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Star Formation Or: What hydrogen can do if given enough time Common Ancestry All stars start out the same way No star forms in isolation Stars vary only in initial mass and, to some extent, initial chemical makeup – The *IMF, or Initial Mass Function, is how many of each kind of star is produced in a cluster These initial conditions determine the life of the star once it achieves the Main Sequence Ingredients: The Interstellar Medium HI 1 atom/cm3 T ~ 100K H2 1 atom/cm3 T ~ 10-30K HII 0.1 atom/cm3 T ~ 104K Stellar Remnants Varying density T ~ 107K Composition: ¾ H, ¼ He 2% everything else, ½ in dust Giant Interstellar Molecular Clouds Precursor to the star-forming *HII regions 105 Msun, 10pc in radius Found mostly in the spiral arms of the galaxy 10-20K By far H2 Also CO2, H2O, Ch3OH (methanol), CH4 (methane), NH3 (ammonia) Where did these C, O, and N come from? Hmmmm… *singly ionized hydrogen Varieties of Nebulae Emission Nebula: HII, star forming regions – If we lived in one the night sky would be as bright as day Reflection Nebula: Hot stars have blown away gas, leaving dust to reflect Dark Nebula: no stars (yet) – If we lived in one it would block out the stars Planetary Nebula: (misnomer, coined by Herschel) Horsehead Nebula both emission and dark Stars Coalescing out of a Nebula Felix Shih’s work Pleiades Orion Nebula HII regions are hidden by clouds opaque to visible light Orion Flythrough Young Star-Forming Region: Serpens Cauda Tarantula Nebula: if it was as close as the Orion Nebula it’d be as big as 60 full moons Bok Globule A cool spot to grow a hot star 10-50 solar masses, a light year across Dust obscures the HII region M16, the Eagle Nebula Stellar ‘eggs’: Evaporating Gaseous Globules, ~100AU Pillar ~ 4LY 3 color overlay, one for O(g), H(r), S(y) IR: new stars YouTube What do you need to cook up a star? Low temperature – Too hot, and the forming elements have too much kinetic energy, i.e. speed Sufficient density/mass – Enough stuff, close enough for gravity to do its job Not too much rotation A trigger Jealous Stars If the star-forming region is too hot, the elements (HII, dust, etc.) are moving too fast for gravity to overcome Sometimes one big star gets started before its neighbors, and it heats the region up so no other stars can form – However, if the first stars to turn on aren’t too hot, their solar winds can be a trigger MJ cs2 G 3 / 2 r 1/ 2 Jean’s Mass Minimum mass for a star forming region Another application of hydrostatic equilibrium – In this case, with no fusion, the mass needed to collapse a cloud of gas into a star – cs is the speed of sound in the gas – G you’ve seen, r is the density Less if cooler, more if hotter Generally, MJ > 0.085 Solar masses – Less than that and you get no fusion – Called a Brown Dwarf Rotation Much like a hurricane The section of the cloud that’s closer to the galactic core moves faster Differential rotation If the cloud is rotating too fast, gravity provides insufficient centripetal force to compact the material Different ways to measure rotation Upper Mass Limit If the initial mass M > 150 Solar Masses, it must come from a large, distended globule Conserving angular momentum, as it grows smaller it will rotate so fast it that it will tear itself apart – In the equation above, w is the spin speed and r is the radius. The subscript i means initial and f means final – Squaring intensifies the effect So 150Msun > M > 0.085Msun But then again, in the LMC… Update: the VLT has found stars up to 300 Msol Apparently, stars can’t form if > 150 Msol, but after formation mergers can occur making super mass stars See: – Blue stragglers – Type Ia supernova The Trigger Filaments of star formation resulting from the shock wave from a supernova Some force is needed to compact the protostellar material This can be: – A galactic density wave – A nearby supernova explosion – A collision between GMCs Once triggered, the result is inevitable The Animation So now you know: As the globule shrinks it: – Spins faster (why?) – Grows hotter (why?) The animation did not show the solar nebula – The left-over disk of material that eventually would form a system of planets, moons, asteroids, and comets That’s another course! Every star that achieves fusion lands on the H-R diagram. When it does we call it ZAMS Named for Ehnar Hertzsprung and Henry Norris Russell, working independently, although various forms of it were floating around at the beginning of the 20th century Spectral Type and surface temperature are here Luminosity in Solar units is here Absolute Magnitude on the right B-V here Spectral Type we’ll discuss in a few slides *Luminosity is how much energy the star emits *Absolute Magnitude is how bright the star would be if it was 10 parsecs away *B-V is a color metric, the difference in magnitude between the blue astronomical filter and the visible light filter *see The Brightness of Stars ppt. The Main Sequence mentioned before is the wavy line going from upper left to bottom right The Sun is here: There are many, many more red dwarfs than sun-like stars, and more sun-like stars than blue giants All the other regions, the Red Giants and the White Dwarfs, are stars that have evolved off the MS Pop Quiz! Where are the big, hot stars? Where are the big, cool stars? Where are the small, hot stars? Where are the small, cool stars? Spectral Types From the Light and Telescopes ppt. you know that atoms give off energy when their electrons fall from one orbital to another Fingerprints Each element has a unique set of orbitals, therefore a unique set of energies it can emit or absorb These energies translate to light colors: the higher the energy, the shorter the wavelength and the bluer the light The sum of these energies is the element’s spectrum and it is a fingerprint for the element For instance, lithium http://jersey.uoregon.edu/vlab/elements/Elements.html A family of spectra Origin of O-B-A-F-G-K-M The history of spectra in Astronomy began in the mid-late 19th century Pickering and Fleming, 2 Harvard Astronomers, classified stars in the 1890s based on the strength of H lines – A for strongest H lines – B for H plus He – C for more He, etc, through Q Pickering’s ‘Computers’ 1901 Annie Cannon, Pickering’s student, looked at the spectra of ¼ million stars (!) and rearranged them according to temperature, eliminating ambiguity and adding subdivisions Mnemonic for The Harvard classification system Oh Be A Fine *Girl Kiss Me Also new L (2000K) and T (<1300K) – Oh Be A Fine Girl Kiss My Left Toe? – The first phrase is risqué enough (for the Victorian 19th century) *or Gentleman, (goat, or gorilla). L had been originally discarded by Cannon Categories Cannon’s Harvard classification system is based on surface temperature, not on spectral content O, A, B were erroneously called ‘early types’ and the F, G, K, and M were ‘late types’ Sub categories B0, B1…B9, A0…A9, etc. The pictures are not quite so pretty This is closer to what Astronomers use Each dip in the line is an absorption Short wavelengths on the left, long on the right – Short, high energy hot – Long, low energy, cool – Hint: take our lab class! How things stack up You can see Pickering and Fleming’s problem with classification: H lines tail off on hotter AND cooler stars *Luminosity Classes I Supergiant II Bright Giant III Giant IV Subgiant V Dwarf (Main Sequence) VI Subdwarf This makes the Sun a G2V star *not shown on this HR diagram Nearby Stars So now you see how Spectral class and surface temperature go together All fusion burning stars make it here. What happens next is another story