1 00:00:01,480 --> 00:00:04,417 We have two open ends that I want to close. 2 00:00:04,417 --> 00:00:10,018 One is I talked about mass transfer as a mechanism for generating blue stragglers. 3 00:00:10,018 --> 00:00:15,551 And the other is that I sort of said that the end of our sun, after the beautiful 4 00:00:15,551 --> 00:00:21,287 light show that is the planetary nebula is a slowly cooling solar mass earth size 5 00:00:21,287 --> 00:00:26,593 chunk of carbon that will eventually when it cools crystalize, so that's a pretty 6 00:00:26,593 --> 00:00:30,785 nice diamond in the sky, but, is a white dwarf really the end for 7 00:00:30,785 --> 00:00:32,750 a star. It will be for our sun, 8 00:00:32,750 --> 00:00:35,848 but it need not be. There's still life in them, there are 9 00:00:35,848 --> 00:00:40,222 white dwarfs despite the fact that they're fusion-wise, completely inert. 10 00:00:40,222 --> 00:00:45,143 And, the trick is the phenomenon of mass transfer, so we need to understand it and 11 00:00:45,143 --> 00:00:50,185 the mathematics is due to our old friend Roche, who we all love and remember for 12 00:00:50,185 --> 00:00:52,130 his work on tidal forces. And so, 13 00:00:52,130 --> 00:00:56,309 thing to remember is that the real question we want to ask in a binary 14 00:00:56,309 --> 00:01:00,724 system, of course, every object in a binary system, is in some sense rotating, 15 00:01:00,724 --> 00:01:05,080 orbiting both stars each star is orbiting the common center of gravity. 16 00:01:05,080 --> 00:01:09,731 And if you throw a rock in there, it'll also orbit in some complicated way, both 17 00:01:09,731 --> 00:01:10,320 stars. But, 18 00:01:10,320 --> 00:01:14,650 we want to describe this in a way that gives us a sense of what orbits which 19 00:01:14,650 --> 00:01:16,927 star, which matter is bound to which star, 20 00:01:16,927 --> 00:01:20,647 and the complication is that the two stars themselves are rotating. 21 00:01:20,647 --> 00:01:25,145 So if you wanted something to be sitting next to a star, it has to not just orbit 22 00:01:25,145 --> 00:01:29,309 that star, but also follow it on its orbit around the common center of mass 23 00:01:29,309 --> 00:01:31,808 and this is the problem that Roche addressed. 24 00:01:31,808 --> 00:01:34,640 And it turns out that if you ask the question, drop, 25 00:01:34,640 --> 00:01:40,130 imagine the whol universe rotating with the system, imagine that the stars are in 26 00:01:40,130 --> 00:01:45,556 circular orbits. And somewhere on this, in this rotating universe and drop a rock 27 00:01:45,556 --> 00:01:50,217 and ask where it will fall, the combination of gravity and the rotational 28 00:01:50,217 --> 00:01:53,090 motion is described by this Roche potential. 29 00:01:53,090 --> 00:01:58,760 here is the Roche potential drawn for a combination of two stars relatively close 30 00:01:58,760 --> 00:02:04,277 to each other one of whom has twice the mass of the other, and so, the surface 31 00:02:04,277 --> 00:02:09,658 here is the potential. So that if you drop something here, it will slide that 32 00:02:09,658 --> 00:02:12,587 way, if you drop something here, it'll slide 33 00:02:12,587 --> 00:02:16,639 down into the star. there's a complication that the rotation 34 00:02:16,639 --> 00:02:19,333 introduces just like the rotation of the earth. 35 00:02:19,333 --> 00:02:24,490 something called the Coriolis Force, for the same reason that winds and artillery 36 00:02:24,490 --> 00:02:27,470 shells in the northern hemisphere turn to the right. 37 00:02:27,470 --> 00:02:31,768 If we imagine that this is the direction which the stars are orbiting, then 38 00:02:31,768 --> 00:02:34,118 indeed, everything will turn to the right. 