It is common to believe stars live in solitary. However, they frequently exist in dynamic relationships within binaries and clusters. These systems offer insights into star formation and evolution, with initiatives like the Gaia mission contributing significantly towards understanding these processes. In today's astrobito we talk about how comprehensive sky surveys are reshaping the way we do astronomy.
Paper details:
Title: Dancing with the stars: a review on stellar multiplicity
Authors: Thibault Merle
First-author Institution: Royal Observatory of Belgium, Brussels, Belgium Status: Accepted for publication in the Bulletin de la Société Royale des Sciences de Liège.
As we gaze into the night sky from a remote and dark location, we are captivated by the multitude of stars that expand across the sky. In such pristine conditions, attempting to enumerate the exact quantity of stars visible to the naked eye becomes almost impossible. Despite the overwhelming number of stars, a common perception arises: that each star resides in solitary within the remote corners of our Universe. This notion appears reasonable given the colossal scale of the sky; stars seem to occupy their own cosmic niches without apparent interaction.
However, the reality is quite different. Many stars do not exist in isolation; instead, they engage in dynamic relationships within binaries, clusters, and association systems. Far from being solitary entities, a significant proportion of stars are accompanied by other celestial objects, interacting altogether. These systems, commonly termed star systems, showcase the rich tapestry of interstellar connections that punctuates the vastness of the universe.
What can we learn from star systems?
Stars like company. They can form binary systems, where two stars orbit each, but can also exist in more complex configurations consisting of three or more stars. The study of these stellar systems, whether in binary pairs or more complex systems, enhances our understanding of star formation and evolution.
Star Formation: The exploration of multiple star systems has significantly advanced our understanding of their formation pathways. The discovery of binary star and compact triple systems, for example, has revealed that they often form through disk fragmentation followed by gas accretion-driven inward migration. In contrast, higher-
order systems seem to primarily form through inside-out processes, where the inner parts of the stars take shape first.
Star Evolution: The wealth of information derived from studying star systems has also highlighted the role of binary systems as fundamental building blocks for comprehending the evolution of higher-order systems. Close binary systems, where stars are in proximity (within approximately less than 10 astronomical units), offer insights into the underlying physics behind of mass and angular momentum transfer and loss between stars. These processes can lead to the formation of chemically unique stars, characterised by enhancements in carbon and heavy elements. Examples include Barium stars (where there’s an overabundance of ionised Ba), CH starts (with strong spectral absorption bands due to the methylidyne radical CH), and carbon-enhanced metal-poor stars.
This knowledge extends beyond merely describing the chemical composition of stars; it also allows us to interpret various astronomical observations resulting from past merger events in higher-order stellar systems. Examples include phenomena like the Betelgeuse cloud high spin, the 19th-century supernova impostor event of Eta Carinae, and even the generation of gravitational waves—all of which could be attributed to merging events within high-order star systems.
Astronomical surveys
Observational astronomy is ushering in a new era for space exploration, driven by our rapid technological progress. The continuous refinement of telescopes, coupled with the integration of machine learning algorithms into data processing pipelines, exemplifies the cutting-edge tools that are propelling our exploration of the cosmos.
Among the ground-breaking initiatives driving this progress are sky surveys, which afford us unprecedented insights into the universe (Figure 1). An astronomical sky survey constitutes a comprehensive map or image of a specific region of the sky. Most surveys have focused on detecting exoplanets and studying the history and formation of the Milky Way. However, these same surveys have also played a pivotal role in allowing systematic searches for binary and higher-order star systems throughout the cosmos.
Figure 1. Example astronomical surveys.
Noteworthy among these large sky surveys is the Gaia mission, aimed at constructing the most extensive and precise three-dimensional map of the Milky Way by surveying about 1% of the galaxy's 100 billion stars. This ambitious survey has resulted in the identification of the largest homogeneous sample of nearly 800,000 binary stars!
Gaia mission has also unveiled the presence of subtle companions within binary systems, including X-ray binaries housing neutron stars and black holes. This revelation has paved the way for a more systematic exploration of such celestial candidates. Notably, Gaia has marked the identification of binary systems hosting black holes, such as BH1, featuring a Sun-like star orbiting a black hole, and BH2, comprising a red giant also in orbit around a black hole. These extraordinary findings underscore the transformative impact of advanced sky surveys in expanding our understanding and demographics of binary and higher-order star systems.
Other sky surveys also actively contributing to study of star systems include the spatial missions Kepler and TESS.
A bright future ahead
The exploration of star systems is undeniably opening a new chapter in our comprehension of stars, encompassing their formation, evolution, and the recognition of occasionally surprising celestial companions. As we venture into the cosmos through extensive sky surveys such as the Gaia mission and other ground-breaking initiatives, our understanding of these stellar companions broadens. Simultaneously, we bear witness to the transformative influence of technological advancements, which continually enhance our perspective on the cosmos. This convergence of scientific exploration and technological innovation propels us towards an ever-deeper appreciation of the celestial wonders that populate our vast universe.