It has been said that scientific progress is accompanied not by cries of “Eureka”,
but instead by murmurs of “huh, that’s weird”. Well, we’ve just observed a faint
point of light on the sky whose weirdness could change the way we think about the universe
on the largest scales.
This particular “huh, that’s weird” takes the form of a white dwarf star that’s
doing some stuff that no white dwarf should ever be able to do. In fact, it has multiple
properties that are so extreme that it almost certainly did NOT form in the way that we
thought all white dwarfs formed. This one peculiar point of faint light may change our
understanding of not just white dwarfs, but of all cosmology. Mysterious, huh?
This particular mystery began at the Zwicky Transient Facility in California - a telescope
dedicated to watching for things that go bump in the night - astrophysical objects that
vary over time. Astronomers using the ZTF caught a white dwarf that, at first glance,
looked suspicious - a bit too massive and spinning a bit too fast. That spin was seen
in its rapid but periodic flickering - suggesting that it was rotating several times every minute.
It got the poetic name ZTF J1901+1458 - we’ll call it Zee for short.
In a criminal investigation, a suspect must be treated as innocent until proven guilty.
In a scientific investigation we should assume a thing to be typical until proven weird.
Well, follow-up observations proved Zee weird beyond reasonable doubt. Observations with
the Hale Telescope confirmed that it’s 1) definitely a white dwarf, and 2) definitely
spinning way too fast to make sense.
Before we get into why this is so weird, let’s review what we know about white dwarfs - or
at least, what we thought we knew. When all but the most massive stars end their lives,
they blast off their outer layers in their final fits of nuclear fusion. This exposes
their naked cores, which by now are insanely hot nuggets of nuclear ash - hyper-dense balls
of mostly carbon and oxygen. These are white dwarfs, the final fate of any star less than
8 or so times the mass of the Sun.
That’s how we thought all white dwarfs were formed. But something is off with Zee - particularly
with how fast it’s spinning. Now we do expect white dwarfs to rotate. After all, stars rotate
and so should their remnant cores. That spin should also increase as the core slowly collapses
under its own gravitational crush due to conservation of angular momentum. But typical white dwarfs
take from a few hours to a few days to rotate. But it’s hard to see how any star could
be rotating fast enough to produce a white dwarf that spins every 7 minutes.
Such a parent star should have torn itself to pieces.
And so the mystery deepens. At this point, astronomers decided that the object was weird enough to
bring some serious firepower to the investigation - the W. M. Keck Telescope in Hawaii, one
of the largest telescopes in the world. Keck was needed to do the spectroscopy - to break
the white dwarf’s light up into component colours. Splitting the light this way makes
it even harder to detect this already faint object, hence the need for a giant telescope.
But it’s worth it because spectroscopy can yield an enormous amount of information - it’s
a full forensic workup. For example, it gives us spectral lines. When electrons in an atom
move between orbitals, they emit or absorb light with very specific wavelengths. That
tells us what kind of atoms are in the object, but also a lot more. In the case of Zee the
wavelengths of the hydrogen absorption lines were shuffled all over the place in a way
that suggests the presence of gigantic magnetic fields. Fields around a billion times stronger
than the earth or sun’s magnetic field. That’s at the top tier of the most magnetic
white dwarfs.
OK, so far so weird. The next step in the star-sleuth’s playbook is to get an accurate
measure of size. Size is hard to measure even for normal stars: most are so far away that
even our highest-resolution telescope cameras see them as single points of light. But astronomers
have a clever trick. If you know how much light a star is churning out - its luminosity
- then you can figure out how big it needs to be in order to shine that brightly. You
also need to know how efficiently it’s shining - how much energy for every unit of surface
area - but that’s just a function of its temperature, and you can measure temperature
from the star’s color.
Temperature is surprisingly easy to measure, but luminosity is less so. Luminosity determines
how bright the star appears to us - but there’s another factor at play there - how far away
the star is. Measure the star’s brightness on the sky, factor in how much that brightness
has been dimmed by distance and you have its luminosity. Then luminosity plus temperature
gives the star’s size.
So the only thing we’re missing from this equation for size is the distance to this star
- and of all of these things distance is the hardest to measure. The most accurate way
to get distances to stars is with stellar parallax. That’s when the motion of the
Earth causes a star to appear to move relative to more distant stars. Until recently it's
only been possible to do this for the most nearby stars.
But the European Space Agency’s GAIA satellite changed that by measuring parallaxes
for a billion stars across the Milky Way, and Zee was one of them. So we have its distance
- around 135 lightyears away.
Combined with our measurements for temperature and brightness, we get a radius for Zee of
2140 kilometers, plus or minus a few hundred. And that is tiny, even by white dwarf standards.
For comparison, a white dwarf the mass of our sun would be around the size of the earth,
this new guy is barely 25% bigger than the moon, making it the smallest known white dwarf.
