Monday 30 January 2012

10 Bahasa Yang Paling Banyak Digunakan Di Dunia


1. Bahasa Mandarin

Tak salah lagi, Bahasa Mandarin adalah bahasa yang paling banyak dituturkan orang di seluruh dunia. Jumlah penduduk di China/Tiongkok saat ini diperkirakan hampir mencapai 1,4 milyar juta jiwa. Dari jumlah ini, semuanya diwajibkan bertutur kata resmi dalam satu bahasa yaitu Bahasa Mandarin. Belum lagi, para imigran Tionghoa di berbagai penjuru dunia yang setia menggunakan bahasa Mandarin sebagai bahasa sehari-harinya.


Asal-usul kata Mandarin

Kata mandarin dalam bahasa Indonesia sendiri sepertinya diserap dari bahasa Inggris yang mendeskripsikan bahasa Cina juga sebagai bahasa Mandarin. Namun sebenarnya, kata Mandarin ini diserap bahasa Inggris dari bahasa Cina sendiri. Mandarin secara harfiah berasal dari sebutan orang asing kepada pembesar-pembesar Dinasti Qing di zaman dulu. Dinasti Qing adalah dinasti yang didirikan oleh suku Manchu, sehingga pembesar-pembesar kekaisaran biasanya disebut sebagai Mandaren (Hanzi: 滿大人) yang berarti Yang Mulia Manchu. Dari sini, bahasa yang digunakan oleh para pejabat Manchu waktu itu juga disebut sebagai bahasa Mandaren. Penulisannya berevolusi menjadi Mandarin di kemudian hari.

Jumlah penutur: sekitar 1,5 milyar jiwa.

Jenis aksara: Karakter Cina
Negara penutur: Cina dan komunitas keturunan Cina lainnya di seluruh dunia. Bahasa resmi PBB.
Untuk menyapa dalam bahasa Mandarin, ucapkan "Ni hao" (Nee HaOW)


2. Bahasa Inggris


Nah, ini dia bahasa paling populer di dunia sekaligus bahasa yang paling banyak diadopsi menjadi bahasa resmi di beberapa negara dan organisasi internasional. Tercatat ada 53 negara dan 10 organisasi internasional yang memakai bahasa Inggris sebagai bahasa resmi. Selain itu, hampir semua negara di dunia menerapkan Bahasa Inggris sebagai bahasa kedua setelah bahasa nasionalnya masing-masing.


Jumlah penutur: sekitar 500 juta jiwa

Jenis aksara: Latin
Negara penutur: Inggris Raya, AS, Afrika Selatan, Antigua & Barbuda, Australia, Bahama, Bangladesh, Barbados, Belize, Botswana, Brunei Darussalam, Dominika, Ethiopia, Eritrea, Fiji, Filipina, Gambia, Ghana, Grenada, Guyana, Hong Kong, India, Irlandia, Jamaika, Kamerun, Kanada, Kenya, Kiribati, Lesotho, Liberia, Malawi, Maladewa, Malta, Marshall Kepulauan, Maritius, Micronesia, Namibia, Nauru, Nigeria, Pakistan, Palau, Papua Nugini, Rwanda, Saint Kitts & Nevs, Saint Lucia, Saint Vincent & Grenada, Samoa, Selandia Baru, Seychelles, Sierra Leone, Singapura, Solomon Kepulauan, Somalia, Sri Lanka, Swaziland, Tanzania, Tonga, Trinidad & Tobago, Tuvalu, Uganda, Vanuatu, Zambia, Zimbabwe. Organisasi Internasional: PBB, Uni Eropa, Persemakmuran, CoE, NATO, NAFTA, OAS, OIC, PIF, UKUSA, dll

3. Bahasa Hindi


India memang unik! Bayangkan saja, negara ini punya penduduk terbanyak kedua di dunia, film-film Bollywood, Taj Mahal, Raja Asoka yang Agung, Mahatma Gandhi.... ehh, ternyata India tidak memiliki bahasa nasional resmi. Bahasa Hindi yang dituturkan oleh sebagian besar masyarakat India hanya diakui sebagai bahasa resmi sehari-hari bersama bahasa Inggris dan bukan sebagai bahasa nasional oleh konsitusinya.


Jumlah penutur: sekitar 497 juta

Jenis aksara: Devanagari
Negara penutur: India, AS (100.000 jiwa), Mauritius (685.170 jiwa), Afrika Selatan (890.292), Yaman (232.760 jiwa), Uganda (147.000 jiwa), Singapura (5.000 jiwa), Selandia Baru (20.000 jiwa), Jerman (30.000 jiwa), Fiji, Nepal, Suriname, Trinidad & Tobago, Guyana dan Uni Emirat Arab.
Untuk menyapa dalam bahasa Hindi, ucapkan “Namaste” (Nah-MAH-stay).


4. Bahasa Spanyol


Setelah Portugal menguasai jalur-jalur maritim di Asia, Afrika dan Amerika pada abad ke 15, gantian Spanyol yang mendominasi wilayah-wilayah tersebut sejak abad ke 16 dan 17. Akibatnya, banyak kebudayaan Spanyol yang tertancap kuat di sana. Saat ini, Bahasa Spanyol banyak dipakai sebagai bahasa resmi negara-negara di Amerika Tengah dan Selatan. Selain itu, bahasa Spanyol juga banyak dituturkan di beberapa negara bagian Amerika Serikat yang berbatasan dengan Meksiko. Bahkan beberapa kosa kata bahasa Inggris dipinjam dari bahasa Spanyol seperti tornado, bonanza, patio, quesadilla, Enchilada, dan taco grande supreme, dll.


Jumlah penutur: sekitar 400 juta

Jenis aksara: Latin
Negara penutur: Spanyol, Argentina, Bolivia, Chile, Dominican Republic, Ecuador, El Salvador, Guinea Katulistiwa, Guatemala, Honduras, Kolombia, Kosta Rika, Kuba, Mexico, Nikaragua, Panama, Paraguay, Peru, Puerto Rico, Uruguay, Venezuela
Untuk menyapa dalam bahasa Spanyol, ucapkan: “Hola”.

5. Bahasa Arab


Bahasa Arab adalah salah satu bahasa tertua di dunia dan merupakan bahasa yang digunakan dalam Al Qur’an. Bahasa Arab banyak meminjamkan kosa katanya ke sejumlah bahasa di Eropa utamanya bahasa Spanyol, Portugis dan Sisilia. Bahasa Arab, seperti juga bahasa Ibrani dan Persia memakai sistem penulisan aksara dari kanan ke kiri. Sejak tahun 1974, bahasa Arab digunakan sebagai salah satu bahasa resmi di PBB.


Jumlah penutur: sekitar 300 juta orang

Jenis aksara: Arab
Negara penutur: Arab Saudi, Aljazair, Bahrain, Chad, Komoro, Djibouti, Mesir, Eritrea, Irak, Israel, Yordania, Kuwait, Lebanon, Libya, Maroko, Niger, Oman, Palestina, Qatar, Somalia, Sudan, Syria, Tunisia, Uni Emirat Arab, Sahara Barat, Yaman, Mauritania, Senegal, Mali. Bahasa resmi PBB
Untuk menyapa dalam bahasa Arab, ucapkan; Assalammualaikum Wr Wb.

