<p>How the name came to be chosen is discussed in <a href="https://arxiv.org/pdf/astro-ph/0305074.pdf" rel="noreferrer">this paper</a><sup>1</sup> (§3, pp. 4–5):</p>
<blockquote>
<p>Eight years after the discovery, one of us (Brown) invented the name
Sgr A<sup>∗</sup> to distinguish the compact source from the other components in
the galactic center and to emphasize the unique nature of this source.</p>
<p>...</p>
<p>When I [Brown] began thinking of the radio source as the “exciting
source” for the cluster of H II regions seen in the VLA maps, the name
Sgr A<sup>∗</sup> occurred to me by analogy brought to mind by my Phd
dissertation, which is in atomic physics and where the nomenclature
for excited state atoms is He<sup>∗</sup>, or Fe<sup>∗</sup> etc.”</p>
</blockquote>
<p><sup>1</sup> Goss, W. M.; Brown, Robert L.; Lo, K. Y. (2003-05-06). &quot;The Discovery of Sgr A<sup>∗</sup>&quot;. <em>Astronomische Nachrichten</em>. 324 (1): 497. doi:10.1002/asna.200385047.</p>

CC BY-SA 4.0
2022-04-18T11:16:48.520Z
2022-04-18T11:16:48.520Z
Alex Hajnal
<p>Sgr A is a radio source from near the centre of the galaxy.  In early radio wavelength maps it wasn't clear if this was a single source or multiple.  With higher resolution it is clear that Sgr A is actually multiple objects:  A supernova remnant, several clouds of ionised gas, and a compact source of radio waves.</p>
<p>Sgr A* was a named (by its co-discover) Robert Brown.  He said, later that he attached a * because he thought this object was &quot;exciting&quot;, and excited states  of atoms have a * attached:</p>
<blockquote>
<p>“ Scratching on a yellow pad one morning I
tried a lot of possible names. When I began thinking of the radio source as the “exciting source” for the
cluster of H II regions seen in the VLA maps, the name Sgr A* occurred to me by analogy brought to mind
by my PhD dissertation, which is in atomic physics and where the nomenclature for excited state atoms is
He*, or Fe*
etc.” (<a href="https://arxiv.org/pdf/astro-ph/0305074.pdf" rel="noreferrer">sourc</a>e)</p>
</blockquote>
<p>On this model, some other supermassive black holes are also designated by a *. But there is no systematic naming of black holes like this.  Many don't seem to have their own name and are merely &quot;The SMBH at the centre of ...&quot; Other black holes were previously known as quasars, and have a quasar designation, or have multiple designations as radio sources, X-ray sources, UV sources. And so appear under different names in different contexts.</p>

CC BY-SA 4.0
2022-04-18T18:13:10.007Z
Out of curiosity, is “exciting” a technical term here (like in physics), or did he mean it colloquially?
Josh Grosso
CC BY-SA 4.0
2022-04-18T19:28:27.543Z
It's a pun, a play on words, a double meaning joke.  Who'd have thought that a scientist could make jokes, eh?
James K
CC BY-SA 4.0
2022-04-18T11:18:37.710Z
2022-04-18T11:18:37.710Z
James K
<p>Asterisk generally indicate that the body is located at the center of galaxy. For e.g. M87's black hole or &quot;M87*&quot; is located at the center of Messier 87 (M87) galaxy. The reason for this notation is explained very well <a href="https://chandra.si.edu/resources/faq/black_hole/bhole-18.html" rel="noreferrer">here</a> by taking the example of Sagittarius A*:</p>
<blockquote>
<p>The name 'Sagittarius A*' refers to the believed location of the supermassive black hole in the center of our galaxy. About 50 years ago, astronomers identified an area within the constellation of Sagittarius that was the strongest region of radio emission thus making it the likely center of the Milky Way. Astronomers dubbed this region 'Sagittarius A' to identify this particular area within the large patch of sky that this constellation covers. As science and technology improved, astronomers were later able to pinpoint a more exact location of the suspected location of the supermassive black hole through a variety of techniques. This improved location of the black hole was then named Sagittarius A* to separate it from the larger region of Sagittarius A.</p>
</blockquote>

CC BY-SA 4.0
2022-04-20T16:05:19.567Z
This A has it backwards. The Q is, "why do some supermassive black holes have an asterisk in their name". This A states that the asterisk denotes bodies at the center of a galaxy. Well, "supermassive black holes" and "the things at the center of galaxies" are more or less the same thing. It does *not* answer *where that asterisk came from* (as the other two answers do). ;-)
DevSolar
CC BY-SA 4.0
2022-04-18T12:09:37.233Z
2022-04-18T17:02:51.540Z
2022-04-18T17:02:51.540Z
Nilay Ghosh
<p><a href="https://en.wikipedia.org/wiki/Sagittarius_A*" rel="noreferrer">Sgr A*</a>, <a href="https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole_M87*" rel="noreferrer">M87*</a>,... Several supermassive black holes (SMBHs) at the center of galaxies have a star (*) in their name. But <a href="https://en.wikipedia.org/wiki/List_of_most_massive_black_holes" rel="noreferrer">many others</a> do not. Is there a physical difference between the SMBHs that are denoted by a star and those that are not?</p>

<supermassive-black-hole><nomenclature>
Why do some supermassive black holes have a * in their name?
usernumber
# Why do some supermassive black holes have a * in their name?

