Stars and planets

Star formation:

• Matter and energy were not evenly spread in the early universe. These density differences caused differences in gravity, drawing matter together: the beginnings of galaxies.
• A galaxy contains a huge number of stars. Galaxies are spread over distances measured in thousands of light years; a single galaxy is 80,000-150,000 light years across.
• A galaxy begins as a large cloud of hydrogen gas called a nebula. The growing nebula develops clumps of gas; these become denser and denser, forming stars. The book places star formation some 5-6 billion years ago.

Light year: a measure of distance, not time. Light travels at about 3,00,000 km per second; the distance it covers in one year, 9.461 × 10¹² km, is one light year. The sun is on average 149,598,000 km from the earth, so its light takes about 8.311 minutes to reach us.

Formation of planets (three stages):

• Stars are local lumps of gas in a nebula. Gravity forms a core, and a huge rotating disc of gas and dust develops around it.
• The gas condenses, and the matter around the core forms small rounded objects, which stick together (cohesion) into planetesimals. Larger bodies form by collision, as gravity makes material stick.
• Finally, the many small planetesimals accrete into a few large bodies: the planets.

Example (new): the Moon is about 3,84,000 km away, so its light reaches us in roughly 3,84,000 ÷ 3,00,000, about 1.3 seconds: a 'light-second' distance, far less than a light year.