Nucleosynthesis

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© 1998-2008 by Glenn Elert -- A Work in Progress
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Discussion

big bang nucleosynthesis

By the first millisecond, the universe had cooled to a few trillion kelvins (10¹² K) and quarks finally had the opportunity to bind together into free protons and neutrons. Free neutrons are unstable with a half-life of about ten minutes (614.8 s) and formed in much smaller numbers. The abundance ratio was about seven protons for every neutron. Before one neutron half-life passed nearly every neutron had paired up with a proton, and nearly every one of these pairs had paired up to form helium. By this time the universe had cooled to a few billion kelvins (10⁹ K) and the rate of nucleosynthesis had slowed down significantly. By the time the universe was three minutes old the process had basically stopped and the relative abundances of the elements was fixed at ratios that didn't change for very long time: 75% hydrogen, 25% helium, with trace amounts of deuterium (hydrogen-2), helium-3, and lithium-7. Big Bang nucleosynthesis produced no elements heavier than lithium. To do that you need stars, which means waiting around for at least 200 billion years.

we are all made of stars

More than ninety per cent of the universe is composed of hydrogen and helium. Both elements have been around since shortly after the beginning of the universe. Yet, hydrogen and helium together won't make anything as complex and as interesting as the earth, or a bacterium, or a refrigerator, or you and I. To do that we need carbon and oxygen and nitrogen and silicon and chlorine and every other naturally occurring element. Almost all the hydrogen and helium present in the universe today (and some of the lithium) were created in the first three minutes after the big bang. All of the other naturally occurring elements were created in stars.

slow neutron capture within stars (s-process) will not synthesize elements heavier than iron
rapid neutron capture during supernova explosions (r-process) heavy hydrogen through uranium

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Stars like the sun

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Details were discussed in the section on Fusion. The basic parts of the reaction are …

Which overall yields …

Stars heavier than the sun use carbon-12 as a catalyst.

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You need really massive stars for this -- say 20 to 120 times the mass of the sun.

Really, really heavy stars do something different.

The Mass-5 and Mass-8 Bottlenecks. There are no stable isotopes (of any element) having atomic masses 5 or 8. But there is always a very small amount of beryllium-8 at any moment that is available to fuse with a third helium to produce carbon-12. This extremely improbable sequence is called the triple-alpha process because the net effect is to combine 3 alpha particles to form a carbon-12 nucleus. The triple-alpha process is not relevant in main sequence (normal) stars like the sun because their core temperatures are too low. However, in the red giant phase, after many stars have accumulated vast amounts of helium in their core, the central temperature can rise high enough (10⁸ K) to initiate the triple-alpha process.

Overall

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In order of increasing alpha number, the following forms of fusion take place …

alpha number	mass number	element(s)		comments
Stable Isotopes Built from Helium Nuclei (Alpha Particles)
1	4	He	helium	formed in all stars
2	8			no stable isotopes with this mass number
3	12	C	carbon	triple alpha process
4	16	O	oxygen
5	20	Ne	neon
6	24	Mg	magnesium
7	28	Si	silicon
8	32	S	sulfur	most abundant isotope of sulfur
9	36	S Ar	sulfur argon	0.02% of all sulfur atoms most abundant isotope of solar argon
10	40	Ar K Ca	argon potassium calcium	most abundant isotope of atmospheric argon 0.01% of all potassium atoms most abundant isotope of calcium
11	44	Ca	calcium	2.1% of all calcium atoms
12	48	Ca T	calcium titanium	0.19% of all calcium atoms
13	52	Cr	chromium
14	56	Fe	iron	nuclear "ash"

Massive Stars

lifetime remaining	core temperature	core reaction
Core Nuclear Reactions in Massive Stars
10,000,000 years		¹H	⇒	⁴He
1,000,000 years	170,000,000 K	⁴He	⇒	¹²C, ¹⁶O
1,000 years		¹²C	⇒	²⁰Ne, ²⁴Mg
10 years	1,500,000,000 K	²⁰Ne	⇒	¹⁶O, ²⁴Mg
1 year	2,000,000,000 K	¹⁶O	⇒	²⁸Si, ³²S
1 day	3,000,000,000 K	²⁸Si, ³²S	⇒	⁵⁶Fe, Ni
1 s		explosive fusion neutron capture	⇒ ⇒	light elements heavy elements

Mix it all up and get everything from hydrogen to uranium (and maybe even up to californium).

