Dokumentasi Gerhana Matahari Cincin 26 Januari 2009

Annular Eclipse on Jan. 26. Credit: M.R. Taufik

Di bawah ini disajikan 4 rekaman video gerhana matahari cincin yang diamati dari Jakarta dan Lampung.

• Video 1 diamati oleh Langit Selatan di Lampung
  

• Video 2 direkam oleh saudara M. Thariq Ziyad, siswa kelas 7 Labschool Kebayoran Jakarta

• Video 3 diperoleh dari alamat ini.

• Keterangan:
• Jakarta, 26 January 2009
• Titik Observasi: Pancoran 12770
• Elevasi: 63 derajat. Arah Barat.
• Waktu: 16.36 pm WIB
• Fase: cincin utuh.

• Video 4 diperoleh dari alamat ini.

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).

January 2009 Observations January 3rd and 4th; 13th and 14th

This report will actually cover a report from the evening of January 3rd and the morning of January 4th and my observations from the evening of January 13th and the morning of the 14th.

To highlight what I have and am using here is a listing of my equipment. I have an Orion Classic XT8 that is my primary scope, while also owning an Orion XT6 that my son and my students use. I have the 25mm and the 10mm Sirius Plossls that came with each scope as well as the following eyepieces:
Orion Sirius Plossls 32mm, 17mm,
Orion Expanse 9mm
Orion Stratus 21mm and 13mm
2x Shorty Barlow
UHC Orion NB Filter
7x35 Binoculars
Telrad
9x50 RA Finderscope

I use the Star Atlas 2000, the Pocket Sky Atlas and several books from time to time in helps in planing my observations.

I observe either from my backyard which is in the western part of the Salt Lake Valley and I have limiting magnitude skies from 5.3 (full moon) to 6.3 to 6.5 when conditions are good. Come spring, summer and fall I have a dark site in the green I go to which is about 45 minutes to an hour from my home.

Sketching is a part of each session as well, and that is why you may see from time to time I do not list eight, ten, twelve or more items in a session, unless I pull a near all night session or an all night session. I will include my sketches though for this entry I will have to return as I haven't scanned them yet.

Why a blog over posting at one of the major amateur astronomy sites on their forums? You'll see I include a lot in a report and I find that in the major astronomy sites that there is a core of people who have posted there and though they are very good at supporting new members, after awhile I just felt like I was either ignored (just to many members) or in terms of sketching, no where near the level of expertise that the core members had. Since these sites don't differentiate between newer members and their observations and sketches and long term members, I've opted to just do a blog. I will continue to read the forums on these sites, and I will participate, I just have decided that for now, blogging on my observations and my sketches is the best way for me to go. Oh, and I have to say, the blog is more for me and a way to keep them online and if someone else leaves advice, I definitely will appreciate it or if someone learns something from it, so much the better.

The night of the 3rd was a good night with seeing conditions at a 3 out of 5. I started out around 9pm and got set up in the backyard. After setting up, I have two pads we use under our sleeping bags in order to lay on the ground. One is 6 feet long so I lay on that, and I keep the other one which is about 3 feet long rolled up to support my head. To dark adapt I take out the 7x35 binocs as I call them and begin observing the sky where I want to observe. I will either do one of two things in my observations. One is to come up with a list of objects and go after them, or pick one part of the sky and make a list of objects and go after them. This night, well you can determine which method I took.

I started at Pollux and jumped down Kappa Gemni and then over to Mu Cancer and then to M44. M44 shows in the 7x35 and is easily seen. From my backyard I can see Asellius Austrailis visually and a faint of a glow above it which is M44. From here I went back to Pollux and went down to Wasat. Using Wasat as the starting point I wanted to observe and went from Wasat to 63 Gemni and began looking for a jump to NGC 2392 (Eskimo Nebula). I've seen NGC 2392 several times before earlier in the late fall early winter, but not at or near zenith. I also went back to M44 the Beehive Cluster and centering on Asellius Asustralis I hopped down to Acubens and went looking for M67. M67 in the binocs was hard to see, but a small glow was visible.

By now I was darked adapted so I put the binocs away and got the XT8 ready to go. I put in the 21mm Stratus, which serves as my finer eyepiece now. I love the wide field of view and have had no major issues with any of my Stratus'. I hopped easily to Wasat and then over to 63 Gemni. From here I got lost. I attempted the jump down to NGC 2392 but failed each time (did a total of 4 attempts). I realized then that I was frustrated and it was time to move on. I also realized that I did a fundemental error in planning this session. I did not look at my previous observations when I successfully had found this. In doing so I would have noted that once I got to 63 Gemni I changed from a low power to the 17mm (in this case I would have used the 13mm) to pull out NGC 2392. I'll have to do that next time (tonight).

