May, to me, is more of a transition season than most others in the night sky. The realm of springtime galaxies slowly cedes to the summer realm of the Milky Way’s star fields, clusters and nebulae, rising in the east. So, if you can get out between late spring cool fronts and storms, and stay out long
enough, you can sample both. Below are a few destinations in both realms, but as always, your own voyage will be unique. Wherever it takes you, enjoy!

W Virginis   rating EASY
variable star in Virgo
RA 13h 26.1m  Dec -3d 22.6′  (2000)
Magnitude 9.5

Usually when amateurs think of Virgo, galaxies come to mind: the great Virgo cluster is a premier galaxy hunting ground. But Virgo also contains some other interesting inhabitants, one of which is variable star W Virginis, located 8 degrees due north of Spica (Alpha Vir) in an area of Virgo
remarkably free of galaxies.

W is the prototype of the W Virginis variables, a subclass of the Cepheid variable stars also called Population II Cepheids. This group of Cepheid variables is physically different than the better known “Classical Cepheids” or “Population I Cepheids,” and this difference caused problems before it
was recognized.

In the early 20th century, when the extragalactic distance scale was being determined, Population II Cepheids were mistakenly grouped with Population I Cepheids. This resulted in an inaccurate period-luminosity relationship that weakened the distance scale. The magnitude differences in the two groups of stars threw off distance calibration significantly.

Finally, after a detailed studies of M31 from Mt. Wilson during WWII blackouts, Walter Baade recognized two distinct stellar populations in both M31 and the Milky Way:  younger population I stars, like those found in galactic disks, and older population II stars, found in the galactic halo and globular clusters. Each has its own Cepheid variables, and an average type I Cepheid is 1.5 magnitudes brighter than a type II Cepheid with the same period. Once this difference was understood and the distance scale was corrected, the known universe roughly doubled in size.

Type II Cepheids can be divided into 3 groups according to their period. Stars with periods of 1-5 days are called BL Herculis variables, stars with periods of 10-20 days are called W Virginis variables, and stars with periods greater than 20 days are called RV Tauri variables.

W Virginis itself varies between magnitudes 9.46 and 10.75 over a period of about 17 days, very suitable for observation by interested amateurs. Variations in the light curve are apparently due to multiple superimposed pulsation periods rather than instabilities. But the dominant pulsation period of 17.27134 days appears to be decreasing, based on 75 years of observations.

NGC 5247   rating MEDIUM
galaxy in Virgo
RA 13h 38.1m  Dec -17d 53.0′  (2000)
Magnitude 10.5

The bulk of the great Virgo SuperCluster rides high in Virgo, north of the celestial equator, but a tail of galaxies trails off to the south, passing just west of Spica on its way down into Hydra. Although it most likely also belongs to the Virgo Supercluster, galaxy NGC 5247 lies in this tail, in far southern Virgo about 7 degrees SSE of Spica.

Discovered by William Herschel on March 17, 1787, NGC 5247 is an unbarred face-on spiral with two dramatic spiral arms that bifurcate (branch out) after wrapping half way around the nucleus. Astronomers call such galaxies “grand design” spirals, the most famous example of which is perhaps M81 in Ursa Major.

Because one of the branches stands out more than the others, NGC 5247 is sometimes described as a 3-armed spiral.  For you fans of oddities, note that astronomer Halton Arp even had a category for such 3-armed beasts in in his collection of peculiar galaxies.

NGC 5247 is easily visible in an 8-inch scope, and some hint of the spiral arms can be glimpsed in a 12-inch. Some amateurs claim to have detected a small odd but sharply defined dust lane that cuts right the gap between northern arm and the core. You can decide for yourself on that one. The galaxy sits 50 to 60 million light-years away.

Palomar 5   rating HARD
globular cluster in Serpens Caput
RA 15h 16.1m  Dec -00d 06.7′  (2000)
Magnitude 11.8

Palomar 5 can be found in Serpens (actually, Serpens Caput) almost right on the celestial equator, just 2 degrees SSW of well-known globular M5. It was discovered by Walter Baade in 1950, and again independently by A.G. Wilson in 1955. He published its discovery along with that of Palomar 3, Palomar 4, Palomar 13, and two new local group galaxies. In fact Palomar 5 itself was initially temporarily mistaken for a nearby elliptical or spherical dwarf galaxy, and suspected to be a Local Group member due to its resolution into stars.

Palomar 5 is currently undergoing a process of tidal disruption by the gravitation of the Milky Way galaxy. Many former cluster member stars are moving away from it, forming tails in opposite directions. These streams have probably existed for several billion years, and now extend to lengths
of over 30,000 light-years. The cluster is about 60,000 light-years from the galactic center, and has a noticeable amount of flattening.

Probably the best way to catch this dwarf globular is by imaging; good amateur images can be found on the web. The Palomar globular list is in general a very challenging project for owners of big scopes. The list of astronomers who first identified them includes a number of famous names,
among them Edwin Hubble, Fritz Zwicky, Halton Arp, and George Abell. Several of the Palomar globulars – including Palomar 6, 7, 9, 10 and 11 – are nearby clusters of average size that just happen to be heavily obscured by dust in our line of sight. Others – including Palomar 3, 4 and 14 – are giant globulars that are very far away in the extreme outer halo of the Milky Way. The list initially included just 13 clusters; Palomar 14 and 15 were added later. The list members vary greatly in degree of observing difficulty, from easy to nearly impossible.