Читать книгу Exploring the Solar System - Peter Bond - Страница 37

The structure and evolution of Sun‐like stars are quite well understood (see Chapter 1). The Sun was formed some 5 billion years ago in a giant cloud of dust and gas, known as a molecular cloud, located in one of the spiral arms of the Milky Way galaxy. The cloud began to contract, possibly triggered by a shock wave from a nearby supernova explosion. Over tens of millions of years, the cloud continued to collapse under its own gravitation while pulling in material from its neighborhood.

As the gas cloud shrank to a fraction of its original size, and as the central density and pressure increased, a protostar began to form, surrounded by a rotating disk, or nebula. Jets and radiation from the growing star cleared away most of the gas in the surrounding cloud. As it shrank in size, it also began to rotate much faster. If this spin up process had continued for long enough, the fledgling star would have flown apart, but it was able to shed much of its angular momentum by accumulating (accreting) slow‐spinning material from the surrounding disk of gas and dust, and by ejecting fast‐spinning material through two powerful bipolar jets that formed perpendicular to the disk and were shaped by the local magnetic field.

When the core of the protostar reached a temperature of 10 million degrees Celsius, hydrogen fusion caused the Sun to begin to emit copious amounts of radiation. The remains of the accretion disk around the star eventually clumped together to form a planetary system. Today our star is approximately halfway through its life.