Kind: captions
Language: en
In 1970, Paul Murden is a young English
astronomer trying to secure his next
job.
>> I was a research fellow. I was coming to
the end of my three-year contract, and I
thought, what can I contribute to
finding out what these things are?
>> Using the largest telescope in England,
he begins searching the area of the
constellation Signis, the Swan.
He decides to hunt for pairs of stars.
Pairs of stars are called binaries.
Many of the stars we see, perhaps half,
are actually binaries, pairs of orbiting
stars locked together by gravity.
But Murden wonders, is it possible there
are binaries where only one of the stars
is visible?
I thought that maybe there was a kind of
a star system in which there was a star,
an or one ordinary star that made light
and then there was another star nearby
that made X-rays.
>> The telltale sign of a binary is that
the stars are moving around each other.
So Murdan begins searching for a visible
star that shows signs of motion.
>> Sometimes it's coming towards you,
sometimes it's coming away. Sometimes
it's coming towards you, sometimes it's
coming away.
When the star is moving toward us, it
appears more blue.
As the wavelength of its light gets
shorter.
Moving away, it appears more red as the
wavelength of its light gets longer.
This is known as Doppler shift.
After looking for color changes in
hundreds of stars in the area of Signis,
Merden spots a possible suspect. A
visible star whose light is shifting as
though moving around.
It very clearly was a binary star, a
double star.
The star was moving around and around
with a period going round once every 5.6
days.
But whatever it's going around can't be
seen.
>> There was no trace in the spectrum of
the second star. There was one star
there. There wasn't the second star
there.
>> Murden has a binary pair in which only
one star is visible. The second object
emits X-rays, has enough mass and
gravity to dramatically move a star, but
gives off no light.
Could it be the corpse of a star massive
enough to become a black hole?
>> The crucial issue in deciding whether
Signis X1 was a black hole was to
measure the mass of the X-ray emitting
object.
>> It would have to be very massive, at
least three times the mass of our sun.
If not, it's probably just a neutron
star,
a collapsed star that's dense, but not
heavy enough to be a black hole. From
his observations, Murden is able to make
an estimate of the mass of the invisible
partner.
>> And the answer came out to be six times
the mass of the sun.
So there was a story then that Signis X1
was a black hole. And the key to the
argument was that the mass of the star
you couldn't see was more than three
solar masses.
When I'd finished writing it all out, I
sat back and thought, "It's a black
hole.