Meanwhile, the shortest wavelengths on the electromagnetic spectrum belong to gamma rays and can be as small as just a tnllionth of a metre. So to see this hidden light, astronomers need a variety of tools, from giant bowl-shaped radio dishes that can be seen from miles around, to detectors in orbit
above Earth's obscuring atmosphere.
This other light remained hidden until its discovery at the beginning of the 19th century. It was the year 1800 when William Herschel used a prism to split light into a spectrum of colours and measured the temperature of each shade. He came to the conclusion that there must be an extra unseen colour beyond red because his thermometer was responding to light in the spectrum that he could not see - the infrared. A year later, the German scientist Johann Ritter conducted a similar experiment, but this time found that unseen light beyond violet had the ability to darken paper soaked with silver chloride. Ultraviolet light had been discovered.
Our planet's atmosphere doesn't want us to see some of this hidden light from the universe and it does a pretty job of absorbing infrared light using its water molecules, while its ozone layer blocks harmful ultraviolet. In fact, other than radio, microwave and infrared astronomy, most of this hidden light needs a very strong space telescope high above the atmosphere to see it. That's partly why our understanding of the universe has grown so much since the Space Age, because we can now launch telescopes far into Earth's orbit to study the mysterious hidden universe.
So lets imagine that we're able to put on infrared goggles and X-ray glasses - what can we see? Our own galaxy, the Milky Way, is a completely different beast when we look at it in other wavelengths. Galaxies, including our own, are made up of lots of hydrogen gas. Atomic hydrogen radiates at a wavelength of 21 centimetres (8.3 inches) and radio astronomers are able to map where this atom is in our galaxy to give them an indication of the size and shape of the Milky Way, along with how fast it's actually spinning.
But there are many more things in the galaxy that spit out radio waves. There are pulsars, which are spinning dead stars discovered by astronomer Jocelyn Bell in 1967. She found a regular 'beep-beep-beep' pulse coming from these objects in the form of radio waves. These dead stars, which come from stellar explosions commonly known as supernovas, emit beams of radio waves and as they spin they flash these beams at us over and over again, creating the dramatic appearance of a pulsing object.
When supernovas explode, they leave behind a nebula of gas and dust blasted out by the explosion. The brightest radio objects in the sky are supernova remnants, such as the Crab Nebula, also known as Ml in the constellation Taurus. Ml is the remains of a massive star that blew itself to smithereens in the year 1054. Supernova remnants like Ml also produce radio waves, when a powerful stream of ultraviolet radiation coming from the pulsar at its heart excites the gas in the nebula and causes it to radiate in radio waves. There's more to Ml than radio though. Let's turn the dial on the electromagnetic spectrum and look at it in infrared light, which tends to come from cooler, lower energy objects. In this remnant, it is the cool dust that was created In the mighty supernova explosion that we can see in infrared light. Now turn the dial again into visible light and what we see is hot gas glowing at tens of thousands of degrees. Move again past visible light and into the ultraviolet and X-ray spectrums and we see that the nebula glows at each of these wavelengths. This tells us that there is a powerful magnetic field around the pulsar in the middle. Subatomic particles, known as electrons, have an electric charge that causes them to spiral around wildly in the magnetic field. As they do so, they give off photons of energy in a process scientists call synchrotron radiation. These photons are what we see in the Crab Nebula at ultraviolet and X-ray light and wherever astronomers see synchrotron emission in the universe, they know that there must be a magnetic field lurking around somewhere.
Massive stars that explode as supernovae are found to glow brightly and possess swelteringly hot temperatures of up to 20,000 degrees Celsius (36,000 degrees Fahrenheit). Things this hot radiate in visible light but also in ultraviolet light and space telescopes such as NASA's Galaxy Evolution Explorer (GALEX) seek out sites of massive star formation, not just in our galaxy but in other galaxies, by looking for their ultraviolet light. On the other hand, cooler stars glow best in infrared light, meaning that telescopes such as NASA's Wide-field Infrared Survey Explorer (WISE), along with their Spitzer Space Telescope, are able to sniff them out. These cool stars include the pint-sized red and brown dwarfs that can be as cool as a planet like Jupiter, or even cooler. WISE has found several brown dwarfs just a few light years away that are very dark in visible light, but are a bit brighter in infrared - at a bearable room temperature, they are found to glow quite faintly.
