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Sunday, November 30, 2008

Mengenal Objek Langit : Horsehead Nebula

One of the most identifiable nebulae in the sky, the Horsehead Nebula in Orion, is part of a large, dark, molecular cloud. Also known as Barnard 33, the unusual shape was first discovered on a photographic plate in the late 1800s. The red glow originates from hydrogen gas predominantly behind the nebula, ionized by the nearby bright star Sigma Orionis. A blue reflection nebula surrounds the bright star at the lower left. The darkness of the Horsehead is caused mostly by thick dust, although the lower part of the Horsehead's neck casts a shadow to the left. Streams of gas leaving the nebula are funneled by a strong magnetic field. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming. Light takes about 1500 years to reach us from the Horsehead Nebula. The above image was taken earlier this month with a 0.6-meter telescope at the Mt. Lemmon SkyCenter in Arizona, USA.

Source : Astronomy Picture of the Day

Trivia : Coba sebutkan nomor NGC dari objek di atas? Apakah objek termasuk muncul di katalog Messier? Jika ya, coba sebutkan nomor katalognya?

Selamat mencoba. Jawaban bisa disampaikan lewat kolom komentar.
Pembaca lain juga boleh mengomentari jawaban pembaca lain sehingga bisa saling mengkoreksi atau menambahkan.

Saturday, November 29, 2008

Menghitung Satuan Astronomi (Astronomical Unit)

Pendahuluan
Mungkin Anda pernah mendengar istilah AU. AU adalah singkatan dari Astronomical Unit.

Satu AU sama dengan jarak RATA-RATA Matahari ke Bumi

Nilai eksak AU yang saat ini diterima adalah 149.597.870.691 ± 30 meter (sekitar 150 juta kilometer atau 93 juta mil). Untuk keperluan perhitungan sehari-hari sering diambil 1,496 x 108 km.

Satuan AU umumnya digunakan untuk menyatakan jarak benda-benda yang ada di tata surya. Alasannya adalah agar jarak benda-benda di tata surya (planet, asteoroid, dll) mudah dibandingkan dengan jarak Matahari-Bumi. Misalkan, planet Jupiter dikatakan berjarak 5,2 AU dari Matahari berarti jarak Jupiter-Matahari 5,2 kali jarak Bumi-Matahari.

Bagaimana menghitung AU?
Sebelum kita mulai membahas bagaimana para ahli menghitung besarnya satu AU, Anda sebaiknya memahami Hukum Kepler, khususnya Hukum Kepler ketiga.

Hukum ini menyatakan bahwa perbandingan pangkat tiga jarak suatu objek terhadap kuadrat periode revolusinya adaah konstan jika mengorbit objek yang sama (misalnya, semua planet memiliki nilai konstanta yang sama karena sama-sama mengitari Matahari tetapi Bulan tidak mempunyai nilai konstanta yang sama dengan planet karena Bulan mengorbit Bumi, bukan Matahari). Jika dinyatakan dalam bentuk persamaan, Hukum Ketiga Kepler berbentuk:

(a3/P2)1 = (a3/P2)2 = konstan

dengan : a = jarak objek dan P = periode orbit.

Sebenarnya banyak cara untuk menentukan jarak Bumi-Matahari. Salah satu teknik yang paling modern yang cukup teliti adalah dengan menggunakan radar.

Pengamatan dengan radar ini pertama kali dilakukan oleh Lincoln Laboratory, Massachusetts Institute of Technology pada tahun 1958 dengan mengirim gelombang radar berfrekuensi 440 Megahertz ke planet Venus.

Untuk penentuan ini diandaikan orbit Bumi dan Venus berbentuk lingkaran. Asumsi ini hanya mengurangi seidkit akurasi perhitungan karena orbit Venus memang hampir berupa lingkaran sempurna (eksentrisitas orbit Venus = 0,00677323).

