<p>Partial answer to share what I've found to date:</p>
<h3>Galaxies</h3>
<p>The term &quot;merger&quot; does have widespread use in <a href="/questions/tagged/galactic-dynamics" class="post-tag" title="show questions tagged &#39;galactic-dynamics&#39;" rel="tag">galactic-dynamics</a>. The word &quot;merger&quot; appears 71 times Wikipedia's <a href="https://en.wikipedia.org/wiki/Galaxy_merger" rel="nofollow noreferrer">Galaxy merger</a> for example, with terms like binary merger, multiple merger, minor merger, major merger, wet merger, dry merger, damp merger, mixed merger, merger history trees all having explicit definitions there.</p>
<h3>Supermassive Black holes</h3>
<p>When galaxies merge, there is the question of what happens to the supermassive black hole (SBH) that may be in the center of each. Since galactic mergers and SBH mergers are inextricably linked I'll list their questions together here:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/6292/7982">Milky Way Formation</a></li>
<li><a href="https://astronomy.stackexchange.com/q/12410/7982">What parameters determine whether galaxies colliding will result in a merger or a hit and run?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/13013/7982">Why do we believe that the super massive black holes at the centers of two merging galaxies would themselves merge?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/15001/7982">Difference in energy released in stellar mass black hole merger and supermassive black hole merger</a></li>
<li><a href="https://astronomy.stackexchange.com/q/32622/7982">Why can&#39;t supermassive black holes merge? (or can they?)</a></li>
<li><a href="https://astronomy.stackexchange.com/q/32634/7982">How do two supermassive black holes reach &quot;the last parsec&quot; in merging galaxies?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/32865/7982">Are there galaxies with 2 or more super massive black holes orbiting each other?</a> &quot;Yes, there are galaxies with two supermassive black holes in the center, see for instance <a href="https://en.wikipedia.org/wiki/4C_%2B37.11" rel="nofollow noreferrer">4C +37.11</a>. Most likely such galaxies are formed by collision and merger of two galaxies, and their cores have not yet merged. <a href="https://aasnova.org/2016/10/05/merging-a-pair-of-supermassive-black-holes/" rel="nofollow noreferrer">Source</a>&quot;</li>
<li><a href="https://astronomy.stackexchange.com/q/33228/7982">Is there a way to calculate how much damage black hole merger shockwaves inflict on nearby objects?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/36686/7982">What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/36687/7982">Why would the merger of spinning black holes within the accretion disk of a supermassive black hole cause them to &quot;shoot straight up&quot; out of the disk?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/43821/7982">How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?</a></li>
</ul>
<h3>Stellar objects</h3>
<p>But for individual stellar objects the situation does get murky. We have had a <a href="/questions/tagged/collision" class="post-tag" title="show questions tagged &#39;collision&#39;" rel="tag">collision</a> tag for a while now, and the &quot;merging&quot; of stars due to collisions happening in the centers of dense clusters is a topic first raised decades ago. Whether they merge, or just exchange matter or something else, it seems infinitely safer to stick with collision.</p>
<p>But as <a href="https://astronomy.stackexchange.com/questions/44256/what-is-the-difference-between-the-terms-collision-and-merger-how-are-they-used?noredirect=1#comment93476_44256">@DaddyKropotkin</a> points out:</p>
<blockquote>
<p>We're just giving you basic sense of how the terminology is used. It is subject-specific and nuanced <strong>since the word &quot;merger&quot; is used in the new field of gravitational wave astronomy</strong>, so you could get many different, inconsistent, opinionated answers.</p>
</blockquote>
