…continued
Exploring the Hubble Sequence by EyeMore Ellipticals
M84 and M86 form two points of a triangle including the somewhat fainter galaxy NGC 4388 to the south. In the center of that triangle is another even fainter galaxy, NGC 4387. This is a more typical lenticular member of the Virgo Cluster and provides a good example of the contrast in sheer size between galaxies that lie at the same distance.
Before leaving the early-type galaxies, it is worth slipping about a degree southeast to M87 and its several companions. M87 is usually called a “giant elliptical” (type E+), and images or photographs reaching to very low light levels show the galaxy to have an extensive but faint corona similar to that of M86. Visually M87 is another case like M84, where the brightness fades without a break until it merges with the sky. How large does the galaxy appear? (For a “yardstick,” note that 5.7 arcminutes separate M87's core and HD 108915, an 8.5-magnitude star due north of the galaxy.)
M87's two companions to the southwest, NGC 4476 and 4478, are again smaller and fainter examples of lenticular and elliptical galaxies. More interesting to me is NGC 4486B, which lies just west of HD 108915. At lower powers in a modest telescope, NGC 4486B looks like a 13.5-magnitude star. But by cranking up the magnification one can see that the object is not quite starlike; rather it resembles a small, high-surface-brightness planetary nebula a few arcseconds across. This is an excellent example of a “compact elliptical” (cE) galaxy. The brightness profile of a compact elliptical is identical in form to that of a giant elliptical, but compact ellipticals are as much as a hundred times less luminous than giant ones. The brightest example of a compact elliptical — it defines the class, in fact — is M32, the Andromeda Galaxy's most conspicuous companion. M32's core is so small it defies resolution (that is, it remains starlike) even with the Hubble Space Telescope (Sky & Telescope: March 2000, page 22).
