Howdy Star Gazers and welcome to this episode
of Star Trails. My name is Drew and I'll be your
guide to the night sky for the week of February
the 22nd to the 28th. This week we wrap up our
month -long series on stars by exploring how
they end their lives. For some it's a quiet process
like embers cooling after a campfire. Others
explode and some turn into black holes. destined
to live the remainder of their lives like cosmic
vampires facing eternity. Later in the show,
we'll take a look at what you can see in the
night sky this week. Whether you're tuning in
from the backyard or the balcony, I'm glad you're
here. So grab a comfortable spot under the night
sky and let's get started. When I was a kid,
I distinctly remember the night when I learned
our son was at some point going to die. And to
be honest, it wasn't a great feeling. Sitting
on the floor in the den of my grandmother's house
as twilight fell, I opened up my favorite astronomy
tome, The Universe by National Geographic. and
read a surprisingly long and detailed account
of how in some five billion years our Sun would
grow hotter, all life on Earth would be vaporized,
and the Sun would expand into a red giant, enveloping
Venus, Mercury, and the Earth. This was terrifying.
It was the first realization that the Sun and
Earth aren't permanent. Sure, I knew I wouldn't
be around in five billion years to see the Sun
collapse, but even as a seven or eight year old
child, knowing that the end was approaching like
a timer counting down, created a sense of existential
dread and anxiety. So tonight, we're wrapping
up our month -long theme about stars. We've talked
about how they form, what they are, and what
they do during their epic lifespans. We've done
some statistical analysis related to star formation
and what they leave behind. But now it's time
to talk about how stars die, including the one
that keeps us alive. The first thing to remember
is most stars don't explode. Our imagination
is filled with supernova. Stellar death is often
portrayed as violent and spectacular. But the
vast majority of stars don't end that way. As
we've mentioned a few times this month, most
stars in the universe are small red dwarfs that
burn their hydrogen so slowly they will outlive
the current age of the cosmos by orders of magnitude.
They won't explode, they'll simply fade away.
Larger stars, like our sun, live for about 10
billion years in what astronomers call the main
sequence phase. During this time, hydrogen fuses
into helium in the core. Fusion releases energy.
That energy produces outward pressure. Gravity
pulls inward and the two forces balance one another
in what is called hydrostatic equilibrium. For
billions of years, the Sun has existed in that
quiet stalemate. But hydrogen in the core isn't
infinite. And this is where the story becomes
more subtle than most of us expect. Counter to
what we might think as a star dies, some, like
our sun, actually grow in size. When hydrogen
in the sun's core is mostly depleted, fusion
in the very center begins to slow. The outward
pressure weakens. Gravity, which never takes
a day off, begins to win. The core starts to
contract. And here's the key piece. When gas
is compressed, it heats up. Gravitational potential
energy is converted into thermal energy as the
core shrinks. Think about how a diesel engine
works. Unlike a normal gas engine which uses
spark plugs to touch off the combustion, a diesel
engine compresses the air to ignite the fuel.
Now imagine that process happening with half
the mass of a star. It's not just a small contained
explosion, it's nuclear fusion. As the core contracts,
it becomes hotter and denser. Fusion in the center
has largely ceased, but the rising temperature
ignites hydrogen in a shell surrounding the inert
helium core. Instead of fusion happening at the
center, it now happens in a thin, intense layer
around it. And that shell fusion is more energetic
than the Sun's original core fusion was. The
increased energy output pushes outward on the
star's outer layers. Those layers respond by
expanding dramatically. Our Sun will swell to
perhaps a hundred times its current diameter,
engulfing the inner planets. Paradoxically, gravity
causes the expansion. Gravity compresses the
core. Compression increases temperature. Higher
temperature ignites shell fusion, and shell fusion
increases energy output. Thus, the outer layers
expand. As the Sun expands, its surface temperature
drops. A larger surface area radiates energy
more efficiently, so the outer layers cool to
around 3 ,000 to 4 ,000 Kelvin. The Sun will
appear redder and dimmer per unit area. even
though its total luminosity increases. And this
is the red giant phase. You can look up at the
night sky right now and see some famous red giants.
Betelgeuse in Orion is the classic example. It's
already in its death throes, hundreds of times
larger than the sun. If placed at the center
of our solar system, it would extend past Mars.
possibly even toward Jupiter, depending on how
you measure its outer atmosphere. This star will
not become a white dwarf. It will explode as
a supernova within the next hundred thousand
years or so, astronomically soon, though not
calendar soon. Aldebaran is closer to the Sun's
eventual fate. It's a red giant, not massive
enough to explode, but expanded and cooling.
This is a preview of what our Sun will look like
in about 5 billion years. It's steady. It's not
violent. It's just old. Eventually in our Sun
the contracting helium core will become hot enough,
about 100 million Kelvin, to ignite helium fusion
into carbon and oxygen. This ignition happens
in what astronomers call the helium flash. a
rapid internal event that barely disturbs the
outer star. After a brief period of renewed stability,
fusion once again shifts outward into shells,
and the outer layers grow increasingly unstable.
