With more energy than that, photons become X-rays, which travel right through you. When photons have a little bit more energy, they become ultraviolet radiation, which you cannot see but which can give you a sunburn. Light is made up of tiny particles called “ photons.” Photons in visible light have a medium amount of energy. Because human eyes can only see visible light, we have to build special telescopes to pick up the rest of that “spectrum” – and then turn them into pictures and graphs that we can see. The electromagnetic spectrum also includes gamma rays, X-rays, ultraviolet radiation, infrared radiation, microwaves, and radio waves. The visible light that your eyes can see is only a tiny portion of what astronomers call the “ electromagnetic spectrum,” the whole range of different light waves that exists. Almost all of these stars are invisible to your eyes, which cannot see the dim light from distant stars. Beyond the Milky Way, astronomers think that about 100 billion more galaxies (each with their own 100 billion stars) exist. About 100 billion more stars, exist just in our galaxy, which is called the Milky Way. But the individual dots you see are all nearby stars. If you live in a dark place far from cities, you can see thousands of them. When you look up at the night sky, you see the bright lights of stars. Together, telescopes that can see different kinds of waves – from radio waves to visible light waves to gamma rays – give a more complete picture of the universe than any one type of telescope can on its own. While scientists can learn a lot from the visible light they detect with regular telescopes, they can detect different objects and events – such as black holes, forming stars, planets in the process of being born, dying stars, and more – using radio telescopes. Since then, astronomers have built better and better telescopes to find these cosmic radio waves and learn more about where they come from and what they can tell us about the universe. Radio astronomy began in 1933 when an engineer named Karl Jansky accidentally discovered that radio waves come not just from inventions we create but also from natural stuff in space.
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