WEBVTT

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sleep at time facts, the silent cosmos, let your shoulders lower, and your breathing, find a

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slow rhythm, the day can remain where it is, while you rest here, in a quiet room, beyond the windows.

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The universe is mostly hush, sound needs matter to carry it, and most of space is too empty

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for ordinary sound to travel, in that great stillness. Stars and planets move without noise,

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reaching across the dark. That silence is spacious. There is nowhere else. You need to be right now,

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as you breathe out. Imagine your thoughts loosening their grip. One by one, your body can soften

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into the bed, supported by the surface beneath you, the cosmos asks nothing of you tonight. It's simply

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continues in quiet, while rest comes closer, let the hush be enough, far beyond the familiar stars

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of the northern sky. In the direction of the constellation boodies, there is a place where the usual

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crowd of galaxies, thinns, almost to silence, it is not a hole in space, and it is not a region where

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the laws of physics fail, it is an enormous volume of the universe, or matter, is spread much more thinly,

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than average astronomers call such places voids, and this one is among the most widely known.

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The scale is difficult to hold in mind the region spans hundreds of millions of light years,

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if a traveler could move from one side to the other at the speed of light. The journey would take

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hundreds of millions of years in a volume that large ordinary expectation would place many thousands

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of bright galaxies, here the count is far lower, the galaxies that do exist, sit at great distances,

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from one another, like lanterns placed along a very wide plane, the void was recognized,

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while astronomers were mapping the positions of galaxies in three dimensions, a telescope can show

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where a galaxy appears on the sky, but distance requires another measurement by studying the light

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from each galaxy, and measuring how its wavelengths are stretched. Astronomers can estimate how far away

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it is when many such distances are gathered. The flat pattern of the night sky becomes a deep map

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in that map. Galaxies gather into sheets and filaments between them, live broad regions,

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with fewer galaxies, the region toward Baudes stood out. Because of its size, this place is not

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completely bare, space within it. Still contains the same basic ingredients found elsewhere.

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There is dark matter. The less densely packed, there is very thin gas. There are also galaxies.

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Just widely separated, some are small and faint, others appear to have formed stars slowly.

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With little disturbance from neighbors, their isolation gives them a different character from galaxies

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in crowded clusters, where encounters and mergers are more common, the quietness of the void

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comes from the way structure grew in the universe long ago. Matter was distributed almost evenly,

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but not perfectly tiny differences in density were present everywhere over immense spans of time.

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Gravity made those differences more pronounced, regions with slightly more matter,

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pulled in material from their surroundings. They became denser, forming the filaments and walls

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that now outline the cosmic web, regions with slightly less matter, did the opposite,

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as matter drifted outward toward the denser boundaries. The interiors became quieter,

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and more open, in a void, expansion has a clearer presence in denser places.

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Gravity binds galaxies together, and slows the stretching of space on local scales inside a large

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under dense region. There is less mass to resist the general expansion. The void does not push

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outward with force, it simply becomes a little more spacious over time. While the surrounding walls

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remain more closely gathered, the motion is slow, measured in the patient language of cosmology,

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rather than in human events, the shape of the region is often described as broadly rounded.

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Yet, it is not a perfect sphere, voids can grow by joining with one another,

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and their boundaries can be uneven, what appears as one great quiet volume.

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May contain smaller low-density pockets, within it, around the edges. Galaxies trace, faint walls,

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and threads, these boundaries, are where the void meets the more populated universe,

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they are gentle borders, not sharp walls, and they guide the slow flow of matter over cosmic time,

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studying such a place, helps astronomers understand how environment, shapes galaxies,

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a galaxy inside a sparse region, evolves, with fewer close encounters, it may keep its gas

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longer, form stars at a steadier pace, or remain small, because it has little material together,

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comparing these galaxies, with those intense clusters, shows how much surroundings matter,

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the void offers a calm extreme, a place where the usual interactions of galactic life

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happen rarely, there is also a simple beauty in the arrangement, the universe is often pictured as a web,

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with bright nodes and connecting strands, the voids are the spaces between those strands

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they give the web its shape without the low-density regions, the filaments would not stand out,

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so clearly the silence of the void is part of the pattern, not an absence of meaning,

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light crossing this region carries information about distance and history, because galaxies are so far

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apart, the light from one may travel a long way before it passes near another, the sky seen

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from within the void would be different from the sky seen from our own neighborhood, fewer bright galaxies

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would be visible, and the night would be dominated by the local stars of any galaxy present,

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the wider universe would appear more distant, its lights separated by wide intervals,

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none of this suggests a place of loss, the void is not a ruin, or a missing piece,

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it is a natural outcome of gravity, working on small differences, over great time, matter has not

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vanished, it is gathered elsewhere, leaving a broad interior, where stillness is the ordinary condition,

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the galaxies within it are not stranded, they follow the same motions that govern all matter,

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moving gently through a space that has room to spare on human timescales, the void changes

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almost imperceptibly, stars within its few galaxies, continue their long lives, gas drifts,

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gravity works slowly, the distances are so large, that even the motions of entire galaxies appear

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settled, this is one of the comest large-scale places known in the cosmos, a region where the universe

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shows how much can be made from patience and distance, guided by the quiet pool of gravity,

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every direction in the night sky, holds a faint micro wave glow, it is not made by stars,

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and it does not gather into bright points, it lies smoothly across the heavens,

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so evenly that early measurements could barely tell one patch from another, this radiation,

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is the coold remnant of a time when the universe was much younger, and far simpler,

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it has traveled for an immense span of time, stretching as space expanded, until it became the

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soft whisper that modern instruments can detect, the story begins when the cosmos was hot,

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and dense enough to behave like a glowing plasma in that early state, light could not travel freely,

