Globular Cluster

How to Crash Stars Together

Article Updated: 24 Dec , 2015


The math is simple: Star + Other star = Bigger star.

While conceptually this works well, it fails to take into account the extremely vast distances between stars. Even in clusters, where the density of stars is significantly higher than in the main disk, the number of stars per unit volume is so low that collisions are scarcely considered by astronomers. Of course, at some point the stellar density must reach a point at which the chance for a collision does become statistically significant. Where is that tipping point and are there any locations that might actually make the cut?

Early in the development of stellar formation models, the necessity of stellar collisions to produce massive stars was not well constrained. Early models of formation via accretion hinted that accretion may be insufficient, but as models became more complex and moved into three dimensional simulations, it became apparent that collisions simply weren’t needed to populate the upper mass regime. The notion fell out of favor.

However, there have been two recent papers that have explored the possibility that, while still certainly rare, there may be some environments in which collisions are likely to occur. The primary mechanism that assists in this is the notion that, as clusters sweep through the interstellar medium, they will inevitably pick up gas and dust, slowly increase in mass. This increase mass will cause the cluster to shrink, increasing the stellar density. The studies suggest that in order for the probability of collision to be statistically significant, a cluster would be required to reach a density of roughly 100 million stars per cubic parsec. (Keep in mind, a parsec is 3.26 light years and is roughly the distance between the sun, and our nearest neighboring star.)

Presently, such a high concentration has never been observed. While some of this is certainly due to the rarity of such densities, observational constraints likely play a crucial role in making such systems difficult to detect. If such high densities were to be achieved, it would require extraordinarily high spatial resolution to distinguish such systems. As such, numerical simulations of extremely dense systems will have to replace direct observations.

While the density necessary is straightforward, the more difficult topic is what sorts of clusters might be capable of meeting such criteria. To investigate this, the teams writing the recent papers conducted Monte Carlo simulations in which they could vary the numbers of stars. This type of simulation is essentially a model of a system that is allowed to play forward repeatedly with slightly different starting configurations (such as the initial positions of the stars) and by averaging the results of numerous simulations, an approximate understanding of the behavior of the system is reached. An initial investigation suggested that such densities could be reached in clusters with as little as a few thousand stars provided gas accumulation were sufficiently rapid (clusters tend to disperse slowly under tidal stripping which can counteract this effect on longer timescales). However, the model they used contained numerous simplifications since the investigation into the feasibility of such interactions was merely preliminary.

The more recent study, uploaded to arXiv yesterday, includes more realistic parameters and finds that the overall number of stars in the clusters would need to be closer to 30,000 before collisions became likely. This team also suggested that there were more conditions that would need to be satisfied including rates of gas expulsion (since not all gas would remain in the cluster as the first team had assumed for simplicity) and the degree of mass segregation (heavier stars sink to the center and lighter ones float to the outside and since heavier ones are larger, this actually decreases the number density while increasing the mass density).ย  While many globular clusters can easily meet the requirement of number of stars, these other conditions would likely not be met. Furthermore, globular clusters spend little time in regions of the galaxy in which they would be likely to encounter sufficiently high densities of gas to allow for accumulation of sufficient mass on the necessary timescales.

But are there any clusters which might achieve sufficient density? The most dense galactic cluster known is the Arches cluster. Sadly, this cluster only reaches a modest ~535 stars per cubic parsec, still far too low to make a large number of collisions likely. However, one run of the simulation code with conditions similar to those in the Arches cluster did predict one collision in ~2 million years.

Overall, these studies seem to confirm that the role of collisions in forming massive stars is small. As pointed out previously, accretion methods seem to account for the broad range of stellar masses. Yet in many young clusters, still forming stars, rarely do astronomers find stars much in excess of ~50 solar masses. The second study this year suggests that this observation may yet leave room for collisions to play some unexpected role.

(NOTE: While it may be suggested that collisions could also be considered to take place as the orbit of binary stars decays due to tidal interactions, such processes are generally referred to as “mergers”. The term “collision” as used in the source materials and this article is used to denote the merging of two stars that are not gravitationally bound.)


Stellar collisions in accreting protoclusters: a Monte Carlo dynamical study

Collisional formation of very massive stars in dense clusters


33 Responses

  1. Torbjorn Larsson OM says:

    they will inevitably pick up gas and dust, slowly increase in mass. This increase mass will cause the cluster to shrink,

    Coolness, relative cooling by “dust busting”.

    Another example of how unintuitive gravity can be. For example, if I was dust busting, I would heat. (Despite that, if professing to an occasional behavior of it, will deduct Manly Points and give them to the Female Team!)

