What's the diameter of a neutron star?

We test the metal of our panelists with some questions of our own! Chris Smith puts them to University of York animal behaviour scientist Eleanor Drinkwater, exercise physiologist Dan Gordon from Anglia Ruskin University, Cambridge University physicist Fran Day, and Cambridge University mathematician and University Challenge icon Bobby Seagull...

Chris - Team 1 are gonna be Fran and Dan, and Team 2 are gonna be Bobby and Eleanor. And Round 1 is called Around The World. So please confer you two, Fran and Dan, Team 1. Which of the following megacities, referred to as cities of more than 10 million people, has the biggest population. Karachi, Moscow, or Istanbul?

Fran - I just don't know. Do you know?

Dan - Well. I would take a guess at Karachi.

Fran - Yeah that sounds plausible. I’ll just go with that.

Chris - You’re going Karachi?

Fran - Yes.

Chris - Yep. That is correct. Karachi’s got 18… Bobby sounds so surprised.

Bobby - I was sort of expecting Istanbul.

Chris - Ah, no. Karachi's got 18 million people. Istanbul's snapping at its heels though, 14.5 million. And Moscow - just over 13. London though, way down the list, nearer 9 million. Team 2, Eleanor and Bobby. According to the Guinness Book of World Records, which is the biggest - you have to listen to these carefully: the smallest country in the world, the smallest republic in the world, or the smallest colony in the world?

Bobby - So smallest country, smallest republic, smallest colony. Smallest country is the Vatican, it’s about 0.44 square kilometres. So the smallest republic and the smallest…

Eleanor - I’d say the smallest colony would probably be a colony of ants, probably. But.... if that's not the answer I’m going to be very disappointed.

Bobby - So which is the largest of the three. Smallest republic - what is a republic? I’m getting into quite political terms here. Are we a republic yet? No we're not. Oof, I don’t know.

Chris - Gonna have to hurry you.

Eleanor - I'm going to go for colony, because if it is the animal route - which I'm obviously predisposed to think it is - then you can talk about supercolonies, and they can stretch across huge distances.

Bobby - Let's go for that.

Chris -  Do you know, I would be tempted to give you a bonus for really clever thinking, OK. It's not actually right. I'm going to have to give you a...

Eleanor - Oh, I’m sorry, I’m sorry!

Chris - But I thought it was ingenious thinking so I’ll give you half a point for that, because... and actually some of those mega-colonies of ants though, Eleanor, are huge though, aren’t they. Which makes them bigger than, in fact, probably Vatican City which is tiny. That's just 0.44 square kilometres. And I'm pretty sure there are ant mega-colonies which span a bigger area than that.

Eleanor - Definitely.

Chris - But the answer to the quiz question is: the smallest republic is Nauru, which is in the Pacific Ocean, 21 square kilometres. Smallest colony? Anyone? Any ideas? Gibraltar. It's 5.8 square kilometres on that one. Okay so we've got, you're just about in the lead Fran and Dan with half a point in it. Next round, Round 2. Tech Yes or Tech No? Which of the following is a ‘tech no’ - in other words which one did we make up? OK, of these three: a bed that tucks you in at night; a frying pan that counts calories in your dinner; or a games console for your pet pooch. Which one is a ‘tech no’?

Dan - Ah, man. They all sound like they could be there, don’t they!

Fran - Yeah. I think that dog games console - if that doesn't exist then I'm gonna make it.

Dan - That’s got to be. Yeah.

Fran - A bed that tucks you in…

Dan - I’ve seen the bed that tucks you in.

Fran - Oh really?

Dan - I'm pretty sure I’ve seen that…

Fran - Isn’t it kind of dangerous and creepy?

Dan - I'm pretty sure I've seen something on Facebook that does this, Facebook shows everything…

Fran - So there’s these gravity blankets that are just really heavy?

Dan - Yeah, I'm sure I've seen something which is designed for people who are disabled that in essence... I don't know how it works. I seem to remember seeing something that, the sheets are able, the blankets are able to pull themselves across.

