Astronomy, the most ancient of sciences, has always been about mapping.
Australian aborigines looked at the constellation we call Orion and saw a canoe carrying two banished brothers. The Finns saw a scythe. In India, it was obviously a deer. For the Babylonians, it was the heavenly shepherd, and for the Greeks, it was the hunter, a primordial giant.
Over the centuries, mapping the cosmos has been a gradual process of locating the brighter objects and then filling in the gaps. We have helped our eyes along by constructing telescopes, some gathering much more than just visible light to illuminate phenomena beyond our wildest imaginations.
Seeing the universe for what it is has required us to overcome many other blind spots, including the one that places ourselves at the center of the map. It took the insight and intellectual conviction of Galileo and Copernicus to challenge the orthodoxy that Earth was at the center of everything. Even then, the notion that our solar system was nonetheless located somewhere at the middle of the visible universe lasted into the first decades of the 20th century.
The discovery, by the astronomer Harlow Shapley in 1918, that our solar system was not even at the center of the Milky Way galaxy opened the floodgates for more revelations in the following decades. The Milky Way, it turned out, is merely one of many galaxies, all flying apart as the universe expands.
So what does our current map look like? It is both three-dimensional and four-dimensional, linked as it is to time. The farther away objects are, the longer their light has taken to reach us, all the way back through the universe’s 13.8-billion-year history. There are so many categories of objects and phenomena, and so much higgledy-piggledy data from several hundred years of telescopic astronomy, the best we can do to begin to grasp what this atlas looks like is to play out a thought experiment.
Let us pretend that a very large box has just been delivered to our doorstep, and we have hauled it inside. It contains an ominous-looking sack filled to bursting. An occasional wisp of gas escapes through the knotted top, and every so often a muffled thump or muted glow comes from within.
This sack contains what we could regard as a representative portion of the universe -- a “fair sample,” a cosmologist would say. If you divided the total mass in the sack by its volume, you would obtain a good estimate of the average density of the universe as a whole. Equally, if you measured just how lumpy the arrangement of galaxies was within this volume, it would be a close match to the universal lumpiness of structure.
The first thing that happens when you cautiously untie the sack is that electromagnetic radiation floods out, together with particles of all kinds. Photons of light are present in huge quantities, from extremely low-frequency radio waves, where a single crest-to-crest distance may span kilometers, to microwave, infrared, visible and ultraviolet frequencies, on to the realm of X-rays and gamma rays. Light may not have mass, but the cosmos is thick with it.
The other particles that come pouring out of the sack are more difficult to quantify. Neutrinos, with less than about a millionth of the mass of electrons, come in a variety of flavors: electron, muon and tau. They have been likened to the “ghosts of the cosmos” because they have very little to do with normal matter, passing through gases, liquids and solids. Here on Earth, every second, roughly 65 billion neutrinos from the sun’s core pass through every square centimeter of your skin.
Most of the recognizable matter is in the form of hydrogen and helium, in the proportion of roughly seven hydrogen atoms to every one of helium. The next most abundant element is oxygen, though there is only one oxygen atom for roughly every 1,500 hydrogen atoms. All the elements that are so critical in making objects like planets, and the molecules that are part of us and all living things, are rare -- cosmic pollutants.
Some of these come zooming out of the sack with considerable speed. In this case they are components of hot gases, often so hot that most of the electrons that usually stick to an atomic nucleus have been stripped away, leaving an electrically positive object known as an ion. A gas in this state is also referred to as plasma, and it can have a temperature of tens of millions of degrees. Other normal matter seeps out at an extremely slow rate -- the components of much, much colder gases, some barely a few degrees above absolute zero.
Within this colder gas are molecules, mainly hydrogen, and traces of compounds such as carbon monoxide, carbon dioxide, water and even alcohols. Roughly 70 percent of all the heavier molecules adrift in the universe contain carbon. Many of them are the same organic compounds we find here on Earth.
There is also something else that we can barely sense: dark matter. It is most likely some variety of subatomic particle that has very weak interactions with “normal” matter. Much like the neutrino, it can drift right through solid material, but it moves slowly, and each particle carries a significant mass. This mysterious stuff outweighs all normal matter by a factor of five, dominating the mass of the universe.
