+ ^ atriptothe galactic center^ Zoom in from the naked-eye view of Sagittarius to the unique objects hidden in the Milky SAGITTARIUS Way’s rich and Galactic center complicated core. + By Angelle Tanner Looking into our galaxy’s depths. John Gleason assem- bled this composite image from pictures he took of the Milky Way, Lassen Volcanic National Park in California, and a star party held by his club at the park’s Bumpass Hell parking lot. The small yellow box shows the area covered by the infrared image at right and the radio view at far right. 44 April 2003 Sky & Telescope ©2003 Sky Publishing Corp. All rights reserved. ^ hat’s^ at the heart of the Milky have been described in this magazine over the Way? Look toward the galaxy’s years, but it can be hard to picture how they center on a dark night when all fit together. So join us on a journey as we Sagittarius is up, and you’ll see traverse 26,000 light-years to take a tour of glowing starclouds that peek tantalizingly out the galactic center. from behind a great, patchy black band of in- We begin by stepping outdoors to look up terstellar dust. And that’s about all you will at the familiar Milky Way toward Sagittarius see. The dust blocks our view of everything (facing page). Our eyes are drawn to a particu- farther than a few thousand light-years from larly bright patch just off the spout of the Sag- us in the galactic center’s direction. ittarius Teapot star pattern. This is the Large In the last few decades, however, astrono- Sagittarius Starcloud, rising from the Teapot’s mers have been able to peer through this ob- spout like a puff of steam. It does not mark scuring dust by observing at infrared, radio, the direction to the galactic center. Rather, and X-ray wavelengths with ever-increasing the center lies a few degrees west and is hid- sensitivity and resolution. In this way, we can den behind opaque clouds. Nothing at all now see clear to the Milky Way’s center — marks it in visible light. and we’ve found a lot going on there. Our Look at this spot with infrared eyes, how- galaxy’s core swarms with exotic phenomena ever, and the view becomes more transparent. and objects large and small. Many of these The zoomed-in view at lower left is from the Near Infrared Radio (90 cm) Inset: 6 cm ^ + Galactic center 1° 1° 450 light-years 450 light-years 2MASS / IPAC W. M. GOSS / C. LANG/ VLA/ NRAO Above, left: Peering through the dust: an infrared view toward the galactic center. This image only hints at the tens of millions of stars filling this 2.8°-wide view. As in all near-infrared color images with this article, blue represents a waveband centered on 1.2 microns, green 1.6 microns, and red 2.2 microns — shifting our normal color vision toward longer wavelengths by a factor of 3. Above, right: The same field as at left, imaged at the radio wavelengths of 90 centimeters and (inset) 6 cm by the Very Large Array in New Mexico. ©2003 Sky Publishing Corp. All rights reserved. Sky & Telescope April 2003 45 Radio (6 cm) ) etic agn s (m ad re ) ead th l of a rc Thr A m r e Arches h t ( Cluster s e h c Quintuplet r A Cluster Thread Background galaxy Sagittarius Circumnuclear A Disk and Mini-Spiral ^ + Radio (6 cm) 5' ^ 38 light-years CORNELIA LANG / VLA / NRAO The most prominent gas features in the galaxy’s cen- tral half degree, imaged here in radio, include the tur- bulent Arches, many thin magnetic threads, and large, powerful Sagittarius A, one of the first radio sources discovered in the early days of radio astronomy. Two Micron All Sky Survey (2MASS). Dense 5′ masses of stars around the galactic center now 38 light-years show through, though they are still considerably FARHAD YUSEF-ZADEH / VLA/ NRAO ^ dimmed and reddened by dust. The large red- A false-color rendering of the 6-cm radio image, dened region glowing behind the patchy fore- bringing out the 4-arcminute (30-light-year) bright ground is the inner part of the galactic bulge, oval inside Sagittarius A. roughly 500 light-years wide. The dark patches are the cores of especially dense light a smooth magnetic field that pervades the What radio reveals is dust clouds. One of them partly galactic-center region. interstellar gas, hides a small, very bright spot: The radio image on the previous page also the galaxy’s dense central star reveals ring-shaped supernova remnants small shaped here into a mass, roughly 30 light-years wide, and large, spawned from the abundance of high- unique collection of where many millions of stars mass, short-lived stars that form in the galaxy’s crowd together toward the center. central