OUR HOME THE MILKY WAY CONTENTS 2 The Milky Way: Earth’s home galaxy Here’s a closer peek at the galaxy we call home. 8 What’s blowing bubbles in the Milky Way? Astronomers peer into the gamma-ray bubbles at our galaxy’s center. 14 What lurks in the monstrous heart of the Milky Way? Unlocking the mysteries of our supermassive black hole. ESO/B. TAFRESHI A supplement to Astronomy magazine THE MILKY WAY Scientists think our gal- axy has four major spiral arms that wind their way out from a central bar. The Sun lies approximately S 27,200 light-years from cu the center. NASA/JPL-CALTECH/R. t HURT (SSC-CALTECH) um -C Earth’s home galaxy e n ta u r Giant clouds of gas and dust sprinkled u s A with splashy star clusters adorn the Milky Way’s r spiral arms, while the galaxy’s vast halo teems m with darker matter. by Francis Reddy m r m A r A s u s i u r e a s t SOONER OR LATER ON ANY inside a disk where dust clouds dim and r t i e CLEAR, DARK NIGHT, AN ETHE­ block starlight. g P a The true scale of the Milky Way Galaxy S REAL BAND CALLED THE MILKY - — and, indeed, the universe as a whole — a WAY ARCHES ACROSS THE SKY. became dramatically clearer in the 1920s. Central bar in Although recognized since antiquity, phi- That’s when a new generation of large tele- r a losophers and scientists could only guess at scopes coupled with photography revealed C what it represented until fairly recently (see that “spiral nebulae” were actually entire “How the Milky Way Galaxy got its name,” galaxies like our own — “island universes” p. 33). With the invention of the telescope, in the evocative parlance of the time. Sur- it became clear that the Milky Way was the veys showed that most disk-shaped galaxies collective glow of stars too faint to be seen possessed winding spiral arms where young by the naked eye. More than a century later, stars, gas, and dust were concentrated. English astronomer Thomas Wright sug- Astronomers assumed our galaxy was a gested that this glowing band was precisely spiral too. In the 1950s, radio telescopes what one would expect to see if the Sun were produced the first crude maps of the Milky Sun embedded in a flat disk of stars. Way’s spiral arms by tracking how gas We now know that the Milky Way is the clouds moved around the galaxy. primary structure of our galaxy seen edge- Over the past two decades, surveys using wise. Additional detail and especially the dust-penetrating infrared light have brought physical scale of the galaxy took another two the general picture of our galaxy into better centuries to work out. The process continues focus. These projects include the ground- today as astronomers wrestle with conflict- based Two Micron All-Sky Survey and Sloan No ing evidence and make new discoveries. Digital Sky Survey (SDSS) as well as two rm Much like mapping a fogbound city from a NASA spacecraft, the Wide-field Infrared a-O single intersection, scientists must decipher Survey Explorer (WISE) and the ute the galaxy’s structure while viewing it from Spitzer Space Telescope. These FAST FACT r Arm observations have helped astron- Francis Reddy is the senior science writer for the omers better define our galaxy’s MERTEN (HEIDELBERG/BOLOGNA) I)/J. C Astrophysics Science Division at NASA’s Goddard spiral arms, take a census of star 27,200 Space Flight Center in Greenbelt, Maryland. clusters and other phenomena in LIGHT-YEARS (PANDORA’S CLUSTER) (PANDORA’S Pandora’s Cluster ESO/D. COE (STS The Sun’s distance 3.52 billion light-years from the galaxy’s billion light-years center 2.75 3 3.25 3.5 © 2015 Kalmbach Publishing Co. This material may not be reproduced in WWW.ASTRONOMY.COM 3 any form without permission from the publisher. www.Astronomy.com HOW THE MILKY WAY GALAXY GOT ITS NAME Spend some time under the stars any clear night far from city lights, and a ghostly band called the Milky Way eventually will come into view. Flecked with some of the brightest stars in our galaxy and cleaved by intervening dust clouds for about a third of its extent, the Milky Way has been recog- nized since antiquity. In mythology, it was frequently associated with a cosmic path- way or heavenly stream. The ancient Greeks called it galaxías kýklos, the “milky circle,” a description that At a distance of 1,350 light-years, the Orion Massive stars live fast and die young, exploding also gave rise to our word “galaxy.” The Nebula (M42) is the nearest large star-forming as supernovae and leaving behind remnants like Romans lifted the concept but gave it a region. Our Sun likely formed in a cloud like this, the Crab Nebula (M1). Such star death