6. Bahasa Rusia


Mikhail Gorbachev, Vladimir Putin, Roman Abramovich, Anna Kournikova, dan si cantik Maria Sharapova adalah para penutur bahasa Rusia yang kita sudah kenal lewat berbagai pemberitaan media. Namun, selain mereka tentu saja ada 270-an juta orang lainnya yang menggunakan bahasa Rusia sebagai bahasa resmi.


Jumlah penutur: sekitar 278 juta jiwa

Jenis aksara: Cyrillic
Negara penutur: Rusia, Belarusia, Kazakhstan, Kyrgyzstan, Moldova. Bahasa resmi PBB
Untuk menyapa dalam bahasa Rusia, ucapkan: “Zdravstvuite” (ZDRAST-vet-yah).


7. Bahasa Melayu (termasuk Bahasa Indonesia)


Nah, ini dia yang ditunggu-tunggu... Bahasa Indonesia yang termasuk dalam rumpun bahasa Melayu ternyata berada pada urutan ke 7 dengan jumlah penutur sekitar 259 juta orang. Hitungan kasar ini didapatkan dari perkiraan jumlah penduduk Indonesia tahun 2009 yang mencapai 230 juta jiwa ditambah penduduk Malaysia 28 juta, penduduk Brunei 388 ribu serta sebagian kecil penduduk Thailand, Singapura dan Timor Timur. Jumlah ini mungkin bisa bertambah karena sejak tahun 2007 Bahasa Indonesia telah ditetapkan sebagai bahasa resmi kedua di Vietnam.


Fakta menarik tentang bahasa Indonesia:


Bahasa Indonesia menduduki peringkat 3 di Asia dan peringkat ke 26 di dunia dalam hal Tata bahasa terumit di dunia.

Bahasa Indonesia juga mendunia di dunia maya, buktinya wikipedia berbahasa Indonesia telah menduduki peringkat 26 dari 250 wikipedia berbahasa asing di dunia dan peringkat 3 di Asia setelah bahasa Jepang dan Mandarin, selain itu bahasa Indonesia menjadi bahasa ke 3 yang paling banyak digunakan dalam postingan blog di wordpress.

8. Bahasa Portugis


Portugal memang hanyalah sebuah negara yang kecil dan bisa dikatakan salah satu negara miskin di Eropa. Namun pada abad ke 15, Portugal merupakan bangsa yang besar karena merekalah yang pertama-tama melakukan penjelajahan maritim ke berbagai penjuru dunia. Berkat jasa para penjelajah seperti Vasco da Gama, Henry the Navigator, Afonso de Albuquerque dan Pedro Álvares Cabral, Portugal menguasai wilayah-wilayah penting di Asia, Afrika dan Amerika Selatan dan kemudian menanamkan pengaruh kebudayaannya di sana.


Jumlah penutur: sekitar 240 juta jiwa

Jenis Aksara: Latin
Negara Penutur: Portugal, Brazil, Angola, Cape Verde, Timor Timur, Guinea-Bissau, Makau, Mozambique, São Tomé e Príncipe.
Untuk menyapa dalam bahasa Portugis, ucapkan: “Bom dia” (Bohn DEE-ah).

9. Bahasa Bengali


Bangladesh adalah sebuah negara dengan wilayah geografis yang kecil, tapi jumlah penduduknya mencapai 162 juta orang. Bahasa resmi Bangladesh adalah Bahasa Bengali. Karena wilayah Bangladesh hampir seluruhnya berbatasan dengan India maka jumlah penutur bahasa Bengali meluas hingga ke beberapa wilayah India.


Jumlah penutur: Sekitar 230 juta jiwa

Jenis Aksara: Bengali
Negara Penutur: Bangladesh, India
Untuk menyapa dalam bahasa Bengali, katakan “Ei Je” (EYE-jay).

10. Bahasa Perancis


Bahasa yang sering disebut-sebut sebagai bahasa paling romantis di dunia ini selain digunakan di Perancis juga menjadi bahasa resmi di beberapa negara yang pernah dijajahnya. Bahasa Perancis juga adalah salah satu bahasa resmi di Perserikatan Bangsa-bangsa (PBB) selain bahasa Inggris, Mandarin, Rusia, Spanyol dan Arab.


Jumlah penutur: sekitar 200 juta jiwa

Jenis Aksara: Latin
Negara Penutur: Perancis, Monaco, Kanada, Swiss, Belgia, Luxemburg, Benin, Burkina Faso, Burundi, Kamerun, Afrika Tengah, Chad, Komoro, Kongo/Zaire, Pantai Gading, Djibouti, Guinea, Guinea Katulistiwa, Gabon, Guernsey, Madagaskar, Mali, Mauritius, Niger, Rwanda, Senegal, Seychelles, Togo, Haiti, Lebanon, Kaledonia Baru, Vanuatu. Polynesia, Martinique, Guadalupe. Bahasa resmi PBB.
Untuk menyapa dalam bahasa Perancis, ucapkan: "Bonjour" (bone-JOOR).

Sumber : Kaskus.us

Facebook Akan Lepas Saham Pekan Depan


JAKARTA-Laman jejaring sosial Facebook segera akan menjadi milik publik para pemilik saham karena pekan depan Facebook dijadwalkan untuk menjual saham-sahamnya.

Kabar penjualan saham Facebook ini diwartakan oleh berbagai media asing seperti koran Financial Times dan BBC.

IPO (initial public offering) alias pengenalan perdana saham Facebook akan dimulai pada pekan dan bakal benar-benar mulai melantai di bursa Wall Street pada Mei tahun ini.

Pada masa IPO, Facebook diperkirakan bakal menyerap dana hingga 10 miliar dolar Amerika hingga akhir tahun 2012, demikian dilaporkan oleh media-media asing tersebut.

Jika benar saham Facebook dijual kepada publik, maka boleh jadi ini adalah penjualan terbesar dalam sejarah bursa saham di Wall Street.

Angka IPO saham Facebook akan melampaui penjualan saham Google pada tahun 2004 yang hanya sekitar 1,9 miliar dolar Amerika.
Tapi nilai saham Facebook masih di bawah saham pabrikan mobil General Motors yang dijual pada November 2010 sekitar 20 miliar lebih tinggi.

Perusahaan Facebook sendiri diperkirakan memiliki nilai sekitar 75-100 miliar dolar Amerika atau setara dengan Rp667,5-890triliun (dengan kurs tukar 1 dolar = Rp8.900)

Perkiraan "harga Facebook" ini memang fantastis, tapi raja media massa Rupert Murdoch meragukannya dengan berkicau di Twitter "Facebook a brilliant achievement, but $75-$100bn? Would make Apple look really cheap" (Facebook adalah pencapaian yang brilian, tapi 75-100 miliar dolar? Itu akan membuat kesan Apple sangat murah).

Facebook bermula ketika Mark Zuckerberg dan kawan-kawannya di Universitas Harvard pada tahun 2004 membuat situs pertemanan lokal "penduduk" Harvard.

Namun dengan cepat situs pertemanan ini menjadi sangat populer dan menjadi salah satu situs paling sering dikunjungi di dunia.Pendapatan terbesar dari Facebook adalah dari iklan yang ditampilkannya.

Menurut Financial Times, pendapatan Facebook dari iklan selama tahun 2011 mencapai 3-6 miliar dolar Amerika.

