This is what I wrote recently (minutes ago) as an answer to a question on Quora:
**Overview**
Astronomers use every form of physical information arriving from far away places that they can possibly get their hands and/or instruments on. These include:
* Physical matter in chunks, dust, molecules or high energy massive particles: meteorites, mainly. Matter is also obtained at great expense by sample return missions such as Apollo (Moon rocks (http://www.collectspace.com/resources/moonrocks_apollo11.html)) or Genesis (https://www.scientificamerican.com/article/nasa-identifies-likely-di/) (Solar wind particles (https://curator.jsc.nasa.gov/genesis/)). Many Earth-orbiting satellites since soon after Sputnik (https://earthsky.org/space/this-date-in-science-launch-of-sputnik-october-4-1957), and spacecraft leaving the vicinity of Earth, carry instruments to detect dust, mass spectrometers to analyze gases and plasmas. An example is the INMS on Cassini that “tasted” the plumes of Enceladus (https://www.nasa.gov/mission_pages/cassini/media/cassini-20080326.html).
* Electromagnetic radiation. This includes light, infrared, radio, x-rays, everything in between and beyond. See detailed list in the next section.
* Gravitational waves. This is brand new! GWs pass through vast stretches of space, empty or filled, are not absorbed or deflected by galaxies, vast stretches of dust, or plasma, or by anything, though they *are* refracted by the gravitational fields of massive objects just like light. For now and the next two decades, our instruments won’t be sensitive or focused enough to observe that. For now, we enjoy observing the strength, timing, and changing orbital periods of black hole and neutron star collisions with LIGO (https://www.ligo.caltech.edu/page/gravitational-waves) and VIRGO (http://www.virgo-gw.eu/).
* Neutrinos. These, like gravitational waves, pass through great stretches of space without being bothered by matter. They too are deflected by gravitational fields, like light and GWs. We’ve been detecting neutrinos (https://www.bnl.gov/science/neutrinos.php) from the Sun and supernova since a few decades ago, but research into new instruments (https://www.sciencemag.org/news/2017/08/milk-jug-sized-detector-captures-neutrinos-whole-new-way) continues
Electromagnetic Waves
The main advantage of EM waves is that we have eyes to see light, if it is bright enough. We have technology to detect fainter sources of light, make accurate measurements, detect changes to sources, and in many cases can be automated and computerized. The physics of photon, their sources, and their interactions with matter (e.g. film (https://www.lonelyspeck.com/the-night-sky-on-film-a-salute-to-the-photographic-process/), CCD arrays, photomultiplier tubes (https://www.testandmeasurementtips.com/the-basics-of-photomultiplier-tubes/)) is well understood. The instruments are expensive, but that’s mostly the surrounding support, data acquisition, mechanical systems, cryogenics, and so on. The real core of the instruments, the detectors, can be fairly cheap. Optical engineering (https://en.wikipedia.org/wiki/Optical_engineering) is one solidly established area of engineering. We have pretty much mastered light handling, and this applies in a wide variety of interesting ways to all wavelengths of EM radiation.
The main disadvantage of EM waves is that along the way from a distant source to our eyes and instruments, a lot can happen. The interstellar medium (http://casswww.ucsd.edu/archive/public/tutorial/ISM.html) absorbs or scatters certain wavelengths. Vast stretches of ionized or neutral gases or dust can absorb light, warm up ever so slightly, and put out their own infrared or millimeter radiation. Near the end of the trip, Earth’s atmosphere refracts all wavelengths and absorbs some.
