A highly magnified star dating just 900 million years after the Big Bang

A gravitational lens is any object that deflects light by gravitation as described by the general theory of relativity; it is analogous to a lens in optics. The prediction of a gravitational lensing effect was confirmed repeatedly in observations on quasars. In 1979 an apparently double quasar was discovered, due to the multiple image of a single quasar caused by gravitational lensing by a galaxy, or cluster of galaxies, along the line of sight between the observer and the quasar.

When we say any object, it includes some of the biggest in the universe, like galaxy clusters. Proportionally, galaxy clusters magnify background objects through a strong gravitational lensing. Typical magnifications for lensed galaxies are factors of a few units, but can also be as high as tens or hundreds, stretching galaxies into giant arcs. Individual stars can attain even higher magnifications given fortuitous alignment with the lensing cluster.

Redshift is the displacement of the spectrum of an astronomical object toward longer (red) wavelengths. It is attributed to the Doppler effect, a change in wavelength that results when a given source of waves (e.g., light or radio waves) and an observer are in motion with respect to each other.

The American astronomer Edwin Powell Hubble reported in 1929 that the distant galaxies were receding from the Milky Way system, in which Earth is located, and that their redshifts increase proportionally with their increasing distance. This generalization became the basis for what is called Hubble’s law, which correlates the recessional velocity of a galaxy with its distance from Earth. That is to say, the greater the redshift manifested by light emanating from such an object, the greater the distance of the object and the larger its recessional velocity. This law of redshifts has been confirmed by subsequent research and provides the cornerstone of modern relativistic cosmological theories that postulate that the universe is expanding.

Extremely large redshifts mean that the object is moving away from Earth at a tremendous velocity (i.e., approximately 90 percent the speed of light) and thereby constitute one of the most distant objects in the universe. Now, a team of researchers reports ¹ observations of a distant and persistent magnified star at a redshift of 6.2 ± 0.1, meaning it dates just 900 million years after the Big Bang. This star is magnified by a factor of thousands by a foreground galaxy cluster lens called WHL0137–08 (redshift 0.566).

The Hubble Space Telescope Treasury Program obtained images of a total of 41 lensing clusters. Included in these observations was a lensed arc of a galaxy at photometric redshift 6.2 ± 0.1 nicknamed the ‘Sunrise Arc’ (officially WHL0137-zD1). Within this galaxy, the researchers identified a highly magnified star sitting atop the lensing critical curve. This object is officially designated WHL0137-LS, but the researchers nicknamed the star ‘Earendel’ from the Old English word meaning ‘morning star’ or ‘rising light’. Earendel’s high magnification has persisted for 3.5 years of observations, thus, it is not a transient event.

The four different lensing models built by the astronomers lead to the conclusion that we are before a single star (or a binary) of mass greater than 50 times the mass of the Sun. The spectral type, temperature and mass of the star remain uncertain. Future spectroscopic observations with James Webb Spatial Telescope will determine these properties for Earendel and place it on the H–R diagram.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.