1 00:00:00,000 --> 00:00:05,577 So as promised, let's start our story by figuring out what the sun had to do to 2 00:00:05,577 --> 00:00:10,096 become a main sequence star. we're going to follow the models. 3 00:00:10,096 --> 00:00:15,744 The models are going to start with a spherical cloud with say, one solar mass 4 00:00:15,744 --> 00:00:20,574 of gas and dust in it. and this is already fragmented possibly 5 00:00:20,574 --> 00:00:24,378 from an even a bigger cloud. We'll talk about that when we talk about 6 00:00:24,378 --> 00:00:27,465 the formation of clusters. We're now big building a star, 7 00:00:27,465 --> 00:00:30,883 this cloud is collasping under the influence of gravity. 8 00:00:30,883 --> 00:00:35,293 And what the model tell us is that within a few thousand years, the interior of 9 00:00:35,293 --> 00:00:39,649 this cloud becomes dense enough that there is an optically thick opaque region 10 00:00:39,649 --> 00:00:40,917 there. this is great. 11 00:00:40,917 --> 00:00:45,217 this means that we can define the interior of this region as the protostar. 12 00:00:45,217 --> 00:00:48,800 The protostar has a surface. It communicates with the rest of the 13 00:00:48,800 --> 00:00:53,319 world through black body radiation. We can plot it on a HR diagram at this 14 00:00:53,319 --> 00:00:56,816 point, we've produced if you want, an accent photo star. 15 00:00:56,816 --> 00:01:02,319 This photospere will start out at about size of about five astronomical units and 16 00:01:02,319 --> 00:01:07,693 a comfortable temperature of 300 Kelvin. And now but the models tells us is that 17 00:01:07,693 --> 00:01:10,801 this thing is contracting and of course heating. 18 00:01:10,801 --> 00:01:15,790 And you compute the luminosity remains the approximately constant over a 19 00:01:15,790 --> 00:01:20,848 contraction from five astronomical units all the way down to about twice the 20 00:01:20,848 --> 00:01:25,250 current solar radius. again, what's contracting is the surface 21 00:01:25,250 --> 00:01:30,834 at which the, the protostar is opaque. Some material, of course, is penetrating 22 00:01:30,834 --> 00:01:35,105 but it's still accreating. The protostar is still growing in mass 23 00:01:35,105 --> 00:01:40,234 and the energy to heat for this heat is to some extent kelvin held hold seating 24 00:01:40,234 --> 00:01:43,230 conversion of gravitational potential energy yo 25 00:01:43,230 --> 00:01:47,248 heat. But also at these temperatures, we begin 26 00:01:47,248 --> 00:01:51,733 to get fusion of deuterium. Whatever amounts of deuterium are 27 00:01:51,733 --> 00:01:54,467 present, the deuterium fuses at a lower 28 00:01:54,467 --> 00:01:59,668 temperature than hydrogen, that's why we use it in terrestrial fusion reactors. 29 00:01:59,668 --> 00:02:05,136 And over the next 600,000 years or so we get this collapse down the twice as to 30 00:02:05,136 --> 00:02:09,070 the radius and the temperature stabilizes at around 4,000 kelvin. 31 00:02:09,070 --> 00:02:14,541 these early stages are built on models. We can try to verify them by looking for 32 00:02:14,541 --> 00:02:19,122 these very young, very cool protostars. This used to be a very difficult 33 00:02:19,122 --> 00:02:23,576 proposition because when a protostar starts forming, around it is the 34 00:02:23,576 --> 00:02:28,475 leftovers of the big cloud, the parts that did not collapse yet. So there's 35 00:02:28,475 --> 00:02:33,555 this gaseous dusty cocoon that hides the protostar to look through the dust, we 36 00:02:33,555 --> 00:02:37,675 use radio and infrared radiation. And in recent years, we have had 37 00:02:37,675 --> 00:02:42,622 detections of these early stages of protostars. 