Next I went back to Pollux and went to Kappa Gemni and then over to Mu Cancer and over to the Beehive. In the 21mm Stratus the Beehive shows itself to be incredible. It fit into the view (barely and only for a few moments before going out of view for a part of the cluster). I observed the cluster using the 21mm, the 13mm Stratus' and enjoyed both views. With the 13mm I saw the hints of stars wanting to pop out and with averted vision some of them did. I spent about 40 minutes sketching after observing for about fifteen minutes. I'll post my sketch when my son is not using the computer attached to our scanner.

After finishing with M44 The Beehive Cluster I went down to Asellus Asutralis and jumped down to Acubens by passing down past Omicron Cancer (two stars next to each other) and then down to Acuberns. At Acuberns I went over past 60 Cancer and up to M67. In my finder M67 was a blurry object and using the 21mm Stratus I captured this small open cluster right away. M67 is a wonderful cluster and though smaller than its neighbor, M44, is a cluster that I enjoyed observing and sketching. I did not see the King Cobra that this cluster is known for but nevertheless, spending around 15 minutes observing and 35 minutes sketching it.

My next object was M48. It was a night when I wanted to bag some of the larger open clusters. I used the Telrad and went to Sirius and from there hopped and jumped up to M46 and M47 which were visible in the binocs and in the finderscope. From here I jumped up to Alpha Monoceros. From Alpha I went up towards Zeta Monoceros and about 3/4 of the way up went over to the left. M48 was easily seen in the finderscope and in the 21mm Stratus. I went over and was amazed by its sheer size. It has a definite triangular shape and in some ways, reminds me of a crocidile. I observed for 18 minutes on this one because I looked at my watch after recording my observations and writing them down, and then just looking. I pulled my glove down to see how long I had been observing and realized I had been spending about 10 minutes just observing. I moved on to my sketch and this one took a little longer, taking about 40 minutes to get my sketch down. I also took more time sketching since I made a 21mm sketch and a 13mm sketch.

My observations of M48 was my last observation of this session. I had seen M44, M67, M41 (no sketch, just looked at it), M46 and M47 (no sketch though I decided I need to redo my orginal sketches of these objects) and M48. I should have added M93 to this session but as it was now around 3a.m. I decided to bag it.

January 4, 2009

I only had about 2 hours to observe since I had to work and be there early the next morning. I got the equipment out and by the time I had done that I had about an hour to observe. I looked at Cassiopeia tonight and wanted to nab M 52 to the list. I am currently working on completing the Messier Astronomical Award for my local clubs award next fall (along with the best of the NGC). I used the Telrad to Caph and from there I jumped to Cassiopeia 6. From 6 I could see Cassiopeia 4 and hopped over to that star. Once at 4 it was really easy to see M 52 off to the left in the finder or to the right in the 21mm Stratus. M 52 was a large cluster and to its left sits the Bubble Nebula NCG 7635. As I had both objects in view I observed and made my notes on M 52 first. I know some call this the scorpian and I can see that, but the first thing that stuck out to me was the Y shape to the cluster. I spent 14 minutes or so observing and another 30 to 35 minutes sketching M 52.

After finishing with M 52 I went over to the Bubble Nebula and made my observation of the Bubble and sketched it also. I spent a good ten minutes observing and probably 20 minutes sketching it. I put it just outside the eye piece circle of M 52. I really enjoyed the both the Bubble and the Cluster. After observing and sketching both items it was time to take everything in and retire for the night.

January 13, 2009

I spent 15 years in business, ten of those as an excutive and now have spent 9 years as a teacher. One thing I will not do is allow a hobby to intervere with my professional career. It's a commitment I made a long time ago and I've stayed pretty true to it. So tonight is a short observation session again. The skies were clear and with smog coming this weekend I'm not sure how conditions will be.

I got set up and ready to go by 7p.m. tonight and was dark adjusted by 7p.m. when the scope was cooled. I started with observing M 42 since I have wanted to make sketches of it using the 21mm and 13mm Stratus eyepieces. I went to M 42 and just admired the view of the entire nebula in the 21mm Stratus. It was clear, sharp and a wonderful site to see. Prior to sketching, I put in my 9mm Orion Expanse. I know some have had issues with kidney beaning and fade outs, and I did too for the first three or four times I used the eyepiece. Now that I have figures out the eye relief (I am nearsighted and in the past have not worn my eye glasses while observing with the Sirius Plossls I have) and the key for me is wearing my eye glasses with the Expanse, I truly love the views the 9mm Expanse gives to me. Next to my Stratus, they are one of my favorites, and probably will be until I can buy a 8mm Stratus.



Anyway, I took the 21mm Stratus out and put in the 9mm Expanse with the 2x Shorty Barlow and looked at the Trapezium. I was shocked, F was clearly visible. Must be decent conditions tonight. After viewing the Trapezium I took the barlow and the 9mm Expanse and put them back in the case and put the 21mm Stratus back in. I observed for about 10 minutes (I've done my first observation back in October early in the a.m.) and began sketch the nebula. I started with the brighter stars and drew them in four quadrants by dividing the circle into fourths. Then I came back and did the smaller stars. Finally I added the nebula. Though not perfect, I was happy with the overall drawing as I used a stump to put in the nebulosity.