region. These blast waves plow into sur- overlapping structures. Radio astronomy gives a clearer rounding interstellar material, compressing it view — though most stars them- enough to trigger ongoing cycles of massive-star selves are invisible at radio wavelengths. What formation, which in turn yield new supernovae. radio reveals is interstellar gas, shaped here into The large bright area at the center is the pow- a unique collection of overlapping structures. erful radio source Sagittarius A, more than 50 Among the most striking of these features are light-years wide in the large image above. It’s the thin “threads” that crisscross the images one of the brightest radio sources in the sky roughly east-west (horizontally) at right angles after the Sun. to the galactic plane. These structures, observed The next few images zoom in on the promi- only at the galactic center, glow in radio by nent bright oval forming much of Sagittarius A. means of synchrotron radiation: emission from The oval’s eastern side, Sagittarius A East, ap- charged particles spiraling at relativistic speeds pears to be a hypernova remnant — the after- around magnetic-field lines. The threads high- math of an especially massive star’s sudden 46 April 2003 Sky & Telescope ©2003 Sky Publishing Corp. All rights reserved. death. In the oval’s western side we discover a clude the Northern Arm, the Western Arc, the feature called the Mini-Spiral, only about an Eastern Arm, and the Bar. The clue was the arcminute (7 light-years) wide. A strong, point- Doppler shifts of emission lines from the gas like radio source offset from the Mini-Spiral’s along these structures. The motion of the North- center is named Sagittarius A* (“A-star”). This ern Arm, for instance, suggested material flowing source marks the true dynamical center of our toward or orbiting around something having Milky Way: a black hole containing the mass of many millions of solar masses at 3 million Suns. the position of Sgr A*. The Mini- The Mini-Spiral The first sign that Sgr A* is something very Spiral was once believed to be consists of gas special came from studies of velocities in the shaped by spiral density waves Mini-Spiral’s different components. These in- similar to those that create the big streams falling spiral arms in galaxies. However, inward toward the the velocity data suggest that its Radio (6 cm) material is falling inward from a central black hole. ring of dense molecular gas just outside the Mini-Spiral called the Circumnuclear Disk. This disk (or ring) orbits the Mini-Spiral at a speed of 100 kilometers per second. Colli- sions between dense clouds within it dissipate orbital angular momentum, thereby causing + Radio (6 cm) SGR A EAST SGR A WEST 30″ IRS 8 3.6 l-y m r A ^ The Sagittarius A bright oval processed to bring out n r e h the Mini-Spiral: streams of gas falling the last few t r light-years toward the central black hole. Courtesy o N Farhad Yusef-Zadeh, VLA, and NRAO. Sgr A* E + as Bar ter n A Circumnuclear rm c r Disk and A n r e t Mini-Spiral s e W Dust ridge ″ rm 30 A n r 3.6 light-years e h t r o A higher-resolution radio view showing the main N Sgr A* components of the Mini-Spiral. Courtesy W. M. Goss, E a s VLA, and NRAO. te rn Ar Bar m enough material to peel off and fall inward to c Mini- r form a 1,000-solar-mass stream racing toward A Spiral n the massive object lurking at Sagittarius A*. r e t s Some interesting objects in and around the e W Mini-Spiral have been seen by giant telescopes working at infrared wavelengths. These include a collection of hot, windy stars embedded in the C ir g) cu in Northern Arm and interacting with its gas and m k (r 30" nuclear Dis dust; a low-density bubble in the Bar called the 3.6 light-years Mini-Cavity, which was blown by one or more hot stars; and a type-M supergiant star known The Circumnuclear Disk, just a few light-years across, encloses and as Infrared Source (IRS) 7. Further inspection feeds the Mini-Spiral. Gas clouds that collide in the disk should lose of the emission around IRS 7 reveals an unex- orbital momentum, allowing parts of them to fall inward. pected surprise — a tail! The wisp of dust trail- ©2003 Sky Publishing Corp. All rights reserved. Sky & Telescope April 2003 47 ing this bloated star is thought to be matter ab- such unusual star-forming conditions, it’s not lated from its outer atmosphere by the windy, surprising that the galactic center contains an hot stars in the cluster IRS 16, the rich swarm unusual abundance of especially massive stars in the central light-year around Sagittarius A*.