Scrutinizing Spirals
Another spring treat for Northern Hemisphere observers, this trio of bright spiral galaxies in Leo illustrates another telltale characteristic of some spirals: an elongated profile that narrows because our sightline nearly crosses the plane of the galaxy's disk. Clockwise from top are NGC 3628, M65, and M66.
Courtesy Akira Fujii.
The remaining galaxies in the Markarian Chain are all early-type galaxies.
NGC 4461 is a lenticular whose prominent bar and rings we view obliquely.
NGC 4477, toward the chain's northeast end, is a face-on lenticular that shows some spiral structure.
M88 is our first definite spiral galaxy (type Sb), and a beauty it is! Medium-size telescopes show a fairly bright oval halo surrounding a moderately large and somewhat brighter oval core. Higher powers best show the very small nucleus. These three components of the visual impression are common to ordinary spiral galaxies. Ellipticals and many lenticulars lack this three-part halo-core-nucleus zonation; instead, they fade from center to edge without any kind of plateau. Relative to the galaxy as a whole, M88's nucleus is not as prominent as those of the lenticulars, but is it brighter or fainter than they in absolute terms? Switch views among some of the similarly bright Markarian Chain lenticulars and compare for yourself.
About 50 arcminutes to the east of M88 lies a barred spiral galaxy, M91. With a Hubble type of SBb, M91 is comparable in a sense to M88, but it shows a larger and brighter bulge relative to the arms, as well as a prominent bar. The difference between the two galaxies is evident even with my 70-mm Pronto, in which M91's bar appears elongated nearly east-west within a faint circular halo. The bar was even more readily visible to Stephen James O'Meara in a 4-inch refractor, according to his 1998 guidebook, The Messier Objects.
Spiral galaxies M99 (left) and M100 (right) sketched by
Ronald J. Buta in 1977 with a 30-inch reflector at McDonald Observatory in Texas. This large telescope was necessary to unambiguously reveal the galaxies' spiral arms. The sketch below shows what can be seen through a smaller telescope. North is up with east to the left.
A few degrees to the north and west, two more Messier objects provide additional glimpses of spiral structure and the halo-core-nucleus pattern.
M99 is of type Sc, and telescopic images show clumpy arms that contain bright star clouds and H II regions (nebulous regions of gas, driven to glow by the ultraviolet light from hot, young stars). The arm extending south from the center clearly stands apart from the main body of the galaxy as it wraps around to the west (as opposed to the innermost arm on the other side of the galaxy, which is much more tightly wrapped). A weak disturbance in the otherwise smooth glow was apparent to me in my 70-mm telescope. Those with 8- and 10-inch telescopes at dark sites
Spiral galaxies M99 (left) and M100 (right) sketched through a 4-inch TeleVue telescope in 1999 by Sky & Telescope contributing editor Stephen James O'Meara. Even a 4-inch (under dark skies) can isolate features that differentiate a spiral galaxy from an elliptical one. North is up with east to the left.
will see the detached arm readily at high power, and larger apertures will reveal a wealth of detail. However, with just about any aperture you should be able to note the relative size and brightness of the circular core (which ranks halfway between those of M91 and M88) and the extensive but relatively faint halo.
More regular in appearance is M100, a bright face-on spiral north of the most crowded parts of the Virgo Cluster. Given its Hubble type (Sbc) we expect to see a small, modestly bright core, which M100 delivers. Interestingly, however, M100 lacks a conspicuous nucleus, as I've noted in several observations I've made with telescope apertures up to 12 inches. Larger telescopes again reveal a number of subtle spiral features, particularly at high power.
