Galaxy

• 1. Introduction

  ›

  A galaxy holds, on average, an estimated 100 million stars, each one orbiting a shared center of mass that none of them will ever reach. The smallest galaxies, the dwarfs, hold fewer than a thousand stars. The largest known, the supergiants, hold one hundred trillion. Yet for all those points of light, the stars are almost beside the point. In a typical galaxy, most of the mass is dark matter, and only a few percent of it shows up as stars and nebulae at all. The word itself comes from the Greek galaxias, meaning milky, a nod to the band of light our own Solar System sits inside. For most of human history, nobody knew what that band was, or whether anything lay beyond it. The questions that follow are the ones astronomers spent centuries chasing. How many of these systems are there. How do you measure something with no edge. And how did the universe come to be filled with them at all.

• 2. Counting The Uncountable

  ›

  Estimates of how many galaxies fill the observable universe run between 200 billion and 2 trillion. That enormous range is itself the story, because galaxies are far easier to suspect than to confirm. The Hubble Deep Field, an extremely long exposure aimed at a relatively empty patch of sky, suggested there were about 125 billion galaxies in the observable universe. A 2016 study led by Christopher Conselice of the University of Nottingham pushed that figure to at least two trillion, a factor of ten more than appear directly in Hubble images. Later observations with the New Horizons space probe, made from outside the zodiacal light, detected less cosmic optical light than Conselice had, while still hinting that direct observations are missing galaxies. Even the catalogues reflect this glut. Millions of galaxies have been recorded, but only a handful carry well-established names. Most are known instead by numbers from catalogues like Messier, the New General Catalogue, or the Sloan Digital Sky Survey, each system assigning the same object a different label. A single spiral, Messier 109, also answers to NGC 3992, UGC 6937, and PGC 37617, among others.

• 3. The Great Debate

  ›

  In 1920, two astronomers named Harlow Shapley and Heber Curtis met to argue over the size of the universe itself, in what became known as the Great Debate. The disagreement was whether the faint spiral nebulae scattered across the sky lay inside the Milky Way or stood apart as separate systems entirely. Curtis backed the so-called island universes hypothesis, pointing to dark lanes in the Great Andromeda Nebula that resembled the dust clouds of the Milky Way, along with a significant Doppler shift. The matter was settled not by debate but by a bigger instrument. Using the 100-inch Mount Wilson telescope, Edwin Hubble resolved the outer parts of some spiral nebulae into individual stars and identified Cepheid variables among them. Those stars let him measure distances, and the nebulae proved far too distant to belong to the Milky Way. The names lagged behind the science. Harlow Shapley had pushed to call these objects galaxies rather than nebulae or universes, but the influential Hubble kept saying nebulae, and the nomenclature did not fully change until after Hubble's death in 1953. In 1926, Hubble produced a classification of galactic morphology still in use today.

• 4. A Family Of Shapes

  ›

  Hubble's classification sorts galaxies into three main types: ellipticals, spirals, and irregulars. Elliptical galaxies are rated by how stretched they look, from E0, nearly spherical, up to E7, highly elongated. They hold little interstellar matter and few young stars, dominated instead by older stars orbiting in random directions, low in heavy elements because star formation ceased after an initial burst. Spiral galaxies turn like pinwheels, with a rotating disk and a central bulge of older stars, their bright arms forming approximate logarithmic spirals. Those arms are thought to be density waves, regions of high-density matter that slow passing stars the way a wave of brake lights ripples down a crowded highway. A majority of spirals, the Milky Way included, carry a bar-shaped band of stars across the core. Between these categories sits the lenticular galaxy, Hubble type S0, which shares traits of both ellipticals and spirals. The largest galaxies of all are the type-cD galaxies, first described in 1964 in a paper by Thomas A. Matthews and others. Popularly called supergiant elliptical galaxies, they wrap a central nucleus in a faint halo of stars stretching to megaparsec scales, and the leading model holds that they grow by swallowing smaller galaxies in dense clusters.

