The Life Cycles of Stars

1. The Life Cycles of Stars Dr. Jim Lochner, NASA/GSFC

2. Stellar Nursery Space is filled with the stuff to make stars.

3. Nebulas--Stars start from clouds Clouds provide the gas and dust from which stars form.

4. Collapse to Protostar • Stars begin with slow accumulation of gas and dust. • Gravitational attraction of Clumps attracts more material. • Contraction causes Temperature and Pressure to slowly increase.

5. Nuclear Fusion ! A Main Sequence Star is born. • At 15 million degrees Celsius in the center of the star, fusion ignites ! • 4 (1H) --> 4He + 2 e+ + 2 neutrinos + energy • Where does the energy come from? • Mass of four 1H > Mass of one 4He

6. A Balancing Act • Energy released from nuclear fusion counter-acts inward force of gravity. Throughout its life, these two forces determine the stages of a star’s life.

7. The Beginning of the End: Red Giants • After Hydrogen is exhausted in core ... • Gravity is no longer counteracted by the energy of nuclear fusion, so… • 1. Core collapses, • Kinetic energy of collapse converted into heat. • This heat expands the outer layers.

8. Meanwhile, as core collapses, • Increasing Temperature and Pressure cause…

9. More Fusion ! • At 100 million degrees Celsius, Helium fuses: • 3 (4He) --> 12C + energy • New energy output sustains the expanded outer layers of the Red Giant

10. The end for Medium-Sized Stars After Helium exhausted, outer layers of star expelled Planetary Nebulae

11. White dwarfs • At center of Planetary Nebula lies a • White Dwarf. • Size of the Earth with Mass of the Sun “A ton per teaspoon” • Inward force of gravity balanced by repulsive force of electrons.

12. A Supergiant You Know

13. Fate of high mass stars--Supergiants • After Helium exhausted, core collapses again until it becomes hot enough to fuse Carbon into Magnesium or Oxygen. • 12C + 12C --> 24Mg OR 12C + 4H --> 16O • Through a combination of processes, successively heavier elements are formed and burned.

14. The End of the Line for Massive Stars • Massive stars burn a succession of elements. • Iron is the most stable element and cannot be fused further. • Instead of releasing energy, it uses energy.

15. Supernova !

16. Supernova Remnants: SN1987A • a) Optical - Feb 2000 • Illuminating material ejected from the star thousands of years before the SN • b) Radio - Sep 1999 • c) X-ray - Oct 1999 • d) X-ray - Jan 2000 • The shock wave from the SN heating the gas a b c d

17. Supernova Remnants: Cas A Optical X-ray

18. What’s Left After the Supernova • Neutron Star (If mass of core < 5 x Sun) • Under collapse, protons and electrons combine to form neutrons. • 10 Km across • Black Hole (If mass of core > 5 x Sun) • Not even compacted neutrons can support weight of very massive stars.

19. Supernovae compress gas and dust which lie between the stars. This gas is also enriched by the expelled material. This compression starts the collapse of gas and dust to form new stars.

20. Materials for Life Cycles of Stars • This presentation, and other materials on the Life Cycles of Stars, are available on the Imagine the Universe! web site at: • http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/stars.html

21. The Hertsprung-Russell Diagram The Sun seen in X-rays

22. The Hertzsprung-Russell Diagram Usually abbreviated to HR Diagram. This is a plot of luminosity (brightness) against color for a selection of stars.

23. Observable properties Brightness: Measure by absolute magnitude, or by the total power output of the star. Color: Measure by spectral class (OBAFGKM), by temperature, or by color index.

27. Types of star Stars are not scattered randomly throughout the HR diagram, but fall into classes. They are • The main sequence • Giants and supergiants • White dwarfs

28. The Main Sequence Most stars reside in a broad band stretching from the top left (hot and luminous) to the bottom right (cold and faint). The Sun lies pretty close to the centre of this band.

29. The Main Sequence • The main sequence consists of stars whose principal source of energy is the nuclear fusion of hydrogen to form helium in the star’s core.

30. Giants and Supergiants • These lie in the upper right of the HR diagram, meaning that they are cool but luminous (bright). • Their luminosity is high because they are very large, and so have a big surface area to radiate from. Typically they may have a radius one hundred times that of the Sun. • The most luminous are known as supergiants.

31. Giants and Supergiants • The giants and supergiants are stars which have exhausted their supply of hydrogen fuel in their cores, and which produce energy by burning heavier nuclei such as helium.

32. White Dwarfs These lie in the lower left of the HR diagram, meaning that they are hot but faint. There are probably very large numbers of these, but they are not easy to detect.

33. White Dwarfs White dwarfs are remnants of stars which have completely exhausted their core nuclear fuel and which have too little gravity to contract further. They have no new source of energy and are cooling into obscurity.