In celebration of the six anniversary of the International Year of Astronomy 2009, I introduce NGC 2419, not one, but a million stars.
NGC 2419 is a massive, luminous globular cluster in the Lynx constellation. It is easy to overlook, because, first, it is located in the opposite direction of the typical globular cluster of our galaxy in a faint, sparce constellation near the Milky Way's Anticenter, making it unlikely to be noticed. Next, it is one of the most remote objects in our Milky Way galaxy. It is even more distant than the Sagittarius Dwarf Elliptical Galaxy and Magellanic Clouds, which are our closest galactic neighbours. It is so distant (around 300, 000 light years from the Solar system), it was thought in Shapley's time (1944) that it could escape the Milky Way's gravity into intergalactic space, for which Shapley nicknamed it the Intergalactic Tramp (O'Meara 2003). However, unlike the Sagittarius Dwarf Elliptical Galaxy and the Magellanic Clouds, NGC 2419 is gravitationally bound to the Milky Way. NGC 2419, finally, is extremely faint, shining with a pale visual magnitude of 10.4. None of its individual stars is visually brighter than 17th magnitude.
Despite this, ever since Herschel discovered it in 1788, it has been the focus of frequent study.
NGC 2419 is a huge star cluster, about 400 light years in diameter, and its total luminosity equals that of 175,000 Suns. Yet, it is so distant at 300, 000 light years, it appears a tiny 4.1' (arc minutes) across and a dim 10.4 magnitude. If you could move it to where M13 is (a mere 25 100 light years from Sol), it would be as big as the full moon!
Radial velocity measurements reveal that most globular clusters move in highly eccentric elliptical orbits that take them far beyond the outer edge of the Milky Way's disk. These orbits generally are tilted to the plane of the disk and do not seem to be associated with each other. The globular clusters form a roughly spherical halo around the galaxy, in the dark matter region that accounts for most of our galaxy's mass. Most globular clusters are actually concentrated near the galactic center, within 25, 000 light years from it. (The Solar system is approximately 26,000 light years from the galactic center.) A few, outliers like NGC 2419, are scattered farther out, away from the globular cluster crowd. NGC 2419 itself is one of the farthest objects in our galaxy. It also happens to be one of our galaxy's largest, most massive and most luminous globular clusters.
Globular clusters are believed to be very old, consisting of 10 thousand to a million stars formed from primordial matter present just after, or even before, the galaxy's formation, about 14 to 16 billion years old, the age of the Universe itself. Typically, all globular clusters are about the same age. The H-R diagrams for globular clusters typically have short main sequences and prominent horizontal branches. This again represents very old stars. The stars are first-generation, meaning they are made mostly of hydrogen with some helium. The disk stars, in contrast, evolve through many cycles of star birth and supernovae, producing concentrations of heavier elements.
NGC 2419 Statistics
Discovered By = William Herschel on December 31, 1788
Normal Location = RA: 07h 38.1m, Dec: +38° 53'
Distance = 295.0 kly from our solar system and 320.6 kly from the galactic center
(~2x as far as the Large Magellanic Cloud)
Position = galactic anticenter, opposite the typical globular cluster position
Radial Velocity = ~ 20 km/sec toward Sol
Velocity wrt Galactic Center = 76.2 mas/yr away
Velocity wrt Galactic Rotation = 0.5 mas/yr against
Velocity wrt Galactic North Pole = 35.9 mas/yr toward
Orbital Period wrt Galactic Center = three billion years
Apparent Dimension = 4.1 arc minutes
Actual Dimension = diameter of about 400 light years
Number of Stars = about one million
Hubble Morphological Class = II
where I = extreme blue (B) side of RR Lyrae variable (V) on the Main Sequence
where VII = extreme red (R) side of RR Lyrae
Hubble Ratio = (B-R)/(B+V+R) = 0.86
where B = length of the Main Sequence left of the object (its B side)
where R = length of the Main Sequence right of the object (its R side)
where B+V+R = entire length of the Main Sequence
Visual Magnitude = 10.39 mag
Absolute Magnitude = -9.58 mag
Surface Brightness = 19.83 J/s at center
Brightest Star = 17 mag
Equivalent-to-Spectral Class = F5
B-V Color Index = 0.66
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