755 words · auto-generated from the episode video
11:08that is at the edge of the universe 13 billion lighty years away, how are we going to do it? Well, we use something called the Doppler broadening of spectral lines. Okay, effectively what's happening is this. We look at a certain spectra, let's say H alpha, which is a intrinsic piece of light that comes out of hydrogen. Okay? Now, if the hydrogen is not moving at all and it's static, that line is going to be really sharp. >> But in a black hole, there's going to be stuff moving around in an accretion disc, right? So, there's going to be hydrogen that's moving away from us and hydrogen that move that's moving towards us. So, there's going to be some bits of Halpha, the light that is going to be blueshifted. So the wavelength will be
11:50shorter and then there's going to be some bits that are redshifted because it's moving away and the wavelength is going to be longer. And what's that going to what that's going to cause is my H alpha line which used to be just a really well- definfined frequency. It's going to broaden. >> I see. >> Right. And that's called Doppler broadening. >> This is what we're seeing here. >> And that's what we're seeing here. We we get a nice Gausian profile of our spectral line. and that gausian profile from there we can then back calculate how fast does the gas need to be moving around to create that broadening right and then you can apply the varial theorem which is something in basic physics the mass is proportional to the velocity squared and from that you get something like a
12:30number that is 10 7 to 10 9* the mass of the sun so it's like you know a billion to 100 million tens of million times the mass of the sun this is for something that is moving at 2,000 km/s. That's what we can calculate back from the broadening is the velocity and from the velocity we calculate back the mass. >> Got it? >> Okay. So, we're getting 10 7 * or 10 the 9 mass of the sun >> as the as the scale of these of of >> how big these black holes should be. >> Should be, >> right? >> How much how big they should be. >> Okay. [snorts] So, you would think that okay, we're good, right? Like they're not galaxies, they're just black holes. >> Mhm. Not so fast. Okay, there's two
13:12problems with these masses. >> For one thing, these black holes are way too big for whatever galaxy this little red dot is. If this little red dot is a galaxy and at the center is this active galactic nuclei that's, you know, churning up a bunch of mass and then spewing it out as radiation, then those black holes are about 10 to 100% the host stellar mass. >> Mhm. [clears throat] >> Okay. In contrast, for our Milky Way, the center of the galaxy where the black hole is, that black hole is only 0.1% of the entire mass of the Milky Way. >> So, this is the math is not math. >> The math is not mathing. Okay. Sagittarius A star at the center of our
13:53Milky Way is only 0.1% of the total mass of the Milky Way. And here I'm getting 10% to 100% the total mass. Like the whole thing is a black hole. >> Black hole. They're way too massive. >> Yeah, >> they're Okay. >> It doesn't make any sense. >> It does. Okay. >> Okay. Yeah. >> The second thing is, okay, so you're you're telling me that these black holes are 100 million times the mass of the sun, right? How did it get that big? Well, the way that black holes get big is through accretion, right? It's like taking in mass and it's eating up mass. And as it eats up all this matter, it gets bigger and bigger. There's a caveat to it, though. Okay. It can't just like eat endlessly. It's not like those video games where you just eat the whole city
14:34around you and >> Yeah. In like a single Yeah. In like a single moment. You can't do that. You've got to take your time with your meal. That's the point. There's something called the Edington limit. Okay. And it says there's a maximum luminosity to accretion rate >> before I'm eating too much.