Why haven't we seen a Milky Way Supernova in 400 years?

Daniel and Katie talk about the mysteries of supernovae, and why we haven't seen a local one since Kepler! 

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Episode Transcript

Speaker 1 (00:09):
Katie, if you want to learn more about some creature
you're planning to feature, what's a good way to get started? Well,
I use this obscure technology called Google, or I could
find one and observe it. That's it. You just read
about it or observe it. You don't like poke it
or anything. I mean, in general, I think in the

(00:30):
field of evolutionary biology, they discourage poking wild animals. Yes.
Does that mean therefore that you also, like, never smashed
two of them together at high speeds? Pretty strongly discouraged. Interesting?
I mean, in my experience, that's a pretty good way
to learn what something's made out of. I think that
someone needs to find some advocacy organization or pro bono

(00:52):
lawyers to represent the protons you guys have been smashing. Oh,
we are facing the biggest class action lawsuit in history. Hi,

(01:15):
I'm Daniel. I'm a particle physicist and a professor at
UC Irvine, and I am not prepared for a lawsuit
from ten to the thirty protons. Hi. I am Katie Golden.
I host the podcast Creature Feature, and I am boning
up on my lawyerism to try to sue the pants

(01:36):
and particles off of Daniel, how are you going to
manage all of those clients? I mean, what if they
disagree about how much money to ask for or you know,
whether to send me to prison or something. I mean,
tended the thirty clients is a lot to wrangle. I mean,
I think I'll just send out a general mailer or
do an ad on TV. That's like, if you're a particle,
and you are a loved particle were involved in a

(01:57):
smashing well, I hope that all the protons out there
are not mad at us for all the times we've
been smashing them together to try to learn about the
nature of the universe. And Welcome to the podcast Daniel
and Jorge Explain the Universe, a production of I Heart
Radio in which we do encourage people to smash stuff
together in order to learn about how the universe is

(02:20):
put together. We try to ask and answer some of
the deepest questions about the very nature of the universe,
the fundamental fabric of reality. What in the end is
the universe made out of our space and time, fundamental
our particles, the basic building blocks of the universe, Or
is it something even weirder, even deeper, even stranger than

(02:41):
our little minds can suppose, and we don't just smash
particles together. We look out into space to see other
things that Mother Nature or Grandpa Universe has smashed together
on our behalf, so that we can ask huge questions
about how those things are put together. The moral of
the story is smash stuff, learn a lot, but of course,

(03:02):
watch out for people's rights and be careful about applying
this to wild animals. My co host and friend Jorge
can't be here today. He is traveling through Spain and Portugal,
so we are delighted to have our regular co host,
Katie Golden. Katie, thanks very much for joining us. Yeah,
of course, I think it is a little bit suspicious
that Jorge is missing when we are talking about smashing

(03:24):
things together to learn more about them. But okay, and
you know, astronomers are always delighted when stuff smashes into
other stuff in outer space. They're not capable of building
a collider like we can. You know, we can smash
protons together to learn about what's inside of them, but
astronomers can't build a black hole collider for example. Yes,

(03:46):
they got dreams, they got plans, but sometimes they get
lucky anyway, and the stuff just sort of happens out
there in the universe for them to watch, and they
don't have to worry about the black holes rights or
it's legal fees or or whether he gets mad because
they're just taking notes. And I wonder, Katie, if the
same thing happens for you. I mean, sometimes I watch

(04:06):
these nature shows about like giraffe battling by slamming their
next together, or you know, big horn rams being in
themselves together. You guys also learn a lot, don't you
when animals smash themselves together. Yeah, I mean, to be fair,
I am not currently doing research, but when I was
a little undergrad, I was definitely involved in some of
these research projects. And one of them was just going

(04:29):
out and watching squirrels and taking notes, and it made
us very popular among the school body, just sitting there
being the squirrel people, watching the squirrels, writing down what
the squirrels were doing. But yes, observing wildlife, observing animals
is extremely instructive for revolutionary biologists. It can be very
difficult because, of course, when you're observing things in nature,

(04:52):
it is not a laboratory environment, so you know, eliminating
the variables that are inevitable when you have sort of
this chaotic natural environment can pose a kind of tricky
thing for research, But yeah, it is. It is a
hugely important side of evolutionary biology is just looking at
what an animal is doing and hoping it doesn't notice

(05:15):
you looking at it. All right, But I want to
clear answer here. Do squirrels smash themselves together or not?
They smash acorns into their little faces, but they don't
smash acorns against each other's no, like squirrel acorn Collision
research program, not exactly. They may smash it into the

(05:35):
ground a little bit to get that buried in there,
but they don't have a squirrel acorn hadron collider quite yet. Well,
you know, there's just a limitation to how much you
can learn by observing stuff. We were just looking at protons,
for example, we never would have understood what was inside
them and seeing this incredible dance of the corks and

(05:56):
the gluons that are all tied together to make this
crazy thing we call a proton. And so it's these
dramatic events when protons smashed together or when larger objects
smashed together. Squirrels black holes for example, a list nobody
has ever made before in the history of time that
consists only of squirrels and black holes seems pretty complete
to me. What else could there be onside a list?

(06:19):
But when these things do happen, you have an incredible
opportunity to learn something about the nature of what's inside
these objects. I mean, we know something about what's inside squirrels,
but we have deep questions about what's at the heart
of neutron stars, what's inside black holes? Hey, what's going
on even inside normal stars? Stars like our sun have

(06:39):
incredible convection zones of plasma slurping and slashing and making
magnetic fields that flip every eleven years, we still don't
even understand what's going on inside a normal star. To
be fair, if you want to know what's going inside
of a squirrel, you can't really do that just by
watching the squirrel. Uh. It involves the process that I

(07:03):
may not want to describe to people right now, but
I imagine it is quite difficult to know in a
similar way what is going on inside something like a
star when you can only look at it and not
dissect it on a table. Yeah, and even star collisions
are pretty rare, But astronomers are lucky that there's another

(07:23):
way to see inside of stars. You don't have to
smash them together. Sometimes they just blow up on their own.
They erupt and deposit all of their innerds now on
their outers, so that we can study them and see
what was inside that star, just like squirrels, just like
got to one too many acorns, right exactly, I'm stuffed.

