Semoga rekaman peristiwa gerhana yang termasuk langka ini bisa disaksikan oleh Anda yang mungkin belum berkesempatan mengamati gerhana tersebut secara langsung (dengan bantuan alat pelindung mata tentunya).
Caption : Volume renderings of the density field in a region of the simulation at 55,000 years of evolution. The left panel shows a polar view, and the right panel shows an equatorial view. The fingers feeding the equatorial disk are clearly visible. Images by Krumholz et al
For a long time, scientists have understood that stars form when interstellar matter inside giant clouds of molecular hydrogen undergoes gravitational collapse. But massive stars–up to 120 times the mass of the Sun—generate strong radiation and stellar winds. How do they maintain the clouds of gas and dust that feed their growth without blowing it all away? The problem, however, turns out to be less mysterious than it once seemed. A study published this week in the journal Science shows how the growth of a massive star can proceed despite outward-flowing radiation pressure that exceeds the gravitational force pulling material inward.
The new findings also explain why massive stars tend to occur in binary or multiple star systems, said lead author Mark Krumholz, an assistant professor of astronomy and astrophysics at the University of California, Santa Cruz. Co-authors are Richard Klein, Christopher McKee, and Stella Offner of UC Berkeley, and Andrew Cunningham of Lawrence Livermore National Laboratory.
Radiation pressure is the force exerted by electromagnetic radiation on the surfaces it strikes. This effect is negligible for ordinary light, but it becomes significant in the interiors of stars due to the intensity of the radiation. In massive stars, radiation pressure is the dominant force counteracting gravity to prevent the further collapse of the star.
"When you apply the radiation pressure from a massive star to the dusty interstellar gas around it, which is much more opaque than the star's internal gas, it should explode the gas cloud," Krumholz said. Earlier studies suggested that radiation pressure would blow away the raw materials of star formation before a star could grow much larger than about 20 times the mass of the Sun. Yet astronomers observe stars much more massive than that.
The research team has spent years developing complex computer codes for simulating the processes of star formation. Combined with advances in computer technology, their latest software (called ORION) enabled them to run a detailed three-dimensional simulation of the collapse of an enormous interstellar gas cloud to form a massive star. The project required months of computing time at the San Diego Supercomputer Center.
The simulation showed that as the dusty gas collapses onto the growing core of a massive star, with radiation pressure pushing outward and gravity pulling material in, instabilities develop that result in channels where radiation blows out through the cloud into interstellar space, while gas continues falling inward through other channels.
Caption : Computer simulation of the formation of a massive star yielded these snapshots showing stages in the process over time. Panels on the left represent a polar view (the axis of rotation is perpendicular to the plane of the image), and panels on the right represent an equatorial view. Plus signs indicate projected positions of stars. Colors represent density. Images by Krumholz et al
"You can see fingers of gas falling in and radiation leaking out between those fingers of gas," Krumholz said. "This shows that you don't need any exotic mechanisms; massive stars can form through accretion processes just like low-mass stars."
The rotation of the gas cloud as it collapses leads to the formation of a disk of material feeding onto the growing "protostar." The disk is gravitationally unstable, however, causing it to clump and form a series of small secondary stars, most of which end up colliding with the central protostar. In the simulation, one secondary star became massive enough to break away and acquire its own disk, growing into a massive companion star. A third small star formed and was ejected into a wide orbit before falling back in and merging with the primary star.
When the researchers stopped the simulation, after allowing it to evolve for 57,000 years of simulated time, the two stars had masses of 41.5 and 29.2 times the mass of the Sun and were circling each other in a fairly wide orbit.
"What formed in the simulation is a common configuration for massive stars," Krumholz said. "I think we can now consider the mystery of how massive stars are able to form to be solved. The age of supercomputers and the ability to simulate the process in three dimensions made the solution possible."
Setelah melihat ada artikel baru di Langit Selatan tentang GMC tanggal 26 Januari 2009, Saya baru ingat bahwa waktunya sudah cukup dekat. Oleh sebab itu, di dalam artikel ini akan coba dibahas lebih dalam seluk beluk tentang gerhana Matahari cincin.
Overview Gerhana Matahari Cincin (Annular Solar Eclipse) adalah peristiwa terhalangnya piringan Matahari oleh piringan Bulan, namun karena Bulan saat itu ada di apogee (jarak terjauhnya dari Bumi) sehingga piringan Bulan sedikit lebih kecil dibandingkan dengan piringan Matahari. Alhasil, piringan Matahari tertutup Bulan tetapi bagian tepinya tidak tertutup dan nampak seperti cincin. Untuk lebih jelasnya, perhatikan ilustrasi di bawah ini:
Sumber : en.wikipedia.org
Simulasi Gerhana Matahari Cincin (GMC) dapat Anda lihat di bawah ini (sumber : www.astrodeneb.org):
Penduduk bumi yang ada di daerah B saja yang pada tanggal 26 Januari nanti bisa mengamati GMC, sisanya (di daerah C) hanya bisa mengamati GM sebagian. Di luar kedua daerah tersebut, tidak akan teramati gerhana apapun. Pada momen GMC 26 Januari 2009 ini, bayang-bayang utama (umbra) Bulan yang jatuh di permukaan Bumi hanya selebar 280 km, sehingga tidak seluruh tempat berkesempatan untuk menyaksikan fase cincin. Momen puncak gerhana sendiri hanya berlangsung kurang dari 8 menit.
