sleep at time facts, the silent cosmos, let your shoulders lower, and your breathing, find a
slow rhythm, the day can remain where it is, while you rest here, in a quiet room, beyond the windows.
The universe is mostly hush, sound needs matter to carry it, and most of space is too empty
for ordinary sound to travel, in that great stillness. Stars and planets move without noise,
reaching across the dark. That silence is spacious. There is nowhere else. You need to be right now,
as you breathe out. Imagine your thoughts loosening their grip. One by one, your body can soften
into the bed, supported by the surface beneath you, the cosmos asks nothing of you tonight. It's simply
continues in quiet, while rest comes closer, let the hush be enough, far beyond the familiar stars
of the northern sky. In the direction of the constellation boodies, there is a place where the usual
crowd of galaxies, thinns, almost to silence, it is not a hole in space, and it is not a region where
the laws of physics fail, it is an enormous volume of the universe, or matter, is spread much more thinly,
than average astronomers call such places voids, and this one is among the most widely known.
The scale is difficult to hold in mind the region spans hundreds of millions of light years,
if a traveler could move from one side to the other at the speed of light. The journey would take
hundreds of millions of years in a volume that large ordinary expectation would place many thousands
of bright galaxies, here the count is far lower, the galaxies that do exist, sit at great distances,
from one another, like lanterns placed along a very wide plane, the void was recognized,
while astronomers were mapping the positions of galaxies in three dimensions, a telescope can show
where a galaxy appears on the sky, but distance requires another measurement by studying the light
from each galaxy, and measuring how its wavelengths are stretched. Astronomers can estimate how far away
it is when many such distances are gathered. The flat pattern of the night sky becomes a deep map
in that map. Galaxies gather into sheets and filaments between them, live broad regions,
with fewer galaxies, the region toward Baudes stood out. Because of its size, this place is not
completely bare, space within it. Still contains the same basic ingredients found elsewhere.
There is dark matter. The less densely packed, there is very thin gas. There are also galaxies.
Just widely separated, some are small and faint, others appear to have formed stars slowly.
With little disturbance from neighbors, their isolation gives them a different character from galaxies
in crowded clusters, where encounters and mergers are more common, the quietness of the void
comes from the way structure grew in the universe long ago. Matter was distributed almost evenly,
but not perfectly tiny differences in density were present everywhere over immense spans of time.
Gravity made those differences more pronounced, regions with slightly more matter,
pulled in material from their surroundings. They became denser, forming the filaments and walls
that now outline the cosmic web, regions with slightly less matter, did the opposite,
as matter drifted outward toward the denser boundaries. The interiors became quieter,
and more open, in a void, expansion has a clearer presence in denser places.
Gravity binds galaxies together, and slows the stretching of space on local scales inside a large
under dense region. There is less mass to resist the general expansion. The void does not push
outward with force, it simply becomes a little more spacious over time. While the surrounding walls
remain more closely gathered, the motion is slow, measured in the patient language of cosmology,
rather than in human events, the shape of the region is often described as broadly rounded.
Yet, it is not a perfect sphere, voids can grow by joining with one another,
and their boundaries can be uneven, what appears as one great quiet volume.
