In the works of ancient astronomers, Theta Eridani is described as one of the brightest stars in the sky. Today, however, it is visible to the naked eye but is no longer especially prominent. Scientists say they may have solved this mystery.
Theta Eridani
Astronomers have been observing the night sky for thousands of years, and ancient records sometimes become valuable material for modern research. This is precisely the case described by phys.org. It concerns a recently published arXiv paper devoted to Theta Eridani.
Eridanus is a southern constellation that would also be clearly visible from the Northern Hemisphere if it contained more bright stars. Ancient astronomers described one of the brightest objects in the night sky as being located within it.
The star in question is Theta Eridani, which today is only the third-brightest star even within its own constellation. In the writings of ancient authors, however, it was described as one of the brightest stars in the sky. In 129 BCE, Hipparchus described it as the brightest star in the River, referring to Eridanus as the river from ancient mythology.
Ptolemy agreed with him in 132 CE, as did the Arab astronomer al-Sufi in 964 CE. The latter even assigned it a brightness of magnitude 1 using a system broadly comparable to the modern stellar-magnitude scale. Yet in the works of European astronomers from the seventeenth century, its brightness was already estimated at magnitude 3, approximately the same value observed by astronomers today.
A Mysterious Star System
At first glance, the difference between magnitudes 1 and 3 may not seem very large. However, the astronomical brightness scale is logarithmic, meaning that the star actually dimmed by a factor of ten. This is extremely difficult to explain, especially because both its earlier and present states appear to have persisted for centuries.
The difference therefore cannot be explained by temporary flares. What, then, caused it? According to astronomers, the answer lies in the nature of Theta Eridani itself. It is actually a system consisting of three stars: a close binary pair and a third star located much farther away, orbiting the common center of mass of the first two.
The first two stars attracted the researchers’ attention. They are massive stars with masses of 2.2 and 2.3 times that of the Sun. The distance between them is only 0.083 astronomical units, less than the distance between Mercury and the Sun. Both stars are very old and have nearly filled their Roche lobes, the regions of space they can occupy before matter begins to flow from one star to the other.
At present, the more massive star has just completed hydrogen burning in its core. The researchers suggest that several centuries ago, when hydrogen was still its main source of energy, the star’s envelope reached the boundary of its Roche lobe. As a result, matter may have flowed onto the companion star, causing a sharp increase in the luminosity of the system. Later, the rate of energy production decreased, the envelope contracted, and the transfer of matter ceased.
According to phys.org
