So today we will be going through all the
various star types, from the tiniest category
of brown dwarf stars up to monstrously huge
Wolf-Rayet stars, and from the normal main
sequence stars to some strictly hypothetical
ones composed of Dark Matter or types of particles
we’re not even sure exist.
We will also be trying to simplify a lot of
the concepts in astronomy that tend to get
used a lot but generally not well explained.
Often the biggest problem people have mastering
astronomy and astrophysics is the terminology
itself, and a lot of that comes from terms
that have antique origins or had to do with
how we discovered them, not so much how they
function.
We will try to clear that away.
So loosely we will discuss the following topics
in the following order.
1.
The Harvard Classification System
2.
The Yerkes Classification System & Lifetime
3.
Population, Metallicity, and Formation
4.
Abnormal Stars
5.
Dead Stars
6.
Hypothetical Stars
        Okay, there’s a decent chance
you have never heard of the Harvard Classification
System but you have heard it used all the
time.
You’ve heard our own sun referred to as
a G-type star or a G2 yellow star, that’s
the Harvard Classification system.
Some quick history is needed here, we have
obviously known about the stars a long time
but we only knew them by how bright they were
and what color they were.
We didn’t know how far away they were for
instance, and we didn’t learn that as soon
as we invented telescopes.
We can’t even see the nearest star for instance,
Proxima Centauri, with the naked eye, but
we can see V762 Cas, which is over 16,000
light years away, 4000 times further off than
Proxima.
We can only see a few thousands stars with
the naked eye, maybe double that if you’ve
got great vision and great observing conditions,
but there are a few billion stars closer to
us than that most distant visible star, V762
Cas.
So visibility doesn’t particularly indicate
proximity.
    Once we had telescopes though we could
obviously see a lot more stars and we could
also zoom in on one, by itself, and set up
some instrument to measure the components
of that light.
How much red or yellow or infrared light was
coming off it for instance.
We could also measure its total brightness,
its total luminosity, across all wavelengths
even those invisible to our eyes by use of
a device called a bolometer.
This gives us the term bolometric luminosity,
and it differs from normal luminosity in that
the former is all the light given off by a
star across every wavelength.
    By and large stars give off light in
a spectrum of wavelengths based off their
surface temperature, but it generally is not
a smooth curve because of various substances
floating around on top of them absorbing some
of that light and re-emitting it at a different
wavelength.
That results in some very jagged spectra,
especially for hydrogen, the main component
of stars, which sucks up light and re-emits
it at specific frequencies.
4 of which are in the visible range of light
and these were discovered Johann Balmer in
1885.
So suddenly they could look at a star’s
spectra and tell how much hydrogen it had.
You can do the same for other elements and
for frequencies outside the visible band,
and also what they absorb, so you will often
hear about absorption and emission lines,
but for our purposes today we don’t care
about that.
That’s just how we identify the stars and
how the system originated.
The original system assigned each star type
a letter based on how much hydrogen could
be seen in their spectra, with A having the
most, B the second most, and so on, for 22
in total, A to V.
This original system, sometimes called the
Draper Catalogue, just measured hydrogen,
and was put together by Edward Pickering and
his assistant Williamina Fleming, best known
for her discovery of the Horsehead Nebula
in 1888.
Draper was an astronomer who did a lot of
the groundwork for photography in astronomy
and after he died his widow agreed to fund
Pickering’s work, though Williamina Fleming
did most of the actual classification.
She got her career in astronomy started as
Pickering’s house maid at Harvard and was
the first member of what became known as the
Harvard Computers, 80 or so women employed
to do calculations back when computer was
a job title not a device.
Another one of them, Annie Cannon, formalized
the classification system later on based on
temperature, but the original version Pickering
and Fleming made was a bit more cumbersome.
Unfortunately she used it as the base so we
got stuck with a bunch of letters that basically
mean nothing anymore and is hard to memorize.
From hottest to coldest they are O, B, A,
F, G, K, and M, and if you are familiar with
those and ever wondered why those letters
were picked in that order, now you know.
