NASA, Japan Release Most Complete Topographic Map of Earth
 
global digital elevation map This image was created by processing and stereo-correlating the 1.3 million-scene ASTER archive of optical images.
› Full image and caption/related images
PASADENA, Calif. - NASA and Japan released a new digital topographic map of Earth Monday that covers more of our planet than ever before. The map was produced with detailed measurements from NASA's Terra spacecraft.

The new global digital elevation model of Earth was created from nearly 1.3 million individual stereo-pair images collected by the Japanese Advanced Spaceborne Thermal Emission and Reflection Radiometer, or Aster, instrument aboard Terra. NASA and Japan's Ministry of Economy, Trade and Industry, known as METI, developed the data set. It is available online to users everywhere at no cost.

"This is the most complete, consistent global digital elevation data yet made available to the world," said Woody Turner, Aster program scientist at NASA Headquarters in Washington. "This unique global set of data will serve users and researchers from a wide array of disciplines that need elevation and terrain information."

According to Mike Abrams, Aster science team leader at NASA's Jet Propulsion Laboratory in Pasadena, Calif., the new topographic information will be of value throughout the Earth sciences and has many practical applications. "Aster's accurate topographic data will be used for engineering, energy exploration, conserving natural resources, environmental management, public works design, firefighting, recreation, geology and city planning, to name just a few areas," Abrams said.

Previously, the most complete topographic set of data publicly available was from NASA's Shuttle Radar Topography Mission. That mission mapped 80 percent of Earth's landmass, between 60 degrees north latitude and 57 degrees south. The new Aster data expand coverage to 99 percent, from 83 degrees north latitude and 83 degrees south. Each elevation measurement point in the new data is 30 meters (98 feet) apart.

"The Aster data fill in many of the voids in the shuttle mission's data, such as in very steep terrains and in some deserts," said Michael Kobrick, Shuttle Radar Topography Mission project scientist at JPL. "NASA is working to combine the Aster data with that of the Shuttle Radar Topography Mission and other sources to produce an even better global topographic map."

NASA and METI are jointly contributing the Aster topographic data to the Group on Earth Observations, an international partnership headquartered at the World Meteorological Organization in Geneva, Switzerland, for use in its Global Earth Observation System of Systems. This "system of systems" is a collaborative, international effort to share and integrate Earth observation data from many different instruments and systems to help monitor and forecast global environmental changes.

NASA, METI and the U.S. Geological Survey validated the data, with support from the U.S. National Geospatial-Intelligence Agency and other collaborators. The data will be distributed by NASA's Land Processes Distributed Active Archive Center at the U.S. Geological Survey's Earth Resources Observation and Science Data Center in Sioux Falls, S.D., and by METI's Earth Remote Sensing Data Analysis Center in Tokyo.

Aster is one of five Earth-observing instruments launched on Terra in December 1999. Aster acquires images from the visible to the thermal infrared wavelength region, with spatial resolutions ranging from about 15 to 90 meters (50 to 300 feet). A joint science team from the U.S. and Japan validates and calibrates the instrument and data products. The U.S. science team is located at JPL.
+ نوشته شده توسط مریم دیانى در دوشنبه بیست و سوم اسفند 1389 , ساعت 10:31 |
NASA Shows Topography of Tsunami-Damaged Japan City
 
Sendai, Japan Region The topography of the earthquake and tsunami-stricken city of Sendai, Japan, is revealed in this radar image from NASA's Shuttle Radar Topography Mission. Image credit: NASA/JPL/NGA
› Full image and caption
› Learn more

The topography surrounding Sendai, Japan is clearly visible in this combined radar image and topographic view generated with data from NASA's Shuttle Radar Topography Mission (SRTM) acquired in 2000. On March 11, 2011, a magnitude 8.9 earthquake struck offshore about 130 kilometers (80 miles) east of Sendai, the capital city of Japan's Miyagi Prefecture, generating a tsunami that devastated the low-lying coastal city of about 1 million residents.

