Astronomy is one of the oldest sciences in existence, with the histories of nearly all cultures finding a connection to the stars. In modern times our fixation with outer space has only grown, with scientists finding new ways to study the universe, the most recent being through the perception of gravitational waves released due to space-time distortions from merging black holes and neutron stars. However, most of our knowledge comes from observing the universe using a somewhat less complicated tool, electromagnetic (EM) radiation.
For the longest time, we relied on what was visible with only our naked eye (optical light or the visible spectrum), but the truth is that there is more to ‘light’ than meets the eye- literally! Now astronomy is based on understanding the universe using a large portion of the electromagnetic spectrum and examining wavelengths of light far beyond what is visible. It is truly amazing that studying different parts of the light spectrum reveals new understandings of the universe.
Studying the Big Picture
Multiwavelength astronomy takes a holistic approach to the electromagnetic spectrum, working on the principles of both wave and ray optics. It uses the fact that light has a dual nature and can exist as both particles and waves (DeBroglie’s wave-particle duality). At long wavelengths (such as radio waves), it is treated as a wave, and at short wavelengths (such as x-rays and gamma-rays), it is treated as a particle, specifically a photon. While building telescopes, this dual nature of light is exploited. Most deep-space telescopes currently under construction will be able to capture at least two forms of light, such as x-rays and gamma-rays, which are frequently paired with each other.
The value of viewing the same object using different wavelengths of light can be seen in the case of the Crab Nebula. A nebula is created when a star explodes. The star collapses and then expands outwards, shooting particles and energy into the space around it. The nebula retains a cloud-like structure since the free particles are still slightly attracted to the star’s cooling core.
The Crab Nebula has been photographed in six different spectra of light. To address a few, visible light is used to identify the elements which compose the nebula, such as oxygen in yellow and sulfur in green. These residues would not normally be visible, but infrared radiation heats them, causing each element to glow in a unique colour profile. Infrared light shows the shape and intensity of the magnetic field and its heat centre. X-rays and gamma rays reveal the small and highly energized pulsar (previously the core of the star that collapsed) along with the spirals and jets of high-energy particles it releases.
Roadblocks in Multiwavelength Astronomy
Unfortunately, there are many difficulties associated with performing multiwavelength astronomy across the entire electromagnetic spectrum. Firstly, there are certain limitations on what part of the electromagnetic spectrum can reach the surface of the earth, which is where the biggest telescopes are. For example, most infra-red, ultra-violet, x-rays, and gamma-rays are absorbed or scattered by the Earth’s atmosphere. In the optical wavelength, we are limited to observing only on nights with clear weather. In an attempt to mitigate these barriers, most of the telescopes used for this type of star gazing are built at high altitudes on Earth, or in outer space (like the Hubble space telescope).
Secondly, for longer wavelengths, the telescopes are diffraction-limited, meaning that the resolution that the telescopes can achieve is limited by the wavelength being perceived and the size of the telescope lens. This is a common phenomenon in wave optics called angular diffraction limit that has been quantified in the formula:
Adf = 1.22 × (λ÷D)
Where Adf is the angular diffraction limit, λ (lambda) is the wavelength of light, and D is the diameter of the reflector (or the surface diameter of the telescope lens).
Therefore, most radio telescopes in the world like the Arecibo telescope in Puerto Rico, and the Five hundred meter Aperture Synthesis Telescope (FAST) in China are very large (300 and 500 meters in diameter, respectively). So while it has its challenges, modern astronomy has also challenged scientists to produce tremendous advancements in technology.
Views of the Future
Multiwavelength astronomy has accelerated our understanding in fields like cosmology (the study of the large scale structure, expansion, and formation of the universe), galaxy evolution, stars and planet formation, exoplanets, transient phenomenon (the explosion of a star), and much more. With the detection of gravitational waves becoming possible, we are now headed into a new era of multiwavelength astronomy. In other words, the universe is about to get a lot bigger!
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Couder, Y. & Fort, E. Probabilities and Trajectories in a Classical Wave-particle duality. Journal of Physics: Conference Series. 2012. doi:10.1088/1742-6596/361/1/012001.
Mickaelian, M. Multiwavelength Astronomy and Big Data. Astronomy Reports 60. 2016. doi:10.1134/S1063772916090043.
Middleton, J., Casella, P, Gandhi, P., et al.. Paving the way to simultaneous multi-wavelength astronomy. New Astronomy Reviews. 2017. 79. doi:10.1016/j.newar.2017.07.002.
Takahashi, T., Uchiyama, Y., & Stawarz, Ł. Multiwavelength astronomy and CTA: X-rays. Astroparticle Physics 43. 2013. doi:10.1016/j.astropartphys.2012.05.010.