'Failed star' delights astronomers

Astronomers have caught a remarkable image of a brown dwarf circling a nearby star. A brown dwarf is too big to be a planet but too small to be a star and although a great many have been detected before, this is the first time one has been directly imaged so close to its companion. It was done using the relatively new technique of adaptive optics, which allows astronomers to get a much clearer view through the Earth's turbulent and distorting atmosphere. The discovery raises questions about how brown dwarfs and planets are formed. Because brown dwarfs are intermediate objects between planets and stars, they are often described as "failed stars"; they are more massive than Jupiter, the largest planet in our Solar System, but fall short of the minimum mass needed to sustain nuclear fusion. This is estimated to be 8% of the Sun's mass. The brown dwarf is orbiting a star called 15 Sagittae (Sge), which is about one to three billion years old, making it slightly younger than our Sun. It is located approximately 58 light-years from Earth. It is separated from its parent star by less than the distance that separates the planet Uranus from the Sun. This makes it the smallest-separation brown-dwarf companion yet seen directly. The research team behind the discovery estimate the mass of the brown dwarf to be 55 to 78 times the mass of planet Jupiter.