Question

This is from an Intro Astrophysics class:

(a) Name the two factors that primarily determine the size of an H II region.

(b) Explain the physical basis for your answer to part (a) in terms of the Str ̈omgren sphere.

(c) The brightness of an H II region depends only on the gas density of the nebula. Explain this phenomenon in terms of what you know about the hydrogen atom. (Hint: Remember the Saha equation.)

Answer 1

a)H II region is a region of interstellar ionized atomic hydrogen in which star formation has recently taken place.

It's size vary from one to hundreds of light years across.

It's size depends on the intensity of the source of ionising photons and the density of the region.

b) A Strömgren sphere is the theoretical sphere which describes the ionized regions. According to this model - a)The hotter and more luminous the exciting star(more intensity), the larger the Strömgren sphere. b)The denser the surrounding hydrogen gas, the smaller the Strömgren sphere. Its clearly

c)The Saha equation is written as:

where:

• is the density of atoms in the i-th state of ionization, that is with i electrons removed.
• is the degeneracy of states for the i-ions
• is the energy required to remove i electrons from a neutral atom, creating an i-level ion.
• is the electron density
• is the thermal de Broglie wavelength of an electron

It describes the degree of ionization for any gas in thermal equilibrium as a function of the temperature, density, and ionization energies of the atoms. Chemically, H II regions consist of about 90% hydrogen. The star formation rates have been greater in the denser central regions, resulting in greater temperature and hence brightness. The strongest hydrogen emission line, the H-alpha line at 656.3 nm, gives H II regions their characteristic red colour. Above argument along with Saha's equation clearly proves that brightness of the H II regions depends on density.

In other words higher density means higher rate of formation of starts resulting more ionization of H atoms and hence higher brightness.