May contain smaller low-density pockets, within it, around the edges. Galaxies trace, faint walls,
and threads, these boundaries, are where the void meets the more populated universe,
they are gentle borders, not sharp walls, and they guide the slow flow of matter over cosmic time,
studying such a place, helps astronomers understand how environment, shapes galaxies,
a galaxy inside a sparse region, evolves, with fewer close encounters, it may keep its gas
longer, form stars at a steadier pace, or remain small, because it has little material together,
comparing these galaxies, with those intense clusters, shows how much surroundings matter,
the void offers a calm extreme, a place where the usual interactions of galactic life
happen rarely, there is also a simple beauty in the arrangement, the universe is often pictured as a web,
with bright nodes and connecting strands, the voids are the spaces between those strands
they give the web its shape without the low-density regions, the filaments would not stand out,
so clearly the silence of the void is part of the pattern, not an absence of meaning,
light crossing this region carries information about distance and history, because galaxies are so far
apart, the light from one may travel a long way before it passes near another, the sky seen
from within the void would be different from the sky seen from our own neighborhood, fewer bright galaxies
would be visible, and the night would be dominated by the local stars of any galaxy present,
the wider universe would appear more distant, its lights separated by wide intervals,
none of this suggests a place of loss, the void is not a ruin, or a missing piece,
it is a natural outcome of gravity, working on small differences, over great time, matter has not
vanished, it is gathered elsewhere, leaving a broad interior, where stillness is the ordinary condition,
the galaxies within it are not stranded, they follow the same motions that govern all matter,
moving gently through a space that has room to spare on human timescales, the void changes
almost imperceptibly, stars within its few galaxies, continue their long lives, gas drifts,
gravity works slowly, the distances are so large, that even the motions of entire galaxies appear
settled, this is one of the comest large-scale places known in the cosmos, a region where the universe
shows how much can be made from patience and distance, guided by the quiet pool of gravity,
every direction in the night sky, holds a faint micro wave glow, it is not made by stars,
and it does not gather into bright points, it lies smoothly across the heavens,
so evenly that early measurements could barely tell one patch from another, this radiation,
is the coold remnant of a time when the universe was much younger, and far simpler,
it has traveled for an immense span of time, stretching as space expanded, until it became the
soft whisper that modern instruments can detect, the story begins when the cosmos was hot,
and dense enough to behave like a glowing plasma in that early state, light could not travel freely,
photons were constantly scattered by free electrons, much as sunlight is scattered inside a thick cloud,
the universe was opaque, filled with a bright fog as expansion continued, the temperature fell,
electrons and protons settled into neutral atoms, and the fog cleared for the first time,
light could move outward without constant interruption, the photons released, then have been traveling
ever since, that first free light was once very warm over billions of years,
the expansion of space stretched its wavelength, visible, and infrared radiation,
gradually shifted into microwaves, today the glow has a temperature of about 2.7 Kelvin,
a little under 3 degrees above absolute zero, it remains a quiet thermal trace,
a low home of energy spread through all of space, one of the most reassuring things about this radiation,
is its spectrum, it follows the smooth curve of a nearly perfect black body, a black body spectrum,
is what physicists expect from light that once shared a common temperature with matter,
exchanging energy until everything settled into balance, the cosmic glow matches that expectation,
with unusual precision that match tells us the early universe was once in a state close to thermal
equilibrium, it also shows that the radiation has traveled through space, without being reheated,
or scrambled by later events, the glow is almost the same in every direction, but not perfectly so,
sensitive maps reveal tiny differences in temperature, variations of only a few parts in 100,000,
some regions appear very slightly warmer, others appear very slightly cooler, these small differences,
our natural imprints of early density variations were matter was a little more concentrated,
gravity had a little more material to work with over long ages, those gentle contrasts,
eventually helped guide the formation of galaxies and clusters, though the background itself
comes from a time long before any star ignited, studying these patterns requires patience,
the signal is weak, and the sky contains other sources of microwave light dust in our own galaxy,
can glow faintly, electrons moving through magnetic fields, can produce microwave emission,
instruments must observe the sky at many frequencies, so these foreground contributions
can be separated from the deeper background, the process is gradual, researchers,
compare maps, and check calibrations, they also look for patterns that remain stable across different
methods the result is a cleaner view of the ancient signal, the background also carries information
