# Can a small black hole orbit a large star?

I recently read about the discovery of a tiny black hole (with only three times the mass of the sun) nicknamed ‘The unicorn’ about 1500 light years from earth. This got me thinking, can this black hole orbit around a star? For example, let’s take UY Scuti with a mass 7-10 times that of the sun.

1. Is it theoretically possible for a small black hole like the unicorn to revolve around a giant star like UY Scuti?

2. Will such a star be stable?

3. Will the black hole take mass from UY Scuti and form an accretion disk?

4. What will happen if both of them collides?

• Well, they'd orbit each other. To be more precise, they'd both orbit their barycenter. Most stellar black hole candidates are in a binary system with a visible star, the more common solitary BHs are hard to detect, as mentioned on Wikipedia. Jun 18, 2021 at 12:33
• FWIW, Wikipedia has a list of the most massive known neutron stars, and a list of the least massive known black holes. Jun 18, 2021 at 15:05
• Why shouldn't it be possible? Jun 19, 2021 at 5:31
• What's your opinion on @PM2Ring 's idea that most are found in such binary systems. Does that meet your requirements for "a black hole orbiting a star?" If not, what's your definition of one object orbiting the other? A common definition used is when both objects orbit a barycenter that is enclosed in one body or the other. Jun 20, 2021 at 4:03
• Related tweet (from the lead author). Jun 20, 2021 at 9:55

A black hole of a given mass will probably have arisen from the collapse/supernova of a much more massive star. In particular, stars with an initial mass of less than around 15-20 solar masses are unlikely to leave a black hole remnant at all. Stars of $$<8$$ solar masses end their lives as white dwarfs and those with $$8$$ to $$\sim 15$$ solar masses likely leave neutron star remnants.

What happens is that (i) the star will lose mass during its life, particularly during the final stages of its life; (ii) if there is a supernova explosion then only a fraction of the star's mass will end up in the compact remnant.

So in principle there is no problem with a 3 solar mass black hole accompanying a more massive "normal" star. It will have started life as more massive (and short-lived) than its companion.

There are however problems in producing close binary systems in such an arrangement. The "normal" star would have to survive the experience of a nearby supernova explosion, but of course the black hole may have formed (in the case of the unicorn example) when the "normal" star was on the main sequence and so their separation would be considerably wider (as a multiple of the stellar radius).

It is quite likely that in a close binary system that an accretion disc would form. Even if the separation was wider, then some of the natural mass loss from the giant companion would likely be captured by the black hole and it has to form a disc because the accreting material has angular momentum.

I'll have to think about the last part, but you could clarify exactly what you mean by "collide".

• By collision, I meant is that if the star and the black hole get closer and closer, what happens in the end? Will they both merge and form a larger black hole? Jun 18, 2021 at 12:57
• stars with a mass of less than around 15-20 solar masses are unlikely to leave a black hole remnant at all Correct me if I'm wrong, but I think you're referring to fallback-limited accretion supernova, which is very uncertain, arguably at least as uncertain as stellar evolution models (which have no real issue with producing low mass black holes) if not more uncertain, because it depends on the uncertainties of supernova physics. LIGO has already observed black holes less than 15 solar masses. And LIGO has observed mass gap events. Jun 18, 2021 at 14:54
• @DaddyKropotkin Stars with initial masses less than 15-20 solar masses have end states which are neutron stars or white dwarfs (according to almost all theoretical studies I have seen). Jun 18, 2021 at 14:57
• OH of course, but in that case I think you should specify that you mean the initial stellar mass, i.e. the ZAMS mass. I was confused about which mass you refer to: ZAMS or pre-collapse... Jun 18, 2021 at 16:00

This is a great series of questions!

Such a low mass black hole (BH) could have originated from a few possibilities: 1) a result of stellar evolution (the resulting black hole mass depends fundamentally on the initial mass and metallicity of the stellar progenitor, among other things); 2) a star collapsed into a neutron star which can accrete matter from its surroundings until its mass overcomes the neutron degeneracy pressure and forms a BH; 3) a result of a binary neutron star merger.

1. Is it theoretically possible for a small black hole like the unicorn to revolve around a giant star like UY Suit?

Yes. In the three cases above, the newly formed low-mass BH could be captured gravitationally by a nearby star, for example if it forms in a dense stellar cluster.

As a different example, consider the life of a stellar binary, a pair of stars that are gravitationally bound to each other, in which they evolve in a kind of kindred evolution where the development of one can affect the development of the other. Generally, one star will be initially more massive than the other, and at the end of their stellar lifetimes they collapse into compact objects - let's assume their initial masses are high enough and metallicity is low enough that a BH forms when each collapse. So, generally, the initially more massive star will collapse into a BH first, leaving the possibility that the binary is composed of a BH and a star. In this system, the bodies are mutually orbiting each other, but from the star's frame of reference, the BH is orbiting it. You can extend this thought experiment by adding more companion bodies, but it that case the bodies would orbit about the common barycenter.

