Stars don’t usually evolve fast enough for humans to notice them change within one lifetime. Even a hundred lifetimes won’t do – astronomical processes are just too slow. But not always. There are some phases of stellar evolution that happen quickly, and when they do, they can be tracked. A new paper posted to ArXiv last week uses astronomical observations found in ancient Roman texts, medieval astronomical logs, and manuscripts from China’s Han Dynasty to trace the recent evolution of several bright stars, including red supergiant Antares, and Betelgeuse: one of the most dynamic stars in our sky. With observations from across the historical record, the paper suggests that Betelgeuse may have just recently passed through the ‘Hertzsprung gap,’ the transitional phase between a main sequence star and its current classification as a red supergiant.

If you were to survey all the stars in the night sky for their color and luminosity, you would see that most stars fall within a distinct pattern known as the main sequence (the hydrogen-burning phase of a star’s life), with a smaller number of stars falling within a second category of giants (dying stars that have used up all the hydrogen in their cores). Surveying stars this way and plotting them on a graph is called a Hertzsprung–Russell diagram, and it is a useful tool for understanding stellar evolution. One of the key features of the diagram is a distinct gap between the main sequence and giant stars, known as the Hertzsprung gap. This gap doesn’t really mean that stars don’t exist within that gap – but rather that stars don’t stay there very long. It is a transitional phase, which can be crossed in a few thousand years, meaning that catching a star in the middle of the phase is rare – hence the gap in the diagram.

A Hertzsprung-Russell diagram, with the Hertzsprung gap between main sequence stars and red giants. This gap indicates a life stage of stars that doesn’t last long, making observations of stars with that luminosity and color rare. Image Credit: Richard Powell (Wikimedia Commons).

With luck, this short-lived transitional phase could theoretically be observed within humanity’s written historical record, for any number of stars.

The key candidates for such a study are bright, nearby red supergiant stars visible to the naked eye – meaning they could have been observed and studied before modern telescopic lenses. Some ideal examples include Antares, a variable red supergiant in the constellation of Scorpius, and Betelgeuse (the right shoulder of Orion), a roughly 10 million-year-old star that is no longer burning hydrogen in its core. Nearing the end of its life, Betelgeuse is expected to explode in a dramatic supernova sometime in the next 100,000 years (astronomically speaking, that is not very long).

Most of our knowledge about these stars comes from modern observations. However, modern remote sensing techniques are not perfect, and it is valuable to have multiple redundant methodologies for calculating phases of stellar evolution. The historical record can therefore help corroborate, or constrain, the predictions of modern astrophysics.

The historical evidence: Betelgeuse in transition, and an unchanging Antares

A key marker of the transition across the Hertzsprung gap is a change in color toward the reddish end of the spectrum. As such, historical descriptions of Betelgeuse or Antares denoting anything other than red would hint at a recent transition.

You might be inclined to dismiss historical texts as potentially misleading or inaccurate – besides, a vague description of a star as ‘reddish’ isn’t very scientifically helpful by today’s standards. But the real value of historical documents occurs when ancient writers make comparisons between distinct astronomical objects: Betelgeuse to Saturn, or Antares to Mars, for example. Those kinds of statements give us a much more measurable, if still approximate, dataset to work with, because we can make the same comparisons in today’s sky with modern equipment.

This is exactly the kind of data the paper’s authors, led by Ralph Neuhäuser (AIU Jena), were able to find. Digging into a variety of historical records, they uncovered several early descriptions of bright supergiants like Betelgeuse and Antares. One of the key sources for Betelgeuse was De Astronomica, a Roman text attributed to Gaius Julius Hyginus (64 BC-AD 17), the keeper of the Palatine library during the reign of Augustus