Turn to page 591-592 for the 8th edition of your Cosmic Perspective textbook. Note the bright reddish color of the emission nebula above the famous dark Horsehead Nebula. This is a hot thin gas primarily made of hydrogen. What is the predominant wavelength of the photons emitted by the gas in this nebula?

Explanation:

Nebula emission are ionized cloud has and they emit there own light as an optical wavelength.

The high energy ultraviolet emitted from a nearby hot star is the most common source of ionization

Type of nebula emission

1. planetary nebulae

2. emission nebulae

3. reflection nebulae

4. dark nebulae

5. supernova remnants.

One of the most common types occurs when an interstellar gas cloud dominated by neutral hydrogen atoms is ionised by nearby O and B type stars.

O and B stars are hot, massive stars of spectral types O or early-type B that form in loosely organized groups called OB associations.

The neutral hydrogen atom is break into hydrogen nuclei and electrons by vast quantities of high energy ultra violet photons.

The hydrogen nuclei and the electrons later recombine to form neutral hydrogen atom, but this time in the excited state.

At optical wavelength, the most important of this transition corresponds to wavelength of Hydrogen(656.3 nm) in the red end of the spectrum. It is this transition that gives the nebula emission their distinctive red colour.

So the predominant wavelength of the photons emitted by the gas in this nebula hydrogen is 656.3nm.