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rank	element		per million kg	per million atoms
Top 20 Elements in the Universe
1	H	hydrogen	750,000	930,000
2	He	helium	230,000	72,000
3	O	oxygen	10,000	800
4	C	carbon	5,000	500
5	Ne	neon	1,300	80
6	Fe	iron	1,100	20
7	N	nitrogen	1,000	90
8	Si	silicon	700	30
9	Mg	magnesium	600	30
10	S	sulfur	500	20
11	Ar	argon	200	6
12	Ca	calcium	70	2
13	Ni	nickel	60	1
14	Al	aluminum	50	2
15	Na	sodium	20	1
16	Cr	chromium	15	0.4
17	Mn	manganese	8	0.2
18	P	phosphorus	7	0.3
19	Co	cobalt	3	0.06
20	K	potassium	3	0.1
…	…	everything else	6	0.2
Source: WebElements

how like a god

Rutherford was the first to transform one element into another.

technetium and promethium

	Discovered by Perrier & Segre in a sample of Molybdenum that had been irradiated by deuterons at the UC Berkeley cyclotron by E.O. Lawrence and then sent to Italy.
	Contamination hazard
	Radioactive tracer
	From New Scientist, "Famed scientist P.W. Merrill fifty years ago observed the signature of live technetium - an element that has no stable isotopes - in the starlight from certain types of stars, thereby proving the then-controversial theory that stars make atoms via a process called nucleosynthesis."
	First synthesized at Ohio State in 1941
	Identified chemically by Larry Glendenin, Jacob Marinsky, and Charles D. Corell in 1944

transuranic, cisuranic, superheavy

element			year	location	process
First Synthesis of Various Artificial Elements (some claims are subject to debate)
43	Tc	technetium	1936	Italy
61	Pm	promethium	1944	ORNL
93	Np	neptunium	1940	LBL
94	Pu	plutonium	1940	LBL
95	Am	americium	1944	Chicago
96	Cm	curium	1944	Chicago
97	Bk	berkelium	1949	LBL
98	Cf	californium	1950	LBL
99	Es	einsteinium	1952	Pacific Ocean	found in radioactive fallout
100	Fm	fermium	1952	Pacific Ocean	found in radioactive fallout
101	Md	mendelevium	1955	LBL
102	No	nobelium	1958	LBL
103	Lw	lawrencium	1961	LBL
104	Rf	rutherfordium	1964	JINR
105	Db	dubnium	1970	LBL
106	Sg	seaborgium	1974	LBL
107	Bh	bohrium	1976	JINR
108	Hs	hassium	1983	GSI
109	Mt	meitnerium	1982	GSI
110	Ds	darmstadtium	1994	GSI
111	Rg	roentgenium	1994	GSI
112	[Uub]	[ununbium]	1996	GSI
113	[Uut]	[ununtrium]	2003	JINR
114	[Uuq]	[ununquadium]	1999	JINR
115	[Uup]	[ununpentium]	2003	JINR
116	[Uuh]	[ununhexium]	2000	JINR
117	[Uus]	[ununseptium]	–	–	not yet synthesized
118	[Uuo]	[ununoctium]	2002	JINR
119	[Uue]	[ununennium]	–	–	not yet synthesized
120	[Ubn]	[unbinilium]	–	–	not yet synthesized
121	[Ubu]	[unbiunium]	–	–	not yet synthesized
122	[Ubb]	[unbibium]	–	–	not yet synthesized
Chicago:		University of Chicago
GSI:		Gesellschaft für Schwerionenforschung; Darmstadt, Germany
LBL:		Lawrence Berkeley National Laboratory; Berkeley, California
ORNL:		Oak Ridge National Laboratory; Oak Ridge, Tennessee
JINR:		Joint Institute for Nuclear Research; Dubna, Russia (Объединенный Институт Ядерных Исследований; Дубна, Россия)