The Interstellar Medium (ISM)

1. The Interstellar Medium (ISM) Observations: gas and dust fills the space between the stars  ISM In the optical Emission and Reflection nebulae Dark clouds (Bok Globules) At radio wavelengths Giant molecular clouds You will meet a tall Bok Globule

2. The Orion Star Formation Nebula

4. Small dust clouds – Bok globules Bart Bok (1906 – 1983)

6. The ‘tadpoles’ of IC410 Image scale ~ 70 light years across Distance ~ 12,000 light years ‘tadpoles’ are ~ 10 light years in length A Bok Globule (The cosmic ‘finger’)

7. The interstellar medium (ISM) Observations: Glowing gas clouds and obscured (dusty) regions Star formation complexes In the optical we see Emission nebulae Reflection nebulae Dark clouds (Bok Globules)

8. Three basic nebula types Nebula type Characteristics Emission red color due to Hydrogen emission (HII regions) lines  made of hot hydrogen gas Reflection blue color: starlight reflected by dust into our line of sight Dark dust cloud absorbs (blocks) light (Bok Globules) in our line of sight After Bart Bok who first studied them in detail

9. Dust clouds About 1% of the mass of the ISM is in the form of small dust grains – 99% of ISM mass is in the form of a gas Observed as dark clouds, (Bok Globules) long columns and “lacy arcs” + dimming of starlight Grains are small: sizes ~ 10-7 m since they are very efficient at scattering blue light Same reason for blue sky on Earth = Rayleigh scattering Definition - Interstellar reddening: stars look reddish because their blue light component has been scattered out of our line of sight by intervening dust clouds

10. Schematic Dust Nebula Dark nebula if seen from this direction + reddening of embedded stars due to blue light scattering Reflection nebula if seen from this direction Blue starlight reflected by dust Bright star cluster Dust cloud Dust bunny: not a cosmic life form

11. Reflection nebula – dust scattering blue light

12. Star reddening due to blue light being scattered from LOS by dust Dust absorption: dark nebula (Bok Globule)

13. M16 The Eagle Nebula Star formation complex containing HII regions, dust pillars and Bok globules

14. A newly forming star with Herbig-Haro jet A dust pillar in Carina

15. Radio telescope observations Wavelengths of a few cm Vibrations in the carbon monoxide (CO) molecule produce radio-wavelength emission line spectrum Surveys find: Giant Molecular Clouds: most massive structures in the galaxy masses ~ 104 - 106 M diameters ~ 10 - 100 pc temperature ~ 20 K -250 oC

16. CO line 350 µm Molecular Line Survey of Orion KL star formation region

17. Giant Molecular Clouds have a complicated structure Contain many complex molecular species: Studied at radio wavelengths – molecule emission lines Water H2O, CO, CO2, ethanol H2CHCOH, Benzene C6H6 Filaments and strands + dense cores complex gas motions (turbulence) produced by newly forming stars and supernovae They contain emission nebulae (HII regions) These identify regions of active star formation – e.g., Orion Nebula GMCs account for ~25% of the mass of ISM This is about 5 billion solar masses of material

18. Star formation – gravity rules Observations show: stars form within low temperature molecular cloud cores in the ISM Key mechanism is gravitational collapse Large, low T, low density cloud Gravitational collapse Small, high T, high density star + 

19. Rotation is also very important Large, low T, low density cloud Gravitational collapse + + Rotation Small, high T, high density star  + Accretion disk • Gas collapses to form a rotating disk about the central star • Material spirals through disk to either fall upon the star or to build-up planets

20. Stars and planet formation Scale ~ 1017m gravity + rotation Eagle Nebula GMC core accretion disk forms Scale ~ 1013 m solar nebula Disk around GG Tau Home Scale ~ 107 m planets

21. GMC core Disk forms due to rotation Planets begin to form in disk Final system star + planets

22. Building The Planets Homage to Slartibartfast and the hyperspatial engineers of Magrathea

23. Making planetary systems Idea is: planets grow in the accretion disks about newly formed low mass stars – the process is a natural part of star formation in general Support for idea: we see disks about many stars our Solar System has disk-like characteristics

24. Observations from our Solar System All planets orbit the central Sun All planets orbit Sun in nearly the same plane The ecliptic plane = plane of Earth’s orbit All planets orbit Sun in the same direction Also: Planets and Sun have same age (4.5 billion yrs) and same basic chemical composition However, 99.9% of mass of Solar System is contained in the Sun

25. Solar nebula hypothesis First suggested by Immanuel Kant in 1775 but idea greatly improved upon since See history of astronomy: STS 232 NB: solar nebula = remnant accretion disk describes a ‘recipe’ by which planets form does not account for the exact number of planets does account for terrestrial and Jovian types Key idea: planets grow by accretion of gas and small planetesimals made of ice and rock From little acorns do mighty Oak trees grow

27. The Solar Nebula Temperature / distance dependent (determines where ice will form) Collision / accretion phase

28. ThePlanetesimals(survivor solar system) Asteroids Comets Comets and asteroids are remnants of the first formed solid objects in the solar system

29. Planetary types Recall: Two major planetary types Terrestrial – small, solid, ‘Earth-like’, inner solar system Jovian – large, gaseous, mostly H and He + ice moons, outer solar system Solar nebula hypothesis Division of types at boundary where ice can form The ice line: Terrestrial planets inside – Jovian planets outside Cold enough for ice to form at about 3 AU Jovian planets built from icy planetesimals + gas beyond ~3 AU Terrestrial planets built from mostly rock/iron planetesimals within 3 AU of Sun

30. The Solar System 134340 Pluto 136199 Eris Terrestrial planets Jovian planets Ice line decreasing temperature in nebula disk

31. Final growth of terrestrial planets Collision of proto-planets Sizes ~ planet Mars: 6-7,000 km across Incredible amounts of energy liberated during encounters Venus: Retrograde spin and tipped over due to a glancing blow Mercury: lost most of outer rocky mantle through direct impact Earth: origin of the Moon Off-center impact by Mars-sized protoplanet with young Earth (~ 50 to 100 million years old) The giant impact hypothesis Similar to solar nebula hypothesis Moon assembles by accretion in a disk of impact debris produced by collision Moon composed mostly of Earth rock

32. The Giant Impact Hypothesis for Moon’s origin • Off-center impact between Earth and a Mars sized proto-planet • Debris cloud forms about the Earth • Debris begins to interact and accrete • Numerous small moonlets form • Finally one moonlet wins the accretion ‘race’ and ends up being our Moon • Timescale to form – a few months! See: http://th.nao.ac.jp/~kokubo/moon/kit/movie.html

33. And with the formation of the Moon, we have come full circle… back to where we began the course some 12 weeks ago, when we looked at the Moon’s motion through the celestial sphere