January 14, 2021
Astronomers turn back the clock on the growing remains of an exploded star nearby. Using NASA's Hubble Space Telescope, they tracked the rapid splinter of the explosion to get a more accurate estimate of the location and time of the star's detonation.
The victim is a star that exploded a long time ago in the small Magellanic Cloud, a satellite galaxy to our Milky Way. The doomed star left behind an expanding, gaseous corpse, a supernova remnant named 1E 0102.2-7219, which NASA's Einstein Observatory first discovered in X-rays. Like detectives, the researchers searched the archive images taken by Hubble and analyzed observations in visible light every 10 years.
This portrait from the Hubble Space Telescope shows the gaseous remains of an exploded massive star that erupted approximately 1,700 years ago. The stellar corpse, a supernova remnant named 1E 0102.2-7219, experienced its demise in the small Magellanic Cloud, a satellite galaxy of our Milky Way. Credits: NASA, ESA and J. Banovetz and D. Milisavljevic (Purdue University)
The research team, led by John Banovetz and Danny Milisavljevic of Purdue University in West Lafayette, Indiana, measured the velocities of 45 tadpole-shaped, oxygen-rich clumps of ejecta ejected from the supernova explosion. Ionized oxygen is an excellent indicator because it glows the brightest in visible light.
To calculate an exact age of the explosion, the astronomers selected the 22 fastest moving ejecta clumps or nodules. The researchers found that these targets were the least slowed by passage through interstellar material. They then followed the movement of the nodes backwards until the ejecta merged at one point and identified the explosion site. Once this was known, they were able to calculate how long it would take the fast knots to get from the center of the explosion to their current location.
They estimate that the light from the explosion came to earth 1,700 years ago during the fall of the Roman Empire. However, the supernova would only have been visible to residents of the southern hemisphere. Unfortunately, there are no known records of this titanic event.
The researchers' results differ from previous observations of the explosion site and the age of the supernova. For example, previous studies reached an explosion age 2,000 and 1,000 years ago. However, Banovetz and Milisavljevic say their analysis is more robust.
This time-lapse video shows the motion of a supernova remnant – the gaseous remnants of an exploded star – that erupted approximately 1,700 years ago. The stellar corpse, a supernova remnant named 1E 0102.2-7219, experienced its demise in the small Magellanic Cloud, a satellite galaxy of our Milky Way. The opening frame of the film shows ribbons of glowing gaseous lumps that make up the rest. The video then alternates between two black and white images of the remnant 10 years apart, showing subtle shifts in the expansion of the ejecta over time. Credits: NASA, ESA, A. Pagan (STScI), J. Banovetz and D. Milisavljevic (Purdue University)
"A previous study compared images years apart from two different cameras on Hubble, the Wide Field Planetary Camera 2 and the Advanced Camera for Surveys (ACS)," said Milisavljevic. “However, our study compares data that was recorded with the same camera, the ACS, which makes the comparison much more robust. The nodes were much easier to trace with the same instrument. It's testament to Hubble's longevity that we can so cleanly compare images taken 10 years apart. "
The astronomers also took advantage of the sharp ACS images in selecting the clumps of ejecta to analyze. In previous studies, the researchers averaged the velocity of all gaseous debris to calculate an explosion age. However, the ACS data showed regions where the ejecta slowed as it hit denser material that the star had shed before it exploded as a supernova. The researchers did not include these nodes in the sample. They needed the ejecta that best reflected their original explosion rates and used them to make an accurate estimate of the age of the supernova explosion.
Hubble also clocked the speed of a suspected neutron star – the crushed core of the doomed star – ejected from the explosion. Based on their estimates, the neutron star would have to travel more than 2 million miles per hour from the center of the explosion to arrive at its current position. The suspected neutron star was identified during observations with the Very Large Telescope of the European Southern Observatory in Chile in combination with data from NASA's Chandra X-ray Observatory.
"That's pretty fast and at the extreme end of the speed a neutron star can move even if it got a kick from the supernova explosion," said Banovetz. “Newer studies question whether the object is actually the surviving neutron star of the supernova explosion. It is possibly just a compact chunk of supernova ejecta that has been illuminated, and our results generally support that conclusion. "
The hunt for the neutron star could still be ongoing. "Our study does not solve the mystery, but gives an estimate of the velocity for the candidate neutron star," said Banovetz.
Banovetz will present the team's results on January 14th at the American Astronomical Society's winter meeting.
The Hubble Space Telescope is an international cooperation project between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.
Credits: NASA, ESA and J. Banovetz and D. Milisavljevic (Purdue University)