(07:46):
And when stars do go boom, it's an incredible opportunity
to learn about the processes that are going on inside
the star, why they suddenly got imbalanced. Plus it's a
pretty dramatic light show. Now you say that, but I
have never gotten to see a star explode, and I
would really like to. So where can I sign up? Well,
one problem is that we don't even really understand why

(08:09):
it happens and when it happens. We don't have a
scientific model that can predict when a given star is
going to go supernova. So all we can do is
scan the night sky. And you know, to me, that's
fascinating because the night sky seems pretty stable. If you
like looking at the stars, you probably are looking at
the same stars that looked pretty much the same as
your parents did, and your grandparents and their ancestors did.

(08:34):
The night sky does not change very much, but when
it does, when that happens, when a supernova goes boom,
it's very dramatic. These things can be as bright as
an entire galaxy, so it's a pretty exciting event when
it does happen. But you know, you're right, it's not
something that we've been able to see very recently, and
even though supernovas are expected to be rare, something of

(08:57):
a mystery about why we haven't seen more supernovas. In fact,
it's been over four hundred years since we have seen
a supernova in our galaxy. It seems strange to me
because even if it's rare, it seems like there's so
much stuff in the galaxy that you would at you know,
you are increasing your odds. Like when they say an

(09:20):
infinite number of monkeys on an infinite number of typewriters.
It's like an infinite maybe not infinite, but many many
exploding monkeys who once in a while explode. You would
think you'd catch a monkey explode. I'm so sorry to
everybody out there who just had the mental image of
an exploding monkey put into their head. I hope that's

(09:41):
a positive addition to your day. Whatever else is going on,
which we don't condone. We do not condone it. We
simply describe it exactly scientifically. We're just observing super monkey nova's.
You know, it's not our fault. We're just you know,
doing our best to learn something from it. So that's
exactly what we're gonna be talking about today. The title
of today's AT episode is why haven't we seen a

(10:07):
Milky Way supernova in more than four hundred years? Have
we just not looked up? Like, has anyone you know,
just kind of checked it out? You mean, have we
been so self involved for the last four hundred years
that we haven't noticed incredible explosions in our sky? Right?
It's these kids with their noses and their smartphones all
the time. They just don't notice them. Well, I think

(10:29):
that the last person to see a supernova in the
Milky Way was Kepler, himself, famous man of astronomy, of course,
and I'm pretty sure that he didn't have a smartphone,
and that in most of the intervening four hundred years,
astronomers have not been distracted by their smartphones. So that's
a good idea. But I think we're gonna have to
look for other explanations for why we haven't seen any

(10:51):
supernovas from the Milky Way in our sky in four
hundred years. It's something of a cosmic mystery that we're
going to dig into today, but before we do, we
were wondering what our listeners thought about this question. People
understand why there hasn't been a supernova in our galaxy
that we've spotted in several hundred years. So thanks very

(11:12):
much to everybody who volunteered to answer random questions. They
heard this question without any chance to prepare, and we
asked them to just speak from the top of their head,
so we get a sense for what you, dear listener,
are thinking about as you listen to this podcast episode.
What you know, what you don't know? What the ideas
are out there. If you'd like to hear your voice
speculating on the podcast for a future episode, please write

(11:34):
to us. We'd love to have you on the show.
Just drop us an email to questions at Daniel and
Jorge dot com. Here's what our listeners had to say.
If I remember rightly, there are a couple hundred billion
stars in the Milky Way, and supernovas are made from
stars that last much shorter than ours. So if they

(11:55):
were all eligible, there the right kind to be a
super nova and being produced at a constant right throughout
the life of the universe, we'd be seeing about one
every two and a half years, if I've done my
math right. Since we haven't seen one in at least
four hundred years, which is about a hundred and sixty

(12:18):
times as long as expected, my guess is that only
about one in a hundred and sixty stars is the
right kind to end up as a supernova. I've actually
thought about this question a couple of times, because, like
most people, I would love to see a supernova in
broad daily. I keep asking beetle juice, but it doesn't listen.

(12:40):
So I think that in individual galaxies they are just
a rare event. Um, there's only so many stars in
a galaxy, so I think they're pretty rare on a
galaxy scale, but on a universal scale, because there's countless galaxies,
they become common events. So that's my guess. Years is

(13:00):
like a blink of an eye in the galactic time scales.
So I think we just haven't seen a supernova in
our Milky Way yet. We haven't seen a Milky Way
supernova in more than four hundred years, because supernova are
not just that frequent. Quite a bit of the Milky

(13:21):
Way galaxy is hi didn't behind various gas clouds and
the central budge, so we don't see what's happening on
the other side of the lisk. But what is the
rate of supernova occurrence? I have no idea. I think
the answer to this is that there's probably a statistical
anomally that if the Sun has been around for five

(13:44):
billion years and there's like a billion stars in our
galaxy or a couple of billion stars in the galaxy,
and that not every star turns into a supernova, you
might just seduce the statistics suggests that one might not
have during this time, that one might happen in a
reasonable period of time. But four under views is not

(14:05):
a reasonable period of time in terms of the galaxy.
This question reminds me of the question that I asked
my grandmother when she was trying to tell me about
God and angels and all that good stuff. And I
would ask her, but Grandma, why can't we see angels?

(14:28):
Why can't we see these things that you're talking about
that used to be a long time ago? And she
was telling me that the reason is that people are bad,
not the people used to be good before then, and
that's why we could see all that stuff. Now we
are bad and they won't. We cannot see them anymore.

