May 2021 1 227 Report
Astrophysics, Stromgren Sphere?

A pure hydrogen gas cloud of number density n=10 cm^-3 surrounds an O5V star that generates a Lyman continuum flux of NL = 3*10^49 sec^-1. The rate at which hydrogen recombines is a function of temperature but can be taken to be α=3*10^-13 cm^3 sec^-1.

a) What is the physical meaning of the Lyman continuum flux, NL?

b) Calculate the size of the Stromgren sphere associated with this star. Express your answer in parsecs.

c) Suppose you replace the O star with a B0 V star, which generates NL = 4*10^46 sec^-1. What is the size of its Stromgren sphere?

d) The Sun (a G2 V star) emits a Lyman continuum flux of NL = 1*10^39 sec^-1. What is the size of its Stromgren sphere? What can you can conclude about the types of main sequence stars that have observable HII regions?


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