Astronomy Now Home
Home Magazine Sky Chart Resources Store

On Sale Now!



The April 2014 issue of Astronomy Now is on sale! Order direct from our store (free 1st class post & to UK addresses). The Astronomy Now iPad/iPhone editions are now available worldwide in the iTunes store.



Top Stories



Earthshine used to test life detection method
...By imagining the Earth as an exoplanet, scientists observing our planet's reflected light on the Moon with ESO's Very Large Telescope have demonstrated a way to detect life on other worlds...
  READ MORE

Solid buckyballs discovered in space
...Astronomers using NASA’s Spitzer Space Telescope have detected a particular type of molecule, given the nickname “buckyball”, in a solid form for the first time...
  READ MORE

Steamy water-world gets the Hubble treatment
...Hubble Space Telescope observations of a 7 Earth-mass planet find an unusual water-rich world swathed in a thick, steamy atmosphere...
  READ MORE








A new star in the Plough
KEITH COOPER
ASTRONOMY NOW
Posted: December 10, 2009


Bookmark and Share

Everyone familiar with the night sky knows of Mizar and Alcor, two stars in the handle of the Plough in Ursa Major that appear very close to one another. What nobody realised until now was that Alcor, the fainter of the two, is itself a binary system, which has been discovered by astronomers using the Hale Telescope at Palomar Observatory.

The very faint Alcor B red dwarf, with he main star Alcor A hidden behind the coronagraph. Image: Project 1640/AMNH/Digital Universe Atlas.

The astronomers, calling themselves Project 1640 and hailing from the American Museum of Natural History, the University of Cambridge and NASA’s Jet Propulsion Laboratory, spotted the tiny companion to Alcor – a cool red dwarf star a quarter of the mass of the Sun – using the 5.1 metre Hale Telescope’s adaptive optics and coronagraph, which blots out the glare of the primary star in the Alcor system.

“Right away I spotted a faint point of light next to the star,” says Neil Zimmerman, a graduate student at Columbia University who is doing his PhD dissertation at the Museum of Natural History. “No one had reported this object before, and it was very close to Alcor, so we realised it was probably an unknown companion star.”

The familiar shape of the Plough in the sky. Image: Nik Szymanek.

To prove this, they took advantage of a technique that had never been used in this way before: parallactic motion. As the Earth orbits the Sun, the position of nearby stars appears to shift slightly relative to more distant background stars; it’s like when you hold your finger up close before you, and you shut one eye, then switch eyes, and your finger appears to move relative to the background because of the angle between it and your eyes. Called parallax, this is usually used for measuring distances to stars (Alcor and its companion are 80 light years away), but Zimmerman and his colleagues realised that if this new star followed Alcor precisely in its parallactic motion, it would confirm that they are gravitationally bound. When they came back to check 103 days later, they found that they were moving together. This is a much faster method of discovering if two objects are orbiting one another, rather than waiting to watch them orbit (the red dwarf orbits Alcor A once every 90 years).

The Alcor and Mizar system is a fascinating one. Mizar itself can be split into two by a telescope, and each component of Mizar is also spectroscopic binary (meaning that the individual stars are too close to be separated, but their existence becomes apparent in shifts in the stars’ spectra), making four stars. Plus, there is some debate as to whether Alcor (and now its red dwarf companion) are orbiting the Mizar quartet. However, they are all moving through space in the same direction, having all been born together about 500 million years ago along with the other stars in the Plough.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
 GET YOUR COPY

Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
 GET YOUR COPY

3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
 GET YOUR COPY


HOME | NEWS ARCHIVE | MAGAZINE | SOLAR SYSTEM | SKY CHART | RESOURCES | STORE | SPACEFLIGHT NOW

© 2014 Pole Star Publications Ltd.