38 00:02:42,622 --> 00:02:48,451 you can learn a lot from the intensity and various wave lengths, the shapes of 39 00:02:48,451 --> 00:02:53,933 the absorption and emission lines. Too much detail for us, but here's a nice 40 00:02:53,933 --> 00:02:58,010 direct imaging of these three nascent infrared sources 41 00:02:58,010 --> 00:03:03,759 luminosities one and 1,000 and 1,000 solar luminosities and 100,000 solar 42 00:03:03,759 --> 00:03:07,642 luminosities. These may be as young as 100,000 years 43 00:03:07,642 --> 00:03:14,362 since they started collapsing so they are fueled by Kelvin-Helmholtz heating and by 44 00:03:14,362 --> 00:03:20,804 the beginnings of deuterium burning, and these photospheres are the dusty hot 45 00:03:20,804 --> 00:03:24,227 opaque regions that we were talking about. 46 00:03:24,227 --> 00:03:31,073 this is a magnified image and it shows you that the IRS9A, the 100,000th solar 47 00:03:31,073 --> 00:03:35,816 luminosity protostar is 5,000 astronomical units 48 00:03:35,816 --> 00:03:38,277 across. It's a very large object, 49 00:03:38,277 --> 00:03:44,431 still it's just beginning to collapse. another interesting object is this one, 50 00:03:44,431 --> 00:03:49,788 L1527 in Taurus. And here we're seeing edge on the disk which the star is 51 00:03:49,788 --> 00:03:52,830 forming. So, if we had a proto planetary disk it 52 00:03:52,830 --> 00:03:57,101 would extend this way. And what were seeing is these bipolar 53 00:03:57,101 --> 00:04:01,890 flows of gas leaving, this is an infrared image from the Spitzer telescope. 54 00:04:01,890 --> 00:04:07,003 And here, we have a mangified Spitzer image which shows you that actually we're 55 00:04:07,003 --> 00:04:12,310 not seeing the protostar in the infrared. And the infrared we're seeing reflected 56 00:04:12,310 --> 00:04:17,034 light off the disk, and the center where the star would be, the disk is so 57 00:04:17,034 --> 00:04:20,569 optically thick that it actually obscures the star. 58 00:04:20,569 --> 00:04:26,853 the contours that are actually showing us the position of the protostar inside this 59 00:04:26,853 --> 00:04:31,210 few hundred astronomical unit disk are, radio emissions. 60 00:04:31,210 --> 00:04:36,494 So, superposing radio along with infrared, we can see both these bipolar 61 00:04:36,494 --> 00:04:40,993 flows that we'll talk about later and the protostar itself. 62 00:04:40,993 --> 00:04:45,350 So, we have now actually seen protostars, we know that they exist. 63 00:04:45,350 --> 00:04:53,929 And at, at, when the protostar has reached the point at which, it's size is 64 00:04:53,929 --> 00:04:59,678 about twice the solar radius. It's temperature is about 4,000 degrees 65 00:04:59,678 --> 00:05:04,495 and deuterium is burning. We now call this a pre-main sequence 66 00:05:04,495 --> 00:05:08,068 star. There is fusion that is fueling the star. 67 00:05:08,068 --> 00:05:14,594 And so we begin to plot the trajectory of the sun as a pre-main sequence star on 68 00:05:14,594 --> 00:05:19,333 the HR diagram. the protostar phase was this horizontal 69 00:05:19,333 --> 00:05:25,082 trajectory that the sun followed. And what we see initially is that having 70 00:05:25,082 --> 00:05:28,040 become a star, it's burning deuterium. 71 00:05:28,040 --> 00:05:30,845 So it's sustained at least partly by fusion. 72 00:05:30,845 --> 00:05:35,309 It's still contracting, and the contraction is along almost a vertical 73 00:05:35,309 --> 00:05:38,051 line. This vertical line is called a Hayashi 74 00:05:38,051 --> 00:05:41,111 Track. The temperature is determined by the fact 75 00:05:41,111 --> 00:05:44,108 that the photosphere is no longer made of dust. 76 00:05:44,108 --> 00:05:47,615 Dust has long since evaporated where it's 4,000 degrees. 77 00:05:47,615 --> 00:05:52,206 What makes the photosphere optically opaque is the presence of negative 78 00:05:52,206 --> 00:05:55,732 hydrogen ions. Negative hydrogen ions, that's a proton 79 00:05:55,732 --> 00:05:59,226 with two electrons. The extra electron would have come from 80 00:05:59,226 --> 00:06:01,595 ionizing metals, calcium, silicone, carbon. 