(14:51):
They won't appear before our eyes. So with the milk,
keep with supernova. Could say, also, it's a combination of
dus distance and dumb luck that we cannot see them,
but also might be because we are bad. I love
the idea that we've just been too naughty to see

(15:12):
a supernova. This is our punishment. It's like some kind
of galactic council has like, oh, humans destroying your environment, naughty, naughty,
no supernovas for you. I know, like we don't deserve
a supernova. You know, we haven't earned it. It's for
like the good aliens, not for us. Right, we got
to clean our rooms and rainforests before we get a supernova. Yeah, exactly.

(15:38):
I like the idea that the universe is judging us.
But I think that the rest of the listener has
really put their finger on sort of the spectrum of
ideas here. You know, it's true that we haven't seen
one in four hundred years, but is that unexpected? Do
we think we should have seen one in four hundred years,
because it's true that four years feels like a long
time to us humans, but it's just a blink on

(15:59):
cosmic iron scales or processes play out over millions and
billions of years. So it's a fair question whether or
not we should have expected to see one, right, because
like if you're you know, if you live in England,
you expect to see rain all the time, but if
you live in Southern California you never see rain ever,

(16:19):
So the different environments between Southern California and England, you
will have kind of a different expectation for these phenomenon. So,
you know, when we don't see stuff in the Milky Way,
I guess I'm asking is the Milky Way like, should
it be exploding all the time? Is it more of
a southern California? What is the weather of the Milky Way? Well,

(16:42):
you know, meteorologists are famously bad at predicting supernovas. You know,
they say they're going to come on Tuesday afternoon and
then boom Wednesday afternoon. When you make your picnic, that's
when all the supernovas come. And I'm always dressed the
wrong way. What is your supernova outfit look like Katie.
It's very shiny, lots of lots of frills and shoulder pads.

(17:05):
Of course, well, I think you put your finger on
the question, and so we are going to talk about
that today. How often we expect to see supernovas in
our galaxy and then ideas for why we are not
seeing as many as we expect. But first maybe we
should talk about the basics, for example, like what is
a supernova, how does it work, what do we know
about it? And why is it even possible to see

(17:26):
one from so far away? I mean from the name,
it sounds like it's a nova, but really big and
super they are in fact the nova and nova is
a Latin word that means new, and so the name
supernova actually comes from another famous man of astronomy, Tico
bra Hey, who observed one in the fifteen hundreds, and
he wrote a book about them, whose title is in

(17:48):
Latin but includes the words nova and stella as in
new star. And the word nova was later used to
describe new things in the sky, including supernova super new
things in the sky. That's really interesting because I think
of a star exploding is kind of like the death
of a star, the violent death of a star. But yeah,

(18:09):
nova being like it's something new, some kind of new thing,
is I guess a nicer way to think about it. Yeah. Well,
often these things that are blowing up are in other galaxies,
really far away things that we couldn't see. The star
that exploded was individually way too dim for us to
see with the naked eye. But once it becomes a supernova,

(18:31):
it can outshine the entire galaxy that is in becoming
visible and bright in the sky. So you're right that
when a star goes supernova, it's at the end of
its life, so it's not really new. It's new to
us because it went from totally invisible in the sky,
one star out of billions in a distant galaxy, to
something that is now visible, so it's new in the sky.

(18:54):
That's interesting to me that it's so bright, because when
I think of something dying, I think of it's sort
of fading, you know, like when a light bulb dies,
it kind of fades, it flickers out, or when a
candle dies, it flickers out. But if it's really bright,
if after it dies it explodes and creates something really bright,
it sounds like it's releasing a huge amount of energy.

(19:16):
It seems counterintuitive to it being a dying star. That's
a good point, and it shines a light right on
what's going on in the supernova, because supernovas are not
like fires that burn gently for a long time and
then eventually just sort of flicker out and fade. They're
more like a bomb, right where the fuse is going
for a long time and then most of the energy

(19:38):
is released at the very very end. Right, it's very
dramatic sort of ending of the life of an object.
And you know, interestingly, the same thing is sort of
true of black holes. Black holes evaporate, you leave them
out in the middle of space, and they do so
by giving off hawking radiation. And the hawking radiation they
give off is brighter as the black hole gets small.

(20:00):
So as the black hole gives off radiation gets smaller
and smaller, it starts to give off more and more radiation.
So the last gasp of a black hole would actually
be very bright. You would like go off in a
bang of glory. That's I love that, that's I'm proud
of them, that's good for them. Is that how you
want to go out, Katie, You don't want to just
podcast to the end, dribbling out a few last words.

(20:22):
You want to have like one dramatic podcast at the
very end of your career. Exactly. Yeah, if I could
go the way a black hole goes, and you know,
just like shootout podcast rays everywhere all at once, that'd
be great, right, the super podcast nova. Well, in order
to understand why supernovas are so bright and so dramatic,

(20:45):
we have to think a little bit about what's going
on inside them. Now, like everything out there in the universe,
it's not something that we understand very well, but we
do have something of a reasonable cartoon description of what's
going on why supernovas are o dramatic. And remember, the
context is how a star works at all, Like why
are there stars anyway? You know? Which you have is

(21:08):
a huge blob of gas and dust in the universe
which gravity has gathered together into a compact, dense object,
and squeezing it together makes it hot. And then because
it's so hot and so dense, it triggers fusion in
the heart of the star. So you squeeze hydrogen together,
for example, and you get helium. You get those two

(21:28):
protons to overcome their positive charges and to pop together
into a new kind of thing. That helium gets fused
together into something even heavier. And the cool thing about
fusion is that it doesn't just make heavier stuff. It
also releases a lot of energy. So gravity pulls this
stuff together and then creates the conditions necessary for fusion,
which pushes back out. So the energy from fusion, the particles,

(21:52):
the photons, pushes back out on the star. And that's
why the star can burn for billions of years. That's
why you get like a stable, say, situation, like why
do stars even happen? Because there's this incredible balance between
gravity pushing in on the star, trying to make it
into a black hole, and fusion pushing out on the star,
basically a bomb blowing the star up. And these two

(22:15):
things can stay in balance for billions of years, depending
on how massive the star is. But one thing I
know about stars, since we have one, and it's called Mr.
Sun or Mrs Son, it gives off heat. So this
to me seems to indicate that, like, you know, this
is not like a self contained system. If I can
feel the Sun, feel the warmth from it, and it

(22:37):
doesn't that mean it is losing energy at some kind
of rate, Like, is it in the same way as
like a fire burns fuel, is it running out of
fuel as it burns? The star definitely is giving off energy.
It's not self contained, so it is using up fuel,
and the fuel four fusion are light elements, so mostly hydrogen.

