Astronomical spectroscopy for research, industry, education and leisure
Sir Isaac Newton (1642-1727) taught us that what we call « white light » is in fact a mixture of different colors. Spectroscopy is a technique that consists in breaking down the light to its component colors, as do rain droplets forming a rainbow, in order to get what is called a spectrum. Breaking down the light in its different colors is not only aesthetic but also allows to get many informations on the physical phenomenons that produced the light. Different kind of lights – a flame, a lightbulb, a calibration lamp, the sunlight – provide differents spectras, and even very different ones sometimes.
Light travels on typically astronomical distances without losing the information it carries. Analysing the light of stars (or of any celestial body) is so an extraordinary way of understanding the physics at works in their core. In fact, spectroscopy is the very basic tool of astrophysicists.
One could imagine that it’s a very complex technique, only affordable for the happy few having access to the biggest telescopes in the greatest observatories in the world. Nothing could be more wrong ! Even a backyard telescope enables you to do spectroscopy. That’s what the entire Shelyak’s products range demonstrates.
For an amateur astronomer, going for spectroscopy, it’s going from contemplation – which is a strong and very decent motivation ! – to the measure and understanding of the very nature of celestial bodies. It’s opening a new window on a radically different sky, and it’s an exciting and endless adventure.
To get the spectrum of a star, you need to install a spectroscope behind a telescope. It’s really as simple as it souds. The telescope collects the light of the star you want to study, and the spectroscope spreads the lights of this star on the sensor (usually a CCD matrix). If you’re familiar with astro imagery, the spectroscope takes place between the telescope and the CCD camera.
Spectroscopy has much more applications than astronomy – chemistry, industry , etc.- But applied to astronomy, it brings three constraints :
First, as you need to set it up behind a telescope, it must be lightweight and of reasonnable size.
Second, you need a pointing and tracking device, in order to be sure that you’re observing the proper star. Nothing looks more alike a star than another star…
Third, even behind a telescope, the amount of collected light is very dim : efficiency is the key, because you can’t afford losing any grain of light…
So, with this kind of setup, what do you get ? You don’t care anymore about the field of view of your telescope, because you focus on a single star from which you want to obtain the spectrum. The light spreads on a line (see left opposite figure). To any column of the picture corresponds a wavelenght, in fact a color. The shortest ones (blue colors) are on the left, the longest (red colors) on the right.
From that raw image, you get the spectral profile through a computer process called « data reduction ». It provides a profile as free as possible of instrumentals artifacts and calibrated in wavelenght (see right opposite picture ).
Beyond technical aspects, spectroscopy tells an amazing story of human knowledge. It took generations of researchers to understand that dark (or sometimes light) indentations in the spectra were connected to energy transitions in the atoms, and so that these lines were in fact the signature of chemical elements. Or even to understand that laws of physics were applying in the same way everywhere in the Universe. Light is deeply connected with matter, so analyzing light is nothing less crucial than understanding matter. Getting in astronomical spectroscopy, it’s boarding into the big adventure of science through the front door. And this adventure still goes on today…
… and become a Partner of Scientific Research !
While getting to spectroscopy, you could check by yourself differences in spectral types of stars, celestial bodies motion through Doppler effect, and even the Universe expansion by measuring redshift of distant galaxies and quasars. Talk about an educational tool ! Applications are just like stars on a clear night : countless !
And you can still go further : there’s still a lot to discover, and you can do your share of these discoveries to come, even with a backyard telescope. All you have to do for this is to join the growing community of spectroscopists : the more we will be, the more our efforts will pay off.