0:00:02 > 0:00:05There is one celestial object that dominates our skies.
0:00:06 > 0:00:08A star that shines so brightly
0:00:08 > 0:00:12it drowns out the light of all other stars in the universe.
0:00:14 > 0:00:17That star is of course the sun.
0:00:22 > 0:00:25We're just past the summer solstice and here in Britain,
0:00:25 > 0:00:28the sun is above the horizon for 16 hours every day.
0:00:28 > 0:00:31So it's a great time to get outside and see the sun
0:00:31 > 0:00:34and ask a few questions - where does it come from?
0:00:34 > 0:00:36How does it fit into the universe?
0:00:36 > 0:00:38And what, if anything, makes it unique?
0:00:38 > 0:00:40Welcome to The Sky At Night.
0:01:06 > 0:01:08Welcome to the Bayfordbury Observatory,
0:01:08 > 0:01:10part of the University Of Hertfordshire,
0:01:10 > 0:01:14a place where astronomers are looking at stars similar to our sun.
0:01:14 > 0:01:17Tonight, we're journeying out into the Milky Way in search
0:01:17 > 0:01:20of a new perspective on our brightest star.
0:01:20 > 0:01:23We'll be exploring the previous lives of the sun,
0:01:23 > 0:01:25looking at how the stars that came before it
0:01:25 > 0:01:28form much of the material that now makes it shine.
0:01:29 > 0:01:32And could life exist here on Earth
0:01:32 > 0:01:37only because the sun is an unusually quiet star?
0:01:37 > 0:01:42Also, what is springtime on Mars like? Or midsummer on Venus?
0:01:42 > 0:01:44Lucie Green tours the solar system
0:01:44 > 0:01:47to see what seasons are like on other worlds.
0:01:47 > 0:01:53At almost 90 degrees, the planet is effectively orbiting on its side.
0:01:53 > 0:01:55Plus stargazing in the daytime,
0:01:55 > 0:02:00the treasures of the night sky that you can see even while the sun is up.
0:02:00 > 0:02:03But first to the sun itself.
0:02:03 > 0:02:05Our sun's what's called a yellow dwarf,
0:02:05 > 0:02:08pretty average as stars go in terms of size and mass,
0:02:08 > 0:02:11and neither exceptionally hot, nor spectacularly cool.
0:02:11 > 0:02:15And it's now settled in to a fairly comfortable middle age.
0:02:18 > 0:02:22It formed more than 4.5 billion years ago
0:02:22 > 0:02:25from a large cloud of gas and dust.
0:02:25 > 0:02:27Most of the cloud became the sun
0:02:27 > 0:02:29with the remnants evolving into the planets
0:02:29 > 0:02:32and the rest of the solar system.
0:02:32 > 0:02:35The sun is enormous.
0:02:35 > 0:02:38You can fit 1.3 million Earths into one solar volume.
0:02:38 > 0:02:41It shines because of the massive temperature and pressures
0:02:41 > 0:02:44at its core. This drives a continuous nuclear reaction
0:02:44 > 0:02:48converting hydrogen into helium and releasing huge amounts of energy.
0:02:50 > 0:02:55This nuclear fusion creates more than 600 million tonnes of helium
0:02:55 > 0:02:58every second, along with plenty of light.
0:02:58 > 0:03:03Light that our eyes soak up as sunlight.
0:03:03 > 0:03:04A little later,
0:03:04 > 0:03:08we'll be using a telescope here to take a close-up look at the sun.
0:03:10 > 0:03:14But first, the earth's orbit isn't perfectly circular.
0:03:14 > 0:03:18This means that the distance between the sun varies throughout the year.
0:03:18 > 0:03:20So you may be surprised to know that now,
0:03:20 > 0:03:22at the height of British summertime,
0:03:22 > 0:03:25we're about as far away from the sun as we'll get this year.
0:03:25 > 0:03:28In fact, that change in distance isn't nearly enough to account
0:03:28 > 0:03:30for the seasons that we experience.
0:03:30 > 0:03:32So what does cause summer and winter?
0:03:32 > 0:03:36And what would seasons be like elsewhere in the solar system?
0:03:36 > 0:03:37Lucie Green investigates.
0:03:40 > 0:03:44To understand how seasons work right across the solar system,
0:03:44 > 0:03:49a good place to start is with our own planet.
0:03:49 > 0:03:52The earth's seasons are driven by our relationship with the sun
0:03:52 > 0:03:55and the way the earth hangs in space.
0:03:55 > 0:04:01But the most important aspect is that the earth is tilted.
0:04:01 > 0:04:05As the earth journeys around the sun on its yearly orbit,
0:04:05 > 0:04:08it spins on an axis that runs from pole to pole,
0:04:08 > 0:04:11but the whole planet is tilted over.
0:04:11 > 0:04:13It's a phenomenon known as axial tilt.
0:04:16 > 0:04:22The earth's axial tilt is 23.5 degrees from the vertical.
