Webb and Our Galaxy

Our galaxy, the Milky Way, contains a supermassive black hole at its core surrounded by a central bulge of old, yellowish stars. Beyond that are bluish spiral arms filled with younger stars, newly forming stars, and dark lanes of dust.

It is just one of billions of galaxies in our universe, but the Milky Way is our galaxy, our home in the universe. The Milky Way contains the closest examples of stars, planets, nebulae, black holes, and other objects that likely reside in every galaxy throughout the cosmos. By studying the Milky Way in the infrared, the Webb Telescope will be able to teach us a great deal about our galaxy and others. Webb will improve our understanding of all stages of star formation — from birth to death and back again to the rise of the next stellar generation. Astronomers know that stars form out of collapsing clouds of gas and dust, but they don't yet know the exact sequence of how stars are born. What triggers a cloud to collapse and a star to begin forming? How much of that mother cloud does a star use up when it forms? How and when do planets begin to form around a newborn star?
At the end of their lives, stars die in a variety of exciting and interesting ways — from gentle exhales of material to violent explosions expelling stellar shrapnel into the galaxy. Many dying stars and stellar corpses are embedded in their ejected material, which shrouds our view in visible light but can be penetrated with Webb's infrared vision. Webb will help us probe and understand both this residual material and the stars that died. It will help astronomers test their theories for how stars end their lives and how the heavier elements forged within these dying stars are recycled into the galactic environment to help create the next generation of stars.

Omega Centuari is one of the largest globular star clusters residing in the outskirts of our galaxy.

Counting Stars

Webb will help us understand just how many stars there are and how those stars are distributed throughout the galaxy. The most common stars in the Milky Way are "dwarf" stars that are often too dim for Hubble to observe in visible light, but that glow brighter in infrared light. Webb will help astronomers get a firmer grip on just how many of these stars exist, and perhaps help us learn more about them. Knowing how many stars there are of different types also tells us how quickly or efficiently stars formed at various stages in the galaxy's history.
Webb will also study giant stellar swarms called globular clusters, which reside at the outskirts of the Milky Way and contain the oldest stars in the galaxy. Webb will analyze the composition of these ancient stars, and perhaps reveal whether globular clusters formed along with our galaxy or originated somewhere else, and were later absorbed into the galaxy.

Looking Inward and Outward

This X-ray image shows the region around our galaxy's central supermassive black hole, known as Sagittarius A* (or Sgr A*). Credit: NASA/CXC/Univ. of Wisconsin/Y. Bai et al.

Webb will help us understand what's going on at the very heart of the Milky Way. In our galaxy's core lies a supermassive black hole surrounded by gas, dust, and a densely packed swarm of stars. However, this central black hole does not seem to be consuming as much material as its peers in other galaxies are. Astronomers aren't sure why. Webb’s infrared observations could give us a clearer view of the material and stars near the black hole, and perhaps uncover the reason why our galaxy's black hole is so quiet. Webb's sharp and powerful infrared vision will allow it to peer farther into the Milky Way with greater clarity than infrared telescopes before it — uncovering parts of the galaxy that were once too dim, too distant, or too concealed to study. These investigations will not only help us understand our own Milky Way, but myriad galaxies throughout the universe.

What Is the Center of Our Galaxy Like?

Hubble's infrared vision reveled more than half a million stars at the core of the Milky Way.

Our solar system resides in one of the spurs off the spiral arms of the Milky Way galaxy, and we tend to think of our experience of the Milky Way as typical. Even in our science fiction films, when the heroes travel between stars, every sky looks the same.

