Double Nucleus of the Andromeda Galaxy, M31

[Right] HST View of Andromeda's Galactic Nucleus

A NASA's Hubble Space Telescope image of a "double nucleus" in the giant spiral galaxy in Andromeda, M31.

Each of the two light-peaks contains a few million densely packed stars. The brighter object is the "classic'' nucleus as studied from the ground. However, HST reveals that the true center of the galaxy is really the dimmer component.

One possible explanation is that the brighter cluster is the leftover remnant of a galaxy cannibalized by M31. Another idea is that the true center of the galaxy has been divided in two by deep dust absorption across the middle, creating the illusion of two peaks.

This green-light image was taken with HST's Wide Field and Planetary Camera (WF/PC), in high resolution mode, on July 6, 1991. The two peaks are separated by 5 light years, The Hubble image is 40 light years across.

[Center] Ground View of Andromeda Galaxy Core

The nucleus is at the center of the bulge of M31, a smooth system of old stars. The bulge is apparent as the bright center of M31 in standard pictures of the galaxy. Individual points of light are globular star clusters each containing several hundred thousand stars. This ground-based telescopic image does not resolve the double nature of the nucleus.

The background of the present picture is dark only because the contrast has been adjusted to show the full detail in the nucleus, which is much brighter than the surrounding bulge.

[Left] Ground View of Andromeda Galaxy

One of the closest neighbors to our own Milky Way galaxy, M31 dominates the Local Group, a small group of galaxies of which our own Milky Way is a member. M31 can be seen with the naked eye as a spindle-shaped "cloud" the width of the full moon.

Credit: T. R. Lauer (NOAO), NASA

The HST has investigated globular cluster G1, the most luminous globular of the Andromeda galaxy, and the whole Local Group.

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Hartmut Frommert
Christine Kronberg
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Last Modification: 31 Jan 2001