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Fundamentals I: Spectra March 10, 2009

Posted by CosmicThespian in Fundamentals.
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This article is the first in another series which will appear interspersed with other articles: the Fundamentals Series.  The goal of this series is to introduce you to some basic concepts in astronomy.  These concepts may not be directly related to extrasolar planets but they are core ideas which are needed if you want to really understand what astronomers are doing.  These articles are meant to support the main articles on exoplanets and hopefully will provide you with a greater insight into how astronomers go about the dirty work of exploring the Universe.

This article describes a concept which has direct bearing on how we find the majority of planets around other stars: the spectrum of a star.  Let’s start simple: what happens when you pass sunlight through a prism?  You get a rainbow!  Sunlight is composed of many different colors – including colors our eyes can’t detect like infrared and ultraviolet – and prisms allow us to spread out those colors so we can see them.

A prism casting a rainbow onto a wall.

A prism casting a rainbow onto a wall.

But the Sun doesn’t put out its energy in all colors equally.  If you were to take that rainbow and precisely measure the intensity of each color, you’d find that the Sun actually funnels most of its energy into the color green.  The intensity in each color drops off as you move away from green (towards red and violet).  Different stars have their peak energy in different colors: some around red, some around blue, and some actually peak in those colors we can’t see.  Where the peak happens is determined by a star’s temperature: the hotter stars peak around blue and ultraviolet; the coolest stars peak around red or infrared.

The distribution of color intensities for three objects at different temperatures.  The numbers are temperatures in degrees Kelvin.

The distribution of color intensities for three objects at different temperatures. The numbers are temperatures in degrees Kelvin. 5500 K is roughly how hot our Sun is.

What’s important to remember is that each color is actually a measure of the wavelength of light.  Light can be thought of as a wave – much like the waves on the ocean.  If you were to stand on a pier and watch the waves move by, you might notice that there’s a few fundamental things about them you could measure.  One would be the frequency of the wave: how many waves pass by you in a second.  Another closely related property is the wavelength: the distance from the peak of one wave to the peak of another.  As the name suggests, it’s quite literally a measure of how long a wave is.  Ocean waves generally have wavelengths of several meters.  The wavelengths of visible light are much smaller: billionths of meters (also called a nanometer).  Green light has a wavelength of roughly 550 nanometers (nm).  Red light is somewhere around 700 nm and violet is closer to 400 nm.  Since each color is defined by a single wavelength, when you look at a rainbow, you’re actually looking at how much of the Sun’s energy goes into each wavelength.  That is what I mean by a star’s spectrum.

As an aside, here’s a rather amazing bit of trivia.  If you take any star’s temperature and multiply it by the wavelength at which that star puts out most of its energy, you always get the same number, regardless of what star you pick!  This is true for not just every star, but any object which radiates energy – including you!  Any object will get you this number.  This law is referred to as Wien’s Law.  Pretty remarkable!

The spectra (plural for spectrum) of stars are probably the most useful tool astronomers have for understanding the Universe.  By measuring the spectra of stars and galaxies, astronomers have the ability to measure things like chemical composition, gas pressures, temperatures, surface gravity, and – this is important for us – how fast objects are moving.

If we took a lightbulb and a prism and then let the light from the prism fall on a sheet of paper, we’d see a lovely rainbow.  Now, let’s take a large, clear container filled with, say, hydrogen gas, and place it between the bulb and the prism.  How would this change the rainbow?  You’d see that a series of narrow, dark lines would appear superimposed on the rainbow as if light from very specific colors had suddenly gone missing!  And that’s exactly what would be happening.  Without going into the quantum mechanics of it all, the hydrogen has the effect of absorbing very specific colors – or very specific wavelengths – of light.  Since the hydrogen atoms are stealing those wavelengths, they show up as missing on the rainbow.  Astronomers refer to those dark wavelengths as absorption lines.

The absorption lines are amazing for a number of reasons.  The most important of these is that they tell us exactly what stars and galaxies are made of!  You see, each atom has a unique set of wavelengths it can absorb which means that every element produces a unique set of absorption lines in a spectrum.  Think of it as an element’s “spectral fingerprint”.  By matching the absorption line patterns to those of elements measured in labs here on Earth, astronomers can identify every element in any star, galaxy, planet, or cloud of gas in the Universe – we can precisely measure what the Universe is made of!

The spectrum of our Sun.  The black absorption lines result from the different chemical species in the Suns atmosphere

The spectrum of our Sun. The black absorption lines result from the different chemical species in the Sun's atmosphere

It’s amazing what we can do!

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