<p>When two objects that are either black holes (BH) or neutron stars (NS) find themselves in extremely close proximity, usually through a process of orbiting each other and spiraling inward due to energy radiation in the form of gravitational waves, and ultimately touch and combine much/most of their masses to form a single object, the last few seconds generates gravitational waves so strong that we can detect, record, and analyze them. These are then also called &quot;mergers&quot; (BH-BH, BH-NS and NS-NS mergers) See Wikipedia's <a href="https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations#List_of_gravitational_wave_events" rel="nofollow noreferrer">List or gravitational wave observations; List of gravitational wave events</a> for example.</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/39371/7982">How likely are planets to form after neutron star collisions?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/26385/7982">GW from merging of neutron stars and black holes</a> &quot;So in the end, the frequency of LIGO detections depends on two things: The real distribution of objects in binaries: NS-NS, NS-BH, and BH-BH and the loudness/brightness of the gravitational waves emitted by the merger.&quot;</li>
</ul>
<p>From <a href="https://link.springer.com/content/pdf/bbm%3A978-1-4757-2928-3%2F1.pdf" rel="nofollow noreferrer">Appendix A: Astronomical Terminology</a>:</p>
<blockquote>
<p>Energies of the order inferred suggest that gamma ray bursts may originate in <strong>the merger of two neutron stars</strong> to form a black hole or the capture of a neutron star by a black hole. <strong>Such mergers</strong> provide almost the only ways in which we can conceive of vast amounts of energy to be liberated rapidly. The potential energy that can be released in <strong>these mergers</strong> is of order M<sub>0</sub>c<sup>2</sup> ~ 10<sup>56</sup> erg.</p>
</blockquote>
<p>As for stars merging:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/42013/7982">Could our Sun be the product of an ancient stellar collision?</a>  &quot;<a href="https://astronomy.stackexchange.com/a/41016/7982">Stellar mergers are certainly possible</a>, but also relatively rare. [...] <a href="https://dspace.library.uu.nl/handle/1874/27801" rel="nofollow noreferrer">Glebbek's dissertation on stellar mergers</a> estimates a rough condition for the orbital angular momentum to exceed the maximum spin angular momentum of the merged star as...&quot;</li>
<li><a href="https://astronomy.stackexchange.com/q/42013/7982">Could our Sun be the product of an ancient stellar collision?</a> &quot;The dynamics of the Solar System and the chemistry of the Solar System bodies don't support a hypothesis of a stellar merger later than formation of the protoplanetary disk&quot; and &quot;<a href="https://astronomy.stackexchange.com/a/41016/7982">Stellar mergers are certainly possible</a>&quot;</li>
<li><a href="https://astronomy.stackexchange.com/q/32654/7982">What would happen if a Sun-like star were to consume a Jupiter-like planet?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/37022/7982">How far from Betelgeuse is its habitable zone?</a> &quot;In addition to being a highly unstable and variable supergiant, it's a runaway star, suggesting that it was formerly a member of a multiple star system with a companion star that went supernova. Its relatively rapid rotation is difficult to explain via single star evolution, suggesting that it has undergone a stellar merger (<a href="https://ui.adsabs.harvard.edu/abs/2017MNRAS.465.2654W/abstract" rel="nofollow noreferrer">Wheeler et al. 2017</a>, <a href="https://ui.adsabs.harvard.edu/abs/2020ApJ...896...50C/abstract" rel="nofollow noreferrer">Chatzopoulos et al. 2020</a>)&quot;</li>
</ul>
<h3>What can be said?</h3>
<ol>
<li>&quot;Merger&quot; is a solid, standard term when it comes to galaxies and their supermassive black holes.</li>
<li>&quot;Merger&quot; is becoming a standard term for the last few moments of NS-NS, NS-BH and BH-BH grabitational wave event.</li>
<li>In the case of stars, it's murky and &quot;collision&quot; seems to at least adequately cover all possible types of events where there is substantial combination of two stars' mass into one &quot;thing&quot; which could be a supernova, a star or neutron star or black hole or something else. While the term &quot;merger&quot; might be used from time to time by some folks, &quot;collision&quot; will be understood by all.</li>
</ol>
<h3>Things smaller than stars (e.g. planets, protoplanets, asteroids, dust...)</h3>
<p>I think that again in this case &quot;collision&quot; will be the right term, though in solar system formation there is plenty of merging of objects to make larger objects. <strong>This needs to be explored further</strong> as this answer identifies itself as a &quot;Partial answer to share what I've found to date&quot;.</p>

