Date: July 14th, 2012
Title: Encore: The Most Massive Neutron Star
Podcaster: Dr. Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
This podcast originally aired on November 22, 2010
Description: A few weeks ago, there was a paper published about the discovery of the most massive known neutron star. But what is a neutron star, anyway? And how do these folks know that this neutron star is so massive? And why is that important?
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
Today’s Sponsor: This episode of 365 Days of Astronomy is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.”
I am Rob Knop, professor of physics at Quest University Canada.
A few weeks ago, there was a paper published by Paul Demorest, with collaborators Pennucci, Ransom, Roberts, and Hessels; this paper came with corresponding press releases, and was about the discovery of the most massive known neutron star. But what is a neutron star, anyway? And how do these folks know that this neutron star is so massive? And why is that important?
A neutron star is what we call a “stellar remnant”. It’s what is left behind by a star that has died. During their life, stars are powered by nuclear fusion. The Sun is a normal star. The core of the Sun is at a temperature of 15 million degrees Celsius. Even though it’s a gas– really, a plasma, which is a gas of charged particles, including electrons and ions– its density is about 100 times the density of water. Most of the gas at the core of the Sun is made up of Hydrogen. At these tremendous temperatures and densities, Hydrogen nuclei– otherwise known as protons– are able to overcome their electrical repulsion, and come close enough together for nuclear forces to take over, allowing nuclear fusion to happen. The process of fusion ultimately combines together Hydrogen nuclei to make Helium nuclei. In the process, a tremendous amount of energy is released. We have harnessed these forces ourselves, to make H-bombs, which are even more powerful than the atomic bombs used in World War II. Fortunately, we’ve only used these H-bombs ourselves in tests. The Sun, however, is doing fusion at a much faster rate than anything we’ve accomplished. That nuclear fusion is what powers the Sun, allowing it to shine, which in turn is ultimately what allows life to continue on the surface of the Earth.
Without fusion at its core, gravity would cause the gaseous Sun to collapse. The tremendous gravity of the Sun is always trying to make it compress in further on itself. Counteracting that gravity is the pressure of the gas, resulting from the temperature of the gas. That temperature is maintained by the continual input of energy by fusion from the core of the Sun.
Later in life, the Sun will become a red giant, as the details of the fusion occurring at its core change. Eventually, a few billion years from now, instead of just making Helium, the Sun will be fusing Helium together to make Carbon. That’s as far as it will go, however. When enough Carbon builds up in the core of the Sun, it will collapse under its own gravity, until it’s held up by basic quantum mechanics– the electrons are in a state we call “degenerate”, which, roughly speaking, means that they are packed together absolutely as close as they can be. In the late stages of the Sun’s evolution, it will shed its outer layers, leaving behind its core as a white dwarf. A white dwarf is a dead star, inert, usually made of Carbon, or Carbon and Oxygen, that sits there for the rest of time, slowly cooling off. Held up by electron degeneracy pressure, it typically has a mass about half that of the Sun, and a size comparable to the size of the Earth. This is an unimaginably dense object, but it’s by no means the most dense object we know of. A white dwarf is what a star less massive than eight solar masses leaves behind after it dies.
The answer to the questions “what is even more dense than a white dwarf?” and “what do stars more massive than eight times the mass of the Sun leave behind?” is one and the same: a neutron star. If a star is more massive than eight times the mass of the Sun, it is able to compress its core enough to trigger even Carbon, Oxygen, and other elements to undergo nuclear fusion. Eventually, in the late stages of its life, a massive star will fuse elements all the way up to iron at its core. However, it won’t perform fusion of iron to anything else. You can get energy out by fusing lighter elements to make iron, but it costs you energy to fuse iron to even heavier elements. As such, at the end of a massive star’s life, an electron degenerate iron core– something like a white dwarf, only made of iron and still surrounded by the rest of the star– builds up at the core. There is an upper limit for the mass of this core, however, called the Chandrasekhar mass; that mass is 1.4 times the mass of the Sun. When the core reaches this mass, electron degeneracy pressure is no longer able to hold it up. The core collapses rapidly under gravity, and the energy released by this collapse is what powers the supernova that marks the death of a massive star.
As the core collapses, electrons combine together with protons to make neutrons. The end result of the process is a neutron star. It’s a star that’s somewhere between one and two times the mass of the Sun, but only ten to fifteen kilometers in radius. It’s more massive than the Sun, but the size of a mere city. If you thought a white dwarf was dense, then it’s probably difficult to describe the density of a neutron star without echoing the introductory theme song of 365 Days of Astronomy– it’s “like uberdense”!
