Click on the questions below to find answers to some cosmic conundrums.

And if you don’t find what you’re looking for, ask us a question using the form at the bottom of the list. What’s the point of knowing all this space stuff if you can’t share it!

Why are planets round?

Will our Sun ever die?

What’s a supernova?

How big is the Universe?

What’s a black hole?

Why isn’t Pluto a planet any more?

What are Saturn’s rings made of?

Why doesn’t the Moon fly off into space?

How was the Sun made?

Why do stars twinkle?

What’s a planetary nebula?

Why are stars different colours?

Do aliens exist?


Earth. The blue marble.


Q. Why are planets round?
A. Because of gravity. All of the stuff that makes a planet (the rocky crust, molten magma and solid core, plus its gaseous atmosphere) are all falling inward, making the planet round.

Q. Will our Sun ever die?
A. Yes it will. But not in a spectacular supernova explosion in a few years time! The sun is middle-aged at around 4.5 billion years old and will end its life when it’s nearing 10 billion years old. Because it’s rather small (compared to other stars), it’ll swell up and become a red giant before puffing away its outer gas layers to end its days as a white dwarf inside a planetary nebula.

Q. What’s a supernova?
A. A supernova happens when a giant star (at least eight times bigger than our Sun) explodes at the end of its life when it runs out of fuel (nuclear fusion). Supernovae (su-per-no-vee) are so bright that when they happen, they outshine the galaxy they’re in. They don’t often happen in our stellar neighbourhood, so to find out what’s really going on, we’ll have to study them a lot more.

Q. How big is the Universe?
A. Very big! We can’t make an accurate measurement because we don’t have a system of measurement big enough (the Sun is about 150,000,000km away, and that’s really close in cosmic terms). We think that it started impossibly small – a moment we call the ‘Big Bang’ – about 13.5 billion years ago, and has been expanding ever since.

Q. What’s a black hole?
A. If it’s a ‘supermassive black hole’, then it takes the form of an invisible plug hole at the centre of most giant galaxies, sucking in anything that comes near, including light! Stephen Hawking used the phrase ‘spaghettification’ to describe what would happen to an astronaut if they went too close and got sucked in. Another form of black hole can form after a ‘core collapse’ supernova, but this type is a lot smaller.

Q. Why isn’t Pluto a planet any more?
A. Because the International Astronomical Union decided that it should be re-classified as a dwarf planet. There are several reasons that make Pluto a bit different: a) it has an inclined (sloping) plane compared to the other eight planets; b) it’s really small; and c) it hasn’t cleared its orbit (it has an orbital companion, making it a binary planet, and four moons). Several other dwarf planets have also been discovered, so Pluto is part of a gang again.

Q. What are Saturn’s rings made of?
A. Ice and dust – mostly ice. The ring particles come in a range of sizes, from hail stones to icebergs! But most are snowball-sized. Astronomers think Saturn’s ring system may have formed after icy moons or comets came a bit too close to the giant planet and got ripped apart (gravity can pull thing together, but it can also pull things apart). This may have happened fairly recently (within a few million years) because they’re so clean and white.

Q. Why doesn’t the Moon fly off into space?
A. Because of the ‘centrapetal force’. The Moon is moving very fast around our planet, but gravity is always tugging it towards the centre of the Earth. Imagine tying a rock to a piece of string and whirling it around you; if you let go of the string the rock will fly off. The string anchors the rock to your centre of gravity, just like the Earth and Moon.

Q. How was the Sun made?
A. From the dust and gas left behind after a big star went bang. This is called a supernova remnant, like the Orion Nebula. The gas and dust form clumps, gravity then comes into play and pulls in lots more. Eventually, when there’s enough matter, the heat and pressure due to gravity becomes so intense that atoms start fusing together. This is called nuclear fusion, and a new star (just like our Sun) is born.

Q. Why do stars twinkle?
A. Imagine light coming from a distant star in our Galaxy. It travels in more or less a straight line until it hits Earth’s atmosphere, which is made up of lots of tiny particles (molecules and pollutants). Light from the far off stars bounces between the particles, which makes the light wobble a bit. Hence the twinkling. Stars nearer the horizon twinkle more because the light has to travel through a thicker band of atmosphere.

Q. What’s a planetary nebula?
A. When a Sun-like star (or smaller) dies, it leaves behind a tiny, but incredible dense, white dwarf star, surrounded by a diffuse cloud of gas (like the Helix Nebula). Planetary nebulae (neb-you-lee) are quite different to supernova remnants because they don’t contain the abundance of elements needed to form new stars and planets.

Q. Why are stars different colours?
A. If you look through a telescope or binoculars, you can see that some stars look white, yellow or orange, and some look blue. Different temperatures on the surface of stars (their ‘photospheres’) give rise to different colours. The blue ones are the hottest, and the reddish orange ones are the coolest. Weird right?

Q. Do aliens exist?
A. Of course they do. We’re aliens to life forms on other planets! Let’s think about some numbers. There may be as many as 4 billion stars in the Milky Way. Recent research suggests that almost all solar systems outside of our own (called extrasolar systems) have multiple planets orbiting their suns. Say there’s an average of two planets orbiting every star in the Milky Way, then there may be 8 billion planets in our galaxy alone. Some extrasolar systems are very young, meaning that life hasn’t had the chance to get going. But many are mature enough for life to have evolved.

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