Astronomers and physicists know an amazing array of information about celestial bodies all throughout the universe. How stars are made and what they are made of. Whether distant planets are rocky or gas and whether they have atmospheres. Ever stop to think “how do they know all that stuff when we are stuck here on Earth and what astronomers study are thousands, millions and billions of light years away?” Good question.  The answer is fascinating and it concerns a technology called “spectroscopy” which dates back to Isaac Newton.

In the late 1600s Isaac Newton passed light through a prism and produced the familiar spectrum of colors – ROY G BIV – and then he passed the spectrum of colors back thru another prism and recovered white light.  This is amazing if you think about it.  Take any other medium of the colors and combine them and you would get a murky brown.  But, colored light reconstitutes white light – kinda cool.  Obviously, the seven colors are not discrete – they blend into each other and there really is a spectrum of visible colors, rather than separate colors.

In the early 1800s it was noticed that light passed thru a prism produced “lines” where colors are absent.  Experiments conducted over the first half of the 19^th century determined that the patterns of lines that emerge from passing light thru a spectrum relate to which element is being burned to produce the light that is being refracted.  So, the light produced from burning hydrogen would produce a different pattern of lines than burning oxygen, etc.  In fact, the element helium (which is the second most abundant element in the universe) was first discovered in a spectrum of the Sun’s light before it was discovered on Earth (“helium” comes from the word “helios” which means “of the sun”).  It later was discovered that similar lines in other areas of non-visible light such as x-rays and microwaves provide similar information about various molecules (i.e. combinations of elements). Other information is derived from the Dopplar shifts of the spectral lines and the magnetic effects of electromagnetic fields on the spectral lines. 

So, what astronomers do is pass both visible and non-visible light through very advanced versions of prisms and from that they can tell a lot of information about faraway stars, planets, nebulas, clouds, etc.  They can determine the chemical composition of a far-away celestial body, its temperature, how fast it is moving away from us – all based on the colors of light emitted and the spectral lines in the light as well as a lot of other information.  In fact, a primary task of the large telescopes both on earth and in space is to collect light spectrum information of celestial objects.

The amount of information about stars, planets and other objects we are able to gain from spectroscopy is astounding. According to astronomer Neil DeGrasse Tyson:  “in short, if it were not for our ability to analyze spectra, we would know next to nothing about what goes on in the universe.” Yet, due to spectroscopy, “one could make a compelling argument that we know more about the universe than the marine biologist knows about the bottom of the ocean or a geologist knows about the center of the Earth.”

Related IFOD on Exoplanets