[비즈한국] If one were to name the two most special moments in a person's life, they would undeniably be the day of birth and the day of passing. The same applies to the life of a star. Stars, like us, are born and die. All space telescopes, including the James Webb and the Hubble, aim their lenses at the brilliant scenes of stellar birth and death. The places where stars are born and die are certainly among the most beautiful sights in the universe that can be observed through a telescope.
The Hubble Space Telescope, which first reached orbit in 1990, has already turned thirty-four. In human years, that is no longer a young age. Thanks to observations spanning a third of a century, the Hubble Space Telescope now shows us how the universe changes. By stitching together images of the same star observed over several years, one can see changes in shape or position, much like watching a GIF.
Of course, the lives of stars unfold much more slowly compared to humans. A star's lifespan ranges from tens of millions to billions of years. Therefore, it is very difficult to observe meaningful changes in the mere 30-odd years we have been watching. However, the moments of a star's birth and death are the most extreme points in its life. Even stars that usually live very slow and calm lives exhibit their most intense and violent behavior during these times. Stars change rapidly enough for the Hubble to detect meaningful fluctuations. Recently, those beautiful images captured by Hubble were unveiled.
There is a star called 'R Aquarii' located 1,000 light-years away in the direction of the Aquarius constellation. By collecting data from the Hubble Space Telescope observed over about nine years from 2014 to 2023, researchers confirmed the existence of energy jets being spewed in all directions from this star in real-time. This place is a symbiotic binary system where a white dwarf that has already finished its evolution is paired with a massive red giant that is in the middle of its own evolution. In particular, it is a system where the mass of the heavier star is rapidly being pulled toward the white dwarf as it passes too closely within the white dwarf's Roche limit.
As a large amount of matter from the red giant is absorbed by the white dwarf, the unstable white dwarf forms long energy jets on both sides. As the nebula surrounding the stars is blown away by these powerful jets, it creates a unique-looking nebula and debris. The two stars that make up this binary orbit each other every 44 years. As they orbit and get closer, the phenomenon where the jets strengthen and the star's brightness amplifies repeats. Looking at the photos captured by Hubble over nine years, one can see the nebula fluctuating at the center, its brightness changing, and matter spreading in all directions, as if watching a real-time video.
Sometimes, as matter is violently ejected, a nova might be observed, making it look as if the star has exploded. However, there has never been an official record of a nova explosion at the exact location of R Aquarii. Around 930 AD, Japanese astronomers left records of a suspected nova in that area, but it remains uncertain. If we trace the flow of the jet ejections identified through this recent Hubble observation backward, there is a possibility that a powerful explosion occurred about 1,100 years ago. Since the central red giant's brightness can increase by up to 750 times in a year, such an event would have been bright enough to be seen with the naked eye even back then. It is possible that it was recorded by people for a long time as it slowly dimmed over several months.
Beyond fluctuating binary systems, a supernova explosion where a star completely explodes and vanishes is an even more powerful phenomenon that cannot be missed. Its light can be seen from great distances. Records of supernovae occasionally witnessed throughout human history have existed for a long time. In 1181, Chinese astronomers discovered a faint star that suddenly appeared in the vicinity of what is now the constellation Cassiopeia. It was a so-called "guest star," appearing suddenly like a visitor that hadn't been there before.
Entering the 21st century, astronomers began searching for traces left by this supernova recorded in ancient Chinese texts. If a supernova explosion had occurred about 850 years ago, its remnants should still be somewhere. Astronomers conducted a citizen science project searching through data from the now-retired infrared space telescope, WISE, with the help of the public. Amateur astronomer Dana Patchick, who participated in the project, discovered a new gas remnant near Cassiopeia. Named Pa 30, meaning it was the thirtieth celestial object he discovered, it is presumed to be the remnant left behind by the supernova explosion recorded in history in 1181.
When Pa 30 was first discovered, astronomers thought it was a planetary nebula left behind by the death of a relatively light star. However, its appearance was very unique. It looked like long, tentacle-like structures stretching straight out from the center in all directions. Also, an extremely hot white dwarf resides at the center. The surface temperature of this white dwarf reaches nearly 200,000 degrees—almost 40 times hotter than the Sun's surface, which is only 5,000 to 6,000 degrees. This is among the hottest levels of any star discovered to date. An expansion of matter spreading outward from the central star at speeds reaching 16,000 km/s is observed, confirming that this is a supernova remnant spreading from a powerful explosion.
However, if this is truly the site of a supernova explosion, an important question follows: How did the central white dwarf remain intact and survive without being destroyed after the explosion?
