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.