A Universe of Galaxies
Our understanding of the cosmos begins from our perspective on Earth, in the light of a single yellow star, in the spiral Milky Way galaxy. In the early 1900s, the Milky Way was seen as the single “island universe” of stars, beyond which there only occasional fuzzy patches of light.
One of these was called Andromeda or M31, and it ignited the “great debate” over whether it was a cloudy nebula within the Milky Way or another, separate galaxy. American astronomer Edwin Hubble settled the debate in 1923, when he observed the star known simply as Hubble variable 1, or V1, in Andromeda; he could tell by its regular pattern of fading and brightness that it was at a distance far beyond the Milky Way, and therefore is a separate galaxy.
This single discovery unveiled a cosmic vastness we still struggle to comprehend, and like Copernicus’ revelation that the Sun does not orbit the Earth, it changed human perception of ourselves and our place in the universe. Today we know that the Andromeda galaxy is only one of countless galaxies in the observable universe, and that galaxies grow and change over time. A majority of known bright galaxies have a spiral shape like the Milky Way and Andromeda. Other galaxies are more spherical and are called elliptical galaxies, due to our two-dimensional perception of them from Earth. Irregular galaxies are the third type; they have a less structured form and they are often much smaller than spiral and elliptical galaxies.
This detailed view of the Andromeda Galaxy represents a new benchmark for precision studies of large spiral galaxies. It contains over 100 million resolved stars and thousands of star clusters, uncovering the fossil record of Andromeda’s formation and evolution. The observations are the basis for interpreting the light of many distant galaxies. CREDIT: NASA, ESA, J. Dalcanton, B.F. Williams, L.C. Johnson (University of Washington), the PHAT team, and R. Gendler.
Resembling a wide-brimmed hat with a tall bulge at the center, galaxy M104 is nicknamed the Sombrero Galaxy. Far larger than any hat on Earth, this Sombrero is 50,000 light-years wide. We see the galaxy nearly edge-on, so the dark dust in its pancake-like disk appears to bisect a large, white, rounded core of stars. Roughly 29 million light-years away, the Sombrero can be spotted with a modest telescope in the constellation Virgo. CREDIT: NASA and the Hubble Heritage Team (STScI, AURA).
The stunningly beautiful galaxy cluster Abell 370 contains an astounding assortment of several hundred galaxies tied together by the mutual pull of gravity. Located approximately 4 billion light-years away in the constellation Cetus, the Sea Monster, this immense cluster is a rich mix of a variety of galaxy shapes. CREDIT: NASA, ESA, and J. Lotz (STScI) and the HFF Team.
The large Whirlpool Galaxy (left) is known for its sharply defined spiral arms. Their prominence could be the result of the Whirlpool's gravitational tug-of-war with its smaller companion galaxy (right). CREDIT: NASA, ESA, S. Beckwith (STScI), and the Hubble Heritage Team (STScI, AURA).
This large, fuzzy-looking galaxy is so diffuse that astronomers call it a “see-through” galaxy because they can clearly see distant galaxies behind it. The ghostly object, catalogued as NGC 1052-DF2, doesn’t have a noticeable central region, or even spiral arms and a disk, typical features of a spiral galaxy. But it doesn’t look like an elliptical galaxy, either. Even its globular clusters are oddballs: they are twice as large as typical stellar groupings seen in other galaxies. CREDIT: NASA, ESA, and P. van Dokkum (Yale University).
Barred spiral galaxies are so named for the central bar of stars that connects the galaxy’s spiral arms. NGC 1300 also has what is known as a "grand design" disk structure in its nucleus–a spiral within a spiral. Credit: NASA, ESA, and the Hubble Heritage Team (STScI, AURA); Acknowledgment: P. Knezek (WIYN).
The galaxy, called NGC 1569, sparkles with the light from millions of newly formed young stars. NGC 1569 is pumping out stars at a rate that is 100 times faster than the rate observed in our Milky Way Galaxy. This frenzied pace has been almost continuous for the past 100 million years. CREDIT: NASA, ESA, the Hubble Heritage Team (STScI, AURA), and A. Aloisi (STScI, ESA).
Three of the galaxies in this famous grouping, Stephan's Quintet, are distorted from their gravitational interactions with one another. One member of the group, NGC 7320 (upper right) is actually seven times closer to Earth than the rest. CREDIT: NASA, ESA, and the Hubble SM4 ERO Team.