(22:58):
Hydrogen is the most plentiful thing in the universe. Most
of the universe that's made out of baryonic matter like
protons and neutrons and our kind of stuff is hydrogen.
So there's no shortage of hydrogen around. And most of
the life of a star is burning that hydrogen and
turning it into something heavier helium, and then if it's
big enough and heavy enough, it can create the conditions
to burn that helium into something heavier, and if it's

(23:20):
then hot enough to burn the results of that fusion,
it can just keep going heavier and heavier and heavier
until it gets up about too iron. In each case, though,
You're right, it's using up some of the energy stored
in those light elements to turn into heavier elements and
give up some of that energy, right, So, like where
does that energy come from? It comes from the original
energy of that hydrogen. You know, you have that hydrogen,

(23:43):
it's floating around in the universe. You now squeezed it
down to a very compact object a star. It's buzzing around,
it's very high temperature. Now you've captured those protons into
helium and in doing so, they give up some of
that energy, which then gets radiated away into space to
make you have a nice summer southern California day. I
don't know about nice, but yes, uh, given how hot

(24:06):
it has been, thanks Son. So I guess like I'm curious,
like when a star dies, is it running out of
fuel or is it like collapse and getting too heavy,
because it seems like that one of those things would
quote unquote kill the star right exactly. It's definitely not
running out of fuel. When a star dies and go supernova,

(24:28):
there's still an enormous amount of hydrogen left over, and
that's why the star doesn't die in a quiet fizzle.
It's not just like burning through all of its fuel.
But what's happening is that this balance between gravity and
fusion is getting upset. And there's really two different kinds
of supernova is that we should talk about. One of
them is called core collapse, and what happens there is

(24:49):
that the star is making this ash, this product of fusion,
making heavier and heavier elements which then settle at the
heart of the star. And if the star is not
heavy enough to create the high temperatures needed to burn
that into something even heavier than it's like inert material
at the heart of the star. So now at the
heart of the star you are no longer producing a

(25:10):
lot of energy, and so this balance between gravity and
fusion tips towards gravity, and gravity rushes in and collapses
the star. The star collapses, but it doesn't just like
suddenly go out. Collapse makes it super dense and so
super hot inside the star and quickly burns through a
huge amount of fuel. So first gravity is the upper

(25:32):
hand to cause this collapse, but then fusion surges back
and blows it outwards again, and it leaves behind a
very dense core which is gonna be a black hole
or a neutron star. So you mentioned earlier that it
is it's this push and shove kind of where gravity
is trying to push it inwards like almost to create
like a black hole, and then there's the push out.

(25:53):
And so once that outward pushing is like outweighed by
the inward pushing, the heaviness of the inward pushing. Why
doesn't it just become a black hole all the time? Right?
So gravity wants to make everything a black hole. Right.
Gravity can only do one thing, which is pulls stuff together,
and if there was only gravity in the universe, eventually
it would just make everything into a black hole. The

(26:16):
reason that things aren't a black holes that there's some
way to resist it. Like why isn't the Earth a
black hole? It's a big blob of mass. Why doesn't
gravity squeeze it down into a peanut and make it
a black hole? The answer is that the Earth is
structurally strong enough to resist gravity. Gravity actually kind of
a weakling, not really a very powerful force. It's like

(26:37):
ten to the thirty times weaker than all the other forces.
So you just need something to be able to resist it.
So fusion can resist it for a long time, but eventually,
as the star gets more and more massive at its core,
gravity is winning and fusion is losing. So what happens
when a supernova collapse? Sometimes you do get a black
hole at its heart. It depends on the mass of
the original star. Sometimes you get a black hole. Sometimes, however,

(27:00):
there's another stage, like a neutron star can be formed,
So the incredible dense core that's left over after the
supernova happens can be another kind of matter which resists
collapse into a black hole. But again, neutron stars are
not something that we understand very well. We did an
episode recently about what's inside a neutron star. It's something
that scientists are still exploring. So it's sort of like

(27:22):
a ladder of ways that you can avoid black hole
collapse if you can make the right kind of matter,
or you can sort of like hold up against the
trash compactor of gravity trying to squeeze you down into
a black hole, So it could become a black hole,
could become something like a neutron star. Do we know
like why it becomes a supernova sometimes, like what would
cause that explosion rather than it just collapsing. So supernova

(27:46):
is when it starts to collapse. It's like an implosion
and you get this shock wave that's traveling towards the
center of the star and then it hits the core
and it bounces back and you get the supersonic shock
wave that comes out and blows out all of this material,
huge amounts of light are released because you have a
lot of fusion happening all at once. You know, a
star is like a very slow burn, using a very

(28:07):
tiny fraction of its fuel every year, and the supernovit
it can use of a big fraction of its fuel
all at once, which is how it can become brighter
than the entire galaxy around it. It also spews a
lot of that material out into space. So you have
this implosion that creates very high temperatures, very briefly, a
lot of really really rapid fusion, and then it blows

(28:29):
up and sends a lot of that energy in terms
of photons and protons and just like raw star stuff
out into space. So just a hypothetical, it'd be like
a kid throwing a ball against a wall really hard,
and again hypothetically the ball like shooting back out because
you threw it against the wall really hard and hitting

(28:50):
you in the face. But that times like a billion
with a billion children and a billion balls. Yeah, and
it's not something that we can under stand because there's
a lot of really strong forces involved and it's all
of very fast physics. So you know, we can't look
at a star and say this one's gonna go supernova
in forty two point two billion years, or this one's

