ShockShock WavesWaves inin InterstellarInterstellar SpaceSpace Prof. Wayne G. Roberge A Passion for Physics Oct. 2, 2008 Outline • A Famous Example • Anatomy of a Shock Wave • Shock Waves on Earth • Shock Waves in Space • What I do Nuclear Explosion shock front What’s going on? Undisturbed Gas Shock Swept-Up Gas Disturbance (cold) Front (hot) (supersonic) (low pressure) (high pressure) F/A 18 in Transonic Flight Prandtl-Glauert Condensation (makes shock visible) Rifle Bullet in Air spherical shock cone-shaped shock Supersonic Object in Air object “bow shock” Battleship Firing Big Guns shock front Shock Wave in Traffic v v v v L L undisturbed fluid shock disturbance front • Initially: uniform speed v and separation, L. • Next: driver on right touches brakes. • Next: a traffic pileup (“shock”) ensues. WHY? Velocity of Information L • Left driver sees light and reacts. • News travels left with speed c relative to traffic. c = L Δt Δt ≅ human reaction time Signal Velocity Rel. to Ground u = v-c • Shock (pileup) if u>0 or v>c. • Now put in some numbers! V = 70 mph = 100 ft/s L = (N+1)h h = 15 ft (one car length) Δt = 1.5 s* FOR HOW MANY CAR LENGTHS (N) does v=c? *Marc Green, ”How Long Does it Take to Stop?”, Transportation Human Factors, vol. 2, p. 195. Earth’s Bow Shock solar wind Protostars Have Jets! v v HH47: 3 trillion Mile Jet A Theoretical Model HH30: Disk Detected! Georg Friedrich Bernhard Riemann 1826 - 1866 The Riemann Problem (fluid dynamics) LEFT GAS RIGHT GAS VL = VELOCITY VR = VELOCITY NL = DENSITY NR = DENSITY PL = PRESSURE PR = PRESSURE x THE RIEMANN PROBLEM (MULTIFLUID MHD) MOLECULAR JET CLOUD V = 0 VBL = 20 KM/S BR 0 4 - 0 4 -3 NL = 4 X 10 CM NR = 4 X 10 CM 3 TR= 10 K TL= 10 K BR= 50 µG BL= 50 µG x *MNRAS, IN PRESS (ARXIV:0708.4260). Summary • Shock waves = hydrodynamical surprises. • Caused by supersonic disturbances. • Found almost everywhere in space. • Light up and heat cold interstellar gas. • Allow us to see, study star-forming material..