Good evening, and welcome to Night 3. Once again, everything on tonight's list is new.
Our route begins with the very first planets ever found beyond the Solar System, circling the corpse of a star. From there we'll stop at a black hole that settled a famous scientific bet, peek beneath Venus's clouds for volcanoes that may still be erupting, stand at the foot of the tallest volcano in the Solar System, watch a comet slam into Jupiter, and end in a far-future era when the last stars finally go out.
Settle in. 🌙
1. 🪐 The first planets ever discovered orbit a dead star
The first confirmed planets found outside our Solar System did not orbit a normal star.
In 1992, astronomers Aleksander Wolszczan and Dale Frail announced planets around PSR B1257+12, a pulsar roughly 2,300 light-years away. A pulsar is the rapidly spinning neutron star left behind after a massive star dies.
The discovery came from exquisite timing. Pulsars emit radio pulses so regularly that the tiny gravitational tugs of orbiting planets cause detectable shifts in when those pulses reach Earth.
At least three planets orbit B1257+12, including worlds a few times the mass of Earth.
How planets ended up circling the aftermath of a supernova is still a fascinating puzzle. They may have formed from debris after the explosion, or from material later supplied by a companion star.
So our first confirmed alien planetary system wasn't a second Solar System. It was a planetary system built around a stellar corpse.
2. ⭐ UY Scuti — a star so large our Solar System helps describe it
The red supergiant UY Scuti is one of the largest stars whose radius has been estimated.
Its exact size is uncertain, because enormous cool stars have diffuse, pulsating atmospheres rather than neat surfaces. Estimates have also shifted as distances and stellar models improved.
But its scale is extraordinary either way. If it sat where our Sun is, its outer layers would stretch hundreds of times farther than the Sun's radius, swallowing the orbits of all the inner planets and reaching toward, or by some estimates past, Jupiter's orbit.
Yet an enormous radius doesn't mean an enormous density. Much of a red supergiant's outer envelope is astonishingly thin.
UY Scuti is less like an oversized ball and more like an immense glowing atmosphere wrapped around a much denser stellar core.
A star can be gigantic and extraordinarily fluffy at the same time.
3. 🕳️ Cygnus X-1 — the black hole behind a famous scientific bet
Cygnus X-1 was one of astronomy's earliest convincing stellar-mass black-hole candidates.
It's a binary system in which an unseen compact object orbits a massive blue supergiant. Gas pulled from the star heats to millions of degrees as it falls toward its invisible companion, producing intense X-rays.
Measurements eventually showed that the compact object is too massive to be an ordinary neutron star, which makes a black hole the natural explanation.
The system became famous outside astronomy because physicists Stephen Hawking and Kip Thorne made a wager over whether Cygnus X-1 really contained a black hole. Hawking bet against it, partly as an insurance policy against his own research, and eventually conceded.
Today its black-hole nature is considered firmly established.
Sometimes the universe settles a bet simply by providing better data.
4. 🌎 Kepler-186f — the first Earth-sized planet found in a habitable zone
In 2014, astronomers announced Kepler-186f, the first roughly Earth-sized planet found orbiting within the habitable zone of another star. Its host is a red dwarf about 580 light-years away.
The planet receives considerably less energy from its star than Earth does from the Sun, but with the right atmosphere, temperatures could potentially allow liquid water.
That does not mean Kepler-186f is Earth-like in every respect. We don't know its atmosphere, its surface conditions, or whether it has any water at all.
Its importance was conceptual. Before Kepler, Earth-sized planets in temperate orbits were mostly theoretical possibilities. Kepler-186f showed that nature really does build them.
The discovery quietly shifted the question from "Do such worlds exist?" toward "How common are they?"
5. 🌙 Ganymede has its own magnetic field
Jupiter's Ganymede is already unusual: it's the largest moon in the Solar System, bigger than the planet Mercury.
But NASA's Galileo spacecraft discovered something even stranger. Ganymede generates its own intrinsic magnetic field. No other moon is currently known to have one.
Producing a global magnetic field generally requires electrically conductive material churning inside an object, which implies that Ganymede still has an active metallic core, despite being a moon.
Its field interacts with Jupiter's enormous magnetosphere, creating a miniature magnetosphere nested inside a vastly larger one.
Evidence also suggests Ganymede holds a deep subsurface ocean, possibly arranged in several layers separated by exotic high-pressure forms of ice.
It's a moon with a metallic dynamo, a hidden ocean and a magnetic personality of its own.
