Sometimes, what you see is not what you think it is. This basic statement can apply to anything, from the YouTube video of a piece of fruit that turns out to be cake to the discovery of a black hole that turns out to just be a naked stellar core.

Back in 2020, researchers led by Thomas Rivinius published what they thought was the discovery of a black hole in a system along with two other stars. This data was based on periodic observations with the European Southern Observatory’s 2.2-meter telescope in Chile, which sampled its forty-day period. The data seemed to indicate that there was a dense object, orbited every forty days by a star, with yet another star even further out.

But the thing with data like this is that sometimes, there is more than one equally valid way to understand the data, and when Ph.D. candidate Julia Bodensteiner re-analyzed their observations, she came up with a different possibility. What if there are only two stars and no black hole, and one of the stars has had its atmosphere pulled off?

I’m amazed at the creativity involved in this “what if.” While we know stars regularly cannibalize one another, we’ve never actually seen a system at the point in its evolution where one of the stars has been completely stripped down to its core.

Bodensteiner’s team in Belgium reached out to Rivinius’ team in Chile to try and figure out the truth, and the two teams realized they couldn’t make progress with their existing data, so together they requested time on the many meter telescopes of the combined Very Large Telescope Interferometer (VLTI). According to collaborator Dietrich Baade: The VLTI was the only facility that would give us the decisive data we needed to distinguish between the two explanations.

Rivinius went on to explain: The scenarios we were looking for were rather clear, very different, and easily distinguishable with the right instrument. We agreed that there were two sources of light in the system, so the question was whether they orbit each other closely, as in the stripped-star scenario, or are far apart from each other, as in the black hole scenario.”

And this suite of bigger telescopes had the answer. Research Abigail Frost explains: [The instrument] confirmed that there was no bright companion in a wider orbit, while [the instrument’s] high spatial resolution was able to resolve two bright sources separated by only one-third of the distance between the Earth and the Sun. These data proved to be the final piece of the puzzle and allowed us to conclude that HR 6819 is a binary system with no black hole.

This is how science is supposed to work: people analyze data, other folks work to replicate their work, and when discrepancies are found, more data or better tools are used to move past those discrepancies.

And while at first glance, it seems like a black hole had to be more interesting, the reality is that what they found is short-lived and rare, and while we have found black holes, we had never found the core of a dead star. As Frost puts it: Catching such a post-interaction phase is extremely difficult as it is so short. This makes our findings for HR 6819 very exciting, as it presents a perfect candidate to study how this vampirism affects the evolution of massive stars, and in turn the formation of their associated phenomena including gravitational waves and violent supernova explosions.

These results are published in Astronomy & Astrophysics, and the paper was led by Frost.

More Information

ESA press release

“HR 6819 is a binary system with no black hole,” A. J. Frost et al., 2022 March 2, Astronomy & Astrophysics