Astronomical spectroscopy

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Isn’t it fascinating that scientists are using astronomical spectroscopy to determine the age of the Universe? How astronomical spectroscopy makes discovering the dark corners of the Universe possible by gaining an understanding of new stars, galaxies and other celestial objects?

To answer all the above questions, we have discussed everything you need to know about Astronomical spectroscopy in this article.

So, continue reading!

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What is astronomical spectroscopy? – Definition

Henry draper was the first scientist to combine the photographic telescope with astronomical spectroscopy. Fraunhofer and Kirchhoff further advanced the field.

“Astronomical spectroscopy is a scientific analytical technique used to study the dark matter content of galaxies. It is based on measuring the electromagnetic radiations belonging to different regions of an electromagnetic spectrum that are emitted by unique chemical elements present within stars, galaxies and/or other celestial bodies under investigation. A spectrograph is plotted by taking the characteristic spectrum obtained by light emission from a particular chemical element.

Astronomical spectroscopy- Instrumentation and equipment

  1. Telescope: It is the most important astronomical instrument which is used to capture images of distant celestial bodies.
  2. Spectrograph: A spectrograph is an instrument used to obtain and record an astronomical spectrum. The spectrograph splits or disperses light from an object into its component wavelengths so that it can be recorded and then analyzed.
  3. Prism/Blazed diffraction gratings:  Nowadays, prisms (used to focus light) are replaced by gratings. To create a blazed grating, a large number of parallel mirrors are mounted to focus and observe only a small portion of light.   
  4. Detector: The spectroscopic data obtained is recorded by a detector.
  5. Display:  The obtained data is plotted in the form of a graph called a spectrum, also known as the spectrogram, which is then displayed on a monitor screen.  

What is the basic working principle of astronomical spectroscopy?  

Like all other spectroscopic techniques, astronomical spectroscopy is based on the interaction of electromagnetic radiations with matter. The results are obtained in the form of a spectrum. The spectrum provides different types of information about the material object, for instance, its physicochemical properties, structure, composition, etc.

Steps to perform astronomical spectroscopy

  • The telescope gathers electromagnetic radiation (light) emitted from the celestial object under observation.
  • This radiation is then passed through a tiny hole or metal plate slit in the spectrograph, which isolates it from the light coming from other objects.
  • A diffraction grating further disperses light into its component wavelengths.
  •  The detector records the results in the form of a spectrogram. 
  • The obtained data is further analyzed in the laboratory to extract the required information about the celestial object under examination.
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The role of spectroscopy in stars

Spectroscopic analysis of stars involves the study of spectral lines obtained by stars to determine their chemical composition, density, temperature, elemental composition, mass, size, magnetic fields, etc.

The speed of a celestial object can also be determined by studying the width of a given line spectrum. In case the lines show a shift forward and/or backwards, it indicates that the star under observation is orbiting around another massive star or object.

In this way, other material objects in the vicinity of the observed star can also be studied via astronomical spectroscopy.

 Similarly, the presence of a nearby neutron star or black hole can also be estimated. The neutron star is a dense celestial object, heavily packed with neutrons, having a very small radius.

By analyzing light radiation from the matter present between the stars, we can consequently get information about the interstellar medium as a whole.

Through astronomical spectroscopy, scientists revealed that space is not empty; rather, scores of dust particles and gas molecules exist between the stars.

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The disc like flow of gas, plasma or dust particles around an astronomical object is referred to as an accretion disk. Thus, astronomical spectroscopy also provides information about accretion discs in between stars.

What are the spectra of stars?

 The spectrum of a star provides information about temperature, brightness, luminous intensity, chemical composition, mass, density, pressure, and movement of a star.

Stars emit certain light radiations of a specific frequency or wavelength, such as visible light ranging from 400-700 nm. These radiations are collected and observed by the spectrometer.

In astronomical spectroscopy, there are three main types of spectra:

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i) Continuous spectrum:

Stars being hot and dense, emit continuous spectra in a certain range of the electromagnetic spectrum having all wavelengths of light. This is the characteristic spectra of each star depending upon its temperature.

ii) Emission spectrum:

The surrounding material of a star like gas clouds or dust particles, absorbs heat from the observed star and gives a characteristic emission spectrum. This spectrum reveals information about the nature and composition of the surrounding matter.

iii) Absorption spectrum or stellar spectrum:

 In the absorption spectrum of a star, dark lines appear due to the absorption of certain wavelengths by the chemical elements present within the star.

All stars have almost a similar elemental composition, so their absorption spectra predominantly vary due to their temperature differences.

The continuous spectrum of stars is not usually studied for astronomical spectroscopic analysis. Rather, the information obtained by the absorption spectrum is more important and lays the foundation for astronomical spectroscopy. In the absorption spectrum, certain wavelengths may be absent as those wavelengths get absorbed by the chemical elements of a star, like helium gas.

As per astronomical spectroscopy, scientists categorize stars into 7 different classes, represented by the letters O, B, A, F, G, K, and M.

Each one of the above classes is further divided into subclasses from 0 to 9. It describes the range of stars from hottest to coolest.

How do astronomers use spectroscopy?

Astronomers use analytical spectroscopic techniques to study the object matter at the cosmos level, contrary to how analytical chemists use spectroscopy, i.e., to study the properties of material particles at the micro level (atoms, elements etc.).

As discussed in detail above, astronomers use spectroscopy to study stars and galaxies. They interpret the spectrum obtained from the different parts of the universe to gain information about the chemical composition, structure, nature, environment, conditions and properties of the cosmos.

Why is astronomical spectroscopy important? –  Applications

Astronomical spectroscopy is a vast field that has applications in all other scientific fields. It finds plenty of applications at the universal or cosmos level, such as:

  • By interpreting a spectrum obtained via astronomical spectroscopy, astronomers can determine the nature of a star, including its elemental composition, brightness, surface temperature, movement in space, etc.
  • Astronomical spectroscopy provides information about density, pressure, and mass at the surface of a star by comparing the width of the absorption spectral lines. An increase in bandwidth denotes an increase in density.
  • It reveals information about the space present between stars and galaxies.
  • The age of stars, galaxies or even of the Universe can be determined using this technique.
  • By interpreting the astronomical spectra, the movement of distant celestial bodies can be observed (Doppler Effect).

In this way, indeed, astronomical spectroscopy paved the way for scientists to uncover the hidden mysteries of the Universe.

Furthermore, astronomers look forward to reaching the pinnacle of scientific research and exploration by discovering new Planets and distant galaxies enroute astronomical spectroscopy.

Check out this video to learn more about the wonders of astronomical spectroscopy.

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1. Appenzeller, Immo. 2012. Introduction to Astronomical Spectroscopy (Cambridge University Press: Cambridge).

2. Massey, Philip, and Margaret M. Hanson. 2013. ‘Astronomical Spectroscopy.’ in Terry D. Oswalt and Howard E. Bond (eds.), Planets, Stars and Stellar Systems: Volume 2: Astronomical Techniques, Software, and Data (Springer Netherlands: Dordrecht).

3. Tennyson, Jonathan. 2019. Astronomical Spectroscopy: An Introduction To The Atomic And Molecular Physics Of Astronomical Spectroscopy.

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