In case you were wondering, the Sun puts out most of its light in the visible range, but also produces infrared, X-rays, radio waves and ultraviolet too. Solar space telescopes like the Solar and Heliosphenc Observatory (SOHO) and the Solar Dynamics Observatory are able to look at the Sun with cameras designed to observe in an array of wavelengths. Hot active regions on the Sun can produce lots of ultraviolet light, while the most powerful solar flares can throw out bursts of energetic X-rays. The Sun also radiates at longer wavelengths, constantly blaring out in radio waves. In fact, our own star was one of the first objects to be studied with radio waves.
So far we've been kicking back with the not-so-energetic radio, but what occurs with the high-energy photons that belong to gamma rays? These come from only the most extreme objects and the first gamma-ray astronomy was done not by a dedicated space telescope, but by military satellites designed to search for gamma rays from tests of nuclear bombs during the Cold War. The satellites did detect gamma rays, but they weren't coming from nuclear bombs. Instead they were travelling from the depths of space and for a quarter of a century nobody had a clue what was causing these bursts, or even whether they were from a local source or far away. Officials realised that if they were coming from far away, then their strength must make them the most powerful outbursts since the Big Bang.
In 1997 the mystery was solved in something of a role reversal, when scientists used the visible universe to reveal details about the hidden universe. The European BeppoSAX satellite detected the gamma rays from one of these mysterious bursts and an alert was swiftly sent out to astronomers all over the world. The William Herschel Telescope in the Canary Islands was able to point toward the scene of the burst and saw a faint afterglow of visible light from a supernova in a distant galaxy, one that lays an astonishing 6 billion light years away. This was no ordinary supernova, but the destruction of one of the most massive stars in existence as it produced a black hole. The gamma rays are created by twin beams of high-energy particles that shoot out from the exploding star in magnetic jets, causing the electrons to spiral around the magnetic fields so strongly that they produce immensely powerful gamma rays. Since then, gamma-ray bursts have been seen to explode from almost 13 billion light years away. These bursts are the most powerful explosions in the universe and we only found them by accident!
Gamma rays don't just come from exploding stars. The Milky Way also sends out these high-energy rays. NASA's Fermi Space Telescope was designed to observe the universe in gamma rays, but what it found in the heart of our galaxy was truly amazing and surprised everyone.
Galaxies shine in light all across the electromagnetic spectrum and the dust in spiral arms is sensitive to infrared, massive star formations glow in ultraviolet and hydrogen gas emits radio waves. Although the supermassive black holes that reside at the heart of most galaxies don't emit any radiation that we can detect, their interactions with the matter surrounding them radiate at practically every wavelength possible. An active black hole, such as the monster inside the giant elliptical galaxy, Messier 87, is a powerful emitter of radio waves. It beams a jet of particles moving at nearly the speed of light that can be seen in visible light but also at other wavelengths, such as X-rays. Much quieter is the Milky Way's black hole, which is not gobbling up much gas at all. Occasionally we will see it flash in X-rays when it swallows something small like an asteroid, which is why astronomers were astonished when Fermi discovered two giant jets of particles, each pointing in opposite directions at the centre of the galaxy. These were spilling out to form bubble shapes shining in gamma rays. Each Fermi Bubble is about 25,000 light years tall and came from a dramatically violent event that happened in the centre of the galaxy several million years ago. This would have happened when the black hole consumed a large amount of gas, causing it to fiercely erupt, or during an intense period of star formation with stars so hot and massive that their stellar winds were able to blow out and heat large amounts of gas from the centre of the galaxy These bubbles are immensely active, but are completely invisible to our eyes in the always surprising hidden universe.
But maybe the biggest thing to be seen in the hidden universe is the Big Bang itself. We cannot see the moment of formation that took place 13.8 billion years ago, but the heat energy from the Big Bang still fills the universe to this day, albeit a lot cooler than in the beginning. Today it is only -270 degrees Celsius (-454 degrees Fahrenheit) and this remnant heat energy shines brightest in microwave light. Telescopes have made maps of this Cosmic Microwave Background (CMB) radiation and from it scientists can figure out things such as how galaxies grew. Funnily enough, we wouldn't need a telescope to detect the CMB radiation because on analogue television sets, a small portion of the static comes from these microwaves.
The cold universe tends to radiate at the longer wavelengths of the electromagnetic spectrum, such as radio, microwaves and infrared. Infrared is sometimes called thermal emission, because things that are warm around us, such as our bodies or a fire, shine brightly in infrared light. Meanwhile, the most energetic action in the universe produces lots of ultraviolet. X-rays and gamma rays. It may seem like astronomers have revealed all of the universe, but there is still plenty that remains unseen. Within the hidden universe, nothing is beyond possibility.