Telescopes and Studying Space

Why we cannot just look harder

For almost all of human history, people studied space with their eyes alone. They mapped the patterns of stars, tracked the planets and predicted eclipses with remarkable accuracy. But the eye has a limit. Your pupil — the dark opening that lets light in — is only a few millimetres wide. It can only gather a small amount of light, so the faintest, most distant objects in the Universe stay invisible no matter how hard you stare.

The breakthrough came around 1609, when Galileo Galilei pointed a new invention, the telescope, at the night sky. Suddenly he could see mountains and craters on the Moon, four moons circling Jupiter and the fact that the Milky Way is made of countless individual stars. These observations changed science forever and helped prove that Earth is not the centre of the Universe. The telescope turned astronomy from naked-eye stargazing into a powerful science.

What a telescope actually does

People often think a telescope's main job is magnification — making things look bigger. That matters, but it is not the most important thing. The real superpower of a telescope is light-gathering.

A telescope has a large lens or mirror at the front, far wider than your pupil. It collects light over its whole surface and funnels it to a point. The bigger that lens or mirror, the more light it catches, and the fainter the objects it can reveal. A telescope with a mirror 20 centimetres wide gathers hundreds of times more light than your eye. This is why the size of the main lens or mirror, called the aperture, is the single most important number for any telescope.

Once the light is gathered, a smaller lens called the eyepiece magnifies the focused image so your eye can examine it. Magnify too much, though, and the image just becomes a dim, blurry smudge — there is only so much detail in the light you collected. Light-gathering comes first; magnification second.

Two ways to build a telescope

There are two basic designs, and the difference is simply how they gather light.

A refracting telescope (a "refractor") uses a curved glass lens at the front. The lens bends — or refracts — the incoming light and brings it to a focus near the back of the tube. These are the classic long telescopes you might picture a sea captain using. They give crisp images but become heavy and expensive at large sizes, because a big lens can only be supported around its edges and tends to sag.

A reflecting telescope (a "reflector") uses a curved mirror at the back of the tube instead. Light travels down the tube, bounces off the mirror, and is focused back up to an eyepiece. Isaac Newton built the first practical reflector in 1668. Mirrors can be made far larger than lenses because they can be supported across their whole back surface, and they only need one polished side. That is why nearly every giant research telescope today, on the ground and in space, is a reflector.

Beyond visible light

Here is something surprising: the light your eyes can see is only a tiny slice of all the light in the Universe. There is also radio, infrared, ultraviolet, X-ray and gamma light. Each kind reveals something different about space, and we cannot see any of them with our eyes.

To capture them, astronomers build different kinds of telescope:

• Radio telescopes are huge metal dishes that collect faint radio waves from space. They can detect cold gas clouds, distant galaxies and pulsing dead stars, even through clouds and in daylight.
• Infrared telescopes detect heat-like light and can peer through dust to see newborn stars and cool, distant objects. The James Webb Space Telescope, launched in 2021, is mainly an infrared telescope.
• X-ray telescopes study the most violent places in the Universe, such as gas swirling into black holes — and because Earth's air blocks X-rays, these telescopes have to be in space.

By combining views in many kinds of light, astronomers build a far richer picture than the eye alone could ever give.

Why some telescopes go to space

Earth's atmosphere is both a blessing and a problem for astronomers. It keeps us alive, but it also blurs and blocks starlight. Twinkling stars look pretty, but that twinkle is the air wobbling the light — bad news if you want a sharp image. The atmosphere also soaks up much of the infrared, ultraviolet and X-ray light coming from space.

The solution is to put telescopes above the atmosphere, in orbit. The famous Hubble Space Telescope, launched in 1990, gave us astonishingly sharp views of galaxies billions of light-years away precisely because it sits above the blurring air. Space telescopes are expensive and hard to repair, but their clear, steady view is worth it.

Looking into space is looking into the past

One of the strangest and most beautiful ideas in astronomy is that telescopes are time machines.

Light is fast — about 300,000 kilometres per second — but space is unimaginably vast, so even light takes time to cross it. Astronomers measure these huge distances in light-years: one light-year is the distance light travels in a year, roughly 9.5 trillion kilometres.

Because light takes time to arrive, you always see distant objects as they were when their light left them, not as they are now. Sunlight you feel left the Sun about 8 minutes ago. The light from the nearest star beyond the Sun left over 4 years ago. When a powerful telescope captures a galaxy that is, say, 50 million light-years away, it is showing that galaxy as it looked 50 million years ago — around the time the dinosaurs had not long disappeared. The deeper we look into space, the further back in time we see. This is how telescopes let us study the history of the Universe itself.

Try it yourself: a backyard observing project

You do not need a research telescope to do real observing. Try this over a week or two.

1. Find a dark spot. The biggest improvement to your view is escaping light pollution — the glow of streetlights and buildings. A garden away from direct lights, or a trip to the countryside, dramatically increases how many stars you can see.
2. Start with the Moon. Even ordinary binoculars reveal its craters and dark plains. Sketch what you see, and notice how the shadows shift as the Moon's phase changes.
3. Hunt for Jupiter. When Jupiter is in the evening sky, steady binoculars (rest your elbows on a wall) will show up to four tiny points of light beside it — the very moons Galileo discovered. Watch them shift position from night to night.
4. Keep an observing log. Note the date, time, what you looked at and what you saw. Real astronomers keep logs too; yours turns casual stargazing into genuine science.

Be patient and let your eyes adapt to the dark for at least 20 minutes before you start. With practice, the night sky opens up into far more than a scatter of dots.

Want to go further? See Galaxies and the Milky Way to learn about the vast star cities telescopes reveal, and Planets of the Solar System to meet the worlds you can spot from your own back garden.