0:04:22 > 0:04:24And, for me, that doesn't feel like very much,
0:04:24 > 0:04:27but actually it has some dramatic consequences.
0:04:29 > 0:04:32'Consequences that drive change across the planet.
0:04:32 > 0:04:36'To see why, I'm going to recreate the solar system right here
0:04:36 > 0:04:39'at the Royal Observatory in Greenwich.'
0:04:39 > 0:04:42This is the earth, of course, and the lamp here represents the sun.
0:04:42 > 0:04:44Now because the earth is spherical,
0:04:44 > 0:04:46that means that the sunlight falls
0:04:46 > 0:04:50with different intensities on different parts of the globe.
0:04:50 > 0:04:52Towards the top, the sunlight comes in at an angle,
0:04:52 > 0:04:54so it's more spread out.
0:04:54 > 0:04:58Towards the middle, the sunlight's falling directly on the planet,
0:04:58 > 0:05:03it's more intense and those regions get hotter. But the earth is tilted.
0:05:03 > 0:05:06So that brings the UK and the northern hemisphere
0:05:06 > 0:05:09into a position where it's pointed towards the sun
0:05:09 > 0:05:12and it's receiving more sunlight than in the south.
0:05:12 > 0:05:15So, for us, we have northern hemisphere summer
0:05:15 > 0:05:20and for the opposite part of the world, they have winter.
0:05:20 > 0:05:22Now the earth's tilt doesn't change,
0:05:22 > 0:05:24well at least on the timescales that we're interested in.
0:05:24 > 0:05:27But our position in space does.
0:05:27 > 0:05:29As we go on our journey around the sun,
0:05:29 > 0:05:32we reach a point where neither hemisphere
0:05:32 > 0:05:34is looking directly at our local star.
0:05:34 > 0:05:37Now sunlight is falling over the equator
0:05:37 > 0:05:41and we have autumn and spring. This is the equinox.
0:05:41 > 0:05:43Fast forward three months and we come around
0:05:43 > 0:05:46and we find that we reverse our initial positions
0:05:46 > 0:05:49and now the southern hemisphere is pointed towards the sun
0:05:49 > 0:05:51and they have summer.
0:05:51 > 0:05:53And so it goes on, orbit after orbit,
0:05:53 > 0:05:55running through the seasons.
0:05:57 > 0:06:00Our tilt and changing seasons have an important effect -
0:06:00 > 0:06:02they regulate our temperature,
0:06:02 > 0:06:06stopping any part from getting too hot or too cold.
0:06:07 > 0:06:12And we've learned that the tilts and the seasons of the other planets
0:06:12 > 0:06:15in the solar system can be very different to our own.
0:06:15 > 0:06:20And that leads to some interesting seasons on other worlds.
0:06:23 > 0:06:25Venus, our next-door neighbour
0:06:25 > 0:06:29and second planet from the sun has virtually no tilt.
0:06:29 > 0:06:33Although, curiously, it rotates backwards.
0:06:35 > 0:06:39The absence of an axial tilt means that there's always more sunlight
0:06:39 > 0:06:44falling on the equator on Venus than there is up towards the poles.
0:06:44 > 0:06:48And that means there is no seasonal variation on this planet.
0:06:49 > 0:06:53Mars is about 1.5 times further from the sun than we are,
0:06:53 > 0:06:55which is part of the reason
0:06:55 > 0:06:59that temperatures on the Red Planet rarely get above freezing.
0:06:59 > 0:07:01But with a tilt of 25 degrees,
0:07:01 > 0:07:06its seasons should be similar to ours, and indeed we do see changes.
0:07:06 > 0:07:09The polar caps shrink and grow.
0:07:09 > 0:07:14Clouds of carbon dioxide form at the polls and winds pick up,
0:07:14 > 0:07:17sometimes creating huge dust storms visible from Earth.
0:07:19 > 0:07:23But there's another factor that affects Mars' seasons -
0:07:23 > 0:07:26its orbit around the sun isn't circular, it's an ellipse.
0:07:26 > 0:07:30And that means that there's a 43 million kilometre difference
0:07:30 > 0:07:34between its closest point to the sun and its most distant.
0:07:37 > 0:07:42This makes Mars' northern summer longer than the southern summer.
0:07:42 > 0:07:46But there is one planet that stands out from all the rest.
0:07:48 > 0:07:52Uranus has the most unusual axial tilt in the whole solar system.
0:07:52 > 0:07:58At almost 90 degrees, the planet is effectively orbiting on its side.
0:07:58 > 0:07:59And that means that when
0:07:59 > 0:08:01it's summer in the northern hemisphere,
0:08:01 > 0:08:03it's constantly bathed in sunlight,
0:08:03 > 0:08:07whilst at the other pole, it's plunged into a frigid winter
0:08:07 > 0:08:10when the sun doesn't rise for decades.
0:08:10 > 0:08:13But this planet has an extraordinary variability
0:08:13 > 0:08:16and as it moves on in its orbit around the sun
0:08:16 > 0:08:18it reaches equinox, and, at this point,
0:08:18 > 0:08:21there is 8.5 hours of sunlight
0:08:21 > 0:08:25and 8.5 hours of darkness all over the planet.
