A spiral galaxy has a flat, spinning disk with a central bulge surrounded by spiral arms. That spinning motion, at speeds of hundreds of kilometers a second, may cause matter in the disk to take on a distinctive spiral shape, like a cosmic pinwheel.
Our Milky Way, like other spiral galaxies, has a linear, starry bar at its center. Elliptical galaxies are shaped as their name suggests: They are generally round but can stretch longer along one axis than along the other, so much so that some take on a cigar-like appearance.
The universe's largest-known galaxies—giant elliptical galaxies—can contain up to a trillion stars and span two million light-years across.
Elliptical galaxies may also be small, in which case they are called dwarf elliptical galaxies. Elliptical galaxies contain many older stars, but little dust and other interstellar matter. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions. Few new stars are known to form in elliptical galaxies. They are common in galaxy clusters. Lenticular galaxies , such as the iconic Sombrero Galaxy , sit between elliptical and spiral galaxies.
Like elliptical galaxies, they have little dust and interstellar matter, and they seem to form more often in densely populated regions of space. Galaxies that are not spiral, lenticular, or elliptical are called irregular galaxies. Irregular galaxies—such as the Large and Small Magellanic Clouds that flank our Milky Way—appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.
They are full of gas and dust, which makes them great nurseries for forming new stars. Some galaxies occur alone or in pairs, but they are more often parts of larger associations known as groups, clusters, and superclusters. Our Milky Way, for instance, is in the Local Group , a galaxy group about 10 million light-years across that also includes the Andromeda galaxy and its satellites.
The Local Group and its neighbor galaxy cluster, the Virgo Cluster , both lie within the larger Virgo Supercluster , a concentration of galaxies that stretches about million light-years across.
The Virgo Supercluster, in turn, is a limb of Laniakea, an even bigger supercluster of , galaxies that astronomers defined in Galaxies in clusters often interact and even merge together in a dynamic cosmic dance of interacting gravity.
When two galaxies collide and intermingle, gases can flow towards the galactic center, which can trigger phenomena like rapid star formation. Our own Milky Way will merge with the Andromeda galaxy in about 4. Because elliptical galaxies contain older stars and less gas than spiral galaxies, it seems that the galaxy types represent part of a natural evolution: As spiral galaxies age, interact, and merge, they lose their familiar shapes and become elliptical galaxies. But astronomers are still working out the specifics, such as why elliptical galaxies follow certain patterns in brightness, size, and chemical composition.
The universe's first stars ignited some million years after the big bang, the explosive moment Filamentary chains of galaxies wind their way around the dark, empty spaces. On large scales, galaxies in clusters and superclusters are like soap bubbles, with galaxies coating the surfaces and voids lying in between. By the end of the s, astronomers had identified the Great Wall, a sheet of galaxies measuring million light-years across. More recently, the Sloan Digital Sky Survey uncovered the Sloan Great Wall, an assemblage of galaxies at least twice the size of the Great Wall, which covers a long dimension of some 1.
As astronomers discovered more and more distant galaxies, they found that some large mass seemed to be tugging on the local universe, pulling us in the direction of the southern constellations Triangulum Australe and Norma.
Called the Great Attractor, this anomaly, some million light-years away, puzzled astronomers. They eventually discovered that an even larger mass in that direction was pulling us. This mammoth structure, called the Shapley Supercluster, is million light-years away and contains the greatest concentration of galaxies in our local part of the cosmos. Additional surprising discoveries have occurred, too.
In , astronomers identified a new supercluster based on the relative motions of galaxies analyzed in a more sophisticated way than ever before. Laniakea, which is also sometimes called the Local Supercluster, contains some , galaxies, including the Local Group and the Milky Way.
This massive cluster and all its members are traveling together through space, but not all of the galaxies within it are gravitationally bound. Some will splinter apart from the rest of the cluster as time rolls on. Altogether, Laniakea contains around galaxy clusters and groups.
And surrounding Laniakea in the local universe are other galaxy superclusters — the Shapley Supercluster, the Hercules Supercluster, the Coma Supercluster, and the Perseus-Pisces Supercluster. Each of these structures holds hundreds of galaxy clusters and are linked by the fabriclike web of cosmic structure. Beginning in the s, astronomers found evidence of structures even larger than superclusters.
In , Scottish astronomer Adrian Webster found what would become known as the Webster Large Quasar Group, a collection of five quasars, or actively feeding black holes, stretching over million light-years. Now, nearly two dozen LQGs are known. A structure known as the Huge LQG contains 73 quasars over a diameter of some 4 billion light-years.
This massive structure, dismissed by some astronomers, may hold the title as the largest collection of related matter in the cosmos. On one hand, the enormity of the universe makes us feel small. Our brief lives happen so quickly, and we wink out, mostly unaware of the incredibly large cosmos around us. But the fact that we are sentient, that we can ponder the stars and galaxies far away from us, makes life in the universe a truly amazing thing.
David J. Eicher is the editor of Astronomy. Register or Log In. The Magazine Shop. Login Register Stay Curious Subscribe. The Sciences. At 11 million light-years away, IC is a close galaxy.
IC has a low surface brightness and lies in Camelopardalis, near the plane of the Milky Way in our sky. Credit: Tony Hallas. Newsletter Sign up for our email newsletter for the latest science news. Sign Up. He was initially excited, believing he had recorded a nova, an exploding star.
He marked the star, which lies between two tick marks he drew at the top right on the plate, with the letter N. The star turned out to be a Cepheid variable, and Hubble used it to prove that the distance to the Andromeda Galaxy was far greater than astronomers thought. NGC in Coma Berenices is the brightest and most prominent galaxy in our sky that is oriented perfectly edge-on to our line of sight. We see its disk as a thin, silvery needle. Some 57 million light-years off, it lies in the Virgo Cluster and has a prominent central bulge, suggesting it may be a barred spiral.
Credit: Adam Block. Finally, theory suggests early galaxies were smaller and fainter to begin with and only later merged to form the colossal structures we see today. Scientists are confident these galaxies exist. Added to existing Hubble observations, their results suggested such galaxies make up 90 percent of the total, leading to a new estimate—that there may be up to two trillion galaxies in the universe. Such estimates, however, are a moving target.
As more observations roll in, scientists can get a better handle on the variables at play and increase the accuracy of their estimates. In a study presented this week at the American Astronomical Society and soon to be published in The Astrophysical Journal , a team led by astronomers Marc Postman and Tod Lauer described what they found after training the New Horizons telescope on seven slivers of empty space to try and measure the level of ambient light in the universe.
Their findings, they say, allowed them to establish an upper limit on the number of galaxies in existence and indicate space may be a little less crowded than previously thought. According to their data, the total number of galaxies is more likely in the hundreds of billions, not trillions.
The New Horizons spacecraft has now escaped the domain of zodiacal light and is gazing at the darkest sky yet imaged. This offers the opportunity to measure the background light from beyond our galaxy and compare it to known and expected sources. Still, the measurement was not straightforward. But the New Horizons team only found about twice as much light.
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