My first Magazine Sky & Telescope - 03.2019 | Page 18

Famous Stars, Part I
Polaris B
spotted Ab because it pairs with Polaris to
create a spectroscopic binary, a stellar duo
detectable by the wavelength shift in the
primary star’s light that’s created as the
two stars move around each other.
Polaris Ab follows a very elongated,
30-year orbit around Polaris. (In fact, it’s
Polaris Ab
making its closest pass this year.) Astrono-
the pole. It has continued closing the gap
mers weren’t able to see this third star
over the subsequent centuries, but its
directly until 2006, when Nancy Evans
approach is nearly done: Polaris will reach
(Harvard-Smithsonian Center for Astro-
its closest point to the pole around the
physics) and colleagues nabbed it with the
year 2100, when it’ll be less than half a
Hubble Space Telescope (S&T: Apr. 2006,
degree away — closer than the breadth of
p. 17). At the time, the researchers used
the full Moon.
the close companion’s motion to pin
Polaris A
Polaris’s mass at the equivalent of 4½
A Merry Band of Fellows
Suns, give or take a Sun. They recently
The ponderous precession of Earth’s
tried to refine the mass measurement, but
axis isn’t the only reason Polaris moves
with Ab so close to Polaris, even Hubble is hitting its limits.
through the sky. Polaris is the brightest member of a triple-
In order to improve the estimate, the team is now using Geor-
star system. The famed William Herschel first spotted the
gia State University’s CHARA telescope array to watch Ab fly
more distant of its companions, Polaris B, in 1780. The yel-
through its closest approach.
low, 8th-magnitude star lies 18 arcseconds from Polaris A, an
Astronomers think that all three stars in the system were
easy split for small scopes. A and B take more than 40,000
likely born together about 70 million years ago — a blink
years to circle each other on the celestial floor.
compared to the 4.6 billion years the Sun has already lived
But Polaris proved more gregarious than astronomers
(S&T: Oct. 2017, p. 22). Massive stars live fast and die young,
thought. In 1929, astronomer Joseph Moore discovered a
and Polaris is already a puffy, aging star that has devoured
second, much closer companion using some three decades’
the supply of hydrogen fuel in its core. Polaris B and Ab are
worth of Lick Observatory data. This star, Polaris Ab, is also a
lightweights in comparison, each maybe 1½ Suns or less.
yellow star and about half as bright as Polaris B. Yet because
They’re both still fusing their core hydrogen and will keep at
Polaris itself is more than 700 times brighter than Ab, the
it for another 5 billion years.
third star easily hid in its companion’s glow. Moore only
However, it’s possible that Polaris A and B are not the
same age. Based on two different distance calculations,
WHY “CEPHEID”?
Evans and collaborators recently estimated the stars’ intrin-
Cepheids take their name from Delta Cephei,
sic brightnesses and combined those with the stars’ surface
the first of their kind to be discovered. English
temperatures to deduce how old each star is. Such work is
astronomer John Goodricke identified Delta Cep’s
possible thanks to the Rosetta stone of stellar astronomy,
periodic pulsations in 1794.
the Hertzsprung-Russell diagram. Astronomers use the H-R
diagram to make sense of the breathtaking diversity of stars.
He
Cepheid Light Curve
lium
The chart plots stars according to how hot and luminous they
da
are. Where a given star falls on the H-R diagram depends on
m
Co
re
ntr
-
where it is in its life cycle, and a star will move across the
fo
ac
rm
tin
s
g
plot — sometimes even back and forth multiple times — as it
St
evolves. If you can peg a star’s location on the H-R diagram,
ar
co
then you know its approximate mass, size, and age.
nt
rac
It turns out that B appears oddly bright for a 70-million-
ts
year-old sun. It could be that the star is actually a binary,
but one tantalizing possibility is that it’s actually more like
2 billion years old. If so, then Polaris itself might have reset its
Time
evolutionary clock by merging with a now-gone star. Which
p CEPHEIDS Due to an unstable helium layer deep within, Cepheids
of the various scenarios is correct remains unclear.
contract and expand in a regular pattern. As the star contracts, the
helium layer compresses, until eventually the heat and pressure beneath
it shove the layer outward in a rebound. Once the heat escapes through
the now-tenuous layer, the star contracts again and the helium barrier
forms anew.
16
M A RCH 2 019 • SK Y & TELESCOPE
A Variable Star
At 70 million years old, Polaris falls in a special place in the
life cycle of a star: the instability strip. This strip is a diagonal
EDWA RD
u TRIPLE STAR Artist’s illustration of the three
stars of the Polaris system. Just above Polaris
is a smaller companion, Polaris Ab, which is 3
billion km (2 billion mi) from Polaris. Lying much
farther away at some 390 billion km (240 billion
mi) is the wide companion Polaris B.