6. 🌌 The Sombrero Galaxy has an unexpectedly enormous halo
The Sombrero Galaxy, M104, looks almost artificial in photographs: a brilliant central bulge wrapped by a razor-thin dark lane of dust.
For decades, astronomers mostly treated it as an unusual spiral galaxy seen nearly edge-on. But deeper observations revealed a more complicated structure. The Sombrero has an enormous extended halo full of surprisingly metal-rich stars, which is hard to explain with a simple story of quiet, isolated evolution. It may have gone through major mergers in its past.
Its central supermassive black hole is also extraordinarily massive, at roughly a billion Suns.
So the familiar telescope image shows only the luminous centerpiece. Like many galaxies, the Sombrero extends far beyond the part our eyes naturally notice.
7. 💥 A supernova can briefly outshine its entire galaxy
A normal star shines by releasing nuclear energy steadily. A supernova releases an enormous reservoir of energy over a dramatically shorter time.
Events such as SN 2016aps, one of the most energetic supernova candidates studied, show how extreme these explosions can get. For weeks or months, a single dying star can rival or exceed the visible light of the billions of ordinary stars in its host galaxy.
Some exceptionally luminous explosions may get extra power when fast-moving debris slams into huge shells of gas the star threw off earlier. Others may involve different mechanisms.
The scale is hard to take in. A galaxy is a vast city of billions of stars. And occasionally one star dies loudly enough to compete with all of them.
8. 🌊 Callisto may hide an ocean beneath an ancient surface
Jupiter's moon Callisto has one of the oldest, most heavily cratered surfaces in the Solar System. At first glance, it looks geologically dead.
But the Galileo spacecraft detected magnetic behavior consistent with a conductive layer beneath the surface. The leading explanation is a salty subsurface ocean.
Unlike its neighbor Europa, Callisto gets relatively little tidal heating, so keeping water liquid there requires a different thermal balance, and possibly antifreeze-like substances such as ammonia.
Its battered exterior may hide a radically different environment kilometers below. Callisto is a reminder that a planetary surface can be deeply misleading.
A frozen, cratered wilderness could be hiding an entire planetary-scale ocean in darkness.
9. 🔭 Hubble's Deep Field found thousands of galaxies in an apparently empty patch of sky
In 1995, astronomers pointed the Hubble Space Telescope at a tiny, seemingly unremarkable patch of sky near the Big Dipper. Then they kept staring for days.
The resulting Hubble Deep Field revealed roughly 3,000 galaxies in a patch of sky about as small as a grain of sand held at arm's length. Many were extremely distant, so we're seeing them as they were billions of years ago. Later deep fields went farther still.
The philosophical impact was enormous. Astronomers had deliberately chosen a patch containing almost nothing obvious, and discovered that the "nothing" was full of galaxies.
A blank-looking piece of darkness is often blank only because our eyes aren't sensitive enough to see what's there.
10. 🌋 Venus may still be volcanically active
Venus has more volcanoes than almost any world we know, but for decades astronomers weren't sure whether any were still erupting.
Then researchers re-examined radar images taken by NASA's Magellan spacecraft in the early 1990s. They spotted a volcanic vent near Maat Mons that appeared to change shape and grow between observations several months apart. The simplest explanation is volcanic activity, and further analyses have found more signs of recent surface changes.
So Venus may not be geologically dead at all. Beneath its crushing carbon-dioxide atmosphere and clouds of sulfuric acid, magma may still be reshaping the landscape today.
Our nearest planetary neighbor could have active volcanoes hidden permanently beneath opaque clouds.
11. 🌀 Quasars can launch jets longer than entire galaxies
A feeding supermassive black hole can create an active galactic nucleus, and some of these produce colossal jets moving at nearly the speed of light.
The jets come from the region around the black hole, not from inside the event horizon, and magnetic fields probably play a crucial role in extracting and channeling the energy.
In extreme radio galaxies and quasars, these jets can stretch for millions of light-years. Recent radio surveys have found systems whose combined jets span distances far beyond the diameter of the Milky Way; the record holder, nicknamed Porphyrion and reported in 2024, measures about 23 million light-years end to end. The particles inside them move close to the speed of light.
The engine powering all of this occupies a region tiny compared with what it builds. A black hole's immediate surroundings can shape intergalactic space on scales millions or even billions of times larger than the event horizon itself.
12. 🧊 Triton probably began life somewhere else
Neptune's largest moon, Triton, orbits backward relative to Neptune's rotation, and that retrograde orbit is an enormous clue.
A large moon forming naturally from a disk around Neptune should orbit in the same general direction that Neptune spins. Triton's path instead strongly suggests it formed independently, probably in the Kuiper Belt, and was later captured by Neptune's gravity.
