Scientists have a problem with Polaris.
From afar, the North Star's permanent
position in the night sky has guided us
for centuries. But from up close, it
seems to behave in confusing and bizarre
ways. And the closer we look, the less
it makes sense. Polaris doesn't act as
our stellar models predict, and it is
surprisingly hard to get basic
measurements for it. How can a star be
so clearly visible to our eyes but
evasive to our scientific instruments?
Sounds pretty ironic. So, let's dive
into everything we know and don't know
about this enigmatic star.
star could reshape our map of the universe. No, really. Wait and see.
It's a moonless summer night, and you've
wandered to your favorite stargazing
spot. Looking up, you recognize Öza
Major, which contains the Big Dipper, a
constellation of seven stars in the
shape of a ladle. You follow the two
pointer stars up and up until you hit a
noticeably bright star in the sky. This
is Polaris.
It sits almost exactly above Earth's
rotational axis. So while all other
stars appear to wheel around the sky,
this one stays put. At the north pole,
Polaris appears nearly directly
overhead. As you move south, it appears
lower and lower in the sky until you
reach the southern hemisphere and it
disappears behind the horizon.
But while we've relied on Polaris to
navigate the world, it turns out that
Polaris itself is far more mysterious
and far more complicated than anyone
once believed. To us down on Earth,
Polaris appears as a solitary,
unchanging point of light. That is
unless you use a telescope. In 1779,
William Herschel did, and he discovered
Polaris had a second star, Polaris B, in
a wide orbit.
120 years later, in 1899,
astronomer William Wallace Campbell
noticed Polaris A had variable radial
velocity, suggesting it might have
another companion star, one not visible
through a telescope. In 1929, a
spectroscopic study confirmed this, and
we only got our first images of the
third Polaris star, Polaris AB, in 2006,
thanks to the Hubble telescope.
These three stars are bound together by
gravity in a triple star system. The
brightest component, Polaris AA, is what
we think of as the North Star. It is an
evolved yellow super giant 5.13 times
the mass of our sun and 46 times wider.
If you placed it in our solar system, it
would reach over halfway to Mercury.
2.8 billion km away is its close
companion, Polaris AB. Together, these
two stars form a binary system that
completes an orbit every 29.6 years. The
third star, Polaris B, circles this
inner duo every 40,000 years at a
distance of 386 billion km. That's about
24 times further than Voyager 1 is from
us after 48 years of travel.
Both Polaris B and A are yellow white
dwarf stars about 500 times fainter than
their primary star. Polaris B is 1.39
times the mass of our sun, while Polaris
AB is slightly smaller at 1.26 solar
masses.
But Polaris's triple star system is the
least interesting thing about it. Its
primary star, Polaris AA, belongs to a
rare class of stars known as seafared
variables, which as it turns out are one
of the keys to mapping the cosmos.
In 1908, an astronomer named Henrieta
Levit was working at Harvard College
Observatory studying the small Magelanic
cloud. She was looking at seafared
variable stars, stars in a specific
phase of stellar evolution which pulsate
regularly growing brighter and dimmer in
predictable cycles. She discovered that
there was a direct relationship between
how bright a seafood appears and how
long its pulsation period lasts. The
brighter the star, the longer it takes
to complete one cycle of pulsation.
This turned seafoods into standard
candles, objects whose true brightness
we can determine just by measuring their
pulsation period. Since we know how
bright they actually are, we can
calculate how far away they must be
based on how bright they appear to us.
It's like knowing the true wattage of a
light bulb. The dimmer it looks, the
further away it must be. Sephiids became
a crucial rung in what astronomers call
the cosmic distance ladder. Our method
for measuring distances throughout the
universe. Before the period luminosity
relationship breakthrough, we could only
measure objects a few hundred lighty
years away using stellar parallax, a
measurement of how far a star shifts in
the sky as visible from Earth.
