Astro 1050 Wed. Apr. 5, 2017 Today: Ch. 17, Star Stuff Reading in Horizons: For Mon.: Finish Ch. 17 Star Stuff Reminders: Rooftop Nighttime Observing Mon, Tues, Wed. 1
Ch.9: Interstellar Medium Since stars die, new ones must somehow be born They must be made out of material like star: H, He, plus a little heavier elements Short-lived massive stars must be near their birthplace Hot massive stars are almost always found near clouds of gas and dust! Three types of interstellar nebulae or clouds Emission nebulae -- Glow with emission lines Reflection nebulae -- Reflect starlight Dark nebulae -- seen in silhouette 2
Emission nebulae The red glow is Hydrogen Balmer α (Hα ) emission Could be from hot gas but relative strength of emission lines not always right Can also get fluorescence: UV photon from bright star boosts electron to high level (or ionizes it) Emission lines created as electron cascades back down through H energy levels The horse is a dark cloud in front of the glowing gas. 3
Reflection nebulae The Pleiades Cluster of new stars Visible to unaided eye in western Taurus Stars form in clusters most of which slowly spread apart. Reflection nebula is reflected sunlight Can see stellar-like spectra with absorption lines Blue light scattered more efficiently than red Pleiades didn t form here just moving through this cloud of dust. 4
Dark Nebula Interstellar Reddening 5
Spectral Measurements Use spectra of stars Ignore broad ( high pressure stellar lines Very narrow (low pressure) lines from interstellar gas This one Ca II = Ca +1 Stronger in more distant stars Stronger when looking through interstellar gas clouds Hydrogen hard to measure remember Balmer rules 6
Measurements at other Wavelengths Infrared Cirrus really slightly warm dust X-Rays of hot gas near exploded stars (supernova) Radio observations of Molecular Clouds Called that because cool and dense enough for molecules to form H 2 also hard to detect CO common and easy to detect Densest have 1000 atoms/cm 3 T as low as 10 K Location of star formation 7
Collapse of Molecular Clouds Barely stable against collapse: Imagine slightly compressing cloud Gravity goes up because material is packed more tightly (R in 1/R 2 is smaller) Tends to make cloud want to collapse more Pressure goes up because material is packed more tightly (P ρt) and ρ higher Tends to make cloud want to expand For smaller clouds Pressure wins (stable) For larger clouds Gravity wins (collapse) As cloud collapses and becomes denser, smaller and smaller parts become unstable Shock wave can also trigger collapse 8
What will a forming star look like in HR diagram? Temperature changes relatively simple Starts out large and relatively cool Must be on red side of diagram It heats up as it contracts Must towards the blue Luminosity more complicated because it depends on T and R Not much energy to start with Luminosity must start out low Collapse releases grav. energy Luminosity will rise Fusion begins, releases more energy Luminosity at a peak Collapse slows, only have fusion now Luminosity declines Finally stabilizes on the main sequence 9
How does mass affect collapse? More massive protostars have stronger gravity Collapse speed will be much faster than for smaller protostars Fast collapse and short lifetime means massive stars can reach end of lifetime while low mass stars in cloud are still forming Supernova shocks may come from earlier generation of stars Sequential Star Formation Energy from supernova and other effects eventually disrupts cloud prevents further collapse. 10
Observations of Young Clusters Young cluster NGC 2264 Few million years old High mass stars have reached main sequence Lower mass stars are still approaching main sequence Known as T Tauri stars Naming of classes of stars: Usually named after first star in class: T Tauri Stars with letters (RR Lyrae) are typically variable stars Earlier stages hidden by dust 11
More details of stellar structure and energy generation Alternatives to the protonproton chain Fusion of Helium to heavier elements Proton-proton reaction slow because: Need two rare events at once High energy collision of 2 protons Conversion of p n during collision 12
The CNO Cycle Gives way around need for p n during the collision Still must happen later but don t need to rare events simultaneously Trade off is need for higher energy collisions (T>16 million K) Add p to some nucleus where new one is still stable Wait for p n while that nucleus just sits around The net effect is still 4 1 H 4 He C just acts like a catalyst 13
