What do stars send us?
Everything in space gives out energy as waves. Light is one kind. The full family is called the electromagnetic spectrum: radio waves, microwaves, infrared, visible light, ultraviolet (UV), X-rays and gamma rays.
Each kind tells us something different:
- Radio: cold gas clouds, spinning neutron stars (pulsars).
- Infrared: warm dust and baby stars hidden in clouds.
- Visible: stars and planets that shine like the Sun.
- UV and X-rays: very hot gas and gas falling into black holes.
- Gamma rays: the most violent blasts.
Hotter things give shorter waves. Long waves have low energy, short waves have high energy.
Optical telescopes: refractors and reflectors
An optical telescope works with visible light. Its job is not mainly to make things bigger. Its job is to collect more light and to show finer detail.
- A refractor uses a big lens at the front. Lenses sag under their weight and bend colours differently, so very large ones are hard to make.
- A reflector uses a curved mirror. A mirror can be held from behind, so it can be very large. All the biggest telescopes are reflectors.
Light gathering depends on the area of the mirror: area = (width)². A mirror 10 times wider than your pupil catches 100 times more light.
Detail (resolution) gets better with a wider mirror and shorter wavelength. Detectors such as a digital camera chip (CCD) record the picture. Long exposures add up faint light.
Radio telescopes
Radio waves can be a metre long, much longer than light. To see detail with long waves, the dish must be very wide. So radio telescopes are huge dishes, often made of metal mesh (the holes are smaller than the wave, so the wave still bounces off).
The dish reflects waves to a receiver at the focus. The signal becomes numbers on a computer. Radio waves pass through clouds, so radio telescopes also work in the day and in bad weather.
Many dishes can be joined into an array that acts like one giant dish as wide as the distance between them.
Neutrino and gravitational-wave detectors
Some messages from space are not waves of light at all.
Neutrinos are tiny particles made in the Sun, in exploding stars and in nuclear reactions. They have almost no mass and no charge, so they pass through a whole planet. Trillions go through you every second. To catch even a few, scientists use a huge tank of water or ice deep underground, away from other particles. When a neutrino rarely hits, it makes a faint blue flash that light sensors see.
Gravitational waves are ripples in space and time made when very heavy objects, like two black holes, spiral together and merge. A detector is a pair of very long tunnels (kilometres) with laser beams. A passing wave stretches one tunnel a tiny bit and squeezes the other. The lasers measure this change, far smaller than a proton. Several detectors in different countries work together to find where the wave came from.
Ground and space telescopes
The air lets visible light, some infrared and radio waves through, but it blocks UV, X-rays and gamma rays and most infrared. It also makes stars twinkle, which blurs pictures.
- Ground telescopes: can be very large and are easier to repair and upgrade. They work best on high, dry, dark mountains.
- Space telescopes: sit above the air. They see all the blocked wavelengths and get sharp pictures, but they cost more and cannot be fixed easily.
Astronomers often use both and join the pictures to get a full story of one object.
Observatories
An observatory is a place with telescopes, instruments and people. Good sites are high (less air above), dry (less water vapour), dark (little city light) and calm. Mountain tops, deserts and cold dry plains are popular. Radio sites need quiet from phones and TV signals.
Observatories also share data openly, so students and scientists anywhere can study the same sky.
Key formulas and definitions
- Light gathering ∝ D² (D = mirror width)
- Light ratio = (D1 / D2)²
- Best detail ≈ wavelength ÷ D (smaller is sharper)
- Telescope ≠ magnifier: it mainly collects light
- Spectrum (long to short wave): radio, infrared, visible, UV, X-ray, gamma
Worked examples
1. A telescope mirror is 2 m wide. The eye pupil is 0.005 m. How many times more light does the mirror catch?
Ratio = (2 / 0.005)² = 400² = 160 000. The mirror catches 160 000 times more light.
2. Telescope A is 4 m wide and telescope B is 8 m wide. How much more light does B gather?
(8 / 4)² = 2² = 4. B gathers 4 times more light, not 2 times.
3. Why do X-ray telescopes orbit the Earth?
The air absorbs X-rays, so none reach the ground. Above the air, the telescope can detect them.
4. Why is a radio dish much larger than an optical mirror for similar detail?
Detail depends on wavelength ÷ width. Radio waves are about a million times longer than light, so the dish must be far wider to get similar detail.
Common mistakes
- Thinking a telescope mainly magnifies. Its main job is to gather light and show fine detail.
- Thinking twice the width gives twice the light. It gives four times, because area grows as width squared.
- Thinking all telescopes use light. Radio, X-ray, neutrino and gravitational-wave detectors use other signals.
- Thinking space telescopes are closer to the stars. The stars are so far that a few hundred km changes nothing; the gain is being above the air.