Sumber : Republika.co.id

Trik Agar Toilet Bersih & Kering


Toilet menjadi salah satu indikator kebersihan seseorang atau sebuah keluarga. Jika ingin mengetahui orang tertentu bersih atau jorok, tengok saja bagaimana toilet di rumahnya. Nah, bagaimana toilet di rumah Anda?
Menurut Ketua Asosiasi Toilet Indonesia, Naning Adiwoso, ada tiga hal yang perlu diperhatikan dari sebuah toilet.
Semuanya berawal dari desain. Ada beberapa hal yang harus diperhatikan ketika mendesain toilet di rumah. Ukuran minimal toilet rumah, yaitu 1,2 meter x 2 meter. "Ukuran ini membuat orang di dalamnya bergerak bebas. Sikut kedua tangan tidak terbentur ke mana-mana," ujar lulusan Studi Interior Architecture di Pratt Institue New York ini.

Pintu kamar mandi harus bisa dibuka penuh. Hindari membuka pintu terbentur bak atau ember sehingga tidak leluasa bergerak. Untuk menghindari terjadi sesuatu di dalam toilet-orang yang masuk pingsan atau sakit-sebaiknya pintu terbuka keluar.

Jarak antara antara toilet dan westafel tidak boleh terlalu dekat, sekitar satu meter. Toilet pun harus memiliki ventilasi sehingga udara tak terperangkap di dalam toilet. Ventilasi harus diarahkan dari toilet ke luar ke ruangan, bukan ke dalam ruangan lain. Bila toilet berada di bagian tengah rumah, banyak orang memasang exhaust fan untuk mengeluarkan udara di dalam toilet.

Toilet perlu pencahayaan yang baik. Yang sangat penting harus cukup air bersih. Air tersebut baik untuk menggelontor pembuangan maupun mencuci tangan.
Di pasaran ada toilet duduk dan jongkok. Pilihan ini disesuaikan dengan kebiasaan si pemakai. Toilet jongkok lebih baik karena lebih bebas. Akan tetapi, bagi yang sudah sepuh kesulitan untuk bangun. Dengan demikian, pada dinding perlu dipasang pegangan dari baja antikarat. Namun, ada juga yang lebih nyaman menggunakan toilet duduk.

Untuk pintu dan kaca, pilihlah yang berkualitas. Kualitas akan memberi kepuasan. Misalnya, pintu tak mudah jebol atau dimakan rayap dan kaca tak mudah pecah.

Keramik dinding, pilihlah yang berpermukaan halus. Dengan begitu, dinding akan mudah dibersihkan. Sementara untuk lantainya, pilihlah yang memiliki permukaan kasar sehingga tak mudah terpeleset. Hindari pemilihan warna suram untuk menghindari kesan kotor dan gelap.

Jangan abaikan kemiringan lantai, minimum dua persen. Tujuannya agar air mudah mengalir ke pembuangan sehingga tak membuat genangan.

"Toilet yang bersih adalah yang kering dan sehat," tutur Naning. Apalagi, Indonesia sebagai negara tropis kondisi toilet harus benar-benar kering. Genangan air sedikit saja membuat toilet menjadi lembap, mengundang dan menyebarkan bibit penyakit.

Naning menambahkan, kesalahan membersihkan toilet dengan menuangkan karbol. Bahan ini tidak hanya baunya menyengat, tetapi menyebabkan bakteri yang ada di septic tank ikut mati. Hal ini dapat juga merusak air tanah.

Letakkan pewangi atau tanaman ruang di sudut. Toilet akan memberikan suasana segar.
Perlu diperhatikan bahwa toilet yang bersih belum tentu higienis. Ketika disiram air, dibersihkan dengan sabun, atau pembersih biasa, kotorannya hilang toilet pun menjadi bersih. Namun, lanjut Nanang, belum tentu kuman-kumannya mati semua. Sebab, banyak kuman yang bersumber dari kamar mandi tidak mati sekadar disiram air atau sabun biasa.

Idealnya membersihkan toilet minimal sehari sekali, namun bisa juga tiga hingga empat kali agar kuman tidak berkembang biak. Untuk pembersihnya, ujar Nanang, pilih yang mengandung sodium hypochlorite, yang efektif membunuh kuman dan bibit penyakit. Bahan ini juga aman di tangan dan tidak merusak permukaan toilet-porselen lantai, keramik. Pilih pula pembersih yang mengandung pewangi alami. Suasana ini membuat nyaman selama melakukan aktivitas di toilet.

Selain toilet perhatikan pula kebersihan keran air, westafel, pegangan pintu, hingga sudut-sudut tersembunyi. Bisa juga menggunakan pengharum ruangan untuk mengusir bau tidak sedap di kamar mandi.

Berikut adalah langkah-langkah membersihkan toilet yang benar, yaitu:
- Gunakan sarung tangan, lalu tuangkan cairan pembersih di sekitar kloset.
- Tunggu beberapa bahan bekerja efektif.
- Sikat bagian kloset sampai merata.
- Siram dengan air.
- Tutup kloset agar tidak menyebarkan kuman penyakit.

Sumber : Republika.co.id

Makanan Yang Sanggup Membakar Lemak Tubuh Anda


Tidak diragukan lagi ada sejumlah makanan yang punya dampak termogenik (mempercepat metabolisme) sangat kuat. Bila menyantap makanan jenis ini, Anda benar-benar langsung membakar kalori ketika sedang mengunyahnya. Ada pula makanan yang mengandung nutrisi dan zat yang juga segera menggenjot kekuatan metabolisme. Berikut di antaranya:

Gandum
Tubuh Anda akan membakar dua kali dari kalori yang masuk dari aneka gandum itu. Ini terutama terjadi pada gandum yang kaya serat seperti nasi merah dan havermut ketimbang makanan yang sudah diproses.

Daging tanpa Lemak
Protein yang terkandung di dalamnya punya efek termogenik yang sangat tinggi. Anda langsung membakar sekitar 30 persen kalori dalam makanan tersebut ketika sedang dicerna. Jadi, dada ayam yang mengandung 300 kalori membutuhkan sekitar 90 kalori untuk mengurainya.

Aneka produk olahan susu
Selain kaya kalsium dan vitamin D, produk seperti ini juga membantu mempertahankan dan membentuk otot sehingga metabolisme dapat terus terjaga.

Teh hijau
Meminum empat cangkir teh hijau setiap hari membantu untuk menurunkan sekitar 2,7 kilogram dalam delapan pekan, demikian laporan dari American Journal of Clinical Nutrition. Ini karena ada kandungan dalam teh hijau yang langsung mempercepat metabolisme begitu Anda meminumnya. Saran penyajian, siapkan satu termos berisi teh hijau dingin dalam kulkas sehingga Anda dapat meningkatkan konsumsinya.


Cabai atau makanan pedas
Capsaicin, unsur pedas dalam cabai, mampu meningkatkan suhu tubuh. Dengan begitu, zat inilah yang dapat melelehkan kalori tambahan. Anda dapat menyantapnya dengan berbagai cara: dilalap, dimasak, dikeringkan, atau dalam bentuk bubuk. Sebaiknya, tambahkan juga saus pedas dalam beragam makanan Anda.
Sumber : Republika.co.id 

Friday 27 January 2012

Emergence of the Sun’s Family



The planets, their moons, the asteroids and the comets – all are part of the Sun’s family. And they are just as ancient as their parent. Evidence suggests that the Solar System’s contents started to form even while the Sun itself was still only a protostar, almost as soon as the Solar Nebula was in place.