39 00:02:34,118 --> 00:02:38,129 So that for example, a stone left here will not actually fall down the 40 00:02:38,129 --> 00:02:42,370 potential, but will go into some kind of crazy orbit around this so called 41 00:02:42,370 --> 00:02:45,540 Lagrange point, but that's not what interests us. 42 00:02:45,540 --> 00:02:50,860 the point is that this allows us by join a particular level surface of this 43 00:02:50,860 --> 00:02:54,500 potential, particular region where you can move from 44 00:02:54,500 --> 00:03:00,315 one to the other without experiencing any gravitational force is this particular 45 00:03:00,315 --> 00:03:05,614 surface that meets itself at a point. This top of this saddle right there or 46 00:03:05,614 --> 00:03:09,369 the bottom, depending which way you count it in a saddle. 47 00:03:09,369 --> 00:03:14,802 And this divides the region near the stars into two lobes, the larger of which 48 00:03:14,802 --> 00:03:20,436 corresponds to the more massive star the point in in between where the two lobes 49 00:03:20,436 --> 00:03:25,250 meet is not quite the center of mass. It's shifted closer to the notice, it's 50 00:03:25,250 --> 00:03:29,856 closer to the lighter star than it is to the more massive star, whereas the center 51 00:03:29,856 --> 00:03:34,237 of mass is closer to the massive star. But the idea is that this is the region 52 00:03:34,237 --> 00:03:36,708 in which the gravity of this star dominates, 53 00:03:36,708 --> 00:03:40,078 this is the region which the gravity of this star dominates. 54 00:03:40,078 --> 00:03:44,066 If you drop something from here, it will go into orbit around this star. 55 00:03:44,066 --> 00:03:47,380 Drop something here, it will go into orbit around this star. 56 00:03:47,380 --> 00:03:51,537 Drop something here and it can move around in an orbit like this, which is 57 00:03:51,537 --> 00:03:55,120 some complicated orbit around both stars. So, 58 00:03:55,120 --> 00:04:00,730 typically for two stars in a binary system this is a very interesting thing 59 00:04:00,730 --> 00:04:05,861 if you're going to add planets for example into the system, but why is this 60 00:04:05,861 --> 00:04:11,129 interesting when you have two stars? Well, here's the picture and Algol, our 61 00:04:11,129 --> 00:04:16,534 friend al Ghul, the ghoulish star in Perseus is the prototypical example and 62 00:04:16,534 --> 00:04:22,104 Algol presents astronomers with a puzzle. We can appreciate the puzzle now. 63 00:04:22,104 --> 00:04:28,052 This Algol has two members. 3.6 solar mass main sequence primary and 64 00:04:28,052 --> 00:04:31,320 a 0.79 solar mass subgiant secondary. Huh? 65 00:04:31,320 --> 00:04:35,200 That can't be right. The reason it can't be right is because 66 00:04:35,200 --> 00:04:39,000 the two members of a binary would have formed from the collapse of the same 67 00:04:39,000 --> 00:04:43,695 cloud absorbing the angular momentum. And if they form together, then the more 68 00:04:43,695 --> 00:04:46,825 massive primary should have evolved earlier. 69 00:04:46,825 --> 00:04:52,374 How is it that a 3.6 solar mass member is on the main sequence while the less 70 00:04:52,374 --> 00:04:58,208 massive, secondary, is already done with its main sequence life and is already a 71 00:04:58,208 --> 00:05:00,455 subgiant? that's a problem. 72 00:05:00,455 --> 00:05:06,060 the hint to the answer is that Algol is a very close binary system. 