Tuesday 25 September 2012

So, as I understand things...

There's this great big cloud of dust and gas. We're talking big, here. The dust and gas swirls around, and sometimes accumulates in dense areas. If there is enough matter in one place, gravitational forces generate enough pressure to start a nuclear reaction - and a star is born.

Stars have a limited shelf life, and will die when all the hydrogen at their core is depleted. Depending on the size of the star, various things happen. If a star is big enough, there will be a rather large explosion - a supernova. Supernovas occur around once a century in our galaxy (the milky way).

In the early universe, only the elements hydrogen and helium existed. When stars die, and their hydrogen supply runs out, new elements are formed under the intense pressure and heat. When stars go supernova, these elements are ejected out into space. Every physical thing we can see, know or imagine is made up of these elements, including every atom in our body. We truly are star dust.

Originally the gas and dust that would become our Sun was the core of a cloud much larger than the solar system, probably several light-years across. The core was slowly rotating at first, but as it collapsed it spun faster, like a spinning ice-skater pulling in her arms. The rotation prevented the material at the core's equator from collapsing as fast as material at the poles, so the core became a spinning disk.

Gas and dust in the disk spiraled gradually in to the center, where it accumulated to form the Sun. But because dust is denser than gas, some of the dust settled to the mid-plane of the disk. These dust particles stuck together to make clumps, then clumps stuck together to make rocks, then rocks collided to make planets. In the case of the "gas giant" planets, Jupiter, Saturn, Uranus and Neptune, the rocky cores were massive enough to also attract some of the gas. The outer layers of these planets are made up of hydrogen and other gases.

So the Sun is the collapsed core of an interstellar gas cloud, and the planets, asteroids and comets are small lumps of dust or ice chunks which stayed in orbit instead of spiraling into the Sun. The planets all formed within a very short period, probably a few million years, about five billion years ago.

The sun comprises around 98% of all the matter in the solar system. It is 875,000 miles in diameter, and so distant that light takes around eight minutes and twenty seconds to travel the 93 million miles which lie between us. The next nearest star to us is the faint, red dwarf star called Proxima Centauri. This star is only 4.2 light-years away. 4.2 light years? That's nothing, surely. Only about 266,000 times further away than the sun. The farthest object we know of in the universe is estimated to be (wait for it) 30,000,000,000 light years away. That's thirty billion light years. So if we shine a torch in that direction, 30 billion years later the light will get there. Phew.

Our galaxy, the milky way, contains an estimated 200 to 400 billion stars. There are an estimated 100 to 200 billion galaxies in the known universe. How many stars and potential planets in total? You do the sums, my head hurts... It does put things into perspective :)


I don't have any lessons until 11:45, so I pop up to see Gary about Tina's bike, and it's good news - he should be able to get the one I want before the week-end.

Tim and Peggy pop round for a cup of tea and we have a pleasant chat. When I go to my lesson, the learner isn't there. I send them a text message, and come home. I'm able to finally sort out a photo project for Lesley. Last week or the week before I asked her if she needed anything for her new flat, and she said a photo of the family would be good. I got a nice frame the other day, and have made a print using five pictures of the four of us at different ages. For once, everything goes smoothly with the printing. I make a second print for Jim, which is also successful, then the printer spits out a message saying one of the cartridges is empty.

I see Nicola at 4:30, our third lesson, things go well. Home for dinner, then meet Ben at the station for a late lesson at 6:30. He has his test tomorrow. I'm wondering if anyone will pass at the moment. Ben should, but so should the last two candidates...