(29:12):
gonna go supernova tomorrow. It's the kind of thing we
just sort of like see happening, and we're like, oh, everybody,
watch that one. Watch that one, and we're trying to
understand what goes on inside it. But modeling the process
of a supernova is not something we're capable of yet.
You know, when we want to understand something in physics,
either we find like a set of equations that describe
it very simply, like F equals m A that ignores

(29:35):
a lot of the details of the particles that's going
on inside, or we do like really complicated calculations using
a computer to model all of those details and see
if we can describe the sort of big picture that
comes out. So far, we haven't found a simple set
of equations to describe supernova, and we don't have the
computing power necessary yet to like come up with a

(29:55):
basic model of the innards and understand how that describes
a supernova. So we're still really learning about how this
stuff works. So it sounds like if we want to
catch a supernova, we have to kind of stay frosty
and keep watching the sky, which I mean, we haven't
done that in like half an hour, so we should
probably take a break and check just to make sure
one hasn't happened while we've been talking, right, absolutely, all right,

(30:32):
I looked outside, bad news, not yet, and you might
wonder maybe Katie missed her supernova? Right. The amazing thing
about a supernova is that they're not just a flash
like they last for sometimes weeks or months. I mean,
there's sort of a flash on the cosmic time scale,
but for a human they can really last for quite

(30:53):
a long time. So if there's a supernova in the sky,
you could miss it today, you could miss it tomorrow.
You might even be able to ignore for a week
and then eventually look up and catch it up there
in the sky shining its photons at you. So it
seems like if we have like billions of observers, we
should catch it if it happens. And have we ever

(31:14):
caught one? So we have seen supernova and in fact,
they are so dramatic that you don't even have to
just look at Like the recent period of modern astronomy,
when we have space telescopes and incredible ground based telescopes
and all these new digital technologies to see supernova. People
have been seeing supernova in the sky for thousands of
years because sometimes they're just obvious. You know, if all

(31:37):
of a sudden there's an incredible bright new star in
the sky outshining everything else, then people are gonna see that,
and they're gonna say something to each other, and it's
gonna show up in historical records that people have done
a deep dive into the history of astronomy and found
times when locals have looked up in the sky and
seen something they didn't understand and wrote it down, and
we can now reinterpret those writings as evidence for supernova.

(32:03):
I can't imagine what I would be thinking like before.
I mean, I mean, if I saw a supernova today,
let's be honest, I would also be surprised and scared.
But back then even more so. If I didn't have
physicists like you telling me what was going on, I'd
probably think this guy was like really angry at me. Yeah,
it's hard to put yourself in the minds of these folks,

(32:24):
which is why it's really interesting to read these things,
like what does it mean for them? How do they
describe it what questions were they asking about this kind
of thing. It gives you not just a picture into
what supernovas are doing and how often they happened, but
also you know what people are doing, what they are thinking,
their relationship with the night sky itself super fun. So

(32:46):
how do we know? Because, like we we have a
history of things. We have written and drawn histories and
also oral histories. But there's a lot I think lost
in translation. And if someone described something, they're not going
to say, oh, saw a supernova last week it was
pretty cool. You know, They're going to describe it in
terms that they understand at the time. So how do

(33:07):
we know what they're describing is something like a supernova?
So we can't always know, but it depends on sort
of the quality of the notes that we're looking at.
And some of these things are a little speculative, and
in other cases people took really good notes and they
described something that we really can't otherwise explain. You know.
It's like, let's get into the details of the earliest
record we have that might be a supernova comes from

(33:29):
forty five hundred b C. This is like sixty hundred
years ago. Is a rock carving found in Kashmir in
India that people think depicts what is a hunting scene
with two very bright objects in the sky. The idea
is that it looks like a sky with two sons.
I mean, if you were out hunting in the old

(33:50):
days and saw a second son in the sky, you
might also be inspired to make a drawing of it.
So it's not exactly a hundred percent, but this might
be the least record of a supernova observation. And then
note taking now I am looking at this drawing and
it does look like two really bright suns in the

(34:10):
sky or or hear me out giant eyeball like eyeball
aliens with tentacles. Yeah, exactly. It's far from something you
would accept is like figure one in a scientific paper,
you know, But it's fun to think about what these
folks were imagining and what it was like for them,
Like was this so bright that they could see it
in the daytime? Itself? Like really a second son in

(34:33):
this guy, It's possible, you know. The most reliable ancient
recording of a supernova comes from Chinese astronomers. They noted
it in one five the appearance of a bright star
in the sky, and they observed that it took about
eight months to fade from the sky, and it sparkled
just like a star, and it didn't move like a comet.

(34:55):
And so this scene really matches what we expect from
a supernova. It's very bright, it doesn't move like a comet,
it sparkles like a star. It fades on the timescale
of weeks or months. So this very likely was a
supernova captured by those Chinese astronomers. So when they're looking
at this doesn't look like just kind of an extra
bright star or is it just astoundingly bright, like almost

(35:19):
having a little sun in the sky at night or something.
It depends exactly on where the supernova is, and so
if it's in another galaxy, remember that our galaxy is
like a hundred thousand light years across, but other galaxies
are millions of light years away. So if a supernova
happens in another galaxy, then that star is going to

(35:40):
go from invisible to visible at night. If it happens
in our galaxy, then the supernova is going to go
from like a star you can see at night to
a star you might be able to see in the daytime,
depending exactly on how close it is so yeah, this
could appear like a second son if a nearby star
goes supernova, because remember us, supernova can be as bright

(36:01):
as the entire galaxy. It can be a hundred billion
times brighter than our sun. Would we be in trouble?
Are there any stars that would cause us a little
bit of trouble here on Earth if it went supernova?
Oh yeah, if Proximus Centauri went supernova, we would be
quite literally toasted because the amount of energy and radiation
would really fry at least one half of the Earth,

(36:23):
you know, the half the Earth that was in the
direction of that star at the time. The rest of us,
having like an entire Earth shielding us from the supernova,
would probably be fine until, of course, the Earth's one
around and roasted the other half. So we're sort of
like rotisserie Earth. We just have to kind of like
all move to that side and keep moving, you know,

