The Nature of StarsSzydagis

03.27.2015

G.H. Rieke

1/11

Some Questions

What keeps a star from collapsing in on itself? A. Gravitational force B. Nuclear fission pressure C. Nuclear fusion pressure D. Dark energy repulsion

What is the best method for determining what a star is comprised of, the most precisely? A. Spectroscopy B. Temperature of surface C. Visible color D. Mass from size

2/11

Pressure vs. Gravity

Stars (~spheres) are held together by hydrostatic equilibrium

Internal pressure pushing outward is finely balanced against a star’s self-gravity pulling all matter inward symmetrically

Stellar interior pressure gradient is equal to the gravitational force per unit volume as a function of the radius

What causes the pressure? Hot gas (technically plasma - that just means ionized gas). No neutral atoms, fully ionized.

At high temperature of sun/stars, nuclei moving around fast

3/11

Source of Energy

Moving fast because lots of kinetic energy. Origin of it?

Nuclear fusion! Star is hot enough (thus nuclei energetic enough) so that one atomic nucleus can merge with another, releasing vast amounts of energy, because of E = mc2

Electrostatic repulsion between (+) protons overcome

Nuclear fission on the other hand occurring here on earth in bombs and reactors is about the energy released in other way: from blowing a nucleus apart into smaller fragments Originally thought incorrectly that fission powers stars, then

realized not enough energy generated (lot less than fusion)

Bombs use both, reactors only fission. Studying fusion4/11

Nuclear Fusion

This is basically alchemy! Unlike chemical processes, nuclear ones *can* convert one element into another. Where the ancient chemists/alchemists got it wrong: difficult if not impossible to achieve temperatures/energies needed on Earth

Most basic fusion-class reaction: 2 hydrogen (recall: the most common element in the universe) nuclei (2 protons) merge, and 1 proton decays into a neutron, positron (anti-electron), and a neutrino (because 2 protons do not form a stable nucleus). Now we have nucleus of deuterium, next heavier isotope of basic hydrogen (1 proton and 1 neutron) Positron annihilates with electron into 2 gammas pretty soon Neutrino escapes. Many are easily capable of reaching Earth5/11

Proton-Proton Process

Deuterium meets a fresh free proton, creating isotope of helium (2 protons, 1 neutron). Another helium nucleus (~quarter of star initially, like universe) comes along to form “standard” helium (2 protons, 2 neutrons) and 2 protons get released, and cycle is fed

(From U. Tenn Astronomy 162, http://csep10.phys.utk.edu/astr162/lect/energy/ppchain.html)

6/11

DEMO…

Mass Differences

Mass of the deuteron = 3.3436 × 10-27 kilograms Mass of 1 free proton = 1.6726 × 10-27 kg Mass of 1 free neutron = 1.6749 x 10-27 kg Mass of 1 proton + 1 neutron = 3.3475 × 10-27 kg The difference = 0.0039 × 10-27 kg (or, 2.2 MeV)

Emitted as gamma (photon, particle of light)

7/11

Nuclear binding energy

Reaction Variations

8/11

CNO cycle (>15 million degrees K). Nobel Prize for Hans Bethe

Triple alpha process

Require >100 million degrees

HyperPhysics (R. Nave)

Spectroscopy

Absorption and emission lines: “striped” light (prism) Different elements possess characteristic spectra because

they have distinct electron shells (“orbital” energy levels)

Tell us chemical composition of a star: metallicity, etc. Blue-, red-shift. Holes punched or bright spots added

9/11

http://physics.weber.edu/palen/Phsx1040/Lectures/Ldistcomp.html

Lee Kwok-san, Tong Shiu-sing, and Janny Leung

Activity

Previously, you did exercise concerning star twinkling

Why is the color of a star related to its temperature? Can you come up with an analogy? How does a star’s luminosity/brightness come into play?

Are larger stars necessarily heavier? Why or why not?

10/11

Quote of Day + Homework

Keep on reading about stars. Book pages xeroxed this time, provided as PDF next to these slides on the web

Laser fusion experiment extracts net energy from fuel!! http://www.nature.com/news/laser-fusion-experiment-extracts-net-energy-from-fuel-1.14710

Don’t forget the written Homework (7) due on Mon.

11/11

Ann Druyan