Presentation: denser and denser-er (revised)

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?'")

First, dense neutron stars.

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 trace of an interesting repeating radio signal she noticed (along with an audio recording of a similar signal). (Video link: "2-10-denseanddenser-discovery.")

These mysterious signals--named "pulsars"--repeat at very precise intervals. The best model we have for explaining these pulsing radio signals is this "lighthouse model," which sends out light beams in certain directions, and the rotation--whether slow or fast--determines the interval between signals. (Video link: "Beacon, San Luis Obispo County Regional Airport, CA.")

Recall that the core of a massive main-sequence star will begin to collapse and implode at the end of its supergiant phase, crushing itself into a neutron star, concentrating its magnetic fields. 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: "The Oblique Rotator Model for a Pulsar.")

Second, "denser-er" black holes.

This is the main outlet for water in Lake Berryessa water to pass through the Monticello Dam, and is a fair analog for a black hole. If you were swimming far from this dangerous portal, you wouldn't notice its presence, but if you were unfortunate to find yourself near it, you would definitely "feel" its presence. Provided you could swim fast enough, you will can make it back to safety, but there is an imaginary boundary around it at which you could not escape, no matter how fast you can swim--this boundary, or point-of-no-return, can be said to be its "event horizon." (Video link: "The Black Hole—The Glory Hole.")

Now instead of funnels and water, consider stars and black holes and space-time. 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 this curved space-time towards the object--which is how gravitational forces work.

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. And like the Lake Berryessa water outlet, you can get near it and get back out to safety provided you can move quickly enough, but there is an imaginary boundary around it at which you could not escape, no matter how fast you can move--this boundary, or point-of-no-return, for everything including light is the black hole's "event horizon." (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.