Observationally, an example is a high-mass X-ray binary system, where a compact object, such as a BH, accretes matter from a stellar companion, and the BH can be thought of as orbiting the star. This system is such an example

2- Will such a star be stable? 3- Will the blackhole take mass from UY Scuti and form an accretion disk? 4- What will happen if both of them collides?

Continuing with our example of the stellar binary: the answer to your question essentially depends on whether the stellar companion of the low mass BH will fill its Roche lobe, which depends on various things. Basically, the Roche potential defines gravitational potential of a classical binary system, and the Roche lobe is the region around a star in that binary system within which a particle is gravitationally bound to that star. So, as a star ages it generally expands, and by the time it evolves into a late-stage gas giant, its radius is huge. To first approximation, the radius of the star's Roche lobe is proportional to the binary separation (and it depends on lots of other things, like the orbit's eccentricity, the star's rotation rate, etc...). Thus, if the separation is small enough, then the companion star can fill its Roche lobe, which causes matter to pass through the first Lagrange point to the BH - this is known as "mass transfer" in binary evolution theory, and is uncertain and complicated, but that's the basics of it. The mass transfer can be stable, dynamically stable, or dynamically unstable (also known as common envelope evolution because the donor star's envelope engulfs the entire binary). Generally, an accretion disk may form in this process.

If the mass transfer is dynamically unstable (meaning the BH's accretion rate cannot keep up with the mass transfer rate from the star), then common envelope evolution may cause the binary to merge as the viscous friction of the binary's motion through the common envelope expels the envelope by shrinking the binary separation. Common envelope evolution is observationally unavoidable, but the physics of it is still uncertain. If such a merger/collision occurred, the BH would likely tidally erupt the star (though this is also complicated!).

• Also, typical population synthesis models produce black holes at or below ~ 15 $M_{\odot}$. Depending on the various input physics, this can be as low as 5 $M_{\odot}$, see for example Fig. 13 of ui.adsabs.harvard.edu/abs/2020A%26A...636A.104B/abstract Jun 18, 2021 at 22:32

If you don't consider how the black hole (BH) has formed, then it is quite possible for a black hole to form a bound state with a massive object like a star. They can circle (or ellipse) around their center of mass (COM). If this COM lies close to the star then the BH will be orbiting around the star. There will be no noticeable difference between a BH and a normal star, insofar as the gravitational effects are considered on distances sufficiently larger than the Schwarzschild radius. It's not necessary for a BH to stay put in spacetime, and it acts like any other massive object (the swallowing of everything passing its event horizon not taken into consideration).

The BH can move inside the curved spacetime of a heavy star just like a star with a mass of three times the mass of the sun can orbit around a star that has a mass of, say, seven solar masses.

Will the BH eat the star? If the BH appeared from outer space, passing the star, then a stable orbit can develop. Their COM will lie closer to the star than to the BH. The distance of both will depend on their relative velocity. If the orbit is a highly eccentric ellipse, then it is possible that a part of the star crosses the event horizon of the BH and this part will be lost. The other part will continue its travel. Upon the next close encounter, another part can get absorbed by the BH. Reducing the mass of the star further, and its velocity too (contrary to a close encounter where no mass is absorbed). This means that their maximum distance with respect to each other is reduced. This process will reinforce itself, so eventually, the star will be absorbed by the BH.

This will not happen in the case of two massive objects, like stars circling one another. If the stars don't touch each other, no matter from one of them will be absorbed by the other. Of course, they can get stretched due to tidal forces. And if the elongated material of one will find itself on the other side of the COM, it will be pulled to the other star. But absorption caused by an event horizon will be out of the question.

• What would happen if a 3 solar mass BH directly hit a 315 solar mass star? Jun 19, 2021 at 22:59
• @Mazura Nice question! I guess it depends on how fast it hits the star. If the velocity is small, then the whole star will be absorbed. If it moves very fast, then the BH will find its way through, absorbing a small part, In between? Jun 19, 2021 at 23:04
• @Mazura 315 $M_\odot$ is huge! It's 50% larger than the most massive star on en.wikipedia.org/wiki/List_of_most_massive_stars It would take a long time for a 3 $M_\odot$ BH to consume that much matter. The area of that BH's event horizon (EH) is just under 987 km², and even if the star matter could cross the EH at light speed, at a density of 150 g/cm³ (roughly solar core density), it would take over 447 years to absorb the whole star. Jun 20, 2021 at 19:41
• (cont) But that can't happen. Matter falling into a BH forms an accretion disk. See astronomy.stackexchange.com/q/8275/16685 & astronomy.stackexchange.com/q/12947/16685 Jun 20, 2021 at 19:42
• Re "If the star has a much higher mass than the star": Can you disambiguate? Or otherwise change it? Jun 20, 2021 at 20:20