73 00:05:06,060 --> 00:05:09,822 The distance between the two stars is, 0.062 AU, 74 00:05:09,822 --> 00:05:16,041 that's only twice the distance between the sun and Mercury and both this star, 75 00:05:16,041 --> 00:05:21,492 because it's a massive main sequence star, and this star because it's a 76 00:05:21,492 --> 00:05:27,006 subgiant, are much larger than the sun. And so, imagine sticking another sun 77 00:05:27,006 --> 00:05:32,132 twice as far out as Mercury is then make them both big, these stars are almost 78 00:05:32,132 --> 00:05:35,328 touching. The answer to the Algol puzzle is that, 79 00:05:35,328 --> 00:05:40,122 just as we talked about for blue stragglers, B actually started out what 80 00:05:40,122 --> 00:05:43,983 is now the secondary, started out as the more massive star. 81 00:05:43,983 --> 00:05:48,519 So how come it's less massive now? Well as long as they were both main 82 00:05:48,519 --> 00:05:53,523 sequence stars, they orbited each other and each lived within its Roche lobe and 83 00:05:53,523 --> 00:05:57,840 so, the atmosphere of each was part of a well-defined object, the star. 84 00:05:57,840 --> 00:06:03,370 But then, when B turned into a subgiant and started to puff up, it leaked its 85 00:06:03,370 --> 00:06:06,890 outer atmosphere, leaked outsides its Roche lobe. 86 00:06:06,890 --> 00:06:11,801 when it leaked outside, some of it was captured onto A, which is now gaining 87 00:06:11,801 --> 00:06:15,454 mass while B loses mass, which means the Roche lobes move. 88 00:06:15,454 --> 00:06:20,617 B's Roche lobe gets smaller because its less massive, A's Roche lobe gets larger 89 00:06:20,617 --> 00:06:24,396 because it's more massive. And eventually, you get a run away 90 00:06:24,396 --> 00:06:29,565 process of mass transfer from B to A to the point where now a large fraction, 91 00:06:29,565 --> 00:06:34,294 remember, B was more massive then A initially, a large fraction of the mass 92 00:06:34,294 --> 00:06:38,576 of star B has been transferred to star A, which is now more massive. 93 00:06:38,576 --> 00:06:43,817 And this matter in the same way that we'll see later on, comes in with a lot 94 00:06:43,817 --> 00:06:48,866 of angular momentum from the rotation. So it forms a excretion disk around star 95 00:06:48,866 --> 00:06:53,723 A, and then, as material looses angular momentum and energy, it spirals in and 96 00:06:53,723 --> 00:06:59,210 eventually falls onto star A and star A's atmosphere is increasing while star B is 97 00:06:59,210 --> 00:07:02,071 losing mass. So this is a curiosity, it's a very 98 00:07:02,071 --> 00:07:06,725 interesting phenomenon. It's fun that we can understand both the riddle and its 99 00:07:06,725 --> 00:07:09,434 solution. What is this got to do with, and it's 100 00:07:09,434 --> 00:07:12,851 clear how this might resolve the issue of blue stragglers. 101 00:07:12,851 --> 00:07:18,034 If you start with a star that's not very massive, then it sucks a lot of mass from 102 00:07:18,034 --> 00:07:22,747 binary partner, then you would find in your globular cluster a star that is more 103 00:07:22,747 --> 00:07:27,341 massive but still on the main sequence, because it used to be a not so massive 104 00:07:27,341 --> 00:07:30,169 star and evolved very slowly. So far, so good. 105 00:07:30,169 --> 00:07:33,303 What has this got to do with white dwarfs? 106 00:07:33,303 --> 00:07:38,720 Well, imagine, if in a close binary system, the more massive partner evolves, 107 00:07:38,720 --> 00:07:44,299 finishes its main sequence life, ejects a planetary nebula and leaves us 108 00:07:44,299 --> 00:07:49,171 with a white dwarf, while the less massive partner is still on the main 109 00:07:49,171 --> 00:07:52,259 sequence. Eventually, the less massive partner also 110 00:07:52,259 --> 00:07:57,543 will load up and become a giant and when it becomes a giant, the less massive 111 00:07:57,543 --> 00:08:03,427 partner now might extend past its Roche lobe and this time its partner is a white 112 00:08:03,427 --> 00:08:06,111 dwarf. So that means that matter can be 113 00:08:06,111 --> 00:08:10,532 transferred, we can have mass transfer from the less massive partner to, 114 00:08:10,532 --> 00:08:14,580 remember, now the white dwarf is probably the less massive object. 