Voyager 2 found another surprise in 1989: dark plumes rising kilometers above Triton's icy surface, probably powered by sunlight warming volatile ices.
So Triton is most likely an immigrant: a dwarf-planet-like body that once traveled around the Sun on its own, until Neptune captured it and made it a moon.
13. 🧲 Pulsars can be used as a galaxy-sized gravitational-wave detector
LIGO detects relatively high-frequency gravitational waves using kilometer-scale instruments on Earth. For much slower waves, astronomers use something vastly larger: the Milky Way itself.
Networks called pulsar timing arrays monitor millisecond pulsars scattered across thousands of light-years. Because their radio pulses arrive with extraordinary regularity, tiny correlated deviations in arrival times can reveal spacetime being stretched by extremely long-wavelength gravitational waves.
In 2023, several collaborations reported compelling evidence for a nanohertz gravitational-wave background. The leading explanation is the combined murmur of many pairs of supermassive black holes orbiting each other across the universe, though the details are still being studied.
We've effectively turned dead stars into clocks and arranged those clocks into a galactic gravitational-wave observatory.
14. ☀️ Solar neutrinos proved that particles can change identity
For decades, experiments detecting neutrinos from the Sun found only about a third to a half of the expected number. Either our understanding of the Sun was wrong, or something stranger was going on.
Eventually, experiments including Super-Kamiokande and the Sudbury Neutrino Observatory showed the answer: neutrinos can switch between different types, or flavors, while they travel.
The early solar experiments were mainly sensitive to electron neutrinos, so neutrinos that had turned into muon and tau flavors appeared to be missing.
This phenomenon, neutrino oscillation, also means neutrinos have mass, contrary to the simplest version of the Standard Model of particle physics.
A mystery that began deep inside the Sun ended up revealing new fundamental physics about matter itself.
15. 🪐 HD 80606 b has one of the wildest planetary orbits known
Most planets follow roughly circular or moderately elliptical paths. HD 80606 b takes eccentricity to the extreme.
Its orbit is a long, stretched ellipse. During its roughly 111-day year, the giant planet swings from relatively far out to dramatically close to its star in only a short time.
At closest approach, the amount of starlight hitting the planet soars. Infrared observations have watched its atmosphere heat up by hundreds of degrees in a matter of hours as it swings in toward the star.
Instead of seasons driven mainly by axial tilt, HD 80606 b goes through something closer to a recurring planet-wide blast furnace, courtesy of its orbit.
A year there includes a regular atmospheric shock treatment.
16. 🌠 The Geminid meteor shower comes from an asteroid-like object, not a normal comet
Most major meteor showers happen when Earth crosses dusty debris left behind by comets. The Geminids are different.
Their parent body is 3200 Phaethon, officially classified as an asteroid, though it shows some comet-like behavior when it passes close to the Sun.
Every December, Earth plows through material linked to Phaethon, producing one of the year's richest meteor showers.
Exactly how Phaethon makes and replenishes its debris is still being researched. Heating near its closest approach to the Sun may crack its rocky surface, and sodium loss and other mechanisms have also been investigated.
The Geminids remind us that the neat textbook line between "asteroid" and "comet" isn't always good enough. Nature doesn't have to respect the categories we invented.
17. 🌌 There may be galaxies almost entirely without stars
Astronomers have searched for dark galaxies: dark-matter halos that hold gas but very few stars, or perhaps essentially none.
Unambiguous examples are hard to establish, because something that gives off radio emission without visible starlight can have other explanations.
One intriguing class consists of clouds of neutral hydrogen detected through their 21-centimeter radio emission, with extraordinarily faint or missing optical counterparts. Some of these candidates may be failed galaxies whose gas never collapsed efficiently into stars.
The broader idea matters because cosmological simulations predict far more small dark-matter halos than we see as obvious, glowing dwarf galaxies. There may be galactic structures all around us that barely count as visible objects at all.
A galaxy doesn't necessarily need billions of shining stars to make its gravity felt.
18. 🌕 The Moon is slowly drifting away from Earth
The Moon is moving away from Earth by roughly 3.8 centimeters per year.
We know this extraordinarily precisely because Apollo astronauts and Soviet robotic missions left retroreflectors on the lunar surface. Scientists fire lasers at them and time the round trip.
The drift happens because tides transfer rotational angular momentum from Earth to the Moon's orbit. As a result, Earth's rotation is gradually slowing too. Hundreds of millions of years ago, days were shorter and the Moon was closer.