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photons were constantly scattered by free electrons, much as sunlight is scattered inside a thick cloud,

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the universe was opaque, filled with a bright fog as expansion continued, the temperature fell,

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electrons and protons settled into neutral atoms, and the fog cleared for the first time,

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light could move outward without constant interruption, the photons released, then have been traveling

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ever since, that first free light was once very warm over billions of years,

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the expansion of space stretched its wavelength, visible, and infrared radiation,

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gradually shifted into microwaves, today the glow has a temperature of about 2.7 Kelvin,

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a little under 3 degrees above absolute zero, it remains a quiet thermal trace,

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a low home of energy spread through all of space, one of the most reassuring things about this radiation,

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is its spectrum, it follows the smooth curve of a nearly perfect black body, a black body spectrum,

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is what physicists expect from light that once shared a common temperature with matter,

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exchanging energy until everything settled into balance, the cosmic glow matches that expectation,

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with unusual precision that match tells us the early universe was once in a state close to thermal

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equilibrium, it also shows that the radiation has traveled through space, without being reheated,

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or scrambled by later events, the glow is almost the same in every direction, but not perfectly so,

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sensitive maps reveal tiny differences in temperature, variations of only a few parts in 100,000,

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some regions appear very slightly warmer, others appear very slightly cooler, these small differences,

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our natural imprints of early density variations were matter was a little more concentrated,

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gravity had a little more material to work with over long ages, those gentle contrasts,

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eventually helped guide the formation of galaxies and clusters, though the background itself

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comes from a time long before any star ignited, studying these patterns requires patience,

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the signal is weak, and the sky contains other sources of microwave light dust in our own galaxy,

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can glow faintly, electrons moving through magnetic fields, can produce microwave emission,

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instruments must observe the sky at many frequencies, so these foreground contributions

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can be separated from the deeper background, the process is gradual, researchers,

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compare maps, and check calibrations, they also look for patterns that remain stable across different

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methods the result is a cleaner view of the ancient signal, the background also carries information

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about motion because the solar system moves through space, the glow appears slightly warmer

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in the direction we are heading, and slightly cooler behind us, this effect is called a dipole,

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it is similar to the way rain, seems to come more directly, at a moving traveler, once this motion is

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accounted for, the underlying sky becomes more symmetrical, the dipole is a useful reminder

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that the radiation is not an abstract idea, it is a physical field that can be measured from within a

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moving galaxy, another reason the glow matters is that it sets a limit on how far back ordinary light

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can see before the fog cleared, photons were trapped in the plasma telescopes that collect visible

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light, cannot look past the first stars, and even infrared instruments eventually reach a boundary,

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the microwave background marks the edge of that older luminous era, it is the earliest light we can

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observe directly, released when the universe became transparent beyond it lie earlier conditions

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that scientists infer through theory, and through the patterns left in the glow itself, those patterns

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have allowed astronomers to measure broad features of the cosmos, with care, the spacing of the

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warm and cool spots, reflect sound waves that moved through the early plasma by studying the

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angular size of the spots, researchers can estimate how space has expanded since the light was released,

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the overall smoothness of the background supports the idea that the universe was once much more

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uniform than it is today, the tiny ripples within it provide a starting point for the slow growth of

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structure, the record is one of gradual change, written in temperature differences to small to

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feel the radiation also helps scientists understand the contents of the universe in a general way,

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the way the patterns formed depends on ordinary matter and dark matter, it also depends on the

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expansion rate by comparing the observed pattern with models, researchers can estimate proportions

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and test whether the same description works across the sky, this proceeds as a slow fitting of evidence,

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like adjusting a map until the coastline's match, the background offers a stable reference

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because it comes from a time when the universe was simple enough for the physics to be clear, modern studies

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continue to refine the picture, better detectors, can measure the same sky with less noise, independent

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teams can compare their results, without needing to assume anyone instrument is perfect, the glow

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remains a steady reference point, it offers a clear view of a time when the universe was young,

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and transparent enough for light to travel that same simplicity. Let's researchers read the first

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patterns with care, far out in the deep sky. Two great clusters of galaxies have been moving

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through one another for a very long time, a cluster is not a crowded place, in the ordinary sense,

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the galaxies inside it are separated by wide stretches of space, and the stars within each

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galaxy are even more widely spaced, what fills the space between galaxies is a thin hot gas,

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so diffuse that it would feel like almost nothing to a hand past through it, around and through all of

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this lies and unseen mass that does not glow, does not absorb light, and does not behave like

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the gas, the encounter lets these different components be seen separately. As if a slow current had

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sorted them, the hot gas is ordinary matter, mostly hydrogen and helium, heated until it shines in

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x-rays in a quiet cluster, this gas can form a soft rounded glow near the center when two clusters

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pass through one another. Their gas clouds meet, they do not meet, like solid objects,

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they drift together, press gently and slow, the particles in the gas interact with one another,

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so they feel one another, some of their motion turns into warmth, and the gas lingers near the middle

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of the encounter, it becomes a broad, bright region, that telescopes can map the galaxies behave

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differently, each galaxy is mostly empty space, stars are far apart compared with their sizes,

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and galaxies themselves are held together by gravity as the clusters pass through. The galaxies

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glide onward, they may be nudged slightly, but they rarely touch, their motion carries them

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forward, leaving the slowed gas behind, in this way, the visible galaxies, and the hot gas begin

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to part company, one part of the system moves ahead, while the other rests more centrally,

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the unseen mass moves even more quietly, it does not shine, and it does not seem to slow in the way

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the gas does, it passes through the encounter, with little resistance, the result is a gentle separation,

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the hot gas gathers in a broad region between the two groups of galaxies, while most of the mass

