m4 – globular cluster
Lupe Rioné, Amateur astronomer (Spain)
Constellation of Scorpion
Globular clusters: Globular clusters are concentrations of thousands to millions of stars joined gravitationally. They all orbit in the halo of a galaxy, although some of them can be located near the center of the Milky Way.
Do you want to know more about stellar clusters?
Discovery and Location
Also called NGC 6121, M4 is the closest globular cluster to the Earth. It was discovered by Swiss astronomer Jean Philippe Loys de Cheseaux in 1746 in the constellation of Scorpion, near the brightest star in this region of the sky: Antares. Later, in 1764, Charles Messier included it in his catalog as M4. This was the first cluster that Messier “solved”, observing its true nature as a collection of tens of thousands of stars. With a magnitude of +5.6 this celestial body can be seen with the naked eye on a very dark night. With small telescopes of moderate aperture, we can see its spectacular shape.
Image of M4 from the El Querol Observatory (Tarragona). skywatcher 200/1000 reflecting telescope
M4 cluster. Credit: Hubble/NASA
M4 contains about 100,000 stars, and like most globular clusters, it is a very old structure, about 12-13 billion years old. If we consider that the first stars formed about 1 billion years after the Big Bang, it can be seen that it is useful to determine the age of the universe and to study its evolution. In addition, it contains 43 variable stars and also quite a few white dwarfs, so it has a good number of the oldest stars in the Milky Way.
A white dwarf is the result of a red giant star that as it ages expels its outer layers into interstellar space .
Its structure is very disordered (or open) and its central mass is relatively small compared to other clusters. The central mass has a diameter of 8 light years, but its gravitational influence, or the stars linked to the cluster, is 140 light years.
Spectacular image of the star Antares and M4. Credit: Randy Carter
Artistic recreation of the planet Gemini orbiting the binary system in M4. Credit: NASA
Pulsar in M4
In 1987 the PSR B1620-26 pulsar was discovered in M4, being the first object of this type discovered in a globular cluster. This pulsar is located on the outskirts of the central region of the cluster and has a period of 3 milliseconds, that is, it turns on its axis every 3 thousandths of a second, 10 times faster than the central pulsar of the Crab Nebula.
A pulsar is a rapidly rotating neutron star with an intense magnetic field, inclined with respect to its axis of rotation.
Image taken from the El Querol observatory in Tarragona where we can see the constellation of Scorpion where the globular Cumulus M4 and the Antares Star are located.
Later, it was discovered that PRS B1620-20 was part of a binary system accompanied by a white dwarf. In addition, in 2003 a giant planet between 3 and 10 times the mass of Jupiter was discovered orbiting this binary system. The planet is known as Gemini or Methuselah, due to its age which is estimated at 12 billion years.
Don’t miss our astronomy videos on youtube (English and Spanish subtitled)
Somewhere, something incredible is waiting to be known