about motion because the solar system moves through space, the glow appears slightly warmer
in the direction we are heading, and slightly cooler behind us, this effect is called a dipole,
it is similar to the way rain, seems to come more directly, at a moving traveler, once this motion is
accounted for, the underlying sky becomes more symmetrical, the dipole is a useful reminder
that the radiation is not an abstract idea, it is a physical field that can be measured from within a
moving galaxy, another reason the glow matters is that it sets a limit on how far back ordinary light
can see before the fog cleared, photons were trapped in the plasma telescopes that collect visible
light, cannot look past the first stars, and even infrared instruments eventually reach a boundary,
the microwave background marks the edge of that older luminous era, it is the earliest light we can
observe directly, released when the universe became transparent beyond it lie earlier conditions
that scientists infer through theory, and through the patterns left in the glow itself, those patterns
have allowed astronomers to measure broad features of the cosmos, with care, the spacing of the
warm and cool spots, reflect sound waves that moved through the early plasma by studying the
angular size of the spots, researchers can estimate how space has expanded since the light was released,
the overall smoothness of the background supports the idea that the universe was once much more
uniform than it is today, the tiny ripples within it provide a starting point for the slow growth of
structure, the record is one of gradual change, written in temperature differences to small to
feel the radiation also helps scientists understand the contents of the universe in a general way,
the way the patterns formed depends on ordinary matter and dark matter, it also depends on the
expansion rate by comparing the observed pattern with models, researchers can estimate proportions
and test whether the same description works across the sky, this proceeds as a slow fitting of evidence,
like adjusting a map until the coastline's match, the background offers a stable reference
because it comes from a time when the universe was simple enough for the physics to be clear, modern studies
continue to refine the picture, better detectors, can measure the same sky with less noise, independent
teams can compare their results, without needing to assume anyone instrument is perfect, the glow
remains a steady reference point, it offers a clear view of a time when the universe was young,
and transparent enough for light to travel that same simplicity. Let's researchers read the first
patterns with care, far out in the deep sky. Two great clusters of galaxies have been moving
through one another for a very long time, a cluster is not a crowded place, in the ordinary sense,
the galaxies inside it are separated by wide stretches of space, and the stars within each
galaxy are even more widely spaced, what fills the space between galaxies is a thin hot gas,
so diffuse that it would feel like almost nothing to a hand past through it, around and through all of
this lies and unseen mass that does not glow, does not absorb light, and does not behave like
the gas, the encounter lets these different components be seen separately. As if a slow current had
sorted them, the hot gas is ordinary matter, mostly hydrogen and helium, heated until it shines in
x-rays in a quiet cluster, this gas can form a soft rounded glow near the center when two clusters
pass through one another. Their gas clouds meet, they do not meet, like solid objects,
they drift together, press gently and slow, the particles in the gas interact with one another,
so they feel one another, some of their motion turns into warmth, and the gas lingers near the middle
of the encounter, it becomes a broad, bright region, that telescopes can map the galaxies behave
differently, each galaxy is mostly empty space, stars are far apart compared with their sizes,
and galaxies themselves are held together by gravity as the clusters pass through. The galaxies
glide onward, they may be nudged slightly, but they rarely touch, their motion carries them
forward, leaving the slowed gas behind, in this way, the visible galaxies, and the hot gas begin
to part company, one part of the system moves ahead, while the other rests more centrally,
the unseen mass moves even more quietly, it does not shine, and it does not seem to slow in the way
the gas does, it passes through the encounter, with little resistance, the result is a gentle separation,
the hot gas gathers in a broad region between the two groups of galaxies, while most of the mass
travels onward with the galaxies, the pattern is subtle, but it is clear when the right kind of
observation is made, the evidence comes from the way gravity bends light, light from galaxies far behind
the cluster, follows the curvature of space caused by the mass in front of it, the foreground mass acts
like a very soft lens, it can stretch, and distort the images of background galaxies,
by tiny amounts, no single background galaxy, gives the whole answer many of them are studied together,
and their slight shaped changes are used to draw a map of where the mass lies, this method does not
depend on the mass being bright, it responds to gravity itself, astronomers call this gravitational
lensing when such maps were made for this encounter, the mass did not sit, where the hot gas was
brightest, the greatest concentrations of mass, laying near the groups of galaxies, on either side
of the central gas, the gas, though it contained a great deal of ordinary matter,