Astronomers find that the supermassive black hole at the center of the most massive local galaxy (M87) is not where it was expected. Their research, conducted using the Hubble Space Telescope, concludes that the supermassive black hole in M87 is displaced from the galaxy center. CREDIT: NASA, ESA, D. Batcheldor and E. Perlman (Florida Institute of Technology), the Hubble Heritage Team (STScI, AURA), and J. Biretta, W. Sparks, and F.D. Macchetto (STScI).
This image shows the interacting galaxy pair called Arp 273. The disk of the larger galaxy is distorted into a rose-like shape by the gravitational pull of the smaller companion galaxy. A swath of blue, jewel-like points along the edge is the combined light from clusters of bright and hot young stars. CREDIT: NASA, ESA, and the Hubble Heritage Team (STScI, AURA).
The two spiral galaxies started to interact a few hundred million years ago, making the Antennae galaxies one of the nearest and youngest examples of a pair of colliding galaxies. During the course of the collision, billions of stars will be formed. The brightest and most compact of these star birth regions are called super star clusters. CREDIT: NASA, ESA, and the Hubble Heritage Team (STScI, AURA)-ESA/Hubble Collaboration; Acknowledgment: B. Whitmore (STScI).
Hercules A has long been known as the brightest radio-emitting object in the constellation Hercules. Spectacular jets, powered by the gravitational energy of a supermassive black hole in the core of the elliptical galaxy, are captured by the Hubble Space Telescope and the Very Large Array (VLA) radio telescope. CREDIT: NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO, AUI, NSF), and the Hubble Heritage Team (STScI, AURA).
MACS J0717 is located about 5.4 billion light-years from Earth. It is one of the most complex galaxy clusters ever seen. In this composite image, hot gas data from NASA's Chandra X-ray Observatory is color-coded to show temperature; the coolest gas is reddish purple, the hottest gas is blue, and the temperatures in between are purple. Galaxies are shown in an optical image from NASA's Hubble Space Telescope. CREDIT: NASA, ESA, CXC, C. Ma, H. Ebeling, and E. Barrett (University of Hawaii, IfA), et al., and STScI.
The First Galaxies
Astronomers know from the Hubble Space Telescope’s “deep field” observations that the population of galaxies in the early universe was much different; then even massive galaxies had a less defined, irregular structure. The James Webb Space Telescope’s infrared technology will push the boundaries of what is observable in the universe farther back in time and space, to the first galaxies that formed after the big bang. The light from these galaxies was emitted over 13 billion years ago, and as it travelled through space it was stretched to longer infrared light wavelengths by the universe’s expansion; this is why infrared instruments are necessary to study this era of the universe.
About 10 billion years ago, galaxies were more chaotic, with more supernovae, 10 times more star formation, and more mergers between galaxies. Astronomers estimate that nearly all massive galaxies have undergone at least one major merger since the universe was 6 billion years old. However, the driving force behind this activity, and the cause of its sharp decline, remain mysteries that astronomers will use the spectroscopic instruments on Webb to begin solving. Follow-up observations using spectroscopy will help researchers understand how elements heavier than hydrogen were formed and built up as galaxy formation proceeded through the ages. These studies will also reveal details about merging galaxies and shed light on the process of galaxy formation itself, including how early stars came together to form the very first galaxies in the universe.
Astronomers also want to learn more about the role of supermassive black holes, which are thought to lie at the heart of most massive galaxies. Did huge early stars collapse and form the first black holes, with galaxies gathering around them? Or did stars pull together first through gravitational attraction, and central black holes form later?
The Hidden Universe
In addition to viewing some of the oldest light in the universe, infrared-detecting instruments like those on Webb are also essential for determining the rotation rate, and from that the mass, of galaxies. As light sources rotate away from the Earth, the light is redshifted to longer wavelengths. From these measurements astronomers determine the total mass of the galaxy needed for gravity to keep it from spinning apart… if this doesn’t match the observed matter in the galaxy, the remainder is known as dark matter, which cannot be detected, but astronomers know it must be present to hold the galaxy together. Dark matter then likely plays a key role in determining a galaxy’s shape and structure, and if so must be an important factor in how galaxies have changed over time.
Astronomers calculate that the total mass of dark matter in the universe is roughly five times that of detectable matter (i.e., atoms). As such, the nature of dark matter represents a huge area of knowledge in which humanity is still “in the dark.” Astronomers believe that looking back through time, in wavelengths of light beyond what our own eyes can observe, will help us begin to answer some of these puzzling questions.
