Object 24: PSR 0820+02
Podcast release date: 29 June 2020
Right ascension: 08:23:09.8
Constellation: Canis Minor
Corresponding Earth location: 90 km northeast of Bitung, Indonesia
The letters "PSR" in the name PSR 0820+02 stand for pulsar, and the numbers are the coordinates for the pulsar in an older coordinate system, which was in use when the pulsar was first identified and when the first science papers were written about it. Just to review, a pulsar is a type of neutron star, which is the dense remnant of a very massive star that has exploded as a supernova. Initially, a neutron star will spin because it retains the angular momentum of the star that it formed out of. The way angular momentum works, if you take a very wide object spinning very slowly and then squeeze it into a very small space, it will spin very fast. Stars line the Sun complete one revolution on its axis every 25 to 30 days or so. Pulsars, which are the same mass as ordinary stars but which can be around 20 km in diameter, can complete a revolution on timescales of a second or less. This spinning combined with strong magnetic fields causes pulsars to emit very short pulses of electromagnetic radiation, but they usually slow down over time, and their magnetic fields usually decay.
PSR 0820+02, which produces a pulse of radiation once every 0.8648 seconds, was discovered in 1977 as part of a radiowave survey of pulsars . At first, it didn't look that different from any other pulsar, but while astronomers monitored the pulsar, they observed that its pulsation period decreased slightly in 1977 and 1978 and then began to increase slightly in 1979. The astronomers quickly figured out that this variation in pulse emission period occurred because the pulsar was in orbit around another star and that the emission was slightly Doppler shifted as it moved around in its orbit . This was rather exciting at the time, as this was only the second pulsar that had been identified to be in a binary star system .
Careful observations of the radio waves from the pulsar demonstrated that the pulsar and its companion orbited each other once every 1232 days . However, it was not immediately clear what the other star was because it did not produce any radio waves. Telescope observations in the visible part of the spectrum eventually identified the companion star as a white dwarf, which is the remnant of a Sun-like star after it evolves first into a red giant and then eventually into a planetary nebula . In other words, this binary star system contains the cores of two dead stars. Further observations demonstrated that the white dwarf has a mass of 0.6 times the mass of the Sun and a surface temperature of about 14700 Celsius .
Eventually, astronomers discovered some inconsistencies in their observations. As I mentioned before, pulsars are expected to slow down over time, and the period of PSR 0820+02 indicated that it should a relatively young pulsar (in astronomical terms) with an age of about 100 million years, or about 35 million years before the dinosaurs died out . However, white dwarfs should cool after they form, and the temperature of the white dwarf in the PSR 0820+02 system indicates that it formed about 200 million years ago [3,4], or just a few million years after dinosaurs first appeared on Earth. Based on what astronomers know about the evolution of stars, the more massive star that formed the pulsar should have died much sooner than the less massive star that formed the white dwarf, so it doesn't make sense that the pulsar seems younger than the white dwarf or most species of stegosaurs.
This inconsistency can be explained if the pulsar was spun up again by some process involving an interaction with the star that became the white dwarf . The model that describes this process occurs in several stages. To begin with, the PSR 0820+02 star system would have started out with two stars that fuse hydrogen into helium in their cores like the Sun, but one of these stars was much more massive than the other one. The more massive star would have used up all of the hydrogen in its core and gone through a series of changes where it would eventually explode as a supernova, leaving behind a neutron star. The neutron star would initially rotate very fast and look like a rapidly-rotating pulsar, but it would eventually slow down. Next, the less massive star would run out of hydrogen in its core, and it would expand to become a red giant. When the less massive star expanded, its outer atmosphere would get close enough to the neutron star that the neutron star would strip the gas away. Because of the conservation of angular momentum, the gas falling into the neutron would initially be moving slowly when it was far away but would move very fast as it fell inwards. This would speed up the rotation of the neutron star, and it would look like a rapidly-rotating pulsar again, possibly spinning even faster than when it first formed. The red giant would eventually form a planetary nebula and leave behind a white dwarf that would continue to orbit the pulsar that, as we observe it today, has not yet slown down.
This process appears to have taken place in a lot of other pulsars in binary star systems as well. It's all very messy and complicated, but astronomers would not be interested in PSR 0820+02 if it was simple and boring.
 Manchester, R. N. et al., Further observations of the long-period binary pulsar PSR 0820+02., 1983, Astrophysical Journal, 268, 832
 Kulkarni, S. R., Optical Identification of Binary Pulsars: Implications for Magnetic Field Decay in Neutron Stars, 1986, Astrophysical Journal Letters, 306, L85
 Koester, D. and Reimers, D., The white dwarf companion to PSR B0820+02, 2000, Astronomy & Astrophysics, 364, L66
 Koester, D. et al., The White Dwarf Companion to PSR 0820+02, 1992, Astrophysical Journal Letters, 395, L107