was not, where most of the gravity was found, the separation was broad and calm, like two crowds,
moving past one another, while a mist remains between them, the picture showed that something unseen
had moved ahead, this arrangement gives a clean way to think about what the mass might be,
if the extra gravity came only from the hot gas, the lensing map would follow the x-ray glow,
it does not, if gravity were tied only to the matter that shines, the offset would need a careful
explanation, the simplest account, is that there is matter that does not interact strongly
with light or with itself, except through gravity that matter is what astronomers call dark matter,
the encounter makes its presence easier to trace, because the ordinary gas has been left behind,
the time scale is long enough, that nothing about it feels hurried, the passage of one cluster,
through another, takes hundreds of millions of years, during that span, the gas warms and spreads,
and the galaxies continue along their paths, the stars do not strike one another, the distances
are too great, instead the whole event unfolds like a slow tide, gravity guides the motion,
and the light from distant background galaxies records the shape of that motion, the hot gas itself
tells a story of ordinary matter, under gentle pressure, as the two gas clouds met, they were compressed
and heated, yet they remained thin by everyday standards, they glow softly in x-rays,
and their shape shows how they were slowed, the galaxies meanwhile, keep the memory of the passage
in their positions and motions, the invisible mass, adds the clearest part of the story,
because it shows where gravity is strongest, without needing to shine, to see the whole system,
astronomers combine different views, one view records the x-ray glow of the hot gas,
another measures the tiny distortions of background galaxies placed together with the visible galaxies,
these views show that the gas rests while the galaxies, and most of the mass drift forward,
the result, is a quiet demonstration of matter that cannot be seen directly, but can be measured
by its effect on light, this kind of encounter, also helps show why clusters hold together,
the amount of mass needed to bend the background light, is larger than the mass in stars
and gas alone, the unseen mass gives the cluster its gravitational shape, without it, the galaxies
would not move as they do, and the cluster would not remain bound in the way observed, the encounter
does not disturb this conclusion, it makes the hidden mass easier to locate, there is no sudden event here,
no sharp moment of contact, the clusters are so large, that their meeting is measured in cosmic time,
light from the background bends around them, in silence, the hot gas settles, and shifts very slowly,
the galaxies continue on paths that will carry them farther apart, or into a new arrangement
through all of this, the lensing map remains a quiet record of mass, showing that much of the universe
is present without being visible, a soft blur of light, hangs below the three stars of Orion's belt,
in the place, often drawn as the hunter's sword, to an unated eye, it looks like a small cloud with steady edges,
pale against the dark, this soft patch, is a nearby region, where interstellar gas is gathered into stars, its light has traveled a little
over a thousand years to reach us, the glow we see left the nebula long before modern instruments existed,
yet the object itself changes so slowly that a human lifetime sees almost no alteration in its form,
its apparent smallness is a matter of distance, the nebula spans tens of light years,
though it covers only a small angle in the sky, telescopes reveal that its bright core is surrounded by fainter wings
of scattered light, much of the surrounding glow comes from dust, catching starlight,
while the central glow comes from gas, warmed by young stars, seen from earth,
the bright part resembles a shallow bowl, with softer material spilling outward,
the nebula belongs to a much larger cloud of molecular gas, and dust that fills much of the
constellation Orion in such clouds, hydrogen usually exists as pairs of atoms bound together,
and the material is cold by everyday standards, dust grains, similar in scale to find smoke,
float between the gas, they block distant starlight, and help the interior stay cool, cool gas moves slowly,
it can collect in broad sheets and thin filaments, drifting with motions left over from the clouds,
earlier history, the cloud is not uniform, it contains dense cores, and quieter envelopes,
along with long thread-like structures, some parts are so thick that visible starlight cannot pass
through them, astronomers call these obscured places dark, nebulae, when they are seen against
brighter backgrounds, in Orion, dark lanes trace channels of dust across the glowing gas,
and mark regions where future stars may still be forming star formation begins,
when a patch of this material becomes a slightly denser than its surroundings, gravity pulls matter
inward, and the pool grows as more mass gathers, the process is slow, a clump may require
long ages to contract by the width of a planetary system, pressure, and magnetic fields resist the
pool, while slow eddies in the gas add support, because the core can radiate warmth away,
it can continue settling, without building too much pressure, the gas does not fall straight inward,
it turns as it moves, forming flattened disks around dense cores within those cores,
hidden inside dust, protostars take shape, a protostar is not yet shining, by the steady fusion,
the defines a mature star, it glows first from the warmth of contraction,