(Asked by: username_0 on 2022-04-18)

[Sgr A\*](https://en.wikipedia.org/wiki/Sagittarius_A*), [M87\*](https://en.wikipedia.org/wiki/Messier_87#Supermassive_black_hole_M87*),... Several supermassive black holes (SMBHs) at the center of galaxies have a star (\*) in their name. But [many others](https://en.wikipedia.org/wiki/List_of_most_massive_black_holes) do not. Is there a physical difference between the SMBHs that are denoted by a star and those that are not?

**Tags:** supermassive-black-hole, nomenclature

---

## 3 answers

### Answer by username_1 on 2022-04-18. Score: 27

How the name came to be chosen is discussed in [this paper](https://arxiv.org/pdf/astro-ph/0305074.pdf)1 (§3, pp. 4–5):

> Eight years after the discovery, one of us (Brown) invented the name
> Sgr A∗ to distinguish the compact source from the other components in
> the galactic center and to emphasize the unique nature of this source.
>
> ...
>
> When I [Brown] began thinking of the radio source as the “exciting
> source” for the cluster of H II regions seen in the VLA maps, the name
> Sgr A∗ occurred to me by analogy brought to mind by my Phd
> dissertation, which is in atomic physics and where the nomenclature
> for excited state atoms is He∗, or Fe∗ etc.”

1 Goss, W. M.; Brown, Robert L.; Lo, K. Y. (2003-05-06). "The Discovery of Sgr A∗". *Astronomische Nachrichten*. 324 (1): 497. doi:10.1002/asna.200385047.


---

### Answer by username_2 on 2022-04-18. Score: 23

Sgr A is a radio source from near the centre of the galaxy. In early radio wavelength maps it wasn't clear if this was a single source or multiple. With higher resolution it is clear that Sgr A is actually multiple objects: A supernova remnant, several clouds of ionised gas, and a compact source of radio waves.

Sgr A\* was a named (by its co-discover) Robert Brown. He said, later that he attached a \* because he thought this object was "exciting", and excited states of atoms have a \* attached:

> “ Scratching on a yellow pad one morning I
> tried a lot of possible names. When I began thinking of the radio source as the “exciting source” for the
> cluster of H II regions seen in the VLA maps, the name Sgr A\* occurred to me by analogy brought to mind
> by my PhD dissertation, which is in atomic physics and where the nomenclature for excited state atoms is
> He\*, or Fe\*
> etc.” ([sourc](https://arxiv.org/pdf/astro-ph/0305074.pdf)e)

On this model, some other supermassive black holes are also designated by a \*. But there is no systematic naming of black holes like this. Many don't seem to have their own name and are merely "The SMBH at the centre of ..." Other black holes were previously known as quasars, and have a quasar designation, or have multiple designations as radio sources, X-ray sources, UV sources. And so appear under different names in different contexts.

> **Comments:**
> **username_3**: Out of curiosity, is “exciting” a technical term here (like in physics), or did he mean it colloquially?  
> **username_2**: It's a pun, a play on words, a double meaning joke.  Who'd have thought that a scientist could make jokes, eh?  

---

### Answer by username_4 on 2022-04-18. Score: 6

Asterisk generally indicate that the body is located at the center of galaxy. For e.g. M87's black hole or "M87\*" is located at the center of Messier 87 (M87) galaxy. The reason for this notation is explained very well [here](https://chandra.si.edu/resources/faq/black_hole/bhole-18.html) by taking the example of Sagittarius A\*:

> The name 'Sagittarius A\*' refers to the believed location of the supermassive black hole in the center of our galaxy. About 50 years ago, astronomers identified an area within the constellation of Sagittarius that was the strongest region of radio emission thus making it the likely center of the Milky Way. Astronomers dubbed this region 'Sagittarius A' to identify this particular area within the large patch of sky that this constellation covers. As science and technology improved, astronomers were later able to pinpoint a more exact location of the suspected location of the supermassive black hole through a variety of techniques. This improved location of the black hole was then named Sagittarius A\* to separate it from the larger region of Sagittarius A.

> **Comments:**
> **username_5**: This A has it backwards. The Q is, "why do some supermassive black holes have an asterisk in their name". This A states that the asterisk denotes bodies at the center of a galaxy. Well, "supermassive black holes" and "the things at the center of galaxies" are more or less the same thing. It does *not* answer *where that asterisk came from* (as the other two answers do). ;-)  

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