CC BY-SA 4.0
2021-11-08T21:40:55.040Z
Query: Under the heading **What can be said?**, in point 2, should it be gra**b**itational, as writen, or gravitational. I suspect it's a typo given b & v are adjacent on the keyboard. I can also see how *grabitational*  might be an invented word to describe an acquisition situation. ;-)
Fred
CC BY-SA 4.0
2021-11-08T21:49:25.850Z
@Fred gee, I like the word so much and I really hope it catches on, so I'm going to leave it as-is in hopes of receiving international acclaim.
uhoh
CC BY-SA 4.0
2021-06-09T22:06:56.643Z
2021-06-09T22:29:00.643Z
2021-06-09T22:29:00.643Z
uhoh
<p>We often hear of mergers of two stellar objects but we also sometimes talk about these or <em>much smaller objects</em> like planets or asteroids <em>colliding.</em></p>
<p><a href="https://astronomy.stackexchange.com/q/44222/7982">What is the actual differences between Astronomy and Cosmology?</a> received several excellent answers for example, but here what I'm looking for is if there can be a fairly easy way to differentiate the concepts of merger from that of collision, or establish the degree of overlap.</p>
<p>I do have an ulterior motive; in meta there is the question <a href="https://astronomy.meta.stackexchange.com/q/755/7982">Do we need a tag for merging? I struggled to find something suitable for supermassive black hole mergers in galaxy collisions</a> but in meta we discuss how the site works and how to maintain or improve it.</p>
<p>Here I'm asking the actual astronomy terminology question:</p>
<p><strong>Question:</strong> What is the difference between the terms collision and merger? How are they used differently in Astronomy?</p>

