WEBVTT FILE 1 00:00:01.220 --> 00:00:12.100 You're watching FreeSchool! 2 00:00:12.100 --> 00:00:19.690 Space: it's vast and full of secrets. 3 00:00:19.690 --> 00:00:25.619 Of all the incredible objects scientists have found, out among the stars, some of the most 4 00:00:25.619 --> 00:00:30.630 spectacular are nebulae. 5 00:00:30.630 --> 00:00:35.399 Scattered throughout the galaxy, these vast clouds have captivated the minds of astronomers 6 00:00:35.399 --> 00:00:39.860 and stargazers for centuries. 7 00:00:39.860 --> 00:00:43.430 Nebulae are the birthplace of stars and planets. 8 00:00:43.430 --> 00:00:48.100 They hold clues to the formation and evolution of our galaxy. 9 00:00:48.100 --> 00:00:56.699 And, these vast cosmic clouds have a serene and mesmerizing beauty. 10 00:00:56.699 --> 00:01:02.559 Although most nebulae are completely invisible to the naked eye, modern telescopes are able 11 00:01:02.559 --> 00:01:08.600 to give us close-up views of some of the most breath-taking objects in the universe. 12 00:01:08.600 --> 00:01:18.150 A nebula, named for the Greek word for 'cloud,' is in fact an interstellar cloud of dust and 13 00:01:18.150 --> 00:01:19.150 gas. 14 00:01:19.150 --> 00:01:26.069 They are made mostly of hydrogen and helium, but they may also contain other gases and 15 00:01:26.069 --> 00:01:29.719 cosmic dust. 16 00:01:29.720 --> 00:01:34.979 The exact composition of a nebula can vary depending on its location and the processes 17 00:01:34.979 --> 00:01:37.959 that formed it. 18 00:01:37.959 --> 00:01:53.879 Like the clouds in the sky, nebulae come in interesting shapes, like flowers, insects, 19 00:01:53.880 --> 00:02:03.840 animals, or people. 20 00:02:03.840 --> 00:02:10.399 This is how many of them get their names. 21 00:02:10.399 --> 00:02:14.090 There are four main types of nebulae. 22 00:02:14.090 --> 00:02:22.349 First, there are star-forming regions, also known as stellar nurseries. 23 00:02:22.349 --> 00:02:30.550 In these huge areas of hydrogen gas, gravity pulls the material together into larger clumps, 24 00:02:30.550 --> 00:02:36.040 which in turn attract more material. 25 00:02:36.040 --> 00:02:45.018 More and more gases squeeze together harder and harder, making it grow hotter and hotter. 26 00:02:45.019 --> 00:02:51.099 When it becomes hot enough, it will be a new star. 27 00:02:51.099 --> 00:02:59.030 Whatever is left over may become planets, circling them. 28 00:02:59.030 --> 00:03:04.589 An example of a star-forming region and one of the most famous nebulae in the sky is the 29 00:03:04.590 --> 00:03:07.739 Orion Nebula. 30 00:03:07.739 --> 00:03:14.800 The Orion Nebula is the most active area of star formation in our galaxy. 31 00:03:14.800 --> 00:03:21.599 Although it is visible to the unaided eye, it is only a vague glow. 32 00:03:21.599 --> 00:03:28.850 Through a telescope, the extent of its wonder becomes visible. 33 00:03:28.850 --> 00:03:37.159 The Orion Nebular is 15 light years across, and is made mostly of hydrogen. 34 00:03:37.159 --> 00:03:45.379 When you look at this nebula, you are looking at stars being born. 35 00:03:45.379 --> 00:03:53.019 This nebula gets its glow from something called the Trapezium, a cluster of four stars, each 36 00:03:53.019 --> 00:03:57.740 thousands of times brighter than our sun. 37 00:03:57.740 --> 00:04:03.079 Their energy has cleared a space in the gases around them. 38 00:04:03.079 --> 00:04:09.598 You can see that see that other stars are being born inside the nebula, too. 39 00:04:09.599 --> 00:04:14.908 These ovals are blankets of dust surrounding newborn stars. 