How do you form a massive black hole binary? (Part 3)

This post brings to an end a three-part series of posts about how massive black hole binary systems form, such as those recently observed to merge by the LIGO gravitational wave detector. The first two posts discussed how such massive black holes could form and how a binary system of black holes could form in isolation. Here I'm going to talk about a much more efficient way to form binary black holes, and that's in a star cluster!

As we discussed last week, forming a binary black hole in isolation is very hard. There are lots of processes that could disrupt the binary system before both stars form black holes, and once the binary has fallen apart it's hard to put it back together. But not so in star clusters! In star clusters there are lots of (gravitational) interactions between stars (or black holes) that, under the right circumstances, can lead to the formation of new binary systems. In fact there have been many simulations to explore exactly this sort of process!

The dense and massive star cluster R136 in the 30 Doradus nebula. The bright blue dots are predominantly massive
OB-type stars, many of which will end their lives as black holes (Credit: HST).

The basic idea is a simple 3-step process first put forward by Sigurdsson & Hernquist in 1993. The first step is that star clusters are full of massive stars, and these massive stars are heavier than normal stars and so they naturally sink to the centres of the clusters (we observe this process, called mass segregation, in many star clusters). The second step is that once these stars reach the ends of their lives they explode as supernovae and leave behind stellar remnants such as black holes. This we also know to regularly take place.

Simulation showing the development of mass segregation in a star cluster after only 2 Myrs (right)
compared to a lack of mass segregation at the beginning of the simulation (left). The massive stars (big circles)
are clearly more centrally concentrated in the right-hand panel (Credit: Marc Freitag).

The third step, which is where the clustered environment of the star cluster is critical, is that close encounters between stars (or black holes) in the centres of star clusters can quickly lead to the formation of binary systems, and therefore potentially black hole binary systems. Outside of star clusters the density of stars in space is very low and close encounters are very rare (which is fortunate for our Sun and solar system otherwise we'd always be bumping into other stars!). But star clusters are dense and the stars within them are moving very fast, meaning that close encounters between stars are common. If the stars come close enough to each other in just the right way then a binary system forms.

Many studies have confirmed this general picture and have estimated that it should take (on average) about 1 Giga-year (1000 million years) for enough interactions to occur between stars and black holes to form a binary black hole system in a dense cluster. This means that such binary black hole systems are most likely to be found in older clusters, such as globular clusters, though it shouldn't be impossible to find them in younger clusters.

Does the Omega Centauri globular cluster house
a central black hole? (Credit: ESO)

There is good evidence that this happens quite often. A recent study of the massive globular cluster Omega Centauri found that the dynamics of its stars suggest a particularly massive central black hole, while another study even found evidence that there might be a black hole within the nearby Orion Nebula Cluster. Confirming these theories is difficult because directly detecting black holes isn't always easy, but the evidence is there to suggest they might be occupying many of our favourite star clusters!

Further dynamical interactions within the star cluster can lead to two important effects for the binary black hole system. The first is that the binary will shrink over time due to dynamical interactions with individual stars - this brings the black holes in the binary closer together, making an eventual merger more likely. The second effect is that these black hole binary systems can be easily ejected from the star cluster, often at quite high velocities, through interactions with other binary systems.

The most likely clusters to produce binary black hole mergers are intermediate age clusters with ages of a few Giga-years. Younger clusters are unlikely to have had enough time to form the binary black hole systems, while older clusters will have formed them but they might have all merged by now.

This process implies that a typical star cluster, which contains many massive stars, will easily produce many black holes, and most likely a few black hole binary systems that will one day merge and release gravitation waves just like those observed in September.

Given how many star clusters there are in the Universe its hardly surprisingly that such a black hole merger was observed so easily!

How do you form a massive black hole binary? (Part 2)

Following the recent discovery of gravitational waves from a merging black hole binary system, I've been dedicating a few posts to exploring how such a system could have formed in the first place. Last time I talked about how such massive black holes could form, and here I want to discuss how a binary black hole system could form.

Artist's impression of a binary system comprised of
two massive OB stars (Credit: Universe Today)

The answer to this question depends a lot on where the black holes formed. It's a lot easier to form a binary black hole system in a star cluster where there might be many black holes than it would be to form such a system in relative isolation.

Lets deal with the more difficult case of forming a binary black hole system in isolation first. Most massive stars (the precursors of black holes) are actually born in (and spend most of their lives in) binary systems composed of two massive stars orbiting each other. However, there are many events during a massive star's life that can disrupt the binary.

Schematic of a binary star system undergoing
common-envelope evolution
(Credit: Adrian Potter)

First, the star swells up and inflates to become a red supergiant. These are the largest types of star known, with diameters hundreds of times their original size and many thousands of times larger than our own Sun. If the red supergiant is in a binary system then it is possible that its outer atmosphere could spill over onto the secondary star, forming an envelope of material that encompasses both stars.