115 00:08:14,580 --> 00:08:17,591 Because it, it lost all of its atmosphere, 116 00:08:17,591 --> 00:08:23,466 but on to the white dwarf and model show us that in a close binary, that rate of 117 00:08:23,466 --> 00:08:29,048 mass transfer can reach a 100 millionth of a solar mass per year that's a 118 00:08:29,048 --> 00:08:32,327 substantial rate of mass flow. What happens to this gas? 119 00:08:32,327 --> 00:08:36,732 Well, when it falls onto the, a white dwarf, this is the white dwarf acquiring 120 00:08:36,732 --> 00:08:40,970 a little bit of a hydrogen atmosphere. Of course, this is nothing like earth's 121 00:08:40,970 --> 00:08:45,486 atmosphere, because gravity is hundreds of thousands of times more intense at the 122 00:08:45,486 --> 00:08:48,999 surface of a white dwarf. And so, remember this is a earth-sized 123 00:08:48,999 --> 00:08:52,558 object with solar mass, and so, the hydrogen is immediately 124 00:08:52,558 --> 00:08:56,805 compressed to degeneracy and heated by the immense surface gravity. 125 00:08:56,805 --> 00:09:00,607 So you have this very tightly bounded degenerate atmosphere. 126 00:09:00,607 --> 00:09:04,536 Moreover, it turns out that it's important that at its base there's 127 00:09:04,536 --> 00:09:09,480 turbulent mixing because of the heating and that enriches the gas with carbon, 128 00:09:09,480 --> 00:09:14,320 nitrogen, and oxygen, the materials out of which the white dwarf is made. 129 00:09:14,320 --> 00:09:19,797 And then what happens is that when the white dwarf has accumulated about a 130 00:09:19,797 --> 00:09:25,433 hundred, a 10,000th of a solar mass of this atmospheric hydrogen the bottom, the 131 00:09:25,433 --> 00:09:31,307 base, the temperature at the base at the bottom of this atmosphere which is very, 132 00:09:31,307 --> 00:09:37,817 very thin in metric terms, but contains a 10,000th of a solar mass the temperature 133 00:09:37,817 --> 00:09:41,310 reaches the magic number 10 million Kelvin. 134 00:09:41,310 --> 00:09:46,713 When the temperature reaches 10 million Kelvin, you can start efficient CNO 135 00:09:46,713 --> 00:09:51,973 hydrogen fusion, the CNO process, and again, we have a degenerate atmosphere, 136 00:09:51,973 --> 00:09:57,376 because it's degenerate, it does not expand when fusion starts, so the fusion 137 00:09:57,376 --> 00:10:02,140 is explosive and essentially the entire atmosphere fuses to helium. 138 00:10:02,140 --> 00:10:07,900 simultaneously, this, well, not the entire atmosphere, only about 10% gets to 139 00:10:07,900 --> 00:10:13,964 fuse, because after 10%, the temperature rises to a 100 million K and the thermal 140 00:10:13,964 --> 00:10:19,106 pressure overcomes degeneracy pressure. At this point, the luminosity of the 141 00:10:19,106 --> 00:10:24,632 object can reach a 100,000 solar luminosities. This is a 100,000 times as 142 00:10:24,632 --> 00:10:27,972 luminous as the sun. And it's not like the helium flash, 143 00:10:27,972 --> 00:10:33,377 because it's not heated, it's exposed, we will see something with a 100,000 solar 144 00:10:33,377 --> 00:10:36,353 luminousities. once you reach that temperature, 145 00:10:36,353 --> 00:10:41,272 radiation pressure ejects the ramainder of the ecreted material. The total energy 146 00:10:41,272 --> 00:10:47,587 released is on the order of 10^38 Jules over a few months that if you think about 147 00:10:47,587 --> 00:10:53,621 it is comparable or slightly more by a factor of ten than the total energy the 148 00:10:53,621 --> 00:11:00,001 sun will release over its entire history. and this can recur, you know, when mass 149 00:11:00,001 --> 00:11:06,776 reaccumulates, so novas can recur every 10,000 or few tens of thousands of years. 