0:08:26 > 0:08:30It takes Uranus 84 years to orbit the sun,
0:08:30 > 0:08:34which means each of its seasons last 21 years.
0:08:34 > 0:08:38To find out what effect its bizarre tilt has on the planet,
0:08:38 > 0:08:42I'm meeting Uranus expert Patrick Irwin.
0:08:42 > 0:08:46Now, Uranus is a very different planet to our own world, isn't it?
0:08:46 > 0:08:48And the most detailed view we've had a Uranus
0:08:48 > 0:08:50was with the Voyager 2 flyby in 1986.
0:08:50 > 0:08:52Now, what were the seasons that were playing out
0:08:52 > 0:08:54when Voyager 2 got there?
0:08:54 > 0:08:57At this moment in time, the south pole was pointed
0:08:57 > 0:09:00almost entirely towards the sun, so it we're southern summer solstice.
0:09:00 > 0:09:02And this is what I really want to know -
0:09:02 > 0:09:05what are consequences on a planet's seasons
0:09:05 > 0:09:08if the axial tilt is at 90 degrees to its orbit around the sun?
0:09:08 > 0:09:12The poles actually receive 50% more sunlight on average
0:09:12 > 0:09:15over the years than the equator. So the equator gets kind of cold,
0:09:15 > 0:09:18whereas at the pole you'd expect it to get very, very hot
0:09:18 > 0:09:20in the summer and very, very cold in the winter.
0:09:20 > 0:09:23But, in fact, what we found was that if you look at the temperature
0:09:23 > 0:09:26all the way across the planet, the temperature at the south pole
0:09:26 > 0:09:28was almost exactly the same as at the north pole.
0:09:28 > 0:09:30Temperatures everywhere were the same, all over the planet,
0:09:30 > 0:09:33- and that was a big surprise. - So the pole that's in sunlight
0:09:33 > 0:09:36was the same temperature as the pole that was in total darkness.
0:09:36 > 0:09:39Yeah. A good analogy is kind of like a black ball going round the sun
0:09:39 > 0:09:42and the sun-lit side's going to get very hot,
0:09:42 > 0:09:45whereas the winter side's going to radiate heat away
0:09:45 > 0:09:46and basically get very cold.
0:09:46 > 0:09:48If you took a ball of metal and did the same thing,
0:09:48 > 0:09:51then the heat would arrive on the sun-lit side
0:09:51 > 0:09:54and would be efficiently conducted through to the dark side.
0:09:54 > 0:09:58So the fact that Uranus is this gas giant is absolutely fundamental
0:09:58 > 0:10:01- to this very uniform temperature that it has.- That's right.
0:10:01 > 0:10:03I mean, the atmosphere is free to move,
0:10:03 > 0:10:05not just at the surface, like it does on the earth
0:10:05 > 0:10:08but it's free to move within the entirety of the planet.
0:10:08 > 0:10:10Now, it's been almost 30 years since Voyager 2 flew past
0:10:10 > 0:10:13and saw this very plain planet.
0:10:13 > 0:10:14What's been happening since then?
0:10:14 > 0:10:17What we found is Uranus is actually a lot more interesting
0:10:17 > 0:10:18than we thought it was.
0:10:18 > 0:10:22This was measured in 2004 and the equinox was in 2007.
0:10:22 > 0:10:26There's this very bright band of cloud around the southern pole
0:10:26 > 0:10:29and then with that there's these small, discrete clouds,
0:10:29 > 0:10:32which we believe are clouds of methane.
0:10:32 > 0:10:34And it seems to be that as the sun comes around
0:10:34 > 0:10:38- to the equinox position... - Which is what we have here. - ..which is what we've got here,
0:10:38 > 0:10:42the north is getting more and more sunshine and the south is getting less and less,
0:10:42 > 0:10:44and that makes the atmosphere unstable.
0:10:44 > 0:10:48So, I suppose this perhaps sluggish character of Uranus
0:10:48 > 0:10:51and the fact that it took quite a while to see these changes
0:10:51 > 0:10:55is an artefact of the 84-year length of Uranus' orbit.
0:10:55 > 0:10:57And it just means that you have to study Uranus
0:10:57 > 0:11:01- for much, much longer.- That's right, yes.- Well, Pat, thank you very much
0:11:01 > 0:11:04- and thanks for bringing these fantastic images.- My pleasure.
0:11:14 > 0:11:17Here at Bayfordbury, they have 11 telescopes
0:11:17 > 0:11:19and they're run by Mark Gallaway.
0:11:21 > 0:11:25He's going to train one onto the sun to reveal a familiar feature
0:11:25 > 0:11:30that we're beginning to see on other stars, too - sunspots.
0:11:31 > 0:11:33How's it looking, Mark?
0:11:33 > 0:11:37Well, we've got a little bit of cloud, but it's looking pretty good.