40 00:04:14.909 --> 00:04:20.580 The rings inside are probably new planetary systems being formed around these stars. 41 00:04:20.580 --> 00:04:31.119 1300 light years away, the Orion Nebula is only a little glow in Orion's sword. 42 00:04:31.120 --> 00:04:37.450 Second, there are planetary nebulae. 43 00:04:37.450 --> 00:04:43.390 This kind of nebula doesn't really have anything to do with planets, but when early astronomers 44 00:04:43.390 --> 00:04:49.380 saw them through their telescopes, they looked so round and bright they thought they must 45 00:04:49.380 --> 00:04:52.720 be planets. 46 00:04:52.720 --> 00:04:59.699 Planetary nebulae are formed by the slow death of ordinary stars. 47 00:04:59.699 --> 00:05:06.520 Near the end of its life, some stars begin to shed their outer layers into space. 48 00:05:06.520 --> 00:05:12.539 These layers are mainly made of hydrogen gas, but as the star begins to die other elements 49 00:05:12.540 --> 00:05:19.910 are produced as well, including oxygen, carbon, and iron. 50 00:05:19.910 --> 00:05:28.050 The hot core of the dying star illuminates the material, causing them to glow. 51 00:05:28.050 --> 00:05:33.819 As the layers of gas and dust expand, they create beautiful and colorful clouds that 52 00:05:33.820 --> 00:05:51.410 may manifest in many different shapes. 53 00:05:51.410 --> 00:05:58.170 One of the most famous examples of a planetary nebula is the Helix Nebula, also known as 54 00:05:58.170 --> 00:06:01.750 the "Eye of God." 55 00:06:01.750 --> 00:06:07.470 It is located about 700 light-years away from Earth and is one of the closest planetary 56 00:06:07.470 --> 00:06:11.039 nebulae to us. 57 00:06:11.039 --> 00:06:15.470 From a distance, it looks like a giant, glowing eye. 58 00:06:15.470 --> 00:06:23.080 Up close, the Helix Nebula is a fantastic sight, with intricate patterns and thousands 59 00:06:23.080 --> 00:06:31.228 of streamers of bright gas extending toward its center. 60 00:06:31.229 --> 00:06:37.240 This beautiful object is known as the butterfly nebula, and it is another example of a planetary 61 00:06:37.240 --> 00:06:40.229 nebula. 62 00:06:40.229 --> 00:06:46.740 Although it looks peaceful, and like a delicate butterfly from afar, it is actually made of 63 00:06:46.740 --> 00:06:55.199 raging clouds of gas, heated to more than 36,000 degrees Fahrenheit. 64 00:06:55.199 --> 00:07:02.690 For comparison, that is more than three times hotter than the surface of the sun! 65 00:07:02.690 --> 00:07:10.690 The gas is also in motion - moving at more than 600,000 miles an hour. 66 00:07:10.690 --> 00:07:20.930 At that speed, you could go from the earth to the moon in just 24 minutes! 67 00:07:20.930 --> 00:07:25.640 One last famous example of a planetary nebula is the Cat's Eye Nebula. 68 00:07:25.640 --> 00:07:32.630 It is a bit further away from Earth, at about 3,000 light-years, but it is still one of 69 00:07:32.630 --> 00:07:37.490 the brightest planetary nebulae in the sky. 70 00:07:37.490 --> 00:07:43.660 The Cat's Eye Nebula is made up of several rings of gas, each with a different color 71 00:07:43.660 --> 00:07:46.409 and pattern. 72 00:07:46.409 --> 00:07:51.909 The central star is so hot that it has ionized the gas around it, creating a bright blue 73 00:07:51.909 --> 00:07:57.789 color in the middle of the nebula. 74 00:07:57.789 --> 00:08:02.569 The third kind of nebulae are called supernova remnants. 