I have been wanting to get M81 and M82 since by the time I started observing last year, it was into September and the Big Dipper was too low for good viewing and more probably, my eyes had not adjusted. I moved the scope around and using the Telrad focused on Dubhe. From there I went out past UM38 and the group of four stars around that star to a bright UM 24 which is part of a long L. Focusing on UM 24 which is at the end of the long L I went east and there was M82 with her long strectch out looking like a cigar. Going over slightly I was ableto see M82 and put both M81 and M82 in the viewing field together. I observed and made notes, noting how M82 looked brighter on the northern side then on its southern side. M81 had a very bright central core area and hints of lighter arms spreading out.

I decided to swap out the 21mm Stratus with the 13mm Stratus and was glad I did. M 82 still showed more brightness on the northern side but it was streched out longer in the 13mm view. M 81 showed far more details in the 13mm. I was able to discern not only a bright core, but several arms stretching out. I also saw some bright stars in the arms, one being very bright on the eastern north/eastern side. M81 has two stars to the NW of it with a smaller star in betwen the two stars. M82 has two stars at and angle to M82 that are near the western bottom of the galaxy. I decided that night to draw M81 and M82 as self contained galaxies, thinking I would come back on my next session to draw them in the same field of view using the 21mm Stratus. As it was now 10:15p.m. I had to wrap ups this session.

January 14th, 2009

Tonight was rather a frustrating night. I set up later in the evening since I took a nap after getting home from work. I did this since I wanted to go back to revisit M 81 and M 82 and to see if I could get M108, M97 and M109.

I got out to M81 and M82 easily and using the 21mm Stratus captured both galaxies in the same sketch. I was rather happy with this view and with the sketch, especially since I felt I captured M 82 better than M81.

After spending time with M81 and M82 and sketching them in the same sketch, I moved down to Merak. From Merak I went down towards Phecda until I came to the first star. I went diagonally down two stars to where I saw a arrow or inverted Y which should have put M108 into view. Nothing. Nothing I did worked. I tried the Q70 32mm, the 21mm Stratus, the 32mm Sirius Plossl and the 13 mm Stratus and 9mm Expanse. Nothing. I didn't expect the high power eyepieces to do anything but the galaxy did not show herself. I though with averted vision I may have had it, but I cannot confirm it so it stays off the list. I tried for M97 both with and without the NB UHC filter I have but no luck there either. I have to assume that since the Big Dipper is to my east and that is where my worse light pollution and worse sky is, that the LP is impacting my ability to see these galaxies and the nebula. Perhaps someone can tell me if I see M 81 and M82 if I should also be able to see M108, M109 and M 87 as well? From what I've read the bad LP can mean I need to go after these at a dark site.

As mentioned, I wet down to Phecda and dropped down to M109 but nothing was there either! I may have to go to my dark site or wait for March to see if position helps then. Since I knew I probably wouldn't find it I also tried to go after M51 but failed at that one also.

Next I went back to M42 and sketched it using the 13mm Stratus. I can't speak enough of the Stratus eyepieces. They are my main eyepieces now for my observing. I'll probably get the 8mm Stratus in time. Plenty of field of view, I can look with my glasses on and I don't have any loss of color to my eye.

From here to close out my observations I went to M93. I use the Telrad to to to Aludra in Canis Major and then hopped over to Zeta in Puppis. From Zeta M93 was easy using the 21mm Stratus. I simply followed the two stars from Zeta that go diagonal and they took me right to M93. M93 is a rather small cluster of stars that to me seems to form the shape of a skunk, or perhaps a racoon with the tail sticking up. Two bright stars seem to form the eyes. There is a definite V pattern to the cluster also.

That was it. Nothing since though I am going out tonight if conditions are good and I hope to stay out late. Last night would have been ok, but I was too tired and just too worn out to go out. I spent the night watching a movie with my wife with the two teens over at a mutal friend to watch movies there.

Clear Skies

Jay

PS I'll post more images of my sketches but probably not until next weekend.

Study Solves Mystery of How Massive Stars Form

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."

Watch movie simulation of star formation.

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."

Source: UC Santa Cruz

Cited from : universe today

Gerhana Matahari Cincin 26 Januari 2009

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.

Source : ESO

Safety procedures lainnya dapat di sini.

Sekali lagi diingatkan :

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.

(diadaptasi dari : Langit Selatan (dan Weblog Astronomi Berbahasa Indonesia), Wikipedia, NASA, Bosscha)

"Lighthouse" Analogy No Longer Works for Pulsars

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.

Watch an animation of the new look at these pulsars.

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."

Source: NASA

Cited from : www.universetoday.com by Nancy Atkinson