Stars falling and shattering
Many stars you see when you look up at the sky have spent at least part of their lives within star clusters. Star clusters are communities that can consist of millions of stars that share a common origin and are bound together by gravity.
Star clusters are of scientific interest because all their members formed at approximately the same time and share similar properties. For example, if the cloud of gas and dust in which the cluster formed is rich in certain elements, the stars that form within the cluster will also contain abundant amounts of those elements.
There are two types of star clusters: open (galactic) and globular clusters. These two clusters are distinguished by several characteristics.
Galactic Star Clusters
Open star clusters are generally irregularly distributed collections of stars whose members can range from a few dozen to a few thousand. They are frequently found in the galactic plane, and most cluster members have been around for less than 10 billion years. Some are even embedded in the remnants of the gas and dust cloud from which the cluster originated.
The stars that make up open clusters are bound to each other by a weak gravitational pull. As a result, the cluster can break apart due to irregular interactions with other objects as it orbits the galaxy, and its members can leave the cluster.
The Seven Sisters: The Pleiades
Because open clusters are relatively more dispersed and irregular than globular clusters, the stars that make up the cluster can be easily resolved individually, even with a simple telescope. Some are even visible with the naked eye. The Pleiades is just one of these clusters.
The Pleiades is one of the most beautiful Messier objects observable with the naked eye. Containing over a thousand stars, the seven bright members of this cluster, known as the "Seven Sisters," are visible even from relatively light-polluted urban areas. Most of the stars that make up this cluster are clustered in an area 440 light-years away.
The Pleiades are moving through the galaxy toward a region near the foot of the constellation Orion (the Hunter). The cluster is expected to remain held together by gravity for the next 250 million years. However, as the cluster moves, its members will move further and further apart, and like many other open star clusters, it will fragment.
What are Open and Globular Star Clusters? Where Are the Stars Born with the Sun Now?
The Universe's Largest Structures: What Are Galaxy Groups, Clusters, and Superclusters?
How Many Galaxies Are There in the Universe?
The Disintegrating Cluster: The Ursa Major Moving Group
The stars that make up most constellations are actually completely independent and unrelated. For example, Bellatrix, one of the stars that make up the famous Orion constellation, is about 250 light-years from Earth, while Rigel, also in the same constellation, is over 800 light-years from Earth.
However, the situation is slightly different for the constellation Ursa Major. The Ursa Major Moving Group (Ursa Major Moving Group) is an example of a fragmenting open star cluster. This cluster formed several hundred million years ago, 78 to 84 light-years away, and is now spread out over an area 30 light-years in diameter. Six of the constellation's brightest members belong to this cluster.
The famous constellation Ursa Major and the Big Dipper. Six of the eight stars that make up the dipper (one of which is a binary star system, as can be seen in the photo) are members of this open star cluster and are almost equally distant. The two stars at the tip of the dipper and the handle are not included in this cluster.
Globular Star Clusters
Globular star clusters are extremely dense stellar populations consisting of thousands or even millions of stars. They mostly orbit the galactic halo and are composed of the galaxy's oldest stars.
Globular star clusters form a spherical shape due to the high gravity of hundreds of thousands of stars. They contain almost no clouds of gas and dust.
The number of globular clusters in a given galaxy appears to be related to the galaxy's mass. There are approximately 150 known globular clusters in the Milky Way. However, it is likely that more clusters lurk behind the galaxy's thick disk. Our nearest galactic neighbor, the Andromeda Galaxy, is home to over 400 globular clusters. M87, a massive elliptical galaxy containing several trillion stars, contains approximately 15,000 globular clusters!
The Milky Way's Largest Star Cluster: Omega Centauri
Omega Centauri, with approximately 10 million stars, is the largest globular star cluster in the Milky Way Galaxy and lies approximately 15,000 light-years from Earth. With a diameter of 150 light-years, this massive cluster is 10 times larger than a typical globular cluster!
Omega Centauri is so dense that the stars in the cluster's core are separated by only 0.16 light-years on average. By comparison, the closest star to the Sun is Proxima Centauri, at only 4.2 light-years away. This means the stars in the cluster's core are extremely close together.
"Intermediate" Black Holes in Globular Clusters
Globular star clusters are extremely dense, making them much denser than the rest of the galaxy. This density increases the presence of compact objects such as black holes, neutron stars, and white dwarfs in globular clusters. In fact, theoretical calculations suggest that the frequency of these objects in globular clusters is approximately 100 times higher than in the rest of the galaxy.
Images obtained with the Hubble Space Telescope and data from the GMOS spectrograph at the Gemini South Observatory in Chile indicate that Omega Centauri may harbor an elusive and rare intermediate-mass black hole at its center. Astronomer Eva Noyola of the Max Planck Institute for Extraterrestrial Physics in Garching explains:
This result demonstrates a continuous mass range for black holes, from supermassive to intermediate-mass to low-mass types.
Noyola and his colleagues measured the motions and brightness of stars at the center of Omega Centauri. The calculations indicate the presence of a black hole with a mass of approximately 40,000 solar masses at the cluster's center.