The original system, the one used in the Draper
Catalog, focused on how much hydrogen they
could detect in the absorption lines and it
stuck around when Cannon made the modern system,
the Harvard Classifications.
These also all correspond to colors, red is
coldest and blue hottest.
She was also kind enough to come up with a
mnemonic for remembering those, “Oh be a
Fine Girl, Kiss Me.”
So now we had a system for temperature but
still none for any other attribute, and an
M-Type star, the coldest, can be either the
smallest stars or the largest, for instance.
It also is a bit vague even about temperature,
so we follow each letter up with a number
0 to 9, 0 being hottest and 9 coldest.
So the hottest star would be an O0 and the
coldest an M9.
Our sun is a G2, making it in the third hottest
zone of the G-type stars, themselves the third
coldest of the original categories.
Again, these categories tell us nothing about
how old a star is or how big it is or bright
it is by itself, for that we needed something
more.
So in the 1943, at Yerkes Observatory in Wisconsin,
Wilson Morgan, Philip Keenan, and Edith Kellman
came up with an additional classification
system to cover total brightness.
This is sometimes known as the Yerkes Spectral
Classification system, for the observatory,
and sometimes the MK system for Morgan and
Keenan, for the two men, and nowadays MKK
to include Edith Kellman.
Like the Harvard Scale this one has been added
to over the years but originally they just
had five, 1 through 5, done in Roman Numerals.
And these just come after the Harvard scale
of a letter, then a number, then a Roman Numeral.
So our sun for instance is a G2-5, not a G2V,
most stars talked about are V’s, or 5s,
because most stars are 5s or Vs.
This indicates a main-sequence star.
1, 2, 3, 4, and 5 are respectively 1, Supergiants,
2, Bright Giants, 3,  Normal Giants, 4, Sub
Giants, and 5, Main Sequence.
Again in Roman Numerals, and after the letter
and number of the Harvard Scale.
You might be wondering why there are no normal
stars or dwarfs on that scale, and in fact
we have added one to it in recent years, for
sub-dwarfs, and another for white dwarfs,
and broke up category one into various extremes
of brightness, but again no dwarfs or normals.
This is an example of terms changing as we
learned more.
Originally category 5 was dwarves, and that’s
essentially a relic of when most stars we
could see were unsurprisingly the brightest
stars.
Our sun gets classified as a Dwarf Star for
this reason, even though it is bigger than
more than 90% of other stars.
And Main Sequence is a weird word too, you’ve
probably heard it enough times that you don’t
really think of it anymore but when we are
talking about size and brightness, supergiant
this and bright giant that, it does seem a
bit bizarre to have the term ‘main sequence’,
like you had a table for describing people’s
height, giant and dwarf, but instead of just
‘normal’ or ‘average’ you have some
weird term like ‘standard doorway clearance’
or ‘comfortable shelf access’.
That’s basically what happened here.
As we began measuring brightness of stars,
their actual brightness, not how dim they
are to us based on distance from us, we began
plotting them.
Back in 1910 Ejnar Hertzsprung and Henry Russel
plotted a bunch of the stars by their Harvard
spectral class, the O, B, A, F, G, K, and
M thingy, and by their Luminosity, their brightness.
Or basically by their temperature versus their
brightness.
This is unsurprisingly a logarithmic scale
since the plot goes from things thousands
of times dimmer than our sun to thousands
of times brighter.
You get some noticeable clumps this way and
one is a big thick line we call the main sequence.
Most stars were on that main sequence, and
the name just stuck, basically as a way of
saying that star was normal.
As we learned more we added to it, but in
the basic original system the biggest brightest
stars were type 1, supergiants, and this could
be a very massive hot star or a large, cooler,
and lower mass red supergiant.
A star’s total brightness is based on its
temperature and diameter.
Double the diameter, quadruple the brightness
or luminosity.
But for temperature it goes up even quicker,
double temperature and you increase luminosity
sixteen-fold.