81 00:06:01,595 --> 00:06:06,511 They have outside electrons that are more weakly bound than the one electron of 82 00:06:06,511 --> 00:06:09,531 hydrogen. They become ionized at lower temperatures 83 00:06:09,531 --> 00:06:12,966 as positive ions. Some of these electrons will be captured 84 00:06:12,966 --> 00:06:17,349 to produce negative hydrogen ions. Negative hydrogen ions are excellent 85 00:06:17,349 --> 00:06:22,087 absorbers of radiation at all wavelengths because the second electron is rather 86 00:06:22,087 --> 00:06:26,647 weakly bound and infrared visible and, certainly ultra-violet radiation will 87 00:06:26,647 --> 00:06:27,240 ionize. So, 88 00:06:27,240 --> 00:06:30,534 or deionize. And so, hydrogen, negatively charged 89 00:06:30,534 --> 00:06:36,002 hydrogen ions are great light valve. A layer of those absorbs all colors of 90 00:06:36,002 --> 00:06:39,016 light. This is now our photosphere and then 91 00:06:39,016 --> 00:06:44,204 remix it as, as black body radiation. And so, this photosphere contracts and 92 00:06:44,204 --> 00:06:48,410 it's temperatures controlled by it's ionization state of 93 00:06:48,410 --> 00:06:52,345 of hydrogen. And so, it contracts essentially a longer 94 00:06:52,345 --> 00:06:57,520 line of constant temperature. And this continues you notice at constant 95 00:06:57,520 --> 00:07:03,059 temperature, that star is contracting therefore its luminosity is decreasing, 96 00:07:03,059 --> 00:07:08,132 until the ionization in the center becomes as the center heats, the 97 00:07:08,132 --> 00:07:12,816 ionization in the core increases. This makes the core less opaque. 98 00:07:12,816 --> 00:07:15,986 This allows a radiative zone to be produced. 99 00:07:15,986 --> 00:07:21,823 This makes the efficiency of transporting heat out of the core into the envelope 100 00:07:21,823 --> 00:07:25,282 higher. And that increases the luminosity and 101 00:07:25,282 --> 00:07:29,461 that is this knee in the track that we see around here, 102 00:07:29,461 --> 00:07:34,506 this is this increased ionization. And this continues to proceed until 103 00:07:34,506 --> 00:07:40,200 temperatures in the core reach sufficient temperatures and densities are such that 104 00:07:40,200 --> 00:07:44,227 hydrogen fusion starts. What happens when hydrogen fusion starts? 105 00:07:44,227 --> 00:07:48,254 Well, what happens is that the core ceases to expand, to contract. 106 00:07:48,254 --> 00:07:52,659 in fact, under the increased radiation pressure, it expands slightly. 107 00:07:52,659 --> 00:07:57,441 The expansion of the core absorbs the, some of the energy that's produced by 108 00:07:57,441 --> 00:08:02,537 fusion and so the initial response, this is one of those, expansion by contraction 109 00:08:02,537 --> 00:08:04,980 in reverse. When when the 110 00:08:04,980 --> 00:08:09,709 fusion is initiated in the core, the reaction of the envelope is that the 111 00:08:09,709 --> 00:08:13,331 luminosity decreases. And that's this final segment of the 112 00:08:13,331 --> 00:08:17,288 track, so this turn here is the initiation of efficient hydrogen fusion 113 00:08:17,288 --> 00:08:20,227 in the core. And the star now settles down by a loss 114 00:08:20,227 --> 00:08:24,750 of luminosity down to the main sequence. It would have taken the sun models tell 115 00:08:24,750 --> 00:08:29,272 us about 40 million years remember this was a few hundred thousand. Then, this 116 00:08:29,272 --> 00:08:33,738 track takes about 40 million years and the sun settles down into main sequence 117 00:08:33,738 --> 00:08:35,999 equilibrium. That time scale is familiar, 118 00:08:35,999 --> 00:08:38,995 ten million years was the Kelvin-Helmholtz Timescale. 