0:11:37 > 0:11:39Yes. So quite a bit of turbulence?
0:11:39 > 0:11:41Yeah, that's just the atmosphere boiling.
0:11:41 > 0:11:44Like you see in a hot road on a summer's day...
0:11:44 > 0:11:46- And twinkling stars, as well. - And twinkling stars,
0:11:46 > 0:11:48exactly the same effect.
0:11:48 > 0:11:51So this is an image in a particular part of the spectrum
0:11:51 > 0:11:52called hydrogen alpha.
0:11:52 > 0:11:55Here, we can see a pair of sunspots, sunspots always come in pairs,
0:11:55 > 0:11:59and they appear as dots. But an interesting feature here,
0:11:59 > 0:12:02which often associates with sunspots, this is a plage.
0:12:02 > 0:12:04This is slightly above the sunspot and it's a lot hotter.
0:12:04 > 0:12:06It appears bright in H-alpha.
0:12:06 > 0:12:09So we can see sunspots on our local star.
0:12:09 > 0:12:12But I assumed it's not possible to see sunspots on other stars,
0:12:12 > 0:12:15because they're too small to get that sort of resolution?
0:12:15 > 0:12:17Well, most stars, no.
0:12:17 > 0:12:19But particularly here at Bayfordbury,
0:12:19 > 0:12:21where we look at very, very small stars called M dwarfs.
0:12:21 > 0:12:25Now what he did on those is we do something called photometry.
0:12:25 > 0:12:27So we look at how the light varies
0:12:27 > 0:12:30and when the sunspot comes into view, the light will dim.
0:12:30 > 0:12:34A darker patch on a bright surface, so you'll get less light.
0:12:34 > 0:12:36Indeed. But, unfortunately,
0:12:36 > 0:12:39that's exactly the same kind of thing which we see on an exoplanet transit.
0:12:39 > 0:12:42- Of course, yes.- We have a technique to distinguish the two.
0:12:42 > 0:12:45What we see is the sunspot appear, as here,
0:12:45 > 0:12:49and then disappear as the star rotates.
0:12:49 > 0:12:51What we see is we see a dip in light.
0:12:51 > 0:12:55- Yes, but that signature looks very much like an exoplanet...- Exactly.
0:12:55 > 0:12:59However, if we looked in the hydrogen alpha band,
0:12:59 > 0:13:00we've got the same animation,
0:13:00 > 0:13:03because we've got this bright layer on top of it where the plage is,
0:13:03 > 0:13:06we actually see, instead of a dimming, a brightening.
0:13:06 > 0:13:08So even though the sunspots are dark,
0:13:08 > 0:13:10the plage is light enough to make the whole thing come up.
0:13:10 > 0:13:11Yeah, that's exactly it.
0:13:11 > 0:13:14And are we finding many spotty stars? Are most stars spotty?
0:13:14 > 0:13:16Well, we don't really know.
0:13:16 > 0:13:19This is one of the reasons why we're doing this long-term monitoring.
0:13:19 > 0:13:23We want to select candidates for exoplanets, those that aren't spotty.
0:13:23 > 0:13:25But they might be going through sunspot cycles like the sun is,
0:13:25 > 0:13:28which means we're going to have to look for these things
0:13:28 > 0:13:30for a very, very long time before we get any real results.
0:13:30 > 0:13:32- So, watch this space.- Indeed.
0:13:36 > 0:13:40Coming up, I'll be asking where does our sun come from?
0:13:40 > 0:13:43But first here's Pete Lawrence with his guide
0:13:43 > 0:13:45to how you can stargaze in the daytime.
0:13:48 > 0:13:51The 22nd June was International Sun-Day
0:13:51 > 0:13:55when astronomical societies in over 20 different countries
0:13:55 > 0:13:59took part in a global solar-observing event.
0:13:59 > 0:14:02So I've come to Regent's Park in London to join
0:14:02 > 0:14:04a troop of fellow sungazers.
0:14:04 > 0:14:07It might seem a bit strange, the concept of daytime astronomy,
0:14:07 > 0:14:10but you certainly shouldn't write it off.
0:14:10 > 0:14:11There's plenty to see up there.
0:14:11 > 0:14:14For example, the most obvious thing being the sun, but of course you've
0:14:14 > 0:14:17also got things like the bright planets and even the distant stars.
0:14:19 > 0:14:22When observing during the day, you need to look after your eyes
0:14:22 > 0:14:26and never look directly at the sun without protective equipment.
0:14:27 > 0:14:30One of the objects which is synonymous with night-time astronomy
0:14:30 > 0:14:34is the moon, but the moon can actually be seen during the day
0:14:34 > 0:14:35and for much of the month.
0:14:37 > 0:14:40During the day, the moon appears as a lovely blend of soft blues
0:14:40 > 0:14:44and whites, making it look eerily transparent.
0:14:44 > 0:14:48And it's easy to see lots of surface detail with great features on view
0:14:48 > 0:14:49through a telescope.
0:14:51 > 0:14:54At first glance, you might think that's it.
