Messier 21
E450596
Messier 21 is a young open star cluster in the constellation Sagittarius, notable for its bright, densely packed hot stars and proximity to the Trifid Nebula.
All labels observed (1)
| Label | Occurrences |
|---|---|
| Messier 21 canonical | 1 |
How this entity was disambiguated
This entity first appeared as the object of triple T4531769 — resolving that mention is where its identity was fixed. The disambiguator weighed these candidate entities and picked the highlighted one (or “None”, minting a new entity). This is how homonymy is resolved: the same surface form can point to different entities.
NED1
Entity disambiguation (via context triple)
gpt-5-mini-2025-08-07
Target entity: Messier 21 Context triple: [Messier 20, nearObjectOnSky, Messier 21]
-
A.
Messier 28
Messier 28 is a dense globular star cluster located in the constellation Sagittarius, composed of hundreds of thousands of ancient stars.
-
B.
Messier 20
Messier 20, also known as the Trifid Nebula, is a bright, star-forming emission and reflection nebula famous for its striking three-lobed appearance.
-
C.
Messier 22
Messier 22 is a bright globular star cluster located near the center of the Milky Way, visible in the constellation Sagittarius and notable for being one of the closest and most easily observed clusters of its kind.
-
D.
Messier 45
Messier 45, commonly known as the Pleiades, is a prominent open star cluster in the constellation Taurus, easily visible to the naked eye and famous in many cultures worldwide.
-
E.
Messier 75
Messier 75 is a densely concentrated globular star cluster located in the constellation Sagittarius, notable for its great distance from Earth and rich population of ancient stars.
- F. None of above. chosen
- G. Unsure - the case is ambiguous/there is not enough information to decide.
NED2
Entity disambiguation (via description)
gpt-5-mini-2025-08-07
Target entity: Messier 21 Target entity description: Messier 21 is a young open star cluster in the constellation Sagittarius, notable for its bright, densely packed hot stars and proximity to the Trifid Nebula.
-
A.
Messier 28
Messier 28 is a dense globular star cluster located in the constellation Sagittarius, composed of hundreds of thousands of ancient stars.
-
B.
Messier 20
Messier 20, also known as the Trifid Nebula, is a bright, star-forming emission and reflection nebula famous for its striking three-lobed appearance.
-
C.
Messier 22
Messier 22 is a bright globular star cluster located near the center of the Milky Way, visible in the constellation Sagittarius and notable for being one of the closest and most easily observed clusters of its kind.
-
D.
Messier 45
Messier 45, commonly known as the Pleiades, is a prominent open star cluster in the constellation Taurus, easily visible to the naked eye and famous in many cultures worldwide.
-
E.
Messier 75
Messier 75 is a densely concentrated globular star cluster located in the constellation Sagittarius, notable for its great distance from Earth and rich population of ancient stars.
- F. None of above. chosen
Statements (47)
| Predicate | Object |
|---|---|
| instanceOf |
Messier object
ⓘ
NGC object ⓘ open star cluster ⓘ |
| age | ~4.6 million years ⓘ |
| angularSize | 13 arcminutes ⓘ |
| apparentBrightnessDescription | moderately bright ⓘ |
| apparentMagnitudeV | 6.5 ⓘ |
| appearsIn | Messier marathon lists ⓘ |
| bestSeenFrom | Northern Hemisphere NERFINISHED ⓘ |
| bestSeenIn | summer ⓘ |
| cataloguedIn |
Messier Catalogue
NERFINISHED
ⓘ
New General Catalogue NERFINISHED ⓘ |
| constellation | Sagittarius NERFINISHED ⓘ |
| contains |
B-type stars
ⓘ
hot blue stars ⓘ pre-main-sequence stars ⓘ |
| declination | −22° 30′ ⓘ |
| discoveredBy | Charles Messier NERFINISHED ⓘ |
| discovererNationality | French ⓘ |
| discoveryDate | 1764 ⓘ |
| distanceFromEarth |
~1.2 kiloparsecs
ⓘ
~4000 light-years ⓘ |
| estimatedNumberOfStars | ~60 ⓘ |
| galacticArm | Sagittarius Arm NERFINISHED ⓘ |
| has |
bright member stars
ⓘ
dense stellar concentration ⓘ |
| interstellarExtinction | moderate ⓘ |
| locatedIn | Milky Way ⓘ |
| locatedNear | galactic plane ⓘ |
| metallicity | near-solar ⓘ |
| near |
Messier 20
NERFINISHED
ⓘ
Trifid Nebula NERFINISHED ⓘ |
| observedIn |
infrared wavelengths
ⓘ
optical wavelengths ⓘ |
| otherName |
C 1800-225
NERFINISHED
ⓘ
Cr 362 NERFINISHED ⓘ M21 NERFINISHED ⓘ NGC 6531 NERFINISHED ⓘ |
| rightAscension | 18h 04m ⓘ |
| separationFromTrifidNebula | ~0.75 degrees ⓘ |
| skyLocationDescription | north of the Trifid Nebula ⓘ |
| stellarPopulation | young ⓘ |
| structure |
compact core
ⓘ
loosely concentrated halo ⓘ |
| TrumplerClass | I3p ⓘ |
| visibleIn |
binoculars
ⓘ
small telescopes ⓘ |
How these facts were elicited
The pipeline generated the facts above by prompting gpt-5.1 with this entity's name + description and the instruction below.
Instruction
You are a knowledge base construction expert. Given a subject entity and a description of it, return factual statements that you know for the subject as a JSON list of dictionaries(triples), where keys must be "subject", "predicate" and "object". The number of facts may be very high, between 25 to 50 or more, for very popular subjects. For less popular subjects, the number of facts can be very low, like 5 or 10. # Requirements - If you don't know the subject at all, return an empty list. - If the subject is not a named entity, return an empty list. - Include at least one triple where predicate is "instanceOf". - Do not get too wordy. - Separate several objects into multiple triples with one object.
Input
Subject: Messier 21 Description of subject: Messier 21 is a young open star cluster in the constellation Sagittarius, notable for its bright, densely packed hot stars and proximity to the Trifid Nebula.
Referenced by (1)
Full triples — surface form annotated when it differs from this entity's canonical label.