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.

Planet-forming discs around young stars truncated by close encounters in Orion

The protoplanetary disk of HL Tau, as observed by the
ALMA Observatory. Discs like those would be continually
eroded by close encounters in dense star clusters like
the Orion Nebula Cluste (Credit: APOD)

Astronomers have for a long time suspected that the environment in which stars form could very strongly influence the formation of planetary systems. A recent paper by Simon Portegies Zwart at the Leiden Observatory shows this to be case and reports results of simulations that explore this effect. The simulations show that close encounters between stars in a dense cluster can truncate the protoplanetary discs around stars where planetary systems are thought to form. His simulations are able to reproduce the properties of protoplanetary discs in the nearby Orion Nebula Cluster, suggesting the stars and discs in this cluster have already felt the impact of their crowded environment. These interactions will not only lead to an erosion of the disc (leaving less material available to form planets) but also reduces the orbital radii at which planets can form, impacting the types of planetary system that might form.

Why are we interested in star clusters?

This blog is all about star clusters, which are pretty stunning and amazing astronomical objects, but some of you might be wondering why astronomers are so interested in them. So in this post I thought I'd give my Top 5 Reasons to Study Star Clusters! I hope you enjoy it!

The Orion Nebula Cluster, seen in X-rays and
optical light. By estimating the masses of the
young stars formed here we can study the
distribution of stellar masses that form
(Credit: Chandra X-ray Observatory)

1. Star formation. Perhaps the most important reason to study star clusters is because we believe the majority of stars form in groups or clusters and so by studying young star clusters we hope to learn something about how stars form. For example, by measuring the masses of all the stars in a cluster we can study the mass distribution of recently-formed stars (known as the initial mass function), which is one of the most important products of the star formation process.

Hubble Space Telescope image of an evaporating
protoplanetary disk (known as a proplyd) in the
Orion Nebula. The disk (dark silhouette in the
centre) is being eroded by radiation from a
nearby bright O-type star leading to the tail of
material stripped off(Credit: NASA/HST)

2. The impact of environment on star formation. Stars form in many different environments, from small groups of only a handful of stars, up to dense clusters with millions of stars and many thousands of luminous and massive OB stars. In dense clusters young stars are very close to each other, which can lead to close encounters that might disrupt binary systems or planetary systems. The bright OB stars that are present in the most massive clusters can also erode the disks around stars in which planets form, potentially hindering the creation of a full solar system like our own. Understanding how the environment that stars form in affects their final properties is therefore very important!

3. Stellar evolution. Just like humans, stars change as they age in many different ways, from subtle changes in their luminosity and slowing their rotation, to dramatic changes as they switch their source of nuclear fuel. To study these changes we need to know how old the stars are, but unfortunately its very difficult to measure how old individual stars are (we can't ask stars how old they are like we do with humans!). If you have a group of stars, however, you can often work out how old the group is by studying which stars have come to the ends of their lives and which haven't. This allows you to estimate the age of the cluster, and therefore all the stars in it. Once you know their ages you can study how the stars have evolved over time, an area of research known as stellar evolution.

The life cycle of a star like our Sun (upper row) and a more massive OB-type star (lower row).
Both types of star form in star forming regions and star clusters, but evolve through different
phases. By studying this process in star clusters with known ages, astronomers can
calculate how long this evolution takes (Image credit: SciOly.org)

4. Star clusters can be used to study star formation in distant galaxies. Its easy to study star formation in our own galaxy, the Milky Way, because we can observe stars forming deep within molecular clouds and the young stars that have recently formed. But in distant galaxies these things are too small and too faint to observe, so our understanding of star formation in other galaxies, whether it is different in any way, and how much star formation has been occurring, is limited. Star clusters however are bright and we can easily observe them in distant galaxies. Its also relatively simple to get a good estimate of their mass, the type of stars in them, and how old they are. This is really useful for astronomers because it means we can study not just the current star formation in these galaxies, but also star formation that occurred in the past. Astronomers call this the star formation history of a galaxy and its useful for understanding how galaxies evolve over cosmic time.

