1 00:00:00,000 --> 00:00:06,350 And now, now we have a sense of the beauty of black holes, 2 00:00:06,350 --> 00:00:09,574 is there any astronomy to this? Do they exist? 3 00:00:09,574 --> 00:00:12,794 Can we see them? Well technically, of course, we can see 4 00:00:12,794 --> 00:00:17,505 them cause no light gets out besides which the star has not yet collapsed past 5 00:00:17,505 --> 00:00:20,606 its horizon. But, black holes produce no light so you 6 00:00:20,606 --> 00:00:23,707 can't actually see a black hole because they're black. 7 00:00:23,707 --> 00:00:30,119 what we can see ss a very dense, massive object. So, we can see we need something 8 00:00:30,119 --> 00:00:32,968 to be orbiting it. Something to be influenced, 9 00:00:32,968 --> 00:00:36,324 to be nearby, to be influenced by the intense gravity 10 00:00:36,324 --> 00:00:42,208 of solar mass, or five, or ten solar mass object such that matter can orbit it 11 00:00:42,208 --> 00:00:46,232 within 300 kilometers. at that distance, you get very intense 12 00:00:46,232 --> 00:00:49,579 gravitational effects. And so if only something would orbit it, 13 00:00:49,579 --> 00:00:53,627 indeed, the way we discover black holes is when they are surrounded with 14 00:00:53,627 --> 00:00:56,488 something else. And for a stellar collapse black hole, 15 00:00:56,488 --> 00:01:01,400 that typically requires that our star was part of a closed binary so that there is 16 00:01:01,400 --> 00:01:04,518 stuff around from the other member of the prime binary. 17 00:01:04,518 --> 00:01:09,167 And the best way to have that happen is have a close binary with mass transfer. 18 00:01:09,167 --> 00:01:13,760 And then, we have mass transfer, then the matter falling onto the black hole forms 19 00:01:13,760 --> 00:01:18,126 the usual accretion disk where it waits to lose its angular momentum to jets 20 00:01:18,126 --> 00:01:22,208 driven by magnetic fields in the disk in the standard way that we don't quite 21 00:01:22,208 --> 00:01:24,759 understand but have grown familiar to saying. 22 00:01:24,759 --> 00:01:28,919 the interesting thing is this disc extends all the way into three shroud 23 00:01:28,919 --> 00:01:32,054 shaped radii. And there's nothing orbiting inside that, 24 00:01:32,054 --> 00:01:36,666 because there are no stable orbits. But three shrouts of radii from a several 25 00:01:36,666 --> 00:01:40,272 solar mass black hole, that's extremely intense gravitational 26 00:01:40,272 --> 00:01:43,169 field. There's intense compression and heating 27 00:01:43,169 --> 00:01:47,900 material is heated to millions of Kelvin. The radiation that it emits are x-rays. 28 00:01:47,900 --> 00:01:52,511 And so, the way to detect black holes, at least initially, was to look for x-ray 29 00:01:52,511 --> 00:01:55,349 sources. And one of the first discovered, and the 30 00:01:55,349 --> 00:01:59,074 most famous, is Cygnus X-1. You can guess what constellation that 31 00:01:59,074 --> 00:02:02,682 sits in. That's an x-ray binary which is some 32 00:02:02,682 --> 00:02:06,844 x-ray source in close binary orbit with a typo super giant. 33 00:02:06,844 --> 00:02:11,461 So, we can see one star and we can see from it's spectrum that it's orbiting 34 00:02:11,461 --> 00:02:16,200 something, and there are x-ray flickers that are clearly too energetic, too hot, 35 00:02:16,200 --> 00:02:21,356 and also too rapid to be emitted from anything as large and flabby as a type-O 36 00:02:21,356 --> 00:02:24,162 super giant. And using the standard Doppler 37 00:02:24,162 --> 00:02:28,992 measurement technology when you see a binary, spectroscopic binary, you can 38 00:02:28,992 --> 00:02:32,909 estimate the mass of this partner to the type-O super giant. 