Milky Way

• 1. Introduction

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  The Milky Way is the galaxy that includes the Solar System, and its name comes from how it looks from Earth: a hazy band of light arching across the night sky. That band is formed from stars in other arms of the galaxy. They sit so far away that the naked eye cannot pick them out one by one. For most of human history, nobody knew what they were looking at. In 1610, Galileo Galilei pointed an optical telescope at that pale ribbon and resolved it into a vast number of faint stars. Even then, the scale stayed hidden. Until the early 1920s, most astronomers thought the Milky Way contained all the stars in the universe. What changed that belief, and what did we learn the band actually is? How big is this galaxy, what holds it together, and where does our own Sun sit inside it? More than one-third of Earth's population can no longer see the Milky Way from their homes because of light pollution, so much of what follows describes a sky most people have never witnessed.

• 2. Resolving The Band

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  Aristotle, who lived from 384 to 322 BC, believed the Milky Way was part of Earth's upper atmosphere, a byproduct of burning stars that lingered in the outermost air. He recorded that earlier Greek philosophers disagreed. Anaxagoras and Democritus proposed instead that the band was the glow of stars not directly visible because of Earth's shadow. The Neoplatonist philosopher Olympiodorus the Younger, who lived around 495 to 570 AD, attacked Aristotle's view. He argued that if the Milky Way were below the Moon, it would show parallax and look different from different places on Earth, and it does neither. The Persian astronomer Al-Biruni, born in 973, called it a collection of countless fragments of the nature of nebulous stars. Nasir al-Din al-Tusi, born in 1201, wrote in his Tadhkira that the Galaxy is made up of a very large number of small, tightly clustered stars, likened to milk in color because of their concentration and smallness. Proof arrived in 1610 through Galileo's telescope. In a treatise in 1755, Immanuel Kant speculated correctly that the Milky Way might be a rotating body of a huge number of stars held together by gravity, seen as a band from our vantage point inside the disk. Kant called both the Milky Way and the distant nebulae island universes, a term still in use up to the 1930s.

• 3. The Great Debate

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  In 1785, William Herschel tried to map the shape of the galaxy by carefully counting stars across different regions of the sky. He drew a diagram that placed the Solar System near the center, an error that would take more than a century to correct. Lord Rosse built a new telescope in 1845 and resolved dim blobs known as nebulae into individual points of light, working out the spiral structure of what is now called the Whirlpool Galaxy. In 1917, Heber Doust Curtis spotted the nova S Andromedae in the Great Andromeda Nebula and found 11 more novae in the photographic record. These novae were on average 10 magnitudes fainter than those inside the Milky Way, which led Curtis to a distance estimate of 150,000 parsecs. In 1920, Curtis faced Harlow Shapley in the Great Debate over whether the spiral nebulae were independent galaxies or part of our own. Edwin Hubble settled it in the early 1920s with the 100-inch Hooker telescope at Mount Wilson. He resolved the outer parts of some spiral nebulae into individual stars and used Cepheid variables to measure distance. He found the Andromeda Nebula sits 275,000 parsecs from the Sun, far too distant to belong to the Milky Way. The galaxy was just one of many.

• 4. Anatomy Of A Spiral

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  The Milky Way is a barred spiral galaxy with a D25 isophotal diameter estimated at 26.8 kpc, give or take 1.1, yet only about 1,000 light-years thick at the spiral arms. The mass distribution closely resembles type Sbc in the Hubble classification, the spirals with relatively loosely wound arms. Astronomers first began to suspect a central bar in the 1960s, and the Spitzer Space Telescope confirmed it in 2005, showing the bar to be larger than previously thought. The interstellar medium and stars beyond the bar organize into four spiral arms, including the Carina-Sagittarius Arm, the Perseus Arm, the Scutum-Centaurus Arm, and the Norma and Outer arm. Two surveys of near-infrared light, which sees red giants without being blocked by dust, found that the Scutum-Centaurus Arm holds about 30% more red giants than expected, while the Carina-Sagittarius Arm showed no such excess. This suggests the galaxy has only two major stellar arms but four arms traced by gas and young stars, and the reason for the discrepancy is unclear. The Near 3 kpc Arm, found in the 1950s through 21 centimeter radio measurements of atomic hydrogen, is expanding away from the bulge at more than 50 km/s. A simulation published in 2011 suggested the spiral structure may come from repeated collisions with the Sagittarius Dwarf Elliptical Galaxy.