CC BY-SA 4.0
2021-06-09T00:43:26.287Z
I'd lean towards "A collision splashes, a merger doesn't."
notovny
CC BY-SA 4.0
2021-06-09T00:58:17.440Z
I think *merger* is an inelastic collision.
Daddy Kropotkin
CC BY-SA 4.0
2021-06-09T01:51:33.957Z
@DaddyKropotkin I don't think that that's a helpful observation; can you cite any example of *elastic* collisions of astronomical bodies that come in contact? When bodies hit each other there's always dissipation, so collision and inelastic collision are the same thing in this context. All collisions dissipate, but some stick and some don't and some of those exchange appreciable mass and some don't.
uhoh
CC BY-SA 4.0
2021-06-09T02:11:55.037Z
The meanings of these words just depends on context, which depends on the specific subfield one considers. I tried to give an abstract notion of the word "merger".... one can consider gravitational scattering of two objects as an elastic collision.
Daddy Kropotkin
CC BY-SA 4.0
2021-06-09T06:35:27.513Z
@planetmaker In Astronomy, merger and collision have "no special meaning beyond what difference is normally understood." I'm not sure I follow. What does "normally understood" mean exactly? Who or what constitutes *normalcy*? Do you mean colloquial use? Or within the context of astronomy, or something else? I think we'll ultimately have a good answer here, and these comments about there not being a difference or it being ambiguous will turn out not to be true. It's like when someone comments "Nobody can answer this" and the next day there are one or two *excellent answers*.
uhoh
CC BY-SA 4.0
2021-06-09T08:52:36.667Z
Sorry trying to make a helpful comment on your question. I shall remember to refrain from doing so
planetmaker
CC BY-SA 4.0
2021-06-09T09:11:18.430Z
@planetmaker Sorry, I couldn't understand it.
uhoh
CC BY-SA 4.0
2021-06-09T13:17:29.603Z
We're not saying it cannot be answered nor that it is a bad question. We're just giving you basic sense of how the terminology is used. It is subject-specific and nuanced since the word "merger" is used in the new field of gravitational wave astronomy, so you could get many different, inconsistent, opinionated answers. Our comments were just trying to point this out. Nothing more. I'd like to see an excellent answer to this. +1 for that effort!
Daddy Kropotkin
CC BY-SA 4.0
2021-06-09T13:57:50.120Z
@DaddyKropotkin I see, thanks for your explanation that helps a lot. Two stellar objects  becoming one (be they stars, neutron stars or black holes or [various pairings](https://astronomy.stackexchange.com/q/29721/7982)) has probably been proposed and studied theoretically starting many decades in the past, way before gravitational wave detectors were built, so I'd assumed that the uses of collision vs merger for stellar objects had already developed into some regularity before the LIGO/Virgo excitement. Perhaps I'm wrong. Looking forward to the posting of a Stack Exchange answer to find out!
uhoh
CC BY-SA 4.0
2021-06-09T14:54:58.777Z
I do not think that assumption is entirely fair, because studies of stellar mergers can involve very disparate fields of research: for example this same problem can be studied by people using hydrodynamical simulations or by people using numerical models, and they often call the same things by different names. Just saying, it might be an unreasonable expectation that these words are well distinguished. If they are, it'll probably be in some very specific situtation, like when two stars merge to form a new star vs when two stars scatter in a collision. In both cases the words can be swap freely
Daddy Kropotkin
CC BY-SA 4.0
2021-06-09T22:09:33.923Z
@DaddyKropotkin I'm more comfortable working in "answer space" than "comment space"; the size limits on comments leads to short truncated passages that can sometimes end up as long comment chains, each clarifying the previous comment. So I've posted a *partial answer* myself in an attempt to break out of the chaining of comments and work towards an actual answer.
uhoh
CC BY-SA 4.0
2021-07-10T05:41:38.353Z
Looking it from a different perspective, a merger can be considered as a gentler coming together, with little, if any, physical damage. Considering galaxies, there's an accommodation of each others star systems. Some planets & stars might be flung to different locations & occasionally they might collide (impact) with one another, resulting in some damage. Similarly when back holes merge, they don't impact each other, but "dance" around each other until they [coalesce](https://www.youtube.com/watch?v=LyPcgCYRYn4). A merger can be regarding as a coalescence, whereas a collision involves impact.
Fred
CC BY-SA 4.0
2021-07-10T06:47:58.183Z
A 'merger' is by definition an inelastic collision, while a collision is more general and thus allows for physics to happen: it can be any, notably it can be elastic and have the two bodies remain separate entities or have one or both destroyed and scattered.
planetmaker
CC BY-SA 4.0
2021-07-10T06:51:09.280Z
@planetmaker physics is not also allowed to happen in inelastic collisions?
uhoh
CC BY-SA 4.0
2021-07-10T08:27:50.497Z
Not allowed, but it does 
planetmaker
<terminology><definition><collision><merger>
What is the difference between the terms collision and merger? How are they used differently in Astronomy?
uhoh
# What is the difference between the terms collision and merger? How are they used differently in Astronomy?

