The unusual firework-like structure of the Pa 30 nebula may result from the merger of two dying stars. Photo by Robert Fesen

The unusual Pa 30 nebula probably comes from the dying star merger observed in 1181.

A Dartmouth professor’s images of the explosive consequences of the collision of two dying stars could help scientists better understand this rare type of astronomical event, and may finally confirm the identity of a bright but short-lived star observed nearly 850 years. Robert Fesen, Professor of Physics and Astronomy, captured telescopic images showing a firework-like burst of thin filaments radiating from a very unusual star at the center of an object called ‘Pa 30,’ according to findings he announced on 12 January at the 241st Meeting of the American Astronomical Society. Fesen is lead author of a paper reporting the findings, which has been submitted to the peer-reviewed journal The Astrophysical Journal Letters for publication.

‘Pa 30’ is a dense region of illuminated gas, dust and other matter, known as a nebula. Fesen and his co-authors report that Pa 30 appears to contain little or no hydrogen and helium, but is instead rich in the elements sulfur and argon. The nebula’s unusual structure and features match the predicted outcome of a collision between end-stage stars known as white dwarfs, Fesen said. White dwarfs are dim, extremely dense Earth-sized stars that contain the mass of the Sun. The merger of two white dwarfs is one proposed explanation for a subclass of supernovae, or star explosions, called Iax events, in which the star is not completely destroyed, Fesen said. “I have never seen any object, and certainly no supernova remnant in the Milky Way galaxy, that looks like this, and neither have any of my colleagues,” Fesen said. “This remnant will allow astronomers to study a particularly interesting type of supernova that until now they could only investigate from theoretical models and examples in distant galaxies.”

The size of ‘Pa 30’ and the speed at which it is expanding, around 2.4 million miles per hour, (3.84 million kilometers per hour) suggest that the explosive collision occurred around the year 1181, report the researchers. That matches observations by Chinese and Japanese astronomers at the time of a very bright star that appeared suddenly in the constellation Cassiopeia and was visible for about six months as it slowly faded. These shooting stars are known as “guest stars.” The images captured by Fesen of the nebula’s structure and luminosity not only provide the most accurate estimate yet of its age, but could also allow astronomers to refine existing models of white dwarf mergers. Pa 30 was discovered in 2013 by co-author and amateur astronomer Dana Patchick, but until now, images of the nebula have shown only an extremely faint and diffuse object, Fesen said. “Our deeper images show that Pa 30 is not only beautiful, but now that we can see the true structure of the nebula, we can investigate its chemical composition and how the central star generated its remarkable appearance, then compare these properties with predictions from specific models of rare white dwarf mergers,” Fesen said.

“Our deeper images show that Pa 30 is not only beautiful, but now that we can see the true structure of the nebula, we can investigate its chemical composition and how the central star generated its remarkable appearance, then compare these properties with predictions from specific models of rare white dwarf mergers,” Fesen said. Fesen took the images of Pa 30 in late 2022 using the 2.4-meter Hiltner Telescope at the MDM Observatory, which is owned by Dartmouth and operates with four other universities, adjacent to Kitt Peak National Observatory in Arizona. Fesen equipped the telescope with an optical filter sensitive to a particular line of sulfur emission. He captured ‘Pa 30′ in three 2000-second exposures under very clear skies and took additional data on the nebula’s structure, size and velocity. Fesen and his co-authors’ study built on work published in 2019 by Russian researchers who found an extremely unusual star near the dead center of Pa 30. That star exhibited several properties suggestive of the collision of two white dwarfs, and had a surface temperature of nearly 400,000 degrees Fahrenheit (222,000 Celsius) with a staggering wind speed of around 35 million miles per hour (56 million kilometers per hour).