NGC 3115 - Spindle Galaxy

NGC 3115 is one of two different objects known as the Spindle Galaxy. It is a bright lenticular galaxy - a smooth disk galaxy without significant spiral structure - located in the constellation Sextans. It lies 4.8° north of Lambda Hydrae. The other galaxy called the Spindle Galaxy is NGC 5866 in the constellation Draco.

William Herschel discovered NGC 3115 on February 22, 1787 and numbered it as H 1.163.

With angular dimensions of 8.3' x 3.2', NGC 3115 is seen almost exactly edge-on. Dreyer describes this galaxy very bright (magnitude 9.2), large, very much elongated, and brightening sharply with an elongated nucleus.

As may be guessed from its appearance and stellar contents, NGC 3115 can hardly be distinguished from an elliptical galaxy, and is occasionally mis-classified as such. In fact it is lenticular, a type S0 "spiral galaxy without spiral structure", where stellar formation has stopped because its interstellar matter has been used up. NGC 3115 appears to contain mostly old stars.

At 32 million light-years away, NGC 3115 is several times bigger than our Milky Way. One supernova has been observed in NGC 3115, SN 1935B; it occurred in April 1935.

In 1992, NGC 3115 was the fourth galaxy in which a supermassive black hole was found. It was one of the largest found up to that time, with 2 billion times the mass of the Sun. The effects of the black hole on nearby stars are easily seen, because the galaxy is transparent (no dust), and made of old stars, so that the measurements are straighforward. The growth of its black hole has also stopped.