We have seen that, in some ways, the Sun formed in much the same manner in which a sculpture is made. What began as a single, large block of material – the giant molecular cloud – was gradually whittled away to reveal a smaller end product. But the planets’ origins are more like those of buildings. They grew bit by bit, from the bottom up, by accumulating steadily larger building blocks. The very first process in the planet-building production line is a familiar concept known as condensation. You can see it in action when somebody wearing spectacles enters a warm room after being outside in the cold. As soon as air-borne water molecules hit the cold lens surfaces, the molecules cool down and stick to the lenses one at a time to produce a thin – and very annoying – film of tiny water droplets. Exactly the same phenomenon was big business in the very earliest stages of the Solar Nebula. As more and more material spiralled from the Solar Nebula into the newly forming Sun, the disc grew less dense. Eventually it became so sparse that its infrared energy could pass through with less hindrance. Thus the heat leaked away into space, the disc began to cool, and its material started to condense – single atoms or molecules grouping together one at a time until they had grown into tiny grains or droplets less than a millionth of a metre across.

But it was not condensation alone that produced the Sun’s family. Condensation is only an e¤icient growth mechanism when the grains or droplets involved are small, because matter is added one atom or molecule at a time. Eventually, as will become clear, the process was replaced by agglomeration and accretion – the building of progressively larger fragments through the accumulation of other fragments, not atoms.

The planet-building processes themselves are reasonably well understood. And yet, even after decades of research, astronomers can agree neither on the timescales involved in the various stages, nor on the sequence in which the events took place. It seems fairly certain that the gas-rich planets Jupiter and possibly Saturn formed very quickly – shortly it will become evident why. The rest, though, is more uncertain. And so what follows represents only one possible sequence in which the various elements of the Sun’s family came into being. This, the second part of our story, begins in the Solar Nebula, after the onset of condensation. Time elapsed since the fragmentation and collapse of the giant molecular cloud: 2 200 000 years.

2 200 000 years
Planetesimals and Protoplanets


The Solar Nebula was a rich soup of many different components. Gases such as hydrogen, helium, carbon and oxygen were common. Thus the disc brimmed with molecules – water, ammonia and methane – made from these available gases. Atoms of silicon – the basis of rock – were also abundant, along with metals. But these metals did not exist uniformly throughout the disc. Close to the protosun, where the temperature was around 2000 Celsius, only the very densest materials, such as iron, could condense. So the grains that grew there had a significant iron content. A bit further out, where it was cooler, silicate particles condensed into grains of rock. And at about 5 AU from the centre, the current location of the planet Jupiter, ices began to gather. Here, at what astronomers call the ‘snow line’, the Solar Nebula was a lot colder – maybe less than _70 Celsius. It was here and beyond where the water, ammonia and methane finally condensed out and froze to form ice crystals.

Thus, with the onset of condensation in the Solar Nebula, the protoplanetary disc soon began to resemble a vast, swirling storm of sand, iron filings and snow, whizzing around the central star at speeds of tens of kilometres per second. Collisions between adjacent particles were of course inevitable. And yet, for the most part, these interactions were fairly gentle, not violent. One way to imagine the scenario is to picture racing cars speeding around their circuit. Naturally the cars travel very fast – relative to the road and the cheering spectators. But, relative to each other, their speeds are much less reduced, hovering around the zero mark. Occasionally one of the cars will nudge up alongside and touch one of the others.

And so it was with the condensed particles in the Solar Nebula. Even though they were moving around so quickly, they were still able to jostle up alongside their neighbours fairly gently. When that happened, many of the particles stuck together, bonded perhaps by electrostatic forces. This is known as agglomeration. Thus, through this process, the first fragments grew steadily larger still. And the results were extremely rapid. Within just a few thousand years of its appearance, the Solar Nebula teamed not only with dust, but also with countless pebble-sized chunks of rubble – rocky and metallic close in, icy beyond the snow line. The planet construction line was underway.

Gradually, through increasing collisions, the great majority of these primordial fragments were deflected towards the mid-plane of the disc. With the fragments thus concentrated into a thinner plane, the rate of collision and agglomeration in the disc then escalated drastically. After only another 1000 years or so, the primordial pebbles had grown to dimensions of several kilometres forming mountain-sized ‘planetesimals’. This marked a turning point in planet construction.

Because of their dimensions, the planetesimals now grew not only by collisions with other fragments, but also by virtue of their own gravity. The larger the planetesimals became, the more matter they attracted. And so, only 10 000–100 000 years after the appearance of the Solar Nebula, the inner disc overflowed with innumerable bodies ranging in size up to that of the modern Moon. These bodies, quite justifiably, are known as ‘protoplanets’.

2–3 million years
Gas Giants and Asteroids



Not all of the protoplanets grew at the same rate. On the snow line, ices were about ten times more abundant than the silicates and metals closer in. Ices are also very adhesive: calculations have shown that they are 20 times stickier than silicates at comparable impact speeds. Thus, with such a wealth of condensed, gluey materials to work with beyond 5 AU, the agglomeration process operated extremely efficiently there. The end product was the first planet to form: Jupiter.

In less than 100 000 years, a protoplanet larger than the modern Earth appeared on the snow line, a gigantic ball of ice and rock. But its growth didn’t stop there, such was the amount of ice. Eventually this icy protoplanet became so large, maybe 15 Earth mases, that it began to suck in even lightweight materials – the gases, principally hydrogen and helium, that still form the greatest part of it today. In this way, the proto-Jupiter gorged itself for several hundred thousand years, after which time it had swept a clear path for itself in the disc. As the planet orbited the Sun, it sucked in gas from either side of the gap it had created, and gradually the reservoir that spawned it began to run dry. What finally stopped Jupiter’s growth in its tracks, though, was not a lack of raw material. It was the Sun. After Jupiter had been growing for about one million years, maybe less, the contracting Sun entered the T-Tauri phase. Its powerful wind surged through the Solar Nebula like a tsunami and blasted the unused gas away, deep into interstellar space. At last Jupiter’s growth was quenched. But by now it had hoarded more than 300 Earth masses. Unable to grow any larger, the giant planet – by now surrounded by its own gigantic disc of gas and dust, similar to the Solar Nebula itself but on a smaller scale – settled down and began to cool. This was about 3 million years down the planetproduction line, long before any of the other planets appeared, with the possible exception of Saturn.

This early appearance of Jupiter spelt trouble for those nearby planetesimals that had not been swept up in the planet’s formation. Those that passed close to Jupiter experienced a tug due to the planet’s gravity. Over time, some of these planetesimals developed chaotic orbits and were flung out of the disc. Those that remained, unable to group together because of the constant bullying of Jupiter’s gravity, survived until the present day in the guise of the asteroids. We shall learn more about these bodies in Part 3.

Saturn, a gas giant similar to Jupiter, came about in a similar manner. But, being twice as far from the Sun, its ice and rock core took longer to form in the relatively sparse surroundings. By the time the solar wind turned on and blasted away the unused gas, Saturn had not had enough time to grow as large as its cousin. A similar fate would meet the next two planets to form, several million years later: Uranus and Neptune.

3–10 million years
Ice Giants and Comets



By about 3 million years, Jupiter and Saturn had formed and were cooling down. But the protoplanetary disc was still very active. Closer to the Sun, the rocky planetesimals were continuing to gather. And much further from the Sun – twice as far out as Saturn is, and beyond – so too were the last of the icy planetesimals. Despite the abundance of ice there, it took longer for icy protoplanets to accrete to the dimensions where, like Jupiter and Saturn, they could pull in gas directly from the disc, because the orbital speeds there were slower. Eventually, though, two more dominant protoplanets of ice and rock did develop. These would become the outermost giants, Uranus and Neptune.

