Stellar Formation

1. Stellar Formation

3. Inter Stellar Matter with a high enough density, and a low enough temperature for proto-stars to form. Protostars form in cold dark nebulae Star Formation

4. Giant Molecular cloud (GMC) in Orion • About 1000 GMCs are known in our galaxy • These clouds lie in the spiral arms of the galaxy, where the dust & gases are.

5. Size of cloud – large, Compression area - small Size: r ~ 50 pc Mass: > 100,000 Msun Temp.: a few 0K Interstellar clouds of mostly molecular hydrogen H Warmer GMCs resist forming stars, kinetic energy opposes the force of gravity to collapse the gas. A cooler gas is needed, and the GMC must be disturbed to induce it to collapse.

6. Star formation is triggered when a sufficiently massive pocket of gas is squeezed by some external event, such as a shock wave Sources of Shock Waves: (1). Since massive stars die young, Supernovae explosions happen near sites of recent star birth. (2) Previous star formation can trigger further star formation. (Stellar winds) (3) Spiral arms rotating can cause shock waves.

7. When you compress a gas it heats up. When a gas expands it cools As a proto-star evolves, it shrinks, its density increases and it temperature rises. Proto-stars are physically larger than the main-sequence stars that they will become. What types of stars are formed? OB – Few AFG – More KM – Many, Many

8. Observations of star formation: 0 Evaporating gaseous globules (“EGGs”): Newly forming stars exposed by the ionizing radiation from nearby massive stars

9. The Birth of Stars

10. near the stars

11. The collapsing protostar eventually heats up, and blows away its cocoon.

13. T Tauri Stars All proto-stars will eject gas before they reach main sequence but the cooler stars G,K, and M, do so more vigorously and are called T Tauri Stars.. Below is a T Tauri star with an accretion disk, and a jet of hot gas.

14. Low-mass stars that eject gas before becoming main sequence stars, may lose as much as 40% of its mass. Some young disks & jets revealed

15. Low-Mass Proto-stars • Collapse is slower for lower masses: • 1 Msun takes ~30 Myr • 0.2 Msun takes ~1 Billion years • When core temperature ~ 10 Million K: • Ignite core P-P chain fusion • Stellar wind blows away the cocoon • Settles slowly onto the Main Sequence • Some of the clump material settles into a rotating disk, from which planets might form .

16. Four protoplanetary disks in the Orion Nebula, 1500 light years away. • The disks are 99% gas and 1% dust. • The dust shows as a dark silhouette against the glowing gas of the nebula. Actual Protoplanetary Disks

17. To Reach Main SequenceAs the core heats up, H fusion runs faster: Core temperature rises Core pressure rises Collapse begins to slow down If the core temperature reaches at least 10 million deg K, the proto-Star becomes a Star • Finally: • Pressure=Gravity & collapse stops. • Star reaches the Zero-Age Main Sequence • (ZAMS).

18. Meanwhile, back in the GMC, things are still happening

19. Meanwhile the original stars are growing

20. Star Form in Clusters Stars do not form isolated, but in large groups, called Open Star Clusters . Our own Sun is part of an open cluster than includes other nearby stars such as Alpha Centauri and Barnard's star. Gravitational interactions between the stars and other objects will cause these clusters to eventually disperse over time

21. HR Diagram Hertzsprung-Russell Diagram:

22. Hertzsprung-Russell Diagram: In 1905, Danish astronomer Einar Hertzsprung, and independently American astronomer Henry Norris Russell, noticed that the luminosity of stars decreased from spectral type O to M. To bring some order into the different types of stars: they organize them in a diagram, the H-R diagram

23. 106 104 102 Luminosity (Lsun) 1 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K) H-R Diagram Basics Each star is represented by a dot.The position of each dot on the diagram corresponds to the star's luminosity and its temperature The vertical position represents the star's luminosity. The horizontal position represents the star's surface temperature.

24. Notice that the plot is not completely random, so there is some sort of relationship. 106 104 102 Luminosity (Lsun) 1 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K) H–R Diagram or L-T Diagram

25. 106 104 102 Luminosity (Lsun) 1 10-2 10-4 40,000 20,000 10,000 5,000 2,500 Temperature (K) H–R Diagram Supergiants Giants Main Sequence White Dwarfs

26. Color Version of H-R Diagram BRIGHT Cool Stars get larger Stars get hotter

27. Luminosity classes • Class Ia,b : Supergiant • Class II: Bright giant • Class III: Giant • Class IV: Sub-giant • Class V:MS • The Sun is a G2 V star

28. Mass-Luminosity relation

29. Radii on the Main Sequence L = 4πR2σT4 If you know L & T, you can calculate R There is a unique mass & radius for each star along the main sequence R  20 R M  30 M R  5 R M  7 M R  1 R M  1 M(sun!) R  0.3 R M  0.2 M