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travels onward with the galaxies, the pattern is subtle, but it is clear when the right kind of

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observation is made, the evidence comes from the way gravity bends light, light from galaxies far behind

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the cluster, follows the curvature of space caused by the mass in front of it, the foreground mass acts

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like a very soft lens, it can stretch, and distort the images of background galaxies,

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by tiny amounts, no single background galaxy, gives the whole answer many of them are studied together,

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and their slight shaped changes are used to draw a map of where the mass lies, this method does not

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depend on the mass being bright, it responds to gravity itself, astronomers call this gravitational

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lensing when such maps were made for this encounter, the mass did not sit, where the hot gas was

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brightest, the greatest concentrations of mass, laying near the groups of galaxies, on either side

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of the central gas, the gas, though it contained a great deal of ordinary matter,

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was not, where most of the gravity was found, the separation was broad and calm, like two crowds,

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moving past one another, while a mist remains between them, the picture showed that something unseen

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had moved ahead, this arrangement gives a clean way to think about what the mass might be,

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if the extra gravity came only from the hot gas, the lensing map would follow the x-ray glow,

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it does not, if gravity were tied only to the matter that shines, the offset would need a careful

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explanation, the simplest account, is that there is matter that does not interact strongly

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with light or with itself, except through gravity that matter is what astronomers call dark matter,

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the encounter makes its presence easier to trace, because the ordinary gas has been left behind,

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the time scale is long enough, that nothing about it feels hurried, the passage of one cluster,

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through another, takes hundreds of millions of years, during that span, the gas warms and spreads,

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and the galaxies continue along their paths, the stars do not strike one another, the distances

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are too great, instead the whole event unfolds like a slow tide, gravity guides the motion,

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and the light from distant background galaxies records the shape of that motion, the hot gas itself

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tells a story of ordinary matter, under gentle pressure, as the two gas clouds met, they were compressed

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and heated, yet they remained thin by everyday standards, they glow softly in x-rays,

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and their shape shows how they were slowed, the galaxies meanwhile, keep the memory of the passage

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in their positions and motions, the invisible mass, adds the clearest part of the story,

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because it shows where gravity is strongest, without needing to shine, to see the whole system,

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astronomers combine different views, one view records the x-ray glow of the hot gas,

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another measures the tiny distortions of background galaxies placed together with the visible galaxies,

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these views show that the gas rests while the galaxies, and most of the mass drift forward,

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the result, is a quiet demonstration of matter that cannot be seen directly, but can be measured

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by its effect on light, this kind of encounter, also helps show why clusters hold together,

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the amount of mass needed to bend the background light, is larger than the mass in stars

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and gas alone, the unseen mass gives the cluster its gravitational shape, without it, the galaxies

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would not move as they do, and the cluster would not remain bound in the way observed, the encounter

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does not disturb this conclusion, it makes the hidden mass easier to locate, there is no sudden event here,

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no sharp moment of contact, the clusters are so large, that their meeting is measured in cosmic time,

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light from the background bends around them, in silence, the hot gas settles, and shifts very slowly,

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the galaxies continue on paths that will carry them farther apart, or into a new arrangement

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through all of this, the lensing map remains a quiet record of mass, showing that much of the universe

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is present without being visible, a soft blur of light, hangs below the three stars of Orion's belt,

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in the place, often drawn as the hunter's sword, to an unated eye, it looks like a small cloud with steady edges,

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pale against the dark, this soft patch, is a nearby region, where interstellar gas is gathered into stars, its light has traveled a little

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over a thousand years to reach us, the glow we see left the nebula long before modern instruments existed,

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yet the object itself changes so slowly that a human lifetime sees almost no alteration in its form,

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its apparent smallness is a matter of distance, the nebula spans tens of light years,

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though it covers only a small angle in the sky, telescopes reveal that its bright core is surrounded by fainter wings

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of scattered light, much of the surrounding glow comes from dust, catching starlight,

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while the central glow comes from gas, warmed by young stars, seen from earth,

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the bright part resembles a shallow bowl, with softer material spilling outward,

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the nebula belongs to a much larger cloud of molecular gas, and dust that fills much of the

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constellation Orion in such clouds, hydrogen usually exists as pairs of atoms bound together,

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and the material is cold by everyday standards, dust grains, similar in scale to find smoke,

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float between the gas, they block distant starlight, and help the interior stay cool, cool gas moves slowly,

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it can collect in broad sheets and thin filaments, drifting with motions left over from the clouds,

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earlier history, the cloud is not uniform, it contains dense cores, and quieter envelopes,

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along with long thread-like structures, some parts are so thick that visible starlight cannot pass

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through them, astronomers call these obscured places dark, nebulae, when they are seen against

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brighter backgrounds, in Orion, dark lanes trace channels of dust across the glowing gas,

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and mark regions where future stars may still be forming star formation begins,

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when a patch of this material becomes a slightly denser than its surroundings, gravity pulls matter

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inward, and the pool grows as more mass gathers, the process is slow, a clump may require

00:30:16.000 --> 00:30:24.960
long ages to contract by the width of a planetary system, pressure, and magnetic fields resist the

00:30:24.960 --> 00:30:32.720
pool, while slow eddies in the gas add support, because the core can radiate warmth away,

00:30:32.720 --> 00:30:41.680
it can continue settling, without building too much pressure, the gas does not fall straight inward,

00:30:41.680 --> 00:30:49.200
it turns as it moves, forming flattened disks around dense cores within those cores,

00:30:49.920 --> 00:30:59.040
hidden inside dust, protostars take shape, a protostar is not yet shining, by the steady fusion,

00:30:59.760 --> 00:31:05.440
the defines a mature star, it glows first from the warmth of contraction,

