How a clue from Einstein helped measure the mass of a star

(ORDO NEWS) — Knowing the mass of a star is difficult: until now, scientists have had to put up with the fact that only the masses of binary stars can be accurately measured by calculating the parameters of their movement in orbits around each other.

But in rare cases, the mass of a star can be estimated by watching the star create the gravitational microlens effect predicted by Albert Einstein in 1936.

Gravitational lenses are called very massive objects – from individual planets and stars to entire galaxies and galaxy clusters – which, with their gravity, bend the trajectory of the propagation of electromagnetic waves.

Light passing near such a lens bends around it, and for an observer in front of the lens, the position of the luminous object behind it or its shape is distorted.

How a clue from Einstein helped measure the mass of a star 2Sometimes gravitational lenses create “copies” of distant light sources – stars, galaxies or clusters of galaxies

Gravitational lenses created by large objects – galaxies and clusters of galaxies – are constantly observed by astronomers. Sometimes such giant lenses act as space observatories, allowing scientists to look into galaxies so far away, the light from which has not yet reached our part of the universe.

But gravitational microlenses – distortions created by individual stars and planets – have so far been observed only once, during the 1990 solar eclipse.

In order for two distant stars to create the effect of a gravitational microlens, it is necessary that the distant light source, the lens and the observer line up on the same straight line.

Then the light from the distant star will go around the distortion of space-time created by the lens, and to the observer it will seem that a ring of light has formed around the lens.

If the stars and the observer line up along the same axis, but not exactly, but approximately, then it will seem to the observer that, due to the lens, a distant luminous object is temporarily slightly away from the place where it is usually observed.

How a clue from Einstein helped measure the mass of a starThe fate of a white dwarf is destined for all stars whose mass is not much greater than the mass of the Sun

There is little chance that at the right time the telescope will be pointed to where the two stars lined up with the Earth in approximately one straight line, which is why Einstein wrote that it is unlikely that mankind will be able to observe the gravitational microlens.

But Einstein did not know that astronomers would have the Hubble telescope. An international team of astrophysicists from the UK, Canada and the US calculated where the next “parade of stars” would take place and sent Hubble to the white dwarf Stein 2051 B, located 18 light years from Earth.

In March 2014, it was supposed to pass right in front of another star. When this happened, the star, which was in the background, shifted from its usual position in the sky; having estimated this shift, astronomers calculated the mass of the white dwarf – it turned out to be equal to 2/3 of the mass of the Sun.


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