- 4127 - STARS and Magnetars?
On a basic level, a star is pretty simple. Gravity squeezes the star
trying to collapse it, which causes the inner core to get extremely hot and
dense. This triggers nuclear fusion, and the heat and pressure from that pushes
back against gravity. The two forces balance each other while a star is in its
main sequence state.
--------------------------------- 4127 - STARS and Magnetars?
- The details of how that star works are
extremely complex. Modeling the interior of a star accurately requires
sophisticated computer models, and even then it can be difficult to match a
model to what we see on the surface of a star.
-
- Although the internal pressure and
gravitational weight of a star are generally in equilibrium, the flow of heat
is not. All the heat and energy generated in a stellar core has to escape in
time, and there are two general ways in which it happens.
-
- The first is through a radiative exchange.
High-energy gamma rays scatter against nuclei in the core, gradually losing
some energy as they migrate to the surface and escape. The interior of a star
is so dense that this can take thousands of years.
-
- The second method is through convective
flow. Hot material near the center of a star tries to expand, pushing its way
toward the surface. Meanwhile, cooler material near the surface condenses and
sinks towards the core. Together this creates a cyclic flow of material that
transfers heat energy to the star’s surface. This convection churns the interior
of a star, and because of things such as viscosity and turbulent vortices, it
is extremely difficult to model.
-
- Stars generally have a radiative zone and a
convective zone. The location and size of these zones depend on a star’s mass.
Small stars are almost entirely convective, while stars like the Sun have an
inner radiative zone and an outer convective zone.
-
- For massive stars, this is flipped, with an
inner convective zone and an outer radiative one. One of the things we know
about convection is that it can cause the surface of a star to fluctuate like a
simmering pot of water. This in turn causes the overall brightness of a star to
flicker.
-
- Convection regions in a star are connected
to the way in which a star flickers.
Sound waves rippling through a star are affected by convective flows,
which in turn change the way a star flickers. This means in principle we can
study the interior of a star by observing its flicker of light, allowing
astronomers to better understand stars.
-
- Astronomers have been able to associate two
seemingly unrelated phenomena: an explosive event known as a “fast radio burst”
and the change in speed of a spinning magnetar. And now new research suggests
that the cause of both of these is the destruction of an asteroid by a
magnetar.
-
- For years
astronomers were stumped by the origins of “fast radio bursts”, which are
flashes of radio energy that last less than a second. Since fast radio bursts
were detected in distant galaxies, they must be incredibly energetic events.
But it wasn’t until astronomers caught a fast radio burst occurring in our own
galaxy that we discovered the likely culprit: magnetars.
-
- Magnetars are a
special kind of “pulsar”, which are rapidly spinning neutron stars. When neutron
stars first form, they can carry with them enormously strong magnetic fields,
the strongest magnetic fields in the entire universe. And so these
super-magnetized neutron stars get a new name: magnetars.
-
- The connection to
fast radio bursts was made when astronomers noted noticed a magnetar glitching.
Magnetars rotate with very precise speeds. But occasionally that speed can
change suddenly, where it shifts to either a slower or faster speed.
Astronomers noticed a glitch in a magnetar around the same time that a fast
radio burst was generated. Since magnetars carry enormous amounts of energy,
they could potentially explain the origin of fast radio bursts.
-
- Asteroids are
thought to be common around magnetars. Since magnetars are the leftovers of
giant stars after they die, parts of their solar systems will remain intact.
And so it’s likely that magnetars are surrounded by a host of asteroids and
other assorted debris.
-
- Occasionally an
asteroid can wander too close to its magnetar. The magnetar, in addition to
having a strong magnetic field, also has an extremely powerful gravitational
force. If the asteroid gets too close, the gravitational force can rip the
asteroid apart.
-
- When the asteroid
gets ripped apart its angular momentum has to go somewhere. If it happens to
follow a path that goes along with the rotation of the magnetar, then it will
increase the speed of the magnetar once it gets disrupted. This causes the
glitch. If the asteroid is moving in the opposite direction it will slightly
slow down the magnetar, leading to what’s known as an anti-glitch.
-
- Either way the
debris of the torn-apart asteroid now gets caught up in the extremely strong
magnetic fields. This causes the magnetic fields to tangle up on themselves and
release their pent up energy in the form of a fast radio burst.
-
- The surviving
debris eventually rains down onto the magnetar surface which releases its own
kind of flares that we can potentially detect.
-
- It is a plausible
scenario to explain how exactly magnetars can lead to fast radio bursts, and it
shows that even the tiniest objects in a solar system, like asteroids, can lead
to very big impacts.
-
-
August 23, 2023 STARS and Magnetars 4127
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