By studying countless stars at various stages of their evolution, astronomers have been able to piece together an understanding of the life cycle of stars and how they interact with their surrounding planetary systems as they age. This research confirms that when a Sun-like star nears the end of its life, it expands anywhere from 100 to 1000 times its original size, eventually engulfing the system’s inner planets. Such events are estimated to occur only a few times each year across the entire Milky Way. Though past observations have confirmed the aftermath of planetary engulfments , astronomers have never caught one in the act, until now.
With the power of the Gemini South Adaptive Optics Imager (GSAOI) on Gemini South, one half of the International Gemini Observatory, operated by NSF’s NOIRLab,  astronomers have observed the first direct evidence of a dying star expanding to engulf one of its planets. Evidence for this event was found in a telltale “long and low-energy” outburst from a star in the Milky Way about 13,000 light-years from Earth. This event, the devouring of a planet by an engorged star, likely presages the ultimate fate of Mercury, Venus, and Earth when our Sun begins its death throes in about five billion years.
“These observations provide a new perspective on finding and studying the billions of stars in our Milky Way that have already consumed their planets,” says Ryan Lau, NOIRLab astronomer and co-author on this study, which is published in the journal Nature.
For most of its life, a Sun-like star fuses hydrogen into helium in its hot, dense core, which allows the star to push back against the crushing weight of its outer layers. When hydrogen in the core runs out, the star begins fusing helium into carbon, and hydrogen fusion migrates to the star’s outer layers, causing them to expand, and changing the Sun-like star into a red giant.
Such a transformation, however, is bad news for any inner-system planets. When the star’s surface eventually expands to engulf one of its planets, their interaction would trigger a spectacular outburst of energy and material. This process would also put the brakes on the planet’s orbital velocity, causing it to plunge into the star.