The city is centered in the image and lies along the coastal plain between the Ohu Mountains and the Pacific Ocean. The eastern part of the city is a low-lying plains area, while the city center is hilly (the city's official elevation is about 43 meters, or 141 feet). Sendai's western areas are mountainous, with its highest point being Mt. Funagata at an elevation of about 1,500 meters (4,921 feet) above sea level.

According to the U.S. Geological Survey, the earthquake occurred as a result of thrust faulting on or near the subduction zone interface plate boundary between the Pacific and North America plates. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a velocity of 83 millimeters (3.3 inches) per year. The Pacific plate thrusts underneath Japan at the Japan Trench, and dips to the west beneath Eurasia. The location, depth and focal mechanism of the March 11 earthquake are consistent with the event having occurred as thrust faulting associated with subduction along this plate boundary.

This image combines a radar image acquired in February 2000 during the SRTM mission, and color-coding by topographic height using data from the same mission. Dark green colors indicate low elevations, rising through yellow and tan, to white at the highest elevations.

The Shuttle Radar Topography Mission is a cooperative project between NASA, the National Geospatial-Intelligence Agency (NGA) of the U.S. Department of Defense and the German and Italian space agencies. It is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate, Washington, D.C.

+ نوشته شده توسط مریم دیانى در دوشنبه بیست و سوم اسفند 1389 , ساعت 10:25 |
شاید بهترین نقطه ی دنیا برای من همین جا ( وبلاگم ) باشد . اگرچه که حدود یک سال و چند ماه هست که به اینجا سر نزده ام ولی این هیچ وقت به معنی جدایی من از دنیای پرسوال ستاره شناسی نبوده و نیست .

چرا که روح و قلب هر انسان زنده دلی خواه ناخواه با این علم گره خورده است. اگر عاشق باشیم چشم از آسمان برنخواهیم داشت و همواره در انتظار یک اتفاق سوال برانگیز دیگرخواهیم نشست و به دنبال جواب سوال هایمان از هر کوی و برزنی نمی گذریم... بلکه با بینش و دقت و با علم اینکه به دنبال جوابی در ذهن جستجوگر مان هستیم آرام آرام قدم برمیداریم و از هر آنچه که میبینیم تنها لذت میبریم و می آموزیم...

 " آه ای حلزون

از کوهستان فوجی بالا برو

اما

آرام

آرام

.

.

. "

اما از آخرین رصدی که رفتم چند ماهی می گذرد.به جرات میتوانم بگویم که بهترین تجربه و بهترین رصد بود . رصدی که با حضور دکتر میرترابی عزیز و آقای احمد کریمی و آقای کاظم کوکرم به مدت دو روز و یک شب شیرین ترین و پر بارترین خاطره ها را برایم رقم زد. ( عکس های رصد در فیس بوک )

شاید یکی از علل کم کار شدنم یا بهتر بگویم بیکار شدنم در وبلاگم همین باشد که منتظر دست دادن موقعیتی مناسب برای رصد بودم...

به هر حال به امید یک شب رصدی فوق العاده... بدرود...

 

"آسمان را که دگر آبی نیست

رنگ آبی بزنیم

عشق و دلدادگی و خوبی را

همه باور بکنیم

همه عاشق باشیم... "

+ نوشته شده توسط مریم دیانى در یکشنبه دهم بهمن 1389 , ساعت 16:58 |
Runaway Star Plows Through Space
01.24.11
 
blue star near the center of this image is Zeta Ophiuchi The blue star near the center of this image is Zeta Ophiuchi. When seen in visible light it appears as a relatively dim red star surrounded by other dim stars and no dust. Image credit: NASA/JPL-Caltech/UCLA
› Full image and caption
› View image gallery

A massive star flung away from its former companion is plowing through space dust. The result is a brilliant bow shock, seen here as a yellow arc in a new image from NASA's Wide-field Infrared Survey Explorer, or WISE.

The star, named Zeta Ophiuchi, is huge, with a mass of about 20 times that of our sun. In this image, in which infrared light has been translated into visible colors we see with our eyes, the star appears as the blue dot inside the bow shock.