Now let us peel away the sack and see how the major ingredients are arranged. The first thing we notice is that space is filled with, well, space. You would have to separate grains of sand by about 60 miles to make an equivalent sparseness. Yet if we step back far enough, vast gatherings of galaxies emerge. The littlest of these may contain 100 million stars and largest, hundreds of billions. Some stars are red and dim, barely a 10th the mass of our sun. Others are bright blue, with 10 times the sun’s mass and 10 to 20 times its girth. The big stars are rarer, though, and rarest of all are giants 100 times the mass of our sun.
Many more star-like objects exist in the galaxies -- including protostars, not yet ready to fuse elements, and the remains of dying stars. White dwarfs are cooling dense lumps of leftover stellar material. Fearsomely dense neutron stars are the descendants of the most massive stars.
Then there are the black holes. In a big galaxy, small black holes, a few times the mass of our sun, may number in tens of thousands. Sometimes matter falls screaming into their gravitational lairs, releasing its energy as brilliant flares of electromagnetic radiation. Rarer, but cosmically more important, are the giant black holes -- the lords of gravity. Billions of times more massive than our sun, they sit imperiously within the deep recesses of galaxies, a great mystery for us to explore.
Our own Milky Way is among the largest of galaxies, containing several hundred billion stars, protostars and stellar remains. About 15 percent of all the galaxies we see are, like the Milky Way, great flattened, disk-like structures with huge curving rivers of stars, as if paint had been dribbled onto a spinning plate. Most of the rest are dandelion-like spheres of stars known as ellipticals. In between are all manner of hybrids and lumpy, distorted galaxies collectively known as irregulars.
Giant ellipticals most often sit at the middle of the great clusters of galaxies. Spiral galaxies, with their young, hot, massive blue stars, lurk in the hinterlands, slowly spinning. Stars in elliptical galaxies fly to and fro like angry hornets in toward the galactic center and then back out again on the other side. In a spiral, the stars out in the great disk follow circular orbits around the center, wobbling as the lumpy system pushes and pulls on them. Our own sun completes a circuit of the Milky Way every 210 million years.
Galaxies themselves stay in constant motion, too, flying in and out of the cluster core. Matter streams along the web-like threads of larger structures, filling the clusters and superclusters with material, as gullies feed mountain lakes.
More than half the stars are orbited by big and small planets, some with their own moons and satellites. Then there are shadowy brown dwarfs, neither big enough to be stars nor small enough to be comfortably called planets.
There is something extraordinary almost anywhere we look. In a remote corner of one galaxy a giant old star is just minutes away from yielding to gravity’s persistent embrace. When it does, its core will collapse inward, then rebound and blow the star apart in a great and brilliant supernova. Over there is a double pulsar: twin neutron stars, each the mass of two suns yet less than 8 miles across, racing about each other like the ends of a furiously spinning dumbbell, beaming radio waves. And off in a majestic spiral galaxy, a small rocky planet orbits a moderately bright star. Its three small moons tug at its great equatorial ocean. On distant shores, water laps over a growing green carpet, home to countless scuttling microscopic forms.
The total number of galaxies in the observable universe probably exceeds 100 billion, and may be closer to 200 billion distinct systems. A single large galaxy such as our own Milky Way may contain upward of 200 billion normal stars.
Estimation is as much art as science. Nonetheless it’s a safe bet that there are a billion trillion individual stars in the observable universe -- and possibly 10 to 100 times more than that, about 10 billion stars for every human ever born.
Why is it this number? It’s a really intriguing question, and in trying to answer it, we’ve posited that black holes may play an active role in sculpting the universe. Our map of forever gives us a jumping-off point for following this line of reasoning. The connections between our atlas and the gravity machines that have helped craft it are there for the taking. We just need to find them.
(Caleb Scharf, an astrophysicist, is the director of Columbia University’s Astrobiology Center and author of the blog Life, Unbounded. This is the first of five excerpts from his new book, “Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars and Life in the Cosmos,” which will be published on Aug. 14 by Farrar, Straus and Giroux. The opinions expressed are his own. Read part 2, part 3, part 4, part 5.)
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