This process is known as common-envelope evolution, so-called because the two stars effectively share their outer envelopes (see the figure to the right). The stars in the common envelope experience a drag on their binary orbits, slowing them down and shrinking the binary system. The phase is typically quite short-lived, but can actually end with the two stars merging!

If the binary system survives the common-envelope phase it may be disrupted by the material the star has ejected during this phase. Red supergiants expel a considerable fraction of their mass through stellar winds. This weakens the binary system by taking away some of the mass holding it together, which causes the binary system to widen. If the system widens sufficiently the stars may actually separate and the binary will be no more!

Finally, the last act in the life of a massive star is a supernova explosion. Again, this expels considerable mass from the star and therefore from the binary system, which could disrupt the binary. In fact this is a commonly-considered mechanism for the disruption of binary systems composed of two massive stars.

This presents a difficult path for a massive binary system to negotiate if it is to become a black hole binary. The system must survive a potential common-envelope phase while one (or both) stars are red supergiants and it must survive the loss of considerable mass from both stars from stellar winds and supernova explosions that can weaken the binary. However, if the system negotiates these obstacles then it could form a binary black hole system just like that observed to merge by LIGO.

Next time I'll talk about a potentially much simpler way to form a massive black hole binary system, and that's in a star cluster!

How do you form a massive black hole binary? (Part 1)

The science news media has been buzzing over the last fortnight after the LIGO (Laser Interferometer Gravitational-wave Observatory) team announced the discovery of gravitational waves. The signal appears to have come from the merger of two black holes, each approximately 30 times the mass of our Sun, that were orbiting each other and have since coalesced. Many posts have been devoted to this amazing discovery and its implications, but I'd like to talk about how the binary black hole system might have formed and what it means for our understanding of black holes, starting in this post with how you might form such massive black holes.

Artists impression of the collision of two black holes, as detected by the Laser Interferometric
Gravitational-wave Observatory (Credit: Vox.com)

First, a little background. Black holes broadly come in two varieties: stellar-mass black holes, with a mass a few times that of our Sun, and supermassive black holes, with millions of times the mass of our Sun. The less massive variety are found throughout galaxies, such as the X-ray binary Cygnus X-1, which was the first black hole discovered (see image below), while their more massive cousins are thought to lie at the centres of most large galaxies like our own.

Artist's impression of Cygnus X-1,
a black hole accreting material from its
companion star (Credit: Chandra X-ray Observatory)

How supermassive black holes form isn't really understood at the moment, but fortunately we do have a good idea how stellar mass black holes form. When very massive stars (O- and B-type stars) have exhausted all the nuclear fuel in their cores then they can no longer produce the necessary energy to support the weight of their heavy outer layers. Under the force of gravity the star collapses in on itself, a process known as core collapse. This collapse is incredibly violent and leads to two things, the first being the release of an immense amount of energy (a supernova) and the second is the creation of a very dense remnant such as a neutron star or a black hole.

This process is well understood and there is considerable evidence for it, as well as for the existence of black holes. However, the gravitational wave source that was observed in September wasn't just a black hole, it was two black holes orbiting each other in a binary system, and each black hole was thirty times the mass of our Sun, which is quite large for a typical black hole! So how do you make a black hole this big?

Wolf-Rayet star #124, as imaged by the Hubble
Space Telescope, showing all the material thrown
off by the star as it nears the end of its life
(Credit: HST)

The main factor determining the mass of a black hole is how much material the massive star that it formed from expelled during its life. This material might be expelled by stellar winds during the late stages of its life, such as while the star is a red supergiant or a Wolf-Rayet star (see image to the right), or much of the material might be expelled when the star explodes as a supernova.

The best way to prevent a star from losing most of its mass is to weaken its stellar winds so that they don't expel much material. The strength of a star's winds is usually proportional to the amount of elements like carbon, nitrogen or oxygen (known as metals to astronomers) that the star has. These elements accelerate the loss of material from stellar winds, so their absence would help to reduce the amount of mass lost and therefore increase the mass of the black hole produced.

Scientists have even gone as far as estimating that to produce a black hole as massive as those in the recent binary black hole merger they'd need to form from a massive star with as little as one hundredth the amount of metals that there are in our own Sun. Since the amount of these metals in a typical star has increased over the lifetime of the Universe, you'd need to form these stars very early in the Universe for them to have few enough metals to produce such massive black holes.

However, you could also make such massive black holes by other processes, possibly starting with a smaller black hole and slowly growing it. Material falling into black holes is absorbed by them, increasing their mass, though this is surprisingly hard to do efficiently, so it would be a slow process. Alternatively you could grow a black hole more rapidly by merging it with another black hole, in exactly the same way that the gravitational wave source that was detected was a merger of two black holes.

Whatever the process, the discovery of this merging black hole binary system suggests that black holes as massive as this must be relatively common in the Universe for scientists to have observed such a merging system so easily. Whether the black holes formed with such masses, or if they grew by accretion or mergers we may never know until we are able to detect and study more black holes and better understand the properties of these elusive but amazing objects.