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.