The amount of information we can glean from a speck of light is fairly staggering. Learning how a rainbow works, a fairly simple concept as physics goes, has shown us what stars are made of, and that spacetime is expanding. Let’s start at the beginning.
For those of you who don’t know, a rainbow is a product of refraction, which is the physical process that makes your arm look bent if you stick it into a pool of water.
An Electromagnetic Spectrum
Depending on the medium, this process will affect a higher frequency more than the lower frequency, refracting the higher frequency at a sharper angle than the lower frequency. This looks like the image on the right.
This is called dispersion. Some mediums affect different portions of the spectrum more than others. For example, flint glass disperses visible light well, but fused quartz disperses ultraviolet frequencies better.
Through this process we can create what we call an electromagnetic spectrum, which allows us to measure what different wavelengths are present in the light that we are looking at and which are not. And by measuring the intensity of different frequencies we can learn things, like that a planet 63 light years away, which is obviously is not directly visible from earth, is blue. They looked at the change within the star’s spectrum as the planet passed in front of it. By looking at the color differences in the star’s spectrum, they figured out the color of the planet orbiting it.
Stars do something very interesting. Stars radiate electromagnetic waves in every frequency. The intensity of the radiation in portions of the electromagnetic spectrum depends on the temperature of the star. This is special because it isn’t normal. Old stars, the ones formed right after the big bang, were made of hydrogen. Hydrogen has a visible emission that looks like this:
An emission spectrum happens because each single atom of an element can only radiate one frequency at a time. It radiates one of a set of possible frequencies that make up the emission spectrum of that element. Those rules go out of the window with high energy objects like stars.
Our sun’s spectrum looks like this from the surface of earth:
Now you might be wondering why there are black lines in here. Those are absorption lines. While the sun does produce a complete electromagnetic spectrum, everything that the light passes through or reflects off of changes its composition, which is the best part.
The suns own composition is becomes apparent because the elements that make it up catch a few frequencies, very notably the hydrogen spectrum shown above, and scatters it in all directions, weakening their intensity relative to other frequencies. Every element that the light comes into contact with will strip out the frequencies that are in its emission spectrum, scattering them. This creates a very complex barcode throughout the whole spectrum for each element, not just in the visible range.
Since we know what elements dominate stellar bodies by looking at the mostly unfiltered light that galaxies give off, we can extrapolate the chemical peculiarities of individual stars in our galaxy quite effectively. This also works for nebulae and chemical clouds that hang out between us and cosmic light sources. For example, we recently found a 288 billion mile wide cloud of methyl alcohol this way. Chemistry is not the limit here though.
A brilliant man named Edwin Hubble figured out that electromagnetic wavelengths change during travel. He looked at electromagnetic spectrums from light that was coming from far away galaxies and determined that their absorption lines, the bar code etched out by the elements composing it, was identical to ours, but moved slightly toward the infrared on the electromagnetic spectrum.
Using the brightness of supernovae, Hubble was able to determine the distance of various galaxies and establish a correlation between redshift and distance. This matters because it proved that older light was more redshifted. Not only does show that spacetime itself is expanding, it gives us a simple tool to determine a rough estimate of the distance and age of the galaxy being observed, without having to wait for a supernova.