And now, now we have a sense of the 
beauty of black holes, 
is there any astronomy to this? 
Do they exist? 
Can we see them? 
Well technically, of course, we can see 
them cause no light gets out besides 
which the star has not yet collapsed past 
its horizon. 
But, black holes produce no light so you 
can't actually see a black hole because 
they're black. 
what we can see ss a very dense, massive 
object. So, we can see we need something 
to be orbiting it. 
Something to be influenced, 
to be nearby, 
to be influenced by the intense gravity 
of solar mass, or five, or ten solar mass 
object such that matter can orbit it 
within 300 kilometers. 
at that distance, you get very intense 
gravitational effects. 
And so if only something would orbit it, 
indeed, the way we discover black holes 
is when they are surrounded with 
something else. 
And for a stellar collapse black hole, 
that typically requires that our star was 
part of a closed binary so that there is 
stuff around from the other member of the 
prime binary. 
And the best way to have that happen is 
have a close binary with mass transfer. 
And then, we have mass transfer, then the 
matter falling onto the black hole forms 
the usual accretion disk where it waits 
to lose its angular momentum to jets 
driven by magnetic fields in the disk in 
the standard way that we don't quite 
understand but have grown familiar to 
saying. 
the interesting thing is this disc 
extends all the way into three shroud 
shaped radii. 
And there's nothing orbiting inside that, 
because there are no stable orbits. 
But three shrouts of radii from a several 
solar mass black hole, 
that's extremely intense gravitational 
field. 
There's intense compression and heating 
material is heated to millions of Kelvin. 
The radiation that it emits are x-rays. 
And so, the way to detect black holes, at 
least initially, was to look for x-ray 
sources. 
And one of the first discovered, and the 
most famous, is Cygnus X-1. 
You can guess what constellation that 
sits in. 
That's an x-ray binary which is some 
x-ray source in close binary orbit with a 
typo super giant. 
So, we can see one star and we can see 
from it's spectrum that it's orbiting 
something, and there are x-ray flickers 
that are clearly too energetic, too hot, 
and also too rapid to be emitted from 
anything as large and flabby as a type-O 
super giant. 
And using the standard Doppler 
measurement technology when you see a 
binary, spectroscopic binary, you can 
estimate the mass of this partner to the 
type-O super giant. 
And the mass estimates depending on how 
it's done, it's not a easy business, come 
in between ten and twenty solar masses. 
But there's, at least, ten solar masses 
in this partners. 
So, that's not very much for a star, 
maybe there's some dead star there that 
we don't see that has a dense star 
producing so many x-rays, and well maybe 
neutron stars produce x-rays. 
But a neutron star can't have a mass of 
bigger than two or maybe optimistically, 
three solar masses. 
What could produce x-rays and have a ten 
have ten solar masses. 
Well, what nails it is that these x-ray 
signals are seen to flicker on very small 
timescales. 
it flickers in as little as a millisecond 
thousandth of a second is the time it 
takes for these xray flickers to turn on 
and off. And that gives you a limit for 
the size of an object that can, that can, 
flicker in that time. Because for the, 
for some object to decide to turn on or 
off altogether, takes at least as long as 
it takes for light to traverse the object 
from side to side because no one 
information can transmit, 
can travel faster. 
So, if something is flickering in a 
millisecond, it cannot possibly be larger 
than 3,000 km across. 
And that's huge for a neutron star, but 
it's tiny for a star. 
You can't fit a star in 3,000 kilometers. 
You cannot even fit a good white dwarf in 
3,000 kilometers 
and something has ten solar masses. The 
only option that we know is a black hole, 
and we have very good reason to believe 
that Cygnus X-1 and many x-ray sources 
that have been found since are in fact 
accretion disks around black holes. If 
there's a black hole around and nothing 
is falling on it, 
we really will not disco, detect it. 
And so, here is a nice recent Chandra 
Observatory x-ray image of Cygnus X-1. 
So what we're seeing is not the blue 
super giant, but the x-ray emissions from 
the accretion disk around the black hole. 
Which is not resolved, we're not seeing a 
disk, we're just seeing the x-ray image. 
this is the x-ray spectrum by energy of 
the photons and the peak is 
characteristic of ionized iron. 
And it's broadened by the, 
you would expect, intense pressures in 
the accretion disk around this very 
compact massive object. And then, for 
another astronomical fun black hole, this 
is a recently detected one in the 
Andromeda Galaxy, and this is a black 
hole that has x-ray flares. 
Its disk is apparently undergoing violent 
changes, and periodically its x-ray 
luminosity increases. And, for the first 
time, the radio image over here on the 
right 
is the, the contours in the middle are 
detecting for the first time. The radio 
emission of gases, compressed, and 
heated, and glowing by the, the, 
collision of the polar jets that come out 
of this accretion disk which we expect to 
happen when you have a black hole. 
So essentially, where black holes, 
stellar mass black holes are not common 
because large stars, massive enough to 
produce the massive cores that collapse 
into black holes are rare in the stellar 
population, but in a galaxy with a 
trillion stars, there's black holes all 
over the place. 
We see the ones that happen to have 
binary partners or some source for matter 
from which they are accreting. 
And as far as we know, there's no mass 
limit for a black hole. 
In fact, we see black holes with masses 
of ten, five, twenty, a hundred solar 
masses. 
And then, there's a gap and then we see 
big holes with masses of millions up to 
billions of solar masses. 
And the issue of intermediate max black 
holes, things that have masses between 
say, a hundred solar masses and a million 
solar masses is a brand new thing. 
We think we have some recent discoveries 
but people are still trying to understand 
where these objects come from. 
They certainly didn't start out as stars. 
Neither did these million solar massive 
black holes. 
Where do they live? 
Well, they live in the center of 
galaxies. 
The image here over on the right is a 
beautiful movie taken of, from images 
over a few years of the proper motions of 
stars near the center of the galaxy, 
near a famous x-ray source known as 
Sagittarius A Star in the constellation 
Sagittarius which is the direction to the 
center of the galaxy from us. 
And it's amazing, you can see stars 
faraway from the center barely move but 
the stars around the center are really 
whipping. 
You can use just their motion to measure 
using Kepler's laws the mass that the 
object are orbiting, and you find some 
tens of millions or 10 million or a few 
million solar masses. 
the size of the object is limited. 
If only by the size of these orbits it is 
clear that it is a very compact object. 
We also see jets and x-ray ignitions from 
a big accretion disk around it. 
we are absolutely confident that at the 
center of the milky way galaxy sits a 
relatively minor four to ten solar mass 
black hole. 
the black hole in the center of N-31, the 
Andromeda galaxy, is probably ten times 
at least more massive. 
And so and is more characteristic black 
holes of masses betwe- of 100 million up 
to a billion solar masses have been found 
and seemed to be in the center of most 
massive galaxies. 
When we talk next week about galaxies, 
we'll ask the important question, does a 
black hole collect a galaxy around it? Or 
does a galaxy produce a black hole? 
The answer to that like many questions 
about galactic evolution is, maybe.