Object 120: Monogem Ring and PSR B0656+14

Podcast release date: 18 March 2024

Right ascension: 06:59:48.2


Epoch: ICRS

Constellation: Gemini

Corresponding Earth location: An agricultural area just north of Choam Ksant, Cambodia

This episodes coordinates point to the center of a giant supernova remnant relatively close to Earth called the Monogem Ring. This object gets its name from the constellations Monoceros and Gemini in which it is located, although it also covers at least part of the constellation of Canis Minor. While it was clever to combine the first syllables of Monoceros and Gemini to create the name of this constellation, it's a little disappointing that they did not get the first syllable of Canis Minor into the name. Also, just to make things interesting, the musician Jen Hirsh adopted the stage name MONOGEM after the name of the Monogem Ring [1].

Anyhow, the ring is actually the outer edges of a giant sphere of gas that, as seen from Earth, is 25 degrees across [2, 3], or about 50 times the diameter of the Moon, or about 1/7 the diameter of the half of the sky that is visible above the horizon at any given time. In terms of physical size, the ring is about 440 light years across, which may not seem like that much compared to the size of the Milky Way, but given that the center of the ring is 939 light years (288 pc) away [4], it ends up looking quite large in the Earth's sky. However, the Monogem Ring primarily emits X-ray emission, so if you try going outside at night to try looking for it, you won't see it. Professional astronomers who have carefully imaged this area of the sky have seen hints of emission from the ring in the visible part of the electromagnetic spectrum, but it's been very hard to detect [5, 6].

The ring, which agin just to emphasize is actually the outer boundaries of a sphere, remained undetected until the development of X-ray astronomy in the second half of the twentieth century. The object was discovered in 1969 by a group led by Alan Bunner, who flew X-ray detectors on a suborbital rocket (or a rocket that got above the Earth's atmosphere but that did not reach orbit) [7]. However, the detectors were unable to really determine the shape of the Monogem Ring; they could only see a big, bright patch of X-ray emission in the direction of Gemini and Monoceros. It wasn't until the launch of X-ray telescopes into space in the late 1970s that astronomers could actually see the ring structure in this supernova remnant [8].

The Monogem Ring is not a perfectly smooth ring but instead made up of a bunch of filamentary arc-like structures and clumpy parts [2, 3]. The ring also seems to produce more X-ray emission from one side than the other. This X-ray emission comes from where the gas in the expanding supernova remnant is colliding with the surrounding interstellar medium. The reason why the ring looks asymmetric is probably because the gas has collided with interstellar gas with different densities.

The star that produced this supernova would have originally been a very big blue star that initially fused hydrogen into helium in its core, but, when the core filled up with helium, it would have fused the helium into carbon and oxygen and then fused those elements into heavier elements until the core filled up with iron, which cannot be fused to produce energy. The stars outer gas layers would have collapsed inwards onto the iron core, and then the rebound shock would have produced the supernova. The explosion forming the Monogem Ring took place somewhere around 86000 years ago, although the estimates are a little uncertain [9]. As best as I can tell, it would have been not only brighter than any star in the Earth's sky but also brighter than the full Moon. However, it took place so long ago that no historical record is likely to exist, even in the form of Cro-Magnon cave art.

Fortunately, it has been possible to identify the leftover core from the star that exploded. This object is the pulsar PSR B0656+14 [4, 8, 10, 11, 12], where the PSR stands for pulsar and the B followed by all of the numbers give the coordinates of this object in the out-of-date 1950 coordinate system. (It's not the best naming system, but it's what pulsar astronomers use.) Pulsars are rapidly rotating neutron stars. Neutron stars are basically objects so dense and with such high gravitational forces that the protons and electrons in the atoms that were in the original stellar core that formed them got squeezed together to form neutrons, so the star basically consists of a giant ball of neutrons. These stars can produce radio emission from their magnetic poles when they rotate, which is why they are called pulsars.

The pulsar PSR B0656+14 was discovered in 1977 by the Second Molongo Pulsar Survey, which was conducted by the Molonglo Radio Observatory in Australia [13]. The survey discovered a total of 155 pulsars which doubled the number of known pulsars at the time [13]. The pulsar rotates about 2.6 times a second [12]. The mass and size of the star is not entirely known, but it's mass is probably somewhere around 1.4 times the mass of the Sun [12], and the diameter is probably around 26 km [12], or slightly larger than the size of the island of Manhattan. (It's normal in astronomy to compare neutron stars to Manhattan. No one would ever compare a neutron star to, for example, the diamter of the city limits of Rome or the diameter of the S50 ring road around Beijing, all of which are very close to the same size.)

