The monster in the Milky Way

The monster in the Milky Way

Artist's impression of a supermassive black hole • ESO/L. Calçada (CC BY 4.0)

Originally published 29 July 1996

Every child at some time in his life has writ­ten his address this way: Bil­ly Smith, 426 Oak Ave., Boston, Mass­a­chu­setts, USA, Earth, Solar Sys­tem, Milky Way, Universe.

Our sense of place is defined by con­cen­tric cir­cles: fam­i­ly, neigh­bors, friends, coun­try­men, species, life, nature, God. Let’s back up one step from the most inclu­sive term of our address — to the Milky Way.

We live in a uni­verse of galax­ies. We exist only because the galax­ies are great reser­voirs of hydro­gen and heli­um. Hydro­gen and heli­um are not the most sta­ble atom­ic forms; car­bon, oxy­gen, sil­i­con, and iron, for exam­ple, are suc­ces­sive­ly more sta­ble. When the nuclei of hydro­gen and heli­um fuse in the cores of stars to form heav­ier, more sta­ble ele­ments, ener­gy is released.

With­out this free ener­gy there could be no life.

In tech­ni­cal terms, the bios­phere of Earth is a non-equi­lib­ri­um ther­mo­dy­nam­ic sys­tem sus­tained by radi­a­tion from one of the tril­lion stars in the Milky Way Galaxy — the sun — a fur­nace in which hydro­gen is burned into heli­um, and ulti­mate­ly into car­bon and oxy­gen. The com­plex­i­ty of life can only be cre­at­ed and sus­tained at the expense of the “run­ning down” of our star.

The Milky Way, then, is that part of our address that puts us square­ly in the cos­mic flow of mat­ter and ener­gy that brings com­plex­i­ty out of chaos.

And what an object! There are as many stars in the Milky Way Galaxy as there are grains in 10,000 box­es of salt, embed­ded in the bil­low­ing neb­u­las of hydro­gen and heli­um out of which they are born. A glit­ter­ing vor­tex of stars. An explod­ing St. Cather­ine’s wheel of supernovas.

At the cen­ter of the vor­tex lurks a monster.

In recent decades, astronomers have become increas­ing­ly con­vinced that galax­ies have super­mas­sive black holes at their cen­ters — small, dense ker­nels with the mass of mil­lions or bil­lions of suns. So pow­er­ful is the grav­i­ty of such an object that noth­ing can escape its pull, not even light.

Gas, dust, and stars orbit the black holes at prodi­gious speeds, some­times being drawn into these vora­cious drains. As they spin into obliv­ion, they emit tell­tale radi­a­tion. Black holes at the cen­ters of galax­ies are pow­er­ful bea­cons of radio and x‑ray energy.

Ear­ly in the his­to­ry of the uni­verse, as the galax­ies and their black-hole ker­nels were form­ing, these bea­cons were brighter than entire galax­ies. We see them today at great dis­tances, and there­fore at those ear­li­er epochs in time, as tiny super-lumi­nous objects called quasars.

As time passed, less mat­ter went down those cen­tral galac­tic drains. The black hole mon­sters grew at a slow­er rate and their vio­lence sub­sided. The blaz­ing bea­cons of the quasars fad­ed. But the black holes remain, hid­den by the light of the sur­round­ing stars, lurk­ing relics of creation.

The most cer­tain way to prove the exis­tence of galac­tic black holes is by their grav­i­ta­tion­al influ­ence on the stars that orbit them. If a black hole resides at the cen­ter of a galaxy, then stars will orbit at high­er veloc­i­ties than if the cen­ter of the galaxy is empty.

The veloc­i­ty of a star can be mea­sured by a change in the fre­quen­cy of its light (the same prin­ci­ple, called the Doppler effect, is employed when a radar gun mea­sures the veloc­i­ty of a car). The high-res­o­lu­tion Hub­ble space tele­scope has been able to make these mea­sure­ments more accu­rate­ly than Earth-based instru­ments, sub­stan­tial­ly strength­en­ing the evi­dence that many galax­ies do indeed have super­mas­sive black holes at their centers.

Our own Milky Way Galaxy is no excep­tion. It too appar­ent­ly has a black hole at its core, with a mass of sev­er­al mil­lion suns and a size small­er than the Earth­’s orbit.

Whirlpools and mon­sters have long been stock items in myth and sto­ry. We remem­ber Scyl­la and Charyb­dis, the mon­ster and whirlpool that threat­ened Odysseus, the Great Krak­en, the whirlpool-caus­ing mon­ster of medieval leg­end, and Melville’s Moby Dick, which drew all but Ish­mael down into its ter­ri­ble vortex.

The whirlpool of the Milky Way has its mon­ster, called Sagit­tar­ius A* by radio astronomers. No Scyl­la and Charyb­dis could be more mag­i­cal than this inven­tion of the physi­cists, revealed by the instru­ments of astronomers, a colos­sal black hole that has fed upon stars, lying still in the pool of night.

On warm sum­mer nights, when the Milky Way drapes its pale ban­ner of light from Cas­siopeia to Sagit­tar­ius, I some­times think of that address that we used to write on the brown-paper cov­ers of our school books: Bil­ly Smith, 426 Oak Ave., Boston, Mass­a­chu­setts, U.S.A., Earth, Solar Sys­tem, Milky Way, Uni­verse. These sum­mer nights are a time to feel at home in the Milky Way, a child of the stars, part of the great spi­ral of mat­ter and ener­gy that spins off life.


In 2017, the super­mas­sive black hole at the cen­ter of the Messier 87 galaxy was suc­cess­ful­ly imaged by the Event Hori­zon Tele­scope. ‑Ed.

Share this Musing:

Reader Comments

  1. Dear Mr. Raymo,

    Thank you.

    I enjoy your writ­ing very much. I became a big fan after read­ing The Soul of the Night. 

    I hope you will keep your blog going, it’s marvelous. 

    John F. Cole

Comments are closed.