Messier 84

Lore & Background

Messier 84 was discovered by Charles Messier in 1781. It has a morphological classification of E1, indicating a flattening of about 10%. The extinction-corrected total luminosity in the visual band is about 7.64×10¹⁰ L☉. The central mass-to-light ratio is 6.5, and this ratio steadily increases away from the core. The visible galaxy is surrounded by a massive dark matter halo.

Radio observations and Hubble Space Telescope images have revealed two jets of matter shooting out from its center, as well as a disk of rapidly rotating gas and stars, indicating the presence of a supermassive black hole with a mass of 1.5 × 10⁹ M☉. The galaxy has a few young stars and star clusters, indicating star formation at a very low rate. The number of globular clusters is 1,775 ± 150, which is much lower than expected for an elliptical galaxy.

Three supernovae have been observed in M84: SN 1957B (Type Ia, mag. 12.5, discovered by Howard S. Gates on 28 April 1957 and independently by Dr. Giuliano Romano on 18 May 1957); SN 1980I (Type Ia, mag. 14, discovered by M. Rosker on 13 June 1980, though historically catalogued as belonging to M84, it may have been in either neighboring galaxy NGC 4387 or M86); and SN 1991bg (Type Ia-pec, mag. 14, discovered visually by Reiki Kushida on 9 December 1991, with a precovery image showing it at magnitude 14.9 on 3 December 1991). This high rate of supernovae is rare for elliptical galaxies, which may indicate a population of stars of intermediate age in M84.

Reader's Guide

Messier 84 is notable as a giant elliptical or lenticular galaxy in the core of the Virgo Cluster, part of the local supercluster. Its discovery by Charles Messier in 1781 adds to the historical catalog of deep-sky objects. The galaxy's morphological classification E1 and its measured luminosity and mass-to-light ratio provide data on its structure and dark matter content. The presence of a supermassive black hole of 1.5 × 10⁹ M☉, inferred from jets and a rotating disk, is a key feature. The low number of globular clusters (1,775 ± 150) compared to expectations for an elliptical galaxy is a notable discrepancy. The observation of three supernovae, including the peculiar Type Ia-pec SN 1991bg now used as a template for 91bg-like events, is rare for elliptical galaxies and suggests an intermediate-age stellar population. The uncertainty regarding SN 1980I's host galaxy is preserved in the record.

Discovery & Place in the Cosmic Neighborhood

Charles Messier identified this object in 1781 while conducting his methodical survey of nebulous objects across the night sky, and it earned the number 84 in his now-famous catalogue. Today it also carries the designation NGC 4374, and it occupies a particularly crowded region of the universe: the dense core of the Virgo Cluster, itself a member of our local supercluster. Astronomers classify M84 as a giant elliptical or lenticular galaxy with a morphological type of E1, meaning its disk is flattened by roughly ten percent. In the visual band, after correcting for interstellar extinction, the galaxy radiates a total luminosity of approximately 7.64 × 10¹⁰ times that of our Sun, placing it firmly among the brighter members of its cluster. Its position in Virgo, a constellation visible from the northern hemisphere, has made it a long-standing target for both professional and amateur observers seeking to explore the rich galactic population of that region.

The Central Engine: Black Hole, Jets, and Dark Matter

Radio observations and imagery from the Hubble Space Telescope have revealed that M84 harbors a remarkably active nucleus. Two powerful jets of matter are seen erupting from the galaxy's center, while a disk of rapidly rotating gas and stars traces the gravitational influence of a supermassive black hole with an estimated mass of 1.5 × 10⁹ solar masses. The central mass-to-light ratio stands at 6.5, and measurements show this ratio steadily climbs as one moves outward from the core, a signature consistent with an extended dark matter distribution. Indeed, the visible galaxy is enveloped in a massive dark matter halo that dominates the gravitational potential well. Together, the jet activity, the rotating inner disk, and the rising mass-to-light profile paint a picture of a galaxy whose center is far more dynamic than a quiescent elliptical would suggest, even though the outer regions remain relatively calm.

Supernovae and a Surprising Stellar Population

Three supernovae have been recorded in M84, an unusually high tally for a galaxy of its type. SN 1957B, a Type Ia event at magnitude 12.5, was spotted in April and May 1957 by Howard S. Gates and independently by Giuliano Romano. SN 1980I, also Type Ia at magnitude 14, was found by M. Rosker in June 1980, though its true host may have been the neighboring NGC 4387 or M86. The most scientifically important of the three is SN 1991bg, a peculiar, underluminous Type Ia discovered visually by Reiki Kushida in December 1991; a precovery image placed it at magnitude 14.9 a week earlier. SN 1991bg has since become the defining template for a whole subclass of events labeled Type Ia-91bg-like. The elevated supernova rate, combined with the presence of a small population of young stars and star clusters, hints that M84 hosts an intermediate-age stellar population and undergoes star formation at a very low but nonzero pace.

Globular Clusters and Angular Structure

Despite its classification as a giant elliptical or lenticular system, M84 carries a globular cluster population of only 1,775 ± 150, a figure notably lower than what one would typically expect for a galaxy of its luminosity and morphological type. This deficit has prompted discussion about the galaxy's formation history and its interactions within the crowded Virgo Cluster core. From Earth, the galaxy's half-light radius—the angular radius within which half of its total light is emitted—measures 72.5 arcseconds, just over one arcminute of sky. That compact apparent size, combined with the dense galactic neighborhood of the Virgo Cluster, makes M84 a challenging but rewarding target for wide-field imaging. The low globular cluster count, the modest angular extent, and the E1 flattening together distinguish M84 from the more extended, cluster-rich ellipticals that dominate the Virgo core.

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