150 00:11:06,776 --> 00:11:11,782 And this ejected matter initially glows at about 9,000 Kelvin, 151 00:11:11,782 --> 00:11:15,500 sort as a characteristic high frequency light signal. 152 00:11:15,500 --> 00:11:20,812 And these novae are pretty common, we see a few per year in the Milky Way, but we 153 00:11:20,812 --> 00:11:24,929 don't see far into the Milky Way, because of dust obstructions. 154 00:11:24,929 --> 00:11:30,440 We see better into, say our neighboring Andromeda Galaxy M-31, and there about 30 155 00:11:30,440 --> 00:11:37,252 novae per year detected in, in 31 and the citizen scientists are instrumental in 156 00:11:37,252 --> 00:11:41,684 discovering these as well, the variable star observers. 157 00:11:41,684 --> 00:11:47,512 Here is a visible light image of a nova that exploded in Cygnus in 2010. 158 00:11:47,512 --> 00:11:51,790 And so, this is three days prior and this is three days 159 00:11:51,790 --> 00:11:54,730 later you see why the name nova was given. 160 00:11:54,730 --> 00:11:57,706 If the star was too dim to be visible before, 161 00:11:57,706 --> 00:12:01,283 this presumably is not the white dwarf, but its partner. 162 00:12:01,283 --> 00:12:05,967 But a star could be too dim to see, and then suddenly, it's 100,000 solar 163 00:12:05,967 --> 00:12:09,544 luminosities. You certainly notice that object at great 164 00:12:09,544 --> 00:12:11,301 distance, nova, for new star. 165 00:12:11,301 --> 00:12:16,570 the nova, these novae, live, leave behind interesting remnants that are being 166 00:12:16,570 --> 00:12:20,147 carefully studied. All these structure in the ejecta. 167 00:12:20,147 --> 00:12:25,481 This is the ejecta still glowing many years after the supernova, after the nova 168 00:12:25,481 --> 00:12:30,893 exploded, and in the center, the white dwarf is getting ready to do it again. 169 00:12:30,893 --> 00:12:34,038 And so, white dwarf need not be the end of the 170 00:12:34,038 --> 00:12:36,660 road if you have a partner to mooch off of. 171 00:12:36,660 --> 00:12:41,574 Now, this brings up an even more interesting possibility. Nova is a very 172 00:12:41,574 --> 00:12:45,490 cool object and very interesting and brilliant, 173 00:12:45,490 --> 00:12:49,854 but there's something even more exciting which goes under the name supernova. 174 00:12:49,854 --> 00:12:53,771 What is super about a supernova? Well, supernova is the answer to the 175 00:12:53,771 --> 00:12:57,296 question, a type 1a supernova, is the answer to the question. 176 00:12:57,296 --> 00:13:00,597 Well, wait a minute. So accretion is adding to the mass of a 177 00:13:00,597 --> 00:13:03,619 white dwarf. What happens if you add so much mass that 178 00:13:03,619 --> 00:13:07,536 it exceeds the trandosacar limit, remember, a white dwarf cannot have a 179 00:13:07,536 --> 00:13:10,390 mass more than 1.44 solar masses, it would collapse. 180 00:13:10,390 --> 00:13:16,888 as modeling and this is a difficult problem shows the mass of a white dwarf 181 00:13:16,888 --> 00:13:22,393 never, doesn't exceed 1.44 solar masses. As the mass becomes close to the 182 00:13:22,393 --> 00:13:27,669 Chandrasekhar limit, the white dwarf compresses, remember, at 1.44 solar 183 00:13:27,669 --> 00:13:31,581 masses, it would have zero radius and zero volume. 184 00:13:31,581 --> 00:13:37,202 The, so as the mass increases, pressure and temperature in this compressed 185 00:13:37,202 --> 00:13:41,745 degenerate carbon-oxygen combination object are increasing. 186 00:13:41,745 --> 00:13:47,366 And eventually, a turbulent convection phase starts out that leads to the 187 00:13:47,366 --> 00:13:52,063 ignition of carbon fusion. And again, your indegenerate matter, 188 00:13:52,063 --> 00:13:56,298 the heating from the fusion does not lead to expansion, 189 00:13:56,298 --> 00:14:02,370 so we get a violent explosive process which fuses a substantial fraction of an 190 00:14:02,370 --> 00:14:05,963 entire solar mass of carbon within a few seconds. 