trade that took place more than 4 billion years ago when a wayward star brushed past our solar system. According to calculations made by Kenyon and astronomer Benjamin Bromley (University of Utah) and published in the Dec. 2, 2004, Nature, the Sun’s gravity plucked asteroid-sized objects from the visiting star. At the same time, the star pulled material from the outer reaches of our solar system into its grasp.

“There may not have been an equal exchange, but there was certainly an exchange,” says Bromley.

A Close Brush
Kenyon and Bromley reached this surprising conclusion while working to explain the mystery object Sedna, a world almost as large as Pluto but located much farther from the Sun. Sedna’s discovery in 2003 puzzled astronomers because of its unusual orbit – a 10,000-year-long oval whose closest approach to the Sun, 70 astronomical units, is well beyond the orbit of Neptune. (One astronomical unit, abbreviated A.U., is the average distance between the Earth and the Sun, or about 93 million miles.)

Understanding Sedna is a challenge because its orbit is far away from the gravitational influence of other planets in our solar system. However, the gravity of a passing star can pull objects beyond the orbit of Neptune, in the Kuiper Belt, into orbits like Sedna’s. Kenyon and Bromley have performed detailed computer simulations to show how this stellar fly-by likely took place.

The fly-by must have met two key requirements. First, the star must have stayed far enough away that it did not disrupt Neptune’s nearly circular orbit. Second, the encounter must have happened late enough in our solar system’s history that Sedna-like objects had time to form within the Kuiper Belt.

Kenyon and Bromley suggest that the near-collision occurred when our Sun was at least 30 million years old, and probably no more than 200 million years old. A fly-by distance of 150-200 A.U. would be close enough to disrupt the outer Kuiper Belt without affecting the inner planets.

According to the simulations, the passing star’s gravity would sweep clear the outer solar system beyond about 50 A.U., even as our Sun’s gravity pulled some of the alien planetoids into its grasp. The model explains both the orbit of Sedna and the observed sharp outer edge of our Kuiper Belt, where few objects reside beyond 50 A.U.

“A close fly-by from another star solves two mysteries at once. It explains both the orbit of Sedna and the outer edge of the Kuiper Belt,” says Bromley.

A Crowded Birthplace
But where did such a star come from, and where did it go? Since the fly-by happened more than 4 billion years ago, any suspects have long since escaped the Sun’s neighborhood. There is no practical way to find the culprit today.

The visitor’s origin may seem equally mystifying because the Sun currently lives in a sparse region of the Milky Way. Our closest neighbor is a distant 4 light-years away, and stellar close encounters are correspondingly rare. However, a near-collision would be much more likely for a young Sun if it were born in a dense star cluster, as recent evidence suggests.

“We believe that 90 percent of all stars form in clusters with a few hundred to a few thousand members,” says astronomer Charles Lada (Harvard-Smithsonian Center for Astrophysics). “The denser the cluster, the more likely the chance for an encounter between member stars.”

“This work is an important piece of evidence that the Sun formed in near proximity to other stars,” he adds.

Searching for Adopted Worlds
Kenyon and Bromley’s simulations indicate that thousands or possibly millions of alien Kuiper Belt Objects were stripped from the passing star. However, none have yet been positively identified. Sedna is probably homegrown, not captured. Among the known Kuiper Belt Objects, an icy rock dubbed 2000 CR105 is the best candidate for capture given its unusually elliptical and highly inclined orbit. But only the detection of objects with orbits inclined more than 40 degrees from the plane of the solar system will clinch the case for the presence of extrasolar planets in our backyard.

Kenyon and Bromley’s next goal is to estimate the sky density of captured objects so that they can make a survey to find such adopted worlds.

“In principle, large telescopes like the MMT Telescope [a joint Smithsonian/University of Arizona observatory] can find them if they’re numerous enough,” says Kenyon.

The calculations reported here were made using about 3,000 cpu-days of computer time at the supercomputing center at the Jet Propulsion Laboratory, Pasadena, Calif.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

Original Source: Harvard CfA News Release