1446 Introductory Astronomy II

1. 1446 Introductory Astronomy II Chapter 12 The Lives of Stars R. S. Rubins Fall 2011

2. Lifecycles of Stars: General • The life of a star is a battle between opposing pressures: the inward pressure of gravitywhich tends to collapse the star, and an outward thermal pressure. • Thermal pressure is produced by nuclear fusion or by gravitational collapse. General rule • Gravitational collapse causes heating, thermal expansion causes cooling. • Degeneracy pressure, a quantum mechanical effect, halts the final collapse of all but the largest stars. • Because of degeneracy pressure, stars with masses of less than 8MSun end their lives as white dwarfs, while stars of masses between 8MSun and 25MSun end their lives as neutron stars.

3. The Interstellar Medium 1 • The interstellar medium is the space between stars, which contains gas and dust at very low densities. • Denser regions of the interstellar medium are known as gas clouds or nebulae. • The gas is mainly H and He, with tiny quantities of heavier elements created by previous generations of stars. • Dust grains are small rod-like clumps of heavier atoms or molecules of a size ( 100 nm) large enough to scatter light. • Star birth occurs in denser regions of nebulae, known as molecular clouds, which are also cool enough (≈ 20 K) to contain many species of molecule, including molecular hydrogen (H2), water (H2O), carbon monoxide (CO), ammonia (NH3), and about 130 species of organic molecule.

4. Detection of Molecular and Atomic H • Molecular hydrogen(H2), the most common constituent of molecular clouds, is hard to detect, but other molecules, particularly carbon monoxide CO, are used as tracers. • Atomic hydrogen (H) is easily detected through its emission of 21 cm radiation, produced by an electron spin-flip.

5. Molecular Cloud in Scorpius

6. Bohr Theory and H Spectra Paschen lines lie in the infrared. Balmer lines lie in the visible range, ending at the n =2 level. The reddish color of emission nebulae is due to the wavelength of the n=3 to n=2 transition. Lyman lines lie in the ultraviolet.

7. Types of Nebula • Emission nebula(ionization nebula or an H II region) This nebula is red, because we observe the n=3 to n=2 Balmer emission line of atomic hydrogen, produced in a two-stage process, known asfluorescence. • Stage 1UV light absorbed from nearby O and B stars excites the electron to a state of higher n. • Stage 2A red photon is emitted as the electron drops from the n=3 to the n=2 level, on its way to the n=1 level. • Reflection nebulaThis nebula is blue, because of the preferential scattering of the shorter wavelength light by the dust grains; i.e. for the same reason that the sky is blue. • Dark nebulaThis nebula is black, because denser dust clouds are opaque, sometimes appearing as dust lanes.

8. Reflection Nebula and Interstellar Reddening • The blue light of the reflection nebula is taken from the light of the star, so that the observer sees light redder than the actual light emitted by the star. This phenomenon is known as interstellar reddening.

9. Emission and Reflection Nebulae • Remember, emission nebulae arered, reflection nebulae areblue, and dark nebulae areblack.

10. Star Cluster and Dark Nebula

11. Barnard 86: a Huge Dark Nebula

12. Flame Nebula, VISTA 2009

13. Giant Molecular Clouds in Orion

14. Orion Nebula in Visible and IR Visible Infrared

15. Horsehead Nebula 1

16. Horsehead Nebula 2

17. Horsehead Nebula 3

18. Eagle Nebula 1

19. Eagle Nebula 2 • The white arrows point towards dense regions of gas, which are particularly suitable for star formation. • The ends of the pillars are then blown away, especially by the radiation emitted by hot O and B stars.

20. Cat’s Paw Nebula

21. Carina Nebula

22. Life-cycle of a Star of One Solar Mass 1 • Molecular cloud to protostar • The following conditions are necessary for star birth: i. high density, which increases the inward pressure of gravity; ii. low temperature, which decreases the outward thermal pressure. • A molecular cloud is cold, and passage through the cloud of a shock wave, produces denser regions, known as dense cores. • As the dense core accretes matter, its central region eventually becomes dense enough to prevent the thermal energy from escaping, so that its temperature and pressure rise rapidly. • The molecular cloud has now become a protostar. • When the core temperature of the protostar reaches about a million K, it becomes an intense IR emitter.

23. Protostars • This false color IR photo of the Eagle Nebula, 7000 ly away, shows protostars (in yellow) not seen in visible light.

24. Molecular Cloud to Protostellar Disk Collapsing molecular cloud Initially rotates relatively slowly. Further collapse increases the rotation rate, producing a protostarsurrounded by an accretion disk. In later stages of collapse, jets of high-speed gas may be emitted.

25. Protostar Outcomes • As the dense core collapses, its rotation rate increases. • If not spinning too fast, a single protostar may be formed, with the accretion disk, which will ultimately becoming a planetary system; e.g. the Solar System. • For a faster spinning core, a close binary system may be formed, in which two protostars in close proximity, orbit each other.