Zeta Ophiuchi once orbited around an even heftier star. But when that star exploded in a supernova, Zeta Ophiuchi shot away like a bullet. It's traveling at a whopping 54,000 miles per hour (or 24 kilometers per second), and heading toward the upper left area of the picture.

As the star tears through space, its powerful winds push gas and dust out of its way and into what is called a bow shock. The material in the bow shock is so compressed that it glows with infrared light that WISE can see. The effect is similar to what happens when a boat speeds through water, pushing a wave in front of it.

This bow shock is completely hidden in visible light. Infrared images like this one from WISE are therefore important for shedding new light on the region.

JPL manages and operates WISE for NASA's Science Mission Directorate, Washington. The principal investigator, Edward Wright, is at UCLA. The mission was competitively selected under NASA's Explorers Program managed by NASA's Goddard Space Flight Center, Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory, Logan, Utah, and the spacecraft was built by Ball Aerospace & Technologies Corp., Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA.

More information is online at http://www.nasa.gov/wise, http://wise.astro.ucla.edu and http://www.jpl.nasa.gov/wise .

+ نوشته شده توسط مریم دیانى در یکشنبه دهم بهمن 1389 , ساعت 16:55 |
An Astronomer's Field of Dreams
01.26.11
 
Long Wavelength Array Multiple antennas of the LWA-1 station of the Long Wavelength Array in central New Mexico, photographed at sunset. Image credit: LWA Project (at UNM) 

Aerial View of the Antennas of the Long Wavelength Array station in New Mexico All 256 antennas of station LWA-1 of the Long Wavelength Array in Central New Mexico taken on April 1, 2010. Image credit: LWA Project (at UNM) 

An innovative new radio telescope array under construction in central New Mexico will eventually harness the power of more than 13,000 antennas and provide a fresh eye to the sky. The antennas, which resemble droopy ceiling fans, form the Long Wavelength Array, designed to survey the sky from horizon to horizon over a wide range of frequencies.

The University of New Mexico leads the project, and NASA's Jet Propulsion Laboratory, Pasadena, Calif., provides the advanced digital electronic systems, which represent a major component of the observatory.

The first station in the Long Wavelength Array, with 256 antennas, is scheduled to start surveying the sky by this summer. When complete, the Long Wavelength Array will consist of 53 stations, with a total of 13,000 antennas strategically placed in an area nearly 400 kilometers (248 miles) in diameter. The antennas will provide sensitive, high-resolution images of a region of the sky hundreds of times larger than the full moon. These images could reveal radio waves coming from planets outside our solar system, and thus would turn out to be a new way to detect these worlds. In addition to planets, the telescope will pick up a host of other cosmic phenomena.

"We'll be looking for the occasional celestial flash," said Joseph Lazio, a radio astronomer at JPL. "These flashes can be anything from explosions on surfaces of nearby stars, deaths of distant stars, exploding black holes, or even perhaps transmissions by other civilizations." JPL scientists are working with multi-institutional teams to explore this new area of astronomy. Lazio is lead author of an article reporting scientific results from the Long Wavelength Demonstrator Array, a precursor to the new array, in the December 2010 issue of Astronomical Journal.

The new Long Wavelength Array will operate in the radio-frequency range of 20 to 80 megahertz, corresponding to wavelengths of 15 meters to 3.8 meters (49.2 feet to 12.5 feet). These frequencies represent one of the last and most poorly explored regions of the electromagnetic spectrum.

In recent years, a few factors have triggered revived interest in radio astronomy at these frequencies. The cost and technology required to build these low-frequency antennas has improved significantly. Also, advances in computing have made the demands of image processing more attainable. The combination of cost-effective hardware and technology gives scientists the ability to return to these wavelengths and obtain a much better view of the universe. The predecessor Long Wavelength Demonstrator Array was also in New Mexico. It was successful in identifying radio flashes, but all of them came from non-astronomy targets -- either the sun, or meteors reflecting TV signals high in Earth's atmosphere. Nonetheless, its findings indicate how future searches using the Long Wavelength Array technology might lead to new discoveries.

