Messier 108
A nearly edge-on barred spiral galaxy with H1 supershells.
Messier 108, also cataloged as NGC 3556 and nicknamed the Surfboard Galaxy, is a barred spiral galaxy located roughly 46 million light-years away in the northern constellation Ursa Major. Discovered by Pierre Méchain in either 1781 or 1782, it appears nearly edge-on when viewed from Earth. The galaxy sits in isolation within the Ursa Major Cloud, a grouping of galaxies in the local supercluster. Its morphological classification is SBbc in the de Vaucouleurs system, indicating a barred spiral with somewhat loosely wound arms. Optically, it spans a maximum angular size of 11.1 by 4.6 arcminutes and is tilted 75 degrees relative to our line of sight. The galaxy’s mass is estimated at 125 billion times that of the Sun, and it contains roughly 290 ± 80 globular clusters. Observations of its neutral hydrogen reveal discrete, expanding shells of gas stretching several kiloparsecs, known as H1 supershells. These may be driven by currents of dark matter, dust, and gas that fuel vigorous star formation and subsequent supernova explosions, or they could result from infalling intergalactic material or radio jets. Data from the Chandra X-ray Observatory have detected 83 X-ray sources, including one at the nucleus. The brightest of these is consistent with an intermediate-mass black hole accreting matter. Diffuse soft X-ray radiation extends within 2.6 arcminutes of the optical galaxy. The core’s spectrum suggests an active galactic nucleus, though observations with the Spitzer Space Telescope found no signs of activity. The supermassive black hole at the center has an estimated mass of 24 million solar masses. Four supernovae have been recorded in M108. SN 1969B, of unknown type and magnitude 16, was discovered by Paul Wild on 6 February 1969, and independently by Balázs four days later, eventually brightening to magnitude 13.9. SPIRITS 16tn, found by the Spitzer Space Telescope in August 2016, was visible only in infrared due to heavy dust obscuration, with an extinction estimated at 8–9 magnitudes, making it one of the most heavily obscured supernovae known. SN 2023dbc, a Type Ic supernova of magnitude 17, was discovered by the Zwicky Transient Facility on 13 March 2023; it is likely a stripped-envelope supernova, lacking hydrogen in its spectra beyond narrow emission from the underlying HII region, and ranks among the nearest Type Ic supernovae found.
Quick Facts
- Epoch
- J2000
- Constellation Name
- Ursa Majorsinnott1988
- Ra
- 11 · 11 · 31.0ned
- Dec
- +55 · 40 · 27
- Z
- 696.1 ± 0.6 km/s
- Dist Ly
- 14.1 Mpc (46 Mly)Sorce2014
- Appmag V
- 10.0seds
- Names
- NGC 3556, PGC 34030, UGC 6225
Facts from the source article.
Lore & Background
Messier 108 was discovered by Pierre Méchain in 1781 or 1782. It is an isolated member of the Ursa Major Cloud of galaxies in the local supercluster. The galaxy has a morphological classification of type SBbc, indicating a barred spiral with somewhat loosely wound arms. Its maximum angular size in the optical band is 11′.1 × 4′.6, and it is inclined 75° to the line of sight, making it appear almost edge-on from Earth. Examination of the distribution of neutral hydrogen in this galaxy shows discrete shells of expanding gas extending for several kiloparsecs, known as H1 supershells. These may be driven by currents of dark matter, dust and gas contributing to large star formation, having caused supernovae explosions, or they may result from an infall from the intergalactic medium or arise from radio jets. Observations with the Chandra X-ray Observatory have identified 83 X-ray sources, including a source at the nucleus; the brightest is consistent with an intermediate-mass black hole accreting matter. The spectrum of the core source is consistent with an active galactic nucleus, but an examination with the Spitzer Space Telescope showed no indication of activity. The supermassive black hole at the core has an estimated mass of 24 million solar masses. Four supernovae have been observed in M108: SN 1969B (type unknown, mag. 16, discovered by Paul Wild and independently by Balázs), SPIRITS 16tn (discovered by the Spitzer Space Telescope in August 2016, visible only in infrared due to heavy dust obscuration), SN 2023dbc (Type Ic, mag. 17, discovered by the Zwicky Transient Facility on 13 March 2023), and SN 2025ahqr (Type Iax[02cx-like], mag. 20.0269, discovered by the Zwicky Transient Facility on 23 November 2025).
Reader's Guide
Messier 108 is significant as a nearby barred spiral galaxy seen nearly edge-on, offering a clear view of its structure and the H1 supershells that may trace star formation or intergalactic interactions. The galaxy's X-ray sources, including a candidate intermediate-mass black hole, and its supermassive black hole of 24 million solar masses, provide targets for studying black hole accretion and activity. The detection of four supernovae, including the heavily obscured SPIRITS 16tn and the nearby Type Ic SN 2023dbc, highlights its value for supernova studies. The uncertainty over the nature of its nuclear activity—consistent with an active galactic nucleus in X-rays but not in infrared—adds to its astrophysical interest. Its isolation within the Ursa Major Cloud makes it a useful object for studying galaxy evolution without strong environmental influences.
Did You Know?
- Messier 108 is nicknamed the Surfboard Galaxy.
- It has about 290 ± 80 globular clusters.
- The galaxy contains H1 supershells of expanding gas extending for several kiloparsecs.
- Four supernovae have been observed in M108, including one only visible in infrared due to dust.
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