00:31:06.560 --> 00:31:14.640
releasing energy, as matter settles onto it, much of this light is infrared, which passes through

00:31:14.640 --> 00:31:24.160
dust more easily than visible light, observations at infrared wavelengths, have allowed astronomers

00:31:24.960 --> 00:31:32.720
to find many young stars inside the nebula that would otherwise remain concealed,

00:31:32.720 --> 00:31:42.560
radio measurements add another view, tracing cool molecules, and dense gas that optical telescopes

00:31:42.560 --> 00:31:51.360
cannot see the visible nebula, owes much of its brightness to a small group of young stars near its

00:31:52.240 --> 00:32:00.320
heart, these stars are known as the trapezium, from the foresighted pattern they make in telescopes,

00:32:00.320 --> 00:32:07.760
their light is energetic enough to excite the surrounding hydrogen when electrons in the gas

00:32:07.760 --> 00:32:17.440
recombine with atoms, they emit light at characteristic colors, giving the nebula soft, reds,

00:32:17.440 --> 00:32:25.680
and greens in long photographs, other elements contribute fainter hues, oxygen can add a

00:32:25.680 --> 00:32:33.840
greenish tone, while nitrogen and hydrogen help shape the deeper reds in small telescopes,

00:32:34.560 --> 00:32:43.360
the nebula often appears gray or greenish to human eyes, because our vision is less sensitive

00:32:43.360 --> 00:32:51.360
to color, in faint light, long exposures collect more photons, and reveal stronger reds,

00:32:51.360 --> 00:32:59.360
this difference between direct view and photograph is not a flaw in the nebula it follows from the

00:32:59.360 --> 00:33:07.600
way eyes, and instruments gather light, the same object can therefore seem quiet at the eyepiece,

00:33:07.760 --> 00:33:17.440
and richly colored in an image made over many minutes, this illumination also shapes the cloud,

00:33:17.440 --> 00:33:25.840
starlight presses on dust and gas, and stellar winds carry particles outward over long ages,

00:33:26.720 --> 00:33:35.120
these influences carve broad hollows in the nebula, and leave denser knots standing in relief,

00:33:35.120 --> 00:33:42.560
dark silhouettes, sometimes called pillars, our places where material remains thicker,

00:33:43.760 --> 00:33:52.320
shielding cooler gas behind it at the edges, gentle compression can encourage new clumps to form

00:33:52.320 --> 00:34:01.360
the same stars that reveal the nebula, by lighting, it also guides slow rearrangement, astronomers

00:34:01.440 --> 00:34:09.600
worked out much of this story, by comparing different kinds of light visible images show the glowing surface,

00:34:10.720 --> 00:34:17.120
where ultraviolet radiation reaches the gas, spectra spread that light into bands,

00:34:18.080 --> 00:34:25.200
revealing which atoms are present, and how the gas moves, small shifts in spectral lines,

00:34:26.080 --> 00:34:34.800
show slow flows, and expanding shells, infrared images, look deeper into the dusty interior,

00:34:34.800 --> 00:34:44.240
radio telescopes, map cold molecules across the wider cloud taken together, these methods turn a

00:34:44.240 --> 00:34:53.280
soft patch of light into a place with depth and motion, and they give a sense of its age,

00:34:53.360 --> 00:35:01.680
the gas is not still, it expands slowly from the central region, and denser knots move at their

00:35:01.680 --> 00:35:09.200
own pace, measurements of these motions help estimate how long the nebula has been illuminated,

00:35:10.720 --> 00:35:18.480
and how quickly its outline may change, the changes are slow compared with a human calendar,

00:35:18.480 --> 00:35:24.000
a telescope image made tonight will look much like one made decades ago,

00:35:25.360 --> 00:35:33.360
though careful comparison can detect subtle differences in brightness and flow around some of the

00:35:33.360 --> 00:35:42.800
youngest stars, the nebula contains disks of gas and dust, these disks are the places where planets

00:35:42.880 --> 00:35:52.320
may later assemble, grain by grain in Orion, bright ultraviolet light can make such disks glow

00:35:52.320 --> 00:36:00.400
at their edges, and slowly disperse their outer material, some appear as small bright knots,

00:36:01.440 --> 00:36:07.920
with tails pointing away from the central stars, these features are often called droplets,

00:36:08.880 --> 00:36:16.400
a short term for protoplanetary disks, their shapes record the direction of nearby starlight,

00:36:17.200 --> 00:36:26.240
and the flow of gas, the nebula, is a passing stage in the life of a cloud, some of its new stars

00:36:26.240 --> 00:36:34.000
will remain near one another for a long time, while others will drift outward into the galaxy,

00:36:34.000 --> 00:36:42.400
the gas that does not become stars, will be warmed, and moved before it gradually disperses

00:36:42.400 --> 00:36:50.720
in later ages, the region may contain only a loose cluster of stars, where a glowing cloud

00:36:50.720 --> 00:36:59.200
once stood, the light that leaves it tonight will continue across space, long after the current arrangement

00:36:59.200 --> 00:37:08.480
of gas has changed, carrying information from a place where stars are still becoming far out

00:37:08.480 --> 00:37:17.920
in the constellation torus, beyond the familiar bright stars of Orion, lies a soft patch of glowing

00:37:17.920 --> 00:37:26.800
gas known to astronomers, as the crab nebula, it is the remains of a star that ended its life

00:37:26.800 --> 00:37:35.840
long ago records kept by sky watchers in the 11th century, describe a new star that shown brightly

00:37:35.840 --> 00:37:47.120
for weeks, and then faded the cloud now seen in telescopes is what remains of that event, still spreading

00:37:47.120 --> 00:37:56.240
outward in slow motion at the center of this pale cloud sits a compact object, turning steadily

