Ancient astronomers could see a star that was 12 times brighter than it is now

The authors of the new study suggest that the ancient observers were not mistaken. The increased brightness may have been sustained by the interaction of two closely spaced stars, which exchanged matter and released energy over a period of around a thousand years.

For the time being, this remains a hypothesis, published as a preprint and not yet peer-reviewed. The researchers have shown that the proposed scenario is physically possible, but have not proven that events unfolded in precisely this way.

Details

Theta Eridani, also known as Akamar, lies approximately 165 light-years from Earth in the constellation Eridanus. Its current apparent magnitude is around 2.9.

Magnitude is an unusual scale: the smaller the number, the brighter the object. Furthermore, it is logarithmic. A difference of nearly three magnitudes signifies not a threefold but a more than tenfold change in brightness.

Today, Theta Eridani is a noticeable star, but one that is otherwise unremarkable to the naked eye. In ancient catalogues, it appeared quite differently.

Around 129 BC, Hipparchus described it as the brightest and southernmost star in the celestial ‘river’ of Eridanus. In 137 CE, Claudius Ptolemy assigned it first-magnitude status and included it among the 13 brightest stars in his catalogue.

Almost 800 years later, in 964, the Persian astronomer Abd al-Rahman al-Sufi also classified Theta Eridani as a first-magnitude star. However, as early as 1603, the Dutch navigator and astronomer Frederick de Houtman classified it as a third-magnitude star — practically the same as modern observers do.

It appears, therefore, that Theta Eridani retained its increased brightness from at least the time of Hipparchus until the time of al-Sufi. Then, between the 10th and the end of the 16th century, it became approximately 10–12 times fainter.

Could the ancient observers have been mistaken?

Over the past century, astronomers have proposed several simple explanations for the discrepancy. Ancient authors may have confused Theta Eridani with another star, made a mistake whilst transcribing the catalogue, or failed to account properly for the influence of the Earth’s atmosphere.

The authors of the new study examined these theories in detail but found no convincing solution.

It would have been difficult to confuse Theta Eridani with the much brighter star Achernar. In the time of Hipparchus and Ptolemy, Achernar was too far to the south and was not visible from the latitudes where they worked. The coordinates and descriptions of Theta’s position also allow the star to be identified with a fair degree of certainty.

The theory of a scribal error also seems insufficient. Ptolemy not only specified the first magnitude in the *Almagest*, but also included Theta Eridani separately in a list of particularly bright stars in another work.

It is more difficult to assess the independence of the historical evidence. Ptolemy drew partly on Hipparchus’s observations, whilst al-Sufi’s catalogue was based on the *Almagest*. However, al-Sufi observed the stars himself, corrected many of Ptolemy’s estimates and described in detail the position of Theta Eridani relative to neighbouring objects. He did not alter its brightness.

There is also the issue of the atmosphere. From Greece and the Middle East, the star was visible low above the horizon, where its light dims noticeably. Yet in ancient catalogues, the brightness of other southern stars is estimated with relative accuracy. Theta Eridani remains the greatest discrepancy amongst the approximately one thousand stars in the *Almagest*.

After correcting for observational conditions, the authors estimated its ancient apparent magnitude to be approximately 0.2. This corresponds to a brightness roughly 12 times greater than today.

What might have happened to the star

Modern observations have shown that Theta Eridani is not a single star, but a system of three stars. Of particular interest is the close inner pair, whose components have been designated Aa and Ab.

Each of them is more than twice as massive as the Sun. Their masses are estimated at approximately 2.3 and 2.2 solar masses, and their radii at 4.3 and 4 solar radii.

The stars complete a full orbit around their common centre of mass in just 4.1 days. The average distance between them is 0.083 astronomical units — almost five times less than the average distance from Mercury to the Sun.

Both stars fill around 80 per cent of their Roche lobes. This is the name given to the notional regions within which matter is still held in place by the gravity of each star. If a star expands beyond this boundary, some of its matter begins to flow over to its neighbour.

One of the stars has recently, by astronomical standards, finished burning hydrogen in its core. At this stage, it began to expand and may have approached a critical limit.

The authors suggest that the pair’s orbit was previously more elongated. Each time they came close together, the expanded star lost some of its mass. At some point, both stars may have been surrounded by a shared gaseous envelope.

Movement within such an envelope slowed the stars down, drew energy from their orbit and converted it into heat and radiation. It is this process, according to the researchers’ hypothesis, that caused the system to shine about ten times brighter than usual.

Gradually, the orbit became more circular, the exchange of matter weakened, the shared envelope dispersed, and the system returned to its current brightness.

Why this is important

If the hypothesis is correct, ancient astronomical catalogues have preserved a description of an extremely rare event that cannot be reconstructed from modern observations. They have, in effect, extended the history of the star’s study by two thousand years.

Usually, noticeable outbursts from interacting stars last for days, months or years. An event lasting around a millennium represents a completely different type of transitional phase in the evolution of close binary systems.

Such systems play an important role in the formation of unusual stars, novae and certain supernovae. However, these stages of mass transfer—which are brief by cosmic standards—are difficult to observe directly.

A search for similar objects in modern sky surveys could reveal whether millennia-long flares actually exist and how frequently they occur.

Limitations of the study

The main limitation is that the paper has so far only been posted on arXiv and has not undergone independent peer review.

Historical brightness estimates were made with the naked eye using a rough six-point scale. These cannot be compared in terms of accuracy with modern photometry, and the catalogues of Hipparchus, Ptolemy and al-Sufi were interlinked.

The proposed mechanism is based on modern stellar characteristics and calculations of available energy. However, the authors have not yet constructed a detailed model that would accurately reproduce the required brightness, the duration of the outburst and its decline between the 10th and 16th centuries.

There is also a statistical difficulty. A millennium-long flare should account for only a negligible fraction of the star system’s lifetime. The fact that it occurred specifically in one of approximately a thousand stars visible to the naked eye, and coincided with the emergence of written catalogues, seems unusual.

It is therefore too early to claim that the mystery has been solved. The study shows that the ancient brightness of Theta Eridani may well have been real, and proposes a plausible scenario that remains to be tested.

Background

The name Eridanus derives from a mythical river. In ancient times, Theta Eridani marked the southern end of the constellation and was given the name Akamar, associated with the expression ‘end of the river’.

Later, European navigators began to observe more southerly regions of the sky. The constellation was extended to include the considerably brighter star Achernar, whose name has a similar origin.

In 1603, in Johann Bayer’s atlas *Uranometria*, Theta Eridani was already depicted as much fainter than Achernar. Observations by Edmond Halley in the 17th century and Nicolas-Louis de Lacaille in the 18th century also correspond to its current brightness.

Thus, historical data point not to a gradual dimming over two thousand years, but to a prolonged period of brightness that ended approximately between 964 and 1597.

Source

The study by Idel Weisberg and Boaz Katz , *The forgotten bright star: Theta Eridani as a millenary stellar transient observed by Hipparchus,* Ptolemy and al-Sufi⁠ was posted on the arXiv preprint server on 29 June 2026.