Pulsars are rotating stars with
electromagnetic radiation emissions that
appear as pulses.
They are different than quasars, which are used in
Very Long Baseline Interferometry.
While a quasar is a black hole in the center of a galaxy
(generating non-stellar radiation, i.e., radiation that is not
pulsars are neutron stars.
Pulsars were discovered in 1967 by Susan Jocelyn Bell
(now Jocelyn Bell Burnell).
Interview with Jocelyn Bell Burnell
Jocelyn Bell Burnell, who co-discovered the first pulsar PSR B1919+21 in 1967,
relates that in the late 1950s a woman viewed the Crab Nebula source
at the University of Chicagos telescope, then open to the public,
and noted that it appeared to be flashing.
The astronomer she spoke to, Elliot Moore,
disregarded the effect as scintillation,
despite the womans protestation that
as a qualified pilot she understood scintillation
and this was something else.
Bell Burnell notes that the 30 Hz frequency
of the Crab Nebula optical pulsar is difficult
for many people to see.
Then one day Schisler noticed something —
the mysterious blip appeared 4 minutes earlier
than the day before. Four minutes meant a lot to
the airman: before being stationed at Clear, he
had been a navigator on a B-47 bomber, and he
knew that stars rise 4 minutes earlier each night
as a result of Earths motion around the Sun.
Air force had early warning of pulsars,
Oblique Magnetic Axes
Pulsars are highly magnetized neutron stars,
emitting radiation from magnetic poles.
The magnetic poles and rotational poles of a pulsar
may be oblique to each other (not coincide),
causing the radiation to sweep around space,
appearing (to us on Earth) like a pulse if
the radiation happens to sweep by the Earth.
Magnetic poles (white axes) of a pulsar,
not in line with the rotational axis (red).
The magnetic poles are not antipodal
(in spherical coordinates),
so that each magnetic pole
has a slightly different axis.
Depiction of a pulsar, with powerful magnetic field
Radiation beams from the magnetic poles (depicted yellow)
can be detected on Earth,
if and when a beam sweeps past Earths position
as the beam forms a swept cone around the pulsars rotation axis.
The beams also emit jets of charged particles,
which travel much less distant than the radiation.
Photon counting camera sequence in slow motion,
detecting periodic image spikes
of the Messier 1 (M1) pulsar in the Crab Nebula.
Crab Nebula pulsar.
Astronomy Picture of the Day, 2014 July 25.
Crab Nebula pulsar.
Astronomy Picture of the Day, 2022 August 21.
Pulsars naturally slow down,
but if galactic material
from a neighboring star is available,
the galactic material will be drawn into the pulsar,
forming an accretion disk,
giving the pulsar more spin and greater energy,
absorbing the other star.
Accretion Disk Animation Video
Animation frame of an X-ray millisecond
pulsar with accretion disk,
radiation beams (depicted as glowing),
and trails of emitted charged particles.
Neighboring star that is being accreted (absorbed)
is in right background.
From NASA video (link above).