39 00:02:32,909 --> 00:02:38,130 And the mass estimates depending on how it's done, it's not a easy business, come 40 00:02:38,130 --> 00:02:43,156 in between ten and twenty solar masses. But there's, at least, ten solar masses 41 00:02:43,156 --> 00:02:46,550 in this partners. So, that's not very much for a star, 42 00:02:46,550 --> 00:02:51,428 maybe there's some dead star there that we don't see that has a dense star 43 00:02:51,428 --> 00:02:55,876 producing so many x-rays, and well maybe neutron stars produce x-rays. 44 00:02:55,876 --> 00:03:01,034 But a neutron star can't have a mass of bigger than two or maybe optimistically, 45 00:03:01,034 --> 00:03:05,095 three solar masses. What could produce x-rays and have a ten 46 00:03:05,095 --> 00:03:09,028 have ten solar masses. Well, what nails it is that these x-ray 47 00:03:09,028 --> 00:03:12,445 signals are seen to flicker on very small timescales. 48 00:03:12,445 --> 00:03:17,947 it flickers in as little as a millisecond thousandth of a second is the time it 49 00:03:17,947 --> 00:03:22,822 takes for these xray flickers to turn on and off. And that gives you a limit for 50 00:03:22,822 --> 00:03:27,343 the size of an object that can, that can, flicker in that time. Because for the, 51 00:03:27,343 --> 00:03:31,924 for some object to decide to turn on or off altogether, takes at least as long as 52 00:03:31,924 --> 00:03:35,741 it takes for light to traverse the object from side to side because no one 53 00:03:35,741 --> 00:03:38,592 information can transmit, can travel faster. 54 00:03:38,592 --> 00:03:43,740 So, if something is flickering in a millisecond, it cannot possibly be larger 55 00:03:43,740 --> 00:03:47,966 than 3,000 km across. And that's huge for a neutron star, but 56 00:03:47,966 --> 00:03:52,038 it's tiny for a star. You can't fit a star in 3,000 kilometers. 57 00:03:52,038 --> 00:03:56,110 You cannot even fit a good white dwarf in 3,000 kilometers 58 00:03:56,110 --> 00:04:00,825 and something has ten solar masses. The only option that we know is a black hole, 59 00:04:00,825 --> 00:04:05,894 and we have very good reason to believe that Cygnus X-1 and many x-ray sources 60 00:04:05,894 --> 00:04:10,550 that have been found since are in fact accretion disks around black holes. If 61 00:04:10,550 --> 00:04:13,910 there's a black hole around and nothing is falling on it, 62 00:04:13,910 --> 00:04:18,379 we really will not disco, detect it. And so, here is a nice recent Chandra 63 00:04:18,379 --> 00:04:23,431 Observatory x-ray image of Cygnus X-1. So what we're seeing is not the blue 64 00:04:23,431 --> 00:04:28,742 super giant, but the x-ray emissions from the accretion disk around the black hole. 65 00:04:28,742 --> 00:04:33,924 Which is not resolved, we're not seeing a disk, we're just seeing the x-ray image. 66 00:04:33,924 --> 00:04:38,717 this is the x-ray spectrum by energy of the photons and the peak is 67 00:04:38,717 --> 00:04:42,410 characteristic of ionized iron. And it's broadened by the, 68 00:04:42,410 --> 00:04:48,095 you would expect, intense pressures in the accretion disk around this very 69 00:04:48,095 --> 00:04:54,001 compact massive object. And then, for another astronomical fun black hole, this 70 00:04:54,001 --> 00:04:59,465 is a recently detected one in the Andromeda Galaxy, and this is a black 71 00:04:59,465 --> 00:05:05,224 hole that has x-ray flares. Its disk is apparently undergoing violent 72 00:05:05,224 --> 00:05:10,909 changes, and periodically its x-ray luminosity increases. And, for the first 73 00:05:10,909 --> 00:05:14,380 time, the radio image over here on the right 74 00:05:14,380 --> 00:05:21,086 is the, the contours in the middle are detecting for the first time. The radio 75 00:05:21,086 --> 00:05:26,975 emission of gases, compressed, and heated, and glowing by the, the, 76 00:05:26,975 --> 00:05:33,764 collision of the polar jets that come out of