Now, one reason that it’s good to know the size of a white dwarf is that it also tells you
its mass. Here we need to learn something that’s weird about all white dwarfs, not
just Zee. We normally think about objects getting bigger the more massive they are.
That’s true of planets and regular stars, but it’s not true of white dwarfs.
In white dwarfs, matter is crushed so close together that the inward gravitational pull
is insane. The only thing holding the star up from absolute collapse is the fact that
if it got any smaller, its electrons would start to overlap - they’d have to occupy
the same energy states. But that’s forbidden by quantum mechanics - specifically, by the
Pauli exclusion principle, which tells us that particles in the fermion family, like elelectrons, can never
occupy the same quantum state.
In atoms, electrons are held in place by the coulomb force - electrostatic attraction to
the nucleus. And those electrons can occupy discrete energy levels, where the higher the
energy, the closer the electron is to escaping the atom. Electrons are bound to the white dwarf
by gravity, but they still have discrete energy levels. A forming white dwarf will collapse
until the electrons are driven down to fill all of the lowest energy states. At that point
it can’t collapse any further because there’s nowhere for the electrons to go.
What happens if you add more mass to a white dwarf? First let’s think about what
happens when you add mass to less weird space-stuff, say a planet or a star. The matter inside
is crushed closer together until there’s enough pressure to resist the extra gravity
- so the original matter contains a smaller volume. But that doesn’t entirely compensate
for the fact that extra matter is added to the surface. The result is that adding matter
usually causes an object to increase in size. But for white dwarfs it’s different - as
you add mass, the white dwarf has to actually shrink in size in order to have enough pressure
to resist the extra gravity. As a result, the more massive the white dwarf, the smaller
in size.
So if Zee is the smallest known white dwarf it must also be the most massive. Doing a
little quantum mechanics, it was found that it must weigh in at 1.32 times the Sun’s
mass. And that’s a lot, at least for a white dwarf. We’ve known for some time that the
absolute maximum mass for a white dwarf is 1.44 solar masses - the Chandrasekhar limit.
Above that and the matter gets packed so close together that one of two things happen. If
a dying star’s core exceeds the Chandrasekhar limit then it collapses into a neutron star
or a black hole. But If you already have a white dwarf and then slowly
add more mass it’ll explode as a type 1a supernova.
Zee is below this mass limit, so it has avoided destruction - so far. We’ll come back to
its final, possibly cataclysmic fate very soon. OK, let’s review the evidence. We have one
weird white dwarf - it’s extremely massive and compact - but that’s not so strange in
itself. The strange part is that it’s rotating extremely quickly and has a crazy strong magnetic
field. We just don’t see these extreme properties in the white dwarfs produced as stars die.
But there is another mechanism that could have done this - one that we know must happen
in the universe, but that we’ve never seen conclusive evidence for it. Zee could be the
result of a white dwarf collision. If two white dwarfs are orbiting each other, we expect
them to slowly spiral together because they emit gravitational radiation that saps away
their orbital energy. We’ve seen the result of this with black holes and neutron stars
when LIGO detected the gravitational waves from the last moment of those inspirals. But
it should happen with white dwarfs too.
If a pair of white dwarfs merge, two things might happen - either their mass adds
up to more than the Chandrasekhar limit and bad things happen that I’ll come back
to. Or it adds up to less and we get a bigger, much weirder white dwarf. That star would
be spinning really really fast because it doesn’t just have the angular momentum from
its spinning parent stars - it has the angular momentum from the orbits of the parent
stars. This process also explains the intense magnetic fields. Magnetic fields in stars
and planets are generated by dynamos - self-sustaining currents of charged particles. A collision
like this could well produce the sort of turbulent motion to jump start a dynamo powerful enough
to produce the observed magnetic field.
OK, so we have a possible origin story for this white dwarf. Remember I said that we
should assume typical until proven weird? Well that still applies. If this one white
dwarf formed this way, that means others probably did also. And it means that white dwarf mergers
really do happen - which actually has really broad-reaching implications. Remember I said
that there are two things that can happen when a stellar remnant exceeds the Chandrasekhar
limit of 1.44 solar masses - absolute collapse for massive stellar cores or absolute explosion
for accreting white dwarfs. So what happens to merging white dwarfs that exceed this limit?
We don’t actually know - it could go either way. But if merging white dwarfs DO explode
then it may well be that many of the type 1a supernovae that we see are NOT from accreting
white dwarfs, as originally believed.
If that's true then it may actually affect our understanding of the universe on the largest
scales. That’s because observations of type 1a supernovae were how we first discovered
the existence of dark energy, and we’ve talked about how that was done previously.
If it turns out that a significant number of those supernovae came from merging white
dwarfs rather than accreting white dwarfs then perhaps our calculations of the amount
of dark energy are wrong. And in case you haven’t been paying attention, there does
seem to be a disagreement between the supernova dark energy measurements and the measurements
from the cosmic microwave background - again, covered previously.
