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Like finding a firefly in a spotlight

Photo: IP

The star "15 in the constellation Arrow" and the brown dwarf surrounding it (the small dot). The brown dwarf, seen here thanks to computer processing, was detected using telescopes with mirrors that constantly change its shape

Astronomers managed to photograph a brown dwarf - a massive, planet-like celestial object - moving in a relatively close orbit around a star far from Earth. Michael Liu, an astronomer at the University of Hawaii, said the distance between the brown dwarf and its host star is slightly less than the distance between the Sun and the planet Uranus. This is the smallest distance ever observed between a brown dwarf and its host star, Liu said.

Using new technology, which sharpens the images obtained from ground-based telescopes, the astronomers found that the brown dwarf moves in an orbit at a distance of about two billion kilometers from the star known as "15 in the constellation Arrow", which is about 58 light years from Earth. The distance between Uranus and the Sun is about 2.9 billion km. A light year is the distance that light travels in a year - about 9.5 trillion km.

"This discovery implies that the distance between brown dwarfs and the sun-like stars they orbit is similar to the distance between the sun and the distant planets in our solar system," Liu said. He announced his discovery on Monday at a meeting of the American Astronomical Society.

A brown dwarf, sometimes called a "failed star", is larger than a planet but smaller than a star. Brown dwarfs are balls of gas that have not managed to accumulate enough mass to shine like a star - a celestial object needs at least 8% of the mass of our Sun in order for an internal pressure to be created with a strength that will wake up the thermonuclear reactions that cause stars to shine.

Liu said the mass of the discovered brown dwarf is 55 to 78 times that of Jupiter, the largest planet in our solar system. The mass of Jupiter is 318 times greater than that of Earth.

Finding a brown dwarf in the close orbit around a Sun-like star is a rare event. The brown dwarfs are difficult to see due to the strong light of the central star. The astronomers compare the operation to finding a firefly in an area illuminated by a spotlight. In the past, ground-based telescopes could not make such observations also because the light coming from distant stars is distorted by the Earth's atmosphere.

According to Liu, the brown dwarf in satellite orbit was detected using telescopes using a new technique called "adaptive optics". The system, installed on two telescopes in Hawaii, uses a flexible mirror that changes its shape constantly to compensate for the distortions caused by the atmosphere.

"Only by using 'adaptive optics' to produce extremely sharp images were we able to find this brown dwarf," Liu said. "It is too pale and too close to its main star to be detected in any other way."

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