(Asked by: username_0 on 2021-06-08)

We often hear of mergers of two stellar objects but we also sometimes talk about these or *much smaller objects* like planets or asteroids *colliding.*

[What is the actual differences between Astronomy and Cosmology?](https://astronomy.stackexchange.com/q/44222/7982) received several excellent answers for example, but here what I'm looking for is if there can be a fairly easy way to differentiate the concepts of merger from that of collision, or establish the degree of overlap.

I do have an ulterior motive; in meta there is the question [Do we need a tag for merging? I struggled to find something suitable for supermassive black hole mergers in galaxy collisions](https://astronomy.meta.stackexchange.com/q/755/7982) but in meta we discuss how the site works and how to maintain or improve it.

Here I'm asking the actual astronomy terminology question:

**Question:** What is the difference between the terms collision and merger? How are they used differently in Astronomy?

**Tags:** terminology, definition, collision, merger
> **Comments:**
> **username_1**: I'd lean towards "A collision splashes, a merger doesn't."  
> **username_2**: I think *merger* is an inelastic collision.  
> **username_0**: @DaddyKropotkin I don't think that that's a helpful observation; can you cite any example of *elastic* collisions of astronomical bodies that come in contact? When bodies hit each other there's always dissipation, so collision and inelastic collision are the same thing in this context. All collisions dissipate, but some stick and some don't and some of those exchange appreciable mass and some don't.  
> **username_2**: The meanings of these words just depends on context, which depends on the specific subfield one considers. I tried to give an abstract notion of the word "merger".... one can consider gravitational scattering of two objects as an elastic collision.  
> **username_0**: @planetmaker In Astronomy, merger and collision have "no special meaning beyond what difference is normally understood." I'm not sure I follow. What does "normally understood" mean exactly? Who or what constitutes *normalcy*? Do you mean colloquial use? Or within the context of astronomy, or something else? I think we'll ultimately have a good answer here, and these comments about there not being a difference or it being ambiguous will turn out not to be true. It's like when someone comments "Nobody can answer this" and the next day there are one or two *excellent answers*.  
> **username_3**: Sorry trying to make a helpful comment on your question. I shall remember to refrain from doing so  
> **username_0**: @planetmaker Sorry, I couldn't understand it.  
> **username_2**: We're not saying it cannot be answered nor that it is a bad question. We're just giving you basic sense of how the terminology is used. It is subject-specific and nuanced since the word "merger" is used in the new field of gravitational wave astronomy, so you could get many different, inconsistent, opinionated answers. Our comments were just trying to point this out. Nothing more. I'd like to see an excellent answer to this. +1 for that effort!  
> **username_0**: @DaddyKropotkin I see, thanks for your explanation that helps a lot. Two stellar objects  becoming one (be they stars, neutron stars or black holes or [various pairings](https://astronomy.stackexchange.com/q/29721/7982)) has probably been proposed and studied theoretically starting many decades in the past, way before gravitational wave detectors were built, so I'd assumed that the uses of collision vs merger for stellar objects had already developed into some regularity before the LIGO/Virgo excitement. Perhaps I'm wrong. Looking forward to the posting of a Stack Exchange answer to find out!  
> **username_2**: I do not think that assumption is entirely fair, because studies of stellar mergers can involve very disparate fields of research: for example this same problem can be studied by people using hydrodynamical simulations or by people using numerical models, and they often call the same things by different names. Just saying, it might be an unreasonable expectation that these words are well distinguished. If they are, it'll probably be in some very specific situtation, like when two stars merge to form a new star vs when two stars scatter in a collision. In both cases the words can be swap freely  
> **username_0**: @DaddyKropotkin I'm more comfortable working in "answer space" than "comment space"; the size limits on comments leads to short truncated passages that can sometimes end up as long comment chains, each clarifying the previous comment. So I've posted a *partial answer* myself in an attempt to break out of the chaining of comments and work towards an actual answer.  
> **username_4**: Looking it from a different perspective, a merger can be considered as a gentler coming together, with little, if any, physical damage. Considering galaxies, there's an accommodation of each others star systems. Some planets & stars might be flung to different locations & occasionally they might collide (impact) with one another, resulting in some damage. Similarly when back holes merge, they don't impact each other, but "dance" around each other until they [coalesce](https://www.youtube.com/watch?v=LyPcgCYRYn4). A merger can be regarding as a coalescence, whereas a collision involves impact.  
> **username_3**: A 'merger' is by definition an inelastic collision, while a collision is more general and thus allows for physics to happen: it can be any, notably it can be elastic and have the two bodies remain separate entities or have one or both destroyed and scattered.  
> **username_0**: @planetmaker physics is not also allowed to happen in inelastic collisions?  
> **username_3**: Not allowed, but it does   