00:37:56.240 --> 00:38:04.560
in the dark, this object is a neutron star, it formed when the core of the old star collapsed

00:38:04.560 --> 00:38:13.200
under its own gravity, while the outer layers drifted away into space, the core did not vanish,

00:38:13.200 --> 00:38:21.760
it was pressed into a sphere, only a few tens of kilometers across, yet holding more mass

00:38:21.760 --> 00:38:29.920
than the sun, matter in such a state, is unlike anything found on Earth, the star is supported

00:38:29.920 --> 00:38:40.080
by the pressure of neutron's pact at extreme density, and it carries a strong magnetic field threaded

00:38:40.080 --> 00:38:47.760
through its rotating body, this neutron star, is the active heart of the nebula, it spins

00:38:47.760 --> 00:38:56.880
about 30 times each second, that rate may sound rapid, but from a distance it appears as a repeating

00:38:56.880 --> 00:39:05.760
signal, the star does not wobble or rush, each turn takes almost exactly the same amount of time

00:39:05.760 --> 00:39:14.560
over many centuries, the rotation has been slowing by a tiny amount, yet the change is so gradual,

00:39:15.440 --> 00:39:23.920
that the pulse remains a useful natural clock, the signal arrives, because the star does not shine

00:39:23.920 --> 00:39:32.720
evenly in all directions, energy gathers near the magnetic poles, which are not lined up with the spin

00:39:32.720 --> 00:39:41.840
axis charged particles, follow the magnetic field, and produce narrow beams of radiation as the

00:39:41.840 --> 00:39:49.920
star turns, those beams sweep through space when one of them points toward Earth, telescopes

00:39:49.920 --> 00:39:57.680
receive a brief pulse, then the beam moves on, and there is a short pause before the next one arrives,

00:39:57.680 --> 00:40:05.360
the effect is much like a lighthouse seen from a ship, or a steady lamp appears to flash,

00:40:06.320 --> 00:40:13.360
because the observer only sees it at certain moments, radio waves, are the most familiar

00:40:13.360 --> 00:40:22.800
part of this pulse, but the pulse are, also sends out pulses, at higher energies, sensitive instruments,

00:40:23.360 --> 00:40:32.160
have recorded its blinking invisible light, x-rays, and gamma rays, the same rotation controls

00:40:32.240 --> 00:40:40.160
them all, this makes the object useful as a natural timing source, astronomers can measure the arrival

00:40:40.160 --> 00:40:49.440
of its pulses with great care, then compare the pattern over months and years, the regularity,

00:40:50.240 --> 00:40:57.600
allows small changes to be noticed, sometimes the star adjusts its spin by a minute amount,

00:40:58.560 --> 00:41:05.200
an event called a glitch, these glitches are thought to come from the way, the interlayers

00:41:05.200 --> 00:41:13.680
of the neutron star, interact with its crust, they do not disrupt the overall rhythm, they simply

00:41:13.680 --> 00:41:21.680
add a slight step to the gradual spin down, the surrounding nebula, is powered by this rotation,

00:41:21.680 --> 00:41:30.800
the pulsar loses a small amount of spin energy, with each turn, and that energy flows outward

00:41:30.800 --> 00:41:39.440
into the gas, it helps keep the nebula glowing, and gives the cloud a faint blue shine in images,

00:41:39.440 --> 00:41:47.120
without the central star, the nebula would be a quieter place, the pulsar acts as a gentle engine,

00:41:47.120 --> 00:41:57.280
feeding the remnant with particles and magnetic fields, while it continues to cool and expand

00:41:57.280 --> 00:42:05.120
the connection between the pulsar and the nebula was worked out through careful observation,

00:42:05.120 --> 00:42:14.080
as radio telescopes improved, they began to detect brief, repeating signals from space, at first,

00:42:14.720 --> 00:42:21.520
such signals were unrecognized, the pattern from the crab nebula was especially rapid,

00:42:21.520 --> 00:42:28.000
its repeats came with a stable spacing, researchers measured the interval between pulses,

00:42:29.440 --> 00:42:37.200
and found it matched, a spinning neutron star, rather than any human-made source, the location,

00:42:38.000 --> 00:42:45.680
also matched the bright remnant known from earlier sky records, this link gave strong support to

00:42:45.680 --> 00:42:55.360
the idea that some stars leave behind dense rotating cores, after their final bright phase, timing

00:42:55.360 --> 00:43:03.760
the pulses, also revealed something about the space between the stars, radio waves of different frequencies,

00:43:04.720 --> 00:43:12.320
do not travel through the thin gas of the galaxy, in exactly the same way, lower frequency waves,

00:43:13.120 --> 00:43:20.560
are delayed a little more than higher frequency waves by measuring that delay. Astronomers

00:43:20.560 --> 00:43:28.560
can estimate the number of free electrons along the line of sight, this pulsar, being bright,

00:43:28.560 --> 00:43:36.560
and steady became a useful probe of this diffuse material, the method does not require

00:43:36.560 --> 00:43:44.880
visiting the region, it only requires listening to the pulse and noting how it arrives, the pulses

00:43:44.880 --> 00:43:53.200
have continued since before telescopes existed, they passed Earth, while people were mapping the sky

00:43:53.280 --> 00:44:02.080
with simple instruments, and they continue now, while large radio dishes listen in quiet valleys,

00:44:02.800 --> 00:44:10.960
and high planes, the star itself, is far too small to see as a disk in ordinary telescopes,

00:44:12.400 --> 00:44:20.160
yet its signal is strong enough to be followed across the galaxy, each pulse carries the same basic

00:44:20.160 --> 00:44:28.960
message, a compact object is turning, and its magnetic field is guiding radiation outward,