releasing energy, as matter settles onto it, much of this light is infrared, which passes through
dust more easily than visible light, observations at infrared wavelengths, have allowed astronomers
to find many young stars inside the nebula that would otherwise remain concealed,
radio measurements add another view, tracing cool molecules, and dense gas that optical telescopes
cannot see the visible nebula, owes much of its brightness to a small group of young stars near its
heart, these stars are known as the trapezium, from the foresighted pattern they make in telescopes,
their light is energetic enough to excite the surrounding hydrogen when electrons in the gas
recombine with atoms, they emit light at characteristic colors, giving the nebula soft, reds,
and greens in long photographs, other elements contribute fainter hues, oxygen can add a
greenish tone, while nitrogen and hydrogen help shape the deeper reds in small telescopes,
the nebula often appears gray or greenish to human eyes, because our vision is less sensitive
to color, in faint light, long exposures collect more photons, and reveal stronger reds,
this difference between direct view and photograph is not a flaw in the nebula it follows from the
way eyes, and instruments gather light, the same object can therefore seem quiet at the eyepiece,
and richly colored in an image made over many minutes, this illumination also shapes the cloud,
starlight presses on dust and gas, and stellar winds carry particles outward over long ages,
these influences carve broad hollows in the nebula, and leave denser knots standing in relief,
dark silhouettes, sometimes called pillars, our places where material remains thicker,
shielding cooler gas behind it at the edges, gentle compression can encourage new clumps to form
the same stars that reveal the nebula, by lighting, it also guides slow rearrangement, astronomers
worked out much of this story, by comparing different kinds of light visible images show the glowing surface,
where ultraviolet radiation reaches the gas, spectra spread that light into bands,
revealing which atoms are present, and how the gas moves, small shifts in spectral lines,
show slow flows, and expanding shells, infrared images, look deeper into the dusty interior,
radio telescopes, map cold molecules across the wider cloud taken together, these methods turn a
soft patch of light into a place with depth and motion, and they give a sense of its age,
the gas is not still, it expands slowly from the central region, and denser knots move at their
own pace, measurements of these motions help estimate how long the nebula has been illuminated,
and how quickly its outline may change, the changes are slow compared with a human calendar,
a telescope image made tonight will look much like one made decades ago,
though careful comparison can detect subtle differences in brightness and flow around some of the
youngest stars, the nebula contains disks of gas and dust, these disks are the places where planets
may later assemble, grain by grain in Orion, bright ultraviolet light can make such disks glow
at their edges, and slowly disperse their outer material, some appear as small bright knots,
with tails pointing away from the central stars, these features are often called droplets,
a short term for protoplanetary disks, their shapes record the direction of nearby starlight,
and the flow of gas, the nebula, is a passing stage in the life of a cloud, some of its new stars
will remain near one another for a long time, while others will drift outward into the galaxy,
the gas that does not become stars, will be warmed, and moved before it gradually disperses
in later ages, the region may contain only a loose cluster of stars, where a glowing cloud
once stood, the light that leaves it tonight will continue across space, long after the current arrangement
of gas has changed, carrying information from a place where stars are still becoming far out
in the constellation torus, beyond the familiar bright stars of Orion, lies a soft patch of glowing
gas known to astronomers, as the crab nebula, it is the remains of a star that ended its life
long ago records kept by sky watchers in the 11th century, describe a new star that shown brightly
for weeks, and then faded the cloud now seen in telescopes is what remains of that event, still spreading
outward in slow motion at the center of this pale cloud sits a compact object, turning steadily
in the dark, this object is a neutron star, it formed when the core of the old star collapsed
under its own gravity, while the outer layers drifted away into space, the core did not vanish,
it was pressed into a sphere, only a few tens of kilometers across, yet holding more mass
than the sun, matter in such a state, is unlike anything found on Earth, the star is supported
by the pressure of neutron's pact at extreme density, and it carries a strong magnetic field threaded
through its rotating body, this neutron star, is the active heart of the nebula, it spins
about 30 times each second, that rate may sound rapid, but from a distance it appears as a repeating
signal, the star does not wobble or rush, each turn takes almost exactly the same amount of time
over many centuries, the rotation has been slowing by a tiny amount, yet the change is so gradual,
that the pulse remains a useful natural clock, the signal arrives, because the star does not shine
evenly in all directions, energy gathers near the magnetic poles, which are not lined up with the spin
axis charged particles, follow the magnetic field, and produce narrow beams of radiation as the
star turns, those beams sweep through space when one of them points toward Earth, telescopes