But the Milky Way isn't quite so uniform. If you lived in the center of the Milky Way, you could look up on a sky thick with stars, a thousand to a million times more than we're used to seeing, depending on how close you were to the core. For Earth's inhabitants, the next closest star to our Sun is about 4 light-years away. For our central Milky Way cousins, that star would be around 0.4–0.04 light-years distant.
The center of the Milky Way consists of the region where the galaxy's spiral arm structure has broken down and transformed into a "bulge" of stars, or roughly the inner 10,000 light-years. At its heart — and the dominant force in that area of the galaxy — is a million-solar-mass black hole we call Sagittarius A*.
It would be an inhospitable area for humanity, rife with radiation emanating from a surplus of massive stars and material being torn apart by the black hole. Plus it would take us more than 25,000 years to reach it, even if we could travel close to the speed of light. Fortunately, the Webb telescope is designed to do our exploring of the galactic center for us.

This simulated image shows a supermassive black hole at the core of a galaxy. The black region in the center represents the event horizon, where no light can escape the massive object's gravitational pull. The black hole's powerful gravity distorts the space around it, stretching light from background stars.

Sleeping Giant

Our central, supermassive black hole is relatively quiet when compared to its counterparts in other galaxies, flaring only occasionally with X-rays and infrared light as objects fall into it. It could be that there's simply not that much material around Sagittarius A*. Webb will investigate our strangely calm central black hole, providing a more accurate measurement of its mass, as well as how much material is falling into it, and when. Furthermore, the mass of our black hole ranges on the low end of normal for galaxies of our size. Webb will examine why that is and the relationship between a black hole and the matter surrounding it.
While Webb helps reveal why we have the kind of black hole that we do, it'll also be shedding light on central, supermassive black holes in other galaxies. Active galactic nuclei (AGN) are a type of extremely bright galaxy core seemingly fueled by powerful black holes actively gobbling large amounts of material. Astronomers would like to know what, exactly, AGN are and if they are triggered by events occurring in the centers of galaxies or by mergers between galaxies.
Webb's investigations of our own black hole and the relationship between black holes and galaxy evolution could help solve a cosmic chicken-and-egg problem: Did black holes come first and galaxies form around them, did galaxies form first and develop black holes, or did the galaxies and black holes develop together?

How Are Stars Born?

Hubble captured the Eagle Nebula in visible light (left) and infrared light (right) in 2015. While the visible-light image shows opaque clouds, infrared light penetrates gas and dust, revealing both stars behind the nebula and those hidden away inside the pillar.

Stars form from collapsing clouds of gas and dust. It's a process that occurred in our distant past and continues to take place today. Astronomers can train their telescopes on giant clouds of hydrogen gas in our own galaxy and find knots of denser, colder gas and dust that are in the process of giving rise to stars. But these dust-thick regions of starbirth are often dark and opaque. The Pillars of Creation in the Eagle Nebula, depicted in one of Hubble's most famous images, is a stellar nursery, but what we see of it looks like a dense cloud.
In 2015, Hubble revisited the Eagle Nebula to create two new images of that famous region — one capturing visible light, and the other near-infrared.

This young cluster of about 3,000 stars in our Milky Way is called Westerlund 2 and contains some of the galaxy's hottest, brightest, and most massive stars. Hubble's infrared vision pierced dust around this stellar nursery to reveal the dense concentration of stars in the central cluster.

The pictures illustrate the striking difference between what visible-light telescopes like Hubble see, and what infrared telescopes like Webb will show us. In the near-infrared image, the clouds are transformed into ghostly outlines and hidden stars blaze forth from both within and beyond. Newborn stars shine dramatically from within the cloud. Infrared light, unlike visible light, travels through dust clouds. And cameras that can capture it can see through such clouds as though they were nearly invisible. Furthermore, Webb's cameras will detect the infrared glow of the dust and gas itself, allowing us to learn what it's made of, how hot and dense it is, and what chemical processes have affected it. These abilities will make Webb a critical tool for learning just how star formation works within those dusty depths.

