METEOR

Meteor

 

A meteor is a bright streak of light that appears briefly in the sky. Observers often call meteors shooting stars or falling stars because they look like stars falling from the sky. People sometimes call the brightest meteors fireballs. A meteor appears when a particle or chunk of metallic or stony matter called a meteoroid enters the earth's atmosphere from outer space. Air friction heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second.

Most meteoroids disintegrate before reaching the earth. But some leave a trail that lasts several minutes. Meteoroids that reach the earth are called meteorites.

Millions of meteors occur in the earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers).

Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second). The earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the earth's atmosphere head-on, the combined speed may reach about 44 miles per second (71 kilometers per second).

Meteor showers

The earth meets a number of streams (trails) or swarms (clusters) of tiny meteoroids at certain times every year. At such times, the sky seems filled with a shower of sparks. Streams and swarms have orbits like those of comets and are believed to be fragments of comets.

The most brilliant meteor shower known took place on Nov. 12-13, 1833. It was one of the Leonid showers, which occur every November and seem to come from the direction of the constellation Leo. 

Meteorites

There are three kinds of meteorites, stony, iron, and stony-iron. Stony meteorites consist of minerals rich in silicon and oxygen, with smaller amounts of iron, magnesium, and other elements. One group of stony meteorites, called chondrites, are pieces of the same material from which the planets formed. Another group of stony meteorites, the achondrites, were once part of a parent body, such as an asteroid, that was large enough to have melted and separated into an iron-rich core and a stony crust. Achondrites come from the outer crust; stony-iron meteorites, from the inner crust; and iron meteorites, from the metallic core. Iron meteorites consist mostly of iron and nickel. Stony-iron meteorites have nearly equal amounts of silicon-based stone and iron-nickel metal.

The size of meteorites varies greatly. Most of them are relatively small. The largest meteorite ever found weighs about 66 short tons (60 metric tons). It fell at Hoba West, a farm near Grootfontein, Namibia. However, much larger bodies, such as asteroids and comets, can also strike the earth and become meteorites.

Meteorites reach the earth's surface because they are the right size to travel through the atmosphere. If they are too small, they will disintegrate in the atmosphere. If they are too large, they may explode before reaching the earth's surface. One such object exploded about 6 miles (10 kilometers) above the Tunguska River in Siberia in 1908, leaving a 20-mile (32-kilometer) area of felled and scorched trees.

Thousands of small meteorites have been found in Antarctica, providing a rich supply of specimens for scientists to study. Scientists study meteorites for clues to the types of material that formed the planets.

Impact craters and basins

When large bodies such as asteroids and comets strike a planet, they produce an impact crater or impact basin. Impact craters are bowl-shaped depressions that measure up to about 10 miles (25 kilometers) in diameter. They have shallow, flat floors and uplifted centers. Impact basins are larger, and inside their rims there are one or more rings on the planet's surface.

Scientists have found more than 120 impact craters and basins on the earth. One of the most famous, the Meteor Crater in Arizona, is about 4,180 feet (1,275 meters) across and 570 feet (175 meters) deep. It formed nearly 50,000 years ago when an iron meteorite weighing 330,000 short tons (300,000 metric tons) struck the earth. 

Most impact craters and basins larger than the Meteor Crater are heavily worn away or have been buried by rocks and dirt as the earth's surface changed. The largest known of these is the Chicxulub (CHEEK shoo loob) Basin centered in Mexico's Yucatan Peninsula. The diameter of the basin is about 190 miles (300 kilometers). Rock samples obtained by drilling into the basin indicate that an asteroid struck the earth there about 65 million years ago. This was about the time the last dinosaurs became extinct. The impact hurled much debris into the sky. Many scientists believe this debris caused climate changes that the dinosaurs could not survive. Contributor: Virgil L. Sharpton, Ph.D., President's Professor, Geophysical Institute, University of Alaska Fairbanks.

Mengukur Jarak dengan Bintang Cepheid

kita dapat menentukan jarak bintang dengan menghitung paralaksnya. Namun metode paralaks itu hanya dapat digunakan untuk bintang-bintang dekat saja karena teknologi yang kita miliki belum dapat menghitung paralaks dengan ketelitian tinggi. Jarak terjauh yang bisa diukur dengan metode paralaks hanya beberapa kiloparsek saja. Lalu bagaimana kita menghitung jarak bintang-bintang yang lebih jauh? Atau bahkan menghitung jarak galaksi-galaksi yang jauh? Salah satu caranya adalah dengan menggunakan hubungan periode-luminositas bintang variabel Cepheid.

