Stars

1. Stars Yr 11 Astronomy Edwards

2. What are Stars? • A Star is a huge ball of hot glowing gas. • They produce their own light and heat by nuclear reactions • The Sun is the closest star to Earth

3. Composition of a Star • Basically, stars are big exploding balls of gas, mostly hydrogen and helium. • Our nearest star, the Sun, is so hot that the huge amount of hydrogen is undergoing a constant star-wide nuclear reaction, like in a hydrogen bomb. • Even though it is constantly exploding in a nuclear reaction, the Sun and other stars are so large and have so much matter in them that it will take billions of years for the explosion to use all the "fuel" in the star.

4. Nuclear Fusion • In the centre of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright. • Nuclear fusion is an atomic reaction that fuels stars. • In fusion, many nuclei (the centres of atoms) combine together to make a larger one (different element). • The result is the release of a lot of energy (the resultant nucleus is smaller in mass than the sum of the ones that made it; the difference in mass is converted into energy)

5. Nuclear Fusion

6. The Life of Stars • A star's life cycle is determined by its mass. (The larger its mass, the shorter its life cycle) • Mass is determined by the amount of matter that in its nebula, the giant cloud of gas and dust from which it was born. • Over time, the hydrogen gas in the nebula is pulled together by gravity and it begins to spin. As the gas spins faster, it heats up and becomes as a protostar. • Eventually the temperature reaches 15,000,000 oC and nuclear fusion occurs in the cloud's core. • The cloud begins to glow brightly, contracts a little, and becomes stable. It is now a main sequence star and will remain in this stage, shining for millions to billions of years.

7. The Life of Stars Continued… • This is the stage our Sun is at right now. • As the main sequence star glows, hydrogen in its core is converted into helium by nuclear fusion. • When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts. • The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and glows red. The star begins the red giant phase. • It is red because it is cooler than it was in the main sequence star stage and it is a giant because the outer shell has expanded outward. • It will eventually collapse and explode. Its fate is determined by the original mass of the star; it will become either a black dwarf, neutron star, or black hole.

9. Constellations • Constellations are names for groups of stars that appear to form shapes in the sky. • They were given their names 100’s of years ago to help us remember which stars are which. • They help by breaking up the sky into more manageable bits. • There are 88 constellations in total. • These contain signs of the zodiac.

10. Star Sign

12. South Celestial Pole • The South Celestial Pole is the point in the sky about which all the stars seen from the Southern Hemisphere rotate. • This can be located using the Southern Cross and pointers.

13. Finding the South Celestial Pole

14. The Sun • Scientists think that the core of the Sun is a 15 million degree Celsius. • Every second, thousands of protons in the Sun's core collide with other protons to produce helium nuclei in a nuclear fusion reaction that releases energy. • Just outside the core, energy moves outward by a process called radiation. • Closer to the surface, the energy moves out by a process called convection - hot gases rise, cool, & sink back down again. • As these masses of gas move, they push off of each other causing "Sun-quakes." These make the material in the Sun vibrate.