The Japan News by The Yomiuri Shimbun

Home > Opinion > Science



Distant beauty of the night sky: viewing stars using radio waves invisible to the human eye

Sumiko Kida
Assistant Professor, Faculty of Science and Engineering, Waseda University

When gazing at the night sky, we are struck by the beauty of its countless gleaming stars. However, there is an even more dynamic sky with far more lights unperceivable to our human eyes. .

Looking at stars with our eyes

What comes to your mind when you hear “astronomical observation?” For many, you probably picture people looking at stars through telescopes—to be a little more precise, through telescopes with their eyes. Of the many types of electromagnetic waves (radio waves, infrared light, visible light, ultraviolet light, X-rays, and gamma rays), the human eye can only detect visible light, which has a wavelength of roughly 400 nm–750 nm. In other words, when we talk about using our eyes to look at stars, what we are seeing is the visible light emitted by these stars. However, stars do not just emit visible light. Although it depends on the type of star, stars emit all types of electromagnetic waves, from radio waves to gamma rays. If we could see electromagnetic waves other than visible light, our entire perception of stars would change

Looking at stars with radio waves

Human beings have been using their eyes to look at stars for over two millennia, unaware for an extremely long time that there was more than visible light coming from the sky. In the 20th century, Karl Jansky, an American radio engineer, picked up unidentified radio waves from the sky by chance, and discovered they had originated in the Milky Way. This was the beginning of radio astronomy.

Figure 1: Waseda University’s Nasu Observatory
Eight radio telescopes with a 20-meter diameter, and one with a 30-meter diameter, perform daily surveys and monitor variances of radio sources.

How can stars be viewed with radio waves? An easy way to conceptualize this is to think of infrared cameras or X-ray images. It becomes possible to look at stars with radio waves after collecting and forming images of their emitted radio waves. To pick up radio waves from stars requires a massive telescope, but unlike the visible-light telescopes that you look into, a radio telescope is a large antenna (Figure 1). As radio waves transmitted by stars are extremely weak and have long wavelengths, the radio telescope must be large in diameter. The current mainstream approach to picking up these radio waves is through technology called radio interferometry, where multiple telescopes are utilized as a single large telescope.

Sometimes, radio waves will show bright stars invisible to the human eye. Conversely, stars that are visible to the human eye will sometimes yield no image on a radio telescope (Figures 2 and 3). Radio waves have longer wavelengths and lower energies compared to visible light. For this reason, radio waves will reveal developing stars that do not yet have enough energy to emit bright visible light. They will also reveal traces of the Big Bang that weakened since the formation of the universe and other phenomena. Radio astronomy has received the most Nobel Prizes out of all branches of astronomy, and has made enormous contributions to the field. The ALMA (Atacama Large Millimeter/submillimeter Array) Telescope set up in Chile’s Atacama Desert, which commenced full operations in 2013, is capable of spotting from Tokyo a one-yen coin on the street in Osaka.*1 This remarkable power is one of the advantages of using radio waves. Radio astronomy is expected to continue making great contributions to the field of astronomy.

Figure 2: Antennae Galaxies (radio observation with the ALMA Telescope)
Varying intensity of radio waves are identified using varying colors
(Image courtesy of National Astronomical Observatory of Japan (NAOJ)) Credit: ALMA (ESO/NAOJ/NRAO)

Figure 3: Antennae Galaxies (superimposition of images produced by radio observation with the ALMA Telescope, and by optical observation with the Hubble Space Telescope)
Different areas light up when using the radio telescope and the optical telescope.
(Image courtesy of NAOJ) Credit: ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope

A contest between radio astronomy and smartphones?

Many of you are probably reading this column on a smartphone. The smartphones that you are holding in your hands right now are in fact closely related to the field of radio astronomy.

We are unable to see many stars in areas surrounded by bright light such as the central part of a city. Similarly, stars can also be obscured when looking at stars with radio waves. Our living environment is full of radio waves flying in all directions. The radio waves that reach Earth from stars are much weaker than those emitted by devices such as smartphones. Strong radio wave interference makes observation increasingly difficult. Legal regulations require frequency allocation for radio transmissions such as radio communication, and radio reception such as radio astronomy. The International Telecommunication Union sets internal regulations for telecommunications while the Ministry of Internal Affairs and Communications oversees frequency allocation in Japan. For radio astronomy, observations are performed exclusively within allocated frequency bands. Among frequencies allocated for radio astronomy, some bands are reserved solely for reception (non-transmission bands), while others are used for both reception and transmission. There have been a number of instances where what was initially thought to be the discovery of the century turned out to be a terrestrial communication signal. I have also had several bitter experiences where interference rendered painstakingly collected observation data useless. The rapid spread of radio communication has presented a danger of radio astronomy.

We are in an age where we need to spread awareness and understanding of radio astronomy to the wider public in order for it to coexist with technologies that enhance daily convenience and safety. Japan’s Radio Act defines radio waves as “electromagnetic waves of frequencies of 3,000,000 MHz or lower.” As the phrase “or lower” indicates, radio waves are a limited resource, a limited asset. I believe a bit of consideration for radio astronomy when using radio communications with devices such as smartphones would go a long way.

  1. ^ *1 ALMA Telescope, NAOJ http://alma.mtk.nao.ac.jp/j/

Sumiko Kida
Assistant Professor, Faculty of Science and Engineering, Waseda University

Sumiko Kida earned her PhD in Science at Waseda University’s Faculty of Science and Engineering’s Department of Pure and Applied Physics at the Graduate School of Advanced Science and Engineering. She became an Assistant Professor in 2013 after spending time as a Research Fellow at the Japan Society for the Promotion of Science, and as a Research Associate at Waseda University’s School of Education. She is also a part-time lecturer at Nihon University’s College of Science and Technology. Her area of expertise is radio astronomy and she is an official member of the Astronomical Society of Japan.