Finally, the Sun will shed those layers entirely,
creating a glowing planetary nebula, a delicate
shell of gas expanding into space, and at the
center remains the exposed core. And this is
where the physics turns truly strange. What remains
after the Sun sheds its outer layers is a white
dwarf, roughly Earth -sized, containing about
half the Sun's current mass. It no longer generates
energy through fusion. It shines because it's
hot, not because it's burning. The density inside
a white dwarf is extraordinary. A teaspoon of
its material would weigh several tons. At this
stage, the matter is packed so tightly that classical
physics alone cannot explain what holds it up.
The answer lies in quantum mechanics. Electrons
obey a rule known as the Pauli exclusion principle,
which states no two electrons can occupy the
same quantum state simultaneously. When gravity
attempts to compress the white dwarf further,
electrons are forced into higher energy states
simply because there are no lower ones available.
This creates a pressure called electron degeneracy
pressure that doesn't depend on temperature.
The white dwarf isn't supported by heat. It's
supported by the structure of quantum reality
itself. Afterwards, it will simply cool over
trillions of years, fading gradually into darkness.
There are planetary nebulae you can see right
now that represent what the sun's far future
could look like. The ring nebula, M57, is the
exposed outer layers of a sun -like star that
died thousands of years ago. At its center is
a white dwarf, the compressed remnant core. The
Dumbbell Nebula is larger and easier to observe
in small telescopes than the Ring Nebula. It's
another Sun -like star shedding its atmosphere.
And while you aren't likely to be able to spot
it, we know of another fairly close white dwarf.
Sirius, the brightest star in our sky, has a
tiny companion, Sirius B. A white dwarf about
the size of Earth, but nearly as massive as the
Sun. It's difficult to see visually because it's
overwhelmed by the glare of Sirius A, but it's
been imaged and studied exclusively by Hubble
and X -ray telescopes. That tiny dot represents
quantum mechanics holding up half a star. If
a star begins its life more massive than the
Sun, the ending changes dramatically. Massive
stars burn through their fuel quickly. fusing
heavier and heavier elements in their cores.
Hydrogen becomes helium, helium becomes carbon,
carbon becomes oxygen, and so on, building an
onion -like structure of nested fusion layers.
This process continues until iron accumulates
in the core. But iron is a dead end because fusing
iron consumes energy instead of releasing it.
When the core becomes iron -rich, fusion can
no longer support it, and gravity wins completely.
The core collapses in less than a second. Electrons
are crushed into protons forming neutrons and
releasing an enormous burst of neutrinos. The
collapsing core rebounds, driving a supernova
explosion that briefly outshines entire galaxies.
What remains depends on the core's mass. If the
remnant core is between roughly one and a half
and about two or three solar masses, collapse
halts at an even more extreme stage. Electrons
and protons merge into neutrons. The result is
a neutron star, typically about 20 kilometers
across, yet containing more mass than the Sun.
Its density is so extreme that a sugar cube -sized
piece would outweigh a mountain. Once again,
quantum mechanics intervenes. Neutron degeneracy
pressure, the same exclusion principle applied
to neutrons, presents further collapse. Inside,
matter may exist as a superfluid, possibly containing
exotic particles or even free quarks. We're still
probing this frontier with gravitational wave
observations and nuclear physics experiments.
Some neutron stars spin rapidly and emit beams
of radiation, appearing to us as pulsars. The
Crab Nebula, M1, is the remnant of a supernova
observed in 1054 AD. At its center lies a neutron
star, the Crab Pulsar, spinning about 30 times
per second. Other neutron stars possess magnetic
fields so intense they defy comprehension. These
are magnetars, neutron stars with extraordinarily
amplified magnetic fields, trillions of times
stronger than Earth's. The leading explanation
is that rapid rotation during collapse amplifies
magnetic fields like a dynamo. The result is
an object capable of releasing immense bursts
of X -rays and gamma rays. They are rare and
violent, but still the product of stellar collapse.
And that finally brings us to black holes. If
the collapsing core exceeds the mass that neutron
degeneracy pressure can support, no known force
can halt the implosion. Gravity overwhelms every
resistance. The core continues collapsing until
an event horizon forms. That's a boundary beyond
which even light cannot escape. A black hole
isn't a cosmic vacuum cleaner. It's a region
of spacetime so curved that all future paths
point inward. At its center, classical general
relativity predicts a singularity, a point of
infinite density and zero volume. But infinities
in physics are usually a sign that the theory
has been pushed beyond its valid domain. Most
physicists suspect that a future theory of quantum
gravity will replace the singularity with something
finite, though we don't yet know what. We tend
to group black holes into categories based on
mass. Stellar mass black holes form from collapsing
massive stars. These typically range from about
3 to perhaps 50 times the mass of the Sun. But
these objects are only tens of kilometers across,
and that's astonishingly compact. Then there
are intermediate -mass black holes, which likely
range from hundreds to hundreds of thousands
of solar masses. Evidence for these is still
emerging, but gravitational wave detections and
observations of dense star clusters do suggest
they exist. And finally, there are supermassive
black holes. Every large galaxy we've studied
appears to host one at its center. The one in
our galaxy, Sagittarius A, has a mass of about
4 million suns. In the far future of the universe,
after stars have burned out, after white dwarfs
have cooled, and after neutron stars have decayed
or collapsed, black holes may be the last major
structures left. This hypothetical scenario is
sometimes called the black hole era of the universe.