30. In the last few years, two new groups were added to the OBAFGKM classification, they are L & T. These stars have been found due to greatly improved infrared detectors aboard satellites. Both L & T are Brown Dwarfs. They are visible in the red, and infrared regions ,

31. Classification of Stars Ia Bright supergiant Ib Supergiant II Bright giant III Giant IV Subgiant V Main sequence star

32. Alternate way of describing temperature:SPECTRAL CLASS O = 40,000 K B = 20,000 K A = 10,000 K F = 7500 K G = 5500 K K = 4500 K M = 3000 K The spectral classes OBAFGKM began as a method of classifying stars according to the appearance of the absorption lines in their spectra. Classification:Spectral Class

33. 900,000 main sequence stars 95,000 white dwarfs 4000 giants 1 supergiant Random Sample of Stars If you took a random sample of 1,000,000 stars from our galaxy. In this sample, you will find, on the average:

34. Main Sequence

35. Pre-main sequence evolutionary tracks Most everything about a star's life depends on its (MASS).

36. Life Tracks for Different Masses • Higher-mass stars form faster • Lower-mass stars form more slowly

37. Stars more massive than 150MSun would blow apart **** Stars less massive than 0.08MSun can’t sustain fusion Luminosity Temperature Increases

38. Main sequence • Zero-age main sequence (ZAMS):ZAMS,phase at which star first gets all its energy from H burning (star no longer contracts). • Main sequence (MS):phase of core hydrogen burning, this is the longest stage in stellar life. • A star spends 90% of their life on the MS

39. Main Sequence Lifetimes(predicted)

40. Normal gas • Pressure is the force exerted by atoms in a gas • Temperature is how fast atoms in a gas move • Hotter atoms move faster higher pressure • Cooler • atoms move slower lower pressure Pressure balances gravity, keeps stars from collapsing

41. Core-Envelope Structure • Outer layers press down on the inner layers. • The deeper you go, the greater the pressure. • Gas Law : Greater pressure = hotter, denser gas • The star develops a Core-Envelope structure: • A hot, dense, compact central COREsurrounded by a cooler, lower density, extended ENVELOPE Where fusion takes place • Supplies gravity to the core

42. Stars on the Main Sequence, are in equilibrium. Gravity pulling inward wants to contract the star. Pressure pushing outwards from fusion wants to make the star expand. When there is a balance between the two, we have a condition of Hydrostatic Equilibrium. In this condition, the star neither expands, nor contracts.

43. Thermodynamics says : Heat always flows from hotter regions into cooler regions. In a star, heat flows from the hot core, out through the cooler envelope, to the surface where it is radiated away as light Radiation Energy is carried by photons. which leave the core, hit atoms or electrons and get scattered. They slowly stagger to the surface. Takes ~1 Million years for a photon to reach the surface.

44. Convection Energy carried from hotter regions to cooler regions above by bulk buoyant motions of the gas. Everyday examples of convection are boiling water.

45. Main-Sequence Starsand Fusion Energy is generated by fusion of 4 1H into 1 4He. There are two nuclear reaction paths by which a star might accomplish this fusion: • Proton-Proton Chain: Low mass stars • Relies on proton-proton reactions • Efficient at low core Temperatures(TC<18M K) • 4 x 1H à 1 x 4He + energy. Fuse 4 protons (1H) into 1 4He nucleus. This reaction produces the following by-products: Gamma-ray photons, 2 positrons , and 2 neutrinos that leave the Sun.

46. 2. CNO Cycle: High mass stars Efficient at high core Temperatures(TC>18MK) In stars that are hotter than 18 million degrees Kelvin, protons are fused into 1 Helium nucleus via a multi-step nuclear reaction , where Carbon is the catalyst.

47. More massive star will have the shorter life time • O & B burn fuel like a bus! • M burn fuel like a compact car! Every M dwarf ever created is still on the main sequence!!

48. X-Treme Stars Largest Star known: LBV 1806-20 Pistol Star 150-200 solar mass Temp 12,300 K Discovered 1995 Radius 500 time sun’s Distance 45,000 ly This star will eject gases into space, and by the time it becomes a main-sequence star, its mass may be 10 solar masses. Coolest White Dwarf SDDSS-J1403 Mass 0.6 solar mass Temperature 4,300 K WD Radius 0.01 times Sun Distance 145 ly Hottest StarWhite DwarfCentral star of NGC 2440 Temperature 211,000 K Mass 0.6 solar mass Radius 0.028 times Sun Distance 7,100 ly

49. Actual Motion from Radial Motion & Proper Motion (vr) Doppler Motion (Radial Motion) v Actual Motion (vt). Line of Sight Proper Motion (Tangential Motion)

50. Radial Velocity • The radial velocity of a star is how fast it is moving directly towards or away from us. (Doppler Effect) • Radial velocities are measured using the Doppler Shift of the star's spectrum: • Star moving towards Earth: Blueshift • Star moving away from Earth: Redshift • Star moving across our line of sight: No Shift • In all cases, the Radial Velocity is Independent of Distance. Earth