Astronomers hypothesize it is a rare type of supernova known as a Type Iax. It is known that these do not occur from the collapse of a single heavy star, but rather from the collision of two white dwarfs. Such an explosion leaves behind a massive, super-giant white dwarf at the center, allowing it to survive partially even after the star explodes. For this reason, it is sometimes called a "zombie star," meaning a star that did not die. The zombie star is very unstable. It is likely spewing powerful stellar winds in all directions. Astronomers estimate that this star may potentially undergo another supernova explosion someday.
Nickel is also detected in Pa 30. The presence of nickel can explain why the central star maintains such an overwhelmingly high temperature. As unstable radioactive isotopes of nickel decay, they can generate enormous heat. However, there is one important issue: the half-life of nickel isotopes undergoing radioactive decay is only about 6 days. Therefore, in normal circumstances, nickel isotopes should not survive for long and should disappear quickly. However, in a collapsed white dwarf where overwhelmingly high pressure is maintained, nickel isotopes can persist. This is because the decaying nickel can continue to capture other electrons, leading to continuous decay over centuries.
Recently, astronomers used the Cosmic Web Imager (KCWI) on the Keck Observatory in Hawaii to observe the Pa 30 supernova remnant more closely. Interestingly, these tentacle structures do not seem to have slowed down since they were first ejected, but continue to spread at high speeds. It is as if matter is spreading like ballistic missiles fired in all directions. This implies that these are traces left by large, heavy material being launched directly from the center a long time ago.
This video is a 3D model of the supernova remnant Pa 30, reconstructed based on recent observation results. In particular, based on spectral data confirmed in the infrared range, astronomers analyzed how fast each tentacle is spreading and in which direction. Through this, they identified tentacles moving toward Earth and those moving away, and implemented this in 3D. In the midst of the various tentacles stretching in all directions, a hot, surviving white dwarf remains clumped together. The material in the tentacles spreading in all directions has been flying straight in one direction each since about 800 years ago. Tracing the speed and direction of the tentacles backward, the point at which the material spreading in all directions converges at the center is approximately 1151 ± 75 years. This means the supernova explosion actually occurred around 1151. It aligns well with the time recorded in Chinese literature.
This additional observation shows another interesting fact. First, there is a relatively empty void with less material in the middle of the supernova remnant. The material spreading out in tentacles did not start spreading from the exact center of the supernova remnant at the very beginning. It started spreading from a point slightly offset from the center. Strictly speaking, it did not spread from the place where the original white dwarf was. It is presumed that as the supernova explosion occurred at the center and the shell-like structure surrounding that area expanded, a very powerful ballistic ejection occurred in all directions.
Meanwhile, a large difference was also found between the speed of the ballistic ejections forming the tentacles and the speed of the gas debris spreading in all directions. While the gas matter ejected from the central white dwarf is spreading at a very fast speed of nearly 16,000 km/s, the speed of the objects spreading while drawing long tentacles remains at 600 to 1,000 km/s, which is not even one-tenth that speed. This can be thought of as the result of the objects subjected to ballistic ejection being much heavier in mass than ordinary gas matter. However, it is difficult to know exactly what these "cannonballs" are.
Even more interesting is that, based on the velocity distribution identified to draw this 3D map, the velocity distribution of this ballistic ejection is not perfectly symmetrical. The speed of the tentacles spreading in all directions varies by up to 40% depending on the direction. It appears there was a quite asymmetrical explosion where matter was ejected faster in certain directions.
Surprisingly, if you look at the night sky at the very spot where ancient humans witnessed something and recorded it with interest, using a clear telescope today, you can see the remnants and traces of something that exploded a long time ago. Although it was an event that happened over a very long interval spanning dozens of generations for humanity, astronomically speaking, we have been looking at the same phenomenon for a thousand years. An explosion occurred long ago, and its remnants are still spreading into space. We are connected to our ancestors of 1,000 years ago through the explosion of one star. The same star that exploded in the same spot still gives wonder to humanity living on Earth for over a millennium.
In particular, the death and birth of stars are the most dynamic moments in the universe. Unlike other phenomena that usually occur on a scale of tens or hundreds of millions of years, the death and birth of stars involve changes on a relatively human scale of decades to centuries. Therefore, if humanity exercises a little patience, we can sufficiently observe the evolving universe in real-time.
References
https://www.keckobservatory.org/dandelion-supernova/
https://iopscience.iop.org/article/10.3847/2041-8213/ad713b
https://esahubble.org/videos/v-r-aquarii_1/
Who is the author Ji Ung-bae? He loves cats and the universe. After watching 'Galaxy Express 999' as a child, he developed a dream to share the beauty of the universe. He is currently researching the evolution of galaxies through their interactions at the Yonsei University Galaxy Evolution Research Center and the Near-Field Cosmology Laboratory. He is engaged in various science communication activities, including lectures and writing. He has written books such as 'The Observatory of Flirting', 'Thinking About the Universe All Day', and 'Stars, the Science of Light'.