• 5. The Galactic Center

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  Sagittarius A* sits at the heart of the galaxy, an intense radio source whose name is pronounced Sagittarius A-star. The motion of material around it points to a supermassive black hole with a mass estimated at 4.1 to 4.5 million times that of the Sun. Its rate of accretion fits an inactive galactic nucleus, roughly one solar mass per year. The inner few kiloparsecs hold a dense, roughly spheroidal concentration of mostly old stars called the bulge. Around the bar may run a structure called the 5 kpc ring, which contains a large fraction of the galaxy's molecular hydrogen and most of its star formation. Since 1970, gamma-ray missions have detected 511-keV gamma rays from the direction of the center, produced when positrons annihilate with electrons. In 2010, the Fermi Gamma-ray Space Telescope revealed two giant spherical bubbles of high-energy gamma emission, one north and one south of the core, each about 25,000 light-years across. They are best read as a magnetized outflow driven by star formation in the central 640 light-years. On the 5th of January 2015, NASA reported an X-ray flare from Sagittarius A* 400 times brighter than usual, a record, possibly caused by an asteroid breaking apart as it fell toward the black hole.

• 6. Counting The Stars

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  Between 100 and 400 billion stars fill the Milky Way, along with at least as many planets. An exact figure depends on counting very-low-mass stars, which are hard to detect beyond 300 light-years from the Sun. The galaxy may hold ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes. A January 2013 study of the five-planet system Kepler-32 supported an average of at least one planet per star, while a separate analysis that month estimated at least 17 billion Earth-sized exoplanets. In November 2013, astronomers reported there could be as many as 40 billion Earth-sized planets in the habitable zones of Sun-like stars and red dwarfs, with 11 billion of those orbiting Sun-like stars. The nearest exoplanet may lie 4.2 light-years away, orbiting the red dwarf Proxima Centauri, according to a 2016 study. Compared with more distant galaxies, the Milky Way has a below average neutrino luminosity, making it a neutrino desert. In June 2023, a team led by Naoko Kurahashi Neilson used a new cascade neutrino technique to detect neutrinos from the galactic plane for the first time, creating the first neutrino view of the galaxy.

• 7. Hidden Mass

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  Stars at the outer edge of the Milky Way move as fast as 254 km/s, measured by the Very Long Baseline Array in 2009. If the galaxy held only the matter visible in stars, gas, and dust, that rotational speed should drop with distance from the center. Instead the observed rotation curve stays relatively flat, the best evidence for dark matter in galaxies. Away from the central bulge or outer rim, the typical stellar orbital speed runs between 200 and 220 km/s. Much of the galaxy's mass appears to be this invisible dark matter, conjectured to spread out beyond a hundred kiloparsecs from the center. Interstellar gas adds its own share, comprising 90% hydrogen and 10% helium by mass, with two-thirds of the hydrogen in atomic form. Estimates of the total mass swing widely depending on method. In March 2019, astronomers put the virial mass at 1.54 times ten to the twelfth solar masses within about 39.5 kpc, suggesting roughly 90% of the galaxy is dark matter. Then in September 2023, using data from the Gaia spacecraft, astronomers reported a virial mass only a tenth that size. Gaia has expanded the number of observed stars from about 2 million in the 1990s to 2 billion.

• 8. Our Place And Our Fate

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  The Sun sits near the inner rim of the Orion Arm, roughly 27,000 light-years from the Galactic Center, inside the Local Fluff of the Local Bubble. Studies of stellar orbits around Sagittarius A* place it at about 8.32 kpc out, though other analyses favor 7.86 kpc. It takes the Solar System about 240 million years to complete one orbit, a galactic year, so the Sun has likely finished 18 to 20 orbits in its lifetime and just 1/1250 of a revolution since humans appeared. The orbital speed is about 220 km/s, which is 0.073% of the speed of light. The Sun passes through the galactic plane roughly 2.7 times per orbit, oscillating like a simple harmonic oscillator, motions once thought to coincide with mass extinctions on Earth. The whole galaxy is in motion too, traveling at about 630 km/s relative to the local co-moving frame, heading toward the Great Attractor and the Shapley Supercluster. Current measurements show the Andromeda Galaxy approaching at 100 to 140 km/s. In 4.3 billion years, the two may collide. The odds of individual stars striking each other are extremely low, and over about six billion years the pair would merge into a single galaxy. Even sooner, simulations suggest the Milky Way's own star formation could fade within about five billion years as it drifts deeper into the green valley.