After an initial imaging sequence on this object it became clear the OIII signal is very weak. So I modified the sequence to acquire much more OIII, as well as more SII, at the expense of Ha, and then continued imaging in a second sequence over several nights. The object is really quite faint and the high signal boost required to extract these low signals really exposed some residual problems with sub-frame calibration at the frame edges, which meant I've had to quite heavily crop them, though the remaining image size is still pretty big.

This object, Sh2-155, is commonly called the Cave Nebula, a name given by Sir Patrick Moore who included it as object number 9 in his Caldwell catalog of objects, first published in Sky and Telescope in December 1995. Apparently the name refers to the arc of glowing gas which seems to resemble the entrance to a cave.

The object is some 2400 light years away, with the cave entrance itself being about 10 light years across.Sh2-155 is a large diffuse nebula including emission, reflection and dark nebulae. The brightest portion, the mouth of the cave, is a region of ionised atomic hydrogen =14pxwhich is emitting deep-red light corresponding to the Balmer series alpha transition, more commonly called H-alpha.

The Balmer series are transitions to the second quantum state: alpha is from the 3rd to the 2nd, beta from the 4th to the second and so on. There are other series as well. The Lyman series features transitions to the first quantum state (the ground state) but the wavelengths of the Lyman series transitions are outside the visible spectrum and the atmospheric transmission window, except in the case of extremely red-shifted, distant cosmic objects.

The obvious question is how hydrogen atoms in interstellar space manage to have their single electron excited to the third quantum state in the first place. It is the result of energisation by external sources of hard electromagnetic radiation, typically UV light from nearby, newly formed, hot stars. To generate the transition from the ground state to the third quantum state requires 12.1 eV, which would be delivered by a photon of exactly this energy (corresponding to a wavelength of 102 nm, well into the UV). However, any photon of wavelength shorter than 91 nm will have an energy of 13.6 eV or more, which is sufficient to totally ionise the atom. This is what usually happens.

When the electron and proton then recombine, as they eventually will, not least due to electromagnetic attraction, the electron will start with arbitrary energy and will cascade down to the lowest quantum state, the ground state. At each transition it will emit the photon of corresponding energy. About half of these recombination transitions will include the transition from the third to the second state, thus generating the H-α photon.

The cloud of ionised hydrogen atoms (this are protons) and the liberated electrons is technically speaking a low-density plasma rather than a low-density gas, and in describing it we use terminology which seems only to be used in astronomy. Neutral (i.e.non-ionised) hydrogen atoms are referred to as H I, while ionised hydrogen atoms (i.e protons) are called H II.