Life Cycle of a Typical Star Objectives How

Life Cycle of a Typical Star

Objectives • How stars are created • How they live • What happens to them at the end of their lives

How Stars Are Created • Cloud of hydrogen is compressed to the right density • Force of gravity is more powerful than the tendency for it to disperse • Cloud then irresistibly collapses • Resultant heating of its interior should eventually ignite the process of thermonuclear fusion that powers stars

Eagle Nebula which astronomers think is the remnants of an exploded star

A Typical Star

Corona A corona is a type of plasma “atmosphere” of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The Greek root of the word corona means crown.

Chromosphere The chromosphere (literally, "color sphere") is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2, 000 kilometers deep.

Photosphere The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photosmeaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived to emit light. It extends into a star's surface until the gas becomes opaque, equivalent to an optical depth of approximately 2/3. In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Equilibrium: Life Goal of a Star This 5 -step process works like this: 1. Nuclear fusion. Gravity = gas pressure (equilibrium) 2. Out of fuel. 3. Fusion stops, temperature drops. 4. Core contracts (gravity pulling atoms in). 5. Increased temperature (more atoms, more collisions) and density in the core reinitiates nuclear fusion, equilibrium is achieved, and the cycle begins again at step 1.

Proton Fusion Two atoms of hydrogen are combined to create helium 4 and energy in several steps: 1. Two protons combine to form a deuterium atom (hydrogen atom with one neutron and one proton), a positron (similar to electron, but with a positive charge) and a neutrino. 2. A proton and a deuterium atom combine to form a helium 3 atom (two protons with one neutron) and a gamma ray. 3. Two helium 3 atoms combine to form a helium 4 atom (two protons and two neutrons) and two protons. These reactions account for 85 percent of the sun's energy. The remaining 15 percent comes from the following reactions: 1. A helium 3 atom and a helium 4 atom combine to form a beryllium 7 (four protons and three neutrons) and a gamma ray. 2. A beryllium 7 atom captures an electron to become lithium 7 atom (three protons and four neutrons) and a neutrino. 3. The lithium 7 combines with a proton to form two helium 4 atoms. The helium 4 atoms are less massive than the two hydrogen atoms that started the process, so the difference in mass is converted to energy as described by Einstein's theory of relativity (E=mc²). The energy is emitted in various forms of light: ultraviolet light, X rays, visible light, infrared, microwaves and radio waves. Note: information taken from “How the Sun Works” by by Julia Layton and Craig Freudenrich, Ph. D.

Proton Fusion

Herzsprung Russell (HR) Diagram

Death of a Star • A low mass star becomes a white dwarf • Medium-mass stars become neutron stars • The largest mass stars may become black holes

White Dwarf

Artist’s Conception of a Black Hole

Summary • Stars form from mostly hydrogen gas + gravity • 2 forces in equilibrium: gravity (inward) + nuclear pressure (outward) • Once proton fusion stops, star begins to die • Size determines death fate of the star