Radio astronomy was born at frequencies below 100 megahertz and developed from there. The discoveries and innovations at this frequency range helped pave the way for modern astronomy. Perhaps one of the most important contributions made in radio astronomy was by a young graduate student at New Hall (since renamed Murray Edwards College) of the University of Cambridge, U.K. Jocelyn Bell discovered the first hints of radio pulsars in 1967, a finding that was later awarded a Nobel Prize. Pulsars are neutron stars that beam radio waves in a manner similar to a lighthouse beacon.

Long before Bell's discovery, astronomers believed that neutron stars, remnants of certain types of supernova explosions, might exist. At the time, however, the prediction was that these cosmic objects would be far too faint to be detected. When Bell went looking for something else, she stumbled upon neutron stars that were in fact pulsing with radio waves -- the pulsars. Today about 2,000 pulsars are known, but within the past decade, a number of discoveries have hinted that the radio sky might be far more dynamic than suggested by just pulsars.

"Because nature is more clever than we are, it's quite possible that we will discover something we haven't thought of," said Lazio.

+ نوشته شده توسط مریم دیانى در یکشنبه دهم بهمن 1389 , ساعت 16:51 |

در سال 2012، دو خورشید در آسمان زمین خواهد درخشید؟!

دانش > نجوم  - «انفجار ستاره‌ای می‌تواند زمین را تا سال 2012 صاحب دو خورشید کند»، «دومین خورشید در راه است»، «ممکن است زمین به زودی خورشید دومی داشته باشد»، «دو خورشید برای زمین»! این تیترهای خبری را دیده‌اید؟

به گزارش سرویس علمی خبرگزاری دانشجویان ایران، ایسنا، برخی روزنامه‌ها و سایت‌های خبری خارجی با استناد به اظهارات یک استاد فیزیک دانشگاه کویینزلند در گفت‌و‌گو با یک سایت استرالیایی، چند روزی است چنین تیترهایی را از احتمال مشاهده دو خورشید در آسمان زمین در پی انفجار ستاره‌ای سرخ‌رنگ و غول‌پیکر منتشر می کنند. به گفته آن‌ها، این پدیده دیدنی و خیره‌کننده ممکن است طی همین یکی دو سال آینده اتفاق بیفتد!

به گفته دکتر براد کارتر، ستاره غول‌پیکر یدالجوزا یا آلفا-جبار که به ابط‌الجوزا (Betelgeuse) نیز مشهور است، در آستانه انفجار ابرنواختری است و در صورت انفجار، تحت تاثیر نور بسیار زیاد آن برای مدتی زمین از دو خورشید برخوردار خواهد بود. به گفته وی در صورت حادث شدن این رویداد، مردم زمین شب و روزهای بسیاری را در این جشن ابرنواختری بیدار خواهند ماند؛ حال آن که بسیاری از مواد پخش شده در فضا در اثر این انفجار از کنار زمین عبور کرده و در معرض دید نخواهند بود.

توضیح عکس زیر: ستاره یدالجوزا، ستاره نارنجی رنگ بالای تصویر است. این تصویر را بابک امین تفرشی از قله گرگش کاشان، محل احداث رصدخانه ملی ایران گرفته است.

آسمان شب

کارتر می‌گوید در هنگام انفجار یک ستاره که معلوم نیست سال 2011 باشد یا 1002011، ابتدا شاهد بارش ذرات ریز نوترینو خواهیم بود که با این‌که 99 درصد انرژی ابرنواختر از طریق این ذرات گسیل می‌شود،‌ هیچ ضرری برای انسان‌ها ندارند.