---

## 1 answers

### Answer by username_0 on 2021-06-09. Score: 1 | Accepted answer

Partial answer to share what I've found to date:

### Galaxies

The term "merger" does have widespread use in [galactic-dynamics](/questions/tagged/galactic-dynamics "show questions tagged 'galactic-dynamics'"). The word "merger" appears 71 times Wikipedia's [Galaxy merger](https://en.wikipedia.org/wiki/Galaxy_merger) for example, with terms like binary merger, multiple merger, minor merger, major merger, wet merger, dry merger, damp merger, mixed merger, merger history trees all having explicit definitions there.

### Supermassive Black holes

When galaxies merge, there is the question of what happens to the supermassive black hole (SBH) that may be in the center of each. Since galactic mergers and SBH mergers are inextricably linked I'll list their questions together here:

* [Milky Way Formation](https://astronomy.stackexchange.com/q/6292/7982)
* [What parameters determine whether galaxies colliding will result in a merger or a hit and run?](https://astronomy.stackexchange.com/q/12410/7982)
* [Why do we believe that the super massive black holes at the centers of two merging galaxies would themselves merge?](https://astronomy.stackexchange.com/q/13013/7982)
* [Difference in energy released in stellar mass black hole merger and supermassive black hole merger](https://astronomy.stackexchange.com/q/15001/7982)
* [Why can't supermassive black holes merge? (or can they?)](https://astronomy.stackexchange.com/q/32622/7982)
* [How do two supermassive black holes reach "the last parsec" in merging galaxies?](https://astronomy.stackexchange.com/q/32634/7982)
* [Are there galaxies with 2 or more super massive black holes orbiting each other?](https://astronomy.stackexchange.com/q/32865/7982) "Yes, there are galaxies with two supermassive black holes in the center, see for instance [4C +37.11](https://en.wikipedia.org/wiki/4C_%2B37.11). Most likely such galaxies are formed by collision and merger of two galaxies, and their cores have not yet merged. [Source](https://aasnova.org/2016/10/05/merging-a-pair-of-supermassive-black-holes/)"
* [Is there a way to calculate how much damage black hole merger shockwaves inflict on nearby objects?](https://astronomy.stackexchange.com/q/33228/7982)
* [What enhances the capture and merge rates of pairs of small black holes orbiting around supermassive black holes?](https://astronomy.stackexchange.com/q/36686/7982)
* [Why would the merger of spinning black holes within the accretion disk of a supermassive black hole cause them to "shoot straight up" out of the disk?](https://astronomy.stackexchange.com/q/36687/7982)
* [How do we know that supermassive black holes can gain mass by means other than merging with other supermassive black holes?](https://astronomy.stackexchange.com/q/43821/7982)

### Stellar objects

But for individual stellar objects the situation does get murky. We have had a [collision](/questions/tagged/collision "show questions tagged 'collision'") tag for a while now, and the "merging" of stars due to collisions happening in the centers of dense clusters is a topic first raised decades ago. Whether they merge, or just exchange matter or something else, it seems infinitely safer to stick with collision.

But as [@DaddyKropotkin](https://astronomy.stackexchange.com/questions/44256/what-is-the-difference-between-the-terms-collision-and-merger-how-are-they-used?noredirect=1#comment93476_44256) points out:

> We're just giving you basic sense of how the terminology is used. It is subject-specific and nuanced **since the word "merger" is used in the new field of gravitational wave astronomy**, so you could get many different, inconsistent, opinionated answers.

When two objects that are either black holes (BH) or neutron stars (NS) find themselves in extremely close proximity, usually through a process of orbiting each other and spiraling inward due to energy radiation in the form of gravitational waves, and ultimately touch and combine much/most of their masses to form a single object, the last few seconds generates gravitational waves so strong that we can detect, record, and analyze them. These are then also called "mergers" (BH-BH, BH-NS and NS-NS mergers) See Wikipedia's [List or gravitational wave observations; List of gravitational wave events](https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations#List_of_gravitational_wave_events) for example.