191 00:14:05,963 --> 00:14:09,044 Oxygen fuses too, although less completely, 192 00:14:09,044 --> 00:14:13,884 this heats the, the, the object to temperatures of a billion Kelvin. 193 00:14:13,884 --> 00:14:19,385 it releases a total of 10^44 Joules, that's like a million novae, 194 00:14:19,385 --> 00:14:24,812 this deserves the title supernova. It blows the star completely away by 195 00:14:24,812 --> 00:14:30,533 releasing a shockwave that ejects material at very high speeds throughout 196 00:14:30,533 --> 00:14:34,456 the neighborhood. the typically, the donating partner, if 197 00:14:34,456 --> 00:14:38,891 there was a partner that was from which the white dwarf was accreting, that 198 00:14:38,891 --> 00:14:43,890 partner would be blown away at high speed and become what's called a runaway star. 199 00:14:43,890 --> 00:14:48,555 luminosity of the supernova can be a billion solar luminosities or more, 200 00:14:48,555 --> 00:14:51,789 that's characteristic of the luminosity of a galaxy. 201 00:14:51,789 --> 00:14:56,764 So you have a very small target, a point object glowing with the luminosity of a 202 00:14:56,764 --> 00:14:59,273 galaxy. You can see these way far off in the 203 00:14:59,273 --> 00:15:04,173 universe and they're characterized by a spectrum that has absorption lines of 204 00:15:04,173 --> 00:15:09,324 silicon which is found inside the white dwarf and is formed by fusion, but very 205 00:15:09,324 --> 00:15:14,350 few hydrogen and helium lines, because this was formed in an object which was 206 00:15:14,350 --> 00:15:17,240 essentially carbon, nitrogen, and oxygen. And 207 00:15:17,240 --> 00:15:24,233 the shock wave drives fusion to heavy elements from carbon to nickel to iron 208 00:15:24,233 --> 00:15:30,468 and beyond uranium and, and other heavy elements are produced within the 209 00:15:30,468 --> 00:15:34,559 shockwave driven by a supernova. This creates a lot of radioactive 210 00:15:34,559 --> 00:15:39,539 nucleotides and their decay sort of contributes to the late time luminosity. 211 00:15:39,539 --> 00:15:44,716 So the energy of the shockwave, some of it, goes into forming these radioactive 212 00:15:44,716 --> 00:15:50,024 nuclei and they release their energy over years, over the next few months or a 213 00:15:50,024 --> 00:15:53,038 year, and so, we can trace the half lives of 214 00:15:53,038 --> 00:15:56,970 the isotopes that created it by tracking the light curve. 215 00:15:56,970 --> 00:16:01,663 Very exciting objects, more exciting than you might think. 216 00:16:01,663 --> 00:16:05,529 What do we know about them? Well, understanding this explosion is 217 00:16:05,529 --> 00:16:08,368 difficult. Understanding how they happen is not 218 00:16:08,368 --> 00:16:11,025 obvious. So, we know that a type 1A supernova 219 00:16:11,025 --> 00:16:14,770 happens when a white dwarf approaches the Chandrasekhar limit. 220 00:16:14,770 --> 00:16:19,602 Where did it gets get its mass? There are two possible suggestions this 221 00:16:19,602 --> 00:16:22,380 might remind you of the blue straggler debate. 222 00:16:22,380 --> 00:16:25,340 One is called the SD or the single degenerate 223 00:16:25,340 --> 00:16:32,225 model, where the donor is a main sequence or giant star in a very close binary pair 224 00:16:32,225 --> 00:16:37,670 with the white dwarf and the donor exceeds its Roche lobe and 225 00:16:37,670 --> 00:16:40,075 that is how matter accretes under the white dwarf. 