Omega Centauri is just one of the globular star clusters that contains a black hole. The Hubble Space Telescope had previously detected the presence of another intermediate-mass black hole with a mass of 20,000 solar masses at the center of the G1 cluster in the Andromeda Galaxy.
Before this observation, we had only one example of an intermediate-mass black hole—located in the nearby G1 globular cluster in the Andromeda Galaxy.
Similarly, data from Hubble suggests that the globular cluster M15 in our galaxy also harbors a black hole with a mass of approximately 4,000 solar masses at its center.
Black holes at the centers of globular clusters could provide answers to questions about the formation of galaxies. Roeland Van Der Marel of the Space Telescope Science Institute in Baltimore says:
With these new data from Hubble, we won't just learn how black holes form. This data will help us connect globular clusters to galaxies and provide insights into one of the most important unsolved problems in astronomy today: how galaxy structure forms in the universe.
Karl Gebhardt of the University of Texas at Austin suggests that the intermediate-mass black holes found by Hubble could be the building blocks of the supermassive black holes found at the centers of most galaxies.
There are two main theories of black hole formation: You can either make a black hole all at once by putting a lot of material in the center of a galaxy as it forms; or you can start with a black hole that grows over time, like a seed. Observational evidence suggests this: starting with a small black hole.
Astronomers have been searching for black holes in globular clusters for about 30 years. Their only obstacle was that ground-based telescopes could not easily resolve the stars closest to the black hole thought to exist at the cluster's center. However, the high resolution of the Hubble Space Telescope has overcome this obstacle. Researchers say the decades-long search for black holes is now over.
Determining Cluster Ages by Stellar Metallicity
The age of a cluster can be determined by the elements its members contain. Stars forming young clusters are richer in metallicity. Conversely, older clusters are considerably poorer in metallicity because they formed before the formation of heavier elements in the universe.
Globular star clusters near the center of the Milky Way Galaxy are older than those more distant. This suggests that the younger clusters formed during mergers with other galaxies, while older clusters remain from the time the galaxy was born.
The Cluster Where the Sun Was Born
The Sun was born 4.5 billion years ago in a loose open cluster of stars that likely contained between 1,000 and 10,000 stars. We know this because stars forming in dense clusters constantly gravitationally interact with each other, causing the disks that will form planets around them to disperse. However, the Sun has a very large planetary system, including Earth.
The Formation of the Sun
In fact, many nebulae are inactive and static until triggered. This was also true for the gas and dust cloud that would eventually form the Solar System. However, millions of years before the Sun came into being, something stimulated the nebula. Astronomers believe this was caused by a supernova. Shock waves from a supernova can affect the nebula, initiating star formation. Studies of meteorites also confirm the idea that the nebula that would form the Solar System was triggered by a supernova.
If you're curious about the formation and evolution of the Sun and the Solar System, you can read our article. We won't go into detail here to stay on topic.
Cluster Fragmentation
Members of open star clusters are bound to each other by a weak gravitational pull. Therefore, open star clusters break up over millions of years as they move.
The Sun completes one complete orbit around the Milky Way Galaxy in 200-250 million years. Therefore, our star has completed 18 complete orbits around the galaxy so far (we are currently in our 19th orbit). This time is more than enough for an open star cluster to break up and disintegrate.
Searching for the Sun's Lost Siblings
Stars formed in the same star cluster bear the traces of the nebula from which the cluster formed. This is because they are composed of similar chemical compositions. This is like a "fingerprint." For example, if you examine the spectral data of the stars in the Pleiades cluster, you can easily tell that they formed from the same cloud of gas and dust.
Spectral data of six stars in the Pleiades obtained by an amateur astronomer. The stellar spectra are generally very similar, except for some details.
As a result, HD 162826, which has the same elemental abundances and orbital parameters as the Sun, was identified as the Sun's sister star. HD 162826 likely originated in the same location as the Sun 4.5 billion years ago.
Ramirez explained:
Finding even a solar sister is exciting. We expected to discover none. We expect the number of such stars to be very small.
While HD 162826 is the Sun's sister, it is not its twin and is 15 percent larger than the Sun.
HD 162826 is located in the constellation Hercules. It is in the vicinity of Vega, the fifth-brightest star in the sky.
According to Ramirez, the existence of a giant planet like Jupiter around HD 162826 is unlikely. However, there may be smaller terrestrial planets such as Earth, Mars, or Venus.
HD 162826's shared birthplace with the Solar System and the possibility that it may harbor terrestrial planets make it a prime target for the search for extraterrestrial life. Ramirez stated that while this possibility is small, it is not zero, and that life-bearing material could travel interstellar, carrying the seeds of life from Earth to planets in other star systems.
It could be argued that solar companions are key candidates in the search for extraterrestrial life.
Gaia Mission
Launched by ESA in 2013, the Gaia mission is collecting data on the motions and distances of 1.3 billion stars in our galaxy using parallax. Measurements from space can yield much better results than ground-based measurements.
The extensive capabilities offered by the Gaia mission will make the search for the Sun's siblings much easier. According to Ramirez, the number of stars they can study will increase by a factor of 10,000!
Every Sun-sister star discovered by science will bring us one step closer to understanding the origins of the Solar System.