A The new galactic order Normal Normal spiral galaxies Sa Sb Sc Elliptical galaxies E0 E3 E7 S0 Making sense of galaxies SBa SBb SBc Barred spiral galaxies Astronomers’ efforts to explain galaxy types NGC 4622, a face- brought them from a tuning fork to a lemon. on spiral galaxy IN 1926, edwin Hubble devised his “tuning fork” scheme for s-shaped r-shaped m in Centaurus, is ⁄ ⁄ ⁄ BY FRANCIS ReddY; ILLUSTRATION BY KeLLIe JAeGeR classifying galaxies. ellipticals, rich in old stars but with little Tea classed as SA(r)ab. E gas to make new ones, give way to lens-shaped lenticulars and g A stronomers have struggled to classify galaxies since the 1920s, it gas-rich normal and barred spirals. Irregular galaxies form a Her when Edwin Hubble at California’s Mount Wilson Observa- separate catch-all category. tory first proved they were “island universes” beyond our Hubble AMilky Way. Hubble found he could lump most galaxies into a few NASA/ A Normal broad types — elliptical, normal spiral, and barred spiral — and he placed them into a tuning-fork-shaped sequence. NGC 3115, an S0-type Galaxies display a huge range of sizes and masses — from nearby B lenticular galaxy, is a Barred dwarfs that contain fewer stars than a globular cluster to mammoths featureless disk like M87. Galaxies show a range of structures, too, including bulges, of old stars. M33 is a nearby, A sLICe of the “lemon” disks, bars, rings, and spiral arms. face-on spiral reveals a wheel of galaxy The subtler details of galactic structure required a more nuanced classed as shapes based on the pres- m scheme than Hubble’s. In the late 1950s, Gérard de Vaucouleurs SA(s)cd. ence of a central stellar bar OELU c and the degree to which the devised a 3-D version of the tuning fork — shown here — based on RE/ d galaxy’s arms look spiral- (s) observations of a few hundred southern galaxies. LLAN NOAO/AURA/NSF i DIT or ring- (r) shaped. U c RE But distant galaxies, seen when the universe was younger, smaller, . c c O t /J.- and more crowded, don’t fit into the scheme. Moreover, new surveys O HT h F P of tens of thousands of galaxies suggest they occupy two distinct zones c when plotted by color and brightness. No one yet knows how to rec- m OELU oncile such observations with the smooth variations astronomers see r-shaped c RE/ among different galaxy types. d LLAN i U GIving Hubble’s tuning fork a half twist creates the lemon-shaped c . “classification volume” Gérard de Vaucouleurs developed in the late c /J.- Relative HT 1950s. Normal galaxies occupy the top half, barred galaxies go on the F number c bottom. different galaxy types occur left to right. Codes describe each of THe PinwheeL galaXY galaxy based on its position within the volume. The “lemon’s” height galaxies (M101), class SAB(rs)cd, reflects the relative number of each galaxy type found in the sample. shows clear and well orga- nized spiral structure. s-shaped m OELU c RE/ d LLAN i U c . m c Tea /J.- F d HT F U c h E t S m GG d th d M87 is a giant elliptical y an ) OELU i galaxy (e0 or e1) in the c c Kell S NOAO/L t Virgo cluster. The galaxy RE/ d oen contains some 15 times : R NGC 6822, an IBm-type LLAN i U ckwith (S c E the Milky Way’s mass. dwarf galaxy, still has . c M87’s unusual properties /J.- many star-forming regions. Astronomy HT F warrant adding the abbre- c THe MILKY WAY is a barred NASA/ESA/S. B viation “pec” — for pecu- M66, an SAB(s)b spiral, spiral, classed SAB(rs)bc. LOOKING BACK in time using the Hubble Space liar — to its classification. wheels in Leo. B Barred Telescope, astronomers see young, ragged galaxies unlike any seen nearby today. These galaxies are E Sa Sb Sc Sd Sm I colliding with each other. Ellipticals Lenticulars Spirals Irregulars © 2010 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com.