receive a brief pulse, then the beam moves on, and there is a short pause before the next one arrives,
the effect is much like a lighthouse seen from a ship, or a steady lamp appears to flash,
because the observer only sees it at certain moments, radio waves, are the most familiar
part of this pulse, but the pulse are, also sends out pulses, at higher energies, sensitive instruments,
have recorded its blinking invisible light, x-rays, and gamma rays, the same rotation controls
them all, this makes the object useful as a natural timing source, astronomers can measure the arrival
of its pulses with great care, then compare the pattern over months and years, the regularity,
allows small changes to be noticed, sometimes the star adjusts its spin by a minute amount,
an event called a glitch, these glitches are thought to come from the way, the interlayers
of the neutron star, interact with its crust, they do not disrupt the overall rhythm, they simply
add a slight step to the gradual spin down, the surrounding nebula, is powered by this rotation,
the pulsar loses a small amount of spin energy, with each turn, and that energy flows outward
into the gas, it helps keep the nebula glowing, and gives the cloud a faint blue shine in images,
without the central star, the nebula would be a quieter place, the pulsar acts as a gentle engine,
feeding the remnant with particles and magnetic fields, while it continues to cool and expand
the connection between the pulsar and the nebula was worked out through careful observation,
as radio telescopes improved, they began to detect brief, repeating signals from space, at first,
such signals were unrecognized, the pattern from the crab nebula was especially rapid,
its repeats came with a stable spacing, researchers measured the interval between pulses,
and found it matched, a spinning neutron star, rather than any human-made source, the location,
also matched the bright remnant known from earlier sky records, this link gave strong support to
the idea that some stars leave behind dense rotating cores, after their final bright phase, timing
the pulses, also revealed something about the space between the stars, radio waves of different frequencies,
do not travel through the thin gas of the galaxy, in exactly the same way, lower frequency waves,
are delayed a little more than higher frequency waves by measuring that delay. Astronomers
can estimate the number of free electrons along the line of sight, this pulsar, being bright,
and steady became a useful probe of this diffuse material, the method does not require
visiting the region, it only requires listening to the pulse and noting how it arrives, the pulses
have continued since before telescopes existed, they passed Earth, while people were mapping the sky
with simple instruments, and they continue now, while large radio dishes listen in quiet valleys,
and high planes, the star itself, is far too small to see as a disk in ordinary telescopes,
yet its signal is strong enough to be followed across the galaxy, each pulse carries the same basic
message, a compact object is turning, and its magnetic field is guiding radiation outward,
the geometry of that beam, happens to cross our line of sight, the rotation is not perfectly smooth,
at the finest levels, the pulse profile can drift slightly, and the radio signal, sometimes,
shows small variations in strength, these changes arise from shifting conditions in the
magnetosphere, the region of magnetic fields, and charged particles around the star, studying them,
helps astronomers map how energy escapes from a neutron star, this neutron star,
therefore remains a laboratory for matter under conditions that cannot be reproduced on Earth,
sending its brief radio notes across the galaxy, one turn at a time at the center of the Milky Way,
hidden behind thick lanes of dust, there is a small radio source that marks the home of the
galaxies, central black hole toward the middle of the galaxy's disk, this object cannot be seen
with ordinary eyes, its signal is faint, produced by gas moving in the strongest part of the
galaxies gravitational field, the object itself, has no solid surface, it is a region where matter has
gathered so densely, that once light or gas crosses a certain boundary, it remains within the
black hole that boundary is called the event horizon beyond it, the black hole is known only by the way
it moves the stars and gas nearby, the mass gathered there is enormous by human standards,
yet small compared with the whole galaxy astronomers estimated at about 4 million times the mass
of the Sun, that sounds immense, and it is, but the Milky Way contains hundreds of billions of stars,
the central black hole is a heavy point at the middle of a broad disk, not a drain drawing the disk
inward, its gravity is strongest, very close in farther away, the combined pool of stars and gas
together with dark matter, shapes the galaxy much more than the black hole alone, a black hole
does not pull matter inward simply because it is a black hole if the Sun were replaced by a black hole
of the same mass, the planets would continue along nearly the same paths, the change would be the absence of
sunlight, not a new inward force, the same principle, applies at the Galactic Center stars that pass near
the central mass, feel its gravity strongly, but stars at a distance, including the Sun,
move according to the total mass of the galaxy, nothing is being dragged toward the middle faster,
then ordinary orbital motion allows the central black hole, was not found by seeing a dark sphere,
it was inferred from motion, radio telescopes, first noticed a compact source near the Galactic Center,