119 00:08:38,995 --> 00:08:43,680 That is how long the sun would have produced its current luminosity 120 00:08:43,680 --> 00:08:48,248 from gravitational potential energy. And ten million years is the time scale 121 00:08:48,248 --> 00:08:51,313 on which gravitational energy is converted to heat, 122 00:08:51,313 --> 00:08:56,122 and we'll come back to that time scale. Embolden by our understanding of the sun, 123 00:08:56,122 --> 00:09:00,570 we can repeat the models with stars of different masses and we find these 124 00:09:00,570 --> 00:09:03,154 tracks. They all, if you notice, start though 125 00:09:03,154 --> 00:09:07,602 it's not obvious in all of them, with approximately the same Hayashi track 126 00:09:07,602 --> 00:09:11,149 where negative ionized hydrogen controls the temperature. 127 00:09:11,149 --> 00:09:17,201 And then move over to the left heating up and contracting for more massive stars at 128 00:09:17,201 --> 00:09:22,235 almost fixed luminosity, all kinds of details they settle down to the main 129 00:09:22,235 --> 00:09:25,433 sequence as we saw with main sequence lifetime. 130 00:09:25,433 --> 00:09:31,012 the models tell us here that bigger stars go faster so the sun took 40 million 131 00:09:31,012 --> 00:09:35,391 years to achieve its current main sequence, or to arrive on 132 00:09:35,391 --> 00:09:39,111 the main sequence. A star with a mass of about five solar 133 00:09:39,111 --> 00:09:43,389 masses would take a million. Star with a mass of fifteen solar masses 134 00:09:43,389 --> 00:09:48,224 would take 100,000 years, and stars with 30 solar masses would get to the main 135 00:09:48,224 --> 00:09:52,936 sequence in tens of thousands of years. So that when a cloud forms the first 136 00:09:52,936 --> 00:09:58,082 stars to reach main sequence are going to be the hot O and B stars, and the slow, 137 00:09:58,082 --> 00:10:01,740 the lightest stars will be the smallest, slowest ones to go. 138 00:10:01,740 --> 00:10:05,061 So, we have seen how a cloud collapses down to a star. 139 00:10:05,061 --> 00:10:07,730 we spoke of the lightest and the heaviest. 140 00:10:07,730 --> 00:10:10,873 What makes these the lightest and those the heaviest? 141 00:10:10,873 --> 00:10:14,254 What happens if you start with a cloud that is too small? 142 00:10:14,254 --> 00:10:18,465 What happen what happens if you start with a cloud that is too big? 143 00:10:18,465 --> 00:10:22,675 Why does the main sequence end? The mechanisms of star formation should 144 00:10:22,675 --> 00:10:25,580 tell us, and they do. first, 145 00:10:25,580 --> 00:10:29,592 below about 1.3% of a solar mass deuterium fusion never initiates. 146 00:10:29,592 --> 00:10:34,159 If an object never initiates deuterium fusion, its collapse mechanisms are 147 00:10:34,159 --> 00:10:38,789 different, we do not call that a star. It's also true that the fragmentation 148 00:10:38,789 --> 00:10:43,665 processes that break the clouds up seem to stop at about 1% of the solar mass. 149 00:10:43,665 --> 00:10:47,732 So, we do not expect stars to attempt to form, much below 1%.. 150 00:10:47,732 --> 00:10:52,688 But below 7.2% of the solar mass, efficient hydrogen fusion never takes 151 00:10:52,688 --> 00:10:55,379 off. And so, these stars do not achieve 152 00:10:55,379 --> 00:10:59,486 hydrostatic equilibrium under hydrogen burning conditions. 153 00:10:59,486 --> 00:11:04,796 So, they do not become main sequence stars, they take their time contracting 154 00:11:04,796 --> 00:11:09,186 and burning deuterium. They run out of deuterium earlier, these 155 00:11:09,186 --> 00:11:14,355 are brown dwarfs type L, T and Y. These are the extra spectral types below 156 00:11:14,355 --> 00:11:18,638 K that were added. they were added by models. 157 00:11:18,638 --> 00:11:23,957 the first actual discovery of a brown dwarf is relatively recent, since 1994, 158 00:11:23,957 --> 00:11:29,237 95, when this object Gliese 229-B was actually discovered both by, by Palomar 159 00:11:29,237 --> 00:11:34,002 and by the Hubble Space Telescope. And after that, infrared surveys started 160 00:11:34,002 --> 00:11:38,059 showing that there were brown dwarfs in the solar neighborhood. 