226 00:16:40,075 --> 00:16:44,019 You have to be careful, you have to make sure matter accretes fast enough that the 227 00:16:44,019 --> 00:16:48,136 white dwarf does not have time to nova it away and before it approaches the 228 00:16:48,136 --> 00:16:51,850 Chandrasekhar limit. the competing proposal is what's called 229 00:16:51,850 --> 00:16:56,550 the DD or double degenerate model where the donor is another white dwarf, and for 230 00:16:56,550 --> 00:17:00,089 some reason imagine two white dwarves orbiting each other. 231 00:17:00,089 --> 00:17:03,803 Well, they're going to orbit each other, nothing is going to happen. 232 00:17:03,803 --> 00:17:08,503 they're not, white dwarves don't grow outside their Roche lobes, they're tiny, 233 00:17:08,503 --> 00:17:13,029 but, if you manage to make them orbit so close that their distance was of the 234 00:17:13,029 --> 00:17:15,949 order of their size, that's the earth's radius that's two 235 00:17:15,949 --> 00:17:19,658 stars rotating within orbiting each other within an order of magnitude of the 236 00:17:19,658 --> 00:17:21,179 earth's size. How do you do that? 237 00:17:21,179 --> 00:17:24,080 Well, if you only had some friction that could slow them down, 238 00:17:24,080 --> 00:17:27,168 but there's no atmosphere because it's a white dwarf. 239 00:17:27,168 --> 00:17:31,889 We'll learn later that there is a kind of friction that could slow them down and 240 00:17:31,889 --> 00:17:35,910 bring them into this very close orbit. When they get close enough, the 241 00:17:35,910 --> 00:17:39,932 gravitational forces of the slightly more massive one will rip apart. 242 00:17:39,932 --> 00:17:44,478 The less massive one, you get dramatic mass transfer through the Roche lobe, 243 00:17:44,478 --> 00:17:47,800 and essentially, you have a merger of two white dwarfs. 244 00:17:47,800 --> 00:17:50,397 And which of these is the correct scenario? 245 00:17:50,397 --> 00:17:53,237 Recent observations suggest that both occur, 246 00:17:53,237 --> 00:17:57,406 but the tide is turning, the single degenerate was the older model 247 00:17:57,406 --> 00:18:02,239 and more and more people are convinced that this at least has to play a role. 248 00:18:02,239 --> 00:18:04,958 We'll see some evidence for that in a minute. 249 00:18:04,958 --> 00:18:09,963 the nature of the explosion itself is very hard to understand, it's very hard 250 00:18:09,963 --> 00:18:13,327 to model such a nonequilibrium turbulant process. 251 00:18:13,327 --> 00:18:18,404 For example, its not even clear whether fusion proceeds as burning defiligration 252 00:18:18,404 --> 00:18:24,332 or an honest detonation where the speed with which the fusion reaction expands 253 00:18:24,332 --> 00:18:30,541 faster than the sound speed, which is quite high inside this dense degenerate 254 00:18:30,541 --> 00:18:34,089 object. is there an actual triggering of the 255 00:18:34,089 --> 00:18:40,540 initial explosion by degenerate flash of degenerate helium in the atmosphere or 256 00:18:40,540 --> 00:18:45,540 does it start, as I suggested, by internal carbon oxygen fusion? 257 00:18:45,540 --> 00:18:50,473 But there's one important fact. And the fact is observational and very critical, 258 00:18:50,473 --> 00:18:54,091 critically important, but we understand what's going on. 259 00:18:54,091 --> 00:18:58,959 Essentially, no matter how you got there, a type 1a supernova is the ex, the 260 00:18:58,959 --> 00:19:04,287 complete fusion of an object made out of carbon and oxygen whose mass is 1.44 261 00:19:04,287 --> 00:19:09,483 solar masses or essentially, as close to the Chandrasekhar limit as this object 262 00:19:09,483 --> 00:19:12,581 can get. So, type 1a supernovae are essentially 263 00:19:12,581 --> 00:19:15,491 all the same. And indeed, when we, they occur in 264 00:19:15,491 --> 00:19:20,364 clusters and galaxies, to which we know the distance, we can compute their 265 00:19:20,364 --> 00:19:25,643 luminosity measuring their brightness and comparing the distance that we know, 266 00:19:25,643 --> 00:19:29,230 and indeed, the luminosity is almost exactly the same. 267 00:19:29,230 --> 00:19:34,035 Amazingly uniform objects compared, say to the differences in luminosity among 268 00:19:34,035 --> 00:19:38,533 stars, and in fact, if you make some corrections based on the shape of the 269 00:19:38,533 --> 00:19:42,907 light curve then you can actually use these things as standard candles. 