Stargazing Live: Professor Brian Cox answers your questions

The TV physicist explains why stars twinkle - and what our universe is expanding into?

What’s the difference between a star and a planet?


Stars have their own energy sources, and shine by nuclear fusion. Every star in the sky is a sun, much like our own, but thousands of light years away. In fact there are 350 billion galaxies in the observable universe each with, on average, 100 billion stars in them.

What is the Milky Way and where is it?

It’s our galaxy and we’re in it. We’re on one of its outer arms so if you looked back into our galaxy through a powerful telescope you would see the vastness of hundreds of billions of stars. What you’re seeing is the glowing light of billions of suns stretched out across the sky.

On a clear night how many stars can I see, unaided, in the night sky?

Most of us won’t be able to see more than a few hundred, maybe a thousand or so, and you’ve only got a chance of seeing more than that in ideal conditions with no light pollution.

On an exceptionally clear night, it is possible to see the Milky Way, and Andromeda, the nearest equivalent galaxy, which is “only” about two million light years away. Except that you can’t actually see separate spots of light in these galaxies, you just see this misty, lighter patch.

Why do stars twinkle?

It’s caused by distortion of the light in the Earth’s atmosphere. You’re looking at stars through a lot of air that’s moving in different directions because of thermals. If you were to leave Earth’s atmosphere and go and sit on the Moon, you would see that the stars actually shine steadily. This atmospheric distortion is why telescopes are usually sited on high mountains – so that they have less atmosphere to look through and therefore less distortion.

Why does the Sun appear larger when it’s on the horizon than when it’s overhead?

That’s an optical illusion. There’s no proven single explanation but we can say that it is to do with the psychology of human vision. It is a subject of continuing debate.

It’s sometimes possible to observe a huge circle around the Moon. What causes this phenomenon?

The Earth’s atmosphere again, scattering light.

What are shooting stars?

They’re bits of debris and are almost always very small… sometimes as small as a speck of dust. They heat up and glow because of the speed they’re travelling as they enter the Earth’s atmosphere but most of them miss us.

Some of them have names and come around at the same time every year. That’s because the Earth passes through fields of space debris.

There are lots of different types – you can have bits of comets, meteorites, some are bits of Mars. Then there are larger meteorites, bits of rock and occasionally huge rocks, like the one that landed in Siberia in 1908. Or the one that wiped out the dinosaurs 65 million years ago.

What is the life expectancy of Earth?

What’s more important is that the Sun will run out of fuel in about five billion years – and we’re about halfway through the Sun’s lifespan. Towards the end of its life, it will expand. It’s not known whether it will swallow the Earth, but it might do. What we can say is that Earth will be destroyed in the Sun’s dying phases.

Which one of our solar system’s planets or moons would you like to travel to and why?

The most likely place I could go in my lifetime is Mars, which would be fascinating. We might just find evidence of primitive life there, which would answer one of the great questions about the universe.

But I’m fascinated by the idea of a trip to Titan, the largest moon of Saturn, to see methane rain. Titan has rivers and lakes and rain and snow but all made of methane. The surface temperature is -178° Celsius.

Is it true that there are objects in the universe so distant that their light will never reach us?

What is certain is that there is more universe out there than we can possibly see. We know the universe is 13.7 billion years old and that in itself puts a limit on how far you can see because light takes time to travel. The Sun’s light takes eight minutes to reach us – put differently, that’s eight light minutes.

The next nearest star is Proxima Centauri, which is four light years away. The next nearest equivalent galaxy to ours is two million light years away and so on. The most distant galaxy we’ve seen is about 13.1 billion light years away, which is at the edge of the observable universe, if you choose to think about it in that way.

So can we look across the universe and see the Big Bang?

The closest we can get is with cosmic microwave background radiation that was produced 380,000 years after the Big Bang. Most of the precision measurements we have about the age of the universe, the way it has expanded and events that happened in the first few billionths of a second after the Big Bang are imprinted on that radiation. It’s one of the great measurements in the history of science.

Which is your favourite star?

Betelgeuse is fascinating. It’s one of the most obvious things in the sky, because it’s very red and very bright. It’s the top left-hand star in the constellation of Orion. You’ll see it to the south during Stargazing Live. It’s red because it’s
a sun at the end of its life. It’s enormous, and would fill most of our solar system if you put it where the Sun is.

We think it’s about to explode soon – well, at some point in the next 100,000 years. If it does, it will be a remarkable sight because it will shine like a second sun.

Why can you see stars in the sky one night and then the next night you can’t see any?

Other than for reasons of cloud cover? The constellations you can see change throughout the year because the Earth is in orbit around the Sun. Different bits of the Earth face different bits of the sky at different times of year. But you will always see the North Star in Britain.

If the universe started expanding with the Big Bang, what is it expanding into?

The current view, drawing on Einstein’s picture of the universe developed in 1915, is that space and time itself are stretching. Think of space being elastic; it warps and curves and actually that provides our best explanation of gravity.

What you’re seeing with gravity is the curvature of space caused by the presence of stars, planets and galaxies.

But the expansion of the universe is actually the expansion of space itself. In the past decade or two, we’ve found out that the rate of expansion is increasing and we think that it’s going to continue. Meaning that the rate of increase in the size of the universe is itself increasing. It’s an odd thought.

It’s a golden age for astronomy and cosmology. We’re finding out more every day because of the experiments that we’re doing. It’s an exciting time to be an astronomer.

This is an edited version of an article from the issue of Radio Times magazine that went on sale 10 January 2012.


Stargazing Live concludes tonight at 8pm on BBC2 and BBC HD