Transcript Stars: Element factories.

Stars: Element Factories
•Review of last class
• Dimitri Mendeleev and the
Periodic Table
•What is an isotope?
•Internal structure of the Sun
•Stars and nuclear fusion
What’s a Mole???
Mole (abbr. mol): An amount of an
element (or compound) whose mass,
expressed in grams, is numerically
equal to its atomic (or molecular)
weight.
Atomic Number (Z )
Atomic Symbol
Atomic Weight
Avogadro’s Number
• Avogadro’s number: the
number of atoms in 1 mole
• 6.022  10
23 mol-1
Dimitri Mendeleev
• At 33, Russian chemist
Dimitri Mendeleev was
appointed as chair of
chemistry at the University
of St. Petersburg.
• In 1871 he published a
periodic table of the
elements and predicted the
properties of 3 new elements
(Gallium, Scandium and
Germanium, discovered in
1875, 1879 and 1886)
Mendeleev’s Periodic Law
• The properties of elements are periodic
functions of their atomic weight
K
Na
Rb
Cs
Julius Meyer, 1870
Mendeleev’s Periodic Law
• The periodicity led him to organize the elements
into the PERIODIC TABLE OF ELEMENTS
Sample calculation
•Atomic weight of copper (Cu): 63.55 g
• A 63.55 g sample of Cu (= 1 mole) contains
6.022  1023 atoms.
• A penny weighing 1 g contains 6.022  10
63.55 =
digits)
23
9.48  1021 atoms (to 3 significant
Atomic isotopes
• Mass number (A): number of protons +
number of neutrons in an atom
•A is often indicated in the top left corner
of the element symbol
16O, 18O
What’s an Isotope???
• Definition: An Isotope is one of two or
more atoms having the same atomic number
but different mass numbers.
Hydrogen (1H)
Deuterium (2H)
Tritium (3H)
Uranium-235
(235U)
Uranium-238
(238U)
Mass
number
(A)
Number of
protons (Z)
Number
of
neutrons
1
2
3
1
1
1
0
1
2
235
92
143
238
92
146
Molecules
• Definition: Molecules are two or more
atoms joined together by chemical bonds.
• E.g., H O
2
• The molecular weight is ~ the sum of the
atomic weights.
•  one mole of water weighs ~ 18.02 g.
Composition of the universe
Helium
(7.8%)
Hydrogen
(92%)
Carbon (.03%)
and Oxygen (.06%)
All other elements (.11%)
How are the elements created?
The Sun
The sun’s size in perspective
To scale
Interior of the Sun
• The sun has multiple
layers (like an onion)
• The sun’s energy
source is generated by
H fusion at the core
• Heat escapes the
core by radiation
• In the outer part of
the sun, heat is
transferred by
convection
Solar Data
Value
Surface temp.
6000oC
Diameter
1.392x106 km
Density
1.41 g/cm3
Mass
1.989x1030 kg
Surface gravity
274.4 m/s2
Luminosity
3.9x1026 Watt
Rotation period
25.4 days
Equator
28.0 days
40o latitude
36.4 days
80o latitude
Compared to
Earth
Hot!
109
0.26
330,000
28
1.2x1014 *
* Compared to
energy
consumption of
U.S.A.
Nuclear fusion reactions
Definition: Nuclear fusion is the process of
combining two (or more) atomic nuclei to
create a new element.
• Requires extremely high temperature and
pressure
• Energy release: E = mc2
Hydrogen fusion: the sun’s
energy source (at the present)
• All naturally occurring
elements are created by
fusion in the interiors of
stars
•4 H nuclei are required to
make 1 He nucleus
• Net mass reduction: 0.7%
• H -> He is a 3-step
process
Evolution of a 15 solar-mass star
Element
fused
Hydrogen
Helium
Carbon
Neon
Fusion
product
Helium
Carbon
O, Ne, Mg
O, Mg
Oxygen
Silicon
Si, S
Iron
Time
Temp. (K)
1010 years
> 106 years
1000 years
A few
years
1 year
A few days
4x106
1x108
6x108
1x109
2x109
3x109
Source: In Quest of the Universe, Kuhn, 1998