Butterfly Cluster
An open cluster in Scorpius named for its butterfly shape.
The Butterfly Cluster, also known as Messier 6, M6, or NGC 6405, is an open star cluster located in the southern constellation Scorpius. It gets its name because its shape looks like a butterfly. The cluster contains a mix of hot, blue B-type stars, but its brightest member is a K-type orange giant named BM Scorpii, which stands out sharply against its blue neighbors in photographs. BM Scorpii is a semiregular variable star, with its brightness fluctuating between magnitudes +5.5 and +7.0.
The first recorded observation of the cluster was made by Giovanni Battista Hodierna in 1654. However, Robert Burnham Jr. suggested that the 2nd-century astronomer Ptolemy might have spotted it with the naked eye while observing the nearby Ptolemy Cluster (M7). Credit for the discovery is usually given to Jean-Philippe Loys de Chéseaux in 1746. Charles Messier observed the cluster on May 23, 1764, and added it to his catalog.
Distance estimates have varied over time. A 2009 study by Wu et al. placed the cluster at about 1,590 light-years away, giving it a spatial diameter of roughly 12 light-years. Modern measurements show its total visual brightness is magnitude 4.2, and it is estimated to be 94.2 million years old. The stars in the cluster have a slightly higher abundance of elements heavier than helium—what astronomers call metallicity—compared to the Sun. A total of 120 stars, down to visual magnitude 15.1, have been identified as likely cluster members. There are also eight candidate chemically peculiar stars.
The cluster lies about 24.59 ± 0.13 thousand light-years (7.54 ± 0.04 kiloparsecs) from the Galactic Center. It follows an orbit through the Milky Way with a low eccentricity of 0.03 and an orbital period of 204.2 million years. Currently, it is about 23 light-years (7 parsecs) below the galactic plane, and it crosses that plane every 29.4 million years.
Quick Facts
- Epoch
- J2000.0
- Constellation
- Scorpius
- Ra
- 17 · 40.1
- Dec
- −32 · 13
- Dist Ly
- 0.487 kpc
- Appmag V
- 4.2
- Radius Ly
- 6 light-years
- Age
- 94.2 Myr
- Names
- Messier 6, NGC 6405, Cr 341, Mel 178, Lund 769, OCL 1030, ESO 455-SC030
Facts from the source article.
Lore & Background
The first astronomer to record the Butterfly Cluster was Giovanni Battista Hodierna in 1654. However, Robert Burnham Jr. has proposed that the 2nd century astronomer Ptolemy may have seen it with the naked eye while observing its neighbor the Ptolemy Cluster (M7). Credit for the discovery is usually given to Jean-Philippe Loys de Chéseaux in 1746. Charles Messier observed the cluster on May 23, 1764, and added it to his Messier Catalog.
Estimates of the Butterfly Cluster's distance have varied over the years. Wu et al. (2009) found a distance estimate of 1,590 light-years, giving it a spatial dimension of some 12 light years. Modern measurements show its total visual brightness to be magnitude 4.2. The cluster is estimated to be 94.2 million years old. Cluster members show a slightly higher abundance of elements heavier than helium compared to the Sun.
120 stars, ranging down to visual magnitude 15.1, have been identified as most likely cluster members. Most of the bright stars are hot, blue B-type stars but the brightest member is a K-type orange giant star, BM Scorpii, which contrasts sharply with its blue neighbours in photographs. BM Scorpii is classed as a semiregular variable star, its brightness varying from magnitude +5.5 to +7.0. There are also eight candidate chemically peculiar stars. The cluster is located 24.59 ± 0.13 kly from the Galactic Center and follows an orbit with a low eccentricity of 0.03 and an orbital period of 204.2 Myr. At present it is 23 ly below the galactic plane, and it will cross the plane every 29.4 Myr.
Reader's Guide
The Butterfly Cluster is significant as one of the open clusters cataloged by Charles Messier, who observed it on May 23, 1764. Its discovery history involves multiple astronomers: first recorded by Giovanni Battista Hodierna in 1654, with a possible earlier naked-eye sighting by Ptolemy, though credit is usually given to Jean-Philippe Loys de Chéseaux in 1746. The cluster's name comes from its butterfly-like shape. Its scientific interest includes a measured distance of 1,590 light-years, an age of 94.2 million years, and a slightly higher metallicity than the Sun. The cluster contains 120 likely members, including hot B-type stars and the contrasting orange giant BM Scorpii, a semiregular variable. Eight candidate chemically peculiar stars add to its astrophysical value. Its orbit around the Galactic Center has a low eccentricity of 0.03 and a period of 204.2 Myr, and it currently lies 23 light-years below the galactic plane, crossing it every 29.4 Myr. The cluster is also linked to a candidate escaped star system, Nu Andromedae.
Did You Know?
- The Butterfly Cluster was first recorded by Giovanni Battista Hodierna in 1654.
- Its brightest star, BM Scorpii, is a K-type orange giant that varies in brightness from magnitude +5.5 to +7.0.
- It is located 23 light-years below the galactic plane and crosses the plane every 29.4 million years.
Frequently Asked Questions
What is the Butterfly Cluster?
The Butterfly Cluster is an open star cluster cataloged as Messier 6 (M6) and NGC 6405, earning its nickname from the wing-like spread of its stars. It sits roughly 1,590 light-years away and contains about 120 identified member stars.
Where can I find the Butterfly Cluster in the sky?
M6 resides in the southern constellation Scorpius, making it best viewed from the Southern Hemisphere. Its apparent magnitude of 4.2 means it's visible to the naked eye under reasonably dark skies.
How old is the Butterfly Cluster?
Astronomers estimate the cluster's age at approximately 94.2 million years, placing it in a relatively young phase for an open cluster. This youth is consistent with the presence of hot, blue B-type stars still burning through their fuel.
What is special about the brightest star in the Butterfly Cluster?
The cluster's standout member is BM Scorpii, a K-type orange giant that contrasts sharply with the surrounding blue B-type stars in photographs. It is a semiregular variable whose brightness fluctuates between magnitudes +5.5 and +7.0.
Who first observed the Butterfly Cluster?
The earliest recorded sighting of M6 was made by Giovanni Battista Hodierna in 1654, well before Messier's catalog formalized its designation. The cluster was later independently noted by other astronomers before joining the Messier list.
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