For centuries, astronomy and astrophysics have relied solely on the light that we were able to capture from events occurring in distant corners of the galaxy or the universe. None of the objects studied beyond the solar system have been visited in situ by any of our instruments. Even those that have been visited have been studied from a distance. We have “landed” on 13 celestial bodies, out of the hundreds that we know in detail. I use quotes because in 4 of them the probe did not survive the landing. And despite all this, the amount of information we know about them is incredible.
Beyond allowing us to visualize them, or at least discern their position in the night sky, the light we receive from the different stars allows us to know characteristics as different as their temperature, their chemical composition, their speed with respect to the Earth or their distance, in the case of more distant objects.
For example, stars, like any other body in the universe, emit light due to their temperature. The hotter they are, the more energetic light they will emit. This light is what is known as blackbody radiation , since it is the only light that would be emitted by a perfectly black body, unable to reflect any light that reaches it. This black body radiation is responsible for the lava glowing reddish when it is still hot or, as we have said, it is responsible for the brightness of the stars. It is also the cause of the different colors that we observe between the different stars . Cooler stars will emit red light, while hotter stars will emit blue light.
Other less conventional objects can even emit light in other parts of the electromagnetic spectrum. The cosmic microwave background, which is the remnant of light that remains to us from when the universe was still young, has the emission of a black body at a temperature of just 2.7 K above absolute 0 , the lowest possible temperature . This light therefore reaches us as microwaves.
In addition to emitting light as a direct result of their temperature, some bodies or structures can also glow by what are known as emission spectra . This is the light that atoms emit when their electrons have been excited. This excitation can happen by violent collisions with other atoms or after absorbing light from another source, for example. Excitation will mean that an electron from that atom will jump to a higher energy level. After a few moments, as this level is unstable, it will fall back to the lower level, emitting light in the process. But not just any light. The energy of the emitted photons will be exactly the difference in energy between the two levels of the electron. There will, of course, be a multitude of levels to which the different electrons of the atom will be able to access, so that light will be emitted with very specific and specific energies for each atom or molecule. This is called the emission spectrum of an atom.
Therefore, by accurately measuring the energy of light (directly related to the color of that light) coming from an interstellar gas cloud or from a planet’s atmosphere, we can identify the chemical composition of these structures.
Thanks to this spectrum we know in such detail the composition of many of the bodies in our solar system, of distant stars and of clouds and gas and dust located thousands of light years away. Thanks to telescopes like the James Webb , and the sensitivity of its instruments, in the future we will be able to characterize the composition of the atmosphere of thousands of exoplanets , perhaps detecting the presence of extraterrestrial life in the process.
But the feat of those who dedicate themselves to studying the universe is not always so direct. Sometimes the light we receive is distorted. Not only is it that some structures can directly block the light, preventing it from reaching our telescopes, but it can also be affected by physical effects, such as the Doppler effect.
When we hear the siren of an ambulance or a police car, we notice that the sound we hear changes when the object approaches or moves away from where we are. This is what is known as the Doppler effect, and it has a very simple explanation. When the train or the ambulance is stationary they will emit waves in all directions, creating circular and concentric wavefronts with each other. If they were the waves of the sea, we would say that these wave fronts represent the crests of the waves, for example. Well, when the sound emitter starts moving , the waves will continue to be emitted in a circular fashion, but each wave front will be emitted from a different position, so the wave fronts will pile up ahead, in the direction that the emitter moves and they will move away from behind. Therefore, if the ambulance approaches us, we will perceive a higher frequency sound, a higher pitched sound and if a lower frequency sound moves away, a more serious sound.
Exactly the same happens for light. If a star emits light while it is approaching the Earth (by a wobble motion, as an effect of its orbit within a multiple star system, for example), its light will appear slightly bluish, while if it is moving away, the light will be darker. you will see orange . This is especially noticeable for emission spectra. As each atom and molecule have a very specific spectrum, consisting of the emission of light of certain very specific colors, if we detect a spectrum with the same distribution as that of, for example, carbon, but all of it shifted towards wavelengths smaller (higher energy and therefore more bluish), we will know that the source of said light not only contained carbon but also came closer to Earth when emitting that light. Depending on how displaced the lines of the spectrum are, we can measure their relative speed.
This “reddening” of light, or red (or blue) shift, can also occur as a result of the expansion of the universe . In this way we observe that distant galaxies show a systematic red shift (that is, their light is less energetic than it should be) and that this shift is greater the further they are from us. We can understand this phenomenon as the expansion of the space that separates the source from the receiver causes an expansion of the wavelength of light itself. Longer wavelength means redder light and hence the observed shift.
Therefore, by analyzing in detail the light coming from different astronomical objects, we can learn a lot about them and, by inference, about their internal functioning.