For the first time, scientists have directly tracked the slow transformation of a dying star over more than a century - revealing it is heating up faster than any other typical star ever observed.
The research, published today in The Astrophysical Journal Letters, tracks 130 years of changes in the "spirograph" Planetary Nebula IC418 - a glowing shell of gas and dust cast off by a dying star about 4000 light years from Earth.
By piecing together observations dating back to 1893, when astronomers first recorded the nebula by eye through a telescope, to today, scientists found the nebula's signature green light, emitted by oxygen atoms, has grown around 2.5 times stronger since Victorian astronomers first studied it.
This change is being driven by the central star's rising temperature, which has increased by around 3,000°C since 1893, or roughly 1,000°C every 40 years. For comparison, the Sun increased by the same amount during its formation, but took 10 million years to do it.
However, although the star is heating faster than ever observed, it is still slower than the latest models had predicted. This challenges current theories of how stars age and die, and may force astronomers to rethink the masses of stars capable of producing carbon - the element essential for life.
A planetary nebula marks one of the final stages in a star's life. As the star's core becomes unstable, it sheds its outer layers into space. The remaining core heats rapidly, energising the surrounding gas and dust to form beautiful structures. In the case of IC418, this creates an intricate, swirling structure, earning its nickname "the spirograph nebula". Our Sun will undergo the same fate in about 5 billion years.
While planetary nebulae usually evolve slowly, the researchers discovered that IC418 is evolving fast enough to track within a human lifetime.
This makes it the most prolonged and rapid transformation ever recorded in a planetary nebula, and possibly any star.
The team examined 130 years of observations from a wide range of telescopes - from the human eye measurements in the late 1800s to the advanced technologies of today. They verified, calibrated, and combined the data before comparing it with detailed models of stellar evolution. This allowed them to measure the star's heating rate, determine its current mass, and even estimate the mass of the star before it began its transformation.
The findings offer a rare insight into of how planetary nebulae evolve and suggest the night sky can change much faster than we usually think.
Co-author, Professor Quentin Parker from the University of Hong Kong, said: "We believe this research is important because it offers unique, direct evidence of how planetary nebulae central stars evolve. It will prompt us to rethink some of our existing models of stellar life cycles.
"It's been a strong joint effort - collecting, verifying, and carefully analysing more than a century's worth of astronomical data and then melding that with stellar evolutionary models. It's a challenging process that goes far beyond simple observation, and we're grateful for the opportunity to contribute to our field in this way."
Journal: The Astrophysical Journal Letters
Full title: The secular evolution of planetary nebula IC 418 and its implications for carbon star formation
DOI: 10.3487/2041-8213/ADF62b
Link: https://doi.org/10.3847/2041-8213/adf62b [doi.org[doi]]