00:44:28.960 --> 00:44:37.280
the geometry of that beam, happens to cross our line of sight, the rotation is not perfectly smooth,

00:44:37.840 --> 00:44:46.000
at the finest levels, the pulse profile can drift slightly, and the radio signal, sometimes,

00:44:46.160 --> 00:44:53.280
shows small variations in strength, these changes arise from shifting conditions in the

00:44:53.280 --> 00:45:02.960
magnetosphere, the region of magnetic fields, and charged particles around the star, studying them,

00:45:03.840 --> 00:45:10.800
helps astronomers map how energy escapes from a neutron star, this neutron star,

00:45:11.680 --> 00:45:19.760
therefore remains a laboratory for matter under conditions that cannot be reproduced on Earth,

00:45:21.360 --> 00:45:30.080
sending its brief radio notes across the galaxy, one turn at a time at the center of the Milky Way,

00:45:31.200 --> 00:45:39.440
hidden behind thick lanes of dust, there is a small radio source that marks the home of the

00:45:39.440 --> 00:45:47.680
galaxies, central black hole toward the middle of the galaxy's disk, this object cannot be seen

00:45:48.400 --> 00:45:56.800
with ordinary eyes, its signal is faint, produced by gas moving in the strongest part of the

00:45:56.800 --> 00:46:05.360
galaxies gravitational field, the object itself, has no solid surface, it is a region where matter has

00:46:05.360 --> 00:46:14.720
gathered so densely, that once light or gas crosses a certain boundary, it remains within the

00:46:14.720 --> 00:46:23.520
black hole that boundary is called the event horizon beyond it, the black hole is known only by the way

00:46:23.520 --> 00:46:31.120
it moves the stars and gas nearby, the mass gathered there is enormous by human standards,

00:46:31.120 --> 00:46:41.200
yet small compared with the whole galaxy astronomers estimated at about 4 million times the mass

00:46:41.200 --> 00:46:50.560
of the Sun, that sounds immense, and it is, but the Milky Way contains hundreds of billions of stars,

00:46:50.560 --> 00:46:59.440
the central black hole is a heavy point at the middle of a broad disk, not a drain drawing the disk

00:46:59.520 --> 00:47:08.400
inward, its gravity is strongest, very close in farther away, the combined pool of stars and gas

00:47:09.360 --> 00:47:17.280
together with dark matter, shapes the galaxy much more than the black hole alone, a black hole

00:47:17.280 --> 00:47:26.000
does not pull matter inward simply because it is a black hole if the Sun were replaced by a black hole

00:47:26.000 --> 00:47:34.880
of the same mass, the planets would continue along nearly the same paths, the change would be the absence of

00:47:34.880 --> 00:47:44.560
sunlight, not a new inward force, the same principle, applies at the Galactic Center stars that pass near

00:47:44.560 --> 00:47:52.800
the central mass, feel its gravity strongly, but stars at a distance, including the Sun,

00:47:52.960 --> 00:48:00.960
move according to the total mass of the galaxy, nothing is being dragged toward the middle faster,

00:48:01.840 --> 00:48:10.080
then ordinary orbital motion allows the central black hole, was not found by seeing a dark sphere,

00:48:10.080 --> 00:48:19.200
it was inferred from motion, radio telescopes, first noticed a compact source near the Galactic Center,

00:48:19.280 --> 00:48:28.320
later infrared instruments looked through the dust and watched individual stars moving around an

00:48:28.320 --> 00:48:36.880
unseen point over many years, those stars traced arcs and ellipses by measuring how fast they moved

00:48:37.760 --> 00:48:45.600
and how large their orbits were, astronomers could calculate the mass inside those paths,

00:48:45.680 --> 00:48:53.920
the answer pointed to a very compact mass of millions of Sun's no ordinary cluster of stars

00:48:54.880 --> 00:49:02.160
could remain so tightly packed without spreading or shining brightly, a black hole

00:49:02.160 --> 00:49:10.160
fit the evidence the stars nearest this object are sometimes called the S stars they follow a

00:49:10.160 --> 00:49:18.000
elliptical orbits around the same invisible focus some complete a full circuit in only a few

00:49:18.000 --> 00:49:26.320
decades which is short for a star their paths have been followed carefully with large telescopes

00:49:27.360 --> 00:49:34.720
and precise instruments the orbits are stable they do not spiral inward during each passage

00:49:34.720 --> 00:49:43.760
they swing close then move outward again much as comets in our own solar system swing around the

00:49:43.760 --> 00:49:52.080
Sun and return the difference is scale the central mass is millions of times heavier than the Sun

00:49:53.440 --> 00:50:00.160
so the inner orbits move with measurable speed their paths repeat near the event horizon

00:50:01.120 --> 00:50:10.080
the environment is more extreme than the space around ordinary stars but it follows the same

00:50:10.080 --> 00:50:19.520
physical laws found elsewhere gas and dust can orbit the black hole for a long time before anything

00:50:19.520 --> 00:50:28.800
crosses the boundary much of the material near the center moves in disks or streams heated by compression

00:50:29.360 --> 00:50:36.400
and friction some of it gives off radio waves other portions shine in infrared light

00:50:37.120 --> 00:50:45.600
or x-rays before settling inward the central source is quiet compared with the brilliant cores

00:50:46.640 --> 00:50:55.920
of some distant galaxies it has very little material falling into it at any given time that is one reason

00:50:56.160 --> 00:51:04.480
it can be studied calmly as a natural object rather than a spectacular outburst the event

00:51:04.480 --> 00:51:14.080
horizon itself is small in galactic terms for a mass of four million sons the radius of the horizon

00:51:14.880 --> 00:51:23.040
is roughly 12 million kilometers the full diameter would fit well inside the orbit of mercury