Some stars are very hot and tiny, like white
dwarfs, other are huge and cool, like red
giants.
The Harvard scale tells us temperature, but
Yerkes doesn’t tell us a specific dimension
like age or width or brightness, it tell us
which area of the HR diagram the star is in.
It’s more like a map location, like continents
or chains of islands.
They’ve just got numbers and names, not
just names like North America or Asia.
Astronomers usually say the names and write
the numbers.
Now there are reasons why the stars are in
these groups rather than randomly and evenly
distributed, and it mostly relates to how
old they are, in terms of how long they will
live.
Stars burn through their hydrogen at a rate
based on their mass.
Only the very smallest stars actually get
to use all that hydrogen since they are fully
convective, which basically just means stuff
near the surface can float down to the core
and stuff in the core where fusion is going
on can drift up to the top.
The smallest stars are fully convective, one
just a bit bigger actually have a big drop
off in this, and it rises again when you get
to stars in our sun’s mass range, but is
still pretty low.
But the bigger stars don’t live nearly as
long, some live only a few million years,
while the smallest stars can live trillions
of years.
You probably already know that and you might
even be familiar with a rule of thumb that
let’s you determine a star’s lifetime
based off its mass.
Most folks think a main sequence star when
it runs out fuel becomes a red giant but there’s
more to it than that.
For instance our sun gets brighter every year,
about 1% brighter every hundred million years.
I’ve never heard anyone phrase it this way
before and it’s probably from my own big
focus on megastructures and Dyson Spheres
but the sun’s brightness increases every
five years by about what is needed to illuminate
the planet Earth.
So if you are building a giant dyson swarm
around your sun you can add about a planet’s
worth of new living area every five years,
or a space the size of the US or Europe every
month or so.
Eventually it will run out of hydrogen in
its core, just the core though, if our sun
used all its hydrogen it would live closer
to a hundred billion years.
Many stars, as they begin getting to the end
of the main sequence, growing brighter, have
a sub-giant phase, class IV on the Yerkes
system.
This is generally a relatively short phase
though its exact characteristics depend on
the star’s mass, but basically the star
just gets decently brighter, noticeably more
than it was already doing just from aging.
All stars have a main sequence phase, their
main phase of their life essentially, and
will pass through a subgiant phase as well
before becoming a giant, though the lightest
stars never become giants.
At least we think so, the fact of the matter
is that virtually no stars less massive than
our own sun have ever left the main sequence.
The Universe isn’t old enough for them to
have even if they were among the first to
form 13 billion years ago.
No M or K class main sequence star, red or
orange dwarf stars, has ever died of old age,
and they are the supermajority of stars.
The only stars to leave the main sequence
yet are ones more massive than our own sun
or those just a little bit less massive which
also formed way back near the beginning of
time.
Most stars are red or orange dwarfs and partially
because they have longevity, big stars don’t
live long.
Anyway we always have to be a bit speculative
when talking about phases for things less
massive than our own sun since even though
that is most stars the universe just isn’t
old enough for us to have actually witnessed
what happens to them when they leave the main
sequence.
As I said though, current thinking is the
very lightest of stars don’t even have a
giant phase.
Leaving the main sequence, and going through
type 4, subgiant, they arrive at normal red
giant phase, type 3.
Now here we have another example of bad terminology.
A red giant is always an old star, one that
left the main sequence and expanded and cooled.
It might have been a G or F or A type star
beforehand but it is an M-type now, and it
has expanded so much it can have enveloped
its inner planets and even a close binary
neighbor.
But we often use the term giant to refer to
very large main sequence stars too, like a
blue giant, and this can get kinda of confusing.
Also, amusingly, while we think of things
getting swallowed and vaporized when this
happens those planets or stars would just
keep orbiting inside it, albeit that orbit
would decay and those planets would be roasted,
but slowly roasted.
This is because while the red giant is quite
hot, it’s thinner than air.
So properly insulated you probably could dive
a spaceship into one for a while.
