From Atoms To Stars
James Maxwell (1831 1879) Finalized the work of others on electricity and magnetism. He formulated Maxwell Equations for the electromagnetic field. His equations predicted the existence of electromagnetic waves that propagated with the speed of light. The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws.
Maxwell Equations
Max Planck (1858 1947) In 1900 he proposed that light is emitted in discreet quanta. Quantum Mechanics followed then Energy of a given quantum of light, or photon, is proportional to its frequency. The coefficient of proportionality, h, is called a Planck constant. h = (6.626,068,96 ± 0.000,000,33)x10-34 J*s
Electromagnetic waves They propagate in vacuum. Waves of different frequencies move with the same speed, the speed of light c. c = 299,792,458 m/s = 1,079,252,849 km/h
Electromagnetic spectrum
Atoms and light Electrons in atoms live at specific places, called orbitals or levels. Electrons on different levels have different energies. Only lowest energy states exist forever.
Atoms and light II Only 2 electrons per level are allowed. When an electron moves from a higher to a lower level, an atom emits a photon (the energy is conserved). When a photon of exactly that energy hits the atom, an electron can jump from a lower to a higher level; the atom then absorbs the photon. W. Pauli (1890-1958)
Atoms and light III All transitions from a particular lower level to all higher levels are called a series. Important series in astrophysics: Lyman series (in UV for hydrogen) Balmer series (in the visible for hydrogen)
Atomic spectra Ionized gas (plasma) emits photons of all wavelengths. Emission and absorption spectra are exact inverts of each other.
Let s Play a Game! On the next page you will see emission spectra of many common chemical elements. Your task is to find two identical ones. The person who finds them first gets A in the class and is released from all future assignments and the final exam. Ready, set, go
Find two identical spectra Text
Black Body spectrum This is a misnomer it is a spectrum emitted by hot, fully ionized plasma.
Astronomy + spectroscopy = astrophysics Solar spectrum
Alchemy in ages Since ancient times alchemists tried to find a way to convert different chemical elements into each other most often, iron, lead and copper into gold and silver. To support their efforts to attain that noble goal, they had to engage into more mundane work (making gunpowder, refining ores, inventing inks and dyes). They never realized, the power they seek was over their heads
Nuclear reactions Different nuclei can fuse together or split apart in nuclear reactions. Energy is either produced or consumed in these reactions. In nuclear reactions chemical elements transform into other elements, but numbers of protons and neutrons remain the same. In chemical reactions atomic nuclei do not change, only their bonds together change.
Binding energy per nucleon Energy required to completely dismember an atomic nucleus is called binding energy. Binding energy per nucleon is what matters in nuclear reactions.
Question In what kind of nuclear reactions does iron burn? A: fusion. B: fission. C: fire. D: none.
Cosmic composition Hydrogen: 74% Helium: 24% Oxygen: 1% Carbon: 0.5% Neon: 0.13% Iron: 0.1% Nitrogen: 0.1% Others: <0.05%
Origin of Chemical Elements For nuclear reactions to occur, the temperature must reach several million degrees Kelvin. Such temperatures occur naturally only in two cases: Inside the stars (and during stellar explosions). In the early universe about 3 mins after the Big Bang.
Early Universe Nucleosynthesis When the temperature in the early universe fell below about 3x10 12 K (1 ms since the Big Bang), protons (= Hydrogen) and neutrons formed from free quarks. Immediately neutrons began to decay. When the temperature fell to about 10 9 K (3 mins since the Big Bang), protons and remaining neutrons combined into Helium About 25% of all matter became He then.
Stellar Nucleosynthesis The rest of chemical elements are created in stars: Elements below Fe are created in stellar interiors, during their normal lifetime. Elements heavier than Fe are created in stellar explosions (novae and supernovae). We know that because calculation of stellar nucleosynthesis agree very well with observations.
BigBang Stellar interior Stellar explosions.
We Are the Star Stuff!!! Only 10% of you is made in the Big Bang Chemical composition of the human body
Question Why didn t nuclear reactions in the early universe go beyond Li? A: The temperature was too low. B: The density was too low. C: Nuclear reactions can only go in the lab. D: Binding energies of nuclei were different back then. E: They were forbidden by EPA.
Proton proton chain Result: 4 H ] 1 He Works in stars like the Sun and smaller. Rather inefficient. billion yrs 1 sec million yrs
CNO cycle Result: 4 H ] 1 He Works in stars more massive than the Sun. Efficient.
How stars generate energy Nuclear reactions in stellar cores convert (4) hydrogens into (1) helium. In massive stars, process can go further: He ] C, O, Ne, Mg C ] Ne, Na, Mg O ] Si, S, P Si, S ] Fe
How stars shine Energy from the nuclear burning core propagates outwards by diffusion or convection. It takes several thousand years for a photon to reach the surface.
Internal structure of stars Stars of different types have different arrangement for radiative and convection zones.