1 00:00:01,000 --> 00:00:06,000 The NASA/ESA Hubble Space Telescope is famous for looking deep into the past of the Universe. 2 00:00:06,000 --> 00:00:08,000 But it can also predict the future. 3 00:00:09,000 --> 00:00:13,000 Pictures made by Hubble over the years show us the fate of the Solar System: 4 00:00:14,000 --> 00:00:17,000 a troubling but beautiful preview of what will happen 5 00:00:17,000 --> 00:00:21,000 when the Sun runs out of fuel more than five billion years from now. 6 00:00:38,000 --> 00:00:42,000 Episode 52: The Death of Stars 7 00:00:43,000 --> 00:00:47,000 Presented by Dr J, aka Dr Joe Liske 8 00:00:50,000 --> 00:00:57,000 Don’t panic just yet! The Sun is about 4 1⁄2 billion years old — that’s old by most standards, 9 00:00:57,000 --> 00:01:00,000 but less than half way through its expected lifespan. 10 00:01:01,000 --> 00:01:07,000 However by observing countless stars similar to the Sun, scientists now have a good idea 11 00:01:07,000 --> 00:01:11,000 of what will happen to the Solar System in the very distant future. 12 00:01:13,000 --> 00:01:22,000 Stars are balls of matter that produce energy mainly by fusing atomic nuclei of hydrogen, forming helium. 13 00:01:23,000 --> 00:01:30,000 Now, when two nuclei fuse together, their combined mass is slightly less than the sum of the two original nuclei, 14 00:01:30,000 --> 00:01:34,000 and the difference is released as energy. 15 00:01:35,000 --> 00:01:38,000 That’s where sunlight comes from. 16 00:01:39,000 --> 00:01:41,000 And it’s also the process that powers thermonuclear bombs. 17 00:01:42,000 --> 00:01:46,000 But while thermonuclear bombs use up their fuel in just a fraction of a second, 18 00:01:46,000 --> 00:01:52,000 stars are big enough to sustain nuclear fusion for millions or indeed billions of years 19 00:01:53,000 --> 00:01:55,000 before they too eventually run out of fuel. 20 00:02:00,000 --> 00:02:02,000 What happens next depends on the size of the star. 21 00:02:04,000 --> 00:02:09,000 Really big stars explode as supernovae after only a few million years... 22 00:02:24,000 --> 00:02:28,000 ...while the smallest stars burn slowly enough to be virtually immortal: 23 00:02:28,000 --> 00:02:32,000 their expected lifespan is much longer than the present age of the Universe, 24 00:02:32,000 --> 00:02:36,000 meaning we’ve never seen one die. 25 00:02:37,000 --> 00:02:39,000 But for stars like the Sun, 26 00:02:39,000 --> 00:02:42,000 which have a lifespan measured in billions of years, 27 00:02:42,000 --> 00:02:46,000 astronomers have made many observations of what happens when the fuel supply runs out. 28 00:02:46,000 --> 00:02:49,000 They end with a whimper, not a bang. 29 00:02:49,000 --> 00:02:52,000 Here’s how it goes – 30 00:02:52,000 --> 00:02:57,000 as revealed by Hubble observations of dozens of stars at different stages of evolution. 31 00:03:00,000 --> 00:03:03,000 First, the star swells up and cools down a little, 32 00:03:03,000 --> 00:03:06,000 becoming a so-called red giant. 33 00:03:06,000 --> 00:03:09,000 When the Sun does this, it will destroy 34 00:03:09,000 --> 00:03:12,000 the inner planets of the Solar System. 35 00:03:16,000 --> 00:03:22,000 Next, the outer layers are puffed out, forming a dense cloud of gas and dust 36 00:03:22,000 --> 00:03:25,000 that totally obscures the visible light from the star. 37 00:03:26,000 --> 00:03:30,000 This stage, called a pre-planetary, or protoplanetary nebula, 38 00:03:30,000 --> 00:03:33,000 is tough to observe as it’s so faint. 39 00:03:33,000 --> 00:03:40,000 only dim infrared emissions from the dust cloud and reflected starlight let astronomers see anything at all. 40 00:03:40,000 --> 00:03:44,000 It’s also a short period in stellar evolution, 41 00:03:44,000 --> 00:03:48,000 just a few thousand years long, so these objects are quite scarce. 42 00:03:50,000 --> 00:03:54,000 Hubble’s images of pre-planetary nebulae show a wide variety of shapes, 43 00:03:54,000 --> 00:03:58,000 hinting at complex dynamics going on inside. 44 00:03:59,000 --> 00:04:03,000 The spiral structure of this nebula is particularly unusual, 45 00:04:03,000 --> 00:04:09,000 and is likely due to a binary star system shaping the cloud of gas and dust. 46 00:04:12,000 --> 00:04:17,000 As the star ejects its outer layers to form the cold pre-planetary nebula, 47 00:04:17,000 --> 00:04:23,000 the core of the star is left behind, leaving a small but very hot remnant. 48 00:04:24,000 --> 00:04:27,000 Over a period of a few thousand years, 49 00:04:27,000 --> 00:04:32,000 radiation from this hot leftover excites the gas in the pre-planetary nebula, 50 00:04:32,000 --> 00:04:35,000 eventually making it light up like a fluorescent sign. 51 00:04:39,000 --> 00:04:45,000 At this point, the faint pre-planetary nebula becomes a bright planetary nebula. 52 00:04:45,000 --> 00:04:50,000 In fact, these are bright enough that astronomers have long been able to see them, 53 00:04:50,000 --> 00:04:53,000 which explains their confusing name. 54 00:04:53,000 --> 00:04:57,000 Because they appear roughly spherical and have a greenish tinge when observed visually, 55 00:04:57,000 --> 00:05:00,000 astronomers using early telescopes 56 00:05:00,000 --> 00:05:04,000 found their appearance reminiscent of the planets of the Solar System. 57 00:05:06,000 --> 00:05:09,000 High resolution observations from modern telescopes including Hubble 58 00:05:09,000 --> 00:05:12,000 show that their shapes are often far from spherical, 59 00:05:12,000 --> 00:05:15,000 and the planet-like appearance is pretty dubious 60 00:05:15,000 --> 00:05:18,000 but the name has stuck. 61 00:05:19,000 --> 00:05:26,000 Eventually, planetary nebulae fade to nothing as the gas and dust is diffused into space. 62 00:05:26,000 --> 00:05:32,000 All that remains is a tiny, dense and dim white dwarf — 63 00:05:32,000 --> 00:05:37,000 the ultimate destination of the Sun, billions of years from now. 64 00:05:39,000 --> 00:05:41,000 But for stars there is life after death. 65 00:05:41,000 --> 00:05:45,000 The matter puffed into space by planetary nebulae 66 00:05:45,000 --> 00:05:50,000 forms the building blocks for new generations of stars and planets.