    [Or rather, it would transfer me from the “Potential Hot” subgroup to the “Potential Mate” one. Not an ideal place in this game!

    “Honey, I keep dropping you heirloom porcelain. It must be the testosterone.”]

  2. IVAN3MAN_AT_LARGE says:

    Hmmm… I did a quick scan and neither of the two papers (linked above) mentions blue straggler stars.

  3. Jon Voisey says:

    Ivan: While blue stragglers could certainly be created by collisional processes like this, the rarity of such collisions to the number of blue stragglers indicates that this mechanism likely isn’t responsible. Rather, blue stragglers are much more likely caused by mergers or mass transfer between binaries.

  4. Lawrence B. Crowell says:

    I remember in 4th grade during the unit on astronomy I asked whether stars ever collided with each other. No, was the answer. I also remember asking if asteroids ever collided with Earth — again no. As Phil Plait says. “It’s a bad universe out there.”


  5. Aqua says:

    If anything ever resembled what might be called a ‘White Hole’…. then Globular Clusters would be closest!

  6. IVAN3MAN_AT_LARGE says:

    The Universe is like a game of golf: despite the seemingly improbable odds that a person can hit a golf ball into a hole-in-one shot from at least 150 yards, it happens all the time.

  7. jimhenson says:

    Not a single star has ever been observed to collide with another star. Galaxies collide everywhere because they have black holes, which is gravity. stars obviously must have field charges that repel each others gravitational attraction. Otherwise we would easily observe many colliding stars in our own galaxy if it was as simple as everybody here says it is.

  8. bluestraggler_Fe says:

    Glad someone else mentioned blue stragglers. They’re one of my favorite topics. Also, do larger globular clusters tend to have black holes at their barycenter? I don’t believe I’ve ever heard that discussed. If so, is there a tipping point or critical mass for that?
    It certainly is a bad universe out there – according to the History Channel !

  9. IVAN3MAN_AT_LARGE says:


    [D]o larger globular clusters tend to have black holes at their barycenter? I don’t believe I’ve ever heard that discussed.

    Well, you’ve heard it now: “Black Hole Discovered In Center Of Enigmatic Omega Centauri“. ๐Ÿ™‚

    Here are some more links (just copy & paste into URL address bar):

    “Hubble Discovers Black Holes in Unexpected Places” —

    “First black hole found in globular star cluster” —

  10. In light of the papers and the article suggesting that such collisions are UNlikely (though theoretically possible with enough “if”s added), perhaps a better headline would be “HOW TO NOT CRACH STARS TOGETHER” …

  11. Olaf says:


    Where would that charge come from and why would the charge stay on the star?
    What is this charge? Positive or negative? Because if you have a negative charged star and a positive charged star it should attract each other very fast meaning that 50% of all stars MUST collide because they attract each other.

    Also stars do collide, look at the many examples of one star sucking the other star dry because they are so near.

  12. Olaf says:

    Also how much is the charge?
    Has bigger stars a bigger charge?
    has blue stars a bigger charge?

    How come the orbit of the stars appears to be attracted to another star and not behave like it is being repelled by another star?

    How come the global clusters and galaxies stay together and do not fly apart because of the charged stars?

    Give us some numbers and formula’s to calculate how big the charge is. Something tells me that you have not even a number or maths to prove your claims.

  13. DrFlimmer says:

    @ Jimhenson

    Well, I have heard similar statements before. Can’t quite remember on which occasions that happened (irony intended).

    To add one question to Olafs’:

    What about angular momentum?

    If the stars just miss each other by a little bit, angular momentum is at hand. And since it needs to be conserved, it is quite hard to get rid of it, and to collide stars (and also other objects, actually). And also: Space is huge and empty. There is enough room for a close miss, and even more for a large miss….

  14. jimhenson says:

    my theory is if Galactic black holes are monopoles of negative charge, and all stars have like + charges that repel each other having the same positive charged H+ solar wind coronal south pole emissions as a result of fusion like our sun, then stars motions swarming galactic orbits are caused by opposite charge attractive forces from the singularity at the galaxy center towards each star, and is without tiny stars colliding with each other.

  15. Lawrence B. Crowell says:

    There is no physics involving charge. The dynamics is simply Newtonian mechanics F = ma with the force of gravity F = -GMm/r^2 between any two masses. You have to do a huge sum over all such masses which attract any particular mass. For a million stars this becomes numerically intensive. There are a number of things which do emerge from some statistical arguments. One of them is the so called Virial theorem, which concludes the summation of the kinetic energies of these bodies is equal in magnitude to half the potential energies of these bodies. This is a sort of equilibrium condition, and for it to be obtained a fair number of stars are initially ejected out. It is a sort of many-body analogue of evaporative cooling. The state of the system in this equilibrium condition is such that the stars will be closer together. Further, this equilibrium condition is not entirely stable, for collisions are not conservative — this is not an ideal model with perfectly elastic bodies that collide without losses. In fact the stars partially stick together. So from a statistical mechanical perspective there is something similar to a chemical potential which changes the number of particles (stars) in the system. This should reduce over time and favor the gravitational clumping of matter into black holes.


