Presentation: denser and denser-er

Before we begin, a recap on the stellar remnants from two previous presentations on the lives and deaths of medium-mass stars and massive stars.

A white dwarf is the remnant of a medium-mass star, but to scale, it is much larger than the two possible remnants of a massive star--a neutron star, and a black hole, which actually has zero size (but is surrounded by an event horizon, which we'll discuss later).

However, while the white dwarf is the largest of these stellar remnants, it is not the most massive--a neutron star is more massive, and a black hole is more massive still.

All of these stellar remnants are incredibly dense, but which stellar remnant is densest? Least dense?

Since we have already discussed the behavior of dense white dwarfs (isolated or in close binary star systems) in a previous presentation, we will look at the other denser and "denser-er" stellar remnants--neutron stars and black holes. ("Dense and 'densibility?'")

The two themes of this presentation will require us to tell a mystery story, where evidence required an explanation, and later to invert this narrative, where an explanation required evidence.

First, neutron stars, a mystery story with unexpected evidence requiring an explanation.

Imagine being in graduate school in astronomy, if you were Jocelyn Bell in the 1960s. Suppose she was given instructions to show up early on the first day of class with all the other students, in a muddy field behind campus, wearing old clothes. She was given gloves, wire cutters, and a sledgehammer, and was directed to a massive pile of wooden stakes and spools of cable. "Welcome to astronomy graduate school--your research project will be radio astronomy--and you will be building the school's first radio telescope."

After weeks of pounding in wooden stakes and stringing up cables to form a radio telescope mesh, Jocelyn Bell got down to actual radio astronomy, listening to whatever radio signals were detected and noting anything unusual or strange. This is the actual printout of an interesting repeating radio signal she noticed...

...and this radio signal, when converted into sound sounds not unlike a pulse. (This is not the same signal Jocelyn Bell actually recorded, but from a similar source, with a slightly more rapid pulse.) (Audio link: "B1933+16_ALFA.wav.")

These mysterious signals--named "pulsars"--happens to repeat itself at very precise intervals. One early theory was that such radio signals were a type of extraterrestrial timekeeping or navigational beacon system, which inspired a brand of watches. (Which James Bond movie featured a Pulsar^TM watch?)

Not all of these radio signals sounds like pulsing heartbeats--this is a signal from a much more rapidly repeating pulsar, again converted into sound. (Audio link: "B0531+21_430MHz.wav.")

So instead of timekeeping or navigational beacons, a more plausible explanation for what would produce these slow or fast repeating radio signals involved literally pulsing stars. A big star might slowly expand and contract, while a small star would expand and contract much more rapidly. While some medium-mass stars do become unstable as giants, expanding and contracting over days or weeks, this explanation cannot explain a radio signal that repeats every few seconds, or up to hundreds of times per second, no matter how small the star.

However, a star cannot expand and contract every few seconds or faster, but it is plausible for it to spin every few seconds or faster. This is the "ice skater effect" discussed in a previous presentation, where a n object with a modest rotation rate will speed up if its size decreases. Recall that the core of a massive star will begin to collapse and implode at the end of its supergiant phase, such that the monthly rotation rate of a massive star core will speed up to as much as hundreds of rotations per second. (Video link: "World Record Figure Skating Spin.")

This speed-up in rotation is one key part of solving the pulsar mystery, where the pulsing radio signals are due to radio waves being emitted in certain directions from a rapidly rotating collapsed supergiant core. In this "lighthouse model," which beams in certain directions, the rotation--whether slow or fast--determines the interval between signals. (Video link: " Beacon, San Luis Obispo County Regional Airport, CA .")

The other key part is to recognize that as the rotation rate speeds up for the collapsing supergiant core, its magnetic fields are also being more concentrated. These strong magnetic fields, with north and south poles, capture stray charged particles, forcing them to emit radio waves in certain directions. No one initially expected that the radio pulse evidence would lead there, but neutron stars turn out to be the answer to the mystery of pulsars. (Video link: "Gamma Rays in Pulsars.")

Second, black holes, a story of "Great Expectations," where instead of evidence requiring an explanation, the explanation came first, and then evidence later.

More than 200 years ago a great "what-if" question was posed. Consider an object with a certain amount of mass. Due to its gravitational forces, something must be thrown at or more than the escape velocity of the massive object in order to move away without eventually being pulled back in. More massive object, stronger gravitational forces, and a faster escape velocity. What if a massive object's gravitational forces were so strong, that the escape velocity requirement was such that not even light would be fast enough? This was termed a "darkstar," where if the gravitational forces prevented light from escaping, then you wouldn't be able to see this massive object. Back in the 1700s there was no evidence for such a "darkstar," but its properties were relatively well understood.

We know "darkstars" today as black holes, and we'll briefly consider the evidence gathered for the existence of these objects later, in order to focus on this common (mis)conception of black holes as a funnel-like entity that sucks everything into itself. (Video link: "The Black Hole—The Glory Hole.")

This shape is not something you can literally "see" (not that you would be able to see a black hole, as no light can escape from it), but it is something that you can "feel." All objects distort and curve space-time around themselves, and if you're far away from the distorted space-time around massive objects, where space-time is flat (in this crude two-dimensional model), you won't "see" this flatness, but you'll "feel" its flatness because you would not experience any gravitational forces. If you were near a massive object's distorted space-time, again you wouldn't be able to "see" this puckering, but you would "feel" it because your motion would tend to slide down curved space-time, which is the perception of gravitational forces. Note the space-time depressions around stars, but also the funnel-shaped distortion caused by something that doesn't seem to be there at all--this is the effect of a black hole on space-time. Remember that black holes can't be seen, but its effects on space-time--its gravitational field--can definitely be felt.(Video link: "Black hole deforms space.")

So what would it be like to get close to a black hole, in the presence of its distorted space-time, and try to enter it?

Your textbook discusses tidal effects and spaghettification, and time dilation effects in more detail, but let's try to model how an object would stretch out while circling closer and closer to a black hole, while apparently taking an infinite amount of time (as observed from afar) to circle and enter the event horizon--the point of no return around itself--where not even light can escape.

Here's that same funnel shape representing distorted space-time caused by a black hole. We'll throw in some marbles, taking care to throw them in a tight clump. At first, the clump of marbles will begin slowly spread out, but after circling the "black hole" the marbles will spread out from each other, forming a long line--spaghettification--due to this funnel shaped distortion of space-time!. Although we can't really show the effect of time dilation, there is a crude analog to this from the nature of this funnel shaped distortion of space-time, as it seems like it takes longer and longer for the marbles to get down further and further into the throat of the funnel. (Video link: " Gravity Well (Reuben H. Fleet Science Center, San Diego, CA).")

So next time you see a charity donation funnel, try this for yourself--don't just roll in one coin, toss in several very closely bunched together, and watch the tidal effects stretch them out, and think about time dilation effects as they seem to take longer and longer to get further and further down the "throat" of curved space-time!

The evidence for black holes, even though we can't "see" them--is to "feel" for them, by looking at the effect of their gravity--their distorted space-time--on nearby companion stars. (Video link: "Black hole and companion star.")

In the subsequent in-class activity you will distinguish between companion stars with compact objects--whether white dwarfs, neutron stars, or black holes.