0:00

Meanwhile, on the optical

Â frontier, the hunt was on

Â for the optical counterpart

Â of our x-ray source. The

Â difficulties were enormous. Centaurus is in a heavily obscured part

Â of the Milky Way galaxy, and remember, our early

Â error boxes for locations of sources were quite crude.

Â So despite our detailed understanding of the orbit based on the x-ray data,

Â it was three years before we found this object optically.

Â But the discovery was unmistakable. A Polish astronomer

Â Voytek Krisminsky, found a hot, massive star

Â that changed its brightness ever so slightly every

Â 2.09 days, in lockstep with

Â Cen X-3's x-ray clips. Why such a small variation?

Â Well maybe Cen X-3 is very small, so during

Â its orbit, Krisminsky's star's brightness doesn't change very much.

Â Indeed, the situation is somewhat more complicated,

Â but it's a good starting point, based

Â on our understanding that GK Per had

Â a white dwarf for it's x-ray emitting component.

Â Come to think of it, let's check to see if

Â a white dwarf can be responsible for Cen X-3 x-ray output.

Â If it's a white dwarf spinning around every 4.8 seconds, we

Â know that its speed on the equator must be v equals

Â 2 pi r over T. Where

Â now, r, let's make this look a little bit more like an r, in fact,

Â 2:08

let's do that. Where now r is the radius of a

Â white dwarf, about 6 times 10 to the 8 centimeters,

Â 6 times 10 to the 8 centimeters,

Â and T now, is 4.8 seconds.

Â 2:39

and it does so in 4.8 seconds, so the velocity

Â just distance divided by time. And in other words, we have an Earth

Â sized object, spinning around every 4.8 seconds.

Â 2:55

If we solve this for V,

Â we get V equaling about 8 times 10 to the 8th,

Â centimeters per second.

Â So the acceleration we would feel, trying

Â to pull us off the star, due to it's rotation, would

Â be a equals v squared over r.

Â We know what v is, we know what r is, so we

Â get about 64 times 10 to the

Â 16 divided by 6 times 10

Â to the 8, and the units on that are

Â centimeters per second squared.

Â This is an acceleration of about 10

Â to the 9 centimeters per

Â second squared. Is gravity up to this task?

Â Can it pull us in from the surface of a white dwarf

Â with enough acceleration so that we won't fly off due to the

Â acceleration that is trying to get us to go away from the center of the star?

Â Let's see what a one solar mass white dwarf has for its

Â gravitational acceleration. So we have a, due to gravity

Â equalling GM over r squared.

Â And that's about equal to 6.7 times 10

Â to the minus 8 times, that's G.

Â Our one solar mass white dwarf has 2

Â times 10 to the 33 grams associated

Â with it. And our distance is 6

Â times 10 to the 8 centimeters squared. And

Â again, our units are going to be centimeters per second squared.

Â 5:17

We do this math, and we get a number 4 times 10 to the

Â 8 centimeters per second squared.

Â Uh-oh, we're in trouble.

Â This number is smaller than this

Â number. Gravity cannot prevent a white dwarf

Â from just flying apart due to its spin.

Â What do we do now? Well,

Â fortunately, since Jocelin Bell's discovery of the radio pulsars a few

Â years before, we had a fairly good idea about what the solution might be.

Â Neutron stars, a new type of object in the cosmos,

Â or at least our understanding of it, would fit the bill.

Â 6:15

This story actually begins with the

Â white dwarfs, and with Subrahmanyan Chandrasekhar,

Â a truly wonderful man with whom I was

Â fortunate enough to spend several days when I

Â was a graduate student at Princeton, and incidentally

Â after whom the Chandra x-ray Satellite was named.

Â In 1930, Chandra, at the age of 19,

Â discovered that there was a limit to the size of a white dwarf.

Â If such a star had a mass in excess of about 1.4 solar

Â masses, it simply could not resist the pull of gravity, and must collapse.

Â 7:03

This was initially met with derision on the part of most astronomers.

Â Where would the star go? Ha, ha.

