Twinkle twinkle little star There aren't many nursery rhymes about astronomy. But 'twinkle twinkle little star' makes a useful point. We can tell which lights in the night sky are stars because they appear to twinkle. Planets, on the other hand, don't, they shine steadily in the sky. Stars twinkle because they are very far away, and so appear as tiny points of light in our night sky. Some of this light is absorbed by moving air in the Earth's atmosphere, making the star appear to sparkle. Planets, like Saturn or Jupiter, don't sparkle. This is because they are a lot closer to the Earth and so they look bigger in our sky than stars. Explore the planets with our travel guide

The brightest star in space - the Pistol Star

Rather than being points of light, planets are small discs. As their light is more spread out, even if some of it is absorbed by our atmosphere, some of the light still filters through, so the planet doesn't twinkle. This means that you can tell the difference between a planet and a star without even needing a telescope, just by seeing if it twinkles! Weighing a star As for 'wondering what they are', starlight also contains an extraordinary amount of information that can answer that very question. Weighing a star isn't as difficult as it sounds as you don't need an enormous set of cosmic scales, just one equation. The more massive a star is, the more energy it gives off. So first of all, astronomers measure the star's 'luminosity', the rate at which it emits energy. Then they can work out its mass. Measuring a star's temperature Once the mass of the star is known, you can calculate the temperature inside its burning core. Stars perform a delicate balancing act between gas pressure pushing outwards and gravity pulling inwards. Mathematical equations can be used to map this. Then by measuring the size and mass of the star, astronomers can calculate the temperature of the core.

The spectrum of a star

What's inside a star? Even more incredibly, just by looking at starlight astronomers can discover what a star is made from. The light that we see is just one kind of radiation known as 'visible light'. Other kinds include X-rays, ultraviolet, microwaves, radio waves and infrared. Different stars give out varying amounts of these signals, known as their 'spectrum'. When measured, the spectrum appears as a series of bright and dark lines positioned at specific points or 'frequencies'. This is the blueprint of a star and provides a wealth of information about what is happening inside.

As elements are heated inside the star, they absorb and emit energy, creating a 'blip' in the star's spectrum. So the position and strength of these lines reveal what elements are inside the star. Stars are classified into 'spectral types' according to the shape of this spectrum. Find out what happens inside a star Looking back in time Although time travel isn't physically possible yet, we can see back in time just by looking out into space. Light from the Sun takes 8 minutes to reach Earth. So we are seeing what the Sun looked like 8 minutes ago. The faint Proxima Centauri, our nearest stellar neighbour, lags behind by 4.2 years. So, the further we look out into space, the further we are looking into the past. The distance that light travels in a year is called a 'light year'. The furthest that astronomers have ever seen are about 12 billion light years away, from a time just after the Big Bang, when the Universe was just a baby. Read about the evolution of the Universe