* [How likely are planets to form after neutron star collisions?](https://astronomy.stackexchange.com/q/39371/7982)
* [GW from merging of neutron stars and black holes](https://astronomy.stackexchange.com/q/26385/7982) "So in the end, the frequency of LIGO detections depends on two things: The real distribution of objects in binaries: NS-NS, NS-BH, and BH-BH and the loudness/brightness of the gravitational waves emitted by the merger."

From [Appendix A: Astronomical Terminology](https://link.springer.com/content/pdf/bbm%3A978-1-4757-2928-3%2F1.pdf):

> Energies of the order inferred suggest that gamma ray bursts may originate in **the merger of two neutron stars** to form a black hole or the capture of a neutron star by a black hole. **Such mergers** provide almost the only ways in which we can conceive of vast amounts of energy to be liberated rapidly. The potential energy that can be released in **these mergers** is of order M0c2 ~ 1056 erg.

As for stars merging:

* [Could our Sun be the product of an ancient stellar collision?](https://astronomy.stackexchange.com/q/42013/7982) "[Stellar mergers are certainly possible](https://astronomy.stackexchange.com/a/41016/7982), but also relatively rare. [...] [Glebbek's dissertation on stellar mergers](https://dspace.library.uu.nl/handle/1874/27801) estimates a rough condition for the orbital angular momentum to exceed the maximum spin angular momentum of the merged star as..."
* [Could our Sun be the product of an ancient stellar collision?](https://astronomy.stackexchange.com/q/42013/7982) "The dynamics of the Solar System and the chemistry of the Solar System bodies don't support a hypothesis of a stellar merger later than formation of the protoplanetary disk" and "[Stellar mergers are certainly possible](https://astronomy.stackexchange.com/a/41016/7982)"
* [What would happen if a Sun-like star were to consume a Jupiter-like planet?](https://astronomy.stackexchange.com/q/32654/7982)
* [How far from Betelgeuse is its habitable zone?](https://astronomy.stackexchange.com/q/37022/7982) "In addition to being a highly unstable and variable supergiant, it's a runaway star, suggesting that it was formerly a member of a multiple star system with a companion star that went supernova. Its relatively rapid rotation is difficult to explain via single star evolution, suggesting that it has undergone a stellar merger ([Wheeler et al. 2017](https://ui.adsabs.harvard.edu/abs/2017MNRAS.465.2654W/abstract), [Chatzopoulos et al. 2020](https://ui.adsabs.harvard.edu/abs/2020ApJ...896...50C/abstract))"

### What can be said?

1. "Merger" is a solid, standard term when it comes to galaxies and their supermassive black holes.
2. "Merger" is becoming a standard term for the last few moments of NS-NS, NS-BH and BH-BH grabitational wave event.
3. In the case of stars, it's murky and "collision" seems to at least adequately cover all possible types of events where there is substantial combination of two stars' mass into one "thing" which could be a supernova, a star or neutron star or black hole or something else. While the term "merger" might be used from time to time by some folks, "collision" will be understood by all.

### Things smaller than stars (e.g. planets, protoplanets, asteroids, dust...)

I think that again in this case "collision" will be the right term, though in solar system formation there is plenty of merging of objects to make larger objects. **This needs to be explored further** as this answer identifies itself as a "Partial answer to share what I've found to date".

> **Comments:**
> **username_4**: Query: Under the heading **What can be said?**, in point 2, should it be gra**b**itational, as writen, or gravitational. I suspect it's a typo given b & v are adjacent on the keyboard. I can also see how *grabitational*  might be an invented word to describe an acquisition situation. ;-)  
> **username_0**: @Fred gee, I like the word so much and I really hope it catches on, so I'm going to leave it as-is in hopes of receiving international acclaim.  

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