How big is Virgo A?

Virgo A is a strong source of radio waves in the constellation Virgo. It comes from M87, also called Messier 87, a giant elliptical galaxy about 55 million light-years away. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The galaxy's bright main body is about 130,000 light-years across, and its faint outer halo of stars is about eight times as wide.

Size

An elliptical galaxy is a round or oval collection of stars with little gas or dust and very little structure. M87 has no disk and no sharp edge. Its glow fades gradually outward, so its size depends on how faint a glow is counted.

Astronomers often measure a galaxy out to a set level of faintness. At that level, the Third Reference Catalogue of Bright Galaxies gives M87 a width of about 8.3 arcminutes, about a quarter of the width of the full Moon. An arcminute is a sixtieth of a degree.

At M87's distance of 55 million light-years, a width of 8.3 arcminutes is about 130,000 light-years. Catalogs differ on exactly where the edge falls, and one, HyperLeda, gives a width about 10% larger.

A 2009 study traced M87's faintest stars using planetary nebulae, the glowing gas thrown off by dying stars. It found that the galaxy's halo of stars ends about half a million light-years from the center. Counting the halo, M87 is roughly a million light-years across.

Size comparisons

• M87's bright main body is about 1.3 times as wide as the disk of the Milky Way, which is about 100,000 light-years across.
• Measured to the same faintness, it is about the same width as the bright disk of the Andromeda Galaxy, one of the next larger objects in the explorer, and about three-quarters the width of the Pinwheel Galaxy.
• If M87's bright main body were shrunk to the size of a basketball, 24 centimeters (9.4 inches) across, the Milky Way's disk would be about 18 centimeters (7 inches) across.
• At that scale, M87's faint outer halo would be about 2 meters (6.5 feet) across.

The black hole at the center

On April 10, 2019, the Event Horizon Telescope team released the first image of a black hole, the one at the center of M87. The team linked eight radio telescopes around the world into one virtual telescope as wide as Earth. The image shows a bright ring around a dark center, the black hole's shadow.

The black hole has a mass of about 6.5 billion Suns, about 1,600 times that of the black hole at the center of the Milky Way. It powers a jet of particles that streams out from the galaxy's center at nearly the speed of light. Much of Virgo A's radio glow comes from plumes of fast-moving gas that the jet pushes outward.

Stars and star clusters

M87 holds several trillion stars and about 15,000 globular clusters, tight groups of some of the oldest stars. For comparison, the Milky Way has a few hundred billion stars and about 150 globular clusters. M87 may have gathered some of its clusters by pulling them away from smaller galaxies nearby.

M87 is the largest galaxy in the Virgo Cluster, a group of about 2,000 galaxies. The astronomer Charles Messier discovered it in 1781.

Sources

• M87: A Nearby Galaxy Metropolis. NASA Chandra X-ray Center, 2008.
• Astronomers Capture First Image of a Black Hole. European Southern Observatory, 2019.
• Messier 87. NASA Hubble Space Telescope, Hubble's Messier Catalog.
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• Third Reference Catalogue of Bright Galaxies (RC3). de Vaucouleurs et al., Springer, 1991, via CDS VizieR.
• HYPERLEDA. I. Identification and designation of galaxies. Paturel et al., Astronomy & Astrophysics, 2003, via CDS VizieR.
• The edge of the M87 halo and the kinematics of the diffuse light in the Virgo cluster core. Doherty et al., Astronomy & Astrophysics, 2009.
• Caldwell 63. NASA Hubble Space Telescope, Hubble's Caldwell Catalog.
• Guide to our galaxy. European Space Agency.
• Our Milky Way Galaxy: How Big is Space?. NASA, 2019.
• The Carnegie-Chicago Hubble Program. VII. The Distance to M101 via the Optical Tip of the Red Giant Branch Method. Beaton et al., The Astrophysical Journal, 2019.
• A Sub-2% Distance to M31 from Photometrically Homogeneous Near-infrared Cepheid Period-Luminosity Relations Measured with the Hubble Space Telescope. Li et al., The Astrophysical Journal, 2021.
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