And it's so far in the future that it makes the
sun's 5 billion remaining years look like a brief
candle flicker. At some point, even black holes
sputter out. In the weird quantum fields of a
black hole, some particles manage to escape.
You've maybe heard this term before. Hawking
radiation. because it was in fact theorized by
physicist Stephen Hawking. Over time, and I do
mean a very long time, the black hole will slowly
lose mass. We're talking time scales that are
slow even by astronomical standards. We need
to shift our perspective by powers of 10 for
it to even make sense. For example, the evaporation
of a stellar mass black hole could take years
on the order of 10 to the 67th power. That's
a 1 with 67 zeros behind it. A supermassive black
hole may take 10 to the 100 years or more to
die. By comparison, the current age of the universe
in years is only about 10 to the 10th power.
Black holes aren't eternal, but for all practical
astrophysical purposes they might as well be.
As they lose mass their temperature increases
and near the very end of their evaporation they
would radiate intensely and vanish in a final
burst of high energy radiation. This final phase
has of course never been observed and remains
theoretical. This brings me back around to where
we started this episode. pondering the dread
of our own sun's death. Our sun feels foundational,
yet it is temporary. But its transformation lies
so far in the future that it functions more as
a philosophical boundary than an impending event.
Humanity has only existed for a tiny fraction
of the sun's lifespan. If the sun were a 70 -year
-old human, our entire species would have only
appeared a few hours ago. When its time comes,
the Sun will not rage. It will compress, ignite
new layers, expand, shed, and settle into a dense
white ember held together by the quantum rules
that make the universe coherent at its smallest
scales. In the meantime, it's humbling to realize
that everything heavier than helium in your body
was forged in stars that lived and died before
ours was even born. Stellar deaths made rocky
planets possible. It made chemistry complex.
And it made us. After a quick break, we'll be
back with this week's Sky Report. Stay with us.
Welcome back. This week's sky offers a rich mix
of wandering worlds and familiar lunar phases,
perfect for both binocular stargazers and people
simply stepping outside after dinner. As the
sun sets, start by finding the moon. This week
it's rotating into view as a thin waxing crescent
early in the week. By February 24th, the moon
reaches first quarter. appearing half illuminated
and hanging high in the southern sky around sunset.
By week's end, it grows into a bright, waxing
gibbous rising earlier each evening and dominating
the early night. This week is especially good
for planets because a slow -building planetary
parade is unfolding in the evening sky. Six planets,
Mercury, Venus, Jupiter, Saturn, Uranus, and
Neptune, will appear along the same stretch of
sky shortly after sunset. From Earth's perspective,
they trace a broad arc along the ecliptic that
can be seen in a single sweep with dark skies
and clear horizons. Start your evening soon after
sunset by looking low toward the western horizon.
There, Venus shines brightest among the inner
planets, a brilliant beacon just above the horizon.
Not far below it is Mercury, still a challenge
because it sits low and close to the Sun's glare,
but visible if you can catch it while the sky
is still dusky and the horizon is clear. Saturn
also lingers in this direction, a softer yellow
point that will benefit from binoculars or a
small telescope to cut through the twilight.
Toward the east and southeast as night deepens,
you'll see Jupiter, the brightest star in the
evening sky this month. It rises early and stays
high well into the night, far easier to spot
than Mercury, Venus, or even Saturn. In a telescope,
Jupiter's cloud bands and its large moons make
for stunning details if the air is steady. The
outer ice giants, Uranus and Neptune are also
part of this extended parade, located between
the brighter worlds. They're too faint to see
with the naked eye. Binoculars or a telescope
will help you tease them out against the star
fields. The best collective view of this six
-planet parade builds toward February 28th, when
all six planets are visible in the same post
-sunset window roughly 30 to 90 minutes after
sundown. Start with the western horizon for Venus,
Mercury, and Saturn, then sweep eastward to catch
Jupiter higher in the sky. Uranus and Neptune
will be between them, subtle, but there. Finally,
even with the moon brightening later in the week,
don't miss a couple of quieter sky -watching
opportunities earlier in the period. On the evening
of February 23, the moon moves through the northern
part of the Pleiades star cluster, subtly occulting
stars as it passes. It makes a fine pairing for
binoculars or a wide -field telescope. Next week
we'll return to our book club covering chapters
6 and 7 in Nightwatch. These chapters cover deep
sky objects and the planets of our solar system.
And like the previous chapters, make excellent
companion readings for this podcast. That's going
to do it for this week. If you found this episode
interesting, please share it with a friend who
might enjoy it. The easiest way to do that is
by sending folks to our website, StarTrails .Show.
And if you'd like to support the show, use the
link on the site to buy me a coffee. That really
helps. Be sure to follow Star Trails on Blue
Sky and YouTube. Links are in the show notes.
Until we meet again beneath the stars, clear
skies everyone.