161 00:11:38,059 --> 00:11:43,146 People started extrapolating those surveys to imagine how many brown dwarfs 162 00:11:43,146 --> 00:11:48,749 there were in the milky way and the going rate about a decade ago was that there 163 00:11:48,749 --> 00:11:51,840 was one brown dwarf for each main sequence star. 164 00:11:51,840 --> 00:11:56,023 So, at that point, the number, estimate of the number of stars in the milky way 165 00:11:56,023 --> 00:12:00,001 essentially doubled because there was going to be as many brown dwarfs 166 00:12:00,001 --> 00:12:03,484 previously undiscovered as there were main sequence stars. 167 00:12:03,484 --> 00:12:08,409 more recent data has cast doubt on this. We found sort of a collection of brown 168 00:12:08,409 --> 00:12:10,991 dwarfs. We've gotten rather good with these 169 00:12:10,991 --> 00:12:16,216 infrared servers at discovering them. And a recent survey by the wise detector 170 00:12:16,216 --> 00:12:20,360 of the solar neighborhood. A tally of these, the calculation of the 171 00:12:20,360 --> 00:12:25,045 efficiency at detecting them suggests that it's more like one brown dwarf for 172 00:12:25,045 --> 00:12:29,309 each five main sequence stars. The number of stars in the Milky Way has 173 00:12:29,309 --> 00:12:32,109 again decreased. Science is an succession of 174 00:12:32,109 --> 00:12:36,084 approximations to truth. this is what cuts off the main sequence 175 00:12:36,084 --> 00:12:41,186 at the bottom end objects lighter than 7% of the solar mass will never actually 176 00:12:41,186 --> 00:12:46,051 manage to heat and be hot and dense up in the core to initiate efficient hydrogen 177 00:12:46,051 --> 00:12:48,721 fusion. What cuts you off at the top end? Well, 178 00:12:48,721 --> 00:12:53,468 if you start with the cloud that is too big, it will not actually manage to make 179 00:12:53,468 --> 00:12:56,968 a star as it collapses, radiation pressure will break it up. 180 00:12:56,968 --> 00:13:02,024 We might talk about the Eddington limit that its involved here later. But again a 181 00:13:02,024 --> 00:13:06,961 few years ago I would have said masses bigger than 100 or 150 solar masses 182 00:13:06,961 --> 00:13:11,700 cannot possibly form a star. that is what models suggested, and indeed 183 00:13:11,700 --> 00:13:15,979 stars with masses as high as 100 solar masses are exceeding rare. 184 00:13:15,979 --> 00:13:21,246 But, it turns out that again observations come to the rescue in the Tarantula 185 00:13:21,246 --> 00:13:25,981 Nebula, a Large Magellanic Cloud. 2010 discovery shows, well here's a 186 00:13:25,981 --> 00:13:31,099 visible image of the Tarantula Nebula and an magnified visible image of the 187 00:13:31,099 --> 00:13:33,978 Tarantula Nebula and over here on the right. 188 00:13:33,978 --> 00:13:39,033 And infrared image which gets rid of all the scattered, light from dust and sees 189 00:13:39,033 --> 00:13:42,616 through the dust. And among these stars that we're 190 00:13:42,616 --> 00:13:46,921 discovering, these main sequence stars is this monster 191 00:13:46,921 --> 00:13:52,681 object R136A, and R136A has a mass estimated about 250 solar mass's. 192 00:13:52,681 --> 00:13:59,345 And, for a size comparison, we see here the sun O-type star perhaps, and then 193 00:13:59,345 --> 00:14:04,225 this new monster. So the current limits are that stars can 194 00:14:04,225 --> 00:14:07,471 form with masses up to about 250 solar masses. 195 00:14:07,471 --> 00:14:11,677 This is still very, very, rare above that radiation pressure will 196 00:14:11,677 --> 00:14:14,793 fragment the cloud. So, we understand what the solar, what 197 00:14:14,793 --> 00:14:17,909 the main sequence is and where these stars come from. 198 00:14:17,909 --> 00:14:21,805 We'll pick it up at the end of the main sequence lifetime in the next.