19:35 UT
ESO 455-11 (less commonly,
Haute Provence 1). This has been one of my favourites for years. It’s a fun cluster because it’s in a brash glittery field while being so timid itself. You can get to it from M6 (the hard way) or from the bottom-most star in Ophiuchus’s knee. As shown on the TriAtlas B chart (#147), you can go from NGC 6304 under the Pipe Nebula to the OC NGC 6360 to two pair of stars that make a nice flat rectangle. Move eastward slightly and there will be a pretty line of half a dozen mag 10 – 11 stars, which leads to another little line of mag 10 – 12 stars, this one with a noticeable bow to it. Beware the nearby line of all-mag 12 stars that is more straight; it’s misled me several times. Above the westernmost star on the upper (rounded) side of the bow you will see a little threesome that resemble the shape of the three isosceles stars of Corvus. Imagine lines joining these stars. Draw perpendiculars through their centres, and HP1 will be where the perpendiculars cross (a little like where the fourth star in Corvus is). HP1 is very tiny, only 1 arcmin visual. It looks like a fuzzy 12.5 mag star. That “star” is actually half a dozen very closely packed red giants that comprise most of HP1’s core. HP1 is the closest globular to the centre of the Galaxy, and has lived its entire 12.5 billion years orbiting in one of the densest mass concentrations in the Galaxy. HP1 weighs in at a feathery 37,000 solar masses (feathery for a globular, anyway), which suggests that it has lost a lot of stars over the eons. Globulars that orbit in a region as dense as the Galactic bulge have an unusual method to keep from being rapidly stripped to bits by the Milky Way’s tidal shears. As the cluster plunges forward at 4 km/sec, stars in its direct path are pulled in toward by the cluster’s gravity. Some are captured. First their orbits are very flat ellipses. These fatten over eons of years into oblate circles as they become more tightly bound to the cluster. But . . . win some, lose some. In the cluster’s wake, the field stars which slipstream around the sides gradually pry away stars from the back side of the cluster. These would be stars already close to the cluster’s tidal radius, where pull and push forces balance. Over time the cluster’s overall mass loss changes little, but has new and old faces. It’s like men losing their hair who grow beards by way of compensation.
19:50 UT
Pal 9 is also known as NGC 6717 (Huey p.109). It is an easy catch in the 152mm refractor. Pal 9 lies about 8 arcmins away from a mag 5 star in a triangle of two other mag 5 stars, an easy find with the naked eye. The globular itself resembles a tight, bright trapezium-like open cluster underlain by a faint halo the same diameter as the cluster. It was long classified as an open cluster until astronomers discovered some telltale signs of an ancient globular, e.g., a blue horizontal branch on the CMD and an [Na/O] anticorrelation between oxygen and sodium in the atmosphere. The [Na/O] relationship in an old star is one of the defining factors that distinguishes a true globular from an ancient open cluster. True globulars formed from two generations of stars in quick succession. The first generation formed with almost no heavy elements. Its massive stars went supernova within 10 million years. These seeded the nearby medium with complex elements, among them a certain proportion of carbon, nitrogen, fluorine, and magnesium. When those elements were captured during the formation of the second generation roughly 250 million years later, each element was processed differently, ending in certain abundances of oxygen and sodium. Over billions of years their proportions changed as a portion of each element was processed into a higher element. Billions of years later, more oxygen was gained as sodium was depleted. Today the ratios are so well known that they are used as one of the tools to date globulars back to their formation era. So Pal 9 turned out to be a GC after all. The glitter we see is from very bright red giants and a confusing pair of field stars. At 60x in a 152mm refractor it presents as a trapezium much tighter than the one in Orion. At double the power in the Mak it looks like the Orion Trapezium somehow got shrunk. Pal 9 is actually two clusters in one: the faint red clump of an old population and four or five red giants about 2 magnitudes brighter. It fairly speckles in the Mak, a teensy disco ball flashing in the sky. It’s only due to photometry that we know it to be a globular – Pal 9 has several RR Lyraes and a moderate-sized helium-burning red horizontal branch, also a sure sign of it being aged <10 billion years or more.
