Transcript
JULIA COOKE
The Sun, Earth, and other planets in our solar system are part of a galaxy called the Milky Way. A galaxy is a system of stars, remnants of stars, gas, dust, and possibly mysterious dark matter, held together by gravity. The word galaxy comes from the Greek word galaxis, which means milky. This description refers to the opaque appearance, punctuated by brighter stars and dark patches, which is how we see our galaxy.
We can see parts of our milky galaxy with the unaided eye, though telescopes provide more detailed images. Because we are inside the galaxy, views such as this are only part of the Milky Way. It is estimated that there are between 200 billion and 2 trillion galaxies in the universe. In the spaces between them is very low density gas.
Galaxies occur in three main types – elliptical, spiral, and irregular. These terms describe the shape of galaxies, which are a product of the way they were formed and their age. The Milky Way is a spiral galaxy. On average, it is about 100 000 light years across. A light year is the distance that light travels in one year and a useful way to describe the vast distances in space. The Earth is about 27 000 light years away from the galaxy’s centre. The Milky Way rotates once every 200 million years. The heavier the galaxy, the faster they rotate.
One of the nearest galaxies to the Milky Way is called Andromeda. It is another spiral galaxy, much bigger than our own. It is 2.5 million light years away. Researchers predict that the Milky Way and Andromeda galaxies will collide in about 4.5 billion years and will eventually form an even bigger galaxy.
As well as dust, galaxies contain trillions of stars. Star formation can be considered as the beginning of structure in the universe. Small variations in the density of matter mean gravity can pull together protons or hydrogen, fusing them in huge numbers, and forming massive outputs of energy. The energy released in this process, called nuclear fusion, is what makes stars shine.
Old and dying stars, such as the one shown here, run out of fuel or hydrogen. They are so large that the heat and pressure they produce allows the formation of new elements by fusing protons. Smaller stars make carbon, silicon, oxygen, and iron, while larger stars, or supernovas, make other elements. These explosions send clouds of matter out into the universe. Everything we know is made of this stardust – planets, our atmosphere, and all of life on Earth, including ourselves.