0:14:54 > 0:14:57But there is another object which can be seen with just your eyes
0:14:57 > 0:14:59when it's in the right position away from the sun
0:14:59 > 0:15:01and that is the planet Venus.
0:15:03 > 0:15:05Venus is incredibly bright,
0:15:05 > 0:15:09allowing it to cut through the blue haze of the daytime sky.
0:15:09 > 0:15:11And, depending where it is in its orbit,
0:15:11 > 0:15:13it can appear at different phases,
0:15:13 > 0:15:16from a full disc through to a thin crescent.
0:15:17 > 0:15:20And with a telescope, other worlds can be seen, too.
0:15:20 > 0:15:24Sadly, Jupiter is a bit too close to the sun for comfort at the moment,
0:15:24 > 0:15:25but, when it's further away,
0:15:25 > 0:15:28it is possible to see it, even in the daytime sky.
0:15:28 > 0:15:32Amazingly, it's also possible to see surface features, as well.
0:15:32 > 0:15:35Features such as its candy-striped weather patterns,
0:15:35 > 0:15:40the Great Red Spot or even shadows cast by four of its largest moons.
0:15:42 > 0:15:44And what about further afield?
0:15:44 > 0:15:47If you know where to look and you've got a telescope,
0:15:47 > 0:15:50it is possible to see even bright stars during the day.
0:15:50 > 0:15:53At this time of year, I'd recommend looking for bright stars
0:15:53 > 0:15:56such as Arcturus, Sirius and Regulus.
0:15:56 > 0:16:00'And, now, I'm going to have a go at finding Regulus.
0:16:00 > 0:16:04'The trick is to start by aligning the telescope with the sun.'
0:16:04 > 0:16:07I've set the setting circles on the telescope mount
0:16:07 > 0:16:09to match the coordinates of the sun.
0:16:09 > 0:16:13And what I have to do now is basically turn the telescope
0:16:13 > 0:16:16so that those setting circles read the coordinates
0:16:16 > 0:16:19of Regulus in the sky.
0:16:22 > 0:16:25That should just about do it.
0:16:25 > 0:16:30So, if I'm lucky, then what I should see is the bright dot of Regulus
0:16:30 > 0:16:32in the field of view of the telescope.
0:16:34 > 0:16:36'But the object that offers the most staggering views
0:16:36 > 0:16:38'in the daytime has to be the sun.'
0:16:41 > 0:16:44This is a little bit different from the other telescopes out there -
0:16:44 > 0:16:47- a pair of binoculars. - Yes, they are 20x80s.
0:16:47 > 0:16:49Right, OK, so big binoculars
0:16:49 > 0:16:51and, of course, they're fitted with solar safety film.
0:16:51 > 0:16:53- These are home-made things.- OK.
0:16:53 > 0:16:56It's a little bit of do-it-yourself, which is quite fun.
0:16:56 > 0:16:59So what sort of things can you see through these?
0:16:59 > 0:17:01Sunspots, had the neighbours come to look at it,
0:17:01 > 0:17:03even track the rotation of the sun with it.
0:17:03 > 0:17:05Well, the filters are very, very easy to make,
0:17:05 > 0:17:09and if you want to make one yourself then we have actually got some
0:17:09 > 0:17:10web clips up on our website
0:17:10 > 0:17:13where you can go and find out how to do it.
0:17:13 > 0:17:17That is absolutely brilliant, wow! So how was that taken?
0:17:17 > 0:17:20It's four-inch reflector, DSLR straight in at prime focus.
0:17:20 > 0:17:24- So just a normal stills camera. - With a webcam, I achieved that.
0:17:24 > 0:17:28So the sunspot there shows the dark portion in the centre
0:17:28 > 0:17:31which is called the umbra. And then around the outside of that
0:17:31 > 0:17:33you've you got what's called the penumbra.
0:17:33 > 0:17:35So is that your first webcam photo of the sun?
0:17:35 > 0:17:38- First webcam... - That's really impressive.
0:17:38 > 0:17:42Yeah. I suffered for that.
0:17:42 > 0:17:45Now there's another treat in store in the sky this month
0:17:45 > 0:17:47and one you don't need a telescope to see
0:17:47 > 0:17:50and that's the focus of this month's Star Guide.
0:17:50 > 0:17:52During July, you might catch
0:17:52 > 0:17:55a display of noctilucent, or night-shining, clouds.
0:17:56 > 0:18:00Located at the edge of space, in a narrow layer 50 miles up,
0:18:00 > 0:18:04they form when water vapour freezes around tiny particles,
0:18:04 > 0:18:08such as those created where meteor vaporises in the atmosphere.
0:18:10 > 0:18:12From their viewpoint, the sun is still above the horizon,
0:18:12 > 0:18:14which is why they appear to shine.
0:18:16 > 0:18:19They may typically be seen a couple of hours after sunset,
0:18:19 > 0:18:21low above the north-west horizon.
0:18:21 > 0:18:25Or a couple of hours before sunrise, low in the north-east.