Type II are the bright giants, and these can
come in any color, any letter on the Harvard
Class, O through M, blue through red.
A Blue Bright giant would be a main sequence
star while a red bright giant would be a dying
star that had been fairly large to begin with.
When stars leave the main sequence they don’t
just expand overnight or contract afterwards,
with the exception of the really big ones
that outright explode, which is not an instant
process either but the timelines are a lot
shorter.
The next category, type 1, are supergiants
and again these can run the whole spectra,
but they are inevitably a very massive star
on the main sequence or a pretty massive star
near the end of its life.
We also further subdivide this category by
how bright they are and you will sometimes
see a type 0 called a hypergiant.
It’s not that there are so many of these
stars out there that we actually needed a
new class, it’s just that we can see them
so far away that they are pretty easy to catalog
and they are often yellow, because stars that
massive have stability issues that tend to
make them physically larger and cooler than
we’d expect.
So it isn’t that color, or temperature,
of a star doesn’t tell you a lot about that
star, it's just that unless you are an astronomer
you are almost always better off looking at
stars using the Harvard plus Yerkes notation
for that little V, the roman numeral 5, and
if you see that you know it is a regular old
hydrogen burner in the prime of its life,
and the letter than tells you its mass based
off the temperature or color.
If you don’t see that V, or 5, at the end,
you will be safe assuming it is either dying,
huge, or weird, or some combination of all
3.
But we talked about dying, and we’ll talk
about it more in a bit, let’s talk about
star formation for a bit.
Stars tend to form out of nebulae, big dense
pockets of gas that generally spawn a whole
bunch of stars when something comes by and
pushes on that gas or otherwise sets off a
wave of star formation.
These stars won’t all form exactly simultaneously
but on astronomical timelines they more or
less do, and they generally will stay together,
occasionally having one meander off or a migrant
join for a while or even permanently.
These clusters and groups of stars tend to
be about the same age but not the same mass.
Stars tend to form in clumps and be of the
same age, and we often refer to these ages
as populations.
Another weird bit of terminology, we have
population 1 and 2 stars and even added 3
in recent years, and this actually refers
to their metal content, but its called a population
because the stars in a group tend to be of
the same age and so you’d be looking at
a big group, or population, of stars and trying
to determine their age by how much metal they
had in them.
Metallicity in astronomy refers to any element
besides hydrogen and helium in a star, not
just things like iron or copper but stuff
like carbon and oxygen too.
As you know the heavier elements mostly come
from stars that died making them and sprayed
them out into the cosmos, so the oldest stars,
ones that formed long ago, have a lot less
of these metals in them while the ones that
formed more recently have a lot more.
Population 1 stars, or metal rich stars, are
younger stars, and our sun is one.
Population 2 are metal poor stars, meaning
they are old, and population 3 is ultra-poor,
and these are the very oldest stars, ones
that formed near the beginning of things.
Now metal content doesn’t just tell us about
age, higher metallicity star are cooler than
equal mass stars with lower metallicity, because
it’s also got those metals down in its hydrogen
burning core interfering with fusion.
Which also make them live longer though as
I said in the starlifitng episode if you want
to extend a star’s lifetime you’d just
remove mass from it, even those metals, since
they extend lifetime but their sheer presence
still adds to the mass and gravity of that
star to increase fusion, just not as much
as hydrogen would, and it would only matter
if the star was on the convective side.
Okay so we have our picture now of the main
types of stars, so let’s talk about some
of the abnormal ones.
On the smaller side we have type-L, in this
case because it is the letter right before
M. These are hot brown dwarf stars, and they
would give off a little bit of light, brown
dwarfs are not brown, but would look a dim
orange-red or magenta.
Next down are T-type, and I’ve never found
out the origin of that one, but these are
Methane Dwarfs.
Not because they are made of all Methane,
but because they have some.
You do not get a lot of molecules forming
on the surfaces of stars, from the heat, but
T-type are quite cool.
Compared to other stars anyway, they are still
warm enough to melt lead.