Don’t get me wrong, this issue with the supernovae would not make dark energy go away
- there’s too much independent evidence - but it’s probably something we want to
sort out anyway. OK, let’s get back to Zee. We know how it might have got here, but one
mystery remains: what is its fate? Will it just cool and fade to a black dwarf over trillions
of years? That’s the usual doom of a white dwarf. But Zee may get its explosive finale
after all. A typical white dwarf is pretty dense, at around a metric ton per cubic centimeter.
Zee is a 1000 times denser still. That means it can support incredibly energetic electrons
in its core - electrons so energetic they are in danger of slamming into protons, which
would turn those protons into neutrons. If that starts to happen then you get a chain
reaction of so-called electron capture which is how you turn a white dwarf into a neutron star.
Zee seems to be safe from that happening, but only barely and that may change. Over
millions of years, heavy isotopes - nuclei with more neutrons than protons, will slowly
sink, or sediment, to the core. These nuclei are more susceptible to electron capture.
So build up enough of that stuff in the center and the electron capture chain reaction may
begin - giving us another path to supernova, and a bad end for our highly suspicious little
star.
So there we have it- ZTF J1901+1458 - Zee - is a moon-sized, highly magnetized white
dwarf probably formed when two low mass white dwarfs spiralled into each other. It teeters
on the edge of explosion, and may force us to rethink how we measure our universe on
the largest scales. It’s a glimmer of weirdness that tells us that the universe isn’t quite what
we thought it was. Perhaps a powerful clue towards a better understanding of this generally
weird space time.
Our Patreon supporters make it possible to keep up the standards of this show. Without
your support and enthusiasm, we’d probably have sunk to space-themed reaction videos and quantum mechanics based pranks.
As always, thanks for everything. And today’s extra special thank you goes to Charlie, who’s supporting
us at the big bang level. Charlie, the fabric of space around us thrums with the infinitesimal
vibrations of countless white dwarfs that have merged since the beginning of time. With
the help of your support we’ve deciphered their song, come to us from a universe away.
It turns out that they sing your praises, Charlie, and extend their white dwarf blessings: may your magnetic
fields stay untangled, your electrons be ever degenerate, and may your mass remain
always sub-chandrasekhar.
Okay, last time we took a magnetic tour of the universe, exploring how magnetism shapes our cosmos
from the scale of planets up to entire galaxies. That magnetic episode .. attracted many questions.
Peter Tiuller asks whether the force that stops him when he runs into a wall is ultimately
electromagnetic. Actually it’s not only that. Electromagnetism is responsible for the strong
bonds between atoms in a solid - so the fact that the wall doesn’t fall apart is due
to electromagnetism. But the reason you can’t just pass through the wall is something else
- it’s the Pauli Exclusion principle, which says that electrons in atoms can’t be shoved
into each other to occupy the same energy levels. Now we’ll actually be coming back to
this when we get into the spin statistic theorem.
Dakronu asks whether magnetic fields have any measurable effect on the orbits of stars
around the galaxy. Not directly. The galactic magnetic field is very weak compared to the
magnetic fields of stars, or even planets. Stars don’t respond to that field directly. However gas does respond
to the galactic magnetic field - it can definitely move gas around and even trigger gas to collapse into stars.
So the locations that stars formed may be influenced by magnetic fields, which in turn affects
their orbits. So the answer is yes, sort of.
Alexander Charlton and Brandon Munshaw ask whether magnetic field lines are really
“lines” versus some sort of continuous thing. Well it's definitely the latter - the field lines
are just the way we represent an underlying continuous field. Like Brandon’s example
of isotherms on a weather map or elevation contours. Field lines are continuous, and
the exact choice of where to draw them is somewhat arbitrary. Unlike isotherms and elevation
contours, they don’t represent lines of constant field. Rather they represent the
lines of steepest gradient in that field. The lines of constant field would be drawn
to intersect the field lines at 90 degrees.
A number of you ask questions about the potential role of magnetic fields in the universe that all have the
answer I just gave - can magnetic fields be used to explain dark matter or to explain dark energy? Or could magnetic
fields be used to power spacecraft? So the answer is no, although magnetic fields
do have important effects on scales from stars to galaxies - they’re still much, much weaker
than gravity. Some people also hopped on to imply that with this episode I’d validated
the electric universe idea - which states that electromagnetism drives the universe
on the largest scales, and gravity is either negligible or doesn’t exist. … Let me
be clear - the electric universe pseudo-theory is about as valid as flat earthism - it’s
just dressed up a bit nicer. But I don’t want to get all negative at the end of the
video - so instead I”ll direct you to the Professor Dave Explains YouTube channel for a satisfyingly
scathing debunking, along with a brutal analysis of the psychology of pseudoscience conspiracy
theories. I will just say this: if electromagnetism really does govern everything, why wasn’t
Magneto the most powerful of all mutants? He should have been able to control all gravity
- not just metal stuff. Personally, that’s enough evidence for me - but if you need more check out Professor
Dave’s episode linked below.