Supernovas – or things that go “bang” in the night!

1. Supernovas – or things that go “bang” in the night! The next-to-last chapter in the lives of large stars.

3. Large stars begin like any other star • Stars that eventually go supernova are from 3 to 100 times the mass of the sun. • A nebula becomes a protostar, becomes a main sequence star (where it lives most of its life), begins to use up its fuel.

4. continued (2) • As the Hydrogen fuel in the core is used up, the core shrinks and heats up, while the outer layers swell and cool – the star becomes a red giant. • As the core shrinks & heats it begins fusing He (the ash from the previous reaction) into C and O.

6. continued (3) • The layer right next to the core begins fusing H into He – so now 2 fusion reactions are occurring. • A small star would stop here, the core not being hot enough to do anything with the C and O.

7. continued (4) • A large star’s core can shrink and heat up further. The core begins to fuse C and O into Neon (Ne), Magnesium (Mg), and Silicon (Si). • The next layer out fuses He into C and O. The layer outside that fuses H into He. The star starts to resemble the layers of an onion.

8. continued (5) • The production of Ne, Mg, and Si continues only for a few hundred years. • Eventually, when all the C and O is used up, the core shrinks once more, heats to over 600,000,000 K and starts fusing Mg + Si into iron (Fe). • Iron is the final ash and doesn’t fuse into anything else.

9. Onionlayers

10. continued (6) • The production of iron from Mg & Si happens very quickly, less than 1 day. • When the iron core gets massive enough it implodes! (The needed mass is 1.4 x the mass of the sun – the Chandrasekhar limit!)

11. Inverse Beta Decay • The gravity is so great in the core, that protons & electrons get squashed together into neutrons: p+ + e- no (This is called inverse beta decay.) • The core becomes a neutron star.

12. Neutron Stars • Neutron stars are formed from stars that were originally 3 – 8 times the mass of the sun. • They’re held up against gravity simply by the neutrons being jammed in tightly next to each other. This is called “neutron degeneracy”.

13. Black Holes • What happens if the star was originally more than 8 solar masses? • Even the pressure of the neutrons is overcome, and the neutron star collapses into a black hole.

14. What about the rest of the star’s layers? • When the iron core collapses into a neutron star or black hole (at nearly the speed of light), the outer layers follow it in. • The outer layers “bounce” or rebound off the immensely hot new neutron star and a gigantic explosion occurs!

16. Supernova Explosions • The most recent close SN explosion happened in 1987 in the Large Magellanic Cloud – a satellite galaxy to the Milky Way. • For a few days, the SN was brighter than the rest of the satellite galaxy combined!

17. How often do SN happen? • On average, about every 100 years for any given galaxy. • Our own galaxy has had several during recorded history:

18. “Recent” Supernovas • July 4th, 1054 in Taurus, 6500 light years away. This resulted in the Crab Nebula. It was recorded by Anasazi Indians in the American southwest.

19. The Crab Nebula in Taurus

20. “Recent” Supernovas (2) • Tycho Brahe saw a supernova in Cassiopeia in 1572 – it was 16,000 light years away. • The Chinese also saw and recorded the appearance of a “guest” star.

21. Theremnant of Cassio- peia A.This is the brightest radio- emitting object in the sky.

22. Evidence for Older SN’s • When a supernova explodes, it leaves behind a cloud or supernova remnant. • These remnants last for hundreds of thousands of years. New (small) stars can be formed from their gases and dust.

24. This is how the whole Cygnus Loop SN remnant looks – remarkably like the cloud from an ordinary explosion.

26. Future Supernovas • At present, astronomers are waiting for at least 2 more stars to go supernova: one in Cassiopeia, and another in the southern sky called “Eta Carinae”.

28. A Very Special Supernova • The only close supernova that astronomers have been able to study in detail is SN 1987A in the Large Magellanic Cloud, 170,000 light years away. • The SN happened in a nebula called the Tarantula Nebula.

29. The Large Magellanic Cloud

30. The Tarantula Nebula

31. SN 1987A (2) • This supernova was different than many – when it exploded, it was an blue supergiant. Guess we don’t know all the details yet about SN’s. • The star was originally called Sanduleak -69o 202. The Sanduleak (pronounced San-doo-lik), was named after Nick Sanduleak, who studied it before it blew up.

32. Astronomer Nick Sanduleak

33. SN 1987A (3) • The first astronomer to actually observe SN 1987A after it exploded was Ian Shelton, from the University of Toronto, Ontario. • At the time, he was observing at Las Campanas Observatory in Chile.

34. Ian Shelton

35. Las Campanas Observatory

36. After the Explosion • The brightness of SN 1987A has been monitored for the past 15 years. It didn’t follow the usual pattern (suddenly bright, with a quick fall-off). • Rather, it got suddenly bright, grew brighter, then faded off gradually.

38. After the Explosion (2) • A couple of years after the supernova faded, it suddenly brightened again. • It wasn’t the supernova itself, but its light reflecting off a cloud of dust behind the SN. This reflected light is called a “light echo”.

40. After the Explosion (3) • This supernova has been observed extensively. Over the years, we’ve seen shock waves from the explosion slam into the clouds of gas that the star gave off just before it exploded. • The shock waves heat the gas, producing rings.

42. After the Explosion (4) • The shock waves heat the shells of gas hot enough to give off X-rays.

44. One last point – supernovas make the chemical elements • We’ve already seen how supernovas can make elements up to the mass of Fe (atomic number 26) before they explode. However, there are 83 elements that are heavier than iron. • How do supernovas make the heavier elements? It’s a process called nucleosynthesis.

45. Nucleosynthesis (2) • During the explosion, there are a lot of very fast, high energy neutrons flying around. Sometimes, one of these neutrons hits an iron nucleus: 5626Fe + no5726Fe

46. Nucleosynthesis (3) • The extra neutron inside the heavy iron nucleus can split into a proton and an electron. This produces the next heavier element: 5726Fe 5727Co

47. Nucleosynthesis (4) • This process of adding a neutron, then the neutron splitting into a proton and electron can happen over and over, producing elements heavier than Uranium. 5727Co + no5828Ni etc.

48. The End • The next time you look at your girlfriend or boyfriend – remember that they truly are made of stars. • So is your lunch today….that’s that funny taste.