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.
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.