this accretion disk which we expect to 77 00:05:33,764 --> 00:05:39,805 happen when you have a black hole. So essentially, where black holes, 78 00:05:39,805 --> 00:05:44,522 stellar mass black holes are not common because large stars, massive enough to 79 00:05:44,522 --> 00:05:49,420 produce the massive cores that collapse into black holes are rare in the stellar 80 00:05:49,420 --> 00:05:53,834 population, but in a galaxy with a trillion stars, there's black holes all 81 00:05:53,834 --> 00:05:56,918 over the place. We see the ones that happen to have 82 00:05:56,918 --> 00:06:01,271 binary partners or some source for matter from which they are accreting. 83 00:06:01,271 --> 00:06:04,960 And as far as we know, there's no mass limit for a black hole. 84 00:06:04,960 --> 00:06:11,348 In fact, we see black holes with masses of ten, five, twenty, a hundred solar 85 00:06:11,348 --> 00:06:14,580 masses. And then, there's a gap and then we see 86 00:06:14,580 --> 00:06:18,684 big holes with masses of millions up to billions of solar masses. 87 00:06:18,684 --> 00:06:24,177 And the issue of intermediate max black holes, things that have masses between 88 00:06:24,177 --> 00:06:28,786 say, a hundred solar masses and a million solar masses is a brand new thing. 89 00:06:28,786 --> 00:06:34,216 We think we have some recent discoveries but people are still trying to understand 90 00:06:34,216 --> 00:06:38,700 where these objects come from. They certainly didn't start out as stars. 91 00:06:38,700 --> 00:06:42,525 Neither did these million solar massive black holes. 92 00:06:42,525 --> 00:06:46,072 Where do they live? Well, they live in the center of 93 00:06:46,072 --> 00:06:49,550 galaxies. The image here over on the right is a 94 00:06:49,550 --> 00:06:55,671 beautiful movie taken of, from images over a few years of the proper motions of 95 00:06:55,671 --> 00:07:00,679 stars near the center of the galaxy, near a famous x-ray source known as 96 00:07:00,679 --> 00:07:06,596 Sagittarius A Star in the constellation Sagittarius which is the direction to the 97 00:07:06,596 --> 00:07:11,272 center of the galaxy from us. And it's amazing, you can see stars 98 00:07:11,272 --> 00:07:16,787 faraway from the center barely move but the stars around the center are really 99 00:07:16,787 --> 00:07:20,277 whipping. You can use just their motion to measure 100 00:07:20,277 --> 00:07:26,210 using Kepler's laws the mass that the object are orbiting, and you find some 101 00:07:26,210 --> 00:07:30,887 tens of millions or 10 million or a few million solar masses. 102 00:07:30,887 --> 00:07:36,351 the size of the object is limited. If only by the size of these orbits it is 103 00:07:36,351 --> 00:07:41,730 clear that it is a very compact object. We also see jets and x-ray ignitions from 104 00:07:41,730 --> 00:07:46,532 a big accretion disk around it. we are absolutely confident that at the 105 00:07:46,532 --> 00:07:51,468 center of the milky way galaxy sits a relatively minor four to ten solar mass 106 00:07:51,468 --> 00:07:54,880 black hole. the black hole in the center of N-31, the 107 00:07:54,880 --> 00:07:58,849 Andromeda galaxy, is probably ten times at least more massive. 108 00:07:58,849 --> 00:08:04,245 And so and is more characteristic black holes of masses betwe- of 100 million up 109 00:08:04,245 --> 00:08:09,145 to a billion solar masses have been found and seemed to be in the center of most 110 00:08:09,145 --> 00:08:12,556 massive galaxies. When we talk next week about galaxies, 111 00:08:12,556 --> 00:08:18,483 we'll ask the important question, does a black hole collect a galaxy around it? Or 112 00:08:18,483 --> 00:08:25,299 does a galaxy produce a black hole? The answer to that like many questions 113 00:08:25,299 --> 00:08:28,708 about galactic evolution is, maybe.