(36:43):
like hamsters and a hamster wheel of death. While I'll
be adding that to my long list of things that
I cannot directly change but cause me existential dreads so great.
It also sounds like a great pitch for a Netflix show.
You know, entire cities on wheels rolling around the planet
to avoid the frying radiation. Yeah, like snow piercer, but

(37:06):
except it's like to escape the heat. Supernova piercer sounds good.
And so if we look back in the historical record,
there's like five of these things observed in like the
last thousand years. There was a time in a thousand
and six when people all over the world noticed one,
from China to Japan. Astronomers in Iraq and Egypt and

(37:27):
also in Europe noticed something they called a guest star
which appeared sorry they called it a guest star. Yeah,
like we have a sudden guest somebody said, another place
for dinner, welcome or not. I do like the idea
of like just on one of these night shows, you know,
Jimmy Fallon or whoever, it's like, oh, we got a
guest star, and then it's a supernova and everyone gets

(37:48):
fried exactly. Sometimes they just blow it all up. And
then as we enter in sort of like the more
modern historical period, we have folks whose names we know
well writing about these things. So Tico Bray noted one
in fifteen seventy two in the sky and took a
lot of great data. He's sort of famous for taking
meticulous notes about what he saw. What's interesting also about

(38:10):
this one is, like we said earlier, it helps us
think about what people thought about supernova at the time
and Europe around this time. Most people thought that the
cosmos beyond the moon and the planets couldn't change. They
thought it was just like out there static. You know,
the whole universe was a bunch of stars hung out
into space. They weren't even aware of the idea of
other galaxies, right. They thought the whole universe was infinitely old,

(38:34):
with stars not moving or changing. That was the idea
at the time, just kind of painted on a big
globe or something. Yeah, so this didn't really fit well
with the idea of supernova because this is like a
change in something. So they thought that probably of something
happening in the atmosphere. They thought supernovas were like weird
bright lights created like, you know, fifty miles above the surface,

(38:56):
instead of super duper far away swamp gas. Well ali exactly.
But you know, Bray took a lot of notes, and
he realized that this thing remains stationary from night tonight,
doesn't change. It's parallax, and so it has to be
really really far away, and so he wrote a book
with all of these details, and it's that book which
gave the name nova to things that appear in the

(39:17):
night sky. But then the last one that we've ever
seen coming from our galaxy was Kepler. He saw one
in sixteen o four, And so you know, here we
have to make a distinction. We are seeing supernova from
all over the universe because they are so bright that
we can also see them from other galaxies, but the
one we saw in sixteen o four was the last
one we've ever seen from the Milky Way itself. So

(39:42):
I haven't necessarily been keeping good track of the dates,
but had we been seeing them at a rate greater
than once every four hundred years before this point or no.
So we have some estimates for how likely it is
to create supernovas from watching other galaxies and from think
about the ones we've seen in our galaxy, and so

(40:03):
we estimate that in the Milky Way we should get
about one supernova every twenty years or so. The galaxy
of about a hundred billion stars should give us a
supernova every twenty years. So in the four hundred years
since Kepler's observation, we should have seen about twenty supernova
in the Milky Way, right, and you mentioned that we
saw like potentially saw like five in the past one

(40:26):
thousand years, So that'd be one every two hundred years, right,
not one every four hundred years. That's right. But we
don't think that the historical record is complete, right. We
don't think that we have found every written record of supernovas,
and also we don't think that people have seen all
the supernovas that were out there. But something we'll dig
into in a minute is like how often we expect

(40:47):
these things to happen, and how likely we are to
see one if it does happen in our galaxy. Right,
So we have this mystery of these missing supernovas, right
that if like we should be seeing one every maybe
twenty years, or at least more frequently than every four years.
But before we waste time trying to figure it out,

(41:09):
just a case it popped up while in the past,
like you know, fifteen minutes while we were discussing, I'm
gonna go out and check again, just make sure, because
then that that would settle it. Right. I'm so impressed
with how dutiful you are as a researcher. Yeah, yeah,
I'm gonna get up on the roof and stare right
into the sun for a little while. I will be
right back. Well, I am back. I stared right into

(41:45):
the sky directly at the Sun just so I wouldn't
miss the supernova if it happened, and I gotta tell
you was not one of my best ideas. I'm glad
that you're still back and you have everything you need
to continue podcasting, because you know, we don't really need
your eyeballs for podcast. Yes, the uh. I basically just
see a big glowing orb all the time, which is great.

(42:07):
So we were talking about there is this mystery because
it seems like we should be seeing these supernovas more often.
Not only in historical records, does it seem like there
were more supernovas, you know, not maybe one every two
hundred years or so, not one every four hundred years,
but also there was a calculation done to see how

(42:27):
often should we be seeing it, and that resulted in
about twenty years, right exactly. And remember we don't understand
supernovas very well, so it's hard to predict, sort of theoretically,
how often they should happen. But we can learn by
looking at all the other galaxies to try to get
a sense for the rate of supernovas. So remember, we
can see supernovas in our galaxy even just using the

(42:49):
naked eye, going back thousands of years. But in the
more recent era, because we have telescopes, we could now
study very thoroughly supernovas in other galaxies. So, starting in
the nine or so, people began these campaigns to scan
the sky for new stars appearing in other galaxies using telescopes,
and they saw dozens and so we have definitely seen

(43:10):
supernovas recently. For example, we saw one in seven that
came from a large Magellanic cloud. This was one you
could actually see with the naked eye, even though it
wasn't in the Milky Way, because a large Magellanic cloud
is a satellite of the Milky Way, so it wasn't
that far away. So we're seeing supernovas in other galaxies,
and that lets us estimate what we think is the

(43:32):
rate of supernovas we should expect in the Milky Way,
being about one every twenty years. You know, there's still
a lot of uncertainty in that number, and so then
the question is, you know, are they not happening in
the Milky Way, or are we just not seeing them,
are they hiding it from us? Or maybe the Milky
Way is super weird and doesn't make supernovas as often

