1. Stellar Evolution Star birth in the Eagle NebulaStar birth in the Eagle Nebula Courtesy of the Space Telescope Science Institutethe Space Telescope Science Institute

2. Introduction Human lifetimes vs. ages of stars How do we know...? –Humans via pictures In one day, take pictures of people, then piece together human behavior & history; similar to finding the life history of stars –High-energy physics Interactions of matter/energy at extremely high temperatures Theories tested, modified, some completely rewritten Many questions remain unanswered

3. The Birth of a Star Nebulae = more than one nebula Vast clouds of gas in space Mainly hydrogen Disturbance –Colliding with other clouds –Blast from nearby supernova explosion

4. The Birth of a Star GRAVITY RULES!!

5. The Birth of a Star Rotating cloud collapses in on itself As center of the cloud becomes more dense, collapse accelerates due to increased gravitational attraction between gas particles Collapsing clouds mark the formation of a protostar (not yet a true star; no nuclear reactions occurring yet) Particles far apart don’t exert much gravity on each other The same particles, now closer together, exert more gravitational force on each other

6. The Birth of a Star As the clouds continue to collapse it begins to warm up When the gas particle collides with the center of the cloud, –it loses kinetic energy because it slows down –It loses potential energy because it isn’t so far away from the middle of the cloud. This energy turns into HEAT Out here, the gas particle has both kinetic & potential energy Center of gas cloud

7. The Birth of a Star Warming occurs slowly at first Center begins to glow, dim to bright When central temperature is high enough (~15 000 K, ~15 273 C) nuclear reactions can begin Protostar has now become a true star As the temperature increases, these hydrogen particles move faster. Eventually, they move so fast that when they collide they’ll stick together.* A helium nucleus has been formed! When this “sticking” (fusion) occurs, a bit of mass is converted to energy as in E = mc 2

8. The fight Fusion pressure pushes OUTWARD from core Gravity pulls INWARD toward core

9. The Birth of a Star Stars can form from extremely large interstellar clouds that have fragmented into smaller clouds. These clusters of stars are called... Star clusters (!) Ex: The Pleiades (Seven Sisters)

10. The Birth of a Star The haze (“nebulosity”) is part of the original gas cloud that’s left over. How long does formation take? –Small low mass stars can take billions of years to form –More massive stars can completely form in a few hundred thousand years

11. Main Sequence Star has settled into the most stable part of its life Converts hydrogen to helium (H => He) Next step depends on the mass of the star Three different examples of stars: 1.Stars similar to our Sun 2.Stars several times more massive than the Sun 3.HUGE HUMONGOUS stars, VERY massive

12. The Life of a Sun-like Star Will remain on the main sequence (H to He) for about 10 billion years As more He is produced, temperature increases and core contracts –We see this as an increase in brightness –Temperature not high enough to sustain He to C fusion Central core then expands as more He is produced Red Giant Star expands, becoming a Red Giant Our sun, as a red giant, will be as large as Earth’s present orbit

13. The Life of a Sun-like Star Over thousands of years, the star’s central region shrinks & heats up. Outer regions are pushed away We see: –a small, dense central star –surrounded by expanding shell of gas The star is now a planetary nebula

14. The Life of a Sun-like Star The object seen at the center of the gas cloud is the core of the original star Still very hot (~100 000 C) Gradually cools & contracts to become a white dwarf Cools even more to become a black dwarf; not much bigger than Earth, but much more dense

15. The Life of a Sun-like Star

16. The Life of a Star Several Times More Massive Than the Sun Enters main sequence (H to He process) at a higher temperature than smaller stars Core is hotter than smaller stars, causing faster “aging” After all H is converted to He, He is fused into carbon (requires 100 million degrees)

17. The Life of a Star Several Times More Massive Than the Sun After all the He is used, C fuses into neon (requires 500 million degrees) As each element is used up, star becomes a red giant.... And so forth, as long as temperatures are high enough to fuse that particular element As particles that are colliding get larger, much more heat (energy) is needed to get them to stick together

18. The Life of a Star Several Times More Massive Than the Sun When an iron core is formed: –Reactions STOP –Iron fusion requires HUGE amounts of energy –Eventually, cools to white dwarf, then black dwarf stage –Different than smaller star’s fate because different elements will compose the core

19. The Life of HUGE Stars As with all other stars, follows main sequence If the star is still large (>1.4 Suns) when the core becomes iron, a supernova results

20. The Life of HUGE Stars Within seconds of running out of nuclear fuel, the HUGE gravitational force (remember, large mass = large gravity) attracts all of the atmosphere into the core. http://ircamera.as.arizona.edu/ NatSci102/movies/corcoll3.gif

21. The Life of HUGE Stars As particles fall to the core they lose kinetic & potential energy and more HEAT results This heat triggers nuclear fusion in the outer layers, and the resulting explosion is the supernova. The energy released can fuse iron and other heavier elements, up to uranium.

22. The Life of HUGE Stars

23. “This next image is one of the most spectacular views of 1987A yet acquired by the HST. The single large bright light is a star beyond the supernova environs. Around the central supernova is a single ring but associated with the expansion of expelled gases are also a pair of rings further away that stand out when imaged at a wavelength that screens out much of this bright light.” Courtesy http://rst.gsfc.nasa.gov/Sect20/A6.htmlhttp://rst.gsfc.nasa.gov/Sect20/A6.html

24. The Life of HUGE Stars The death of the largest stars results in a core more dense than anything we know on earth This core has such a large gravitational force that light cannot escape it.... Hence the name, black hole Picture here

25. Caption: In this image, X-ray contours are overlaid on an optical image. The X-ray contours and the colors in the optical image represent brightness levels of the X-ray and optical emission, respectively. When viewed with an optical telescope this galaxy, located 2.5 billion light years from Earth, appears normal. But the Chandra observation discovered an unusually strong source of X rays concentrated in the central regions of the galaxy. The X-ray source could be another example of a veiled black hole associated with a Type 2 Quasar. This discovery adds to a growing body of evidence that our census of energetic black hole sources in galaxies is far from complete. CXO 0312 Fiore P3 (CXOUJ031238.9- 765134): A possible Type 2 quasar veiled black hole.(Credit: X-ray: NASA/CXC/SAO; Optical: ESO/La Silla) From http://chandra.harvard.edu/photo/2000/0312/0312_hand.html

26. Some artists’ conceptions of a black hole

27. The Life of HUGE Stars How do we know a black hole exists? Evidence –Strong x-ray emissions from charged particles accelerating REALLY fast –Gravitational lensing Light from stars is bent when a black hole is between us & the stars Usually form in binary star systems

28. We are all made of stars...