Stars: Old Age, Death, and New Life

1. Stars: Old Age, Death, and New Life BC Science Probe 9 Section 13.4 Pages 431-436

2. Stars • Stars age as they use up their hydrogen. • After millions or billions of years, a star will enter the last stages of its life. • At the end, a star can either fade out of existence, or explode in a life-renewing cycle!

3. Hertzsprung-Russell • A star’s mass determines its brightness, colour, size, and how long it will live. • This information is organized in a diagram called the Hertzsprung-Russell (H-R) diagram which plots the lives of stars.

5. Hertzsprung-Russell • An H-R diagram shows the temperature and luminosity of stars. • Luminosity is energy output and the Sun is assigned the value of 1.

6. Hertzsprung-Russell • Most stars fit into the main sequence of the H-R diagram. • This is the diagonal band on the diagram. • Where it fits on the main sequence depends on its mass.

7. Hertzsprung-Russell • Our Sun has 1 solar mass, and it is the star to which all others are compared.

8. Hertzsprung-Russell • In the lower right of the diagram are the cooler, reddish stars that are small and dim.

9. Hertzsprung-Russell • In the upper left are the massive, very bright, hot, bluish stars. • The blue giants.

10. Hertzsprung-Russell • The cooler red giants (0.4-10 solar masses) and the super giants (10-70 solar masses) are found off the main sequence to the upper right.

11. Hertzsprung-Russell • The white dwarfs which are very hot and about 1/3 of a solar mass are in the lower left.

12. Hertzsprung-Russell • The stars in the H-R diagram represent different stages in the lives of stars as their fuel is consumed.

13. Red Giant to White Dwarf • After 10 billion years as a main sequence star, most of the Sun’s hydrogen will be converted to helium. • The helium forms a core inside a shell of the remaining hydrogen.

14. Red Giant to White Dwarf • Less hydrogen = less energy = less outward flow of energy. • This causes the core to shrink and the shrinking/contraction reheats the rest of the hydrogen and starts fusion again!

15. Red Giant to White Dwarf • Even though the core is shrinking, the outer layers will expand and then cool. • This expanded, cooler Sun will eventually be a red giant.

17. Red Giant to White Dwarf • The Sun will expand for millions of years. • It will engulf Mercury, Venus, and maybe even the Earth!

19. Red Giant to White Dwarf • Once the hydrogen is actually all used up, the core will heat to a temperature high enough to begin helium fusion.

20. Red Giant to White Dwarf • Helium fusion will continue the expansion. • It will also produce heavier elements like Carbon and Oxygen.

21. Red Giant to White Dwarf • Now it is a fully formed red giant. • It will have several thousand times its original brightness!

22. Red Giant to White Dwarf • Red giants give off dust and gas, so they begin to lose mass. • Even a star that starts at a mass of up to10 solar masses will eventually lose enough mass to become a stable white dwarf.

23. Red Giant to White Dwarf • A white dwarf will have a mass no larger than 1.4 solar masses and it is compressed to the size of the Earth.

24. Red Giant to White Dwarf • A star that has a mass over 10 solar masses will explode as a supernova.

25. Red Giant to White Dwarf • The Sun is 1 solar mass, so it will become a white dwarf. • It will release particles that will collide with the matter it shed during its last stages as a red giant.

26. Red Giant to White Dwarf • The energy from these collisions of particles will illuminate the clouds of gas and dust and create a nebula… • What can a nebula form?

28. Red Giant to White Dwarf • The white dwarf, or the remains of the Sun, will keep its place in the Milky Way until it burns out and becomes a black dwarf. • It will no longer be visible.

29. Supernovas • The explosion of a star in which the star may reach a maximum intrinsic luminosity one billion times that of the Sun.

31. Supernovas • They may only last a few months. • The energy that they generate can drive the fusion and formation of all of the elements on the Periodic Table.

32. Neutron Stars • If a star starts out at 10-50 solar masses, the supernova will produce a neutron star.

33. Neutron Stars • The core of a neutron star is made mostly of neutrons. • They are so tightly packed together that 250 ml (1 cup) of the core would have a mass of millions of kilograms!

34. Neutron Stars • A pulsar is a kind of neutron star that sends out light and a beam of very high-energy radio waves. • It rotates giving off the beam of energy kind of like a light house.

35. Black Holes • If a star has an initial mass of over 50 solar masses, it will become a supernova and produce very heavy elements like iron and nickel.

36. Black Holes • If the mass left behind is more than 4 solar masses, the core will collapse in on itself.

37. Black Holes • The object’s mass is still immense, so its gravitational pull is also immense. • Not even light can escape. • It has become a black hole.

39. Black Holes • A black hole of 10 solar masses would only be 60 km in diameter!

40. Black Holes • Black holes are very difficult to find because they are small and do not give off light. • Many stars exist as binaries ( 2 stars circling each other), this was how the first black hole was discovered. • A blue giant and its invisible companion.

42. Black Holes • The first black hole was found in the constellation Cygnus. • The black hole pulls gas from the blue giant. • The gas heats up and emits X-rays which allowed the black hole to be detected. • The blue giant has a solar mass of 27 and the black hole (Cygnus X-1) has a solar mass of 14.