11/8/2010AST207, F2010 1Helium Production in Big Bang8 Nov• Homework 8 is on angel. Due noon on Mon, 15 Nov.• Homework 9 will be due Fri, 19 Nov at start of class. No late papers. Covered on Test 3 (22 Nov).– Long assignment. Start early.• A fossil is a remnant or trace of the past. What is a fossil from the Big Bang?– There are 7 protons for every neutron– The surface of the sun is 25% He and 75% H.• What does that fossil tell about the BB?Fossil from Burgess ShaleFossil from Big BangpHepppppppppppObjectives• Describe how the abundance of neutrons is a fossil of the Big Bang.• What part does the radiation from the Big Bang play?• Why did the abundance of neutrons change before the “fossil was laid down” and not afterwards?• How do astronomers “collect the fossils?”11/8/2010AST207, F2010 21H4He3He100Helium formed when universe was 3 minutes old• How & where were the elements made?– Carbon, Iron, Calcium in stars– Hydrogen is primordial– Helium is too abundant to have been made in stars.– Helium was made at 3min.Jim Peebles• What part does the radiation from the Big Bang play?11/8/2010AST207, F2010 3Book‐burning Universe• When the universe was smaller by a factor a (when the distance between us and some object was smaller), the temperature was hotter by a factor 1/a. 2.7K/.• At one time, the universe was too hot to have paper.– Paper burns at 451 F = 500 K.– In reality, there was no carbon and no paper at that time.2. Hoag’s object is 300 Mpc from the Milky Way. How far was it when the U was just hot enough to burn paper?A. 30 Mpc, a=1/10, T=2.7 * 10 = 27KB. 10 Mpc, a=1/30C. 3 Mpc, a= 1/100D. 1 Mpc, a= 1/300Book‐burning Universe• Key idea: When the universe was smaller (when the distance between us and some object was smaller), the temperature was hotter. There is no obvious limit to the temperature.• At one time, the universe was too hot to have paper. 1. What other familiar things were not possible at one time? What other reactions might have occurred when the universe was smaller & hotter.11/8/2010AST207, F2010 4Book‐burning Universe• Key idea: When the universe was smaller (when the distance between us and some object was smaller), the temperature was hotter. There is no obvious limit to the temperature.• What other reactions might have occurred when the universe was smaller & hotter?• Events in the universe’s life (in reverse order)• First stars formed– When U cooled enough, gravity was able to overcome pressure.• Recombination: U changed from opaque to transparent– Ionization & rec ombination– Free p + e hydrogen atom• Production of the first nuclei other than H– Nuclear reaction– Free protons + neutrons helium nucleus• Why did the abundance of neutrons change before the “fossil was laid down” and not afterwards?11/8/2010AST207, F2010 5Neutrons and protons change identity• Neutrons can change into protons & vice versa. 1. When hydrogen fuses to become helium in the sun, does the ratio #n/#p change?A. Yes.B. No.• (number of neutrons)/(number of protons) changes in fusion reactions.p pn np1. Does the (number of neutrons)/(number of protons) change for 16O, the most abundant isotope of oxygen? for 14C14N+e‐+?A. YYB. YNC. NYD. NN• (number of neutrons)/(number of protons) does not change for stable nuclei.11/8/2010AST207, F2010 6#n/#p at the present time• In the outer parts of the sun, the material is nearly primordial.• He is 25% of the mass, H is 75%. 12 H atoms for every He atom– Mass He = 4– Mass H = 12– Total mass = 161. #n / #p =a. 1:1b. 1:12c. 2:14d. 4:12e. 2:3mass He=25%Mostly made in BB in 3 minp pn npFollow the neutrons• #n/#p=2/14=1/7 now• Processing in stars changes #n/#p slightly.– #n/#p in H and O• 8n/(800p+8p)=1/101• #n/#p has been 1/7 from 3min to now. This is the fossil.– As the fossils in the Burgess Shale have not changed their shape since the animal died 505Myr ago, #n/#p has not changed significantly since the universe was 3min.
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