Astrochemistry: Spectroscopy in Space
At its most fundamental level, astrochemistry deals with where substances in the universe come from. A main way that this is studied is through the use of spectroscopy, which is the study of the emission and interaction of a substance with electromagnetic radiation and light, or photons. Radio telescopes measure these photons and create spectra with the data.
There are two subcategories of spectroscopy. The first is atomic spectroscopy, which measures absorption or emission of a photon at a specific wavelength when an electron undergoes a change in energy levels. If an electron transitions from a higher to a lower energy level, then energy is released, and if an electron transitions from a lower energy level to a higher energy level, then energy is absorbed because the electron requires energy to move further away from the atom's nucleus.
The other type of spectroscopy is molecular spectroscopy, and it measures the energy emission or absorption with a change in the rotational energy state of a molecule. The rotational energy state of a molecule depends on the spin of its electrons. When the spin of an electron changes due to interaction with energy, the rotational energy state of the molecule changes and it emits or absorbs a photon.
These interactions with energy can be measured and shown on spectra. Emission happens when the source being studied releases a photon, and emission lines show up as colored lines on a black background when shown as a spectrum because the receiver is receiving the photons emitted at the wavelengths shown. On the other hand, absorption happens when the source “uses up” or absorbs energy. Absorption lines show up as black lines on an otherwise continuous spectrum. Since the energy absorbed or emitted by each element has a specific wavelength and the emission or absorption line appears at the wavelength of the photon emitted or absorbed, elements can be identified using spectra.
But what types of things in space release or absorb energy? Blackbodies, which are extremely hot and dense gases or solids that act as a light source like stars, release energy on a continuous spectrum. If a cool, transparent gas is between that blackbody and Earth, an absorption line will be shown on the otherwise continuous spectrum because that gas absorbed photons released by the light source at certain wavelengths that correspond to the elements within it. If there is solely a hot, transparent gas, the gas will release energy at wavelengths corresponding to the elements within it and create emission lines.
These spectra can be plotted on a graph comparing the wavelength and intensity. An emission line shows up as a spike because there is more light being received at that wavelength, and an absorption line shows up as a valley because there is more of an absence of light at that wavelength. Also, the wavelength of the emission or absorption line shown on the graph is equal to the wavelength or the photon emitted or absorbed by the atom.
Observing the spectral lines of the same substance in space multiple times can also give information on where and how fast it is moving. If spectral lines are shifted towards a shorter wavelength, or left, the object is moving towards Earth. This is called a blueshift. If the object is moving farther away from Earth, the spectral lines will shift towards a longer wavelength, called a redshift. This is due to the Doppler Effect, as the frequency and wavelength of the object are changing as it moves farther away or closer. The further the shift in spectral lines, the greater the object’s velocity and the faster it is moving.
Observations from Spectral Lines
Spectral lines tell us about the relative abundance of the material, so a larger spike means that there is more of that element in the star, gas, or substance. The spectral lines can also give information about the area around the substance. Surroundings with a higher temperature or higher pressure will cause the spectral lines taken of the star to broaden because individual molecules experience Doppler shifts. Finally, faster rotation of an object will cause spectral lines to broaden because the blueshift and redshift of each half of the object will be more drastic, spreading out the spectral lines.
Spectral lines can also be used as probes to give information about the environment it is in or the substances around it. Temperature probes give insight as to what temperature a region is. Electrons of certain elements in high temperatures will become excited then return to the ground state, and these spectral lines can be measured. It can also show the density of a region because some spectral lines are only found in dense environments.
Spectral lines can be used to examine stars, gas, ice, dust, and more. One particular use is to determine if an explosion in space is a supernova, which is an explosion that takes place towards the end of a star’s life cycle. Looking at the spectral lines of the remnants of an explosion is the only way to determine if it was a supernova and to determine what type of supernova it is. Looking at the spectral lines, if there is an emission line indicating the presence of hydrogen, then the supernova is Type II. If there is no hydrogen, then it is Type I, but there are subcategories of Type I supernovae. If there is silicon but no hydrogen then it is Type Ia, if there is hydrogen but no silicon then it is Type Ib, and if there is neither hydrogen nor silicon then it is Type Ic. Types II, Ib, and Ic do not occur in elliptical galaxies and normally form in spiral arms. These arise from massive stars that went through a core collapse and results in a neutron star or black hole. Type Ia supernovae form in any galaxy and do not require the presence of spiral arms. These come from carbon-oxygen white dwarfs that gained matter from a nearby star and underwent thermonuclear runaway, a reaction that ejects matter into space.
The ejections of gas into space from supernovae are the most violent and therefore hottest ejections of gas into space. The space that it is ejected into contains matter and radiation, and it is called the interstellar medium. Because supernovae remnants are so hot, they emit most of their light in the x-ray portion of the light spectrum. The explosion also release a shockwave and stellar material that have velocities as fast as one percent of the speed of light. This speed causes gas to heat up and form plasma. The shockwave also sweeps up gases and material in its path and moves it, possibly causing reactions with other elements.
Polycyclic Aromatic Hydrocarbons (PAHs) in the Interstellar Medium
Once the shockwave and expelled gas cools, it becomes incorporated into the interstellar medium as it interacts with other gases and space clouds. Many of these materials are made of polycyclic aromatic hydrocarbons because it is hot, hydrogen-rich, and carbon-rich. These molecules are variations of benzene and sometimes have different groups attached to the outer carbons. The conditions of the interstellar medium, as it is hot and dense, create an environment that favors the bonding of small hydrocarbons so that polycyclic aromatic hydrocarbons are formed.
These compounds are shown in spectral lines as infrared radiation. When UV light is absorbed by these polycyclic aromatic hydrocarbons, their electrons are excited, and then return to the ground state, releasing infrared energy. Most of these compounds are found in ice dust that is mostly H2O. If these molecules have extra hydrogens on the outer atoms, which is plausible in the hot gas clouds, then the hybridization of the atoms is sp3 instead of the normal sp2. This can cause changes in the spectral lines of the substance. An example of this is infrared emission in the C–H stretching region from the Orion Bar, which has ionized hydrogen, compared with a lab-taken spectrum of hexahydropyrene, or H6-pyrene. The large difference in the spectral lines can be explained by the tetrahedral bonding of the outer atoms because it has different rotational energy states.
When this ice with water and polycyclic aromatic hydrocarbons is exposed to radiation, the polycyclic aromatic hydrocarbons react, especially the outer hydrogens or groups. If oxygen is incorporated in the reaction, then ethers, alcohols, and ketones can be produced, and some of them are of astrobiological interest. Those of the most interest in astrobiology are the aromatic ketones, or the polycyclic aromatic hydrocarbons with CO groups on their outer edges, because they contain quinones, which play roles in biochemical processes in living organisms on Earth. For example, consider the two-ring polycyclic aromatic hydrocarbon naphthalene. When this is in ice containing H2O and exposed to UV radiation, oxygen atoms can be added to the edges of the naphthalene, forming products like the aromatic alcohols 1-naphthol and 2-naphthol, and the quinone 1,4-naphthoquinone.
Overall, spectroscopy can be used to identify the compounds in space, inform about their motion, give insight into the environment around them, and be used to examine stars and the interstellar medium.