In time these kernels of rock and ice, each about as massive as the modern Earth, began to stockpile hydrogen and helium, just as the larger cores of the gas giants had done a couple of million years earlier. But they had arrived on the scene too late. The Sun was by now past its T-Tauri phase, and very little gas remained in the protoplanetary disc. For a few more million years Uranus and Neptune seized what little gas they could from the ever-diminishing supply, but their growth ceased after about 10 million years – the exact time remains uncertain. The end result was a pair of planets a little over one-third the diameter of Jupiter and only 5 per cent of its mass. And yet, despite their diminutive statures compared with Jupiter, Uranus and Neptune are each still heavier than 15 Earths. They were more than capable of joining in the game of cosmic billiards demonstrated earlier by Jupiter and Saturn. While Uranus and Neptune were still forming, those icy planetesimals that they could not sweep up were instead tossed away like toys that no longer pleased. Today, these fragments, known as comets, surround the Sun in two extensive reservoirs. One, the Kuiper belt, extends a little beyond the orbit of Neptune and is constrained largely to the plane of the Solar System; these fragments are also known as trans-Neptunian objects. Meanwhile, much, much further out, trillions more comets orbit the Sun in a gigantic spherical shell known as the Oort cloud, perhaps more than a light-year in diameter.

In some respects, Uranus and Neptune are like Jupiter and Saturn, but without those planets’ gaseous mantles of hydrogen and helium. And so, with a much smaller gas content compared with the proportion of icy substances such as water, methane and ammonia, Uranus and Neptune are not true gas giants. They are best referred to as the ice giants.

3–10 million years
Regular Satellites



While the four giant planets were forming, they were not doing it alone. As each of the giant protoplanets stole gas from the Solar Nebula, the material had swirled around the icy kernels to form gas discs like the Solar Nebula on a much smaller scale. Exactly as in the Solar Nebula itself, the particles in these discs had begun to lump together into larger building blocks – and new, independent worlds had started to appear in orbit around the planets. These would become the giant planets’ satellite systems – their moons. Because these moons formed from discs, like the planets, they now tend to orbit their planetary hosts in a thin plane, each in the same direction as the others and in fairly circular paths. Moons with these orbital characteristics also tend to be large. They are known as regular satellites.

It is probable that the regular satellites grew to maturity very quickly, even before their planets did. Why? Simply a question of scale. The discs that surrounded the newly emerging giant planets were much smaller than the Solar Nebula, so they had correspondingly shorter orbital timescales. Their rich cargoes of icy volatiles grew to protoplanet dimensions much more quickly than the planets did. But not all of the moons formed at the same time. The Jovian disc, right on the snow line, would have been the richest. So Jupiter’s regular satellites – Io, Europa, Ganymede and Callisto – no doubt formed first, alongside their planet, at T-plus 2–3 million years. These are known today as the Galilean moons, after their discoverer. The next moons to form were the seven or eight largest satellites of Saturn, followed by Uranus’ biggest five, and finishing with the moons of distant Neptune several million years after the appearance of the Galileans. Today, however, Neptune does not have a regular satellite system. It is possible, as we shall see later, that its original moons were destroyed when Neptune’s gravity netted a rogue protoplanet called Triton. This worldlet went into a retrograde, or backwards, orbit around Neptune and collided with or gravitationally ejected those moons already present. Triton remains today as Neptune’s only large satellite, though it is not regular because it did not accrete in a disc around that planet. Triton is a so-called irregular satellite, one of many found in orbit not only around Neptune, but also around all of the other giants. Triton aside, these irregular moons are mostly small lumps of ice and rock that were captured by the planets long after they had formed.

At last, with the giant planets, the regular satellites, the asteroids and the comets in place, the outer regions of the Solar System quietened down. Ten million years had passed. But there was a long way to go. Closer to the Sun, the planet-building factory was still in full swing. There, playing catch-up, the terrestrial planets were emerging.

10–100 million years
Terrestrial Planets



The terrestrial planets were latecomers. Because ices could not condense near the Sun, the materials (rock and metal) from which these planets coalesced were a lot less abundant than those that formed the giants further out. So, while the gas planets had formed within a million years – or at most a few million years – and the ice giants took maybe ten million years, for the terrestrials the formation process was even longer.

At least the initial growth of the terrestrial planets, within a few astronomical units of the Sun, had been very fast. Once the first rocky planetesimals had appeared, they had begun gravitationally to attract smaller bits of nearby debris. As we have seen, these first planetesimals grew to dimensions of hundreds or thousands of kilometres in less than 100 000 years. After about one million years the innermost regions of the Solar Nebula were populated by several large rocky and metallic protoplanets approaching the size of Mercury. And by 10 million years these protoplanets had grouped together through gravitation so that only four dominant spheres remained. These, at last, were the primitive terrestrial planets: from the Sun outwards, Mercury, Venus, Earth and Mars. But even after all four of the giants and their satellites had emerged, the terrestrial planets had grown to only half their eventual masses. And they had a very long way to go to make up that missing half – because the supply of available fragments in the disc was now much lower. Moreover, the terrestrial protoplanets had become large enough for the addition of more planetesimals to have a smaller and smaller effect on their size as they continued to accrete. Thus the growth of the terrestrial planets slowed very significantly.

Tens of millions of years after Neptune and Uranus had formed in the frigid, far reaches of the protoplanetary disc, even after the Sun had started on the main sequence, the terrestrials kept on growing, more and more slowly. In total, it took perhaps 100 million years for the terrestrial planets to mop up the debris, double their masses and swell to their present diameters. But because they never did grow large enough to pull in discs of gas from the Solar Nebula, not one of the terrestrial planets has any regular satellites. (We shall see, however, that Mars did capture two planetesimal moons, and that the Earth’s Moon is a specialcase.) Earth and Venus ended up with roughly equal masses, while Mars acquired only a little more than a tenth of that mass – later we shall see why. Meanwhile, we shall learn that Mercury might have started off with more mass, but lost much of its outermost regions in a gigantic collision with another protoplanet.

100–1300 million years
The Heavy Bombardment

 
The planets had finally finished growing. Now they would begin their long process of evolution towards the way we see them today. By now, about 100 million years had passed and the Solar Nebula was relatively sparse. Yet its activity did not stop completely. For the Solar System was still littered with fragments of debris that had not yet been ejected from the system by the giants or been swept up by the terrestrials. It was at this point that the Solar System entered what astronomers call, quite justifiably, the heavy bombardment phase.

For hundreds of millions of years, leftover scraps continued to rain down on the planets and their satellites. This is the battering that shaped the planets’ and moons’ crusts, and the majority of it occurred in the first 600 million years or so of their creation. A glance at the surface of the Moon gives ample reminder of this violent phase in the Solar System’s history. Many of the craters there are well over 100 kilometres across. One of them is about 12 kilometres deep and 2500 kilometres across – greater than half the Moon’s diameter. Called the Aitken basin, it is the largest known impact structure in the entire Solar System, carved out when the Moon was struck a glancing blow from a piece of rock and metal some 200 kilometres across. This constant barrage meant that the crusts of the terrestrial planets and moons oscillated between molten and solid states for many hundreds of millions of years. The heaviest elements sank to their centres, while the lighter substances, buoyed up, stayed near the surfaces. In this way the terrestrial planets and the satellites developed differential structures: in the planets, crusts and mantles of rock now surround molten cores of denser metal; and in the moons, the central cores are primarily rocky, with lightweight ices fashioning the mantles and crusts.