00:51:23.760 --> 00:51:31.040
around the Sun compared with the width of the Milky Way which spans tens of thousands of

00:51:31.040 --> 00:51:39.920
light years that is almost too small to notice the black holes influence grows with closeness

00:51:39.920 --> 00:51:49.200
at great distances its gravity behaves like the gravity of any other object with the same mass this

00:51:49.200 --> 00:51:57.360
difference in scale is central to understanding why the galaxy remains settled the reach in

00:51:57.360 --> 00:52:07.120
around the black hole contains old stars and younger stars along with warm gas and dust clouds

00:52:07.120 --> 00:52:15.680
the black hole sits among them not apart from them in a dramatic sense its presence is revealed

00:52:15.760 --> 00:52:23.120
by the order of the motions around it stars do not crowd into the horizon they keep their distances

00:52:23.920 --> 00:52:31.120
according to their angular momentum and their orbital energy when a little gas drifts too close

00:52:32.320 --> 00:52:40.640
it can add a brief flicker of a mission but the overall pattern remains steady this quietness helps

00:52:40.640 --> 00:52:49.680
explain how the black hole can anchor the central region without disrupting the wider galaxy

00:52:49.680 --> 00:52:57.600
it provides a common gravitational center for the innermost stars it helps to find the midpoint

00:52:58.240 --> 00:53:07.120
of the Milky Way's rotation yet the rotation of the galaxy as a hole depends on mass spread across

00:53:07.120 --> 00:53:15.360
a broad disk and halo the central black hole is a small part of that total if it were removed

00:53:16.000 --> 00:53:25.360
while the rest of the galaxy remained unchanged most stars would continue much as before the comparison

00:53:25.360 --> 00:53:34.800
shows how local the black hole's direct effect becomes once one moves away from the center our own

00:53:34.800 --> 00:53:43.920
son is far from the center it circles the galaxy at a distance of tens of thousands of light years

00:53:45.200 --> 00:53:53.760
taking hundreds of millions of years to complete one orbit from that vantage the central black hole

00:53:54.480 --> 00:54:01.760
is a remote reference point it does not reach into the solar system it does not alter the seasons

00:54:02.560 --> 00:54:10.080
or the motions of the planets its role is galactic it belongs to the architecture of the Milky Way

00:54:11.360 --> 00:54:20.160
not to the immediate neighborhood of earth astronomers continue to refine the picture orbits are measured

00:54:20.160 --> 00:54:28.880
with greater precision and gas behavior near the inner edge is watched carefully comparisons with other

00:54:28.880 --> 00:54:38.720
galaxies show that central black holes come in different masses often related to the sizes

00:54:39.360 --> 00:54:48.480
of their host galaxies the one at our center is moderate as supermassive black holes go it is large enough

00:54:48.480 --> 00:54:57.600
to organize the innermost stellar paths and small enough that it does not dominate the entire galaxy

00:54:57.680 --> 00:55:05.440
the name given to this object is a label from radio astronomy but the object itself

00:55:06.240 --> 00:55:14.480
is older than labels it formed through processes that are still being studied it may have grown from early

00:55:14.480 --> 00:55:24.720
black holes left by massive stars and it may have gained mass from gas and mergers over cosmic time

00:55:24.800 --> 00:55:33.520
the accepted view is that it grew slowly alongside the galaxy its history is written in the motions

00:55:33.520 --> 00:55:42.800
of the stars around it and in the amount of mass it holds today because the central black hole marks the

00:55:42.800 --> 00:55:51.920
galaxy's midpoint so clearly it gives astronomers a reference for mapping the Milky Way coordinates

00:55:51.920 --> 00:56:01.600
used for the galaxy can be tied to this central direction and motions near that point help calibrate

00:56:01.600 --> 00:56:09.920
the larger rotation of the disk far beyond the quiet edges of the Milky Way two galaxies move

00:56:09.920 --> 00:56:18.320
through a shared path so gradual that no human lifetime could notice a change they are known to

00:56:18.320 --> 00:56:31.760
catalogs as NGC 4030 and NGC 4039 and to many observers simply as the antennae their name comes from

00:56:31.760 --> 00:56:41.360
too long streamers that are away from them thin in appearance and graceful in outline these filaments

00:56:41.360 --> 00:56:50.640
are made of stars and gas with dust mixed through drawn outward by gravity working over immense

00:56:50.640 --> 00:56:59.680
spans of time they lie in the constellation Corvus tens of millions of light years away at that distance

00:57:00.480 --> 00:57:08.320
their light left them long before modern telescopes existed yet it still carries a clear picture

00:57:08.960 --> 00:57:16.320
of what they are doing the two galaxies are not separate in the way they once were they have come near

00:57:16.320 --> 00:57:24.240
enough for their gravity to reach into each other and rearrange their shapes the result is a slow

00:57:24.240 --> 00:57:34.480
exchange of material a stretching and folding that resembles a dance before this encounter each system

00:57:34.480 --> 00:57:43.600
likely had its own rotating disk of stars and gas as they passed near one another gravity

00:57:43.600 --> 00:57:51.360
began to raise tides in those disks these are gravitational tides gentle differences in pole

00:57:52.160 --> 00:58:02.160
from one side of a galaxy to another over many orbits the tides pulled stars and gas outward into the

00:58:02.160 --> 00:58:09.760
long tails now visible because the spaces between stars are so wide the stars themselves

00:58:10.560 --> 00:58:19.840
almost never meet the encounter reorganizes paths stars follow new curves around shared centers of

00:58:19.840 --> 00:58:29.280
gravity clouds of gas drift closer together and gather in those gathered places new stars form