Far in the future, total solar eclipses like the ones we see today will stop happening, because the Moon will look too small to cover the Sun completely.
Every human lifetime takes place during a tiny instant of a billions-of-years-long gravitational dance.
19. 🟤 Brown dwarfs have clouds made of minerals
Brown dwarfs occupy the strange territory between giant planets and stars. They form the way stars do but lack the mass to sustain ordinary hydrogen fusion in their cores.
Some have atmospheres cool enough for substances such as silicates and iron to condense into clouds, which means their weather can involve mineral particles.
As brown dwarfs rotate, patchy clouds cause measurable changes in brightness, letting astronomers map atmospheric structure on objects many light-years away. The cooler ones even show molecules such as water and methane.
They're not quite stars and not quite planets. And their skies may hold storms made from vaporized rock and metal.
20. 🌀 The Cartwheel Galaxy records the aftermath of a cosmic collision
The Cartwheel Galaxy looks like a gigantic glowing wheel.
Its spectacular ring probably formed when another galaxy plunged through or near the center of a disk galaxy hundreds of millions of years ago. The gravitational jolt sent a density wave rippling outward, a bit like ripples spreading across water after a stone drops in. As the wave swept through gas, it triggered intense star formation, producing the bright outer ring.
The collision didn't simply smash the galaxy apart. Gravity reorganized it into a new structure spanning well over 100,000 light-years.
Galaxies are surprisingly resilient. Even catastrophic collisions can become engines for whole new generations of stars.
21. 🧊 Mercury has water ice despite being closest to the Sun
Mercury's daytime surface can top 400°C. Yet spacecraft observations have confirmed deposits of water ice near its poles.
The trick is geography. Mercury's axis is barely tilted, so the floors of some deep polar craters never get direct sunlight. These permanently shadowed regions stay cold enough for ice to survive for extremely long periods.
Radar observations from Earth first hinted at the deposits, and NASA's MESSENGER spacecraft later provided strong evidence. Some of the ice is even covered by a dark layer of carbon-rich material that helps insulate it.
So the planet closest to the Sun has places that have stayed in permanent night for billions of years.
22. 🕳️ Stars can be ripped apart by black holes
If a star strays too close to a massive black hole, gravity on its near side becomes dramatically stronger than on its far side. Eventually those tidal forces can overwhelm the star's own gravity.
The result is a tidal disruption event. The star is stretched and torn apart. Some of its material escapes, while the rest forms an extremely hot stream around the black hole, producing a luminous flare that can be seen across enormous distances.
Astronomers now find these events routinely in wide-field sky surveys, and they offer rare chances to study supermassive black holes that would otherwise be quiet.
The unfortunate star ends up lighting up its destroyer. A black hole that was almost invisible can suddenly announce itself because a star wandered too close.
23. 🧲 Earth's magnetic poles really do reverse
Geological records show that Earth's magnetic field has flipped polarity hundreds of times. During a reversal, magnetic north and south swap places.
We know this partly because molten volcanic rock records the direction of Earth's magnetic field as it cools. Stripes of alternating magnetic orientation on the ocean floor became crucial evidence for both magnetic reversals and plate tectonics.
The last full reversal, the Brunhes–Matuyama reversal, happened about 780,000 years ago. Reversals are irregular, and there's no reliable schedule for the next one.
A reversal also doesn't mean the field simply switches off. Instead, the geodynamo goes through a complicated transition lasting thousands of years.
Our planet carries a magnetic history written directly into its rocks.
24. 🌋 Olympus Mons towers far above Mount Everest
Mars is home to Olympus Mons, the largest known volcano in the Solar System. It rises roughly 20-plus kilometers above the surrounding plains and spans around 600 kilometers.
It could grow so large partly because Mars lacks Earth's active plate tectonics. On Earth, a tectonic plate slides over a relatively stationary volcanic hotspot, creating chains of islands such as Hawaii. On Mars, the crust could stay parked over a volcanic source for immense periods, letting lava flow after lava flow pile up in one place.
Mars's lower gravity also lets mountains grow bigger before their own weight becomes a structural limit.
Olympus Mons is so broad that if you stood on its lower slopes, you might not recognize it as a mountain at all. Its summit could lie beyond your local horizon.
25. 🌌 Galaxy clusters contain gas hotter than the stars inside them
Look at a galaxy cluster in visible light and you see hundreds or thousands of galaxies. Look in X-rays and another structure appears.
The space between those galaxies holds enormous amounts of intracluster gas heated to tens of millions of degrees.