* Visible light is great because we see it and can understand things visible by light. If we can see even the weirdest astrophysical objects, it’s a sort of understanding of it, a sense of knowing what it is. We like light. With a prism or diffraction grating, we can see emission and absorption lines that tell us quite a lot about what heavenly bodies are made of. Helium was discovered (https://www.universetoday.com/53563/who-discovered-helium/) by the observation of unexpected lines in the Sun’s spectrum. But this is only one octave out of the whole spectrum of EM waves that we can handle scientifically and technologically. The oldest non-visible EM radiation known to science is:
* Infrared (http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/importance.html) – this is great for astronomy though we can’t directly sense it though we might feel warmth from strong sources, and we experience its effects in the Greenhouse Effect. Get into a car with a dark interior on a sunny day. Infrared is absorbed, or greatly reduced, by our atmosphere, so IR scopes are usually on mountains or flown on airplanes (https://www.sofia.usra.edu/) including fighter jets (https://solarsystem.nasa.gov/people/36/alan-stern/). Infrared tells us about temperatures of heavenly bodies, especially those that aren’t white hot or red hot, the presence of various molecules through spectroscopy (just like with visible light), and though we may dislike IR from our favorite astrophysical sources being absorbed, that can be seen as a tool for measuring the interstellar medium. Some stars, class L and class T, (http://stars.astro.illinois.edu/sow/spectra.html) are detectable only in infrared All in all, IR is just visible light a lower wavelengths. We can go to much longer wavelengths:
* Radio Astronomy covers a wide range of wavelengths. Like infrared, radio wavelength and microwave radiation can tell us a bit about bulk thermodynamic properties of distant matter. It can tell us about plasma densities, magnetic fields, sizes of ionized dust grains, all-natural masers (http://astronomy.swin.edu.au/cosmos/M/Masers), much more. Radio astronomers often measure radio strengths at several wavelengths and then see how they fit a power law. With this they can distinguish between various sources and identify intervening material. Changes, blips, and repeating pulses in radio and microwave wavelengths are the main way of how we learn about neutron stars and their surroundings, and certain types of stars that flare up. Microwave astronomy is, if you haven’t heard by now, a main tool for exploring the big bang (https://wmap.gsfc.nasa.gov/universe/bb_tests_cmb.html). BTW, we should rename “microwave” since the wavelengths are micro-meters, but a few cm to a few meters. The current name looks especially stupid next to:
* Millimeter Wave – this is a fairly new area in astronomy, fitting in between microwaves and infrared. Wavelengths are from about 1cm to a fraction of a millimeter. You get what it says on the tin. Are you old enough to remember the TV ads for dish detergent where the lady says “You’re soaking in it” (https://www.youtube.com/watch?v=_bEkq7JCbik)? Well, you’re soaking in it. Infrared is from hot objects. MW is just lower-energy, longer wavelength EM radiation from room temperature objects. We now have magnificent instruments for observing MW – my favorite is ALMA (https://public.nrao.edu/telescopes/alma/). Science of ALMA and other mm wave instruments and satellites is revealing wonderful details of stars, nebula, surroundings of more exotic objects, such detail we just can’t obtain at lower microwave and radio wavelengths, or with infrared. But one thing we’re not soaking in, or shouldn’t be exposed to very much, is:
* Ultraviolet (https://stardate.org/astro-guide/ultraviolet-astronomy) – sadly, does not make it through Earth’s atmosphere, though the Sun puts out so much that even after most of it is lost in the air, we still get sunburns and skin cancer. For astronomy, viewing the Sun in UV with Earth-orbiting satellites is a great joy for solar scientists. UV tells us about temperatures, plasma activity, high energy particles. In visible light, we see mottling, resembling convection cells like boiling fluid, but not the magnetically influenced arches of plasma leaping up, twisting and vanishing, and strange bright spots in UV that don’t look like anything in visible light. It takes pictures in several forms of radiation to understand what’s going on with the Sun. UV is also useful for classifying and studying the workings of stars in general.
* X-Ray astronomy is good for detecting and studying the more violent activities around certain intense massive heavenly bodies. The accretions disks and jets around black holes, crazy things (https://www.nextbigfuture.com/2017/11/one-in-ten-neutron-stars-are-magnetars-with-magnet-fields-1000-times-stronger.html) happening around neutron stars, supernova. Requires satellites or very high altitude balloons, since X-rays won’t go far in air.
* Gamma Ray astronomy – like X-Ray astronomy, but amped up another factor of a hundred, thousand, or more. Gamma ray sources can be seen only by satellites in orbit. Our atmosphere is basically opaque to gamma rays, as it is to X-rays. If you enjoy having plenty of things to worry about, you’ll be glad to learn about Gamma Ray Bursts (https://www.astrobio.net/news-exclusive/deadly-nearby-gamma-ray-burst/).
There’s much more to say about the details of each of these information channels of knowing about astronomically distant places, but I think the point is made: astronomers rely on many sources of information, EM and other, each for its own reasons having to do with physics at the source, physics in between, and physics here on Earth.
Not just that, but using multiple information channels simultaneously is necessary for certain extreme edge areas of science. With one famouse neutron star collision, we sensed the gravitational waves (https://www.ligo.caltech.edu/news/ligo20190502) with LIGO while other instruments picked bursts of visible light, radio waves and X-rays (https://www.nytimes.com/2017/10/16/science/ligo-neutron-stars-collision.html). When multiple modes of observation fit together, relate in ways expected by established theory, or maybe deviate in some small way, it is great progress for astrophysics.
Wiggle Wave 