<p>I don't believe that O0 is a real classification(see <a href="http://www.atlasoftheuniverse.com/startype.html" rel="nofollow noreferrer">this chart</a>), but if it were following the temperature steps it would probably be around 80,000-90,000 degrees Kelvin. The hottest star we know of is WR 102, which is 210,000 degrees Kelvin, and that is much, much hotter than my predicted O0 temperature. So the short answer is yes, O0 temperature stars exist, but O0 isn't truly a real classification. (The hottest real classification is O2, at 53,000 Kelvin and up) Hope I helped!</p>
<p>Edit: I forgot to add a closing parentheses and my Markdown was messed up
Edit 2: I was looking at an outdated document, the real hottest classification is O2 not O3.</p>

CC BY-SA 4.0
2020-11-03T13:53:16.900Z
Ok, so if we ignore WR, would they be O0 or 1?
WarpPrime
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2020-11-03T13:54:05.573Z
What do you mean would "they" be O0 or 1?
Anna Wood
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2020-11-03T14:08:22.407Z
@fasterthanlight I mean, what/who is they?
Anna Wood
CC BY-SA 4.0
2020-11-03T15:03:21.970Z
Wikipedia gave examples of three O2 stars and as far as I can tell these are hotter than O3 stars, so I think your answer is inaccurate.
StephenG - Help Ukraine
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2020-11-03T15:20:29.847Z
@StephenG Ah right, I think that graph may be outdated. I found O2 becoming a real classification in 2002, but still nothing about O1 or O0. I'll edit my answer shortly, but it doesn't really change much.
Anna Wood
CC BY-SA 4.0
2020-11-03T15:22:52.370Z
@StephenG I fixed it now! Would you mind removing your downvote?
Anna Wood
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2020-11-03T15:27:37.533Z
@StephenG Actually, a potential correction of my previous self-I found several website that appear to mention O0 and O1 stars, but they appear to be incredibly old, before 2002(when O2 stars were made an official thing), thus I'm not sure if they're just making it up or if they're a real thing. I can't find any date or anything like that so I'm fairly certain they were just making those star classifications up.
Anna Wood
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2020-11-03T16:32:31.897Z
@StephenG Thanks for removing the upvote! I agree, it is kind of confusing. I will say though I haven't seen any big sources(such as NASA, SpaceX, space.com, etc.) mention O1 or O0 stars, so I think they're just making it up.
Anna Wood
CC BY-SA 4.0
2020-11-03T16:35:55.363Z
@fasterthanlight Ah, I think I understand your question now. I think if the scales kept going up to another letter, it would be several letters past O. But we're assuming it doesn't do that, so I think it would be classified as a O0 star since I think anything above 90,000 degrees Kelvin would be considered O0(if that classification existed) Technically, in reality WR 102 is classified as a O2 star.
Anna Wood
CC BY-SA 4.0
2020-11-03T13:34:00.813Z
2020-11-03T15:22:24.717Z
2020-11-03T15:22:24.717Z
Anna Wood
<p>This is what <a href="https://en.wikipedia.org/wiki/Stellar_classification#Class_O" rel="noreferrer">Wikipedia</a> says about it:</p>
<blockquote>
<p>When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.</p>
</blockquote>
<p>It references the paper <a href="https://discovery.ucl.ac.uk/id/eprint/9632/1/9632.pdf" rel="noreferrer">A New Spectral Classification System for the Earliest O Stars: Definition of Type O2</a></p>
<blockquote>
<p>Thus, a new earliest spectral type has been defined.</p>
</blockquote>
<p>So unless that information is outdated, there's no such thing as O1 or O0.</p>

CC BY-SA 4.0
2020-11-03T16:19:48.087Z
I can find some places claiming there is an O1 type, but nowhere that references an actual example of an O1 star.  People may simply be assuming the classification exists.  Unless someone can provide a reference to an O1 *star* I think it's a mute point.
StephenG - Help Ukraine
CC BY-SA 4.0
2020-11-03T15:20:58.030Z
2020-11-03T15:20:58.030Z
Glorfindel
<p>Okay, we've seen the super hot Wolf-Rayet stars, especially WR 102 and 142, and the &quot;slash stars,&quot; many of which are early O (O2-4.5/WN). We know the temperatures of these Wolf Rayet stars. So if we know that, what would their spectral types be in the standard Morgan-Keenan spectral classification (i.e. O2V or B0Ia<sup>+</sup>), and are there known stars with spectral class earlier than O2?</p>

<star><temperature><spectral-type>
Is there an O1 or O0 star?
WarpPrime
# Is there an O1 or O0 star?

