Physical Astronomy Professor Lee Carkner Lecture 11 Magnetic Activity Physical Astronomy Professor Lee Carkner Lecture 11
Questions A brown dwarf has L = 4X1022 W, M = 1X1029 kg, and R = 7X107 m. How many years could it shine from gravitational energy? E ~ -(3/10)(GM2/R) E ~ -(3/10)(6.67X10-11)(1X1029)2/(7X107) E ~ 2.86X1039 J T = E/L = 2.86X1039/4X1022 = 7.15X1016 s T = 2.27X109 years = 2.3 billion years
Magnetism In the regions above the photosphere, magnetic fields provide: Responsible for: Corona Magnetic field given as B Often use Gauss (1 T = 10000 G)
The Corona Above the photosphere is the corona Strongly ionized Can only be seen by eye during total eclipse Produces radio and X-ray emission
Emission Lines Corona produces forbidden lines Also produces X-ray line emission from strongly ionized heavy elements
Free-Free Emission Will emit a photon Sometimes called braking radiation or Bremsstrahlung Produces radio and X-ray emission in the sun
Synchrotron Emission As the electron spirals around it changes velocity and thus emits photons Produces radio emission from corona
Zeeman Effect Dn = eB/4pme The energy levels are slightly altered by the presence of the B field Dn = eB/4pme Where Dn is the difference between the normal line the magnetically altered one
vs = (gP/r)½ = (gkT/mmH)½ MHD Need to apply magnetohydrodynamics (MHD) vs = (gP/r)½ = (gkT/mmH)½ Where P, T and r are pressure, temperature and density and g is the ratio of specific heats for the gas vs is the “normal” wave speed Waves traveling faster than this will create a shock wave
Magnetic Pressure um = B2/2m0 Pm =B2/2m0 Where m0 is the permeability of free space = 4p X 10-7 N A-2 They don’t want to be compressed Pm =B2/2m0 This pressure is like the tension in a string
Alfven Waves The velocity of the Alfven wave is given by vm like a wave on a string The velocity of the Alfven wave is given by vm vm = B/(m0r)½ As they change the B field, they create currents that produce Joule heating
Mass Ejections Open magnetic fields produce the solar wind Particles spiral out away from the sun and into the solar system Speed ~750 km/s Large, random ejections of material
Sunspots Regions where a strong magnetic field is inhibiting convection B ~ 1/10 T T ~4000 K Up to 5 times fainter
Starspots But we can see variations in luminosity due to spots rotating in and out of our field of view Very magnetically active stars can have spots covering half the star
Sunspot Cycles Takes 11 years for Sun’s magnetic field to get wound up and then break and reconnect There is also a longer term cycle where the number of spots at peak is low for many cycles Temperatures were also anomalously low
Next Time Read 12.1-12.2 Homework: 11.13, 11.14, 11.16, 12.1, 12.10