A greater percentage of systems are multiple star systems, meaning that they are systems containing two or more stars, and in particular, binary star systems, which specifically contain two stars, are exceptionally common. When we can see both of the stars, they are called visual binaries, and if the plane of their orbit happens to coincide with our line of sight, such that they pass in front of each other, they are called eclipsing binaries. This situation allows us to gather important information about the system, and is actually one of the best ways to detect them in the first place, as we can chart the reliably periodic change in luminosity first as the larger star passes in front of the smaller, and then as the smaller star passes in front of the larger, in cyclical fashion.
But beyond this, we also know that stars come in many different varieties. Therefore, the possible combinations that exist for the two types of stars that comprise a binary system are even more numerous. What are the most interesting combinations we have been able to find for types of binary star systems, and what can they teach us about the universe? Certainly, there are many binary systems that involve very sun-like stars orbiting each other at quite a distance. An example of this is Alpha Centauri A and B, which we examined in a previous tutorial.
But a far more interesting case involves close binary systems. This is when the stars are very close together, orbiting around their center of mass quite rapidly. Sometimes they are so close that the gravitational distortion produced causes their stellar atmospheres to exchange material, or they could even be so close that they are in direct contact with one another, such as this system with two large, hot, main sequence stars practically overlapping. This is most fascinating when one star is a compact object, like a white dwarf star, neutron star, or black hole, as this object will begin to pull matter away from the other object in the system until a dramatic event occurs. If a white dwarf is causing the accretion of gas from another star, it becomes a cataclysmic variable star, where the incoming gas gets very hot and emits radiation.
We sometimes call such an object a vampiric star, as it is almost as though it is sucking the essence out of its companion star like a vampire. If instead the compact object is a neutron star or black hole, this is called an X-ray binary, which can be either a low-mass or high-mass X-ray binary depending on the mass of the donor star, which is the other star in the system, feeding the compact object with material. One fascinating binary system is called AR Scorpii. This is a binary pulsar, which consists of a white dwarf-pulsar about the size of Earth, and a red dwarf star. Pulsars are highly magnetized objects that emit powerful beams of radiation in a rapid, periodic manner.
Typically pulsars are neutron stars, but sometimes they can be white dwarfs as well, though being much less compact than neutron stars they rotate more slowly. AR Scorpii contains the first object of this type that was ever discovered. We should note that binary systems can have complicated evolutions. Take for example a binary system with two fairly large main sequence stars, which form with around 15 and 20 solar masses respectively.
We know that stars of this size evolve rapidly, since the inward gravitational pressure is so strong that fusion occurs at a furious pace, burning through the hydrogen in the core much faster than in smaller stars. Eventually one star enters a phase of expansion and will exceed its Roche lobe, meaning that it juts far enough into the gravitational field of the other star that one will begin to pull material away from the other. This material may form an accretion disc, as we saw previously, but it can also be absorbed through direct impact, as shown here. We see the blue star acting as the vampiric star, rotating faster and flattening out.
We also see that an enormous percentage of the other star’s mass is being transferred in the process, actually the majority of its mass. This star is now so much more massive that fusion increases even more dramatically, which generates a stellar wind that causes the other star to become very small. Eventually this star will go supernova and leave a tiny neutron star behind, potentially escaping the system altogether. The vampiric star will then reach a red supergiant phase where it expands immensely, after which it too will go supernova and leave a neutron star behind. So we can see that stellar evolution is much more complicated in binary systems due to the influence each star has on the other. There are so many fascinating systems out there, not just binary systems, but triple-star systems and beyond.