The Interstellar Medium (ISM) and Starbirth Intro to Stars Starbirth 1 How do we know that starbirth is going on now? •We see stars more massive than Sun. • Massive stars live only a few million years • Starbirth must have occurred ‘recently’ and is most likely occurring now Intro to Stars Starbirth 2 ISM = Interstellar Medium matter that’s between the stars (gas and dust) Interstellar gas: • mostly hydrogen (H) • clumps into clouds • hot, dilute gas between the clouds • neutral atoms, ions, electrons, molecules • very tenuous Intro to Stars Starbirth 3 Nebulae : plural of nebula (meaning cloud) Emission Nebulae : bright nebulae, caused by hot gases Spectrum would be ________ spectrum. • Energy from O and B stars that are inside the nebulae or near them • UV photons from the stars are absorbed by the gases in the nebulae, then given off as emission lines • Example: Orion Nebula - lines of H, O, He Intro to Stars Starbirth 4 The Rosette Nebula RED = Hydrogen GREEN = Oxygen BLUE = Sulphur APOD Intro to Stars Starbirth 5 North American Nebula APOD Intro to Stars Starbirth 6 Trifid Nebula APOD Intro to Stars Starbirth 7 M16 The Eagle Nebula more emission nebulae Intro to Stars Starbirth 8 O Star Hot - 30,000 K ionizes the H gas out to a few light years HII Region - Ionized Hydrogen HI - Neutral Zeilik 6/e Hydrogen Intro to Stars Starbirth 9 HI Region - clouds of neutral hydrogen • imagine these particles as spinning tops p+ • can spin in same direction or opposite e- Less energy to spin opposite Collisions cause them to be aligned Zeilik 6/e Intro to Stars Starbirth 10 SPACE • neutral H clouds • space between clouds filled with H gas, thin, neutral • even thinner, hotter gas (10,000 K) - ionized gases - coronal interstellar gas seen in X-ray wavelengths Intro to Stars Starbirth 11 Interstellar Molecules atoms combine at cold temperatures can vibrate or rotate, emitting or absorbing a photon vibrational => IR photon rotational => radio photon spinning => radio photon (mm wavelength) Most abundant is H2 - molecular hydrogen in UV and IR, about 2000 K Intro to Stars Starbirth 12 APOD Radio Map of Our Galaxy in CO Intro to Stars Starbirth 13 Molecular Clouds dark, dense, cold often near HII regions one sits behind the Orion Nebula Zeilik 6/e Intro to Stars Starbirth 14 Giant Molecular Clouds bulk of the ISM held together by gravity mostly H2 density ~ few hundred million kg / m3 few tens of light years 104 - 107 solar masses 10 K in center (that’s COLD!) Intro to Stars Starbirth 15 Giant H II region (ionized hydrogen) surrounds young, massive stars - found near molecular cloud complexes => play a role in star formation Zeilik 6/e Intro to Stars Starbirth 16 Table 15.2 Indicator Temperature Density Molecular clouds CO 10-50K 108 - 1015 H I Regions 21-cm rad. 50-100K 106 - 5x107 Intercloud Gas 21-cm rad. 7,000-10,000K 105 Intercloud Coronal Gas O VI 1,000,000 102 - 103 H II Regions H alpha 10,000 107 - 1010 Intro to Stars Starbirth 17 DUST (who would have thought!) One particle per football field sized cube 1% of the mass of all interstellar matter Yet it can totally block out what’s behind it. Dark Nebulae Intro to Stars Starbirth 18 The Horsehead Nebula APOD Intro to Stars Starbirth 19 APOD Intro to Stars Starbirth 20 APOD Intro to Stars Starbirth 21 Horsehead Nebula in IR APOD Intro to Stars Starbirth 22 Our Milky Way Galaxy dark nebulae Intro to Stars Starbirth 23 Dust • extinction - dimming of starlight • reddening - scattering of blue wavelength of light Reflection Nebulae • clouds of dust that reflect light from nearby stars • spectrum from a reflection nebula is the absorption spectrum of those stars • bluish in color due to scattering from dust grains Intro to Stars Starbirth 24 The Witchhead Nebula APOD Intro to Stars Starbirth 25 Merope, the Pleiades APOD Intro to Stars Starbirth 26 The Pleiades APOD Intro to Stars Starbirth 27 The Orion Nebula APOD more reflection nebulae Intro to Stars Starbirth 28 InfraRed