The Gaia Space Telescope helps astronomers image hidden objects around bright stars

Scientists have directly imaged eight dim objects in Gaia’s data catalog that accompany very bright stars, including so-called “failed stars,” also known as brown dwarfs.

The stars and their companions were originally identified from millions of stars in the Gaia catalog. They were considered ideal for follow-up research with the ground-based GRAVITY instrument, an advanced near-infrared interferometer at the Very Large Telescope (VLT) at the summit of Cerro Paranal in Chile. By combining infrared light from multiple telescopes, a process called interferometry, GRAVITY has already achieved the first direct observation of an extrasolar planet, or ‘exoplanet’.

Following Gaia observations, GRAVITY directly observed light signals from companions around the eight bright stars, seven of which were theoretical objects previously undiscovered.

Three of the companion objects are small and faint stars, and the other five are brown dwarfs. The latter form as stars and have more mass than gas giant planets, but do not have enough mass to initiate the fusion of hydrogen into helium in their cores, as main sequence stars do. That’s where their nickname ‘failed stars’ comes from.

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One of the brown dwarfs spotted by GRAVITY orbits its parent star at a distance approximately equal to the distance between Earth and the Sun. This is the first time that one of these failed stars has been directly observed so close to its host star.

“We have shown that it is possible to image a faint companion even if it orbits very close to its bright host,” says team leader and European Southern Observatory (ESO) scientist Thomas Winterhalder , in a statement. “This achievement highlights the remarkable synergy between Gaia and GRAVITY. Only Gaia can identify such tight systems that host a star and a ‘hidden’ companion, and then GRAVITY can take over to image the smaller and fainter object with unprecedented accuracy. “

A firefly on a lighthouse

Directly observing faint objects such as small, faint stars or brown dwarfs around bright stars is no easy task. In fact, noticing their light signals is like seeing the light of a firefly perched on a shining lighthouse. Understandably, any attempt to image the firefly’s light is blurred by the brighter light of the lighthouse, and the same goes for bright stars and their faint companions.

Although Gaia cannot directly observe these stars’ faint companions, the space telescope was able to infer their presence. This is because when a brown dwarf, or a small star in general, orbits a larger, brighter star, gravity pulls on the parent star and this causes a “wobble” in the motion of the larger, brighter star.

As that Earth star (and Gaia) “wobbles,” the wavelength of light stretches, shifting it toward the red end of the electromagnetic spectrum. Conversely, when it wobbles toward Earth, the wavelengths of the light are shortened, causing the light to shift toward the blue end of the electromagnetic spectrum.

This redshift and blueshift effect is analogous to the Doppler shift, the phenomenon that affects sound waves on Earth. For example, if an ambulance rushes towards you with its siren blaring, the sound waves are compressed and the siren sounds higher-pitched, similar to blue shift. As the ambulance passes you, the wavelengths of sound stretch and the siren is low, as is the redshift of the star’s light as it moves away.

An illustration of the Doppler effect.  As the ambulance moves away from the pedestrian, the sound is stretched out and has a low frequency.  As it approaches, the sound waves are compressed and the siren is high frequency

An illustration of the Doppler effect. As the ambulance moves away from the pedestrian, the sound is stretched out and has a low frequency. As it approaches, the sound waves are compressed and the siren is high frequency

This redshift and blueshift effect is small, but Gaia is sensitive enough to notice it. The small companions of these stars in the Gaia sample are within close distances of their bright stellar parents of a few tens of milliarcseconds, which is about the size of a quarter when viewed from a distance of about 100 kilometers away.

“In our observations, Gaia data acts as a kind of signpost,” Thomas explains. ‘The part of the sky we can see with GRAVITY is very small, so we need to know where to look. Gaia’s unparalleled precision measurements of star movements and positions are essential to pointing our instrument in the right direction in the sky. “

The collaboration between Gaia and GRAVITY helped the team go beyond just detecting these companions. The two data sets also allowed the team to distinguish the masses of the stars and the masses of their companions. Furthermore, by measuring the differences in wavelengths of light from the stars and their companion bodies, and combining this information with the aforementioned mass estimates, the team was able to determine the ages of the companions.

This showed that the brown dwarfs were less luminous than expected at the observed ages and masses, implying that these bodies themselves could be brought into orbit by another smaller and even fainter companion, perhaps even elusive exomoons.


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The power of the Gaia-GRAVITY tag team means that scientists will soon be able to use these two instruments to image smaller companions around bright stars, namely exoplanets.

“The ability to unravel the tiny movements of nearby pairs in the sky is unique to the Gaia mission. The next catalogue, to be made available as part of the fourth data release (DR4), will provide an even richer collection of stars with potentially smaller companions,” said Gaia scientist Johannes Sahlmann of the European Space Agency (ESA). “This result is groundbreaking in the hunt for planets in our Milky Way and promises us a glimpse of new distant worlds.”

The team’s research was published June 10 in the journal Astronomy and Astrophysics.

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