(43:54):
as these other neighboring galaxies. Right these are like just
the basic questions we have about supernove is because we're
so clueless about the thing that triggers them, the moment
that the star decides to collapse, and exactly what's going
on inside. I mean, it seems odd though, for like
an entire galaxy to have different rules from another galaxy,

(44:17):
or is that not so odd? It would be odd,
But there are odd galaxies out there. There are galaxies
that have lots of dark matter and galaxies that have
very little dark matter. They're huge galaxies and little galaxies.
There are galaxies that are still making stars and galaxies
that are quenched that will no longer be producing stars.
And we just don't know if the Milky Way is
typical or not, the same way we don't know if

(44:38):
Earth is typical or if it's weird and rare in
the universe. These are the kind of questions we get
to ask as we look out deep into the universe
to try to figure out whether our context is normal
or really really weird. But we can do more than
just look for supernovas as they happen. We can actually
also look for supernovas that have happened that we might

(44:59):
have missed because we can see supernova remnants. When a
supernova happens, it's very bright, it's very dramatic, but also
it creates this shock wave that shoots out into the universe,
leaving a sort of interesting fingerprint that you can continue
to see four hundreds or thousands of years later. So
it's kind of like looking at a star's kind of
fossil record exactly. Because when a supernova happens, remember, it

(45:23):
shoots out like several solar masses worth of stuff, right, Like,
think about what that means. A solar mass is an
entire sun's worth of stuff. And so now you have
a supernova shooting out like several times the mass of
the Sun, just in plasma and hot gas out into
the universe and incredibly high speeds, like significant fractions of

(45:45):
the speed of light. So what happens is that it's
going to smash into all the stuff around it. Space
is not totally empty, it's filled with the interstellar medium.
Which is like a very dilute gas, but when the
supernova remnants smash into it, you get this expanding shell
of a shock wave. So that's what we call a
supernova remnant. It's very distinctive. And scientists have looked out

(46:06):
into the night sky and for example, in our galaxy
in Cassiopeia, they see a remnant which looks to be
about three hundred and twenty five years old, which means
they think there was a supernova there three hundred years
ago that everybody missed. Nobody saw it, but we can
see evidence that had happened, and it happened fairly recently,

(46:27):
so it's a It's an interesting thing because like as
kind of postmortem detectives of this star explosion, you can't
necessarily look just at the sight of where the star was.
You're looking at where the remnants go, right, So it's
this expanding force. It's not just like looking at where

(46:47):
the star died. You have to look far afield of
where the star has exploded to yeah, precisely. And this
stuff moves out, but it moves out much slower than
the speed of light. You know, it's pretty fast and
pretty energetic, but it sort of in the vicinity of
where it happened. It's not like looking at black hole collisions,
and we're getting evidence of that from gravitational waves that

(47:08):
travels at the speed of light directly to us. Now
we're looking at like stuff spewing out sideways from the
supernova ramming into something else and then sending us light
from that collision. So we're seeing this like shock wave
emanate from the supernova, and we're seeing light from that
shock wave as it bounces into gas and heats it up.
So it's pretty cool. You can tell that as supernova

(47:30):
was there. It's sort of like you missed an explosion,
but now you're looking at the burn marks on the
ground or something like that. Yeah, yeah, that's really interesting. Yeah,
and people have done scans for these kind of remnants
in our galaxies and they found a few. They found
this one in Cassiopeia. That found another one that they're
pretty sure happened at the end of the nineteenth century.
But you know, there's no records that anybody saw the

(47:52):
actual supernova. So all this kind of stuff lends credence
to the idea that maybe supernova's actually are happening in
our galaxy. We're just not seeing them right that they're
out there, they're blowing up, but we are missing the party.
But wait just a minute, because you said that they're
super bright. There's a lot of energy released, and our

(48:13):
galaxy isn't you know, too far away from us. So
how could we miss that? Did we podcast over it? This?
This is what I was worried about. It's because you
were napping. You know, you took a break, a nap
in a coffee, and you missed all the important datah
man I got like fomo on a galactic scale. Now, no, no,

(48:33):
don't feel bad. It's not because we're lazy. The issue,
like everything else, is probably location. You know, the galaxy
is not that big on a universal link scale, but
it's also not that transparent. You know, the galaxy is
a lot of gas and dust in it, and if
something happens on the other side of the galaxy, then
the center of the galaxy, which is a swirling maelstrom

(48:54):
of intensity and gas and dust and it's pretty opaque,
then we might not be able to see it. And
so some of these things might just be happening behind
big dust clouds or big gas clouds, and this dust
is really good at blocking the light. And unfortunately, the
places that have the most dust are the densest places,
which are also the places that supernova are most likely

(49:17):
to happen. So this really was a punishment for us
not cleaning our rooms. I knew it it might be.
You know, this is still really speculative. Is the kind
of explanation people are trying to understand if it works.
And I read a paper last week exploring this in detail,
trying to say, like, can we explain not having seen
supernova based on where the gas and dust are in

(49:40):
the galaxy? And they did a really interesting, very thorough study.
They have a model for where they think supernova should
happen based on where are the stars in the galaxy.
From that they get a sense for how much light
should have arrived on Earth from each of these supernova,
you know, how bright it was, how far away it was.
Then we have maps actually of where or the dust

(50:00):
is in the galaxy, right like where these clouds of
choking gas and dust that would obscure our view are sitting.
And then we can do a calculation and say where
do we expect to see supernova in our galaxy? And
where do we expect not to see them because the
gas and dust are blocking them, and drum roll, do
we have those results. We do have those results. I'm

(50:21):
looking at them right now from this paper, and interestingly,
the results are kind of a surprise. Most of the
supernova that we have seen in our galaxy actually land
in places where we didn't expect to see them because
either there actually was a lot of gas and dust,
which somehow they are mysteriously overpowering, or it's a place
where you don't expect to see many supernovas. So we