75 00:08:02.569 --> 00:08:09.360 Sometimes when a star dies it doesn't die slowly - it dies with a bang! 76 00:08:09.360 --> 00:08:15.280 A supernova is a huge explosion that spreads the material from the star across a huge expanse 77 00:08:15.280 --> 00:08:17.389 of space. 78 00:08:17.389 --> 00:08:26.659 The leftovers form a nebula, which may over time collect and form new stars. 79 00:08:26.659 --> 00:08:31.520 The Crab Nebula is an example of a supernova remnant. 80 00:08:31.520 --> 00:08:40.909 This nebula is especially exciting because we know when the star that created it exploded! 81 00:08:40.909 --> 00:08:47.980 Nearly 1,000 years ago, Chinese astronomers noticed a bright new star in the sky. 82 00:08:47.980 --> 00:08:54.420 It stayed there for almost two years, and then slowly vanished. 83 00:08:54.420 --> 00:09:00.010 Modern astronomers studying the Crab Nebula realized that the bright, temporary star described 84 00:09:00.010 --> 00:09:04.980 in the ancient record was an excellent match for the the supernova that would have created 85 00:09:04.980 --> 00:09:08.760 the Crab Nebula. 86 00:09:08.760 --> 00:09:14.640 Knowing when the star went nova allowed scientists to learn a great deal about the way this type 87 00:09:14.640 --> 00:09:19.320 of nebula develops. 88 00:09:19.320 --> 00:09:24.730 In addition to the Crab Nebula, there are many other supernova remnants found throughout 89 00:09:24.730 --> 00:09:31.770 our galaxy, each with their own unique characteristics and properties. 90 00:09:31.770 --> 00:09:36.970 Some of the most famous supernova remnants include the Veil Nebula, the remnant of a 91 00:09:36.970 --> 00:09:43.950 supernova that occurred around 5,000 years ago, and the famous Tycho's Nova, a nebula 92 00:09:43.950 --> 00:09:52.059 formed by a supernova which was observed in 1572. 93 00:09:52.059 --> 00:09:55.199 The fourth kind of nebulae are dark nebulae. 94 00:09:55.200 --> 00:10:03.070 A dark nebula, also called an absorption nebula, is a cloud of gas and dust that is only visible 95 00:10:03.070 --> 00:10:08.730 because of bright nebulae and stars behind it. 96 00:10:08.730 --> 00:10:14.970 Typically much denser than bright nebulae, dark nebulae are difficult to observe because 97 00:10:14.970 --> 00:10:18.970 they do not produce their own light. 98 00:10:18.970 --> 00:10:27.929 Like a shadow or a silhouette, they take their shape from the light we do not see. 99 00:10:27.929 --> 00:10:32.819 One of the most famous dark nebulae is the Horsehead Nebula. 100 00:10:32.820 --> 00:10:37.059 It is easy to see where this nebula got its name! 101 00:10:37.059 --> 00:10:42.768 The Horsehead Nebula is made of thick dust clouds, so dense that they block the light 102 00:10:42.769 --> 00:10:47.130 of stars behind them. 103 00:10:47.130 --> 00:10:53.390 Just because the cloud is dark, does not mean that it is cold. 104 00:10:53.390 --> 00:10:57.190 Looking at the nebula through an infrared camera that allows us to look at the heat 105 00:10:57.190 --> 00:11:04.760 it gives off, we can see that the Horsehead Nebula is very hot, indeed. 106 00:11:04.760 --> 00:11:13.760 There are even bright spots visible, which shows where new baby stars are forming. 107 00:11:13.760 --> 00:11:19.708 No matter what type they are, nebulae are among the most dazzling celestial objects 108 00:11:19.708 --> 00:11:22.699 to be found in the sky. 109 00:11:22.700 --> 00:11:27.390 They are a window into the life cycle of the universe. 110 00:11:27.390 --> 00:11:34.920 In the end, the same raw materials that form stars are left behind when stars die, to form 111 00:11:34.920 --> 00:11:39.890 in turn new stars - and the cycle begins again.