Lintasan Gerhana Gerhana Matahari pertama di tahun 2009 ini terjadi saat Bulan ada di titik simpul naiknya (ascending node) di daerah rasi Capricornus. Gerhana ini termasuk dalam rangkaian gerhana Saros 131. Gerhana Matahari Cincin akan dapat teramati pada daerah yang cukup luas, melewati Samudera Hindia dan bagian barat Indonesia. Gerhana Matahari Sebagian akan dapat diamati dari daerah yang lebih luas, mencakup bagian selatan Afrika, Madagaskar, Australia kecuali daerah Tasmania, tenggara India, Asia tenggara dan Indonesia.
Pada gambar di atas ini, kedua garis biru masing-masing menandai batas paling utara ("atas") dan paling selatan ("bawah") untuk dapat menyaksikan fase cincin GMC. Garis merah adalah jejak greatest eclipse, yaitu momen yang berlangsung ketika jarak sumbu bayang-bayang Bulan dengan pusat Bumi mencapai maksimum. Animasi di bawah ini menunjukkan daerah mana saja yang dapat mengamati gerhana tersebut (sumber: NASA).
Di Indonesia sendiri, daerah yang dilalui lintasan gerhana Matahari cincin ini akan melintasi bagian selatan pulau Sumatera (Lampung dan sekitarnya), bagian barat pulau Jawa (Cilegon, Serang, Anyer, dan sekitarnya) dan bagian tengah pulau Kalimantan. Sedangkan pengamat di wilayah Indonesia lainnya hanya bisa menyaksikan gerhana Matahari sebagian.
Peristiwa GMC 26 Januari 2009 Proses GMC itu akan diawali dengan tertutupnya piringan Matahari oleh Bulan pada pukul 15.21 WIB. Kemudian Matahari akan berubah menjadi bentuk sabit hingga akhirnya seluruh piringan Bulan sudah berada di dalam piringan Matahari. Inilah yang disebut dengan puncak GMC, yang akan terjadi pada pukul 16.40 WIB. Kita akan melihat Matahari berbentuk cincin selama sekitar 6 menit. Setelah itu Bulan mulai keluar dari piringan Matahari hingga pada pukul 17.52 WIB Bulan sudah benar-benar meninggalkan piringan Matahari sebagai tanda bahwa peristiwa GMC ini sudah berakhir. Jadi dari perhitungan di atas, berarti waktu yang kita miliki untuk melihat Bulan menutupi Matahari adalah sekitar 90 menit.
Di Tanjung Karang (Lampung), fase cincin dimulai pukul 16.38 WIB, puncak gerhana 16.41 WIB, dan fase cincin berakhir pada pukul 16.44 WIB.Si Samarinda (Kaltim), fase cincin dimulai pukul 17.48 WITA, puncak gerhana 17.49 WITA, dan fase cincin berakhir pada pukul 17.50 WITA.Sebagian kecil daratan di Sulawesi juga dilintasi bayang-bayang Bulan. Di Manado, awal gerhana (kontak I) dimulai pukul 16.42 WITA, momen puncak gerhana 17.49 WITA, dan akhir gerhana (kontak 4) pada pukul 18.50 WITA.
Untuk simulasi GMC ini dari berbagai daerah, silakan dilihat di sini.
Pengamatan Gerhana Untuk mengamati gerhana ini, ada beberapa hal yang harus Anda perhatikan:
1. Safety Procedures. Mengamati gerhana Matahari dengan cara yang salah (tanpa pelindung, baik dengan mata langsung maupun dengan menggunakan binokular, teropong, teleskop tak berpelindung) dapat mengakibatkan KEBUTAAN PERMANEN. Hal ini disebabkan pada saat kita mengamati piringan Matahari (meskipun sudah tertutup sebagian) tetap memancarkan energi yang besar yang dapat mengurangi kepekaan mata bahkan membakar retina kita!. Tapi, tentunya hal ini jangan membuat Anda menjadi paranoid atau takut untuk mengamati gerhana.
Cara yang aman untuk mengamati Gerhana Matahari antara lain dengan : a. Kamera lobang jarum.
b. Filter Matahari Filter ini umumnya sering digunakan untuk mengamati gerhana matahari dengan alat pengamatan seperti teleskop.
Filter Matahari ini juga ada yang dapat dipakai untuk pengamatan langsung.
Anda juga dapat membuat filter Matahari ini sendiri. Lihat penjelasannya di sini.
c. Mengamati gerhana lewat proyeksinya. Cara ini juga murah dan sederhana untuk dilakukan jika kita mempunyai dan ingin menggunakan alat bantu optik (misalnya teleskop) untuk mengamati gerhana. Anda dapat meletakkan selembar kertas putih di belakang teleskop untuk menangkap proyeksi peristiwa gerhana tersebut.