انفجار ستارگان در اثر پدیده ابرنواختر پدیده‌ای کاملا شناخته شده و معمول در عرصه اخترشناسی است؛ اما نکته قابل تامل در خبر اخیر، رویکرد سایت استرالیایی منبع اولیه خبر و برخی سایت‌های انتشاردهنده آن است که با «هالیوودی کردن» و پر و بال دادن به اظهارات کارتر، خبر را به نحوی تنظیم کرده‌اند که این تصور در مخاطبان کم دقت ایجاد می‌شود که زمان انفجار این ستاره بسیار نزدیک (سال 2012) خواهد بود؛ رویکردی که شاید چندان بی ارتباط با خرافات مطرح در غرب درباره سال 2012 و فیلم‌ها و قصه‌پردازی‌های صورت گرفته در مورد آن نباشد.

دکتر رضا منصوری، استاد فیزیک دانشگاه صنعتی شریف و مجری طرح رصدخانه ملی ایران در گفت‌و‌گو با ایسنا با اشاره به این‌که انفجار ابرنواختری ستارگان به دلیل تحولات درونشان، پدیده‌ای طبیعی در روند حیات آنها از جمله ستاره یدالجوزا است، اظهار کرد: «مساله مهم این است که زمان این پدیده در ابعاد میلیون سال قابل تخمین است و ای‌که گفته شود چنین پدیده‌ای مثلا تا سال آینده به وقوع می پیوندد کاملا بی اساس و فاقد مبنای علمی است».

وی انتشار چنین خبری را ناشی از بی دقتی در نقل اظهارات فیزیک‌دان استرالیایی یا تمایل برخی به جنجال‌سازی به منظور کسب شهرت خواند و بر ضرورت توجه رسانه‌ها به مبانی روزنامه‌نگاری علمی و پرهیز از نقل مطالب غیرعلمی و شبه‌علم تاکید کرد.

یدالجوزا یا ابط‌الجوزا، ستاره‌ای با قطر 900 برابر خورشید (تقریبا به بزرگی مدار سیاره مشتری، 4 برابر بزرگتر از مدار سیاره زمین به دور خورشید) است که دومین ستاره پرنور این صورت‌فلکی جبار یا شکارچی است و 590 سال‌نوری از خورشید ما فاصله دارد. رنگ این ستاره نارنجی است و این شب‌ها در آغاز شب، می‌توان آن را در افق شرقی دید که بالاتر از سه ستاره کمربند جبار واقع شده و به سادگی قابل تشخیص است.

وضعیت این ستاره غول‌سرخ سال‌هاست که برای دانشمندان شناخته شده است. مرگ این ستاره به صورت انفجاری ابرنواختری طی هزاران سال آینده هم دور از ذهن نیست و مطمئنا در صورت وقوع این انفجار با توجه به فاصله نسبتا نزدیک این ستاره به زمین، منظره‌ای بسیار تماشایی خلق خواهد شد؛ اما واقعیت این است که فعلا نمی‌توان تخمین صحیحی از زمان مرگ باشکوه ابرغول قرمز شانه «شکارچی» داشت و تا آن زمان، طلوع قریب‌الوقوع خورشیدی دیگر تنها در آسمان وهم گرفته شبه‌علم و جنجال‌های رسانه‌ای قابل مشاهده است.

+ نوشته شده توسط مریم دیانى در یکشنبه دهم بهمن 1389 , ساعت 16:46 |
سلااااااااااام

بعد از مدتها دوباره یک رصد عااااااااالی منو مجبور کرد که بنویسم .

۵شنبه ی هفته ی گذشته به همراه بچه های دانشگاه   دکتر میرترابی    آقای کاظم کوکرم و آقای احمد کریمی به سمت نیاسر حرکت کردیم و به جرات میتونم بگم که بهترین رصد در این چند سال بود .

آسمون عالی بود  هوا هم خیلی سرد نشد .

از لحظه ایی که رسیدیم تا اذان صبح رصد کردیم و رصد کردیم و رصد کردیم و رصد کردیم و رصد کردیم ...

فقط میتونم بگم که خیییییییییییییلی خوش گذشت .

-برای مشاهده ی عکس ها به صفحه ی  facebook من سربزنین.