• 5. The Unseen Mass

  ›

  In the 1970s, Vera Rubin found a discrepancy that should not have existed. Galaxies were rotating faster than the visible mass of their stars and gas could explain. Today that galaxy rotation problem is attributed to large quantities of unseen dark matter. Most of the mass in a typical galaxy is dark matter, with only a few percent visible as stars and nebulae. A study of 27 Milky Way neighbors sharpened the point. In every dwarf galaxy examined, the central mass came out to roughly 10 million solar masses, whether the galaxy held thousands of stars or millions. That consistency suggests galaxies are largely shaped by dark matter, and that a minimum size may point to a form of warm dark matter unable to coalesce on smaller scales. The pull of dark matter scales up beyond single galaxies. In clusters, about 70 to 80 percent of the mass is dark matter, with 10 to 30 percent in heated gas reaching temperatures of 30 to 100 megakelvins, and only a few percent in the galaxies themselves. These associations formed early, as clumps of dark matter drew their galaxies together.

• 6. Cores That Outshine Their Galaxies

  ›

  Some galaxies pour out more energy from a single point at their center than from all their stars combined. These are the active galaxies, defined by an active galactic nucleus where a supermassive black hole sits at the core. Radiation pours from the gravitational energy of matter falling toward the black hole through an accretion disc, and the luminosity depends on the black hole's mass and how fast matter falls in. The least luminous of these are called Seyfert galaxies, which look like ordinary galaxies with a bright star pasted over the core. The most luminous are the quasars, the most energetic and distant active nuclei of all, capable of shining 100 times brighter than the Milky Way. The nearest known quasar, Markarian 231, sits about 581 million light-years from Earth, while others reach as far as UHZ1, roughly 13.2 billion light-years distant. In about 10 percent of active galaxies, a pair of opposed jets fires particles from the core at velocities close to the speed of light, by a mechanism still not well understood. Push those jets far enough and you get a radio galaxy. Alcyoneus, an FR II class radio galaxy, has the largest observed radio emission, its lobes spanning 5 megaparsecs.

• 7. Webs, Sheets, And Voids

  ›

  Only about 5 percent of surveyed galaxies are truly isolated, having avoided significant interaction with comparable galaxies for the past few billion years. The rest are bound into groups, clusters, and superclusters arranged in a fractal-like hierarchy. A group of galaxies is the most common type and holds the majority of galaxies in the universe. Clusters gather hundreds to thousands, often dominated by a single giant elliptical, the brightest cluster galaxy, which tidally destroys its satellites and absorbs their mass over time. Superclusters reach into the tens of thousands of galaxies, and at that scale matter spreads into sheets and filaments wrapped around immense empty voids. The Hercules-Corona Borealis Great Wall, the largest structure found so far, stretches 10 billion light-years, three gigaparsecs, in length. The Milky Way lives in the Local Group, about one megaparsec across, which it shares with the Andromeda Galaxy as the two brightest members. That group belongs to the Virgo Supercluster, which in turn forms part of the Laniakea Supercluster.

• 8. How Galaxies Begin And End

  ›

  About 300,000 years after the Big Bang, atoms of hydrogen and helium formed in an event called recombination, beginning a period known as the dark ages when neutral hydrogen absorbed light and no stars yet shone. From tiny density fluctuations in that primordial matter, baryonic gas condensed within cold dark matter halos, forming protogalaxies. The first stars to appear inside them, the Population III stars, were made almost entirely of hydrogen and helium. Their ultraviolet light re-ionized the surrounding neutral hydrogen in expanding spheres, an event called reionization, and their deaths in supernovae seeded the interstellar medium with heavier elements. Within a billion years, a galaxy grows globular clusters, a central supermassive black hole, and a bulge of metal-poor Population II stars. Galaxies do not stop changing. The Milky Way and Andromeda are approaching each other at about 130 km/s and may collide in roughly five to six billion years. Cosmological simulations suggest the Milky Way merged with a particularly large galaxy, labeled the Kraken, around 11 billion years ago. The current era of star formation is expected to last up to one hundred billion years. After that, across ten trillion to one hundred trillion years, the smallest red dwarfs will fade, and galaxies will be left as brown dwarfs, cold black dwarfs, neutron stars, and black holes, their remaining stars eventually falling into central black holes or flung into intergalactic space.