  16. jimhenson says:

    n A correct solar system model, assuming I’m suggesting one, would retain all positive charged solar wind protons. Heerikhuisen found a ribbon shaped magnetic wall at 100 AU exists where the heliopause edge boundary is, deflects back all positive protons H+ in a U-Turn style coming from the suns solar wind emission back to the solar system where they came from probably retaining a postive charge for our sun or solar system. charges borders between our sun and the galaxys magnetic field are identified seen where at 100 AU the ribbon exchanges charges producing ENA’s. Assuming all stars would be like our sun, then their positive winds too would be reflected back retaining + charge too, unless it became a supernova expulsion into the intergalactic medium.

  17. jimhenson says:

    IBEX discovered that inside the solar wind positive charged protons forms a huge 200 AU round heliosphere bubble emitting ENAs that travel large straight distances with minimal changes. it could transport energy and angular momentum (that appears to be gravity) to the heliosphere of every star, this includes the sun and its solar system, as it orbits the center of the galaxy. ENA images produce different plasma object MASSES and energy ranges. ENAs emit no light and have large gravity effects. Its believed that the galactic magnetic field shapes the heliosphere as it drapes over it. The ribbon runs perpendicular to the direction of the galactic magnetic field just outside the heliosphere, and Dave McComas of IBEX says there’s a missing fundamental aspect of the interaction between the heliosphere and the rest of the entire galaxy.

  18. DrFlimmer says:

    @ Jimhenson

    Your model of a charged star (or in our case the sun) is simply refuted by the solar wind: electrons and protons are floating outward with the same velocity. This means that the sun cannot be charged, otherwise a large electric field would attract one species and repel the other. This would result in different velocities of protons and electrons, which is not observed.

  19. bluestraggler_Fe says:

    Thanks for the links. Fascinating stuff!

    Others: re charged stars – if stars carried an overall charge, what would be the implications for neutron stars where protons and electrons have been forced together into neutrons. And then what about strange matter stars (if they exist.)

  20. Olaf says:

    bluestraggler_Fe , just ignore the imhenson stiff, he is making up things as he goes but has no that is scientifically basis to check.

    A star cannot be charged as EU proponents would you want to believe.
    The problem is how would the charge be of a star positive or negative?

    Ok let’s assume it is negative. The charge will repel each other so only at the surface would there be a negative charge. I
    If it were a solid planet, the charge would stay there a bit, but in time it will release its charge gradually.
    In the case of a star the charge gets disposed very fast because of the solar wind carrying the charge.

    What could happen on a star because the star is turbulent gas, you might have a charge build-up in one location and a inverse charge build-up at a different location. But the complete charge will cancel each other out and be zero.

    In regard to your neutron star, if electrons and protons have crushed together, that can only mean that the charge is neutral. Equal number of protons and neutrons.

  21. Olaf says:

    To illustrate the insanity of a electric star theory.

    Let’s assume that the Sun is + charge, then earth must be – charged otherwise it will get repelled. Here comes the problem, what charge has the moon?

    OK let’s say that the Sun has +++ charge, the Earth has — charge and the moon – charge. So the moon still gets attracted to Earth. Now what would the charge of Jupiter be? And specially his many moons., what would each charge of each moon be and how come none of it’s moon moves in a none-elliptical orbit since the charges would attract-repel-attract-repel depending on the location of the moon and the location of all other moons.

    And it gets worse and worse this theory when you take into account that the planets would lose their charge.

  22. Lawrence B. Crowell says:

    Stars are not appreciably charged. No astrophysicist considers such ideas. This has noting to do with the diffuse plasma at the heliosphere boundary. The motion of large bodies in aggregates of this sort, solar systems, clusters, galaxies and so forth obey Newton’s laws and Newtonian gravity. One does not even need to consider general relativity, unless one is concerned with dynamics close to a black hole.


  23. Olaf says:

    As usual the tons of blablabla and fuzzy logic taken out of context taken but again no clear numbers no clear maths. And the typical tricks to sidestep the discussion with other none related fuzzy stuff like the heliosphere.