often trig- twist appropriate for a civilization fond of one capable of producing 1,000 to 10,000 stars. gers future star formation as shock waves com- road construction, calling it via lactea, the NASA/ESA/M. ROBBERTO (STSCI/ESA)/THE HST ORION TREASURY PROJECT TEAM press surrounding gas. NASA/ESA/J. HESTER AND A. LOLL (ASU) “milky way.” Galileo Galilei took the first step in understanding what it actually The Carina Nebula (NGC 3372) ranks among represented in 1610, when his new and the Milky Way’s biggest stellar nurseries. It lies passage to reach planetary orbits, but the The night sky distorts our picture of the improved spyglass revealed that the pale about 7,500 light-years from Earth and burst to system’s low mass — just one-sixth the galaxy’s stars in another way, too. Of the life when its first stars ignited some 3 million light came from individual faint stars “so years ago. Today, it holds nine stars with lumi- numerous as almost to surpass belief.” Sun’s — and its path through the outer 100 brightest stars in the sky, a third lie nosities at least a million times that of the Sun. Over the next two centuries, as astrono- Oort Cloud argue against any significant within 100 light-years. These include Sirius, NASA/ESA/N. SMITH (UCB)/THE HUBBLE HERITAGE TEAM (STSCI/AURA) mers began to understand that the Milky comet enhancement. the night sky’s brightest, 8.6 light-years Way was part of an “island universe” that Dim, low-mass M dwarfs like Scholz’s away; Procyon, 11 light-years away; Vega included the Sun and other visible stars, the Star and Proxima Centauri actually typify and Fomalhaut, both 25 light-years off; name for a mythical cosmic pathway was Exploration of Radio Astrometry (VERA), transferred to our galactic home. — F. R. the Milky Way’s stellar population. Most Castor (52); Aldebaran (65); and Regulus are using this capability to pinpoint the of the galaxy’s roughly 400 billion stars (77). But another third lie more than 400 locations and motions of regions where are likely M dwarfs, but because they emit light-years away, including Polaris (430), new stars are forming in order to trace little visible light, we’re still finding those Antares (600), Betelgeuse (640), Rigel (860), our galaxy’s spiral structure. complete, the Sun’s position oscillates close to the solar system through infrared and Deneb (2,600). All these stars have above and below the galactic plane. Other surveys like WISE. For stars, mass is des- masses more than seven times the Sun’s and Movin’ out stars in our vicinity follow slightly different tiny. M dwarfs may be dim, but their low are tens of thousands of times more lumi- The frontier of the galaxy lies at the outer paths, which means that the distribution masses mean they burn their nous. Consequently, they burn fringe of the Oort Cloud of comets, about and composition of our stellar neighbor- nuclear fuel sparingly and will through their hydrogen fuel at 100,000 astronomical units (AU; the aver- hood gradually changes. Stars routinely keep shining billions of FAST FACT a faster clip. Long before our age Earth-Sun distance) or 1.6 light-years pass much closer to the Sun than Proxima years after the Sun dies. Sun’s fires quench, these away (see “From AU to light-year,” p. 34). Centauri is now. Some stars barely stars will end their days The Pleiades star cluster (M45) resides 440 light-years from Earth in the constellation Taurus. A proto- Here, the Sun’s gravitational pull weakens For example, in 2014, astronomer Ralf- shine at all. They never 150,000 in spectacular supernova typical open cluster, it spans about 15 light-years and holds some 500 stars. These luminaries will to the level of nearby stars, and comets Dieter Scholz of the Leibniz Institute for generate energy in their LIGHT-YEARS explosions. disperse over the next few hundred million years. NASA/ESA AND AURA/CALTECH whose orbits take them this far may drift Astrophysics in Potsdam, Germany, dis- cores though true hydro- out of the Sun’s grasp entirely. Although covered that a faint M dwarf star detected gen fusion, the power Diameter of the From stars to the nearest star today is Proxima Centauri, by WISE was about 20 light-years away, source that heats stars galaxy’s disk clusters dust-obscured regions of the disk, and But Gaia largely covers optical wave- 4.22 light-years away, other stars played this making it a previously unknown close for most of their lives, but Going further up the reveal that the central component of our lengths, which means that intervening dust role in the past and will do so in the future.