Astronomical spectroscopy

In many fields, scientists use different technics to analyze different types of samples. Most of them can visit and collect samples and analyze them in labs.  But astronomers are not that lucky. They have to observe and analyze astronomical objects from far away.

How do we know that Sun is made up of 74 percent hydrogen or that its surface temperature is almost 6,000 kelvins? The atomic composition of distant stars and planets? Have you ever wondered how we know all these details? All these questions are answered by a technique called spectroscopy!

Figure 1: Electromagnetic Spectrum (coe.edu/principles-of-structural-chemistry/relationship-between-light-and-matter/electromagnetic-spectrum)

Spectroscopy is the study of light and its interaction with matter. In 1700 Sir Isaac Newton showed that white light could split into seven colors. Colors of a rainbow. The different colors produced have a different wavelength.

The visible range (From red to violet) wavelength is between 400nm to 700nm range. But the whole electromagnetic spectrum is vast. It has a range of wavelengths from a few kilometers (Radio waves) to picometer(10-12 meters) range (gamma rays). So electromagnetic spectrum contains details about all energies of light.


The Temperature of a Star

The Sun produces a spectrum that is a close approximation to a blackbody spectrum; so do other stars. A blackbody is an imaginary physical body that can absorb all incident electromagnetic radiation, regardless of frequency or angle of incidence. Also, black bodies emit light of all wavelengths just at vastly different intensities(Black body emission). We can assume that a star's surface is to a near-perfect blackbody.

By analyzing a star's spectrum, we can predict how hot it is because the light they emit will depend solely on their temperature. Wien's Displacement Law states that when temperature increases, overall radiated energy will increase, and the peak of the radiation curve moves to shorter wavelengths in a black body. Hotter the star, the smaller the peak wavelength, so the energy of the light is high.

 Figure 2 shows how the peak wavelength reduced with increasing temperature.

T = 0.0029 / λmax

T = temperature(in Kelvin),

 λmax = wavelength of maximum emission (in meters).

By analyzing the electromagnetic spectrum of a star, we can calculate the star's temperature using its peak wavelength.


Figure 2: The intensity of blackbody radiation versus the wavelength of the emitted radiation. (https://cnx.org/contents/NP3Ov7lW@2.48:OjoA1o37@7/6-1-Blackbody-Radiation)

Without any equipment, we can tell if a star is hot or cool. If we observe stars, we can see them in different colors. Some are red, blue, or yellow. Blue lies closer to high energy in the electromagnetic spectrum (Figure 01), and red lies closer to lower energies. So cool stars emit light closer to red color. Betelgeuse has a temperature of 3500K and appears red. Hot stars emit light closer to blue color. Rigel has a temperature of 15000K and looks bluish. So next time you observe the sky, try to guess their temperatures.

Analyzing the atomic composition

Figure 3: Hygrogen energy levels

We know that a black body like Sun can produce light in every wavelength (continues spectrum). If we look at different elements, can they produce a spectrum? To do that, we take an individual element and heat it to a high temperature. This will excite the electrons and will move to higher energy levels. When they come back to lower energy levels, they will emit energy as radiation (figure3). If we observe these energies using a spectrometer, it will look like a few lines in different wavelengths. 

Electrons in an element can only stay in specific energy levels. These energy levels are different from atom to atom. So, electrons can only excite between specific energy levels. And when they release and come to a lower energy level, the energy released is equal to the energy difference between the two energy levels. And this released energy has a unique wavelength, which will be detected by the spectrometer. It is more like a fingerprint to each element. These are called emission spectrums. (not only in the visible range at any wavelength) No two types of atoms or molecules give the same patterns. In other words, each particular element can absorb or emit only specific wavelengths of the light particular to that element.


Figure 4: Continuous Spectrum and Line Spectra from Different Elements (https://courses.lumenlearning.com/astronomy/chapter/spectroscopy-in-astronomy/)

We previously stated that Sun can emit wavelengths of any range. But there is a catch. In 1802 William Wollaston Look at the actual spectrum from Sun using a high-resolution spectrometer. He did not observe a continuous spectrum. He saw some colors are missing from the spectrum.

Imagine Sun has Hydrogen gas. As previously stated, Hydrogen can excite between specific energy levels (absorption spectra). When sunlight passes through Hydrogen molecules, it will absorb energy from light from corresponding wavelengths. So light that comes to the spectrometer is missing some specific wavelengths. We know most of the emission lines of elements by experiments we have done on Earth. If we compare the wavelengths missing from the Sun's spectrum, we can determine which element is present in the Sun because every element has its unique absorption spectrum.

Figure 5: Visible Spectrum of the Sun (https://courses.lumenlearning.com/astronomy/chapter/spectroscopy-in-astronomy/)

This is one of the most significant discoveries in astronomy. We can deduce the chemical composition of many objects far away without ever traveling them. It does not have to be a star. Starlight passing through a planet's atmosphere will absorb the wavelengths according to chemical composition.

Most of the space probes or telescopes have a spectrometer built-in. Curiosity rover has ChemCam to analyze the chemical composition of rocks and soil. ChemCam fires a laser and then analyzes the composition of vaporized materials from the surface of Martian rocks and soils.

 

Mars 2020 rover has an instrument called SHERLOC (Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals) SHERLOC instrument also uses spectrometers, a laser, and a camera to identify organics and minerals in watery environments and may be signs of past microbial life.

Using the Doppler effect, we can measure a star's speed or a galaxy with respect to Earth. In distant stars measuring its spectrum and identifying sudden decreases in light intensity will indicate that something is traveling between us and the observing star. This technique is used to identify exoplanets. Spectra also contain information on the magnetic field present in the object, the matter's composition, and much more. 


Article by :  Nipun Chandrasiri - Faculty of Science

Design :  Kasun Madushan

References