How Old is the Sun?
by FRASER CAIN on SEPTEMBER 16, 2008
How old is the Sun? The age of the Sun is 4.6 billion years old. In fact, astronomers are fairly certain that the entire Solar System formed at the same time – 4.6 billion years ago.
Before we had a Solar System, there was just a giant cloud of cold molecular hydrogen. And then something happened to the cloud that caused it to collapse. Perhaps it was the interaction with a passing star, or maybe the shockwave from a nearby supernova triggered the collapse. After that, the cloud began to collapse because of its collective gravity.
As the gas and dust collected together, it started to spin and flatten out. Most of the mass collected in the center, while the rest spun out into the flattened protoplanetary disk (here’s where the planets, moons, and asteroids formed). The Sun ignited fusion in its core once it had enough temperature and pressure. For the first 50 million years, the Sun was probably quite active, firing off enormous flares. And then it settled down into the main sequence star we know and love today.
So how do astronomers know the age of the Sun (and the rest of the Solar System) with such accuracy? In short… meteorites. Astronomers have collected fallen meteorites around the planet, and calculated that they all formed within a few million years of one another.
They use a technique called radiometric dating. Since the decay rate of elements is very well known, they just calculated the ratio of uranium to lead, for example, in a meteorite sample. By comparing these ratios to the known decay rate of uranium, scientists are able to calculate when the meteorite formed.
And this is how we know how old the Sun is.
That is a question that cuts to the heart of it all. By studying several things, mostly meteorites, and using radioactive dating techniques, specifically looking at daughter isotopes, scientists have determined that the Solar System is 4.6 billion years old. Well, give or take a few million years. That age can be extended to most of the objects and material in the Solar System.
The United States Geological Survey(USGS) website has a lot of indepth material about how the age of the Solar System was determined. The basics of it are that all material radioactively decays into a stable isotope. Some elements decay within nanoseconds while others have projected half-lives of over 100 billion years. The USGS based their study on minerals that naturally occur in rocks and have half-lives of 700 million to 100 billion years. These dating techniques, known as radiometric dating, are firmly grounded in physics and are used to measure the last time that the rock being dated was either melted or disturbed sufficiently to re-homogenize its radioactive elements. This techniques returned an approximate age for meteorites of 4.6 billion years and Earth bound rocks around 4.3 billion years. The USGS admits that they were unable to find any rock that had not been altered by the Earths tectonic plates, so the age of the Earth could be refined in the future.
When the gasses of the early solar nebula began to cool, the first materials to condense into solid particles were rich in calcium and aluminum. Eventually solid particles of different elements clumped together to form the common building blocks of comets, asteroids, and planets. Astronomers have long thought that some of the Solar System’s oldest asteroids should be more enriched in calcium and aluminum, but, none had been identified until recently. The the Allende meteorite of 1969 was the first to show inclusions that were extremely rich in calcium and aluminum. It took 40 years for the spectra of the inclusions to be discovered and then extrapolates to very old asteroids still in orbit around the Sun. Astronomer Jessica Sunshine and colleagues made this discovery with the support of NASA and the National Science Foundation
Additionally, the Universe is thought to have been created about 13.7 billion years ago. Measuring two long-lived radioactive elements in meteorites, uranium-238 and thorium-232, has placed the age of the Milky Way at in the same time frame. From these measurements, it appears that large scale structures like galaxies formed relatively quickly after the Big Bang.
Indeks 
15th June 2011, 15:37