JANGAN SEKALIPUN MENGAMATI MATAHARI TANPA FILTER PENAPIS CAHAYA MATAHARI, BAIK MEMAKAI MATA TELANJANG, (apalagi) TELESKOP, ATAU ALAT BANTU OPTIK LAINNYA, KARENA AKAN MERUSAK DAN BAHKAN MEMBUTAKAN MATA ANDA SECARA PERMANEN.
2. Lokasi pengamatan Seperti yang sudah dijelaskan di atas, GMC ini hanya bisa diamati dari daerah-daerah tertentu saja. Jika Anda tidak tinggal di daerah yang dilewati lintasan umbra Bulan, maka Anda harus melakukan perjalanan luar kota. Hal yang perlu diperhatikan adalah transportasi dan akomodasi Anda selama di sana.
3. Cuaca Kita tidak bisa memastikan bahwa cuaca pada hari-H akan cerah. Oleh karena itu kita harus mempersiapkan diri jika terjadi hujan besar saat itu. Peralatan yang kita bawa harus bebas dari resiko basah terkena hujan. Anda dapat melengkapi bawaan dengan sejumlah tas plastik besar (trash bag) sebagai antisipasi darurat kala hujan turun tiba-tiba.
NASA's Fermi Gamma-ray Space Telescope has found 12 previously unknown gamma-ray only pulsars, as well as identifying gamma-ray emissions from 18 known or suspected radio pulsars. And what the telescope is finding is changing the way we think of these stellar cinders. The old analogy for pulsars was a lighthouse: gamma-rays were thought to pulse out in a narrow beam from the neutron star's magnetic poles. But this new research shows that cannot be the case. A new class of gamma-ray-only pulsars shows that the gamma rays must form in a broader region than the lighthouse-like radio beam. "We used to think the gamma rays emerged near the neutron star's surface from the polar cap, where the radio beams form," says Alice Harding of NASA's Goddard Space Flight Center. "The new gamma-ray-only pulsars put that idea to rest." She and Roger Romani from Stanford University in California spoke today at the American Astronomical Society meeting.
A pulsar is a rapidly spinning and highly magnetized neutron star, the crushed core left behind when a massive sun explodes. Most were found through their pulses at radio wavelengths, and were thought to be caused by narrow, lighthouse-like beams emanating from the star's magnetic poles. If the magnetic poles and the star's spin axis don't align exactly, the spinning pulsar sweeps the beams across the sky. Radio telescopes on Earth detect a signal if one of those beams happens to swing our way. Unfortunately, any census of pulsars is automatically biased because we only see those whose beams sweep past Earth. "That has colored our understanding of neutron stars for 40 years," Romani says. The radio beams are easy to detect, but they represent only a few parts per million of a pulsar's total power. Its gamma rays, on the other hand, account for 10 percent or more. "For the first time, Fermi is giving us an independent look at what heavy stars do," he adds.
Pulsars are phenomenal cosmic dynamos. Through processes not fully understood, a pulsar's intense electric and magnetic fields and rapid spin accelerate particles to speeds near that of light. Gamma rays let astronomers glimpse the particle accelerator's heart.
Astronomers now believe the pulsed gamma rays arise far above the neutron star. Particles produce gamma rays as they accelerate along arcs of open magnetic field. For the Vela pulsar, the brightest persistent gamma-ray source in the sky, the emission region is thought to lie about 300 miles from the star, which is only 20 miles across.
Existing models place the gamma-ray emission along the boundary between open and closed magnetic field lines. One version starts at high altitudes; the other implies emission from the star's surface all the way out. "So far, Fermi observations to date cannot distinguish which of these models is correct," Harding says.
Because rotation powers their emissions, isolated pulsars slow as they age. The 10,000-year-old CTA 1 pulsar, which the Fermi team announced in October, slows by about a second every 87,000 years.
Fermi also picked up pulsed gamma rays from seven millisecond pulsars, so called because they spin between 100 and 1,000 times a second. Far older than pulsars like Vela and CTA 1, these seemingly paradoxical objects get to break the rules by residing in binary systems containing a normal star. Stellar matter accreted from the companion can spin up the pulsar until its surface moves at an appreciable fraction of light speed.
"We know of 1,800 pulsars, but until Fermi we saw only little wisps of energy from all but a handful of them," said Romani. "Now, for dozens of pulsars, we're seeing the actual power of these machines."
This blog is intended for students who want to participate in astronomy competitions and olympiads (OSN, IAO, IOAA, APAO). Tutors and teachers can use this blog as a teaching resource. Amateur astronomers can find this blog useful,too. Its content varies from problem-set, course slide, try out, news, textbook recommendation, astronomy software review, trivia, video, and many more.
Annular Eclipse on Jan. 26. Credit: M.R. Taufik
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