Fran - So how would the frying pan thing work though? Because the way I know to see how many calories are in something is to burn it. Then you wouldn't have any dinner, so that’s not ideal.

Dan - So have we talked ourselves into it? Shall we go with the frying pan?

Fran - Yeah, let’s go with the frying pan.

Chris - You’re going ‘tech no’ is the frying pan?

Fran - Yeah.

Dan - Oh, no.

Chris - No actually it is the bed. The bed is the ‘tech no’. The frying pan comes with its own app and a built-in scale so it can weigh and then quantify how many calories you're allegedly going to get. The games console, the idea of that is that your dog does a little puzzle and it gets a treat in return for its labours. So unfortunately no points for that one. So currently the scores stand at one point to Dan and Fran, half a point to Eleanor and Bobby. Your chance to take the lead here, Eleanor and Bobby. Same question, which of the following is a ‘tech no’: a mask for your mouth to stop you annoying everyone around you having loud phone conversations; a phone soap; or smart hair bobbles, they tell you when they're about to snap before your hairdo collapses.

Bobby - A mask for your mouth?

Eleanor - But also the hair bobbles thing - if you're wearing it in your hair, then if it's about to snap, you wouldn't be able to tell because it would be in your hair.

Bobby - So it sounds ludicrous, you think.

Eleanor - Yeah. You know, as someone who wears hair bobbles, this sounds not very useful. But then again just because it's not very useful doesn't mean that it doesn't exist.

Chris - What are you going to go for then?

Bobby - Is it the hair bobbles?

Eleanor - Yeah, but then again the mask for your mouth sounds bizarre as well.

Bobby - I want to invent that, that sounds... shall we go for the hair bobbles?

Eleanor -  OK, we can go for the hair bobbles.

Chris - You’re gonna go hair bobbles? Gonna go hair bobbles… it is… correct! So one and a half points to Eleanor and Bobby so far, so they’re half a point ahead. It could all be on this one. Here we go, OK, Round 3. This is potentially the decider. This round is called On The Other Foot. This is where we ask the team something about something that the other team probably know quite a bit about, and so you have to keep very quiet, other team; and then you can gloat when they do or don't get it right. So Fran and Dan, according to Wikipedia which of these is the smallest mammal: the bumblebee bat, which is a vulnerable bat species with a reddish-brown or gray coat and piglike snout; the Etruscan shrew, which looks a bit like a mouse but has an elongated nose and eats mainly insects; or the long-tailed planigale, which is a small marsupial, also looks a bit like a mouse, has a flattened head and it can get through very narrow cracks. What do you think?

Dan - They all sound small. All of the above.

Fran - Well the last one must be quite small to get through very narrow cracks, in the description.

Dan - But you said that the shrew was apparently mouse-sized as well.

Fran - Oh. Yeah. Maybe it's the bat, maybe it’s... because it’s called the bumblebee bat, and bumblebees are smaller than mice.

Dan - Shall we just do it?

Dan - OK, we think it’s the bat.

Chris - You’re going to go with the bat?

Fran - Yep. Oh, yay!

Dan - High five!

Chris - Yes. Well Eleanor and Bobby: what's the diameter of a neutron star? Is it 20 kilometres, 20,000 kilometres, or 20 million kilometres across?

Bobby - Neutron star, presumably it’s quite small.

Eleanor - I have no clue.

Bobby- So 20, 20,000 or what was the last one, 20 million?

Eleanor - Yeah.

Bobby - 20 kilometres sounds too... that sounds more black holey. How about 20,000?

Eleanor - I… yes. I agree.

Bobby - The middle option!

Chris - Going for 20,000?

Eleanor - No!

Chris - Did you know that Fran?

Fran - Yeah, it's 20 kilometres.

Chris - It is 20 kilometres, yeah.

Bobby - It really is that! Oh my god, I thought it was...

Chris - Neutron stars are actually very small, they’re actually roughly the size of a city in terms of diameter, and a teaspoonful full of one would weigh about 10 million tons. Our own sun in comparison: 1.4 million kilometres in diameter. So there you go, our big brain winners of the week are Dan and Fran. Give yourselves a big round of applause, very well done.