0:18:25 > 0:18:28A bright display may remain visible all night long.
0:18:32 > 0:18:34Next, it's a remarkable thought,
0:18:34 > 0:18:39but the material that makes up our sun has had previous lives.
0:18:39 > 0:18:42Much of the material in the sun was formed
0:18:42 > 0:18:46in other stars that then exploded, ceding clouds of gas
0:18:46 > 0:18:50that in turn became the nurseries for new stars.
0:18:50 > 0:18:53To find out how the cycle of star formation and death
0:18:53 > 0:18:56creates the elements that make us up,
0:18:56 > 0:19:00I'm talking to galactic archaeologist Sean Ryan.
0:19:00 > 0:19:03You know, it's hard to remember that we live in a very strange place
0:19:03 > 0:19:06in the universe, but everything around us is made of carbon
0:19:06 > 0:19:09and oxygen and nitrogen and these heavy elements,
0:19:09 > 0:19:11and even the sun isn't just pristine hydrogen.
0:19:11 > 0:19:14So where do all these elements come from?
0:19:14 > 0:19:17Stars throughout the age of the galaxy have had a key role
0:19:17 > 0:19:21in the formation of the elements that we see around us now.
0:19:21 > 0:19:25If you go all the way back to the Big Bang, you had hydrogen, helium
0:19:25 > 0:19:27and the tiniest amount of lithium being produced.
0:19:28 > 0:19:30Nothing else of any consequence.
0:19:30 > 0:19:33And so at successive generations of stars,
0:19:33 > 0:19:36where those elements have been produced,
0:19:36 > 0:19:41that production mechanism is one involving nuclear reaction.
0:19:41 > 0:19:44So the star starting off with a very simple composition
0:19:44 > 0:19:47of hydrogen and helium can work its way up to
0:19:47 > 0:19:50almost the full suite of chemical elements.
0:19:50 > 0:19:52And all of this is happening at the centre of the star,
0:19:52 > 0:19:56so we need to get them out. And we've got a picture of how that happens.
0:19:56 > 0:19:58This is the Crab Nebula.
0:19:58 > 0:20:01It's quite a challenge for that material to get off,
0:20:01 > 0:20:03so there's a range of masses of stars,
0:20:03 > 0:20:06somewhere between perhaps 10 times the mass of the sun
0:20:06 > 0:20:09and perhaps 25 to 30 times the mass of the sun,
0:20:09 > 0:20:13in which stars can produce these heavy elements
0:20:13 > 0:20:14during their lifetime,
0:20:14 > 0:20:18eject in a supernova explosion and then they can be folded into the gas
0:20:18 > 0:20:21from which subsequent generations of stars form.
0:20:21 > 0:20:24So what can we say about the sun's predecessors?
0:20:24 > 0:20:28Can we write down the sequence of stars that have led us to the sun?
0:20:28 > 0:20:30Astronomers sometimes think of the sun
0:20:30 > 0:20:32as being a third-generation of star.
0:20:32 > 0:20:36They don't mean from that they were just one, two, three stars,
0:20:36 > 0:20:39but the very first stars came out of the Big Bang,
0:20:39 > 0:20:42if you like the first generation, made of almost pristine hydrogen,
0:20:42 > 0:20:45helium and a little bit of the lithium.
0:20:45 > 0:20:47Subsequent generations of stars,
0:20:47 > 0:20:50which would still be amongst the oldest in our galaxy,
0:20:50 > 0:20:54had a slightly higher content of heavier elements.
0:20:54 > 0:20:56Which will have come from those first stars.
0:20:56 > 0:21:01That's right, and then, ultimately, once you get up to stars formed,
0:21:01 > 0:21:04perhaps over the last five to seven, five to eight billion years,
0:21:04 > 0:21:08then indeed you find the kind of composition which we see in the sun.
0:21:08 > 0:21:10And so we've got this idea that, by looking at the elements
0:21:10 > 0:21:13within a star, you can say something about its history.
0:21:13 > 0:21:15There's one nice example of this.
0:21:15 > 0:21:20This is a star called HD 162826, which I'm sure you're familiar with!
0:21:20 > 0:21:23This was in the news because it was announced
0:21:23 > 0:21:26as a likely solar sibling, a twin of our sun.
0:21:26 > 0:21:29So, what does that mean, and why is it exciting?
0:21:29 > 0:21:32We have a whole range of elements which we can observe in stars,
0:21:32 > 0:21:36so we can measure the composition of carbon, of nitrogen, of oxygen,
0:21:36 > 0:21:39and if you do a match between the measurements
0:21:39 > 0:21:44of the composition of this particular star, HD 162...
0:21:44 > 0:21:46..826.
0:21:46 > 0:21:49..and the measurements we can make in the sun,
0:21:49 > 0:21:52you find they're an incredibly close match.
0:21:52 > 0:21:57But, more than that, this particular star also has the same motion
0:21:57 > 0:22:00through the galaxy as the sun does,
0:22:00 > 0:22:04which suggests that they perhaps formed out of the same gas
0:22:04 > 0:22:07cloud, therefore giving rise to both the same composition
0:22:07 > 0:22:09and the same motion through the galaxy.