Sejarah metode penghitungan jarak ini berawal dari sebuah penelitian tentang hasil pengamatan terhadap bintang variabel (bintang yang kecerlangannya berubah-ubah) yang ada di galaksi Awan Magellan Besar dan Awan Magellan Kecil (LMC dan SMC). Saat itu Henrietta Leavitt, astronom wanita asal Amerika Serikat, membuat katalog yang berisi 1777 bintang variabel dari penelitian tersebut. Dari katalog yang ia buat diketahui bahwa terdapat beberapa bintang yang menunjukkan hubungan antara kecerlangan dengan periode variabilitas. Bintang yang memiliki kecerlangan lebih besar ternyata memiliki periode varibilitas yang lebih lama dan begitu pula sebaliknya. Bentuk kurva cahaya bintang variabel jenis ini juga unik dan serupa, yang ditandai dengan naiknya kecerlangan bintang secara cepat dan kemudian turun secara perlahan.

Bentuk kurva cahaya seperti itu ternyata sama dengan kurva cahaya bintang delta Cephei yang diamati pada tahun 1784. Karena itulah bintang variabel jenis ini diberi nama bintang variabel Cepheid. Penamaan ini tidak berubah walaupun belakangan ditemukan juga kurva cahaya yang sama dari bintang Eta Aquilae yang diamati beberapa bulan sebelum pengamatan delta Cephei.

Hubungan sederhana antara periode dan luminositas bintang variabel Cepheid ini bisa digunakan dalam menentukan jarak karena astronom sudah mengetahui adanya hubungan antara luminositas dengan kecerlangan/magnitudo semu bintang yang bergantung pada jarak. Dari pengamatan bintang Cepheid kita bisa dapatkan periode variabilitas dan magnitudonya. Kemudian periode yang kita peroleh bisa digunakan untuk menghitung luminositas/magnitudo mutlak bintangnya dengan formula M = -2,81 log(P)-1,43. Karena luminositas/magnitudo mutlak dan magnitudo semu berhubungan erat dalam formula Pogson (modulus jarak), maka pada akhirnya kita bisa dapatkan nilai jarak untuk bintang tersebut.

Kunci penentu agar metode ini dapat digunakan adalah harus ada setidaknya satu bintang variabel Cepheid yang jaraknya bisa ditentukan dengan cara lain, misalnya dari metode paralaks trigonometri . Jarak bintang akan digunakan untuk menghitung luminositasnya dan selanjutnya bisa digunakan sebagai pembanding untuk semua bintang Cepheid. Oleh karena itu, astronom sampai sekarang masih terus berusaha agar proses kalibrasi ini dilakukan dengan ketelitian yang tinggi supaya metode penentuan jarak ini memberikan hasil dengan akurasi tinggi pula.

Menghitung jarak bintang variabel Cepheid menjadi sangat penting karena kita jadi bisa menentukan jarak gugus bintang atau galaksi yang jauh asalkan di situ ada bintang Cepheid yang masih bisa kita deteksi kurva cahayanya. Di sinilah keunggulan metode ini dibandingkan dengan paralaks, yang hanya bisa digunakan untuk bintang-bintang dekat saja.

Lalu apa sebenarnya yang terjadi pada bintang Cepheid? Bintang ini mengalami perubahan luminositas karena radiusnya berubah membesar dan mengecil. Proses ini terjadi pada salah satu tahapan evolusi bintang, yaitu ketika sebuah bintang berada pada fase raksasa atau maharaksasa merah. Jadi dengan mempelajari bintang variabel Cepheid kita bisa menghitung jarak sekaligus mempelajari salah satu tahapan evolusi bintang

Galaksi BIMASAKTI :)

