<p>According to <a href="http://adsabs.harvard.edu/full/1998ASPC..134..405K" rel="noreferrer">Spectroscopic Properties of Ultra-cool Dwarfs and Brown Dwarf</a> by J.D. Kirkpatrick, the working definition of ultra-cool dwarf is:</p>

<blockquote>
  <p>dwarfs having classifications of M7 or later.</p>
</blockquote>

<p>That is referring to the <a href="https://en.wikipedia.org/wiki/Stellar_classification" rel="noreferrer">stellar classification</a> system which categorizes stars by their spectral characteristics. Class M stars have a surface temperature less than 4000 K, and "M7 or later" means surface temperature less than 2500 K. Though it should be noted that this is a working definition, and the Wikipedia article identifies ultra-cool dwarfs as objects with surface temperature less than 2700 K. It is possible that the term is still used somewhat loosely in scientific papers.</p>

CC BY-SA 3.0
2017-02-22T19:30:37.797Z
2017-02-22T19:30:37.797Z
called2voyage
<p>Breaking the phrase down: </p>

<ul>
<li><a href="https://en.wikipedia.org/wiki/Dwarf_star" rel="nofollow noreferrer"><em>Dwarf star</em></a> - a term I will never understand - is used to describe relatively small, dim stars. Unfortunately, this encompasses most main-sequence stars, which are indeed dwarfs compared to large giants and supergiants.</li>
<li><p><em>Ultra-cool</em>, as <a href="https://astronomy.stackexchange.com/a/20128/2153">called2voyage</a> already discussed, <a href="http://www.eso.org/public/archives/releases/sciencepapers/eso1615/eso1615a.pdf" rel="nofollow noreferrer">means that</a> the star has an</p>

<blockquote>
  <p>effective temperatures of less than 2,700 kelvin.</p>
</blockquote>

<p>This is about half of the Sun's surface temperature. Spectroscopically, it means that these stars are of class <a href="http://iopscience.iop.org/article/10.1086/510132/meta" rel="nofollow noreferrer">M7-L8</a>, i.e. really, really cool stars that just barely reach the threshold for nuclear fusion. Actually, some L dwarfs will never fuse hydrogen as normal stars do, and will become brown dwarfs. Therefore, "ultra-cool dwarf" doesn't necessarily refer to just main sequence stars.</p></li>
</ul>

<p>Other characteristics in some cases (from <a href="http://iopscience.iop.org/article/10.1086/510132/meta" rel="nofollow noreferrer">Cruz et al</a>.):</p>

<ul>
<li>Lithium in some L-type objects, indicative of low temperatures, low masses, and young ages in those particular objects.</li>
<li>Very low surface gravities in late M-type dwarfs (though likely not as much in an M7 dwarf like TRAPPIST-1).</li>
<li>Very low metallicity in certain other L-type objects.</li>
</ul>

<p>TRAPPIST-1 actually appears to be a rather massive ultra-cool dwarf, then, in comparison to some of these other objects.</p>

<p>Here's an annotated Hertzsprung-Russell diagram, with a box for ultra-cool dwarfs and a circle for where TRAPPIST-1 is, approximately:</p>

<p><a href="https://i.sstatic.net/ZOlPP.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZOlPP.png" alt="enter image description here"></a>
<br>
<sup>Image from <a href="https://en.wikipedia.org/wiki/File:HRDiagram.png" rel="nofollow noreferrer">Wikipedia</a> courtesy of user Saibo, under <a href="https://creativecommons.org/licenses/by-sa/2.5/deed.en" rel="nofollow noreferrer">the Creative Commons Attribution-Share Alike 2.5 Generic license</a>.</sup></p>

CC BY-SA 3.0
2017-02-23T17:30:25.073Z
Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/54135/discussion-on-answer-by-hde-226868-so-what-exactly-is-an-ultra-cool-dwarf-sta).
called2voyage
CC BY-SA 3.0
2017-02-23T20:20:14.167Z
Funny detail: Our sun is also considered to be a dwarf star.
AtmosphericPrisonEscape
CC BY-SA 3.0
2017-02-22T19:48:10.733Z
2017-02-24T20:09:00.337Z
2017-04-13T12:59:07.017Z
HDE 226868
<p>The <a href="https://exoplanets.nasa.gov/news/1419/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around-single-star/" rel="noreferrer">TRAPPIST-1 system</a> is around an ultra-cool dwarf star. I went looking for more information on that kind of star, and found very little. The <a href="https://en.wikipedia.org/wiki/Ultra-cool_dwarf" rel="noreferrer">Wikipedia article</a> on it lengthened from a minimal stub to one paragraph in the 15 minutes from when I began the search, after the press conference on the discovery of 7 Earth-sized planets in orbit around TRAPPIST-1.</p>

<p>What kind of star is this?</p>

<star><terminology><dwarf-star><trappist-1>
So, what exactly is an 'ultra-cool' dwarf star?
kim holder
# So, what exactly is an 'ultra-cool' dwarf star?