0:22:09 > 0:22:11It's rather fun and there must be more of them out there.
0:22:11 > 0:22:14- Sean, thanks a lot.- Thank you.
0:22:20 > 0:22:23There's been a lot happening in the solar system this month,
0:22:23 > 0:22:25so it's time for some astro-news.
0:22:25 > 0:22:28I feel really privileged to have seen one of the transits of Venus,
0:22:28 > 0:22:31but this month there was a transit of Mercury, but it wasn't seen
0:22:31 > 0:22:35from Earth, it was seen from Mars, picked up by the Curiosity Rover.
0:22:35 > 0:22:38We have some images, but don't be underwhelmed.
0:22:38 > 0:22:39This is the first one.
0:22:39 > 0:22:42Now, the two rather large sunspots are a distraction.
0:22:42 > 0:22:46It's X marks the spot. That is Mercury crossing the sun.
0:22:46 > 0:22:49And just to show you it's a transit, here's another image.
0:22:49 > 0:22:52What's exciting about this is that this is the first time
0:22:52 > 0:22:55we've seen the transit of another planet from another planet.
0:22:55 > 0:22:56Pretty unique.
0:22:56 > 0:22:59The mission I'm most excited about at the minute is Rosetta,
0:22:59 > 0:23:02which is on its way to Comet Churyumov-Gerasimenko,
0:23:02 > 0:23:04out in the outer solar system.
0:23:04 > 0:23:08It meets the comet in August and then will fly into the inner solar system,
0:23:08 > 0:23:11landing a probe on the comet's surface in November.
0:23:11 > 0:23:14This first image is from April 30th from Rosetta,
0:23:14 > 0:23:17and there's the comet, you can see it's this fuzzy blob.
0:23:17 > 0:23:19It's already showing signs of activity.
0:23:19 > 0:23:22But if we go to the next image, which was taken on June 4th,
0:23:22 > 0:23:25you can see that that activity has ceased,
0:23:25 > 0:23:28the comet has gone back to a nice, quiet state.
0:23:28 > 0:23:31- Yeah, the tail's gone!- Exactly, it doesn't look very cometary at all.
0:23:31 > 0:23:34That's good news for Rosetta, which wants it to be nice and quiet
0:23:34 > 0:23:37when it arrives so we can watch the comet waking up,
0:23:37 > 0:23:38but it's also a bit confusing -
0:23:38 > 0:23:41we're not sure why the comet would have shown activity
0:23:41 > 0:23:43and then quietened down again.
0:23:43 > 0:23:46It's these questions that Rosetta will help us answer.
0:23:49 > 0:23:51But now back to the sun.
0:23:51 > 0:23:53It's often called an average star,
0:23:53 > 0:23:57but it's the only star that we know of that supports life.
0:23:57 > 0:23:59So what makes it so special?
0:23:59 > 0:24:01And what does that mean for the search for life
0:24:01 > 0:24:03elsewhere in the universe?
0:24:03 > 0:24:07I am talking to public astronomer Marek Kukula.
0:24:07 > 0:24:11One of the interesting things about this is where life does exist,
0:24:11 > 0:24:14and we generally sort of look for life around other stars
0:24:14 > 0:24:15in the Goldilocks zone.
0:24:15 > 0:24:17We've got a diagram of that here.
0:24:17 > 0:24:19And this is where we find liquid water?
0:24:19 > 0:24:24That's right, so this is the range of distances around the sun
0:24:24 > 0:24:27where the temperature is just right for water to be liquid,
0:24:27 > 0:24:29so not so hot that it boils away as a vapour,
0:24:29 > 0:24:31and not so cold that it freezes into ice.
0:24:31 > 0:24:34And you can see that the earth is slap bang in the middle of it.
0:24:34 > 0:24:37Of course, that's not the only thing that makes a planet habitable,
0:24:37 > 0:24:40because Mars and Venus don't have liquid water on them now,
0:24:40 > 0:24:42so there are other things going on,
0:24:42 > 0:24:45but certainly this seems to be the most sensible place to look for life
0:24:45 > 0:24:48because we know that our sun, although it's a fairly stable star,
0:24:48 > 0:24:52has been increasing in brightness by about 10% every billion years.
0:24:52 > 0:24:55But also, the sun has other forms of activity,
0:24:55 > 0:24:57and it spits out great clouds of material.
0:24:57 > 0:25:00Speaking of stability, our Sun can be quite active,
0:25:00 > 0:25:03and I think this is a dramatic picture showing just that.
0:25:03 > 0:25:06Yes, absolutely, and this sort of thing is going on on the sun
0:25:06 > 0:25:10all the time, this enormous prominence arching off the surface,
0:25:10 > 0:25:13explosive flares going on on the surface.
0:25:13 > 0:25:16But some other stars that we know have planets going around them,
0:25:16 > 0:25:20we see activity which is a thousand or even a million times
0:25:20 > 0:25:21more violent than this.
