In the past, cartographers often placed monsters on their maps to mark unexplored areas and potentially dangerous regions. A famous example is the 1570 map Theatrum Orbis Terrarum, which depicts sea serpents and other sea monsters.
Today, in the present day, an unexplored star cluster in the Milky Way suggests that astronomers should also adopt this tradition. The cluster is known as Barbá 2 and lies only about 24,000 light years from Earth. A study using the Gaia star-observing telescope found that Barbá 2 is full of red supergiant stars. These are stars that can be hundreds of times larger than the Sun and up to a million times as bright as the Sun.
“There are many open clusters in the galaxy. However, not all open clusters are equally interesting to astronomers,” Ignacio Negueruela, a researcher at the University of Alicante who was part of the team that discovered the supergiants in Barbá 2, told Space.com. “Star clusters rich in red supergiants are very rare and tend to be very distant, but they play a crucial role in understanding important aspects of the evolution of massive stars.”
The intimidating size and power of supergiants means that these monster stars burn through their nuclear fuel much faster than stars like the Sun. While our star in its main sequence phase exists for about 10 billion years, supergiants are estimated to have a lifespan of just a few million years.
Because of the short lifespan of supergiants, open star clusters like Barbá 2 are common (more than 1,100 have been discovered in the Milky Way alone), but it is extremely rare to find a star cluster full of red supergiants.
Related: How do extreme “blue supergiant stars” form? Astronomers may finally know
Negueruela added that studying open clusters like Barbá 2, which are rich in monster-sized stars, could be important in understanding how they become or do not become red supergiants and how this affects their ultimate fate.
“There are only a handful of star clusters in the Milky Way that are rich in red supergiants,” he said. “The discovery of a cluster like Barbá 2 that can be observed with a medium-sized telescope is a significant and exciting discovery for astronomers. Because the cluster is so far away and is suffering from moderate extinction, it does not look particularly good in optical images.”
Barbá 2 was actually discovered about a decade ago by astronomer Rodolfo Barbá, but after his death in 2021, the discovery was not published until now. So it is only fitting that the cluster now bears his name.
“Rodolfo was known among his colleagues for being very slow in publishing his work. He made many important discoveries that he shared at conferences or discussed with colleagues, but often did not publish,” said Negueruela. “After Rodolfo’s death, his close collaborator Jesús Maíz took over the task of making sure his work was published.”
Nothing lasts forever, even for the most monstrous stars. But what remains is, as with outstanding scientists like Barbá, of crucial importance.
Legacy of a red supergiant: black hole or neutron star?
Although the lifetime of stellar bodies of all sizes makes that of our stars seem truly insignificant in terms of duration, this stellar life also comes to a standstill when the stars run out of fuel for nuclear fusion in their core.
The life of any star is a delicate balancing act between the outward radiative force generated by nuclear fusion in its core and the inward pressure of its own gravity. Whether this battle rages for millions or billions of years, gravity inevitably wins, but the results of that victory vary.
For example, stars the size of the sun give up their fight against gravity when the hydrogen reserves in their core are exhausted and they can no longer convert it into helium.
The gravitational collapse of these smaller stars creates a stellar remnant called a white dwarf. Further collapse of this stellar remnant is prevented by a quantum phenomenon called “electron degeneracy pressure,” which essentially prevents all electrons from occupying the same state.
If a star begins this process while it has at least eight times the mass of the Sun, and can retain at least 1.44 times the mass of the Sun during its initial collapse (the so-called Chandrasekhar limit), it can generate enough pressure in its core during the collapse to fuse helium into heavier elements, thus breathing new life into the star.
When the helium is used up, this process repeats itself; the star collapses again and again, fusing heavier and heavier elements, until the massive star has a core of iron, an element that no star can fuse into heavier elements. The star undergoes a final collapse, triggering a massive supernova explosion that blasts away its outer layers. However, this collapse can have two consequences.