Seeing Stars

For instance, astronomers know collapsing clouds have a point of no return, when they become so dense and so cold that they cannot hold themselves together against collapse. Above this threshold stars form, below it they don't. But the gas drifting between stars isn't dense enough by itself to trigger star formation. Is star formation triggered mainly by shockwaves from exploding stars, or the pressure created by radiation and stellar winds from massive stars — or can those processes get in the way of the collapse? Could star formation begin with the collision and accumulation of sparse pockets of dust and gas? Do stars compete for material in the cloud, or do they form mostly in isolation? Does the entire cloud collapse into stars at the same time, or do stars form in groups? With its powerful infrared sensitivity and resolution, Webb will be able to peer into star-forming regions across the entire Milky Way galaxy, where previous infrared telescopes were limited to dust clouds within our own galactic neighborhood. Webb will collect a wide array of examples to give astronomers plenty of star-formation regions to compare.

Dwarfing the Giants

What Makes Brown Dwarfs Unique?

Credit: NASA/JPL-Caltech

Twinkle, twinkle, little star
How I wonder what you are
It’s not easy to tell a star from a planet when you look up at the night sky. Ancient astronomers noted that some lights moved across the sky, while others appeared to remain in a fixed position. The Greeks, picking up the work of these earlier scientists, called such a travelling point of light planēs – wanderer. We still call them planets today.
But other than orbiting around a star, what makes a planet a planet? As telescopes become more sophisticated and we learn more about the universe, the less some old definitions make sense. We now know that some planets are rocky, like Earth, while others are so-called gas giants, like Jupiter.
We also know that our middle-aged Sun is one type of a variety of stars, classified by their phase in a lifecycle we are still in the process of understanding. A star shines by producing its own light from nuclear fusion in its dense, hot core. Planets shine—to our eyes on Earth—by reflecting the light of stars.
These were the simple, sharp definitions of stars and planets until the discovery of a brown dwarf in 1995. Theorized as early as the 1960s, this new type of celestial body blurred the line between star and planet, requiring an exciting re-thinking of the universe.

An artist’s depiction of the relative sizes of the Sun, a low mass star, a brown dwarf, Jupiter, and the Earth. Credit: NASA/JPL-Caltech/UCB

Despite their name, brown dwarfs can be up to 70 times more massive than gas giants like Jupiter. Brown dwarfs form like stars do, by the contraction of gas that collapses into a dense core under the force of its own gravity, whereas planets form from the accumulation of leftover debris from these stellar births. However, brown dwarfs do not have enough mass for their cores to burn nuclear fuel and radiate starlight. This is why they are sometimes referred to as “failed stars.” They are smaller and cooler than the Sun, and have complex planet-like outer atmospheres, including clouds and molecules such as H₂O. Astronomers now disagree on whether some “free-floating” bodies detected in space – not orbiting a star, but also not shining like a star – should be called planets or brown dwarfs. A brown dwarf atmosphere is easier to study than that of an exoplanet, which is typically obscured in the blinding light of its parent star. But to study brown dwarfs you first have to find them. Their dim light makes this difficult, and eventually the visible light left over from their birth fades completely beyond the red end of the visible spectrum, and they emit only infrared light.

Credit: STScI

Difficult to detect, brown dwarfs hint at the many undiscovered wonders the universe still holds, hidden for centuries beyond the bounds of visible light. Much of the mass that holds the universe together with its gravity has thus far been undetectable, and is known as dark matter. In 2019, the James Webb Space Telescope will continue the work of NASA’s Hubble Space Telescope and infrared Spitzer Space Telescope in probing the furthest and “darkest” regions of the universe. Webb will see farther and in higher resolution, with unprecedentedly powerful infrared cameras and spectrographs. When Hubble launched, the only planets we knew of were those in our own Solar System. There were no images of brown dwarfs. Webb will take a detailed look at the atmospheres of brown dwarfs and exoplanets, determining their temperatures and chemical compositions. Do the traditional boundaries between planets and stars still make sense? Once purely philosophical, these questions now loom large in science. With infrared observations, the Webb Telescope will add to our understanding of the universe’s ongoing evolution, and the place of Earth and our Solar System within that bigger picture.