(Asked by: username_0 on 2020-11-01)

Okay, we've seen the super hot Wolf-Rayet stars, especially WR 102 and 142, and the "slash stars," many of which are early O (O2-4.5/WN). We know the temperatures of these Wolf Rayet stars. So if we know that, what would their spectral types be in the standard Morgan-Keenan spectral classification (i.e. O2V or B0Ia+), and are there known stars with spectral class earlier than O2?

**Tags:** star, temperature, spectral-type

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## 2 answers

### Answer by username_3 on 2020-11-03. Score: 6 | Accepted answer

This is what [Wikipedia](https://en.wikipedia.org/wiki/Stellar_classification#Class_O) says about it:

> When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.

It references the paper [A New Spectral Classification System for the Earliest O Stars: Definition of Type O2](https://discovery.ucl.ac.uk/id/eprint/9632/1/9632.pdf)

> Thus, a new earliest spectral type has been defined.

So unless that information is outdated, there's no such thing as O1 or O0.

> **Comments:**
> **username_2**: I can find some places claiming there is an O1 type, but nowhere that references an actual example of an O1 star.  People may simply be assuming the classification exists.  Unless someone can provide a reference to an O1 *star* I think it's a mute point.  

---

### Answer by username_1 on 2020-11-03. Score: 2

I don't believe that O0 is a real classification(see [this chart](http://www.atlasoftheuniverse.com/startype.html)), but if it were following the temperature steps it would probably be around 80,000-90,000 degrees Kelvin. The hottest star we know of is WR 102, which is 210,000 degrees Kelvin, and that is much, much hotter than my predicted O0 temperature. So the short answer is yes, O0 temperature stars exist, but O0 isn't truly a real classification. (The hottest real classification is O2, at 53,000 Kelvin and up) Hope I helped!

Edit: I forgot to add a closing parentheses and my Markdown was messed up
Edit 2: I was looking at an outdated document, the real hottest classification is O2 not O3.

> **Comments:**
> **username_0**: Ok, so if we ignore WR, would they be O0 or 1?  
> **username_1**: What do you mean would "they" be O0 or 1?  
> **username_1**: @fasterthanlight I mean, what/who is they?  
> **username_2**: Wikipedia gave examples of three O2 stars and as far as I can tell these are hotter than O3 stars, so I think your answer is inaccurate.  
> **username_1**: @StephenG Ah right, I think that graph may be outdated. I found O2 becoming a real classification in 2002, but still nothing about O1 or O0. I'll edit my answer shortly, but it doesn't really change much.  
> **username_1**: @StephenG I fixed it now! Would you mind removing your downvote?  
> **username_1**: @StephenG Actually, a potential correction of my previous self-I found several website that appear to mention O0 and O1 stars, but they appear to be incredibly old, before 2002(when O2 stars were made an official thing), thus I'm not sure if they're just making it up or if they're a real thing. I can't find any date or anything like that so I'm fairly certain they were just making those star classifications up.  
> **username_1**: @StephenG Thanks for removing the upvote! I agree, it is kind of confusing. I will say though I haven't seen any big sources(such as NASA, SpaceX, space.com, etc.) mention O1 or O0 stars, so I think they're just making it up.  
> **username_1**: @fasterthanlight Ah, I think I understand your question now. I think if the scales kept going up to another letter, it would be several letters past O. But we're assuming it doesn't do that, so I think it would be classified as a O0 star since I think anything above 90,000 degrees Kelvin would be considered O0(if that classification existed) Technically, in reality WR 102 is classified as a O2 star.  

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