The early Solar System saw troubled times. But gradually, as more and more planetesimals collided with the planets and satellites, and were thus removed from the scene, the cratering rate began to drop. Some 1200 million years after the last of the planets had appeared, the craters were occurring perhaps 30 or 40 times less frequently than they had been 400 million years earlier. This point in history, about 3300 million years ago, marked the end of the heavy bombardment phase. The cratering did continue after this, but at a more or less constant although substantially reduced rate.

It was during the last few hundred million years of the heavy bombardment that the planets and satellites of the newly formed Solar System, after aeons of turmoil, began to develop their atmospheres.

700–1300 million years
Building the Atmospheres



Most if not all of the planets developed primitive atmospheres while they were still forming. The giants, as we saw, got their hydrogen–helium atmospheres by pulling in these gases from the Solar Nebula, and these have remained essentially unchanged since. Similarly, the terrestrial planets scooped thin veils of hydrogen and helium from the protoplanetary disc as they moved around within it. But these planets, having much punier gravitational pulls than their giant cousins far from the Sun, were unable to retain these lightweight, primitive skies. Slowly, they slipped away into space, their loss hastened along by the Sun’s solar wind.

Gradually, though, as the rate of impacts in the inner Solar System dropped after several hundred million years, the terrestrial planets started to cool. It was during these cooling stages that they developed their secondary atmospheres, via a process known as outgassing. These new skies came from the planets themselves. How? All rocks contain traces of compounds such as water or carbon dioxide that are chemically sealed within the mineral structure of the rock. When these rocks are heated sufficiently, those chemical bonds begin to sever and the trapped gases are released. The terrestrial planets were molten and extremely hot after they had first formed. And so, over hundreds of millions of years during the heavy bom-bardment phase, these hot balls of rock began to release their locked up vapours through volcanic fissures as they started to cool. Carbon dioxide, carbon monoxide, nitrogen, water vapour, and perhaps hydrogen sulphide were released in this way. In addition to outgassing, planetesimals from the orbit of Jupiter and beyond ventured regularly into the inner Solar System, thrown inwards by the mighty gravities of the giant planets far from the Sun. These comets and asteroids no doubt added a significant water content to the planets’ atmospheres – and in fact helped to seed the oceans on Earth.

The secondary atmospheres were in place within several hundred million years of the formation of the planets, while they were still sustaining heavy bombardment. As a result of that bombardment, lightweight Mars ultimately lost 99 per cent of its original secondary atmosphere, which was blasted away into space. And neither Mercury nor the Moon could retain their secondary atmospheres because they did not have sufficient gravity to hold on to even the slow-moving, heavy gases. Over time, all of the planets’ atmospheres have evolved. Today, Venus’ atmosphere is 100 times more substantial than Earth’s, which in turn is 100 times more substantial than that of Mars. But these are stories for Part 3.

4500 million years?
Formation of the Ring Systems



With the emergence and subsequent evolution of the planetary atmospheres, the Solar System was almost complete. Only two things remained to be added: the rings of the giant planets, and some of the smaller, irregular satellites. The irregular satellites were probably acquired early in the history of the Solar System, when the giant planets captured icy planetesimals from the thinning Solar Nebula. Some are no doubt of more recent origin. The origins of the rings, however, are more difficult to pin down.

The most famous ring system is Saturn’s. Consisting of countless boulder-sized, and smaller, icy chunks in individual orbits about the planet, the rings are exceedingly thin – with relative dimensions like those of a sheet of paper the size of a football pitch. But Saturn is not alone, because each of the other giant planets has similar accoutrements, albeit with different characteristics. Indeed, research has shown that no two systems are alike: they differ from each other in terms of diameter, brightness, and in the sizes and compositions of the particles th t constitute them. This is a clue to their formation. But the biggest hint is that most of the rings surround their planetary hosts inside their respective ‘Roche limits’. This is the distance from a given planet at which gravitational forces tear apart any body held together mostly by gravity. These clues could mean that the rings are the unassembled ruins of moons that strayed within this danger zone and got ripped to shreds, or the remains of comets that got too close and suffered a similar fate. Such a hypothesis neatly explains the differences in the rings: they depend on the constituents of the bodies that were destroyed in their making. Alternatively, the rings could be relics from the discs that surrounded the giant planets in the early Solar System, from which the regular moons formed. But this is unlikely. First, Saturn’s icy particles would have evaporated long ago while the planet was still a hot ball of gas. Second, computer simulations of particle orbits suggests that ring systems are unstable over long periods of time.

If these dynamical studies are correct, then the rings are of relatively recent origin – probably dating to less than 100 million years ago. But even this has its problems. How is it that we are alive at just the right time to witness the existence of not just one, but four ring systems, if they are all transient? The best answer is that the rings have existed for longer, but their particles are continually replaced by the break-ups of small moons and comets.

4660 million years
The Modern Solar System


At last we come to the present day. We have journeyed over 4 billion years in time to get to where we are now. But as we peer out into the depths of the Solar System that is our home, we can easily see the evidence of its formation. We see near-circular orbits, most of which lie in the same plane – a relic of the Solar Nebula. We see worlds with battered surfaces – the scars that betray the long and troubled period of meteoritic rain known as the heavy bombardment. And, because of the way the Solar System was made, we can now count five distinct zones within it.

The first zone lies within 1.7 AU of the Sun. This is home to the four terrestrial planets, Mercury, Venus, Earth and Mars. These are small worlds of rock and iron, forged from the hottest fires of the Solar Nebula. The expanse from about 2–3.3 AU marks the second zone, that of the asteroids. Some of these stony or metallic bodies have not been modified extensively in over 4 billion years, which means that they contain some of the most primitive materials in the Solar System. Zone three is much larger, the realm of the giants. Its innermost boundary is marked by the planet Jupiter, almost twice as far from the Sun as Mars is; its outermost boundary lies at Neptune, fully six times further from the Sun than even Jupiter. All of the giant planets are far bigger than the terrestrials, with compositions of ices and gases – and comparatively little rock. The fourth zone is the Kuiper belt of comets, or the so-called trans-Neptunian objects. This extends from roughly the orbit of Neptune to an unknown distance, but perhaps as far as 1000 AU. This icy wasteland is also home to the tiny worldlet known as Pluto, which we will meet in Part 3. The fifth and last zone in the Solar System is the largest by three orders of magnitude. It is the spherical shell of icy comets called the Oort cloud that surrounds the Sun at a distance that might even exceed 50 000 AU – a large fraction of a light-year. The comets in both the Oort cloud and the Kuiper belt owe their presence to the gravities of Neptune and Uranus.

And so we come to the known boundary of the Sun’s family. Somehow it is fitting that the phantom Oort cloud now surrounds the Sun on such a vast scale. It is similar in scale to that of the frigid globule of gas and dust from which everything in the Solar System sprang so long ago.

Source : 


Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.




Wednesday 25 January 2012

Genesis of the Sun and Solar Nebula


Thirty million to 50 million years. That’s all the time it took to form the star we call the Sun. This may sound like a long time, but let’s put it in perspective. Since the last dinosaurs walked the planet, enough time has passed for at least one and possibly two stars like the Sun to have formed, one after the other – utterly from scratch. The details of this miraculous creation are not exceptionally well understood, but astronomers at least have a good grounding in the basics. Perhaps ironically, one star’s birth starts at the other end of the line – when other stars die.