Terdapat banyak bintang, nebula, dan gugus bintang yang bisa diamati di langit setiap malamnya. Semua objek tersebut berada di dalam galaksi kita. Di beberapa bagian bintang nampak padat sehingga ketika langit cerah, bersih dari awan, dan kondisi sekitar yang gelap, kita bisa melihat pita berwarna putih yang memanjang dan melintasi beberapa rasi seperti Sagittarius (arah pusat Galaksi), Scorpius, Ophiucus, Aquila, Cassiopeia, Auriga, Crux, dan Centaurus. Sementara di bagian yang lain tampak celah-celah gelap yang menunjukkan adanya materi antar bintang yang tebal. Itulah (bidang) galaksi yang kita tinggali. Bentuknya yang seperti itu kemudian menginspirasi orang untuk menamakannya dengan sebutan Milky Way. Kata galaksi dan milky way itu sendiri diadaptasi dari bahasa Yunani “galaxias” dan Latin “via lactea” dengan kata dasar lactea yang berarti susu. Sedangkan menurut orang Indonesia, galaksi kita diberi nama Bimasakti. Menurut salah satu sumber dari Observatorium Bosscha, sejarah penamaan ini berasal ketika Presiden RI pertama, Soekarno, ditunjukkan citra galaksi oleh salah seorang astronom Indonesia. Ternyata, Soekarno melihat salah satu bagian gelap di foto tersebut menyerupai tokoh Bima Sakti. Namun tidak diketahui bagian gelap mana yang dimaksud.

Galaksi spiral tersusun atas 3 bagian utama, yaitu bagian bulge, halo, dan piringan. Ketiganya memiliki bentuk, ukuran, dan objek penyusun yang berbeda-beda. Bahkan, bagian bulge dan piringan menjadi penentu dalam klasifikasi galaksi yang dibuat oleh Hubble (diagram garpu tala).

Bagian bulge adalah daerah di galaksi yang kepadatan bintangnya paling tinggi. Bintang-bintang tua lebih banyak ditemukan daripada bintang muda, karena sangat sedikit materi pembentuk bintang yang terdapat di sini. Bulge ini berbentuk elipsoid seperti bola rugby. Bintang-bintang di dalamnya bergerak dengan kecepatan tinggi dan orbit yang acak, tidak sebidang dengan bidang galaksi. Dari perhitungan kecepatan orbit bintang-bintang di dalamnya, diperoleh kesimpulan bahwa terdapat sebuah benda bermassa sangat besar yang berada di pusat Galaksi yang jauh lebih besar daripada perkiraan sebelumnya. Benda tersebut diyakini adalah sebuah lubang hitam supermasif, yang diperkirakan terdapat di bagian pusat semua galaksi spiral. Termasuk juga di galaksi Andromeda, galaksi spiral terdekat dari Galaksi kita.

Komponen kedua adalah halo. Berbentuk bola, ukuran komponen ini sangat besar hingga jauh membentang melingkupi bulge dan piringan, bahkan mungkin lebih jauh daripada batas terluar piringan galaksi yang bisa kita amati. Objek yang menjadi penyusun halo dibagi menjadi dua kelompok, yaitu stellar halo dan dark halo. Yang dimaksud dengan stellar halo adalah bintang-bintang yang berada di bagian halo. Namun hanya sedikit ditemukan bintang individu di bagian ini. Yang lebih dominan adalah kelompok bintang-bintang tua yang jumlah bintang anggotanya mencapai jutaan buah, yang disebut dengan gugus bola (globular cluster).

Di bagian piringan terdapat bintang-bintang muda serta gas dan debu antar bintang yang terletak di lengan spiral. Banyak ditemukannya bintang muda dan gas antar bintang sangat berkaitan erat, karena gas adalah materi utama pembentuk bintang. Di beberapa lokasi bahkan ditemukan bintang-bintang muda yang masih diselimuti gas, yang menandakan bahwa bintang-bintang tersebut baru terbentuk. Sedangkan banyaknya debu di piringan membuat pengamat di Bumi kesulitan untuk melakukan pengamatan visual di sekitar bidang Galaksi, terutama ke arah pusat Galaksi (lihat gambar di atas). Karenanya, pengamatan di sekitar bidang Galaksi akan memberikan hasil yang lebih baik jika dilakukan di daerah panjang gelombang radio dan infra merah yang tidak terpengaruh oleh debu antar bintang (lihat gambar di bawah).