later infrared instruments looked through the dust and watched individual stars moving around an
unseen point over many years, those stars traced arcs and ellipses by measuring how fast they moved
and how large their orbits were, astronomers could calculate the mass inside those paths,
the answer pointed to a very compact mass of millions of Sun's no ordinary cluster of stars
could remain so tightly packed without spreading or shining brightly, a black hole
fit the evidence the stars nearest this object are sometimes called the S stars they follow a
elliptical orbits around the same invisible focus some complete a full circuit in only a few
decades which is short for a star their paths have been followed carefully with large telescopes
and precise instruments the orbits are stable they do not spiral inward during each passage
they swing close then move outward again much as comets in our own solar system swing around the
Sun and return the difference is scale the central mass is millions of times heavier than the Sun
so the inner orbits move with measurable speed their paths repeat near the event horizon
the environment is more extreme than the space around ordinary stars but it follows the same
physical laws found elsewhere gas and dust can orbit the black hole for a long time before anything
crosses the boundary much of the material near the center moves in disks or streams heated by compression
and friction some of it gives off radio waves other portions shine in infrared light
or x-rays before settling inward the central source is quiet compared with the brilliant cores
of some distant galaxies it has very little material falling into it at any given time that is one reason
it can be studied calmly as a natural object rather than a spectacular outburst the event
horizon itself is small in galactic terms for a mass of four million sons the radius of the horizon
is roughly 12 million kilometers the full diameter would fit well inside the orbit of mercury
around the Sun compared with the width of the Milky Way which spans tens of thousands of
light years that is almost too small to notice the black holes influence grows with closeness
at great distances its gravity behaves like the gravity of any other object with the same mass this
difference in scale is central to understanding why the galaxy remains settled the reach in
around the black hole contains old stars and younger stars along with warm gas and dust clouds
the black hole sits among them not apart from them in a dramatic sense its presence is revealed
by the order of the motions around it stars do not crowd into the horizon they keep their distances
according to their angular momentum and their orbital energy when a little gas drifts too close
it can add a brief flicker of a mission but the overall pattern remains steady this quietness helps
explain how the black hole can anchor the central region without disrupting the wider galaxy
it provides a common gravitational center for the innermost stars it helps to find the midpoint
of the Milky Way's rotation yet the rotation of the galaxy as a hole depends on mass spread across
a broad disk and halo the central black hole is a small part of that total if it were removed
while the rest of the galaxy remained unchanged most stars would continue much as before the comparison
shows how local the black hole's direct effect becomes once one moves away from the center our own
son is far from the center it circles the galaxy at a distance of tens of thousands of light years
taking hundreds of millions of years to complete one orbit from that vantage the central black hole
is a remote reference point it does not reach into the solar system it does not alter the seasons
or the motions of the planets its role is galactic it belongs to the architecture of the Milky Way
not to the immediate neighborhood of earth astronomers continue to refine the picture orbits are measured
with greater precision and gas behavior near the inner edge is watched carefully comparisons with other
galaxies show that central black holes come in different masses often related to the sizes
of their host galaxies the one at our center is moderate as supermassive black holes go it is large enough
to organize the innermost stellar paths and small enough that it does not dominate the entire galaxy
the name given to this object is a label from radio astronomy but the object itself
is older than labels it formed through processes that are still being studied it may have grown from early
black holes left by massive stars and it may have gained mass from gas and mergers over cosmic time
the accepted view is that it grew slowly alongside the galaxy its history is written in the motions
of the stars around it and in the amount of mass it holds today because the central black hole marks the
galaxy's midpoint so clearly it gives astronomers a reference for mapping the Milky Way coordinates
used for the galaxy can be tied to this central direction and motions near that point help calibrate
the larger rotation of the disk far beyond the quiet edges of the Milky Way two galaxies move
through a shared path so gradual that no human lifetime could notice a change they are known to
catalogs as NGC 4030 and NGC 4039 and to many observers simply as the antennae their name comes from
too long streamers that are away from them thin in appearance and graceful in outline these filaments
are made of stars and gas with dust mixed through drawn outward by gravity working over immense
spans of time they lie in the constellation Corvus tens of millions of light years away at that distance
