Stellar Evolution

1. Stellar Evolution From protostars to supernovas

2. Protostars • Large nebulas of gas that begin to collapse or contract heat up as atoms interact, causing them to glow. • Once an opaque structure forms, it is considered a protostar.

3. Protostar Structure • As a protostar collapses, an accretion disk forms around the protostar, and jets of electromagnetic radiation erupt from the poles. • Jet • Accretion Disk

4. Brown Dwarf • When a protostar doesn’t have enough mass to cause large scale hydrogen fusion, it forms a small Brown Dwarf star. • Brown Dwarfs glow dimly, and are only a little bit larger than some planets.

5. Red Dwarf • A Red Dwarf is less than half a solar mass. • It has convection currents in its core and envelope. • It is relatively cool and dim. • They are the most common stars that are visible.

6. Sun-like stars • Sun like stars are similar in mass to our Sun. • The sun is a yellow dwarf star. • It has several layers.

7. Corona • Corona – The outermost layer, it is the second hottest at 4000 K. It is made of gases moving away from the sun, also known as the solar wind.

8. Chromosphere • The second layer of the sun. • The chromosphere is the thin lower layer of the atmosphere. • Solar flares and prominencesoriginate in the chromosphere.

9. Photosphere • The gaseous surface of the sun. • The photosphere absorbs heat energy from lower layers, and then releases the energy as light (electromagnetic energy.) • It is the part of the sun that visibly glows. • Cooler areas in the photosphere are darker, and are called sunspots.

10. Convection Layer • The next layer absorbs light and heat from below. • As it warms, convection currents are created that transfer heat from the inner layers to the photosphere.

11. Radiative layer • The radiative layer is extremely dense. • Electromagnetic x-rays are absorbed from the core, re-emitted, and reabsorbed. • It takes light millions of years to work its way through the radiative layer.

12. The core • The core is under such intense pressure from the layers pressing from above due to gravity. • The pressure is sufficient to cause nuclear fusion. • This releases tremendous amounts of energy as gamma ray electromagnetic radiation. • The outward force from fusion reactions keeps gravity from further collapsing the star. • The inward pressure from gravity keeps the fusion reactions from exploding the star.

13. Red Giants • These are stars that are between 0.3 to 6 solar masses. • Red giants form when all the hydrogen in a star’s core has fused to make helium. At this point, fusion stops, and gravity becomes the dominant force. • As the core contracts due to gravity, it heats up, releasing more energy to the radiative layer.

14. Red Giants 2 • As the radiative layer absorbs energy from the collapsing core, it also heats up until the pressure is enough to start hydrogen fusion in the outer layers. • The outward pressure from energy released by fusion in the outer layers causes the star to expand, causing it to grow to as much as 200 times larger.

15. White dwarf • White dwarves form when a Red Giant fuses the last of its hydrogen in its outer layers. • The outer layers heat up and expand to form a planetary nebula, which slowly drifts apart. • The inner core is left, slowly cooling and becoming darker until it forms a Black Dwarf.

16. Supernovas: Type 1a • If a white dwarf star absorbs enough new matter, its mass can increase enough to allow gravity to cause the heavy elements in the core to begin fusion. • Sometimes the resulting energy release is then enough to blow the star apart in a massive explosion.

17. Blue Supergiants • Blue supergiants are extremely hot and extremely dense. • They have enough gravity to fuse heavier elements such as Helium and Lithium to make elements as heavy as Iron. • They have very short lifespans before they fuse all the elements they can.