Dari pengamatan, diketahui bahwa periode orbit Bumi adalah,
PB = 365,25 hari
Periode orbit Venus adalah,
PV = 224,7 hari

Dari hukum Kepler ke-3 (a3 ~ P2)
aV/aB = (PV/PB)2/3

Dari data di atas :
aV/aB = (224,7/365,25)2/3 = 0,72
atau, aV = 0,72 aB

Ilustrasi perhitungan ini ditunjukkan dalam gambar di bawah ini:
Dengan menggunakan aturan cosinus:
Lalu, subtistusikan : aV = 0,72 aB ke dalam persamaan di atas dan akan diperoleh:
Nilai d dapat ditentukan dengan radar. Gelombang radar dipancarkan dari Bumi dan ditangkap sinyal pantulannya. Selang waktu kedua sinyal (sinyal utama dan sinyal pantulan) dapat digunakan sebagai skala jarak. Persamaan yang digunakan adalah:
t = 2d c
dengan :
  • d = jarak antara objek pemancar sinyal (Bumi) dan objek pemantul sinyal (Venus) (diambil pada saat jarak terdekat Bumi-Venus)
  • t = waktu yang ditempuh oleh gelombang radar Bumi- Venus- Bumi
  • c = kecepatan gelombang elektromagnetik (cahaya, gelombang radar, dll) = 299.792.458 m/s

Nilai α juga dapat diamati dari Bumi karena α menyatakan jarak sudut (sudut pisah) antara Venus dan Matahari saat teramati dari Bumi (jarak sudut adalah jarak antara dua objek dinyatakan dalam satuan sudut). Nilai α tergantung pada posisi Bumi-Venus pada saat pengukuran.

Dengan memasukan harga d dan hasil pengamatan, diperoleh,
aB = 1,496 x 1013 cm = 1 AU

Beberapa pendekatan yang digunakan dalam perhitungan ini adalah:
  1. Orbit Bumi dan orbit Venus mengedari Matahari tidak berupa lingkaran sempurna, tapi berupa elips dengan eksentrisitasnya sangat kecil (eksentrisitas orbit Venus = 0,00677323). Jadi, orbit Bumi dan orbit Venus praktis dapat dianggap berupa lingkaran.
  2. Selain itu juga bidang orbit Venus tidak sebidang dengan bidang orbit Bumi, tetapi membentuk sudut 3o23’. Kemiringan bidang orbit ini cukup kecil.
Jadi, sekarang Anda sudah tahu bagaimana salah satu metode menghitung jarak Bumi-Matahari yang dijadikan unit (satuan) jarak benda-benda lain di tata surya kita ini.

Salah satu metode lain untuk menghitung AU dengan ketelitian yang lebih rendah adalah menggunakan peristiwa transit Venus. Ketelitian yang lebih rendah disebabkan sulitnya menentukan secara pasti waktu kontak I (saat piringan Venus menyentuh piringan Matahari) dan kontak IV (saat piringan Venus meninggalkan piringan Matahari). Efek ini dikenal dengan blackdrop effect. Transit Venus adalah peristiwa ketika piringan Venus lewat di "depan" piringan Matahari (nampak seperti noda hitam di permukaan Matahari).


Peristiwa ini terakhir terjadi pada tanggal 8 Juni 2004 dan akan terjadi lagi pada tanggal 6 Juni 2012. Transit berikutnya lagi akan terjadi ratusan tahun lagi (pada 11 Desember 2117).

Data hasil pengamatan transit Venus tahun 1761 dan 1769 (oleh Edmund Halley):
The Observed Tracks of Venus across the Face of the Sun during the Transits of 1761 and 1769

Peristiwa transit ini dapat digunakan untuk menghitung paralaks Matahari.
Metode perhitungannya diajukan oleh Halley pada tahun 1776:

Dari gambar di atas dapat ditemukan hubungan:
D = d . Lv/(LE - Lv ) (dari hubungan kesebangunan segitiga)

Ratio Lv/LE dapat diketahui dari Hukum Ketiga Kepler. Secara kasar, nilai sama dengan sin θ (dimana θ adalah sudut elongansi terbesar planet Venus - lihat gambar di bawah ini)
Jadi, D = d . sin θ. LE/(LE( 1 - sin θ)) = d . sin θ/( 1 - sin θ)

Oleh sebab itu, dari ratio D/H (lihat gambar sebelumnya), nilai H (diameter Matahari) dapat dhitung. Kemudian, nilai paralaks Matahari dan jarak Matahari dari Bumi (1 AU) dapat dihitung. Memang metode ini, nampak lebih sulit dibandingkan metode dengan radar tetapi pada saat itu belum ada cara langsung untuk mengukur jarak Bumi ke Venus.

Sekarang Anda diminta untuk mencoba menghitung jarak Bumi-Matahari berdasarkan metode Halley. Diketahui paralaks Matahari hasil perhitungan Halley antara 8.55" sampai 8.88". Jawabannya boleh disampaikan lewat kolom komentar.