(50:44):
like saw a super rare supernova. It's not something that
we understand, and so it's the kind of moment in
science where we go, well, that didn't work. You know,
we have this data, have a basic model that doesn't
explain it. What's wrong? Probably something simple is wrong with
our model, our estimate of how often the supernova happen,

(51:04):
or our guests for where the dust is or how
transparent that dust is to the light from the supernova.
Something is going wrong with these models. But the upshot
is that we can't explain it. We do not understand
why we are seeing the supernova that we are seeing
and why we are not seeing supernova in some areas
of the galaxy. So the model was saying not necessarily

(51:26):
that the supernova was in the location where it couldn't happen,
but that it was unlikely for us to be able
to see it at this location. Some of the ones
that we have seen are happening in places where they
should be very very rare. Not impossible, but you know,
you should expect to see more supernova where there are
more stars. And if you look at the historical distribution

(51:48):
of supernova that we have seen in our galaxy, there's
a lot of them in places where there aren't very
many stars, which is kind of confusing. Are supernova more
likely to happen there, or maybe there's some structure to
the galaxy that we haven't described in our model, you know,
some like clusters of gas and dust that are creating
supernovas or blocking supernovas. Something else is missing from our

(52:12):
model to add to this recipe to give us a
depiction that matches what we actually see out there in
the night sky. Yeah, that's really interesting. Like I wonder
if it could even be something like, you know, if
a star that's more likely to go supernova is more
likely to be a loner. It could be because we
know also that there's a lot of difference in sort

(52:33):
of the wrong ingredients in the galaxy. You know, in
the center of the galaxy there are more heavy metals,
so the stars there are more metallic, and so the
galactic conditions are very different in the center of the
galaxy and the spiral arms, and then above the galactic plane.
So this is the kind of next round of research
is understanding, like what are the conditions to make supernova?

(52:54):
What are the conditions in the galaxy? Can we make
a model that explains why you might be getting more
supernova or maybe just why we're not seeing them? You know,
maybe this more structure to this gas and dust than
we thought we understood. But it's a great opportunity. Every
time you have something you don't understand is a chance
to refine your models and learn something new about the universe.
Maybe it's just something about the map of the gas

(53:15):
and dust, or you know, maybe like the discoveries of
dark matter, this is not something that we can resolve
with like a little tweak to our models. Maybe it's
gonna require something really big and new in our understanding
of the universe. Right, so you gotta double and maybe
even triple. Check your mouth first, make sure someone didn't
just forget to carry a one. And then maybe it's

(53:37):
a sign of some really interesting discovery or mystery about
the universe. And now we are lucky enough to have
other ways to see supernova, not just through light. Supernova
also produce a lot of radiation in new trinos for example,
In fact, most of the energy produced in a supernova
comes from new trinos something like that. The radiation from

(54:01):
the supernova is in the form of neutrinos, which are
these weird little particles that mostly pass through matter, ignoring us.
And in the supernova, we actually saw the new trinos
from the supernova first because they're produced at the heart
of the supernova. But then they fly right out through
the craziness and get to Earth before the photons do,

(54:22):
because the photons have to make their way sort of
through the star before they get to the surface and
can get emitted. So neutrinos are a new way to
see supernova. So now we have like new kinds of
supernova eyeballs, so that even when you're nappying, Katie, we
are watching the sky for supernovas. Thank you. That does
make me feel a little more relaxed. So I guess

(54:42):
you use some kind of like neutrino glasses so that
you can see these If these are not actual visible
light particles, Yeah, you cannot see new grina's very easily.
But particle physicists have neutrino experiments deep underground to try
to understand how neutrinos changed from one kind due to another,
or to do other kinds of neutrino experiments. We recently

(55:03):
did a whole episode about how neutrinos can teach us
about supernova, so go check that out. But one of
my favorite facts about neutrinos is that they used it
to take a picture of the Sun. Remember, neutrinos interact
very very rarely with us, but there's huge numbers of them,
Like a hundred billion neutrinos from our sun passed through
your fingernail every second. But you're lucky if you can

(55:25):
even detect one in a huge specialized device. So they
pointed this thing at the Sun for like months and
they got a picture of the Sun in neutrinos, which
is is kind of cool because it's like another way
to see the Sun. I just get really excited every
time humans build another kind of eyeball to look out
into the universe. I'm guessing though, like neutrino glasses that

(55:45):
I can wear and look and spot neutrinos myself are
a little bit far from the market, maybe another ten
years or so. Yeah. Currently these things have like thousands
of tons of heavy water in them, And so unless
you're very strong or willing to put on extremely large
glasses and you're not going to be seeing neutrinos with

(56:06):
your eyeballs. I'll work on my neck exercises. But in
the meantime, the universe continues to churn and burn, and
supernovas are happening out there, blowing out these incredible cosmic
engines of the night sky. You know, we are grateful
that stars last for as long as they do, that
they balance on this nice edge between gravity and fusion

(56:26):
for so many billion years to light up the night
sky and to provide life here on Earth, and just
to provide us with something nice to look at as
we shiver in front of the fire and sip our
hot coco on our camping trips. But eventually those stars
do give up their life and they do go out
in incredible cosmic explosions, which then give us another opportunity

(56:47):
to learn what's going on inside the heart of those stars.
And so the more things blow up, the more we
learn about them. A bunch of drama queens. So if
you're out there interested in supernova, to keep an eye
on the night sky. You might see one with the
naked eye. Or if you're excited about squirrels, keep watching
to see how many acorns they eat. Maybe you'll see

(57:08):
you one of those blow up. I do not endorse
that message. Well, thanks Katie very much for joining us
on this tour of cosmic catastrophes. Thanks for having me,
and thank you all for listening. Tune in next time.

(57:30):
Thanks for listening, and remember that Daniel and Jorge Explain
the Universe is a production of I Heart Radio. Or
more podcast from my Heart Radio, visit the I Heart
Radio app, Apple Podcasts, or wherever you listen to your
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