The last new type, Y-class, are called Ultra-Cool
Brown Dwarfs.
They are not hot enough to melt lead, though
still quite hot.
I have no idea where the Y comes from either,
but these are cool enough that some might
even have water clouds in their atmospheres.
Brown dwarf stars, the smallest stars and
arguably not stars at all, can be anywhere
between 13 and 80 Jupiter Masses, or about
4000 to 27,000 times more massive than Earth
or about 0.13% to 0.8% of the Sun’s mass.
In spite of this almost all of them are about
the same size as Jupiter.
They just keep getting denser and denser as
they get more massive until they are massive
enough for fusion.
Once they get over 80 Jupiter Masses they
become the dimmest of red dwarf stars, or
M9’s.
Hypothetically they can burn deuterium for
a few million years, as it takes less effort
to fuse, but there wouldn’t be a lot of
it.
If you’re in the star making business you
could probably make rather smalls stars made
only out of it deuterium but in nature it
occurs at part per million.
We don’t know just yet how common brown
dwarf stars are, they definitely are not uncommon
but they could outnumber all other stars.
People wonder about living around a brown
dwarf, like if a large moon could be habitable.
The answer is probably yes since you could
have a lot of heating from both tidal forces
on the moon, something we discussed back in
the Habitable Planets series, and from some
light off the brown dwarf, even if virtually
all of it was in the infrared range.
We also have, back on the other side of the
size scale, Class W, for Wolf-Rayet Stars,
these are so big and huge that you can see
them thousands of light years away with your
naked eye.
We used to think no star could form above
a certain mass, because we didn’t observe
any beyond a certain luminosity, known as
the Eddington Limit.
But we noticed some of the brightest acted
a bit weird.
Their spectra wasn’t right, too much helium
and nitrogen and carbon.
We realized these were a lot more massive
than their luminosity indicated, and we have
known of them for a long time, they are very
uncommon but so bright we can actually see
two of them with the naked eye.
One of them, Gamma Velorum, is one of the
brightest stars in the night sky, but they’re
both in the southern hemisphere unlike 90%
of my audience.
What happens with these stars is that they
are too big to be stable.
They have very high solar winds and regularly
belch off swathes of matter.
That’s why they seem to have too much helium,
carbon, and nitrogen, their outer layers get
flung off.
You will also hear a lot about Variable Stars,
particularly Cepheid Variables.
A variable star is just one whose brightness
varies, and there are tons of types.
These come in two major categories though,
intrinsic and extrinsic, extrinsic variable
stars have something causing us to see its
luminosity change.
Intrinsically variable stars have something
about the star actually altering its luminosity,
which is true of every star but we use the
term for big changes, like the star swells
in size then shrinks.
There are dozens of classes of variables but
the Cepheid, named for the first we noticed
this with, the star Delta Cephei, is historically
significant.
Cepheids expand and contract in a very regular
way and are big bright stars, so they were
instrumental for two big things.
First, Edwin Hubble used them to show that
distant nebulae were actually galaxies, something
that was already proposed but they settled
the matter for good, and second they were
key to the original determination that the
Universe was expanding, again something Hubble
found.
There are too many of these to list right
now and most are important just for astronomy
so I’ll bypass the others, but that’s
what variable stars are and why they are so
important.
Time to move onto dead stars, the three popularly
known ones are white dwarfs, neutron stars,
and black holes but we have quite a few other
including some hypothetical ones like Iron
stars, so this will spill over into our final
category.
White Dwarfs are the most common fate of a
star, anything less massive than 8 times our
sun’s mass end up as one, which means virtually
all stars.
Not only are stars bigger than that less common
in the sky because they are short-lived, they
also don’t form as often as smaller ones.
Also, as mentioned earlier, the only white
dwarfs around are those from stars as massive
as our own or bigger so exact behavior of
the ones that will come as less massive stars
die in the future is still a bit speculative.
In the future we would probably have white
dwarfs composed mostly of helium from stars
that were not massive enough to fuse helium
into carbon and oxygen and other elements.
