Tuesday, January 25, Phobos, a moon of mars

Transcription

1 Phobos, a moon of mars

2 Phobos, a moon of mars

3 A Polar Ring Galaxy

4 Neutrinos

5 The Sun s Power Source Mid-19th Century Debate: Darwin Lord Kelvin Darwin: Earth must be at least 300 Million years old to account for geological and evolutionary changes evident in the fossil record. Lord Kelvin: If the Sun was powered by gravitational contraction, its age could not exceed 30 Million years. Today we know that Darwin was correct; Lord Kelvin did not know about nuclear fusion!

6 Steps 1905: A. Einstein proposed the equivalence of mass and energy: E = mc : F. W. Aston: measured the mass of the Helium nucleus: less than 4 Hyrdogen nuclei! 1920: Arthur Eddington proposes that fusion of Hydrogen into Helium powers the Sun 1938: Hans Bethe identified the actual nuclear reaction chains for this process.

7 Why does E=mc 2 work c is a very large number, c 2 even larger! The total worldwide energy consumption for 2010 was around 500 exajoules (exa=10 18!). So Joules. 1 Joule/second=a Watt. So your hairdryer = 1000 J/s. The mass energy equivalency of all this? 5563kg, or about 6 tons.

8 Why does E=mc 2 work 6 tons of gravel c is a very large number, c 2 even larger! The total worldwide energy consumption for 2010 was around 500 exajoules (exa=10 18!). So Joules. 1 Joule/second=a Watt. So your hairdryer = 1000 J/s. The mass energy equivalency of all this? 5563kg, or about 6 tons.

9 Sun s Layers

10 And composition 0.8% 28% 70% H He O C Fe 0.3% 0.2%

11 Making Fusion Work Nuclear Fusion: heavier elements made by fusing lighter ones All the elements (like Hydrogen) are ionized: i.e. stripped of their electrons. Nuclei (e.g. two protons) repel each other, as do all charged particles. Get them close enough, and the strong force takes over, binding them.

12 Making Fusion Work Nuclear Fusion: heavier elements made by fusing lighter ones All the elements (like Hydrogen) are ionized: i.e. stripped of their electrons. Nuclei (e.g. two protons) repel each other, as do all charged particles. Get them close enough, and the strong force takes over, binding them.

13 Making Fusion Work Nuclear Fusion: heavier elements made by fusing lighter ones All the elements (like Hydrogen) are ionized: i.e. stripped of their electrons. Nuclei (e.g. two protons) repel each other, as do all charged particles. Get them close enough, and the strong force takes over, binding them.

14 Eddington on heat To overcome the strong repulsive forces of electromagnetism, you need fast moving nuclei: very high temperatures. But quantum mechanics allows you to tunnel through the barrier. I am aware that many critics consider the stars are not hot enough. The critics lay themselves open to an obvious retort; we tell them to go and find a hotter place.

15 Not all nuclear created equal

16 The simplest recipe

17 The Neutron and neutrino A neutral particle adds mass, but not charge, to a nucleus. E.g. Helium has two protons, two neutrons. Free neutrons are NOT stable: n p + + e - +! e ("-decay) p + n + e + +! e (inverse "-decay) νe is a neutrino, nearly massless, almost noninteracting. Travels near the speed of light. 1930: Wolfgang Pauli Postulated, to explain missing energy in β decays.

18 Solar neutrinos Neutrinos do not have charge, and very little mass. They interact almost entirely through the weak force. Which is weak: the electromagnetic force ;). Every conversion of 4H 1He liberates 2 neutrinos, which escape the sun directly. If we can detect them, we can see deep inside the sun!

19 Interaction Problem: A hundred billion neutrinos from the sun pass through your thumbnail every second, and you don t notice! To block a neutrino with a 50% probability, you d need to pass it through 1 light-year of lead.

20 Homestake Mine: 100,000 gallons of dry-cleaning fluid, 1960 s, Ray Davis Underground, to prevent contamination by other radiation. Chlorine gets changed into argon. Argon collected, and counted. Results: only 1/3 of the expected neutrinos found!

21 Homestake Mine: 100,000 gallons of dry-cleaning fluid, 1960 s, Ray Davis Underground, to prevent contamination by other radiation. Chlorine gets changed into argon. Argon collected, and counted. Results: only 1/3 of the expected neutrinos found!

22 Missing Neutrinos but Only one flavor of neutrinos detected

23 From the sun? Super Kamiokande can get a direction for each neutrino event. A 500 day exposure centered on the sun shows that s where they originate.

24 Many experiments confirm: roughly 1/3 as many as theory of the Sun predicts.

25 Super Kamiokande A Japanese experiment now detects all three flavors of neutrinos: missing solar neutrinos found! Electron neutrinos have changed into muon and pion neutrinos on their trip from the sun. Which means: Neutrinos have mass!

26 Neutrinos elsewhere Neutrinos have (tiny, but nonzero) mass: could they explain missing mass in the universe (the so-called Dark Matter problem)? The big-bang produced copious neutrinos: your body, right now, contains 10,000,000 relic neutrinos from the creation of the universe! Almost impossible to detect!

27 Supernova Neutrinos In 1987, a Supernova exploded in The large Magellanic Cloud. Three hours before the light of the SN arrived, three separate neutrino observatories detected a burst of neutrinos (24 observed events, total!). First direct neutrino observation from outside the solar system!

28 Betelgeuse: supernova in <100,000 yrs (not in 2012). Harmless to earth, but excellent for studying neutrinos, SN ejecta, etc.

29 Assignments Read: Popular/secrets.htm Watch: v=md1ckuqp04q Note: Link up from Luminis website default page.