Astronomy - Dust • dust blocks starlight for optical astronomers • IR radiation penetrates the dust • IR allows us to see the dust (glows) Dust: small, solid particles (grains) act like blackbody radiators temperature 100 K - peak in IR Intro to Stars Starbirth 29 In IR we see emission from cold dust (70K) at or near the center of the molecular cloud => dust is heated by something 70,000 Solar Lum. Zeilik 6/e Intro to Stars Starbirth 30 Dust H, O, C, N, Si H2O, CO2, NH4, silicates (O and Si) Core-Mantle Model Zeilik 6/e Intro to Stars Starbirth 31 Dust and Molecular Clouds Find a molecular cloud, you find dust. Dust is what allows the atoms to form into molecules. Gas in space is too dilute to allow atoms to be close enough to bond. Stuck on the dust grain, they can bond. Molecules form, then leave. More complex molecules form by UV processing. Intro to Stars Starbirth 32 Where is all this dust coming from? Denser grains are made in the atmosphere of supergiant stars and cool giant stars. Material streams outwards, temperature drops, solids condense. Star dust …. Intro to Stars Starbirth 33 Starbirth : The Models Born from interstellar clouds by gravitational collapse. • cloud has enough mass • low temperature • contracts from its own gravity Gravitational potential energy => kinetic energy (protostar) What happens as a result ? Intro to Stars Starbirth 34 Temperature gets hot enough for fusion reactions and voila! A Star is Born !! Intro to Stars Starbirth 35 After birth, a star evolves: luminosity temperature change over time size One characteristic drives how the star will evolve, how the star will live, how the star will die. MASS, MASS, MASS Intro to Stars Starbirth 36 Common stages of star formation: • cloud of gas, some light years across • collapse is fast, controlled by gravity • central region collapses faster than outer • small condensation forms in the center, the protostar one million years to get to this point radiates in the IR look for small, bright IR sources near known dense clouds of gas and dust • material accretes from the envelope onto the core Intro to Stars Starbirth 37 Dust-free regions clears out around the star. IR photons are emitted by the dusty shell. Star becomes visible when all the dust is Zeilik 6/e cleared away. (50 million years from collapse to pre-MS) Intro to Stars Starbirth 38 Ah! But what if it’s rotating while it’s collapsing? Rotating and spinning - the cloud will flatten into a disk with a central condenstation (protostar). This dusty disk fits the model for the formation of planetary systems. Intro to Stars Starbirth 39 Observations: Better chance of observing birth of massive stars - why? These are surrounded by dust so we need to use radio telescopes to observe them. What we look for: • molecular clouds - mm wavelengths • dust warmed to 30 K - IR (100 microns) • interior dust reaches 300 K - IR (microns) • MS - ionizes the H gas, forming H II region - radio (cm) • HII region expands to blow off the dust - star seen in optical Intro to Stars Starbirth 40 Zeilik 6/e Hot stars forming in a molecular cloud create an expanding H II region and shock wave that drives the collapse of more of the molecular cloud to make another cluster of stars. Intro to Stars Starbirth 41 APOD Evaporating Gaseous Globules - Eagle Nebula Intro to Stars Starbirth 42 Giant H II region (ionized hydrogen) surrounds young, massive stars - found near molecular cloud complexes => play a role in star formation Zeilik 6/e Intro to Stars Starbirth 43 NGC 3603 It’s all happenin’ here! APOD Intro to Stars Starbirth 44 Planetary Systems forming in the Orion Nebula (Proplyds) APOD Our Solar System Intro to Stars Starbirth 45 Milky Way in IR APOD Intro to Stars Starbirth 46 30 Doradus Nebula Intro to Stars Starbirth 47 APOD 30 Doradus Nebula APOD Intro to Stars Starbirth 48 Orion Nebula APOD Intro to Stars Starbirth 49 Flame Nebula APOD Intro to Stars Starbirth 50 NGC 891 APOD Intro to Stars Starbirth 51 .