270 00:19:42,907 --> 00:19:47,589 Here's how you build a standard candle, here are the light curves of several 271 00:19:47,589 --> 00:19:52,580 supernovae. You can see that this one was considerably more luminous than that one. 272 00:19:52,580 --> 00:19:57,783 But, after making corrections for subtleties in the spectrum and the shape 273 00:19:57,783 --> 00:20:01,426 of the light curve, put them all on this one exact curve, 274 00:20:01,426 --> 00:20:06,662 this is brilliant, because this means, if you measure a type 1a supernova at its 275 00:20:06,662 --> 00:20:11,836 peak, then you know the luminosity of the object you measured and this is an object 276 00:20:11,836 --> 00:20:16,574 with a luminosity of a billion suns. So you can see these things way far off 277 00:20:16,574 --> 00:20:19,878 in the universe, these are measuring sticks for large 278 00:20:19,878 --> 00:20:24,865 distances and much of what we've learned about the structure of the universe at 279 00:20:24,865 --> 00:20:29,353 large distances is due to this understanding of type 1a supernova being 280 00:20:29,353 --> 00:20:33,266 the standard candle. We'll come back to that aspect. I was 281 00:20:33,266 --> 00:20:38,800 talking about the double generate model, these are recent observations that lend 282 00:20:38,800 --> 00:20:42,418 it some credence. the image on the left is a beautiful 283 00:20:42,418 --> 00:20:46,821 image of the remnant of a supernova. And what is observed is that we can 284 00:20:46,821 --> 00:20:51,774 pinpoint the direction in which we should have seen the white dwarf remnant the, 285 00:20:51,774 --> 00:20:56,483 the, the partner remnant had there been a partner, the white dwarf blew itself 286 00:20:56,483 --> 00:20:59,223 apart. The remnant should have been there. 287 00:20:59,223 --> 00:21:04,308 The, the other star, had it not been completely ripped to smithereens and 288 00:21:04,308 --> 00:21:07,292 sensitive searches have found nothing there. 289 00:21:07,292 --> 00:21:12,717 And even more exciting version of this is these x-ray data from the Swift 290 00:21:12,717 --> 00:21:18,141 satellite, where basically they took 53 type 1a supernovae and superposed them on 291 00:21:18,141 --> 00:21:23,295 top of each other, so that, so that their centers completely coincide and look for 292 00:21:23,295 --> 00:21:26,429 the x-ray signal. And what they find is nothing, 293 00:21:26,429 --> 00:21:31,017 in other words, were there a surviving partner to these supernovae, then that 294 00:21:31,017 --> 00:21:35,299 partner should have been heated to extremely high temperatures by the 295 00:21:35,299 --> 00:21:38,357 explosion. You would have expected an x-ray signal, 296 00:21:38,357 --> 00:21:41,660 there is none. What that means is that the partner was 297 00:21:41,660 --> 00:21:45,270 completely destroyed. these are both evidence, but not 298 00:21:45,270 --> 00:21:49,735 conclusive for the DD model. Computing the number of supernovae we see, 299 00:21:49,735 --> 00:21:55,656 comparing it to the probability of binary binary white dwarf pairs still suggests 300 00:21:55,656 --> 00:22:01,123 that the single degenerate model has to make a contribution to these amazing 301 00:22:01,123 --> 00:22:01,691 objects.