The collapse of the massive star’s core forces electrons and protons together, creating a neutron star, a stellar remnant filled with a sea of neutrons. Neutrons are neutral particles that normally occur in atomic nuclei with protons. Further collapse is prevented by “neutron degeneracy pressure,” the pressure that each neutron exerts on the neutrons around it. But this too can be overcome if a star has enough mass.
If a complete collapse occurs, the star turns into a stellar-mass black hole. This is a region of space whose mass is so dense at its center that there is a boundary around it that not even light traveling at sufficient speed can escape: the so-called “event horizon.”
The cutoff point at which the neutron degeneracy pressure can be overcome is called the Tolman-Oppenheimer-Volkoff limit and is thought to be between 2.2 and 2.9 times the mass of the Sun. This means that it is not as clearly defined as the Chandrasekhar limit and scientists would like to establish this cutoff point with greater certainty.
Open star clusters full of red supergiants that could undergo such transformations could be the ideal laboratory to study in depth the formation of neutron stars or black holes and to investigate why a star takes one path and not another.
“The red supergiant-dominated clusters are probably young open clusters with a large concentration of stars. They provide us with information about the properties of red supergiants,” explained Negueruela. “These clusters are valuable to astronomers because they help us understand red supergiants, which are otherwise challenging to study on their own.”
Supercluster is super mysterious
Negueruela further explained that isolated red supergiants are difficult to characterize accurately because their distance from us is often not known exactly and it is difficult to determine basic properties such as their mass and age.
“The reason for this is that red supergiants with different intrinsic properties can look very similar at different stages of their lives,” he added.
The stars in open clusters are thought to have all formed simultaneously from the same collapsing cloud of gas and dust. This means that astronomers can determine the age of the cluster and then compare the properties of the older red supergiants in the cluster with those of the younger blue stars in the cluster.
“The masses of these blue stars are much easier to determine, which helps us learn more about the red supergiants,” said Negueruela. “Our models suggest that the number of red supergiants is directly related to the mass of the cluster.”
This means that astronomers expect about five supergiants per ten thousand solar masses. Negueruela pointed out that there appears to be another factor at play that we do not yet understand.
“Sometimes we find clusters of stars with the same age and mass, but one cluster is full of red supergiants while another has only one or two,” he continued. “There is an element of chance here, because the red supergiant phase in a star’s life is very short and we are dealing with small numbers where small changes can have big effects.”
The team’s models, which predict five red supergiants for a cluster of a given mass, also suggest that it’s not unusual to see between two and eight supergiants in a cluster. Still, the team suspects that this could be influenced by something else. Negueruela suggests that it could be related to how many stars in the cluster are part of binary or multiple star systems, or even the properties of those binary stars.
Not only is there a mystery about star clusters rich in supergiants that has yet to be explored, but there are also some unexpected problems related to the Barbá 2 star cluster.
“First, all known star clusters rich in red supergiants, with the exception of NGC 7419, are located in the central regions of the Milky Way,” Negueruela added. “This makes sense, since star formation is more intense in the inner regions of the galaxy, but it also means that they are all more difficult to study due to strong extinction (light blocking) by dust and gas along the line of sight. Barbá 2, on the other hand, is located in a completely different part of the Milky Way, further out.”
Another interesting point, according to the researcher, is that the area of the sky where Barbá 2 is located is very well studied, as it contains many fascinating objects. The discovery of this star cluster rich in supergiants therefore suggests that, despite years of intensive searching, there are still some hidden treasures waiting to be discovered.
“I’m a little surprised that no one else has come across Barbá 2,” said Negueruela. “This discovery shows that there is still room for improvement in our search methods.”
“The discovery of one of these clusters is only the first step. To fully exploit their potential as astrophysical laboratories, we need to combine stellar models and observations,” Negueruela concludes. “We will try to obtain more spectra to accurately determine the age of the cluster and thus its total mass.”
“In addition, we hope to gain insights from the properties of this cluster to refine our techniques for discovering similar clusters in the future.”
The team’s work is published on the paper repository site arXiv.