their light left them long before modern telescopes existed yet it still carries a clear picture
of what they are doing the two galaxies are not separate in the way they once were they have come near
enough for their gravity to reach into each other and rearrange their shapes the result is a slow
exchange of material a stretching and folding that resembles a dance before this encounter each system
likely had its own rotating disk of stars and gas as they passed near one another gravity
began to raise tides in those disks these are gravitational tides gentle differences in pole
from one side of a galaxy to another over many orbits the tides pulled stars and gas outward into the
long tails now visible because the spaces between stars are so wide the stars themselves
almost never meet the encounter reorganizes paths stars follow new curves around shared centers of
gravity clouds of gas drift closer together and gather in those gathered places new stars form
the antennae are full of young stars patches of bright blue light mark regions
or star formation has increased dense knots of gas have contracted into clusters
some containing many young sons these clusters shine brightly in images and they help astronomers
trace where the galaxies have been compressed by their mutual pull the two streamers
that give the pair their name extend far beyond the bright centers they are made of material
lifted out of the galaxy's disks in a close passage the side of a galaxy near its companion
feels a slightly stronger pole than the far side that difference stretches the disk some material swings
outward then settles into a long looping tail the tails can persist for a very long time
slowly thinning as their stars spread along new orbits this process unfolds on a scale
that is difficult to hold in mind the galaxies do not rush together the process is slow they have
already spent many millions of years approaching passing and reshaping one another the changes
visible in a telescope are a single frame from a long sequence if one could watch from a distance
the motion would seem still only by comparing the positions of stars over epochs
far longer than human history with the pattern become plain invisible light the central regions
of the two galaxies appear softened by dust dark lanes curve through the brighter glow
marking places where thick clouds block starlight around them pale pink and blue patches show
where gas is shining under the light of young stars the overall shape is no longer the neat spiral
of an undisturbed galaxy it is open extended and full of gentle irregularity the colors carry
information they record different temperatures and ages of stars observations beyond visible
light add more detail without changing the quiet story infrared light reveals dust
that has been warmed by clusters of new stars radio observations trace the distribution of cool gas
the material from which future stars may form together these views show that the encounter
changes more than outline it redistributes the raw material of galaxies astronomers have learned to
read such forms by combining observations with models images at different wavelengths show where stars are
and where dust lies other measurements reveal glowing gas and show how material moves
within the reshaped disks computer simulations can begin with two ordinary galaxies
and let gravity act on them across simulated time when the simulated encounter is tuned to match the
antenna it produces tails and bright star forming regions much like those seen in the sky the
long tails also contain their own small bright knots some of these are clusters of stars born within
the pulled out material as the tail stretch these knots move along with them like beads carried
on a slow current in some models of interacting galaxies dense clumps in tidal tails
can remain bound and continue shining for a very long time the antennae give astronomers a nearby
place to study such possibilities some knots may later drift as small companions near the merged
galaxy in time the two systems will settle into a single broader galaxy the spiral patterns
that once belong to each love softened into a more rounded shape the tidal tails will fade
as their stars disperse into wider paths new star formation may slow as gas is spread out the result
will be a calm system lit by older stars and moving quietly through space the older stars
will continue their orbits in a broad steady pattern the antennae offer a clear example
of how galaxies can change through gravitational influence they show that large structures can be
reshaped without haste their long encounter helps explain why many galaxies are not fixed
inform gravity given enough time can turn disks into loops and guide stars onto new paths the long
streamers are records of motion they mark where gravity has lifted material and set it on a wider course
the cosmos remains in steady motion empty regions extend across space in darkness
and the faint after glow of the early universe travels on without hurry far away pulsars turn
with patient rhythm sending regular signals through space these patterns do not require attention
to continue they persist while breathing slows and thoughts soften sleep comes as a natural pause
like the hush between one star's light and the next gravity also continues gently
guiding matter over long spans as sleep deepens the body settles in the mind can let the days
details rest sleep is part of the body's daily rhythm the silent cosmos will carry on through
the night its distances calm and its motions reliable you can softly let the day settle into stillness now good night