Messier 32 (NGC 221) is one of the two bright elliptical companion galaxies to M 31, the Great Andromeda Galaxy; the other is M 110. M 32 and M 110 are the closest bright elliptical galaxies, and therefore among the best investigated.
Discovery and Observation
M 32 was the first elliptical galaxy ever discovered, by Guillaume Joseph Le Gentil on October 29, 1749. Charles Messier first saw M 32 in 1757, and cataloged it in 1764. Messier included both M 32 and M 110 in his drawing of Andromeda's "Great Nebula". These galaxies were first resolved into stars by Walter Baade in 1944 with the 100-inch telescope on Mt. Wilson. Baade recognized that their stars were about as bright, and thus at roughly the same distance, as M 31's - thus confirming their proximity to their large spiral neighbor.
M 32 can be easily seen in small telescopes, 22' south of M 31's central region. Superimposed over the outskirts of M 31's spiral arms, M 32 appears as a bright oval-shaped patch, 3' x 2', with its long axis pointing toward the core of M 31. At magnitude 8.1, M 32 is brighter but smaller than M 110 (NGC 205) on the opposite side of M 31. Its brightness increases gradually to a stellar nucleus. M 32 has an overall spectral type of G3 and B-V color index of +0.75.
Properties and Structure
At a distance of 2.5 million light years, M 31 has a diameter of about 6,500 light years - very small compared to its giant spiral neighbor. Spectroscopic investigations have not shown any of the absorption which would be expected if its light had passed through M 31's disk, confirming that M 32 is closer to us than M 31. The radial velocity of M 32 has been measured at 203 km/sec toward our solar system. Corrected for galactic rotation, M 32 is currently at rest with respect to the Milky Way's Galactic Center. This is about 100 km/sec slower than the Andromeda Galaxy, so M 32 is approaching M 31 at this velocity.
M 32 is an dwarf elliptical galaxy, classified as E2, and contains about 3 billion solar masses. Typical of much larger elliptical galaxies, M 32 is composed of older red and yellow stars. There is practically no interstellar dust or gas, and consequently no star formation. Remarkably, M 32 has no globular clusters (whereas M 110 has 8, and M 31 more than 450).
Nevertheless, M 32 has a nucleus comparable to M 31's. About 100 million solar masses - 5000 stars per cubic parsec - are in rapid motion around a supermassive central black hole. The night sky seen from the center of M 32 would be dominated by this object, and filled with M 32's member stars. From its outskirts, M 32's stars would only fill one hemisphere, the other showing either intergalactic space or a fascinating view toward M 31.
Interaction with M 31
It seems possible that M 32 was once much larger, but lost many of its outer stars, and all of its globular clusters, in past close encounters with M 31. Recent simulations suggest a scenario where the strong tidal field of M 31 transformed a small spiral galaxy into a compact elliptical. As the small spiral fell into the central parts of M 31, most its outer disk was stripped away, but its central bulge was much less affected, and retained its morphology. Tidal effects triggered a massive star burst, resulting in the dense core that we observe today.
M 32 has also had a profound impact on its larger neighbor galaxy, for M 31's structure near M 32 is distorted. These disturbances in the big galaxy's spiral pattern also suggest that M 32 has recently undergone a close encounter with M 31.