Its individual particles are incredibly sparse, so a spacecraft wouldn't experience it like an earthly furnace. But taken together, this plasma holds an enormous amount of ordinary matter, often more than all the cluster's stars combined.
Its heat comes largely from gravitational energy released as matter falls into the cluster's deep gravitational well.
So a galaxy cluster isn't just a collection of galaxies. Those galaxies are embedded in a gigantic, nearly invisible ocean of superheated plasma.
26. ☄️ Shoemaker–Levy 9 let us watch a planetary collision
In 1994, fragments of Comet Shoemaker–Levy 9 slammed into Jupiter. Astronomers knew the impacts were coming and had telescopes ready.
One after another, the fragments struck Jupiter at roughly 60 kilometers per second, throwing up enormous plumes and leaving dark scars in the atmosphere, some larger than Earth.
It was the first time we had directly watched a collision between Solar System bodies on that scale. The event showed dramatically that major impacts aren't just relics of ancient planetary history. The Solar System is still dynamically active, and large objects still collide.
For one extraordinary week, astronomers watched celestial mechanics turn into planetary violence in real time.
27. ⭐ Some stars pulse so predictably that they became cosmic rulers
Cepheid variable stars rhythmically expand and contract. More importantly, the astronomer Henrietta Swan Leavitt discovered that the period of a Cepheid's pulsation is tied to its true luminosity.
Measure how quickly it pulses, and you can work out how bright it really is. Compare that with how faint it looks, and you can estimate its distance.
This period–luminosity relationship became one of astronomy's foundational tools for measuring distance. Edwin Hubble later used Cepheids in the Andromeda "nebula" to show that Andromeda lies far outside the Milky Way, helping establish that our galaxy is just one among many.
Stars that literally brighten and fade became measuring sticks long enough to reveal the scale of the universe.
28. 🛰️ Voyager 1 crossed into interstellar space
Launched in 1977, Voyager 1 flew past Jupiter and Saturn and kept heading outward.
In 2012, its measurements showed that it had crossed the heliopause, the boundary where the solar wind's dominance gives way to the interstellar environment. That made Voyager 1 the first human-made spacecraft to enter interstellar space.
That doesn't mean it has left the Solar System by every definition. The distant Oort Cloud extends vastly farther.
Its radio transmitter puts out only about 20 watts, yet huge antennas on Earth can still pick up its extraordinarily faint signal from nearly a light-day away, a milestone it reaches later this year.
A machine launched when Jimmy Carter was U.S. president is now sampling the material between the stars.
29. 🕳️ Black holes can steal material without immediately swallowing it
In some binary systems, a black hole orbits close enough to a normal star to pull gas away from it. But that gas usually can't fall straight into the event horizon, because it carries angular momentum. Instead, it forms an accretion disk.
Friction, turbulence and magnetic processes gradually move angular momentum outward, letting matter spiral inward. Along the way, gravitational energy turns into heat. Accretion disks around black holes can reach millions of degrees and pour out X-rays.
So, counterintuitively, a black hole can be surrounded by some of the brightest matter in the universe.
The black hole itself stays dark. Its dinner is what shines.
30. 🌌 The universe may eventually enter an era with no shining stars
Stars can't form forever. They need cold gas, and the universe's usable supply is gradually being locked up in stellar remnants, blown out, heated, or otherwise made unavailable for new stars.
Far in the future, the last long-lived red dwarfs should run out of fuel. If our current understanding is broadly right, the cosmos will then enter what physicists call the Degenerate Era, dominated by remnants such as white dwarfs, neutron stars and black holes.
Over even longer times, the black holes themselves should evaporate through Hawking radiation, assuming the theory holds as expected. What's left after that depends on unresolved questions, such as whether protons are stable and the ultimate nature of dark energy.
But one conclusion is remarkably robust: the brilliant, star-filled universe we live in is temporary.
We happen to be here during the comparatively brief cosmic epoch when the universe is full of light.
🌙 One last thing before sleep
A photon leaving the Sun takes about eight minutes to reach Earth. A photon from the nearest stars takes years. A photon from Andromeda travels about 2.5 million years.
And some of the photons entering astronomical telescopes tonight began their journeys more than 13 billion years ago.
They crossed expanding space, passed galaxies that no longer look the same, survived billions of years without being absorbed, and finally ended their journeys inside a detector built by a species that evolved on one small planet.
Astronomy is strange that way. Almost everything we know about the distant universe comes from catching ancient messengers that happened to arrive tonight.
That's all for tonight. Thanks for staying up with me again. Rest well, and clear skies. 🌌
— Kasi