The third brightest star in the constellation Columba (Gamma Columbae), about 900 light years from Earth kept a sectret – which has now been disclosed by Dr. Andreas Irrgang from the Dr. Karl Remeis Observatory in Bamberg in collaboration with Prof. Dr. Norbert Przybilla, a former Remeis astronomer, now at Innsbruck University. The team discovered that the star once formed the heart of a binary star system and lost its shell when it engulfed its companion.
Credit: Collage: Prof. Dr. Norbert Przybilla (Innsbruck): https://www.eso.org/public/images/eso2010a/ https://supernova.eso.org/exhibition/images/0418_cno-1080/)
Their findings were recently published in the renowned journal Nature Astronomy.
Since the late 1930s, it has been known that stars generate their enormous radiation energy through nuclear fusion of hydrogen into helium at temperatures of many millions of degrees Celsius deep inside the star. This fusion takes place in a cyclic process in which carbon, nitrogen and oxygen act as catalysts. This results in a characteristic enrichment of nitrogen in the core of the star.
Because massive stars such as Gamma Col are wasteful with their nuclear energy supply, they only exist for a few million years before they pass away in a gigantic explosion, a supernova. This nuclear evolution is usually not directly observable because the very dense stellar envelopes shield the hot central fusion reactor. Predictions of stellar evolution models can therefore only be tested by observations of stellar surfaces of numerous stars.
For many years, Irrgang Przybilla have been observing massive stars by analysing their spectra using sophisticated model atmospheres. Now they made a unique discovery: the massive star Gamma Columbae in the southern constellation Columba showed anomalies in the chemical composition of its surface – deviating from what is expected for stars with similar mass.
Together with Prof. Dr. Georges Meynet, a leading stellar evolution theorist from the University of Geneva, they found the explanation.
Gamma Columbae once belonged to a binary system and orbited with another star around a common centre of mass and lost its envelope when it engulfed its companion star.
If the two stars were close together, the gravitational wave radiation caused the stellar orbits to shrink. When the stars were close enough to interact, a common envelope around both stars formed and was finally ejected. In this way the stellar core, the heart of the star, so to speak, was exposed. This suggests that Gamma Columbae may be the exposed core of what was originally a much more massive star in a former binary system.
Based on the composition found, the star formed with a mass of 12 solar masses and lost no less than 7 solar masses. It can be assumed that Gamma Columbae has reached about 90 per cent of its estimated lifetime of a good ten million years. This means that it should have less than two million years to live before it explodes.
Gamma Columbae offers even more: It shows complex light variations (“Pulsations”) caused by waves propagating through its envelope. Like earthquake waves are used to probe the interior of the Earth, the pulsations allow the deeper layers of Gamma Columbae to be probed. Futher observations of these pulsations will give insight into the past and future of binary stars in detail.
Original publication: doi.org/10.1038/s41550-022-01809-6
Andreas Irrgang
Andreas.Irrgang@sternwarte.uni-erlangen.de
+49 951 95222 16
Related press releases:
- Swiss National foundation on idw:
https://idw-online.de/de/news803874 - University of Geneva:
https://www.unige.ch/communication/communiques/en/2022/histoire-dun-strip-tease-cosmique