The universe in multi-color

In the planetarium, the camp participants were offered a tour of the universe in multi-colour. It was the opportunity to discuss why astronomers and astrophysicists are first of all not observing nowadays with their eyes, but rather with instruments, and second of all, why they use instruments able to “see” other wavelengths or “colors” than our eyes can do. 

The planetarium offering truly stellar views. (Image: Sebastian Käser, Physics Olympiad)

To the first question, the answer is easy: precision. Our eye is an incredibly complex biological instrument. If it is great and very adaptable for all kinds of situations in everyday life, this adaptability also has drawbacks for astronomy. If you reach a high altitude, say on top of a mountain, you will have less oxygen available, your eyes will not perform as well as on the ground level and the same star will appear dimmer. If we measure the world only by means of our natural eyesight, this leads to a lack of replicability.

Are you a teacher looking to introduce your students to the fascinating science behind planets? On the website of NCCR PlanetS, you can find a number of teaching materials.

On the contrary, electronic instruments are insensitive to this parameter. We even tend to prefer to install instruments and telescopes on top of mountains to reduce the amount of atmosphere between them and the target star. This effectively reduces the absorption of light due to the atmosphere, and increases the light received from a star. Moreover, in astronomy and astrophysics, more light means more information! 

Apart from the lack of replicability, our eyes also lack precision. Let’s talk about exoplanets, which are planets orbiting around other stars than our Sun. One of the main methods of detecting them is to observe the luminosity of a star. If a planet passes in front of it, even if we don’t see the planet itself, we detect a slight temporary decrease of luminosity. To give an order of magnitude, a planet the size of Jupiter in front of a star the size of our Sun would block a mere 1% of the luminosity for a few minutes up to a few hours depending on its orbital period. For a planet the size of the Earth, which is 10 times smaller than Jupiter, it would only be a 0.01% decrease in luminosity! This is far below what a human eye is able to notice. 

To give another example, seeing a planet directly next to its star is extremely challenging for technology, but doable for very young and still warm and bright giant planets. Yet, you need instruments able to distinguish two objects with a brightness contrast of over a million. For a more relatable comparison, imagine standing 1km away from a lighthouse pointed in your direction, and trying to distinguish the dim firefly standing 10 cm away from the bright light. 

Finally, we can also mention the limited range of colors our eye is able to perceive. We can perceive a plethora of colors, from red to purple, which is what we call the “visible” spectrum. But this represents only a minute fraction of the whole spectrum of light, or in other words, of the electromagnetic spectrum. Just beyond purple we have ultraviolet, and just below red we have infrared - “colors” that some animals such as snakes or birds can see. The whole electromagnetic spectrum is usually divided in seven main parts, from the lowest energy to the highest energy: radio waves, microwaves, infrared, visible light, ultra violet, X-rays and gamma rays, each with further subdivisions such as near, mid and far infrared, or soft and hard X-rays. And yes, in terms of physics, a radio wave, or visible light are the same thing. They are an electromagnetic wave, but with a different frequency and wavelength.

This brings us to our second question: why is it important to observe more than only the visible light? In short: we can see more things. 

Think of a piece of steel. At room temperature it will seem gray and not emitting any light. In fact, it emits infrared light just like any of us do. We can’t perceive it but it is there. If you put the steel in a forge and warm it up, as its temperature increases, the steel will start to glow red, orange, yellow and even white. 

Now you can already guess one of the answers to our question: Being able to perceive electromagnetic radiation other than the visible light, enables us to observe astronomical objects at different temperatures. As such, a sun-like star mainly emits visible light, which is why our eyes evolved to be the most sensitive to it. A small « M-dwarf » star in comparison, will mainly emit infrared light.

We can see objects at different temperatures - and at different speeds. Here the explanation lies, in part, in the Doppler-Fizeau effect. In everyday life, you experience it every time a car passes next to you, and even more so with ambulances for instance: the pitch of engine roaring or of the siren, seems to change depending if the car comes in your direction (higher pitch), or goes away (lower pitch). This effect exists in every type of waves such as sound waves in this example, or such as electromagnetic waves in the case of stars. In the case of distant galaxies, this is amplified (in a way) because of the expansion of the universe. The visible light emitted by stars or galaxies might thus be shifted to other wavelengths or “colors”.

A representation of the electromagnetic spectrum. (Credit: NASA, Joseph Olmsted, STScI via Roger Thibaut, PlanetS)

Another aspect of light, which requires us to explore the universe in multicolour, are spectral lines and bands, either in emission, reflection or absorption. We will focus on the latter. If you put a gas in front of a light source emitting all colors at once, such as a star does on a limited range, the gas will absorb specific colours. Those colors depend on the chemical composition of the gas and act as some sort of fingerprint. Thanks to those “fingerprints”, we can detect specific atoms and molecules in space, in stars or even in planets' atmospheres. The specific colours absorbed depend on the electrons of an atom, the vibration of atoms in a molecules, or their rotation. However, many of those “fingerprints” are outside the visible spectrum despite their great use as diagnosis tools, as we can use such lines for instance to determine with precision the temperature, or activity of a star. Going to the infrared enables us to detect complex molecules which are the building bricks of life, while UV might for instance enable us to detect the specific line of ionised hydrogen from a planet's atmosphere being evaporated by the irradiation of its close-by host star.

An example of various « fingerprints » from atoms in the spectrum of a star. (Credit: NASA, ESA, Leah Hustak, STScI via Roger Thibaut, PlanetS)

Apart from the inflatable planetarium, the participants of the physics camp also got to observe a simple but insightful experiment on gravity and learn about how the University of Bern developed and built an instrument for the ESA space probe Juice. This mission has been on its way to Jupiter since April 2023 to explore the largest planet in our solar system and three of its more than 80 moons: the large, icy moons Ganymede, Callisto and Europa.

Finally, just like X-rays allow you to see through your body or luggage but not through materials like metal or bones, observing the universe in all parts of the electromagnetic spectrum allows astronomers and astrophysicists to see through other objects. The example we provided in the planetarium is the Crab Nebula. One of the most majestic astronomical objects in visible light, it is the remnant of a supernova, or if you prefer, a star that exploded. In lieu of the massive star at the center though, still remains a neutron star, surrounded by the nebula itself: a cloud of gas and dust ejected from the star. Observing it in radio enables it to pierce through that cloud and reveal a strange phenomenon: the central neutron star, in radio, pulse every few milliseconds, which is what we call a pulsar.

First image: The Crab Nebula as seen in visible light by the space telescope Hubble. Second image: Observing the Crab Nebula in X-ray like with the Chandra satellite here, displays a completely different picture and enables to glimpse at the pulsar in the center. (via Roger Thibaut, PlanetS).

Just like a lighthouse turning and emitted light in only two directions, a pulsar rotates on itself and emits radio only in two directions. And just like lighthouses are extremely useful for sailors at sea, pulsars are extremely useful guide objects for astronomers and astrophysicists. Not observing in radio such an object would make you miss an important piece of the puzzle, to better understand stars and the universe.

This article was written by Thibaut Roger from NCCR PlanetS. The series was supported by Sophie Krummenacher from NCCR PlanetS/CSH. CSH is an international research team at the University of Bern which fosters interdisciplinary interactions on the formation, detection and characterization of other worlds within and beyond the Solar System. NCCR PlanetS is a project funded by the Swiss National Science Foundation involving the Universities of Bern, Geneva and Zurich as well as ETH Zurich.

Learn more

• Physics Olympiad
• Center for Space and Habitability
• NCCR PlanetS
• Part Two: Exploring the universe's stage