00:58:29.280 --> 00:58:35.200
the antennae are full of young stars patches of bright blue light mark regions

00:58:36.080 --> 00:58:43.280
or star formation has increased dense knots of gas have contracted into clusters

00:58:44.560 --> 00:58:53.280
some containing many young sons these clusters shine brightly in images and they help astronomers

00:58:53.280 --> 00:58:59.920
trace where the galaxies have been compressed by their mutual pull the two streamers

00:58:59.920 --> 00:59:07.280
that give the pair their name extend far beyond the bright centers they are made of material

00:59:08.000 --> 00:59:15.520
lifted out of the galaxy's disks in a close passage the side of a galaxy near its companion

00:59:15.600 --> 00:59:24.800
feels a slightly stronger pole than the far side that difference stretches the disk some material swings

00:59:24.800 --> 00:59:33.040
outward then settles into a long looping tail the tails can persist for a very long time

00:59:34.320 --> 00:59:42.320
slowly thinning as their stars spread along new orbits this process unfolds on a scale

00:59:43.200 --> 00:59:51.200
that is difficult to hold in mind the galaxies do not rush together the process is slow they have

00:59:51.200 --> 00:59:59.840
already spent many millions of years approaching passing and reshaping one another the changes

00:59:59.840 --> 01:00:07.680
visible in a telescope are a single frame from a long sequence if one could watch from a distance

01:00:08.480 --> 01:00:14.800
the motion would seem still only by comparing the positions of stars over epochs

01:00:15.680 --> 01:00:24.320
far longer than human history with the pattern become plain invisible light the central regions

01:00:24.320 --> 01:00:31.120
of the two galaxies appear softened by dust dark lanes curve through the brighter glow

01:00:31.120 --> 01:00:41.280
marking places where thick clouds block starlight around them pale pink and blue patches show

01:00:41.280 --> 01:00:49.520
where gas is shining under the light of young stars the overall shape is no longer the neat spiral

01:00:50.320 --> 01:00:59.440
of an undisturbed galaxy it is open extended and full of gentle irregularity the colors carry

01:00:59.520 --> 01:01:07.760
information they record different temperatures and ages of stars observations beyond visible

01:01:07.760 --> 01:01:16.080
light add more detail without changing the quiet story infrared light reveals dust

01:01:17.040 --> 01:01:24.960
that has been warmed by clusters of new stars radio observations trace the distribution of cool gas

01:01:25.680 --> 01:01:34.000
the material from which future stars may form together these views show that the encounter

01:01:34.000 --> 01:01:42.400
changes more than outline it redistributes the raw material of galaxies astronomers have learned to

01:01:42.400 --> 01:01:52.000
read such forms by combining observations with models images at different wavelengths show where stars are

01:01:52.880 --> 01:02:01.040
and where dust lies other measurements reveal glowing gas and show how material moves

01:02:01.040 --> 01:02:08.000
within the reshaped disks computer simulations can begin with two ordinary galaxies

01:02:09.440 --> 01:02:17.760
and let gravity act on them across simulated time when the simulated encounter is tuned to match the

01:02:17.760 --> 01:02:27.200
antenna it produces tails and bright star forming regions much like those seen in the sky the

01:02:27.200 --> 01:02:36.400
long tails also contain their own small bright knots some of these are clusters of stars born within

01:02:36.400 --> 01:02:44.240
the pulled out material as the tail stretch these knots move along with them like beads carried

01:02:44.240 --> 01:02:51.600
on a slow current in some models of interacting galaxies dense clumps in tidal tails

01:02:52.320 --> 01:03:01.680
can remain bound and continue shining for a very long time the antennae give astronomers a nearby

01:03:01.680 --> 01:03:10.880
place to study such possibilities some knots may later drift as small companions near the merged

01:03:10.880 --> 01:03:19.920
galaxy in time the two systems will settle into a single broader galaxy the spiral patterns

01:03:19.920 --> 01:03:27.680
that once belong to each love softened into a more rounded shape the tidal tails will fade

01:03:28.720 --> 01:03:38.240
as their stars disperse into wider paths new star formation may slow as gas is spread out the result

01:03:38.240 --> 01:03:47.280
will be a calm system lit by older stars and moving quietly through space the older stars

01:03:47.280 --> 01:03:55.280
will continue their orbits in a broad steady pattern the antennae offer a clear example

01:03:56.320 --> 01:04:03.040
of how galaxies can change through gravitational influence they show that large structures can be

01:04:03.120 --> 01:04:12.080
reshaped without haste their long encounter helps explain why many galaxies are not fixed

01:04:12.080 --> 01:04:21.840
inform gravity given enough time can turn disks into loops and guide stars onto new paths the long

01:04:21.840 --> 01:04:31.280
streamers are records of motion they mark where gravity has lifted material and set it on a wider course

01:04:31.280 --> 01:04:38.960
the cosmos remains in steady motion empty regions extend across space in darkness

01:04:40.240 --> 01:04:49.280
and the faint after glow of the early universe travels on without hurry far away pulsars turn

01:04:49.280 --> 01:04:57.360
with patient rhythm sending regular signals through space these patterns do not require attention

01:04:57.440 --> 01:05:04.960
to continue they persist while breathing slows and thoughts soften sleep comes as a natural pause

01:05:06.080 --> 01:05:12.880
like the hush between one star's light and the next gravity also continues gently

01:05:13.760 --> 01:05:22.480
guiding matter over long spans as sleep deepens the body settles in the mind can let the days

01:05:22.480 --> 01:05:29.760
details rest sleep is part of the body's daily rhythm the silent cosmos will carry on through

01:05:29.760 --> 01:05:40.480
the night its distances calm and its motions reliable you can softly let the day settle into stillness now good night