Seberapa besar Galaksi kita? Di bagian pusat Galaksi, bulge hanya memiliki diameter 6 kpc dan tebal 4 kpc (kpc = kiloparsek, 1 parsek = 3,26 tahun cahaya = 206265 SA = 3,086 x 10^13 km). Jarak dari pusat hingga ke bagian tepi Galaksi (jari-jari) adalah 15 kpc dengan ketebalan rata-rata sebesar 300 pc. Sedangkan Matahari berada pada jarak 8 kpc dari pusat. Di posisi itu, Matahari sedang bergerak mengelilingi pusat Galaksi dengan bentuk orbit yang hampir melingkar. Laju orbitnya adalah sekitar 250 km/detik sehingga matahari memerlukan waktu 220 juta tahun untuk berkeliling satu kali. Jika umur matahari adalah 4,6 milyar tahun, berarti tata surya kita sudah mengorbit pusat Galaksi sebanyak 20 kali.

Galaksi kita sebenarnya berada pada sebuah kelompok galaksi yang disebut dengan Grup Lokal, yang ukurannya mencapai 1 MPc dan beranggotakan lebih dari 30 galaksi. Galaksi spiral yang ada di kelompok ini hanya tiga, yaitu Bimasakti, Andromeda, dan Triangulum. Sisanya adalah galaksi yang lebih kecil dengan bentuk elips atau tak beraturan. Grup Lokal ini termasuk kelompok galaksi yang dinamis. Maksudnya adalah bahwa galaksi-galaksi di kelompok ini mengalami interaksi gravitasi, termasuk Galaksi kita dengan galaksi Andromeda. Interaksi tersebut diperkirakan akan mengakibatkan terjadinya tabrakan antara Galaksi kita dengan Andromeda dan kemudian membentuk galaksi elips. Namun jangan terlalu khawatir karena peristiwa tersebut tidak akan terjadi hingga 2 milyar tahun lagi.

The Milkyway

The Milky Way Galaxy

A spiral galaxy, type Sbc, centered in Sagittarius

The Milky Way is the galaxy which is the home of our Solar System together with at least 200 billion other stars (more recent estimates have given numbers around 400 billion) and their planets, and thousands of clusters and nebulae, including at least almost all objects of Messier's catalog which are not galaxies on their own (one might consider two globular clusters as possible exceptions, as probably they are just being, or have recently been, incorporated or imported into our Galaxy from dwarf galaxies which are currently in close encounters with the Milky Way: M54 from SagDEG, and possibly M79 from the Canis Major Dwarf). See our Messier Objects in the Milky Way page, where details are given for each object to which part of our Galaxy it is related. All the objects in the Milky Way Galaxy orbit their common center of mass, called the Galactic Center (see below).

As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses, and its diameter is about 100,000 light years. Radio astronomial investigations of the distribution of hydrogen clouds have revealed that the Milky Way is a spiral galaxy of Hubble type Sb or Sc. Therefore, our galaxy has both a pronounced disk component exhibiting a spiral structure, and a prominent nuclear reagion which is part of a notable bulge/halo component. Decade-long observations have brought up more and more evidence that the Milky Way may also have a bar structure (so that it would be type SB), so that it may look like M61 or M83, and is perhaps best classified as SABbc. Recent investigations have brought up support for the assumption that the Milky Way may even have a pronounced central bar like barred spiral galaxies M58, M91, M95, or M109, and thus be of Hubble type SBb or SBc.

# More on the structure of the Milky Way

The Milky Way Galaxy belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy M31) but perhaps the most massive member of this group. M31, at about 2.9 million light years, is the nearest large galaxy, but a number of faint galaxies are much closer: Many of the dwarf Local Group members are satellites or companions of the Milky Way. The two closest neighbors, both already mentioned, have only recently been discovered: The nearest of all, discovered in 2003, is an already almost disrupted dwarf galaxy, the Canis Major Dwarf, the nucleus of which is about 25,000 light-years away from us and about 45,000 light-years from the Galactic Center. Second comes SagDEG at about 88,000 light years from us and some 50,000 light years from the Galactic Center. These two dwarfs are currently in close encounters with our Galaxy and in sections of their orbits situated well within the volume ocupied by our Milky Way. They are followed in distance by the more conspicuous Large and Small Magellanic Cloud, at 179,000 and 210,000 light years, respectively.

The spiral arms of our Milky Way contain interstellar matter, diffuse nebulae, and young stars and open star clusters emerging from this matter. On the other hand, the bulge component consists of old stars and contains the globular star clusters; our galaxy has probably about 200 globulars, of which we know about 150. These globular clusters are strongly concentrated toward the Galactic Center: From their apparent distribution in the sky, Harlow Shapley has concluded that this center of the Milky Way lies at a considerable distance (which he overestimated by factors) in the direction of Sagittarius and not rather close to us, as had been thought previously.