(Asked by: username_0 on 2017-02-22)

The [TRAPPIST-1 system](https://exoplanets.nasa.gov/news/1419/nasa-telescope-reveals-largest-batch-of-earth-size-habitable-zone-planets-around-single-star/) is around an ultra-cool dwarf star. I went looking for more information on that kind of star, and found very little. The [Wikipedia article](https://en.wikipedia.org/wiki/Ultra-cool_dwarf) on it lengthened from a minimal stub to one paragraph in the 15 minutes from when I began the search, after the press conference on the discovery of 7 Earth-sized planets in orbit around TRAPPIST-1.

What kind of star is this?

**Tags:** star, terminology, dwarf-star, trappist-1

---

## 2 answers

### Answer by username_2 on 2017-02-22. Score: 22 | Accepted answer

Breaking the phrase down:

* [*Dwarf star*](https://en.wikipedia.org/wiki/Dwarf_star) - a term I will never understand - is used to describe relatively small, dim stars. Unfortunately, this encompasses most main-sequence stars, which are indeed dwarfs compared to large giants and supergiants.
* *Ultra-cool*, as [called2voyage](https://astronomy.stackexchange.com/a/20128/2153) already discussed, [means that](http://www.eso.org/public/archives/releases/sciencepapers/eso1615/eso1615a.pdf) the star has an

  > effective temperatures of less than 2,700 kelvin.

  This is about half of the Sun's surface temperature. Spectroscopically, it means that these stars are of class [M7-L8](http://iopscience.iop.org/article/10.1086/510132/meta), i.e. really, really cool stars that just barely reach the threshold for nuclear fusion. Actually, some L dwarfs will never fuse hydrogen as normal stars do, and will become brown dwarfs. Therefore, "ultra-cool dwarf" doesn't necessarily refer to just main sequence stars.

Other characteristics in some cases (from [Cruz et al](http://iopscience.iop.org/article/10.1086/510132/meta).):

* Lithium in some L-type objects, indicative of low temperatures, low masses, and young ages in those particular objects.
* Very low surface gravities in late M-type dwarfs (though likely not as much in an M7 dwarf like TRAPPIST-1).
* Very low metallicity in certain other L-type objects.

TRAPPIST-1 actually appears to be a rather massive ultra-cool dwarf, then, in comparison to some of these other objects.

Here's an annotated Hertzsprung-Russell diagram, with a box for ultra-cool dwarfs and a circle for where TRAPPIST-1 is, approximately:

[![enter image description here](https://i.sstatic.net/ZOlPP.png)](https://i.sstatic.net/ZOlPP.png)
  
Image from [Wikipedia](https://en.wikipedia.org/wiki/File:HRDiagram.png) courtesy of user Saibo, under [the Creative Commons Attribution-Share Alike 2.5 Generic license](https://creativecommons.org/licenses/by-sa/2.5/deed.en).

> **Comments:**
> **username_1**: Comments are not for extended discussion; this conversation has been [moved to chat](http://chat.stackexchange.com/rooms/54135/discussion-on-answer-by-hde-226868-so-what-exactly-is-an-ultra-cool-dwarf-sta).  
> **username_3**: Funny detail: Our sun is also considered to be a dwarf star.  

---

### Answer by username_1 on 2017-02-22. Score: 8

According to [Spectroscopic Properties of Ultra-cool Dwarfs and Brown Dwarf](http://adsabs.harvard.edu/full/1998ASPC..134..405K) by J.D. Kirkpatrick, the working definition of ultra-cool dwarf is:

> dwarfs having classifications of M7 or later.

That is referring to the [stellar classification](https://en.wikipedia.org/wiki/Stellar_classification) system which categorizes stars by their spectral characteristics. Class M stars have a surface temperature less than 4000 K, and "M7 or later" means surface temperature less than 2500 K. Though it should be noted that this is a working definition, and the Wikipedia article identifies ultra-cool dwarfs as objects with surface temperature less than 2700 K. It is possible that the term is still used somewhat loosely in scientific papers.


---

