Why just settle for one hidden world? When we look at the clear night sky we see a scattering of distant stars. But we can only see a small portion of the electromagnetic spectrum. There’s a lot more of the Universe out there to see, but we need a little help!
Our eyes have evolved to cope with a certain range of electromagnetic radiation. Light waves have a wavelength, and this wavelength characterises the light. A shorter wavelength means more energetic photons, while in a longer wave the photons are of a lower energy.
The spectrum is continuous and spreads from very low energy waves such as radio waves, through microwaves and infrared, visible light, ultraviolet, and extending up to higher energy waves such as X-rays and gamma rays. Our eyes have evolved to make the most of the light output by our Sun, which peaks at around 500 nanometres – surprisingly enough, this means the Sun’s peak output is green light!
Since the first serious attempts using a telescope by Galileo in the 1600s, astronomers were limited to searching for distant stellar objects using the visible spectrum. It wasn’t until the early 19th Century that scientists began to find ‘invisible light’ – what we now know as the other regions of the electromagnetic spectrum – and began experimenting with different kinds of telescope.
Radio waves are the lowest energy region of the spectrum, but have contributed much to our understanding of the Universe. Observations using the radio spectrum have revealed distant, exotic objects called quasars, thought to be some of the most distant (and therefore oldest) objects in the Universe. We have also seen pulsars, extremely dense remnants of exploded stars which rotate with such speed they emit regular pulses of electromagnetic radiation. Both of these discoveries have increased our knowledge of the life cycle of stars.
While initially picked up as interference by equipment looking for radio signals, the Cosmic Microwave Background (CMB) is one of the most significant discoveries of 20th Century Physics. By observing the sky in all directions, the microwave background gathered shows the state of the Universe when it was around 380,000 years old – that’s roughly 13 billion years ago! The Universe is surprisingly uniform at this time, possibly indicating that the early Universe was a lot smaller. The CMB is a key piece of evidence for the Big Bang model.
Ultraviolet (UV) radiation is more energetic than visible light, so objects emitting UV light tend to be very hot. Most stars emit mainly in the visible or infrared regions with very little UV radiation (our Sun peaks in the middle of the visible spectrum), so when we look at the Universe in the UV region most of the stars become less noticeable. Instead, we can see the youngest and hottest stars being formed inside enormous galactic nebulae. These stars are shrouded in observations in the visible region so observing in UV allows us to see how stars form.
X-rays and Gamma-rays carry even more energy. X-rays have led us to discover immense hot gas clouds between galaxies. Gamma-rays, as the most energetic region of the spectrum, are associated with the most extreme galactic events. Supernovae, which occur when a dying star collapses under its own gravity, emit bursts of X- and Gamma-rays. They are also characteristic of pulsars and black holes, both of which are very dense and energetic environments.
So much of our understanding of the evolution of stars, galaxies and the Universe as a whole has come from opening our eyes to forms of light that we’re unable to see. From the early formation of the Universe to the last moments of a dying star, visible light simply cannot give us the complete picture. There really is more to the Universe than meets the eye.