 

 

+ نوشته شده توسط مریم دیانى در یکشنبه هشتم آذر 1388 , ساعت 12:28 |
 zodiac signs

Aries

Aries
March 21 -
April 19

Taurus

Taurus
April 20 -
May 20

Gemini

Gemini
May 21 -
June 20

Cancer

Cancer
June 21 -
July 22

Leo

Leo
July 23 -
August 22

Virgo

Virgo
August 23 -
September 22

Libra

Libra
September 23 -
October 22

Scorpio

Scorpio
October 23 -
November 21

Sagittarius

Sagittarius
November 22 -
December 21

Capricorn

Capricorn
December 22 -
January 19

Aquarius

Aquarius
January 20 -
February 18

pisces

Pisces
February 19 -
March 20

+ نوشته شده توسط مریم دیانى در پنجشنبه چهاردهم آبان 1388 , ساعت 21:38 |
Space Porch Open for Business

August 7, 2009: The International Space Station has a new "engawa"—and it's open for business.

Engawa is Japanese for "porch," and while that might seem like a strange thing for a space station to have, researchers have been looking forward to the addition for a long time. Space shuttle Endeavour delivered the Japanese-built platform to the ISS on July 22nd and astronauts attached it to Japan's Kibo1 science lab a day later. Now, when a science experiment requires a dose of hard vacuum or radiation, it can be set "out on the porch" for exposure.

And that's just for starters.

On the new 'Japanese Exposed Facility' [JEF for short], researchers can stage experiments to look up at the cosmos, down at Earth, or around at the environment the ISS voyages through," says Julie Robinson, ISS Program Scientist at NASA's Johnson Space Center. "Besides resembling a porch, this structure has unique features that differentiate it from the experiment exposure points2 located elsewhere on the station."

For instance, the JEF offers temperature control. Like the space station's other external experiment locations, it has a warming plate for thermal control, but unlike the others the JEF includes a cooling feature.3 Another advantage is that JEF experiments can be serviced by Kibo’s robotic arm4.

"The beauty of this is that payloads can be designed to be 'plug and play,'" says Robinson, "so the robotic arm can install them -- no space walk required."

On July 24th, Kibo's arm deftly delivered the first two JEF experiments from the Shuttle payload bay to the porch and positioned them5. These Japanese experiments are the SEDA-AP6, short for Space Environment Data Acquisition equipment-Attached Payload, and MAXI7, or the Monitor of All-sky X-ray Image.

"SEDA-AP's sensors will measure the space environment of low Earth orbit -- neutrons, plasma, heavy ions, high-energy light particles, atomic oxygen, and cosmic dust," explains Robinson.

With this experiment, researchers can test the mettle of materials and equipment exposed to the UV light, deep space radiation, and extreme temperatures of space. SEDA-AP will monitor material degradation to help researchers choose the hardiest materials for building future space instruments, equipment, and vehicles.

MAXI is an all-sky X-ray scanner with super-sensitive X-ray slit cameras to search continuously for exploding stars, black holes, and other hot cosmic X-ray sources. Earth's atmosphere absorbs X-rays (lucky for us), so astronomers have to send their sensors to orbit.

"MAXI will look at more than 1000 different X-ray sources and cover the entire sky," says Tai Nakamura8 of JAXA (Japan Aerospace Exploration Agency). Data from MAXI will be broadcast on the Internet. Upon detecting an X-ray source, MAXI's ground communication system will speed alerts to observers across the globe within 30 seconds.

The U.S. has two experiments destined for the JEF this fall: HREP-RAIDS, or the Remote Atmospheric and Ionospheric Detection System, and HREP-HICO, or the Hyperspectral Imager for the Coastal Ocean.9

"RAIDS will tell us about upper layers of Earth's atmosphere called the thermosphere and ionosphere," says Robinson. "These layers are tremendously imortant because that is where many spacecraft and satellites orbit. According to the Naval Research Laboratory, RAIDS is the most comprehensive survey of the thermosphere and ionosphere in 20 years."

