James Webb captured latest moments of a supermassive star

15 thousand light-years from Earth, in the constellation Sagittarius, is WR 124, a Wolf-Rayet star representing the last moments of the life of the homonymous star before it explodes as a supernova.

A new Wolf-Rayet for Webb

This is not the first time James Webb has observed such a star: in October 2022 the telescope observed WR 124, but with very little details (mostly very grainy), except for the 18 arcs of dust emitted by the powerful solar winds.

The new telescope image, however, observes the same star in impressive detail, revealing all the nuances of the gas surrounding the dying star, which can instead be seen shining brightly at the center of the immense coloured cloud, which is 10 light-years in size.

WR 124 is a star of 30 solar masses, 10 of which have already been lost to space in the form of dust and gas. The perfect geometry of the stellar gas is due to the regular ejection of material by the dying star, which, according to ESA, creates spectacular patterns that have been superbly captured by James Webb.

Technologies to majestically photograph a WR

The image of WR 124 was made possible by the use of two very important instruments on board the James Webb: MIRI and NIRCam.

MIRI, which stands for Mid-Infrared Instrument, is a device that allowed Webb to see the structure of the dust surrounding the star in great detail, the Near-Infrared Camera (NIRCam), on the other hand, combined with MIRI and very high quality filters, allowed Webb to detect the presence of polycyclic hydrocarbons, methane, silicates and molecular hydrogen, i.e. those responsible for the coloring of the stellar cloud.

WR: Will every star becomes one?

Before answering the question that gives this paragraph its title, it is necessary to make an important clarification. A star is a sphere of plasma which, through nuclear fusion processes in its core, generates energy that is constantly radiated into space in the form of electromagnetic radiation (light), elementary particle fluxes (stellar wind) and neutrinos.

To fuel the nuclear fusion processes, the star burns hydrogen. When the hydrogen supply is exhausted, the star begins to burn heavier chemical elements, mainly helium. During this phase, the star begins to grow larger. This is the fate of the Sun, for example. The evolution of a star depends only on its mass.

Extremely massive stars (≥ 30 solar masses), after passing through a temporary ‘blue variable’ phase (also called S-Doradus variable) and running out of hydrogen reserves, become so unstable that they become blue supergiants or even blue hypergiants. Such a star will have a surface temperature of over 50,000 degrees Celsius and a color tending towards white.

Stars of this type will have accumulated so much iron in the core during the post-main sequence phase that they will become Wolf-Rayet stars, stars whose temperature is between 30,000 and 200,000 degrees Celsius, giving the star its typical whitish or blue color. Needless to say, Wolf-Rayet stars are massive stars, but not very bright in the visible light band. This is because Wolf-Rayets, being eruptive stars, emit very strong stellar winds, and the radiation emitted is in the spectrometer range from X-rays to ultraviolet.

Wolf-Rayet in the galactic neighborhood

The first Wolf-Rayets were discovered in 1867 by the French astronomers of the same name, Charles Wolf and Georges Rayet, in the constellation of the Swan, while the first detailed description of such an object dates from the early 2000s.

The brightest Wolf-Rayet to date is still Gamma Velorum, located in the constellation Sagittarius at 1256 light-years from Earth, while the record for the largest Wolf-Rayet ever is held by the binary star WR 104, a system located 8000 light-years from Earth in the constellation Sagittarius, whose stellar winds produce a cloud so large that it occupies 20 solar systems.

About 500 Wolf-Rayets have been identified in the Milky Way, with others in other galaxies because of their remarkably pronounced emission lines. For example, 134 WRs have been cataloged in the Large Magellanic Cloud, but only 12 in the Small Magellanic Cloud, due to the low average metallicity of the galaxy itself. 206 Wolf-Rayets have been detected in the Triangle Galaxy and 154 in the Andromeda Galaxy.

Although it would be virtually impossible to detect them all, it is estimated that there are only a few thousand Wolf-Rayet stars within the Local Group. Outside the group, however, especially in the starburst galaxies, there are thousands and thousands of such stars (more than a thousand have been detected in the Pinwheel Galaxy, for example).