Page 45 - All About Space 68 - 2017 UK

Sun's twin

and more powerful telescopes and surveys have
The Oort cloud at the Solar
developed, they’ve searched our neighbourhood for
System's edge may contain
objects that could be identified as Nemesis. trillions of icy bodies
The most intensive surveys of our solar
neighbourhood have come from NASA’s Wide-
field Infrared Survey Explorer (WISE). This space
telescope, launched into orbit in 2009, has been used
to map the entire sky around Earth in infrared. In the
process, it has discovered thousands of new asteroids
and comets, and even some nearby brown dwarfs –
so-called failed stars that have not been able to ignite
nuclear fusion in their cores.
Perhaps one of WISE’s most important discoveries
came in 2013. That’s when Kevin Luhman, an
astronomer at Penn State University, found the
closest star system in over a century. Together with
his colleagues, he discovered a brown dwarf pair 6.5
light years away. However, the chance of this system
being Nemesis was quickly quashed, with Luhman
saying “we can rule out that the new brown dwarf
system is such an object because it is moving across
the sky much too fast to be in orbit around the Sun."
But the discovery was important for another
reason. Brown dwarfs are, by their nature, extremely
dim. Had a star with the characteristics of Nemesis
been in orbit around our Sun, it would presumably
not be too difficult to spot, being much closer than
this particular brown dwarf system. WISE, though,
has now been running for several years in orbit, and
in that time it has found no evidence for a star to be
in the orbit of Nemesis. This dealt a pretty major blow
to the idea that a star in orbit around our Sun might
be causing mass extinctions.
Our Sun, though, may very well have had a twin.
In a paper published in the Monthly Notices of the
Royal Astronomical Society in June 2017, a team led
by Steven Stahler from the University of California, ©Mark Garlick/Science Photo Library, John Colosimo/ESO
Berkeley, suggested that all stars like our Sun formed
in pairs. This was based on the discovery that our
Sun did have a companion. Understandably, this
brought the idea of Nemesis back to the forefront.
“We are saying, yes, there probably was a Nemesis a
long time ago,” Stahler said at the time in a statement.

Protostars Hotting up Nuclear fusion
The core collapses, forming multiple protostars. As gas falls onto the protostar, it begins to heat After a few million years, the core of the protostar
They take shape from clumps of matter that have up. This rapidly increases its size until, eventually, ignites nuclear fusion, and the object becomes a
up to 50 times the mass of our Sun. it stops accreting gas and it becomes stable. fully-fledged star with a fixed mass.

45