But with this new discovery, suddenly
scientists could measure intergalactic
distances of millions of light years.
Now we could use nearby methods like
parallax to calibrate seafoods. Then use
sephiids to measure distances to other
galaxies, which in turn helps us
calibrate even more distant markers like
type 1A supernova.
As the closest seafood to Earth, you'd
think that would make Polaris a gold
mine for astronomical research. It could
help us calibrate the size, age, and
fate of the universe. Everything from
the distance to Andromeda to the rate of
the universe's expansion.
But here's where things get complicated.
There's still some debate about how fast
exactly our universe is expanding.
Measure it one way and the number is 73
km/s per mega parcel. But measure it
another and you get 67 km/s
per mega parcel. In an article from our
special 35% off discount for Astramm viewers.
Turns out Polaris AA is quite a weird
seafood for a number of reasons.
First off, its amplitude is erratic.
Scientists noticed the difference
between its brightest and dimst points
declined through the 1960s to 1990s,
down from above 0.1 to below 0.05
magnitude.
It leveled off and remained stable for a
while before starting to creep up again
in the year 2000. A pattern never seen
in any other seafood.
But it's not just his amplitude that's
raised eyebrows. For the 150 years we've
been observing it, Polaris's period of
around 4 days has been increasing by 4.5
seconds per year.
That's an incredibly fast rate of period
change for a seafood, which suggests
Polaris is evolving quickly.
One theory astronomers have for this
unusual behavior is that Polaris AA
might be entering an unstable phase of
its life. All stars go through an
unstable phase when they leave the main
sequence, but they don't necessarily all
become seafoods. What kind of pulsating
star they become depends on several
characteristics including its mass,
luminosity and pulsation period among
other things. Most seafoods go through
three big instability events in their
lives. The first happens very fast
lasting from 10 to 100,000 years. It
occurs when hydrogen fusion in the
center has stopped and the star leaves
the main sequence moving towards
becoming a red giant or super giant.
It's the least studied of the
instability phases because it's so brief
relatively speaking. The second
instability occurs when the star begins
burning helium in its core, stabilizing
the star and making it burn hotter and
bluer in color. This transition can take
anywhere from 100,000 to 10 million
years. When the star finally runs out of
helium, expands again and becomes
unstable for the third time with a
similar timeline as the second phase.
Nearly all the seafoods we've seen in
these unstable phases have been in the
second or third phase, making their
behavior relatively predictable. But
catching a seafood on their first
instability is extremely rare and their
behavior in this state is much less
understood. There isn't a consensus on
which instability stage Polaris is in.
Some claim it's in the first while
others say it must be in the third. And
just as these theories emerged to
explain the changing period, Polaris
suddenly reversed the trend. Since 2010,
its period has been getting shorter year
after year, and no one knows why. We
think it might be due to Polaris AB
disturbing it whenever they pass each
other at their closest point, but no one
knows for sure. But even more
fundamental than that, there's still a
key piece of the puzzle missing to fully
understanding Polaris' properties.
something embarrassingly simple that's
proven brutally challenging to solve.
[Music]
You might expect that with modern
technology, measuring the distance to
our north star would be straightforward,
but it's actually trickier than you
might imagine for a few reasons.
Firstly, Polaris is hundreds of light
years away. The further a star is, the
smaller its parallax shift is. For
context, the Hypocus mission from the
'90s measured Polaris' parallax as
0.00754
arcsec or 2.094
* 10 -6°.
An unimaginably small movement. On top
of that, Polaris is so bright it can
saturate our detectors, making our
readings unreliable.
Couple that with the fact it is a
pulsating seafared star with a close
companion, and brightness-based distance
readings get even more complicated.
All this makes Polaris' distance from
Earth surprisingly hard to pin down.
Estimates range from 323 to 520 lighty
years away and there's still no
universal agreement in the scientific
community. For example, Hippacos mission
placed Polaris at 434 lighty years while
the more recent Gaia data from ISSa puts
it at 447 lighty years away. Without a
clear idea of how far Polaris is,
estimating other properties like mass,
radius, and luminosity is complicated.