Your average white Dwarf is about the size
of Earth because it is a leftover stellar
core of something that blew all it outer shell
away when it went into final helium burning
modes.
We wouldn’t expect ones actually composed
of just helium to be quite the same.
Now when a red giant occurs you basically
get a crunch in the center as the core starts
burning helium or the inner shell burns hydrogen,
when that is all done that big thin cloud
that makes up virtually all the volume and
a fair chunk of the mass turns into a Planetary
Nebula.
The rest settles down as a dead but super
dense and super hot planet sized white dwarf
that is slowly cooling.
Eventually it would become a red dwarf again
for instance, in terms of being tiny and red,
and eventually a black dwarf, one too cold
to radiate light.
The universe isn’t old enough for either
of those yet either.
In the meantime it is a super-hot super-dense
thing you wouldn't want to dump hydrogen on
because it can cause a nova.
Just a regular old nova not the super kind.
Nova being latin for new, a nova star, or
new star, sometimes pops up in the night sky
from a place where we couldn’t see a star
before because it was too dim.
Novae are quite bright for a little while
and a lot more common than a supernova.
Unlike supernovae that happen maybe a couple
times a century in our galaxy, regular novae
happen a couple times a month, but they are
not so bright they you can see them across
the galaxy.
Still one happens close enough to make a new
star visible to the naked eye every few years.
Novae can vary a lot in brightness, since
it just depends on how much hydrogen lands
on them, and they can happen multiple times
to the same star.
Now a supernova is a more extreme event and
can happen two ways, the first you know, it
is when a really big star dies, but the more
common kind is when a white dwarf absorbs
enough mass to cross a threshold, about half
again what the Sun masses, and it is too much
for even the electron degenerate matter to
handle and the white dwarf simply explodes.
We are not quite sure as to the exact process
but we think that when it crosses that mass
boundary it’s enough to ignite carbon and
oxygen fusion and get some convection going
on to stir the star, and that after about
a thousand years that results in a detonation,
ripping the white dwarf apart rather than
just causing it to collapse into a neutron
star.
In any event that is a Type 1A Supernova.
Now not all Type 1a Supernovae tear the star
apart, sometimes, we think, they leave behind
a remnant called a Zombie Star, one that might
have about half the mass left over.
This would be a lot like a white dwarf, just
a less massive one, but still basically a
white dwarf.
A Superhot super-dense planet-sized object
that is going to slowly cool off since it
no longer has any fusion going on.
You could conceivably live around one of these
but they are a lot dimmer than normal stars
and keep getting dimmer and dimmer.
Incidentally, Supernova Types 1b and 1c involve
very large stars too, like the Type 2’s,
the Wolf-Rayet stars I mentioned earlier,
so type 1a, the most common type, are the
only ones involving white dwarf stars.
There used to just be types 1 and 2, and type
1 meant we didn’t see any hydrogen in the
explosion, it all got fused or scattered long
before.
Type 2 does have hydrogen in it.
These are the classic supernova you are most
familiar with, big stars that explode and
leave behind a neutron star or a black hole.
These occur in stars heavy enough to ignite
more fusion, and the highest element you can
form this way is iron so the very biggest
stars end as supernova when their core turns
into iron, these then get imploded by the
supernova into neutrons stars.
We estimate there are 100 million to one billion
neutron stars in our galaxy out of many hundreds
of billion of stars, but many would be old
and have cooled down.
They’re quite small, no bigger than a small
moon or large mountain, so they only emit
a lot of light when they are very young and
hot.
They also tend to rotate very quickly, having
kept most of the angular momentum of the parent
star but squeezed in tight like when an ice
skater twirling around bring their arms in
and spins faster.
So they rotate several times a second, or
even hundreds of times a second, unlike most
stars which rotate in days or months.
This causes beams of electromagnetic radiation
to come off them which we can see if we’re
at the right angle and when they are young
and hot.