The Antennae Galaxies, two interacting galaxies with a rich and vibrant star formation history.
Astronomers have been able to study its star formation history by observing the many
star clusters (bright blue dots surrounded by red clouds) (Credit: Hubble Space Telescope)

The Jewel box Cluster (NGC 4755), one of many
clusters that are important for measuring distances
in astronomy! (Credit: APOD)

5. Star clusters are an important step on the cosmic distance ladder. In actual fact, they're two steps! The first step is a 'local' step on the cosmic distance ladder, and it comes from a process known as the moving cluster method, which is essentially a perspective effect whereby if you know the direction that all the stars in a cluster are moving you can estimate how far away it is. The second step is a much more 'distant' step on the ladder, which uses the luminosity of globular clusters to estimate the distance of the galaxy that they're in. The method stems from the assumption that the brightness globular cluster in a galaxy usually has the same luminosity as the brightest globular cluster in another galaxy. Based on this, if you can measure how bright the globular clusters in a galaxy are, you can estimate how far away the galaxy is.

There are many other reasons to study star clusters, but these are some of the most important and wide-ranging, spanning the formation of planetary systems to the size of the Universe!

So next time you hear about star clusters or new research into our understanding of these amazing objects, think about all the different scientific topics that might be influenced by those new results!

Improving our census of star clusters in the Large Magellanic Cloud

A new paper published last week by Romita et al. presents results of a search for new star clusters in the nearest galaxy to the Milky Way, the Large Magellanic Cloud (LMC).

The LMC is a dwarf satellite galaxy currently orbiting our galaxy that has a number of key differences to the Milky Way. For a start the LMC is much smaller than the Milky Way, but critically it is also less chemically evolved, meaning that it has fewer 'metals', which is the name astronomers give to anything other than hydrogen or helium. By studying the distribution of star clusters in this galaxy we can try to understand whether star formation and the evolution of star clusters has proceeded any differently in this environment compared to in our own Galaxy.

The authors have targeted a 1.65 square degree area of the LMC that includes the massive star forming region 30 Doradus, the largest region of star formation in the LMC, and larger than anything in our own galaxy. It's a rich field of star formation as the images below show, and a good place to be hunting for new star clusters.

Images of the area of the LMC studied in this paper. On the left is a colour image compiled from the infrared observations used in the study, while on the right a black and white image is marked with the positions of the newly-discovered star clusters (red dots) relative to the positions of known molecular clouds (black ellipses). (Credit: Romita et al. 2016)

Using infrared images of the LMC the authors identify 65 embedded star clusters, 45 of which are new discoveries. Using their observations the authors are able to estimate the sizes, masses and luminosities of these clusters, all key properties of star clusters.

The authors compare the distribution of these star cluster properties with their distribution in our own Galaxy, and find that the LMC clusters are generally larger, more massive, and more luminous. Since these three quantities are often well-correlated with each other, it's not a surprise that all three properties are bigger in the LMC, but this does clearly show that LMC star clusters are typically more massive than those in our galaxy.

The authors also find the density of clusters in the LMC is 3 times higher than in the Milky Way, and that the mass of clusters in this area of the LMC is 40 times higher than an equivalent area in out galaxy. Both these results suggest that the LMC is producing star clusters at a much higher rate than in our own Galaxy.

These two results are actually linked. If a galaxy is forming more stars and producing more star clusters then it is likely that it will, on average, produce larger and more massive clusters than a galaxy that it is forming fewer star clusters. It is clear that the LMC is very actively forming stars and clusters at the moment.

However the authors note that this shouldn't surprise us because the LMC contains many more molecular clouds than the Milky Way, and since stars form in molecular clouds then more molecular clouds should mean more star clusters! They find that both galaxies display the same relationship (known as the star formation rate scaling law) between the amount of dense gas and the amount of stars (and star clusters) that are forming.

This means that while the environments of the two galaxies may be different, the star formation process that takes place within them isn't. We can therefore take what we've learnt about star formation in the Milky Way and apply it to other galaxies. This is an important step forward for understanding star formation across the Universe!