26. Life-cycle of a Star of One Solar Mass 2 • Protostar to Star • As the protostar accretesmatter, it continually moves to the left on the HR diagram, since its surface gets hotter through the heat created by gravitational collapse. • However, its luminosity given by L = R2T4, may increase or decrease, depending on how the increase in T4 compares with the decrease in R2. • T increases because gravitational collapse causes heating, while R decreases because the increase of density of the central region of the protostar increases the gravitational pull on the outer regions. • When the central region (or core) of the protostar exceeds about ten million K (107K), thermonuclear fusion begins.

27. Protostar Development on HR Diagram

28. Evolution of an Open Cluster

29. Main Sequence Masses 1 • Most Main sequence stars have masses from 0.1 to 100 MSun. • A star of mass 120 Msun was first observed in 2008, when it was thought that a star of mass greater than about 150 MSun would be unstable, and immediately eject its outer layers. • In 2010 British astronomers found more massive stars inside two young star clusters, with surface temperatures of about 40,000 K. • The largest of these stars, R136a1, with a mass of about 265 MSun, is about 165,000 ly away, in the Tarantula Nebula of the Large Magellanic Cloud. • Estimated to be about a million years old, and shedding mass at a rapid rate, R136a1 was deduced to have had a birth mass of over 300 Msun.

30. Main Sequence Masses 2 • The brightest star R136a1, is just the largest of several stars in this cluster with masses greater than 150 MSun.

31. At the other extreme, a protostar of mass less than about 0.08 MSun (or about 80 MJupiter) has a core temperature below ten million K, which is too cool for nuclear fusion of 1H to occur (but fusion of the much rarer isotope 2H may occur). The resulting planet-like object is known as a brown dwarf. Main Sequence Masses 3 Gliese 229B: a brown dwarf

32. Approximate Mass-Lifetime Equation

33. Main Sequence Lifetimes

34. Main Sequence Core Evolution At birth (percentages by numbers of H and He nuclei) At 5 billion years (for a star of MSun) At 10 billion years

35. The Sun as a Main-Sequence Star • The Sun will spend 90% (or 10 billion years) of its energy-producing life as a main-sequence star, during which time it moves only slightly on the H-R diagram, towards higher energy and luminosity. • This stability has allowed the evolution of life on Earth. • Since it entered the main sequence about 5 billion years ago, the following changes have occurred in the Sun: • i. its luminosity has increased by about 40%; • ii. its surface temperature has increased from about 5500 K to 5800 K; • iii the percentage by mass of He in the core has increased from about 25%to over 50%.

36. Life-cycle of a Star of One Solar Mass 4

37. Life-cycle of a Star of One Solar Mass 5 • SummaryIn the red giant phase, the following occur: i. the core contracts and gets hotter; ii. the surface expands and cools.

38. The Sun Now and as a Red Giant

39. Quantum Theory and Nuclear Fusion in the Sun • The term thermonuclear fusion,or simply fusion, refers to a process in which two or more nuclei combine to form a heavier nucleus. • Calculations show that the core of the Sun would have to be more than 1000 times hotter for the first stage of H fusion to readily occur, otherwise two protons would not have the kinetic energy to overcome the electric repulsion between them. • Fortunately for us, quantum theory allows two protons to fuse with a very small probability (even at a temperature as low as ten million K), at just the rate needed to sustain life on Earth.

40. Cepheid Variables 1 • A variable Star is one which shows significant intensity variations. • The Cepheid andRR Lyrae variablesare pulsating variables, which alternately expand and contract at a steady rate. • Working at Pickering’s Harvard observatory, Henrietta Leavitt discovered in 1908 that the more luminous Cephied variables had longer periods. • Returning to Harvard in 1912, Leavitt published the period-luminosity relationship, which showed the existence of other galaxies, and extended the range of distance measurements.

41. Henrietta Leavitt’s period-luminosity relationship illustrated. Cepheid Variables 2

42. The period-luminosity relationship allowed astronomers to deduce that a gigantic universe exists outside the Milky Way. Cepheid Variables 3

43. Delta Cephei

44. Star Clusters • In general, all the stars in a cluster were formed from the same molecular cloud, so that they all • i.were formed at roughly the same time; • ii. have the same chemical composition; • iii. are at roughly the same distance (d) from Earth, so that the differences in their absolute magnitudes (M) equal those in their apparent magnitudes (m). M = m – 5 log (d/10). • An open clusteris a young cluster, typically containing 103 to 104 stars, which is only a million to ten million years old. • A globular clusteris an old cluster, typically containing 105 to 106 stars, which is a billion to ten billion years old.

45. Open and Globular Clusters Open cluster M6 in Scorpius Globular cluster 47 Tucanae

46. Smaller A, F, G, K and M stars have not yet reached the main sequence, indicating an age of about 2 million years. A Very Young Cluster: NGC 2264

47. The smaller stars have all reached the main sequence, while the larger O, B and A stars have left the main sequence, indicating an age of about 100 million years. An Open Cluster: the Pleides

48. Open Cluster NGC 3603

49. Globular Cluster M13 • Globular clusters are spherical distributions of 105 to 106 mostly older stars, with little new star creation occurring. • 2007 measurements on the globular cluster NGC 2808, have shown the presence of three generations of stars.

50. A Globular Cluster: M55 O, B, A and F stars have left the main sequence, indicating an age of about 10 billion years.