Heavy Element Fusion Triple Alpha process 4 He + 4 He 8 Be + γ 8 Be + 4 He 12 C + γ Similar type reactions create heavy elements above 600 Million K Plot to left gives: x: # of neutrons y: # of protons Right one add neutron Up one add proton Diagonal p n or reverse Jumps: add 4 He or more 14
Models of Stellar Structure Divide star into thin shells,calculate how following vary from shell to shell (i.e. as function of radius r) P (Pressure) T (Temperature) ρ (Density) To do this also need to find: M (Mass) contained within any r L (Luminosity) generated within any r P example: dp dr GM r = 2 ρ = gρ 15
Numerical Stellar Models 16
Why don t stars collapse? Limiting case: Assume no nuclear fusion, only energy source is gravity. Star is almost in hydrostatic equilibrium Star radiates energy: If nothing else happened T would drop, P would drop, star would shrink. Star does shrink, but in doing so gravitational energy is converted to heat, preventing T from continuing to drop. In fact, since star is now more compact, gravity is stronger and it actually needs higher P (so higher T) to prevent catastrophic collapse As star shrinks, ½ of gravitational energy goes into heating up star, ½ gets radiated away Rate at which it radiates energy, so rate at which it shrinks, is limited by how insulating intermediate layers are 17
Why do we get steady fusion rates? Strange counterintuitive result: As star radiates away thermal energy it actually heats up (because as it shrinks gravity supplies even more energy) Star continues to shrink till it gets hot enough inside for fusion (rather than gravity) to balance energy being radiated away. Nuclear thermostat If fusion reactions took place in a box with fixed walls: Fusion more energy higher T more fusion (explosion) If fusion reactions take place in sun with soft gravity walls : If fusion rate is too high T tries to go up but star expands and actually ends up cooling off slowing down fusion. (steady rate) 18
Mass-Luminosity relationship L M 3.5 Why? Higher mass means higher internal pressure Higher pressure goes with higher temperature Higher temperature means heat leaks out faster Star shrinks until T inside is high enough for fusion rate (which is very sensitive to temperature) to balance heat leak rate 19
Lifetime on Main Sequence L M 3.5 T fuel / L = M/M 3.5 = M -2.5 Example: M=2 M Sun L = 11.3 L Sun T =1/5.7 T Sun Spectral Type Mass (Sun = 1) Luminosity (Sun = 1) Years on Main Sequence O5 40 405,000 1 10 6 B0 15 13,000 11 10 6 A0 3.5 80 440 10 6 F0 1.7 6.1 3 10 9 G0 1.1 1.4 8 10 9 K0 0.8 0.46 17 10 9 M0 0.5 0.08 56 10 9 20
How about a 0.5 solar mass star? M = 0.5 M sun Time = Luminosity = 21
How about a 0.5 solar mass star? M = 0.5 M sun Time = 5.7 times solar lifetime Luminosity = 0.09 solar luminosity 22
Width of Main Sequence and Stellar Aging As star converts H to He you have more massive nuclei Pressure related to number of nuclei Gravity related to mass of nuclei Pressure would tend to drop unless something else happens Temperature must rise (slightly) to compensate Luminosity must rise (slightly) as heat leaks out faster 23
Orion Nebula: A Star-Forming Region Red light = Hydrogen emission Blue light = reflection nebula Dark lanes = dust Astronomy Picture of the Day: http://antwrp.gsfc.nasa.gov/apod Massive (short-lived) stars are hot and produce lots of UV photons UV photons ionize Hydrogen but recombination produces Balmer photons (remember Hydrogen atom s structure). Detailed observations reveal ongoing star formation! Hubble Space Telescope Image 24
Protoplanetary Disks in the Orion Nebula Dusty disk seen in silhouette against background emission nebula Central star more visible in infrared since these photons can emerge from dusty cloud Hubble Space Telescope Image 25
Herbig-Haro objects: Jets of emission line gas from protostars As clouds try to collapse angular momentum makes them spin faster A disk forms around the protostar Material is ejected along the rotation axis 26
Example: Herbig-Haro 34 in Orion Jet along the axis visible as red (Balmer emission) Lobes at each end (shown in green) where jets run into surrounding gas clouds Radio data show a small, dense disk of gas 27
Motion of Herbig-Haro 34 in Orion Hubble Space Telescope Image Can actually see the knots in the jet move with time and show object is very young (few Million years) In time jets, UV photons, supernova, will disrupt the stellar nursery 28