Another mystery about stellar evolution may have an answer. A group of astronomers looking at globular clusters think they have figured out the origin of a particular type of star known as a blue straggler. The evidence is not concrete but definitely seems to present a plausible answer to a plaguing mystery.

Globular clusters are tightly bound groups of stars that live on the outskirts of galaxies. They make for interesting places of study since usually most of the stars within a particular cluster formed around the same age, so studying different clusters of different ages gives us information about star’s evolutionary paths. Most globular clusters are quite old though, as opposed to open clusters which are usually much younger, and globular clusters are not producing any new stars. Almost every star in a globular cluster is an older star, at least several billion years old. So herein lay the mystery.

In many globular clusters a certain type of star was observed, blue stragglers. Blue stragglers are very massive hot blue stars. Normally, hot massive blue stars are considered young when they are observed because if it is born that hot and massive it will burn out its fuel generally in several million to a few hundred million years. So the problem with finding them in these globular clusters is that since we know the clusters are much older than a few hundred million years these stars should not exist there. So how is it that these stars are where they are and look how they look??

There were two general theories about how these stars are formed. The first involved collisions between stars. You have two stars of medium mass colliding to make one massive star. The other theory was that of stellar cannibalism, in which one star in a binary system feeds of the mass of the other star. Binary systems are just those that contain two stars orbiting around and interacting with each other.

Researchers set out to answer this question by looking at 56 globular clusters. They found that the predicted number of collisions did not match that which was required to give the number of blue stragglers, thus dispelling that theory. They did, however, notice a correlation between the mass contained within the core of the cluster and the number of blue stragglers. It is known that the more massive the core is the higher numbers of binary systems exist within it. Thus they could infer a relationship between number of binary systems and the number of blue stragglers, seeming to support the second theory. This conclusion is also supported by direct observation of the number of binary systems in cluster cores. All of this points toward stellar cannibalism as the explanation. This would not be the only instance of stellar cannibalism in the galaxy. It has been seen many times in binary systems where one star is massive, usually a red giant, and the other is a white dwarf, or already dead star. The smaller white dwarf accretes matter from its larger partner until nuclear burning reignites on the star causing a nova explosion.

The next step for these researchers is to try and find out some information about the original two stars in the binary system, or the parents of the blue straggler. There must be something special about these binary systems that initiates the cannibalism. Are they mostly isolated, or could dynamical interactions between the system and nearby stars be a factor? It’s interesting that we do know a lot about stellar evolution and dynamics but there is always new and interesting ways in which the universe is trying to stump us.

For many years astronomers have debated and speculated about the existence of intermediate mass black holes. Well an answer to the question may now be available. It looks as though one of these mysterious types of black holes has been discovered in one of the Milky Way’s globular clusters. However, this discovery may not have all the answers to the mystery.

For many years the only kinds of black holes that have been found are stellar sized ones and super massive ones. Stellar sized black holes form as a result of the death of a massive star and can range up to maybe 50 solar masses. Super massive black holes reside at the center of almost all galaxies and weigh millions if not billions of solar masses. But observations were lacking for any black holes in between these sizes.

Globular Clusters are dense formulations of stars that orbit in the outskirts of a galaxy. They reside in what is known as the halo of the galaxy. They tend to be very old and are generally no longer creating new stars. They are fairly common, we know of about 200 hundred that belong to the Milky Way.

So it was in the globular cluster known as Omega Centauri that this intermediate black hole is thought to reside. Omega Centauri is one of the largest and most massive clusters belonging to the Milky Way, and is about 17,000 light years from earth. Using NASA’s Hubble space telescope and the Gemini observatory in Chile astronomers were able to note that the stars at the center of the cluster were orbiting something with very fast speeds. These fast speeds and the absence of anything we can see suggests it is a black hole that the stars are orbiting. The astronomers then used theoretical models combined with this data to calculate the mass of this black hole at about 40,000 solar masses.

This is actually the second discovery of a medium sized black hole suggesting the first was not just a fluke and that they may be common. However, no theory currently exists to explain why they exist, that is, to explain how they could have formed. However, there existence might be important for the theory of how super massive black holes form. One of the current theories states that in an early galaxy a “seed” black hole would be needed of about this size. The black hole would feed to grow to super massive size.

However, one issue might be that Omega Centauri is not a normal globular cluster, and many theorize that it is actually the remnant of a dwarf galaxy that was gobbled up by the Milky Way. Models that compare data of galaxy mass with super massive black hole mass show that a dwarf galaxy like the one predicted for Omega Centauri would have had a black hole of this size. So this would suggest not that these intermediate black holes are the seeds for super massive ones but actually play the same role as the super massive ones on a smaller scale.

More of these intermediate sized black holes will need to be discovered before any questions about formation or purpose can be answered.