The mass and radius estimates I shared
earlier are based on the Gaia data. But
if we were to use different inputs, we'd
get pretty different predictions.
To make matters worse, the estimates we
do have of Plarus' age, mass, and
pulsation period don't seem to match our
stellar evolution models whatsoever. And
again, we don't really know why. For
example, Polaris AA appears to be much
less massive than we expected and much
younger than its companion, Polaris B.
We'll come back to this age discrepancy
in a moment.
One possible explanation is the
limitations of the stellar evolution
models themselves. Firstly, all stars
rotate. This rotation can have profound
effects on stars structure, evolution,
and pulsation behavior. However,
modeling rotation is incredibly complex
and involves many unknowns. So, many
researchers don't include this crucial
factor in their stellar evolution
models. For a star like Polaris AA, this
emission could be leading to significant
errors in our evolutionary predictions.
Another possibility is that Polaris AA
has lost mass during its evolution,
perhaps through solar winds or
interactions with its companions. Mass
loss would explain the stars rapid
period change and could account for some
of the discrepancies with evolutionary
models. Researchers have tried to
estimate mass loss with some studies
placing it at 2 septillion kg per year.
That's a two with 24 zeros behind it.
But so far there hasn't been much
convincing evidence to support this
theory. And finally, 2024 observations
using the Charara array detected giant
star spots on Polaris AA surface. These
spots are consistent with the presence
of a magnetic field which might also
affect the stars pulsations.
If we're missing crucial physics in our
understanding of how these stars evolve
and behave, it could affect the accuracy
of seafoods as standard candles. And if
our model of seafoods is wrong, so is
our map of the universe.
All right, Polaris is mysteriously
pulsing faster and faster. We're not
sure how far away it is, and it's making
us rethink our stellar evolution models.
As if that wasn't enough, Polaris has
another ace up its sleeve. Remember that
age discrepancy?
If Polaris A a A and Polaris B are
gravitationally bound as they seem to
be, they most likely form together. This
means we'd expect them to be the same
age. However, our observations indicate
the opposite.
Assuming a distance of 447 lighty years,
Polaris AA appears to be about 50
million years old. But estimates put
Polaris B at 2 billion years old. So
what happened here? A couple of theories
have attempted to untangle this knot.
One is simply that Polaris B is not a
companion of Polaris A after all,
despite strong evidence to the contrary.
But another more enticing idea hinges on
a phenomenon we've known about since the
1950s.
Globular clusters are dense environments
of stars that all formed around the same
time. And yet in 1953, Alan Sandage
noticed some stars appeared much, much
younger than others, bluer, hotter, and
more massive. He named them blue
stragglers. We think they form as the
result of star collisions when binary
stars merge. The result is a star that
appears much younger than its true age.
It's possible that this is what happened
to Polaris AA. It could have been
rejuvenated by a collision with another
star originally in close proximity to
it. While this merger hypothesis is
speculative, it demonstrates how the
Polaris system challenges our
understanding of stellar evolution and
binary star formation. At the end of the
day, this age gap remains one of the big
enigmas surrounding the Polaris system.
While the Polaris star is our north star
today, it won't stay that way forever.
Earth's rotational axis wobbles much
like a spinning top. As a result, our
celestial pole wanders in a closed
circle, sweeping past different stars.
About 14,000 years ago, the celestial
pole pointed towards the bright star
Vega, and it will once again return to
that position in about 12,000 years. For
millennia, Polaris has guided humanity
across oceans and deserts. But today, it
guides us in a different way, forcing us
to confront what we still don't know
about the stars, the universe, and even
our own scientific limits. And as the
next round of Gaia data approaches, set
to release in mid 2026,
we may finally get some more conclusive
insight into this perplexing star
system.