Generally speaking, stars make their exit in one of two ways. A low-mass star like the Sun eventually expands its outermost layers until the star becomes a gross, bloated caricature of itself: a red giant. Gradually, the star’s envelope expands outwards, all the time becoming thinner, until the dense core of the star is revealed. Such an object is known as a white dwarf. It is a tiny and, at first, white-hot object with a stellar mass – yet confined to live out the rest of its existence within the limits of a planet’s radius. The rest of the star meanwhile, the cast-o¤ atmosphere, grows larger and larger. Eventually it becomes nothing but a thin fog of gas spread over more than a light-year. This is the fate that awaits our Sun, as we shall see in detail in Part 4. By contrast, a heavier star dies much more spectacularly. It blows itself to smithereens in a star-shattering explosion called a supernova. The star’s gases are jettisoned into space where, again, they disperse. Whichever way a star finally meets its doom, much of its material has the same ultimate destiny: it is flung back into the galaxy. Over billions of years, these stellar remains accumulate and assemble themselves into the enormous clouds that astronomers refer to collectively as interstellar matter.

But that is not the end of the story. In fact, it is our starting point. For the Universe is the ultimate recycling machine. Starting around 4660 million years ago, from the ashes of dead stars, a new one eventually grew: a star known as the Sun.

Time zero
Giant Molecular Cloud


Before 4660 million years ago, our Solar System existed as little more than a cloud of raw materials. The Sun, the planets, trees, people, the AIDS virus – all came from this single, rarefied cloud of gas and dust particles. These patches of interstellar fog were as common billions of years ago as they are now. They are known as giant molecular clouds.

Orbiting the nucleus of a galaxy called the Milky Way, about twothirds of the way out from the centre, this ancient cloud from which the Solar System sprang was about 50–100 light-years across, similar in size to its modern cousins. And again, like today’s giant molecular clouds, it presumably contained enough material to outweigh millions of stars like the Sun. Most of its mass, about 73 per cent of it, was made up of molecular hydrogen, a gas in which the hydrogen atoms are glued together in twos to make simple molecules. The rest of the cloud’s material was in the form of helium, with traces of heavier elements such as carbon, nitrogen and oxygen, and particles of silicate materials – fragments that astronomers like to lump under the category of ‘dust’. With between a few thousand and a million gas molecules per cubic centimetre, the cloud would have been recognised as better than a first-class vacuum by today’s standards. And it was very cold, around _250 Celsius, barely hotter than interstellar space itself. Molecular hydrogen cannot survive at very much higher temperatures, because the energy shakes the molecules apart. So the cold kept the molecules intact. But the cloud was nevertheless in danger of destruction.

A molecular cloud is like an interstellar house of cards, forever on the verge of disintegration. A push, a pull, anything could have triggered this ancient cloud’s demise – and there are lots of potential triggers spread over 100 light-years of interstellar space. The cloud might have passed close to a massive star whose gravitational tug stirred up the molecules within the nebula. Or the cloud could perhaps have drifted within close range of a supernova explosion, the shockwaves from the dying star burrowing into the cosmic smog and compressing its gases. It would have taken only one such event to collapse the house of cards, to make the cloud fall in on itself under gravity.

Something like this must have happened to our ancient molecular cloud about 4660 million years ago. It was the first step in the process that would eventually lead to the formation of a certain star.

2 000 000 years
Solar Globule


Once the collapse of the giant molecular cloud had started, it continued under its own momentum. By the time two million years had passed, a multitude of nuclei had developed in the cloud, regions where the density was higher than average. These concentrations began to pull in more gas from their surroundings by virtue of their stronger gravity, and the original cloud fragmented into hundreds or even thousands of small, dense cores. Most of them would later form stars. One of them was destined to become the Sun.

By now, the cloud core from which the Sun would form was perhaps a tenth of a light-year across, more than a hundred times the present size of the Solar System out to Pluto. Gradually, this tight clump of gas continued to fall in on itself like a slow-motion demolished chimney stack, a process known as gravitational freefall. The innermost regions fell the fastest; they were closest to the central condensation where the gravitational pull was greatest. The outermost edges of the cloud core took longer to succumb to their inevitable fall. Thus, because of these differences in infall rates, the cloud’s contraction essentially amounted to an implosion, an explosion in reverse. In time, as the gas closest to the centre plunged inward and accelerated, the material there grew steadily hotter, the atoms and molecules within it rubbing against each other frantically. After perhaps millions of years in a deep freeze, the molecular cloud was finally warming up. The
eventual result was a gas and dust cocoon: a shell of dark material surrounding a denser, warmer core. Such an object is known as a globule. It was the Sun’s incubator.

As with all globules, the solar globule was dark. It emitted no light at all. But a bit later in its evolution, as it gradually warmed, it was a strong emitter of heat radiation or infrared. Only an infrared telescope, and possibly a radio telescope, would have been able to penetrate the gas and dust and home in on the low-energy radiation coming from the globule’s gently warming core, and see the first, feeble stirrings of the yellow star that the globule would one day become.

2 030 000 years
Protosun


Over tens of thousands of years, the gases inside the globule continued to fall away from the inside edge of the cocoon, pulled inexorably towards that dense core at the centre. By now, the core of the globule was taking on a definite shape – a gargantuan ball, about the size of the present-day Solar System out to Pluto. Its surface was still too cold to glow optically. But, at last, its central regions had warmed up significantly – to about 10 000 Celsius – and the molecules there had split into atoms of hydrogen.

This marked an important point in the development of the Sun. At this temperature, the cloud core was now hot enough for the radiation it emitted to carry a significant punch. Radiation is composed of tiny packets of energy called photons, each of which can be likened to a subatomic particle. If there are enough of these photons emitted every second they can hit like a hail of bullets, a barrage of electromagnetic force known as radiation pressure. Before this point the core of the globule had been emitting too few photons to exert a noticeable force. Now, though, as the growing waves of radiation streamed away from the warming core they slammed into the outermost regions of the globule where the gases were less dense, and slightly hindered their inbound journey. Thus the contraction of the core slowed, but it did not stop, so overwhelming was the inward pull of gravity. The very centre of the core was also dense enough now that it was beginning to become opaque to the heat radiation generated inside it. The energy could no longer escape so easily, so from here on the nucleus heated up much faster as it shrank. The build-up of heat thus slowed the contraction ever more, and the core grew at a much slower pace. It had reached a configuration that astronomers ennoble with the term ‘protostar’.

By this time, the protostar – ‘protosun’ in this case – had developed a marked rotation. Just as water being sucked down a plug hole spirals around before it falls in, so the gases that had fallen into the protosun had begun to swirl about. And in the same way that a yo-yo spun around on its string spins faster as the string winds around a finger – owing to a concept known as the conservation of angular momentum – so the infalling gases had increased their angular speed as their long journey inwards had progressed. As the protosun grew smaller and hotter, therefore, it began to spin faster and faster.

2 130 000 years
Solar Nebula


The protosun’s collapse continued. Within 100 000 years or so it had become a swollen semi-spherical mass, flattened at the poles by rotation. Its surface temperature of the order of a few thousand degrees, the protosun was at last glowing visibly for the first time. And its diameter was by now roughly equal to that of the present orbit of Mercury – about 100 million kilometres. But the newly forming star was no longer alone. Over the aeons the rapid rotation of the infalling matter had flattened out the gases like a pizza dough spun in the air. Now, a huge pancake of turbulent, swirling gas and dust surrounded the protosun right down to its surface. Thinner near the centre, flared vertically at the edges, this structure is
known as the Solar Nebula.

