Stars usually do not evolve fast enough for humans to notice that they change within a lifetime. Not even a hundred lives are enough – astronomical processes are far too slow. But not always. There are certain phases of stellar evolution that occur rapidly, and when they do, they can be traced. A new paper published on ArXiv last week uses astronomical observations found in ancient Roman texts, medieval astronomical logs and manuscripts from China’s Han dynasty to trace the latest developments of several bright stars, including the red supergiant Antares and Betelgeuse: one of the most dynamic stars. in our heaven. With observations from the entire historical history, the newspaper suggests that Betelgeuse may have recently passed through the “Hertzsprung gap”, the transition phase between a main sequence star and its current classification as a red supergiant.
If you were to examine all the stars in the night sky for their color and brightness, you would see that most stars fall within a distinct pattern called 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 consumed all the hydrogen in their nuclei). Mapping stars in this way and plotting them on a graph is called a Hertzsprung – Russell chart, and it is a useful tool for understanding stellar evolution. One of the key features of the chart is a distinct gap between the main sequence and giant stars, known as the Hertzsprung gap. This gap does not really mean that stars do not exist within that gap – but rather that stars do not stay there for very long. It is a transition phase, which can be passed in a few thousand years, which means that it is rare to catch a star in the middle of the phase – hence the gap in the diagram.
Fortunately, this short-lived transition phase could theoretically be observed within the written historical record of mankind, for any number of stars.
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The key candidates for such a study are bright, nearby red supergiant stars that are 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 Scorpius, and Betelgeuse (the right axis of Orion), a star about 10 million years old that no longer burns hydrogen at its core. Towards the end of its life, Betelgeuse is expected to explode in a dramatic supernova sometime in the next 100,000 years (astronomically speaking, it is not very long).
Most of our knowledge of these stars comes from modern observations. However, modern remote sensing techniques are not perfect, and it is valuable to have several redundant methods for calculating phases of stellar evolution. The historical record can therefore help to confirm, or limit, the predictions of modern astrophysics.
The historical evidence: Betelgeuse in transition, and an immutable Antares
A key marker for the transition across the Hertzsprung gap is a change in color towards the reddish end of the spectrum. As such, historical descriptions of Betelgeuse or Antares denoting anything other than red would suggest a new transition.
You may be inclined to dismiss historical texts as potentially misleading or incorrect – in addition, a vague description of a star as “reddish” is not very scientifically useful by today’s standards. But the real value of historical documents arises when ancient writers made comparisons between distinct astronomical objects: Betelgeuse to Saturn, or Antares to Mars, for example. That kind of statement gives us a much more measurable, albeit approximate, data set to work with, because we can make the same comparisons in today’s skies with modern equipment.
This is exactly the kind of data that the magazine’s authors, led by Ralph Neuhäuser (AIU Jena), could find. They dug into a variety of historical documents and discovered several early descriptions of bright supergiants such as Betelgeuse and Antares. One of the key sources for Betelgeuse was The Astronomicaa Roman text attributed to Gaius Julius Hyginus (64 BC-17 AD), the owner of the Palatine Library during the reign of Augustus Caesar. The Astronomica says, in a literal translation, that “The star of the sun … body is great [i.e. bright], and color / staining fiery / burning; resembles the star in the right axis of Orion [i.e. Betelgeuse]… Many have said that this star is [the star] of Saturn. “
In addition, the tradition of calling Saturn “the star of the sun”, as Hyginus does, can be traced as far back as early Babylonian texts, and may have originated because Saturn’s motion in the sky is the most stable of all planets, and its synodic period (its apparent motion in the sky) agrees well with the length of the solar year. Hyginus describes the color of Betelgeuse as Saturn-like, which is clearly not red (Mars would be the obvious comparison for a red star). This suggests that almost two thousand years ago, Betelgeuse may not yet have entered its current stage of life as a red supergiant.
A second Roman source from a century later, the Almagestlists the brightest red stars in the sky – including Antares – but Betelgeuse is clearly missing from the list.
At the same time, all over the world, Sima Qian (BC 145-87), a “senior archivist” in the West Chinese Han Dynasty, wrote a treatise on celestial bodies called Tianguan shu. In this manuscript, Sima Qian describes Betelgeuse as yellow, while Antares was red. This affirmative statement from a completely different culture strengthens the arguments for a change of color in Betelgeuse over the last 2000 years.
Nearly 1,000 years later, Ibn Qutayba (828–889 AD), an Islamic scholar from the Abbasid caliphate, still described Betegeuse as reddish, as did astronomer Tycho Brahe (1546–1601 AD) a few centuries later. Oral tradition from the native Hawaiians also describes Betelgeuse as red. These three examples clearly characterize the star differently from its older peers, and more in line with modern observations.
Over the course of recorded history, if one is to believe these stories, Antares seems to have remained consistently bright red, which Betelgeuse has switched from yellow to red.
Drawing conclusions: the challenges of historical astronomy
Combining history with astronomy can provide valuable insights into the latest (astronomically speaking) evolution of the night sky, but it is not a perfect science and must be done carefully. One of the challenges with this method is the difficulty of accurately dating old texts. Most ancient manuscripts do not survive in the original, but rather as copies transcribed over the centuries in monasteries, libraries and scriptoriums. As such, the exact dates may be uncertain and works may be attributed to authors incorrectly. For example, there is a chance that The Astronomica incorrectly attributed to Hyginus, and is in fact a document from the 2000s, not a 1st century, as it seems to borrow part of its structure from the 200s Almagest. The good news is, on an astronomical scale, a century or two, give or take, does not matter much.
Another thing that can strike up modern researchers is the cultural influences that shape the language of ancient writers. The Tianguan Shu, for example, the star colors are grouped into 5 categories: red, blue, yellow, black and white. These colors actually do not match visual descriptions (“black” stars are not very meaningful literally, although it can mean “dark” or “dark”). Instead, the five colors come from Chinese Wuxing philosophy, where the colors are in line with five elements (Earth, Wood, Metal, Fire and Water) that emphasize cyclical changes in nature, politics and human physiology. Wuxing color groupings are not reliable markers of objective observation tones. Nevertheless, they represent distinct categories that can be used for comparison – red stars are clearly different from blue stars, for example. This means that Betelgeuse’s placement in a category other than Antares probably reflects a real observed difference, although the exact nuance of each category is unknown.
Comparing old text with modern observations: what do we know?
Modern estimates suggest that Betelgeuse has been in the red supergiant of its life cycle for at least a few thousand years, and could have been as long as 140,000 years (best estimates say it is about 40,000 years). The historical data suggest that the truth may lie in the latter part of that interval. Although not conclusive evidence, the historical document should not be easily discarded either. After all, modern astronomical knowledge, to paraphrase Isaac Newton, ‘stands on the shoulders of the giants:’ our current understanding is only possible because of the insights made by generations before us. From the writings and oral histories they left behind, our ancestors may still have something to teach us.
Read the paper here: R. Neuhäuser et al., “Color evolution of Betelgeuse and Antares over two millennia, derived from historical records, as a new limitation of mass and age.” ArXiv oppression.
Feature Image: Artist’s Impression of Antares. Credit: M. Kornmesser / ESO.
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