Credit: Aldo Zanetti
Astronomical and Imaging Data
| RA: | 16h 23m 35.22s |
| DEC: | -26° 31′ 32.7″ |
| MAG: | 5.36 |
| Diameter: | 36′ |
| Const: | Sco |
| OTA | Takahashi Epsilon 160ED |
| Focal Length | 536 |
| Camera | QHY600M |
| Site | Atacama, Chile |
| Sky Quality | Bortle 1 |
Useful Informations
NGC 6121, more commonly known as Messier 4 (M4), is a large and relatively loosely concentrated globular cluster located in the constellation Scorpius. It’s one of the closest globular clusters to Earth, making it a key object for astronomical study.
Key Physical Properties
- Proximity and Apparent Size: At a distance of approximately 7,200 light-years, M4 is the nearest globular cluster to our solar system. This proximity allows astronomers to resolve individual stars with a clarity not possible for most other clusters, making it an ideal “cosmic laboratory” for stellar evolution studies.
- Structure: M4 is classified as a Shapley-Sawyer Concentration Class IX cluster, indicating a loose, less dense structure compared to more compact globulars. A distinctive and unusual feature is a linear “bar” of brighter stars that crosses its core. This feature, first noted by William Herschel in 1783, is a rare sight among globular clusters.
- Age and Metallicity: The cluster’s estimated age is around 12.2 billion years, making it one of the oldest objects in the Milky Way. Its stars are metal-poor, with a low abundance of elements heavier than hydrogen and helium. However, studies of its stellar population have revealed the presence of two distinct stellar generations with slightly different chemical compositions, a phenomenon observed in many globular clusters.
Notable Stellar Content
- Pulsar Planet System: M4 is home to a unique and scientifically significant system: a millisecond pulsar, designated PSR B1620-26, in a binary orbit with a white dwarf. This system is orbited by a third object, an exoplanet with a mass about 2.5 times that of Jupiter. This is one of the oldest known planetary systems in the universe, providing insights into planet formation and survival in dense stellar environments.
- White Dwarfs: The proximity of M4 has enabled detailed studies of its white dwarf population using the Hubble Space Telescope. These studies have found some of the oldest white dwarfs in the galaxy, providing a powerful tool for estimating the age of the cluster and, by extension, the universe itself.
- Blue Stragglers: The cluster contains a significant number of blue straggler stars—stars that appear younger and bluer than other stars of the same age. Their existence is likely the result of stellar collisions or mass transfer within binary systems, a common occurrence in the dense core of a globular cluster.