Neutron stars also tend to have very large magnetic fields. All of the magnetic field from the core of the star is now squished together in the very small size of the neutron star, making it much more powerful. Likewise, neutron stars tend to be spinning very rapidly, for the same reason that ice skaters spin faster when they pull in their arms and legs. The angular momentum from the star’s core is now in a much smaller volume, so that volume must be spinning faster. When you put these two facts together, you get a pulsar. The magnetic fields collect together charged particles and funnel them towards the magnetic poles of the neutron star, where bright jets of particles and radiation shoot out. If the magnetic poles are misaligned with the rotation, then as the neutron star rotates it will point its beams of radiation in different directions, much like a lighthouse. From our point of view, we’ll see a pulse every time one of the beams is pointing at us.
So how do you measure the mass of a neutron star? Measuring the mass of stars in general can be tricky. The only way you can really measure it at all is if something is orbiting around the star. We can measure the masses of stars in binary star systems, or if there are planets orbiting the star. This does leave some ambiguity, though, for if the plane of the orbits is inclined relative to us, rather than edge-on, we don’t measure the mass directly, but merely set a lower limit on the mass. What’s more, in a binary star system, if we can’t separately measure something such as orbital velocities from both stars, it can be difficult to disentangle the two masses of the stars.
There are known binary pulsar systems, where two pulsars orbit around each other; these typically represent the only precisely measured neutron star masses, and from them we knew that neutron stars could get up to about one and a half times the mass of the Sun. We make these measurements by timing the pulses from the pulsar. When the pulsar is in the phase of its orbit such that it’s coming towards us, the pulses will arrive closer together, as each subsequent pulse has a slight “head start” over the previous pulses. When the pulsar is moving away from us, the pulses will arrive further apart. This allows us to measure the velocity of the pulsar, and from that we can deduce something about the mass of it and its companion.
The neutron star measured with a radio telescope by Demorest and colleagues is actually orbiting a white dwarf star. Because the white dwarf itself doesn’t pulse, it’s much harder to get a speed directly for the white dwarf, so from timing of the pulses they could only get the speed of the neutron star in its orbit. However, they were able to use another effect to measure the mass of the white dwarf directly, and that’s something called the “Shapiro Delay”.
General Relativity tells us that a massive object curves spacetime around that object. One result of this is that when light travels near a massive object, it has more space to go through than you would think just from standard Euclidean geometry. Another way to say this is that the diameter of a circle around a massive object is greater than the circumference divided by pi! That means that light travelling through that diameter will take longer than you would think it would, thus giving a delay in the arrival time of that light. If the neutron star and white dwarf orbit each other in orbits that are close to edge-on to our point of view, then the pulses from the neutron star will have to pass close enough to the white dwarf that the Shapiro Delay due to the white dwarf’s curving of spacetime will be measurable. And, indeed, that is the case in this system. Measuring the Shapiro Delay allowed Demorest and colleagues to determine that the white dwarf has very close to exactly half the mass of the Sun, which is a typical sort of mass for a white dwarf. Knowing that, they could use this value with the other data about the orbit from pulsar timing to determine the mass of the neutron star. The result was that the neutron star is 1.97 plus or minus 0.04 times the mass of the Sun. This is the most massive neutron star that has a precisely measured mass.
A mass this big for a neutron star is greater than some people had expected. The center of a neutron star is an exotic place indeed. Much of the bulk of the neutron star is made up of a bizarre gas of neutrons, held up by “neutron degeneracy pressure” much as a white dwarf is held up by electron degeneracy pressure. Right down at the center, however, some have theorized that there might be a quark-gluon plasma, or other forms of exotic matter. We don’t know enough about how nuclear matter behaves under such tremendous gravity and pressure to be sure, and at the moment neutron stars are the only laboratories where we can test these things. Various models for neutron stars based on these theories predict maximum masses for neutron stars… and most of those models predict a maximum mass less than twice the mass of the Sun. The existence of this two solar mass neutron star rules out those models. It is a very strong indication that likely it remains just nuclear matter, not something exotic like a quark-gluon plasma, all the way down to the center of a neutron star.
You might ask, surely there is a limit to how massive a neutron star can be, even taking into account models that are consistent with this new neutron star? Indeed there is; our best understanding from even the most forgiving plausible model suggests that a neutron star can’t be more than two and a half or three times the mass of the Sun. So, what happens if you try to cram together more mass than that into a single star? The answer is that you get a black hole… but that’s a topic for another time.
End of podcast:
365 Days of Astronomy
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