Life Cycle of a Star Notes Write in Cornell Notes format

Star (estrella) Ball of gas (mostly hydrogen) Produces light by nuclear reactions

Solar Composition 70 % of Hydrogen 28 % of helium and a small amount of other elements

Nuclear Fusion (fusión nuclear) A reaction that happens when two atomic nuclei combine to form a nucleus with a higher mass Energy is released

Nuclear Fusion

Equilibrium (equilibrio) Stars are in equilibrium: – The balance between the gravity squeezing inward and the pressure from nuclear fusion pushing outward.

The Balance Between Pressure and Gravity Fusion reactions produce outward pressure and causes expansion. Gravity pulls particles toward each other and causes contraction. The life of a star is determined by the balance of these forces.

The Balance Between Pressure and Gravity

Temperature and Wavelengths Emitted

Star Colors (colores de las estrellas) Blue Star: hottest and largest Yellow Star: medium temperature and size Red Star: least hot and smallest

Classifying Stars—The H-R Diagram Two astronomers independently developed diagrams of how absolute magnitude, or luminosity, is related to the temperature of stars.

Classifying Stars—The H-R Diagram 90% of stars are in the main sequence. The other stars fall into one of three other groups. –Red giants –Supergiants –White dwarfs

Life Cycle of a Star

1. Nebula (nebulosa) A large cloud of gas and dust in space where new stars form No nuclear fusion No equilibrium

2. Protostar (protoestrella) A large mass that forms by contraction out of the gas of a nebula. As it contracts, it becomes spherical and increases in temperature. Core not yet hot enough for fusion. No nuclear fusion No equilibrium

3. Main Sequence (estrella de secuencia principal) Nuclear Fusion of hydrogen to helium Equilibrium is balanced Our sun! Our Sun

How Stars Come to an End Eventually a star converts all its hydrogen to helium. In smaller stars, fusion will continue to convert helium into carbon, nitrogen, and oxygen. In very massive stars, fusion reactions continue to produce heavier elements. When fusion stops, there is no longer any force to balance gravity.

The Life Cycle of Small Mass Stars

4a. Red Giant (gigante roja) A low mass star at the end of its life, as hydrogen fuel begins to run out Equilibrium becomes unstable, it expands Red because its surface temperature cools as it expands

5a. Planetary Nebula (nebulosa planetaria) Dying star The outer atmosphere blows away No nuclear fusion No equilibrium

6a. White Dwarf (enana blanca) The small, dense, core of a giant star that remains after the star has lost its exterior matter Our Sun will become a white dwarf No nuclear fusion No equilibrium

7a. Black Dwarf (enano negro) Burnt out cold core of a white dwarf (The universe isn’t old enough to have any black dwarfs yet.) No nuclear fusion No equilibrium

The Life Cycle of High-Mass Stars A supernova forms when a supergiant explodes before dying.

4b. Supernova (supernova) A supergiant star that explodes before dying

5b. Neutron Star (estrella neutrón) A star composed primarily of neutrons Very dense Left over after a supernova explosion

6b. Black Hole (hoyo negro) A region of space from which no matter or radiation can escape Created when a neutron star collapses and all its mass is concentrated into a single point Can be detected by their influence on other nearby objects