The companion is a small, red dwarf

Dec 10, 2009 08:45 GMT  ·  By

The asterism (pattern) of seven stars that is known as the Big Dipper has recently just received another companion star. It did not receive it per se, as the red dwarf has been at the location for several million years. Astronomers were only recently able to discover it, accompanying a relatively young star known as Alcor, which is twice the mass of the Sun. The latter celestial body is located in the ladle's handle, astronomers reveal, and is estimated to be about 500 million years old.

Alcor, like most of the other stars in the Big Dipper, is believed to have formed around roughly the same time, from the same cloud of matter. This is very peculiar for the simple fact that most asterisms are not made up of stars in the same group, but rather out of unrelated celestial objects. It's a big coincidence that these ones manage to form the Big Dipper, which is one of the most famous patterns of stars in the sky. The structure is formed out of the seven brightest members of the formal constellation Ursa Major, Space reports.

Viewed from perspectives on Earth, Alcor appears to be roughly the same star as another celestial body nearby, known as Mizar. In ancient times, the sight of warriors used to be tested by asking them to make out the two individual stars. In more recent times, one of Galileo's colleagues hypothesized that Mizar must be in a binary system, after seeing it with a telescope. Years later, after more thorough observations, it was determined that the two components of the binary system, Mizar A and B, were in themselves binary systems, which meant that Mizar was a quadruple system.

Centuries after these observations were made, Alcor was proven to have a companion as well, a small, red dwarf. The find was made this March, as a group of astronomers used the 200-inch Hale Telescope, located at the Palomar Observatory, in California, to look at Mizar. The modern instrument uses adaptive optics – a technique that counteracts the distortions caused by the Earth's atmosphere – to obtain high-quality images of very distant objects. The method acts by changing the shape of the mirrors on the telescope in real time.

“Right away I spotted a faint point of light next to the star. No one had reported this object before, and it was very close to Alcor, so we realized it was probably an unknown companion star,” American Museum of Natural History PhD student Neil Zimmerman says. “We went back 103 days later and found the companion had the same motion as Alcor,” AMNH Department of Astrophysics curator and Professor Ben R. Oppenheimer adds. The two stars, Alcor A and B, orbit each other every 90 years or so. Both are located some 80 light-years (24.5 parsecs) away from the Earth.

“Red dwarfs are not commonly reported around the brighter higher mass type of star that Alcor is, but we have a hunch that they are actually fairly common. This discovery shows that even the brightest and most familiar stars in the sky hold secrets we have yet to reveal.” the professor says. Details of the team's work appear in the latest issue of the Astrophysical Journal.

“We hope to use the same technique to check that other objects we find like exoplanets are truly bound [to] their host stars. In fact, we anticipate other research groups hunting for exoplanets will also use this technique to speed up the discovery process,” Zimmerman concludes.