Yesterday I talked about quark stars. These are hypothetical stars similar to neutron stars, but smaller and a bit more massive. Their gravity and density would be enough that instead of being largely made of neutrons (which are made of up and down quarks), they would be made of free up, down and strange quarks. If such quark stars exist, they would lie between neutron stars and solar mass black holes.
Equations of State
Imagine a star twice as massive as the Sun, compressed to the size of a city. All that matter squeezed into a sphere about 15 miles wide. Such an object is known as a neutron star. The matter of a neutron star is so dense and its gravity is so strong that atoms cannot support themselves. Instead they collapse, with the electrons being squeezed into the nuclei until what remains is a mass of neutrons. Hence the name.
Glitch and Anti-Glitch
About a year ago in Nature astronomers reported evidence of an anti-glitch in the magnetar 1E 2259+586. You might remember from yesterday’s post that a magnetar is a neutron star with an extremely strong magnetic field. This particular magnetar is also a pulsar, meaning that the intense x-ray beams that stream from the magnetar’s polar region happen to be aligned so that we see it flash regularly. You can think of a pulsar as a kind of cosmic lighthouse, if you will.
Magnetars, Pulsars, and X-rays, Oh My!
One of the differences between astronomy and astrophysics is that astronomy is based upon observation, while astrophysics is about the underlying mechanism behind those observations. For this reason, many types of phenomena in the universe have multiple names depending on how we observe them. The reason for this is that typically astronomers start observing different phenomena, give them names, and then only later do astrophysicists figure out that they are different examples of the same thing. By then the names have already stuck.
Over the Limit
Neutron stars typically form when a large star dies in a supernova explosion. The outer layers of the star are cast outward to form a supernova remnant, while the core of the star collapses into a dense neutron star. What keeps a neutron star from collapsing under its own weight is the pressure of the neutrons pushing against each other, similar to the way electron pressure works in a white dwarf star. But there is a limit to how much weight the neutrons can counter, known as the Tolman-Oppenheimer-Volkoff (TOV) limit. This limit means that a neutron star can’t be more massive than about three solar masses. More than that, and it would collapse into a black hole.
Little Green Men
In 1967 a PhD student named Jocelyn Bell detected a radio signal with an odd regularity. Patterns can be heard in all sorts of radio signals, but this particular signal was unusual in that it was a pulse with a period of 1.33 seconds. You can see this pattern in the figure above, and you can hear what the signal sounds like here.
Weeble Wobble
A magnetar is a neutron star with an extremely strong magnetic field, a billion times stronger than the strongest fields we can create on Earth. As a neutron star, magnetars also have very strong gravitational fields, with a surface gravity a hundred billion times that of Earth. Such a high gravity would seem to ensure that a magnetar is spherical, but a magnetar’s strong gravitational field could distort the star, making it more of an oblate spheroid. We’ve suspected that such a magnetic distortion could occur with magnetars, but now a research team seems to have found an example of this phenomenon.
How CERN’s Discovery of Exotic Particles May Affect Astrophysics
You may have heard that CERN announced the discovery (confirmation, actually. See addendum below.) of a strange particle known as Z(4430). A paper summarizing the results has been published on the physics arxiv, which is a repository for preprint (not yet peer reviewed) physics papers. The new particle is about 4 times more massive than a proton, has a negative charge, and appears to be a theoretical particle known as a tetraquark. The results are still young, but if this discovery holds up it could have implications for our understanding of neutron stars.
Starquake
Given their extraordinarily high density and gravity, you might think there is no way a neutron star can be geologically active. But in fact we know that they are active, and even prone to “starquakes”.
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