The Solar Nebula measured about 100 to 200 astronomical units (AU) across, where 1 AU is defined as the current distance from the Earth to the Sun, 150 million kilometres. The disc would have contained about 1–10 per cent of the current mass of the Sun – most of it in the form of gas, with about 0.1 per cent of a solar mass locked up inside particles of dust. Near the centre of the disc, close to the seething protosun, the temperature may have exceeded 2000 Celsius. Here, where things were hot and important, the disc may have been hot enough to emit its own visible radiation – in any case it would have shined optically by virtue of the light it reflected from the protosun. Further out in the disc the temperature dropped rapidly with distance, though, and it would have shone only in the infrared. At about 5 AU, the current location of the planet Jupiter, the temperature dipped below _70 Celsius. And on the outside edges, where the material was more rarefied and the disc vertically flared, it was even colder. This
vast reservoir of material was the raw substance out of which the planets would soon begin to condense, as will become evident in Part 2. It is thus known as a protoplanetary disc, or proplyd for short.

By now, much of the original globule had been consumed. Most of it had fallen into the protosun, and the rest into the disc. At last, with the globule eaten away, the newly forming star was revealed to the exterior cosmos for the first time, as it prepared itself for the next – and most violent – stage in its formation: the T-Tauri phase.

3 million years
T-Tauri Phase


By 3 million years or thereabouts – about 1 million years after the initial collapse of the globule – the protosun had shrunk to a few solar radii. Its temperature at the centre was now around 5 million degrees Celsius, while the surface seethed and bubbled at around 4500 Celsius. At last the object had crossed the line that separates protostars from true stellar objects. It joined the ranks as what astronomers call a T-Tauri star.
Named after a prototypical young stellar object in the constellation Taurus, the T-Tauri phase is one of extreme fury. And as with all T-Tauri stars, this earliest form of solar activity would have been driven – at least in part – by a powerful magnetic field. Because the gases inside the young star were by now fully ionised – a soup of positively and negatively charged elements – their movement as the star rotated effectively amounted to a series of gigantic electric currents. Thus the spinning star developed a global magnetic field in the same way that a wire carrying an electric current does – just as the Sun generates its field even today. During the Sun’s T-Tauri phase, though, the star would have been spinning very quickly – once in 8 days compared with once in 30 days – spun up by the swirling
gases that had ploughed into it earlier. This means that the T-Tauri Sun’s magnetic field was much mightier than at present, and this is what made this phase in the Sun’s formation so violent. The Sun was still surrounded by its protoplanetary disc. So, as the Sun whirled around, it dragged its magnetic field through this disc. Where the field and disc connected, vast globs of gas were wrenched out of the surface of the disc and sucked along the field lines, right into the young Sun. And where these packets of gas hit, the troubled star responded with the violent flares that are the hallmarks of the T-Tauri phase of star formation.

Thus the adolescent Sun was very much more violent than the star we know today. It looked the part too. Its larger, cooler surface meant it glowed an angry red, not a soft yellow. And the sunspots that dotted the solar surface then were very much larger than their modern counterparts. Sunspots are generated when the Sun’s rotation tangles its magnetic field and creates localised regions of enhanced magnetic field strength. Where these entanglements are greatest, the increased magnetism hinders the flow of gases on the surface and cools those regions down – and they appear as dark patches. Today, the Sun’s spots cover less than 1 per cent of its surface. But the T-Tauri Sun would have had sunspot ‘continents’ covering great stretches of its bloated face.

Perhaps the most awesome aspect of the T-Tauri phase, however, was the molecular outflow. This would come next.

3 million years
Outflow and Post-T-Tauri Phase


Almost as soon as the Sun hit the T-Tauri stage – possibly even slightly before – it developed what astronomers call a stellar wind. The modern Sun also has one: a sea of charged particles that streams away from the solar surface, out into the depths of the Solar System. But T-Tauri winds are much more furious and contain more mass, moving at speeds of up to 200 kilometres per second.

How T-Tauri winds are generated is still very poorly understood. A possible cause is again the rapid rotation. Some of the gas dragged out of the Solar Nebula disc would have plummeted towards the star’s surface. But not all of it. Because the Sun was by now spinning very quickly, some of the gas pulled out of the disc plane was hurled radially outwards, much as water is spun out of the wet clothes in a spin dryer. The result was a steady flow of gas away from the star’s surface. However the Sun’s T-Tauri wind arose, its effects would have been quite dramatic. As the wind blasted away from the young Sun’s surface it banged into the disc and was deflected through a sharp angle out of the disc’s orbital plane. The disc might have been threaded by magnetic field lines as well, and these would have channelled the flowing gas further away from the disc and ‘up’ and ‘down’ into space. The result was a ‘beam’ of charged particles blustering away from the young Sun in two opposed directions, perpendicular to the protoplanetary disc. Astronomers call this a bipolar molecular outflow. By now the Sun had stopped amassing material and in fact would lose a significant fraction of its original mass, throughout the life of the wind, via the outflow.

By the time the wind had ceased, a fleeting 10 000 years since it had started, the Sun’s mass had begun to stabilise. However, it continued to shrink under gravity because the pressure at its core, though great, was not yet adequate to stop the contraction. All the time the Sun was slowly contracting it was also gradually approaching its modern temperature and luminosity. This was by far the slowest period in the Sun’s formation. Even as the Sun’s violence ended and it entered the relative calm of the post- T-Tauri phase, a couple of million years after it had started, the Sun still had tens of millions of years to go before reaching full maturity.

30–50 million years
The Main Sequence


At last, after a period of perhaps 30 to 50 million years – astronomers still cannot agree on their numbers – the Sun’s contraction finally came to an end. Why? Because the Sun’s internal temperature had reached an all-time high of 15 000 000 Celsius – and something had begun to happen to its supply of hydrogen.

Hydrogen is the simplest of all elements. Each atom contains just a single subatomic particle called a proton in its nucleus, positively charged. Orbiting this, meanwhile, is a single much smaller particle with exactly the opposite electric charge: an electron. Inside the Sun, these atoms are ionised: the electrons are detached and roam freely in the sea of hydrogen nuclei or protons. Very often, two of these hydrogen nuclei come together. Just as two magnetic poles of like polarity repel each other, so too do two protons. But not if they are brought together with sufficient speed. The speed of particles in a gas can be measured by the gas’s temperature. And at 15 000 000 Celsius, the positively charged hydrogen nuclei at the Sun’s core were now moving so quickly that when they smashed together they overcame their electrostatic repulsion, and fused as stronger nuclear forces took over. At last, the hydrogen was being consumed, gradually converted
into helium in the Sun’s core via a chain of nuclear reactions. Energy is a by-product of these reactions. And so the Sun now began to generate a significant amount of power in its core. The pressure of this virgin radiation was so intense that for the first time since the original gas cloud had started to contract, tens of millions of years earlier, the force of gravity had finally met its match. Exactly balanced against further contraction, and slowly metamorphosing hydrogen into helium in its core, the Sun at last got its first taste of the so-called main sequence. It had become a stable star, in a state that astronomers call hydrostatic equilibrium.

This, the ignition of core hydrogen, was the point at which the Sun as we know it was truly born. Called the main sequence, or hydrogen-burning phase, this is by far the longest-lived stage in the life of a star. It took tens of millions of years for the Sun to get to this point – yet so far it has existed for about 100 times longer than that with little change. About 4600 million years later, it is not quite halfway through its main-sequence journey. It still has a long life ahead.