Those ones are called Pulsars, we’ve found
about 2000 in our galaxy, and there are presumably
at least ten times as many that are just at
the wrong angle for us to see, but a Pulsar
is just a young neutron star titled at the
right angle relative to us.
Now as you know a black hole is hypothesized
to be what you get when an even bigger star
goes off and we have talked about them a lot
before in other videos so we’ll skim it
for now and slide into hypothetical stars
because many of them are thought to be intermediary
stages between neutron stars and black holes.
Neutron stars get their name because they
are made up of nothing but neutrons.
White dwarfs are under so much gravity and
pressure, without any fusion to push back,
that you can’t really have atoms anymore,
just a sea of atomic nuclei and electrons,
electron-degenerate matter, it’s too crunched
up for electrons to wrap around nuclei and
space atoms out.
Neutrons stars are so dense you can't even
have protons and electrons anymore, just neutrons.
A neutron left to itself usually decays into
a proton and electron, or their antiparticles,
this is essentially the reverse process.
But protons and neutrons are both made up
of quarks, and under enough pressure it might
be that even neutrons can’t exist anymore
and you get a big mess of quarks.
Or a quark star.
This might be a short phase right before a
black hole forms or maybe a stable one where
inside a neutron star is a core of raw quarks.
Probably Strange Quarks too, rather than the
up and down quarks that make up most matter.
We do have a few neutron stars that seem a
bit too dense, and it’s been suggested they
might be quark stars, or have a quark core
at least.
They don’t necessarily drop into black holes
after that either.
We still have four more possible steps.
One option is the Electroweak Star, named
for the unified electromagnetic and weak nuclear
forces that occur at high enough energies.
Here the collapsing neutron or quark star
would have a super-dense core about the size
of your fist with a couple planet’s worth
of mass jammed in, and the rest all around
it still at the lower but still absurdly high
densities of neutron or quark stars.
Gravity is too strong for even quarks to exist
at that core and those quarks are being converted
into leptons… electrons are a type of lepton,
they have no quarks.
This energy keeps the star from collapsing
further and it’s been suggested this could
go on for millions of years before the jig
is up.
From the outside it would still look like
a neutron or quark star presumably.
Another option is a Boson Star.
Bosons are particles that can be in the same
place as other particles.
Fermions, what regular matter is made out
of, cannot be in the exact same place as another
fermion.
Quarks, protons, neutrons, and even leptons
like the electron are fermions and cannot
occupy the exact same place as another fermion.
A boson can, and photons, the particle of
light, are an example of a boson.
Considering a radio wave, or radio photon,
can be bigger than your house, it would be
kind of hard to listen to the radio otherwise.
One of the candidates for Dark Matter, which
we discussed in its own episode, is the Axion,
and it has been suggested it could be a type
of boson that could form this sort of star
or even start a star off this way if it were
made back in the Big Bang.
Boson Stars are also a candidate for Dark
Matter, though I skipped them in that episode
for here.
We also have other options for Dark Matter
stars, or Dark Stars, which we’ll get to
in a moment.
Another option is a Preon Star, Preons are
another hypothetical particle, originally
suggested to be what quarks and leptons were
composed of.
There’s no evidence they exist but there’s
always an understandable desire for some most
basic particle all the others are built out
of and Preons were one of the more popular
ones, much like Strings from String Theory.
Like Boson stars they are also a candidate
for dark matter and could have been formed
in the Big Bang, or in a supernova.
Both of these are much denser than even a
neutron or quark star.
The densest though is the Planck Star, and
these are not a last step before a black hole
but thought to be what might be at the center
of a black hole.
We don’t know that a black hole is actually
point-like, and I mean the mass at the middle,
the actual black hole is just the region around
it where gravity is too high for light to
escape, that rises linearly with mass, double
the mass, double the diameter of that event
horizon, but we don’t know that matter keeps
compacting after that until it simply becomes
an tiny dot of zero width.
A Planck Star is one option for the core of
a black hole, and they are insanely dense.
