Interstellar Gas
H II Regions
H II Regions are interstellar clouds of extremely hot ionized hydrogen gas(temperatures typically around 10,000 K) near young stars. Young stars tend to be extremely hot compared to their older counterparts and so they end up heating these regions of interstellar hydrogen gas to very high temperatures. Young stars also emit intense ultraviolet radiation which causes the electrons in the neutral hydrogen to strip away, making the gas ionized. The characteristic red colors within these regions are due to the fact that when you ionize a neutral hydrogen atom, a photon with a wavelength of 656 nm is ejected, which corresponds to the color red. In case you are wondering why the regions are called "H II" regions,
I(Roman Numeral for 1) denotes a neutral atom, with no electrons removed.
II(Roman Numeral for 2) denotes an atom that has been ionized to now have a +1 charge(1 electron removed).
III(Roman Numeral for 3) denotes an atom that has been ionized to now have a +2 charge(2 electrons removed)
This format follows through for however many electrons must be removed. This is easier for scientists to use if they work with spectra because chemically, ionization is denoted through the charge of the resulting ion but electrical charge isn't really a useful quantity for scientists who study space to use.
Ultra-Hot Interstellar Gas
Many scientists believed that H II Regions were the hottest regions of interstellar gas that one could find throughout the cosmos. However, this became disproven when they discovered regions of gas that had temperatures of millions of degrees. This may seem like an exaggeration but it truly wasn't and scientists were baffled by this because these extremely hot regions had no nearby sources of heat that could supply such temperatures. Most of the gas in these regions consisted of oxygen atoms with five out of their total six electrons removed. In order to remove 5 electrons from oxygen, it would require at least temperatures like the ones found in these regions. Remember that the fifth electron to be removed from an oxygen atom would be extremely strongly attracted to the atomic nucleus so it would take more energy to remove it.
These regions of ultra-hot interstellar gas are now believed to be caused by supernova remnants, the shock wave structures that result from supernovae. When supernovae occur, they release temperatures of about millions of degrees but these temperatures last over millions of years, causing ultra-hot interstellar gas regions to stay hot even if it doesn't seem like there would be an immediate source for their heat.
Molecular Clouds
Astronomers can detect certain atoms through their unique spectral lines. The same can be said with complex molecules in interstellar space as their rotational and vibrational energy can be analyzed through infrared and radio astronomy. Through the observations these techniques make, we can then find out how much of and what type of complex molecules are found within the interstellar regions of the universe.
In interstellar space, one can sometimes find large regions of dense molecular clouds(densities around thousands of atoms per cubic centimeter) which lead to the formation of complex molecules. This may sound like any other region of the interstellar medium but these regions have a few distinct traits from just any other region in the interstellar medium. First off, these molecular clouds give birth to new stars, which is they sometimes are called stellar nurseries. Also, these clouds are very distinct from ionized gas clouds, like H II regions and ultra-hot interstellar gas regions because most of the material in these clouds strictly consists of complex molecules and molecular hydrogen, not ionized gas. Lastly, these clouds are very cold(near 10 K, or -263 C) because their densities cause them to block out starlight so they don't get heated sufficiently.
The densities of these regions are on the order of thousands of atoms per cubic centimeter which means that they are dense enough to significantly block starlight. This also means that they don't get sufficiently heated by starlight so they are extremely cold.
Most of the matter in a molecular cloud is believed to be molecular hydrogen(H2) and carbon monoxide(CO) but there are also traces of other complex molecules, especially hydrocarbons, organic molecules, water(H2O), and ammonia(NH3).
Molecular hydrogen is extremely difficult to detect in molecular clouds because it is nonpolar, so it doesn't have a dipole. In case you're unsure about the concept of molecular polarity, check out this link. Since molecular hydrogen has no dipole, it doesn't rotate when an electric field is applied to it. This means that we can't analyze its molecular rotations through infrared astronomy and it can't be detected easily. However, CO can be detected fairly easily, and oftentimes, the presence of CO is used to infer the presence of H2.
Many interstellar clouds contain small traces of cyanoacetylene (HC3N) and acetaldehyde (CH3CHO), both of which are heavily associated with the formations of amino acids. While this certainly doesn't imply the existence of life out there in space(especially under temperatures near absolute zero), it does convey significant evidence that the amino acids that make some of the proteins essential to life can be viably formed under extreme conditions outside Earth, which is noteworthy in its own right.
Citations/Attributions
Astronomy. Provided by: Openstax. Located at: https://openstax.org/books/astronomy/pages/1-introduction License: CC BY 4.0