EL: Be able to describe the evolution of stars. Stellar Evolution EL: Be able to describe the evolution of stars.

Our Sun Like all stars, our sun is made up of several layers that complete cycles of their own. The energy of the sun radiates from nuclear fusion taking place in the core We see the outer layers which have fascinating cycles that affect earth such as in the case of an aurora.

Auroras Ionized particles from the sun interact with Earth’s magnetosphere creating a beautiful display of lights. This Solar Weather also interrupts communication devices.

The Sun Today! www.spaceweather.com

Star Birth Stellar Nursery Stars begin to form in Dense areas of gas and dust due to gravitational pull. This process takes millions of years!!

Protostar A protostar is formed! This is a region in which the density of the interstellar medium is increasing due to gravitational effects. As the gravitational collapse continues the protostar will build enough heat and pressure to begin reactions within its core.

HR Diagram and the Main sequence

HR DIAGRAMS AN HR DIAGRAM PLOTS 4 VARIABLES LUMINOSITY (total energy output of the star) TEMPERATURE (surface temp) ABSOLUTE MAGNITUDE (brightness as it appears from 10 parsec away) B-V COLOR (color index)

Low and High Mass Stars Nuclear reactions begin to occur within the protostar igniting it. If a protostar doesn’t have enough mass to really ignite it becomes a brown dwarf, living it’s life as a cool burning star. Brown dwarfs and protostars are only detectable in the Infrared. If the protostar is massive enough the star will move onto the main sequence to live out the majority of it’s life.

HR Diagram and the Main sequence

The end of a low mass star Once a medium-size star has reached the red giant phase, its helium atoms fuse into carbon. The fusion releases energy, granting the star a temporary reprieve. The atomic structure of carbon is too strong to be further compressed by the mass of the surrounding material. No more fusion can happen. The core is stabilized and the star now begins to shed its outer layers (Cepheid phase) as a diffuse cloud called a planetary nebula. What remains is a white dwarf

Other options for low mass stars If the star is a low mass star (0.3 Solar mass) The star will form no planetary nebula, and simply evaporate, leaving little more than a brown dwarf. A star with less than about half a solar mass will never be able to fuse helium, even after the core ceases hydrogen fusion. These are the red dwarf stars, such as Proxima Centauri, which live for hundreds of billions of years then fade into brown dwarves

High Mass Star Once the outer layers of a star that is greater than 8 solar masses have swollen into a red supergiant the core begins to yield to gravity and starts to shrink. As it shrinks, it grows hotter and denser, and a new series of nuclear reactions begins to occur, creating progressively heavier elements, temporarily halting the collapse of the core. Then silicon fuses to iron, but iron is too heavy to fuse into any new element.

Supernova There is suddenly no energy outflow to counteract the enormous forces of gravity, and the star collapses. What happens next is a supernova explosion that takes less than a fraction of a second.

Neutron stars Massive stars become a nuetron stars after they supernova. These stars often will pulsate radio waves. (Pulsars) If the remaining neutron star is massive enough it will continue to collapse on itself to form a black hole. We call this dense area a singularity.