    So I ask the questions to those promoting EU/PC again: if the sun is charged what is its charge and what are the charged for the planets and moons in out solar system? How come the planets move move in an elliptical orbit like there is only one force and it is attracting, never repulsing? Give us numbers and some formula’s!

  24. bluestraggler_Fe says:

    Since I don’t quite know who any of you are at this point, I was being very careful about how I said that the “charged star” notion seems counterintuitive. You may all be astrophysists for all I know. My knowledge is limited to a minor in chemistry with my biology major many years ago. I do read a lot of the articles in Universe Today – I’ve been fascinated by astronomy for a very long time. That said, my point about the neutron star was that if stars had charges, how would you explain a neutron star, which by definition is all neutrons, absent anything but neutral charged particles, ergo no charge. The idea of any bodies on the scale of planets and stars having a charge doesn’t seem realistic. This may be a stretch, but wouldn’t the fact that the strong force is magnitudes greater than gravity imlpy a whole different set of stellar dynamics than what we observe, especially in close proximity like binaries and even globular clusters?

  25. IVAN3MAN_AT_LARGE says:


    I presume that, when you referred to “strong force”, you had meant to say electromagnetic force which is, indeed, 10^36 times stronger than gravity. However, electrostatic attraction is not relevant for large celestial bodies — e.g., moons, planets, stars, and galaxies — for the simple reason that such bodies contain equal numbers of protons (+) and electrons (-); therefore, the protons and electrons conspire to cancel out so perfectly that they have a net electric charge of zero.

  26. IVAN3MAN_AT_LARGE says:

    Addendum: However, nothing “cancels” gravity because it is only attractive, unlike electric forces that can either attract or repel, but all objects having mass are subject to the gravitational force — which only attracts. Therefore, gravitation is the dominant force on the large scale structure of the Universe.

  27. Olaf says:


    You might not realize but a neutron is basically proton+electron+electron neutrino
    A neutron cancels the proton + charge with the addition of an electron which is -. The electron neutrino has no charge.

    Since all atoms are generally neutral in charge (equal number of protons and electrons) when you press them together to become a neutron star all electrons are pushed inside the nuclear core too. So the net charge stays zero.

    I was thinking first that by some mechanism somehow all electrons could be ejected during an explosion, but when you think about it. That would mean that the star becomes very positive charged and will attract back the ejected electrons ending as a zero charge in the next few thousands years.

    It is insane to go the EU/PC route it only displays that you are very ignorant of physics. It is only possible in a transient state.

    But look deeper, imagine the Sun ejecting a huge solar flare all electrons. What happens is e.g. 1.000.000 Negative charged electrons fly away. Now the sun becomes 1.000.000 charges more positive. The negative and positive attract each other, so the freshly ejected electrons will get attracted back and the positive charged cores will be attracted towards the electrons moving away. I think a ring should be formed around the sun if that were true. of particles that become neutral again (the + and – found each other) and are falling back because of gravity.

  28. Jon Voisey says:

    OLAF: Electrons aren’t the only things streaming off the Sun. The protons that were the nucleus of the atoms from which they were ejected also get blown out of the Sun. Thus, net charge is conserved.

  29. Olaf says:

    @Jon Voisey

    I know!
    I was just doing a thought experiment to check it.
    I was just checking out if is remotely possible and what you should observe if it were true. But the complications get so big very fast that you would be a nut-job if you believed EU to be true. It would only be possible in a works in a SF story.

    A ring around the sun was something I did not expect, it would be cool to create some maths and models for this. But I lack the mathematics skills in this case.

  30. DrFlimmer says:

    @ Olaf

    You might not realize but a neutron is basically proton+electron+electron neutrino

    I’m not quite sure, how you meant this (I guess correctly, but I just want to make sure), so let me add a few words.
    A neutron does not consist of one proton, one electron and one electron neutrino (your words could imply this). You can merge a proton and an electron and what (might) happen then is that one of the up-quarks of the proton changes into a down-quark, consuming the electron and releasing an electron neutrino. This is done by the weak interaction and the proton has become a neutron.

    On a second thought, I hope this was not too technically involved ๐Ÿ˜‰

  31. Olaf says:


    What DrFlimmer said! LOL
    I thought the quark explanation was too technical so I choose to explain it in “charges” terms.

    I was referring to this: Free neutrons decay by emission of an electron and an electron antineutrino to become a proton, a process known as beta decay:

    To indicate that protons and electrons could become charge less neutrons neutrons by absorbing the electron when pressed hard enough

    If the pressure is not there because of the neutron stars gravity it decays back in the + and – charge.

    But your explanation is better of course ๐Ÿ™‚

  32. DrFlimmer says:

    You’re very welcome! ๐Ÿ˜‰

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