HICO, also built by Naval Research Laboratory, is a hyperspectral imager for mapping coastal areas.10 That simply means it collects detailed information on the light reflected from these locations. Traditional multispectral sensors, like Landsat, lump the light measured into only a few bands; hyperspectral sensors have hundreds of bands.

"Hyperspectral sensors are like Landsat on steroids," says Robinson. "But HICO is a test unit that lacks Landsat's spatial resolution. Similar imagers have flown on aircraft, and another hyperspectral imager is on NASA's Earth Observing-1 satellite as a technology demonstration."

"The JEF will help us figure out whether HICO would be feasible for a satellite platform. The 'porch' is perfect for proving imaging technologies in space before investing in sophisticated optics for instruments and putting them on satellites. If HICO passes with flying colors and an operational imager is developed, that new imager could provide unprecedented maps of coastal features."

The JEF can host nine different experiments at once and has places for communications equipment, storage, and for berthing Japan's HTV-exposed pallet.

+ نوشته شده توسط مریم دیانى در چهارشنبه بیست و یکم مرداد 1388 , ساعت 20:32 |
Star clusters point to black holes ejected from host galaxies
Hypercompact stellar systems result when a supermassive black hole is violently ejected from a galaxy, following a merger with another supermassive black hole.
Provided by Rochester Institute of Technology, NY
July 10, 2009
The tight cluster of stars surrounding a supermassive black hole after it has been violently kicked out of a galaxy represents a new kind of astronomical object and a fossil record of the kick.

A paper in The Astrophysical Journal discusses the theoretical properties of "hypercompact stellar systems" and suggests that hundreds of these faint star clusters might be detected at optical wavelengths in our immediate cosmic environment. Some of these objects may already have been picked up in astronomical surveys, reports David Merritt, from Rochester Institute of Technology (RIT), Jeremy Schnittman, from Johns Hopkins University, and Stefanie Komossa, from the Max-Planck-Institute for Extraterrestrial Physics in Germany.

Hypercompact stellar systems result when a supermassive black hole is violently ejected from a galaxy, following a merger with another supermassive black hole. The evicted black hole rips stars from the galaxy as it is thrown out. The stars closest to the black hole move in tandem with the massive object and become a permanent record of the velocity at which the kick occurred.

"You can measure how big the kick was by measuring how fast the stars are moving around the black hole," said Merritt, professor of physics at RIT. "Only stars orbiting faster than the kick velocity remain attached to the black hole after the kick."

These stars carry with them a kind of fossil record of the kick, even after the black hole has slowed down. In principle, you can reconstruct the properties of the kick, which is nice because there would be no other way to do it."

"Finding these objects would be like discovering DNA from a long-extinct species," said Komossa.

The best place to find hypercompact stellar systems, the authors said, is in cluster of galaxies like the nearby Coma and Virgo clusters. These dense regions of space contain thousands of galaxies that have been merging for a long time. Merging galaxies result in merging black holes, which is a prerequisite for the kicks.

"Even if the black hole gets kicked out of one galaxy, it's still going to be gravitationally bound to the whole cluster of galaxies," Merritt said. "The total gravity of all the galaxies is acting on that black hole. If it was ever produced, it's still going to be there somewhere in that cluster."

Merritt and his co-authors think that scientists may have already seen hypercompact stellar systems and not realized it. These objects would be easy to mistake for common star systems like globular clusters. The key signature making hypercompact stellar systems unique is a high internal velocity. This is detectable only by measuring the velocities of stars moving around the black hole, a difficult measurement that would require a long time exposure on a large telescope.

From time to time, a hypercompact stellar system will make its presence known in a much more dramatic way, when one of the stars is tidally disrupted by the supermassive black hole. In this case, gravity stretches the star and sucks it into the black hole. The star is torn apart, causing a beacon-like flare that signals a black hole.

"The only contact of these floating black holes with the rest of the universe is through their armada of stars with an occasional display of stellar fireworks to signal 'here we are,'" Merritt said.
+ نوشته شده توسط مریم دیانى در جمعه نوزدهم تیر 1388 , ساعت 20:0 |