0:25:21 > 0:25:23We call them super flares,
0:25:23 > 0:25:25and if one of those happened in our solar system,
0:25:25 > 0:25:28certainly it would be rather unpleasant for our civilisation.
0:25:28 > 0:25:32So the Goldilocks zone sort of lies between Venus and Mars
0:25:32 > 0:25:35here around our sun, but how about one of the stars?
0:25:35 > 0:25:38Is it likely to lie in the same place?
0:25:38 > 0:25:42Well, depending on how bright that star is, the Goldilocks zone
0:25:42 > 0:25:46will be either nearer or further away from the star itself,
0:25:46 > 0:25:48so if you want to have liquid water
0:25:48 > 0:25:50you kind of have to be in that regime.
0:25:50 > 0:25:53But then, of course, if the star is different in other ways,
0:25:53 > 0:25:55if it does have these super flares
0:25:55 > 0:25:58or if it has various forms of activity that are dangerous,
0:25:58 > 0:26:01then perhaps being in the habitable zone might be great for water
0:26:01 > 0:26:04but it might still not be very good for life, because you might be
0:26:04 > 0:26:07too close to where all of the nasty stuff is going on.
0:26:07 > 0:26:10There are all sorts of different ways that stars can behave
0:26:10 > 0:26:13and misbehave, and we're not sure at the moment
0:26:13 > 0:26:15where our sun falls in that spectrum of behaviour.
0:26:15 > 0:26:19- Is it good, or is it bad? - Absolutely.- I like that idea.
0:26:19 > 0:26:22What we really need now is a proper census of lots
0:26:22 > 0:26:25and lots of stars, thousands or even millions of stars,
0:26:25 > 0:26:27to find out what their general behaviours are
0:26:27 > 0:26:30so that we can see where the sun fits into the overall pattern.
0:26:30 > 0:26:34And there are things like the Kepler Mission and the Gaia Mission,
0:26:34 > 0:26:35which are doing that.
0:26:35 > 0:26:38So, until then, we probably don't know if our sun is unique or not?
0:26:38 > 0:26:42We know that our sun is special to us, and that's maybe good enough,
0:26:42 > 0:26:45but it would be nice to know how it compares to other stars.
0:26:45 > 0:26:48Well, that's brilliant. Thank you very much, Marek.
0:26:57 > 0:27:01Last month, we asked you to join us on a hunt for asteroids
0:27:01 > 0:27:03that could collide with our planet.
0:27:03 > 0:27:07We think we know the position of just 1% of the asteroids
0:27:07 > 0:27:09that are close to Earth or cross its orbit,
0:27:09 > 0:27:12and we want your help to change that.
0:27:13 > 0:27:15We had an unexpected delay in getting the site ready,
0:27:15 > 0:27:18but it's there now and you can try and find an asteroid
0:27:18 > 0:27:22by going to asteroidzoo.org, or to the Sky At Night webpage,
0:27:22 > 0:27:25and when we come back next month we'll show you what results
0:27:25 > 0:27:27we've all managed to achieve.
0:27:27 > 0:27:29But, before we go, we've got one last treat.
0:27:29 > 0:27:32A few months ago we ran a competition to give one of you
0:27:32 > 0:27:34the opportunity to select a location
0:27:34 > 0:27:37to be imaged by the Mars Reconnaissance Orbiter.
0:27:37 > 0:27:38The winner was John Green,
0:27:38 > 0:27:42and he chose to image a region of Mars called Hebes Chasma.
0:27:42 > 0:27:44It's this canyon system here
0:27:44 > 0:27:47just above the more famous Valles Marineris.
0:27:47 > 0:27:49And so his image has now been beamed back to Earth
0:27:49 > 0:27:51from the Mars Reconnaissance Orbiter
0:27:51 > 0:27:55and here it is, this stunning view of wind-sculpted features.
0:27:55 > 0:27:58These long, thin ridges, which look good enough to walk along,
0:27:58 > 0:28:02are the result of the rest of the rock being scoured away
0:28:02 > 0:28:03by the action of the wind.
0:28:03 > 0:28:06But, whatever they are, it's a beautiful image.
0:28:06 > 0:28:08It is. I always think of the Martian atmosphere
0:28:08 > 0:28:09as quite thin compared with Earth,
0:28:09 > 0:28:12but the fact it was able to do this is quite amazing.
0:28:13 > 0:28:15That's it for now.
0:28:15 > 0:28:17Next month, we'll be looking at Rosetta,
0:28:17 > 0:28:20the European mission that aims to put a probe on a comet
0:28:20 > 0:28:22and then follow it as it orbits the sun.
0:28:22 > 0:28:25It's so exciting that Rosetta, after an 11 year journey,
0:28:25 > 0:28:27has nearly reached its comet,
0:28:27 > 0:28:30and we'll bring you some of the first images next month.
0:28:30 > 0:28:33- In the meantime, get outside and get looking up.- Good night.