About 10^97 kilograms per cubic meter, which
for scale, the whole observable universe is
only about 10^52 kilograms so if squeezed
down to Planck Density it would fit into single
atomic nucleus, and remember an atomic nuclei
is very small compared to an atom, like the
Sun is to the solar system.
A Planck Star would be similarly tiny compared
to an atomic nuclei, but with a whole star’s
mass crammed into them.
Needless to say these are also a candidate
for dark matter though again the key notion
is that they might be the actual center of
a black hole.
Now the last two we are going to look at are
not dead stars, although one is probably an
extinct class that no longer exists and the
other is a kind that won’t exist until everything
else is dead.
Dark Stars, stars made entirely of dark matter,
are considered something that might have existed
early on in the Universe when things were
more packed together.
Depending on how dark matter actually behaves
you could get clumps of it mixed with normal
matter that prevented that matter from achieving
the density necessary for fusion.
Such stars would radiate very little light
even though they could be bigger than our
solar system.
We talked about these more in the Dark Matter
episode but they are hypothetical and almost
certainly no longer exist.
Our last is Iron Stars, and they are the ones
we’d probably focus on if I ever do a sequel
to the Civilizations at the End of Time video
that focused on Black Holes.
The Star Forming era of the Universe will
end in about a hundred trillions years, we’re
about 1% of 1% of the way through it.
We talked in that episode about how potentially
that might be just the beginning of real civilization
as expansion drives cooling that allows hyper-efficient
computing.
But that era lasted at most 10^100 years,
just a trillion, trillion, trillion, trillion,
trillion, trillion, trillion, times longer
than the Stellar Phase of which we are again
only a percent of a percent through.
Some folks asked me if things could keep going
after that and there are a few options.
For matter that hasn’t been swallowed into
a black hole during all that time, say an
old white dwarf or neutron star left on its
own, and assuming protons do not decay, given
enough time, and we are talking timelines
that make the entire black hole era seem as
short to it as a second is to that era, you
can have cold fusion occur.
The insane improbability of a cold fusion
event occurring by quantum tunneling is one
of the reasons physicists roll their eyes
at this or that cold fusion device.
It’s easier at super-cold temperatures but
it is still so improbable that to convert
an entire stellar mass object into iron would
take timelines only meaningful in scientific
notation.
10^1500 years.
That’s a ten followed by an entire page
of zeroes.
As mentioned earlier, iron is the heaviest
element formed by fusion so eventually, given
a long enough time, and assuming protons don’t
decay, everything should end as iron.
Of course that isn’t necessarily the end
of the universe but we will save that for
another time, I think.
Iron stars are the last of our hypothetical
stars both for the Universe and this video.
Speaking of future episodes, this episode
was originally planned for next week, as you
may know we were going to do Colonizing the
Outer Solar System this week, but Fraser Cain,
the Publisher of Universe Today, offered to
collaborate on a video.
We both latched onto discussing colonizing
the outer, and inner, solar system and while
at first we thought we could get it done by
the original planned air date we decided to
give ourselves a bit more breathing room,
so that is next week.
And I suspect we will both be doing a lot
of more in-depth looks at that topic so make
sure to subscribe to my channel, and his as
well, for updates when that and other videos
come out.
As mentioned this episode was also the third
Patreon winner we had, the topic being selected
by Neo Navras, our prior two being the Spaceship
Propulsion Compendium selected by Drew McTygue
and the second being Starlifting with Bill
Mains.
They were all great picks and I really enjoyed
the process, Drew and I spent a couple hours
on the phone bouncing ideas around for instance,
so it is definitely something we will be continuing
though I still haven't decided exactly how.
Needless to say I am very grateful to the
channels Patreon supporters for paying the
bills for the channel and if you’d like
to become one there’s a link to the channel’s
patreon account in the episode description.
Again next week is Colonizing the Solar System.
In the meantime you can try out some of the
other episodes on this channel.
If you enjoyed this episode, don’t forget
to hit the like button and share it with others,
and until next time, thanks for watching,
and have a great day!
