My first Magazine Sky & Telescope - 04.2019 | Página 38

Famous Stars, Part II
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A PR I L 2 019 • SK Y & TELESCOPE
each other every 80 years, with a close approach of 11.3 astro-
nomical units and a maximum separation of 35.7 a.u. For
comparison, if A were our Sun, B’s orbit would swing from
3
just beyond Saturn to a bit farther than Neptune.
Being so close to Earth, the pair can teach astronomers
more in less observing time than other stars can. “It makes
1
observing programs a lot easier to get approved,” says Tom
Ayres (University of Colorado, Boulder), who has studied
Alpha Centauri since the 1970s. In 2005 Ayres started using
0.3
NASA’s Chandra X-ray Observatory to monitor Alpha Cen-
tauri while trying to explain a sudden drop of X-ray emission
from A detected by the European Space Agency’s space tele-
0.1
scope XMM-Newton. “It just disappeared from view,” Ayres
1995
2000
2005
2010
2015
Year
says. “It was completely unprecedented.”
Sun-like stars produce X-rays in their coronas, their
p CYCLIC BEHAVIOR X-ray observations of Alpha Cen A and B show
outermost atmospheres, where magnetic fi elds interact with
that B has a regular, roughly 8-year activity cycle (green), similar to the
material superheated to millions of degrees. Looking at these
Sun’s 11-year cycle (gray). The star A (yellow) might have a 19-year
X-rays, astronomers can probe the inner workings of stars,
cycle, or the drop in the late 2000s could have been an anomaly similar
revealing how they evolve with age.
to the Sun’s Maunder Minimum between 1645 and 1715.
Chandra obtained a complete spectrum of both Alpha
Centauri A and B in 2007, which looked strikingly differ-
ent than data captured in 1999. A’s coronal temperature
We now know that Alpha Centauri is actually a triple
had dropped from more than 2 million degrees, typical of
system (probably), and that at least one exoplanet orbits its
Sun-like stars, to under 1 million degrees, only emitting in
smallest stellar member. Only about 4.3 light-years away
from us, the triplet’s proximity — and the possibility of study- the less energetic wavelengths of the X-ray spectrum. The
change indicated that the star was going through a magnetic
ing its planets — has captured the imagination of scientists,
minimum similar to the solar magnetic cycle. “This is exactly
astronomy buffs, and at least one Silicon Valley mogul who is
what happens on the Sun,” Ayres says. “At solar minimum
willing to invest part of his fortune to fund its exploration.
you just have this kind of fuzz.”
Since then, Ayres has continued to request Chandra obser-
Meet the Neighbors
vations of Alpha Centauri every six months or so. His persis-
It was Jesuit missionary Jean Richaud who discovered that
tence has revealed a “beautiful eight-year
Alpha Centauri is a binary. In 1689 he was
cycle” for B’s magnetic activity, a bit shorter
observing a comet from Pondicherry in
than the solar 11-year cycle. B typically
India when he realized that the star could
NOT JUST A
emits more energetic X-ray radiation than
be split into two using a small telescope.
CENTAUR
the Sun, too, which might make things less
“The two stars . . . seemed to be practically
For the aboriginal
hospitable for life on nearby planets.
touching each other,” he wrote.
Maoris of New
Things aren’t as clear-cut for Alpha
The binary comprises two Sun-like stars
Zealand, Alpha
Centauri A. “From 2005 to 2010 the X-rays
orbiting each other in close quarters, Alpha
Centauri pairs with
were very fl at, and we don’t know whether
Centauri A and B, which together are the
Beta Centauri to
that’s just a natural part of a long cycle or
brightest star in the southern constella-
make the anchor
if that is a sort of a mini–Maunder Mini-
tion of Centaurus. The pair is particularly
line connecting the
mum,” says Ayres, referring to a 70-year
interesting for astrophysicists, since both
great canoe of the
period of extremely low magnetic activity
stars are similar to our Sun. Alpha Centauri
Milky Way and its
on the Sun when very few sunspots were
A is 10% more massive and 50% brighter
bright stars to its
observed (S&T: Jan. 2018, p. 18). “We don’t
than the Sun and of a similar spectral
anchor, the Southern
have enough observations of the Sun or of
type, which means it’s a yellow star with
Cross. The Inca
other stars to have really any experience in
a surface temperature of nearly 6000K. B
saw Alpha and Beta
these things,” he adds.
is smaller and dimmer, an orange K dwarf
Cen as the eyes of
that’s 10% less massive and half as bright
the nursing mother
as our star. In a way, they’re like alternate
A Triple System?
llama, which with its
scenarios of the Sun, if it had just had
More than 200 years after Richaud split
baby composes the
slightly different properties.
AB in his scope, Robert Innes, studying
most important Inca
Both stars are older than the Sun, with
photographic plates at the Union Observa-
constellation.
an age of roughly 6 billion years. They circle
tory in South Africa in 1915, found that a