A Cosmic Mystery: Astronomers Capture Dying Star-Blowing Smoke Rings

Astronomers have captured a red giant star going through its final death throes in unprecedented detail, revealing an unusual feature. The star, known as V Hydrae (or V Hya for short), ejected six different rings of material, according to a preprint accepted for publication in the Astrophysical Journal. The specific mechanism of these mysterious “smoke rings” formed is not yet understood. Still, the observation could shake up current models for this particular late stage of stellar evolution and shed more light on the fate of our own sun.

“V Hydrae has been caught shedding its atmosphere — eventually most of its mass — which is what most late-stage red giants do,” said study co-author Mark Morris, an astronomer at UCLA. the first and only time a series of expanding rings has been seen around a star in the throes of death – a series of expanding ‘smoke rings’ that we’ve calculated are blown up every few hundred years.”

Red giants are one of the last stages of stellar evolution. Once a star’s core stops converting hydrogen to helium via nuclear fusion, gravity begins to compress the star, increasing its internal temperature. This process ignites a shell of hydrogen that burns around an inert core. Ultimately, the compression and heating in the core causes the star to expand significantly, reaching diameters between 62 million and 620 million miles (100 million to 1 billion kilometers). The surface temperatures are relatively cool by stellar standards: just 4,000 to 5,800 degrees F (2,200 to 3,200 degrees C). So these stars look orange-red, hence the red giant name.

Eventually, the helium in a red giant’s core will be consumed and the core will shrink again. The star then becomes an asymptotic giant branch (AGB) star (the last red giant stage). The internal structure of an AGB star consists of a central core of carbon and oxygen, a shell where fusion turns helium into carbon, and another shell where hydrogen turns into helium. These stars typically produce dramatic pulses of increased brightness every 100 to 1000 days. In addition, intense surface winds create a gas cloud known as a circumstellar envelope around the star.

Those intense stellar winds will eventually dislodge the atmosphere and the star envelope, and the star will become a white dwarf star in a planetary nebula. The faster an AGB star loses mass, the closer it gets to that final transit. Our sun will eventually become a red giant in about 5 billion years, eventually evolving into an AGB before finally evolving into a planetary nebula with a white dwarf star at the center.

That is the process as astronomers have understood it for years. However, V Hya’s unusual features make them think again. V Hya, located 1,300 light-years away in the constellation Hydra, is a carbon-rich star, meaning its atmosphere contains more carbon than oxygen. It has a high loss rate for its mass, so astronomers suspect it is probably shedding its atmosphere to become a planetary nebula.

This AGB star is also intriguing because it has major plasma outbursts about every eight years and a sharp decrease in brightness about every 17 years. These events suggest the presence of a companion star that is barely visible. (The dips in brightness could be caused by a cloud associated with this second star passing in front of V Hya.)

This latest study combines data from the Hubble Space Telescope with observations using the Atacama Large Millimeter/submillimeter Array (ALMA), which incorporates infrared, optical and ultraviolet data to capture V Hya’s death throes across multiple wavelengths. The star is distant and surrounded by dense dust, but ALMA’s higher resolution capabilities revealed its rings and outflow in detail.

The timing was also serendipitous. “V Hya is in the short but critical transition phase that dying stars go through at the end of their lives,” said study co-author Raghvendra Sahai, an astronomer at NASA’s Jet Propulsion Laboratory. “It’s the stage where they lose most of their mass. It’s likely that this phase won’t last very long, so it’s hard to catch them in the act. We were lucky with V Hya and were able to image all the different activities in and around this star to better understand how dying stars lose mass at the end of their lives.”

Sahai and his co-authors found that the star repels its atmosphere by blasting a series of smoke rings, which have expanded outward over the past 2,100 years to form a dusty disk-like region around V Hya. The team called that structure DUDE (disk expanding dynamically).

Their observations also revealed high-speed explosions of gas from the star in opposite directions, perpendicular to the smoke rings, forming two hourglass-shaped structures. These structures are expanding rapidly at more than half a million miles per hour (240 km/s). “The discovery that this process can involve the ejection of gas rings, simultaneously with the production of intermittent high-speed jets of material, brings a new and fascinating ripple in our understanding of how stars end their lives,” Morris said.

All this suggests that the star is undergoing a particularly rapid evolution, which is contrary to the current model. “Our study dramatically reveals that the traditional model of how AGB stars die — from the massive release of fuel via a slow, relatively constant spherical wind for 100,000 years or more — is incomplete at best or incorrect at worst.” Said Said. “It is very likely that a close stellar or substellar companion played an important role in their deaths. In the case of V Hya, the combination of a nearby and a hypothetical distant companion star is responsible, at least to some extent, for the presence of its six rings and the rapid outpourings that cause the star’s miraculous death.”