At the center of the bulge is the galaxy’s anchor, the object everything else orbits — a supermassive black hole weighing about 4 million solar masses. Regular monitoring of the galactic center shows that it often flares in X-rays — the signature of matter falling toward its doom — but this pales in comparison to what we know a monster black hole can do, and there is evidence it has been more active in the past. In 2010, data from NASA’s Fermi Gamma-ray Space Telescope revealed ginormous gamma-ray-emitting bubbles reaching 25,000 light-years above and below the galactic center, likely the smoking gun of a powerful outburst millions of years ago.
The precise structure of the disk remains poorly known, including the number and position of its spiral arms. Recent radio studies of thousands of sources — stars in embedded clusters detected in the infrared, nebulae set aglow by young stars, giant molecular clouds, and water and methanol masers — seem to show that the Milky Way has four major spiral arms that originate near the galactic center and wind outward. In order from the center moving toward the Sun, they are the Norma-Outer Arm, the Scutum-Centaurus Arm, and the Carina-Sagittarius Arm. Farther out, we encounter the Perseus Arm, and farther still, the outer arc of the Norma-Outer Arm.
Astronomers long thought that the solar system resided in a starry spur located near the inner edge of the Perseus Arm. Yet one of the first surprising results from the BESSEL and VERA projects is that our “spur” is a significant structure, sporting as much massive star formation as the adjacent major arms. At this point, astronomers aren’t sure whether to classify our local patch of the galaxy as a branch of the Perseus Arm or an independent segment.
And the disk holds more surprises. A 2015 study of SDSS data led by Yan Xu at the Chinese Academy of Sciences in Beijing has extended its size by about 50 percent over previous values. The number of stars in the disk had seemed to drop off around 50,000 light-years from the center, but then SDSS found what appeared to be a vast ring of stars about 10,000 light-years farther out. The new study shows this is an illusion caused by at least four ripples that displace stars in the disk above and below the galactic plane. When we look out of the galaxy from the solar system, the disk is perturbed up a few hundred light-years, then down, then up, and then down again, starting about 6,500 light-years from the Sun and reaching at least 50,000 light-years away. Additional ripples may yet be found.
Small galaxies orbiting our own may have produced the ripples. One in particular, known as the Sagittarius Dwarf Spheroidal, has passed through the disk multiple times and is gradually dissolving into streams of stars as it orbits the Milky Way. Like a stone tossed into still water, the gravitational effect of a satellite galaxy plunging through the disk could produce ripples. Simulations suggest that satellite galaxies tearing through the disk can play a role in creating spiral structure. And intriguingly, the newfound ripples align closely with the Milky Way’s spiral arms.
The disk sits within a spherical volume called the galactic halo, a place ruled by globular clusters and satellite galaxies, as well as strewn with stars stripped from them. Our galaxy — indeed, most galaxies — may have been built by gobbling up many smaller galaxies. Today we see streams of stars linked to several small satellites, and the Milky Way appears to have swiped several globular clusters from the Sagittarius Dwarf Spheroidal. The largest and brightest globular cluster, named Omega Centauri and located about 17,000 light-years away, has a more complex stellar makeup than others. Researchers suspect it is the leftover bulge of a dwarf galaxy long ago shredded by our own.
Yet most of the Milky Way’s mass remains unseen. The motions of stars around our galaxy and others reveal a gravitational influence extending far beyond the structures we can see. Studies show that the Milky Way resides in a roughly spherical halo of invisible material — called dark matter — reaching 900,000 light-years across, or about six times the disk’s diameter. This stuff makes up 27 percent of the cosmos and created the gravitational infrastructure that coaxed ordinary matter into structures that eventually built galaxies like our own.
The current phase of galactic exploration already has provided major new insights, but many questions remain. As astronomers consolidate the results of this research over the next decade, an accurate 3-D portrait of the Milky Way will emerge, enabling us for the first time to view our island universe in the same way we see other galaxies: as a complete cosmic object — a whole greater than the sum of its parts.