Our solar system is thus situated within the outer regions of this galaxy, well within the disk and only about 20 light years "above" the equatorial symmetry plane (to the direction of the Galactic North Pole, see below), but about 28,000 light years from the Galactic Center. Therefore, the Milky Way shows up as luminous band spanning all around the sky along this symmetry plane, which is also called the "Galactic Equator". Its center lies in the direction of the constellation Sagittarius, but very close to the border of both neighbor constellations Scorpius and Ophiuchus. The distance of 28,000 light years has recently (1997) been confirmed by the data of ESA's astrometric satellite Hipparcos. Other investigations published consequently have disputed this value and propose a smaller value of some 25,000 light years, based on stellar dynamics; a recent investigation (McNamara et.al 2000, based on RR Lyrae variables) yields roughly 26,000 light years. These data, if of significance, wouldn't immediately effect values for distances of particular objects in the Milky Way or beyond.

The solar system is situated within a smaller spiral arm, called the Local or Orion Arm, which is merely connection between the inner and outer next more massive arms, the Sagittarius Arm and the Perseus Arm; see our Milky Way Spiral Structure page.

Similar to other galaxies, there occur supernovae in the Milky Way at irregular intervals of time. If they are not too heavily obscurred by interstellar matter, they can be, and have been seen as spectacular events from Earth. Unfortunately, none has yet appeared since the invention of the telescope (the last well observed supernova was studied by Johannes Kepler in 1604).

Milky Way pictures are wide-field exposures. Besides being attractive and often colorful, they are often suited to view the Milky Way objects (including nebulae and star clusters) in their celestial surroundings of field stars. Some fields include lots of Messier objects and thus included here:

Scutum, and map of the Milky Way Central Region, by Bill Keel of the University of Alabama

* Milky Way in Sagittarius, including portions of Scorpius and Ophiuchus

* Milky Way around M17, M18, and M24

Our image was obtained by David Malin of the Anglo-Australian Observatory, and shows the many Messier objects around the direction of the Galactic Center. It is copyrighted and may be used for private purpose only. For any other kind of use, including internet mirroring and storing on CD-ROM, please contact the Photo Permissions Department (photo at aaoepp.aao.gov.au) of the Anglo Australian Observatory.

# More information on this image by David Malin

# Old style AAT image

In order to obtain a picture of the whole Milky Way as it appears from Earth, one must either compose a mosaic of many photographs (optionally computer-processed), or create a drawing; fine examples may be accessed below:

* A good Milky Way photo mosaic

* Knut Lundmark's drawing of the Milky Way

In the infrared light, the structure of the Milky Way can be better investigated, as the obscurring dust clouds are of better transparency for long wavelength IR than for the visible light. The Cobe satellite has provided an infrared image of the Milky Way's central region.

The central region of the Milky Way, as those of many other galaxies, is more densely crouded with stars than the outer region, and contains a massive central object, Sagittarius A*.

Below we give some data for the Galactic Center (this and all following positions for epoch 2000.0):

Right ascension 17 : 45.6 (h : m)

Declination -28 : 56 (deg : m)

Distance 28 (kly)

The Galactic North Pole is at

Right ascension 12 : 51.4 (h : m)

Declination +27 : 07 (deg : m)

The coordinate data given here were extracted from the online coordinate calculator at Nasa/IPAC's Extragalactical Database (NED) (also available by telnet).

Our Sun, together with the whole Solar System, is orbiting the Galactic Center at the distance given, on a nearly circular orbit. We are moving at about 250 km/sec, and need about 220 million years to complete one orbit (so the Solar System has orbited the Galactic Center about 20 to 21 times since its formation about 4.6 billion years ago).

In addition to the overall Galactic Rotation, the solar system is moving between the neighboring stars (peculiar motion) at a velocity of about 20 km/s, to a direction called "Solar Apex," at the approximate position RA=18:01, Dec=+26 (2000.0); this motion has been discovered by William Herschel in 1783.

Considering the sense of rotation, the Galaxy, at the Sun's position, is rotating toward the direction of Right Ascension 21:12.0, Declination +48:19. This shows that it rotates "backward" in the Galactic coordinate system, i.e. the Galactic North Pole is actually a physical South Pole with respect to galactic rotation (defined by the direction of the angular momentum vector).