Â Would it disappear?

Â Hah, ha, ha.

Â Well the universe seems to be ultimately impervious to prejudice.

Â Just four years after Chandra put forward his bold idea, Walter

Â Baade and Fritz Zwicky proposed the existence of a neutron star.

Â If the white dwarf had to collapse, the electrons would

Â be the first to go, and the star would consist

Â essentially of a giant nucleus. The neutron had just been

Â discovered a year earlier by James Chadwick, so slamming an electron

Â into a proton to form a neutron seemed at least plausible.

Â 7:54

But, wasn't this preposterous?

Â Nuclear densities without the support of the

Â vast empty space between it and the electrons

Â would mean that a teaspoon full of this star, remember

Â our teaspoon? Remember our white dwarf?

Â 8:33

What does it mean? It's a big number.

Â Well, a typical African elephant weighs about

Â 5,000 kilograms, so our teaspoon-full of

Â this neutron star stuff has the same mass as about

Â 100 million African elephants. Absolutely

Â astounding.

Â But how does this solve our Cen X-3 problem?

Â Well, the radius of this star, according to a calculation that you can and should

Â do by simply scaling up a neutron to the mass of our star, would be

Â about 10 kilometers, about half the length of Manhattan island.

Â Oh, I know that Manhattan isn't circular or

Â spherical, but you get the idea, right? So let's imagine a two

Â solar mass neutron star, the radius of which is

Â 10 kilometers. Would gravity be sufficient to

Â hold this star together? Let's find out.

Â With a period of 4.8 seconds and a radius of 10 kilometers, the speed

Â of a test mass on the equator of the star would have the following

Â velocity: V equals 2 pi r

Â over T, where now T is 4.8

Â seconds, and r is 10 kilometers.

Â 2 pi is about six. And that turns out to

Â 10:21

be 1.3 times 10 to the 6 centimeters

Â per second. So, our

Â acceleration,

Â v squared over r, would

Â be 1.3 times 10 to the 6,

Â squared, divided by 10 to the

Â 6 centimeters for the radius of the star

Â 11:32

GM over r squared, 6.7 times

Â 10 to the minus 8 times 4 times 10 to the 33

Â grams, 2 solar masses, divided by r squared which

Â is 10 to the 12 also, centimetres per second squared.

Â And this turns out to be approximately

Â 2.7 times 10

Â to the 14 centimetres per second squared! Yes,

Â no problem. The acceleration due to gravity is

Â much, much stronger than the acceleration trying

Â to get the star to fly apart. This

Â is no problem, but an astonishing number anyway.

Â What this means is that if an object were dropped at a distance of one meter

Â from the surface of such a neutron star, it would hit the surface

Â in about a millionth of a second, and it would arrive

Â at the surface traveling almost 3,000 kilometers

Â per second. Calculate what that is in miles per hour.

Â 13:00

But what exactly is it that provides our clock?

Â It turns out that the most likely possibility

Â is similar to what causes aurorae on the Earth.

Â In that case, charged particles

Â from the sun are funneled into the magnetic polar regions of our planet

Â where they collide with each other and give rise to spectacular light show.

Â We believe that this is likely to happen in

Â the outer reaches of a neutron star's environment as well.

Â Material form the companion star is concentrated to,

Â by the magnetic field of the neutron star and

Â this, coupled with the intense gravitational field,

Â heats up the gas to incredible temperatures.

Â If the rotational axis is inclined to the magnetic field, similar

Â to the way that the Earth's magnetic poles are not coincident

Â with the geographic North and South Poles, we have a natural

Â way to provide a beacon of light that we can see every

Â 4.8 seconds. Essentially, the magnetic poles become hot

Â spots that radiate copious amounts of x-rays because of the

Â tremendous amount of gravitational energy that can be converted into light.

Â And you can see in a little cartoon what that's likely to look like.

Â 14:37

Now, let's go one step further. Let's revisit

Â Cen X-3, 15 years after

Â the EXOSAT observation we

Â just examined, and see what

Â Chandra can add to the picture.

Â