Sumber :
  1. Slide kuliah Astrofisika 1 : Bab Besaran Mendasar dalam Astrofisika, oleh Dr. Djoni N. Dawanas, ITB.
  2. wikipedia.org
  3. The transit of Venus and The Quest For the Solar Parallax, by David Sellers (Leeds, England)
  4. NASA
Di bawah ini juga disediakan 2 file artikel tentang perhitungan AU yang dapat Anda download dari link di bawah ini:
  1. Venus Transit
  2. Quest for the Astronomical Unit
  3. Transit of Venus 2004, Calculation of the Astronomical Unit from the transit durations at two different locations
Selamat belajar.

Thursday, November 20, 2008

Kompetisi Karya Tulis Astronomi


Buat rekan-rekan muda pecinta astronomi, ada sebuah berita gembira untuk anda. The International Year of Astronomy 2009 bersama International Astronomical Union dan UNESCO bekerja sama membawa penduduk bumi untuk menemukan kembali tempatnya di alam semesta melalui keindahan langit malam maupun cerahnya angkasa di siang hari. Selain itu diharapkan berlangsungnya tahun astronomi 2009 diharapkan dapat menumbuhkan ketertarikan pribadi akan kemegahan alam semesta maupun sebuah penemuan baru.

Kali ini Space Generation Advisory Council (SGAC) mengadakan sebuah kompetisi essay dengan tema :

“What Astronomy means to you?”

Kompetisi ini didedikasikan pada astronom muda untuk mengekspresikan perasaan dan pikirannya secara kreatif. 2 Pemenang pertama akan diundang ke Paris untuk berpartisipasi dalam acara “Launch Conference of the International Year of Astronomy” pada tanggal 14-18 Jan 2009. Panitia akan menanggung biaya selama di Paris, namun biaya perjalanan menjadi tanggung jawab penulis (pemenang).Biaya perjalanan dan akomodasi akan menjadi tanggungan panitia.

Syarat keikutsertaan :

  1. Penulis harus berusia 18-22 tahun.
  2. Essay maksimum memuat 2500 kata, dengan abstrak maksimum 200 kata harus disertakan saat pendaftaran.
  3. Essays ditulis dalam bahasa Inggris. Tidak diperlukan pengejaan tingkat tinggi untuk para penulis yang bahasa ibunya bukan bahasa Inggris.
  4. Essay ditulis dengan huruf (font) Times New Roman 11, spasi dobel dan dikirim dalam format .doc atau .pdf
  5. Essay saat diserahkan sudah dalam keadaaan siap dipublikasikan tanpa catatan kaki.

Panitia penilaian akan menilai berdasarkan kriteria berikut:

  • Inspirasi akan astronomi
  • inovasi dan originalitas
  • keterkaitan dengan IYA2009
  • Kualitas dalam gaya penulisan dan struktur penulisan.

Tenggat Waktu:
Tulisan / essay sudah harus dikirimkan melalui email ke :
bee@spacegeneration.org

28 November 2008, 17:00 GMT atau 23.00 wib

Keterangan lebih lengkap bisa dibaca di :
http://www.spacegeneration.org/node/2202

Sumber: Langit Selatan - Situs Astronomi Indonesia

Saturday, November 15, 2008

Deepest Ultraviolet Image Shows a Sea of Distant Galaxies























Caption : A Pool of Distant Galaxies. Credit: ESO

Dive right in to this image that contains a sea of distant galaxies! The Very Large Telescope has obtained the deepest ground-based image in the ultraviolet band, and here, you can see this patch of the sky is almost completely covered by galaxies, each one, like our own Milky Way galaxy, and home of hundreds of billions of stars. A few notable things about this image: galaxies were detected that are a billion times fainter than the unaided eye can see, and also in colors not directly observable by the human eye. In this image, a large number of new galaxies were discovered that are so far away that they are seen as they were when the Universe was only 2 billion years old! Also…

This image contains more than 27 million pixels and is the result of 55 hours of observation, made primarily with the Visible Multi Object Spectrograph (VIMOS) instrument. To get the full glory of this image, here's where you can download the full resolution version. It's worth the wait while it downloads. Or click here to be able to zoom around the image.

In this sea of galaxies – or island universes as they are sometimes called – only a very few stars belonging to the Milky Way are seen. One of them is so close that it moves very fast on the sky. This "high proper motion star" is visible to the left of the second brightest star in the image. It appears as a funny elongated rainbow because the star moved while the data were being taken in the different filters over several years.

