tailieunhanh - khí quyển sao p5

Khi bức xạ cuộc đấu tranh để làm cho trở lên theo cách của mình thông qua một bầu không khí sao, nó có thể được bị suy yếu do sự hấp thụ và tán xạ. Tác động kết hợp hấp thụ và tán xạ gọi là tuyệt chủng. Tán xạ chỉ đơn giản là có thể bởi sự phản xạ từ các hạt bụi. | 1 CHAPTER 5 ABSORPTION SCATTERING EXTINCTION AND THE EQUATION OF TRANSFER Introduction As radiation struggles to make its way upwards through a stellar atmosphere it may be weakened by absorption and scattering. The combined effect of absorption and scattering is called extinction. Scattering may simply be by reflection from dust particles. If the radiation interacts with an atom the atom may be excited to a higher energy level and almost immediately typically on a time-scale of nanoseconds the atom drops down to its original level and emits a photon of the same frequency as the one it absorbed. Such a process - temporary absorption followed almost immediately by re-emission without change in wavelength - is probably best described in the present context as scattering. Individual atoms in a stellar atmosphere generally radiate dipole radiation however since many randomly oriented atoms take place in the process the scattering can be regarded as isotropic. If however the excited atom collides with another atom before re-emission the collision may be super-elastic as the atom falls to a lower state the energy it gives up instead of being radiated as a photon goes to kinetic energy of the colliding atoms. The radiation has been converted to kinetic energy. This process is absorption. Absorption To start with let us suppose that the predominating mechanism is absorption with no scattering. We can define a linear absorption coefficient a as follows. Let the specific intensity at some level in an atmosphere be I. At a level in the atmosphere higher by a distance dx the specific intensity has dropped as a result of absorption to I dI . Here dI by the convention of differential calculus means the increase in I and it is in this case negative. The quantity -dx which is positive is the decrease in I. The linear absorption coefficient a is defined such that the fractional decrease in the specific intensity over a distance dx is given by dI - adx I The .