Brief reminder of Part 1 (without mistake ) Our galaxy , located in the Milky Way , is one of the largest still observable . It is home to the solar system and therefore to the Earth . The Sun is mainly composed of gas . Its volume is 1000000 , times that of the Earth . With its energy , it provides us with light , but how does it reach us ? To understand its creation , we need to explore the sun , which has a composition similar to an ognion , with its different layers . The outer leyer is called the solar corona , where the temperature is over 1000000 , degrees . The second layer is the chromosphere , which is cooler at 8000 , to 10000 , degrees . Then come the third , coolest layer , the photosphere , with a milder temperature of 5000 , to 6000 , degrees . At the centre of the Sun is the solar core , where energy is produced by nuclear fusion . These atomic explosions (1 per second ) gives rise to the light that travels outwards very slowly (10000 , or even 100000 , years ) from the surface of the nucleus to reech us . Eight minutes is the time it takes for light to travel , at 300000 , km/second , the 150000 , million kilometres that separate the Sun from the Earth . Unfortunately , the yellow star is not eternal . One day , it will disapear forever . During its 4500 , billion year lifespan , hydrogen will gradually run out . In the absence of hydrogen , it will expand and thin out . The core will cool and then shrink , while the solar envelope will expand . It will become immense , 10 times/100 times larger than it is today , swalowing up the nearest planets . Then , after a few million years , the enveloppe will be eject and disperse into the universe , leaving only the hard core , which will become a ' white dwarf ' The Sun ' s decline will take place without an audience . But rest assured , the Sun still has a bright future ahaed of it , estimated at 5 billion years !
