Hertzsprung-Russell Diagram

2. There are thousands of stars that we can see with the naked eye. Stars can be classified into different categories Hertzsprung-Russell Diagram

3. Hertzsprung-Russell Diagram • Different stars have different masses. • Among all stars with the same mass, almost all have the same properties. • The most massive stars are the most luminous

4. The HR diagram explained • The HR diagram shows four groups of stars. The narrow band across the center is the "main sequence" of stars, which contains about 90% of stars. Main sequence stars are normal hydrogen-burning stars like our Sun. A star's position along the main sequence is determined entirely by its mass. Bigger stars are hotter and brighter - class O stars can have 60-100 times the Sun's mass. Smaller stars are cooler and dimmer - class M stars can have one-tenth the Sun's mass. • The stars above and to the right of the main sequence are giant and supergiant stars - older stars that have run out of hydrogen, and now burn heavier elements. These are the brightest stars in the sky. • The stars below and to the left of the main sequence are white dwarfs - giant stars that ran out of all their nuclear fuel and collapsed. They glow hot because of the energy left over from their collapse. They are very faint and hard to detect. The nearest white dwarf is Sirius B, which orbits the bright star Sirius. Sirius B is about 8.6 light-years away and has an apparent magnitude of about 8.5.

7. The different colours of the Stars • O Hottest and bluest but rare • B Hot and blue, e.g. Rigel (in Orion) • A White, contribute much light to the night sky, e.g.Sirius • F Yellow – white, e.g. Canopus and Procyon • G Yellow, e.g. the Sun, Alpha Centauri (the first pointer to the Southern Cross) • K Orange, e.g.Aldebaran • M Cool and red, e.g.Betelgeuse blue and white hot 7500C – 50 000Cyellow warm 5000C - 7500Corange and red cool 2000C - 5000C

8. The Birth of Stars This takes place in nebulae – made up of clouds of interstellar gas and dust – they are very beautiful. Look at these following examples.

9. Pillars of Creation – The Eagle Nebula These pillar-like structures in the Eagle nebula. These eerie, dark pillar-like structures are columns of cool, interstellar hydrogen gas and dust that serve as incubators for new stars.

10. Eagle Nebula A birth place of stars

11. Orion Nebula

12. Orion nebula

13. This is a blob of hot, ionized gas and is part of a nebula of gas and dust inside the Large Magellanic Cloud outside the Milky Way galaxy.

14. Horse head nebula

15. Horsehead Nebula

16. Swan Nebula

17. Trifid Nebula

18. Jets from young stars

19. The Pleiades – young stars