Anyhow, once the Monogem Ring was imaged in X-ray emission in the early 1980s, it was natural to associate it with the pulsar PSR B0656+14 [8]. The pulsar sat in roughly the center of the ring, and as best as astronomers could determine, both the ring and the pulsar had about the same ages. However, some initial distance measurements to the pulsar seemed to indicate that it was more than twice as far away as the ring [14, and initial measurements of the motion of the star relative to the ring seemed to indicate that they were inconsistent [15]. In the early 2000s, astronomers were able to more accurately measure the distance to the pulsar by using the Very Long Baseline Array to make really high resolution images of the object, and they finally associated the pulsar with the ring [4]. Since then, PSR B0656+14 has continued to be a popular target for astronomers to study because it is definitely known to be a "middle aged" pulsar and because it's also a very close one.

One of the final things that can be said about the Monogem Ring is that it is a major source of cosmic rays. If you aren't familiar with cosmic rays, they gave the Fantastic Four their superpowers [16]. Also, they are subatomic particles such as electrons and protons that are moving at nearly the speed of light, and, as you would imagine, any supernova explosion is naturally going to be a source of such high speed subatomic particles. The Monogem Ring is not the only source of cosmic rays reaching the Earth, but it does seem to be one of the strongest sources of high energy cosmic rays in our Solar System's neighborhood [11, 17, 18].


[1] Hirsh, Jen, B I O | monogem, 2024, MONOGEM

[2] Plucinsky, Paul P. et al., ROSAT Survey Observations of the Monogem Ring, 1996, Astrophysical Journal, 463, 224

[3] Knies, Jonathan R. et al., Suzaku observations of the Monogem Ring and the origin of the Gemini H α ring, 2018, Monthly Notices of the Royal Astronomical Society, 477, 4414

[4] Brisken, Walter F. et al., The Distance and Radius of the Neutron Star PSR B0656+14, 2003, Astrophysical Journal Letters, 593, L89

[5] Reimers, D. and Wendker, H. J., Possible optical detection of a filament from the Monogem ring., 1984, Astronomy & Astrophysics, 131, 375

[6] Weinberger, R. et al., Detection of an optical filament in the Monogem Ring, 2006, Astronomy & Astrophysics, 448, 1095

[7] Bunner, A. N. et al., Low-Energy Diffuse X-Rays, 1971, Astrophysical Journal Letters, 167, L3

[8] Nousek, J. A. et al., The Gem-Mon X-ray enhancement : a giant X-ray ring., 1981, Astrophysical Journal, 248, 152

[9] Plucinsky, P. P., The Monogem Ring: A Nearby SNR Similar to the Local Bubble ?, 2009, in The Local Bubble and Beyond II, 1156, 231

[10] Cordova, F. A. et al., Soft X-Ray Emission from the Radio Pulsar PSR 0656+14, 1989, Astrophysical Journal, 345, 451

[11] Thorsett, S. E. et al., Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the "Knee'' in the Primary Cosmic-Ray Spectrum, 2003, Astrophysical Journal Letters, 592, L71

[12] Zharikov, S. et al., PSR B0656+14: the unified outlook from the infrared to X-rays, 2021, Monthly Notices of the Royal Astronomical Society, 502, 2005

[13] Manchester, R. N. et al., The second Molonglo pulsar survey - discovery of 155 pulsars., 1978, Monthly Notices of the Royal Astronomical Society, 185, 409

[14] Kaspi, V. M., Radio pulsar/supernova remnant associations, 1998, Advances in Space Research, 21, 167

[15] Thompson, R. J., Jr. and Cordova, F. A., Revised Proper-Motion Results for the Soft X-Ray--emitting Radio Pulsar PSR 0656+14, 1994, Astrophysical Journal Letters, 421, L13

[16] Lee, Stan, et al., The Fantastic Four, 1961, Fantastic Four, 1

[17] Fang, Kun et al., Discriminating local sources of high-energy cosmic electrons and positrons by current and future anisotropy measurements, 2018, Monthly Notices of the Royal Astronomical Society, 478, 5660

[18] Luo, Qing et al., Statistical Study of the Optimal Local Sources for Cosmic Ray Nuclei and Electrons, 2022, Astrophysical Journal, 930, 82


Podcast and Website: George J. Bendo

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