The VLT folks describe this image as a "uniquely beautiful patchwork image, with its myriad of brightly coloured galaxies." It shows the Chandra Deep Field South (CDF-S), one of the most observed and best studied regions in the entire sky. The CDF-S is one of the two regions selected as part of the Great Observatories Origins Deep Survey (GOODS), an effort of the worldwide astronomical community that unites the deepest observations from ground- and space-based facilities at all wavelengths from X-ray to radio. Its primary purpose is to provide astronomers with the most sensitive census of the distant Universe to assist in their study of the formation and evolution of galaxies.

The image encompasses 40 hours of observations with the VLT, just staring at the same region of the sky. The VIMOS R-band image was obtained co-adding a large number of archival images totaling 15 hours of exposure.

Source: ESO
Cited from : universetoday

Monday, November 10, 2008

Chandra Telescope Searches for Antimatter

The Bullet Cluster.
Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.;
Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

Say the word "antimatter" and immediately people think of science fiction – anti-universes, fuel for the Enterprise's warp-speed engines and so forth. But Captain, we can't change the laws of physics; antimatter is the real deal. Antimatter is made up of elementary particles, each of which has the same mass as their corresponding matter counterparts –protons, neutrons and electrons — but the opposite charges and magnetic properties. When matter and antimatter particles collide, they annihilate each other and produce energy according to Einstein's famous equation, E=mc2. But antimatter isn't something that's available on every corner drugstore (and neither is plutonium, to continue with the movie theme) and there's not very much of it around, so it seems. But, according to theory, it wasn't always that way, and scientists are using the Chandra X-ray Observatory to hunt for evidence of antimatter that was present in the very early universe. And it's not an easy job…

According to the Big Bang model, the Universe was awash in particles of both matter and antimatter shortly after the Big Bang. Most of this material annihilated, but because there was slightly more matter than antimatter - less than one part per billion - only matter was left behind, at least in the local Universe.

Trace amounts of antimatter are believed to be produced by powerful phenomena such as relativistic jets powered by black holes and pulsars, but no evidence has yet been found for antimatter remaining from the infant Universe.

How could any primordial antimatter have survived? Just after the Big Bang there was believed to be an extraordinary period, called inflation, when the Universe expanded exponentially in just a fraction of a second.

"If clumps of matter and antimatter existed next to each other before inflation, they may now be separated by more than the scale of the observable Universe, so we would never see them meet," said Gary Steigman of The Ohio State University, who conducted

the study. "But, they might be separated on smaller scales, such as those of superclusters or clusters, which is a much more interesting possibility."

Illustration of Antimatter/Matter Annihilation. (NASA/CXC/M. Weiss)

In that case, collisions between two galaxy clusters, the largest gravitationally-bound structures in the Universe, might show evidence for antimatter. X-ray emission shows how much hot gas is involved in such a collision. If some of the gas from either cluster has particles of antimatter, then there will be annihilation and the X-rays will be accompanied by gamma rays. Steigman used data obtained by Chandra and now de-orbited Compton Gamma Ray Observatory to study the Bullet Cluster, where two large clusters of galaxies have crashed into one another at extremely high velocities. At a relatively close distance and with a favorable side-on orientation as viewed from Earth, the Bullet Cluster provides an excellent test site to search for the signal for antimatter.

"This is the largest scale over which this test for antimatter has ever been done," said Steigman, whose paper was published in the Journal of Cosmology and Astroparticle Physics. "I'm looking to see if there could be any clusters of galaxies which are made of large amounts of antimatter." The observed amount of X-rays from Chandra and the non-detection of gamma rays from the Compton data show that the antimatter fraction in the Bullet Cluster is less than three parts per million. Moreover, simulations of the Bullet Cluster merger show that these results rule out any significant amounts of antimatter over scales of about 65 million light years, an estimate of the original separation of the two colliding clusters. "The collision of matter and antimatter is the most efficient process for generating energy in the Universe, but it just may not happen on very large scales," said Steigman. "But, I'm not giving up yet as I'm planning to look at other colliding galaxy clusters that have recently been discovered." Finding antimatter in the Universe might tell scientists about how long the period of inflation lasted. "Success in this experiment, although a long shot, would teach us a lot about the earliest stages of the Universe," said Steigman. Tighter constraints have been placed by Steigman on the presence of antimatter on smaller scales by looking at single galaxy clusters that do not involve such large, recent collisions.

Source: Chandra/Harvard and written by Nancy Atkinson

Cited from : universetoday.com