Around 4.6 billion years ago, our solar system formed from a collapsing cloud of gas and dust. As special as it seems to us, our Sun was just one of many new stars being born in that area of the Milky Way Galaxy.
When astronomers want to study the formation of stars and planets today, they look to giant clouds of hydrogen gas – the birthplace of stars. But when we point the most powerful telescope currently in space, the Hubble Space Telescope, at these clouds, we see mostly shadow. The star fields are obscured by gas and dust, which scatters visible light and makes these regions appear opaque. So how can we study these cocoons of star birth?
Stars emit different types of light, and not all light will be stopped by such clouds. Infrared light penetrates clouds of gas and dust, and by detecting it we can see the warm objects within these regions. To an infrared telescope like the Spitzer Space Telescope, a stellar nursery is a realm of gas and dust in the process of coalescing into the young stars that are hidden from Hubble.
But Spitzer’s infrared images cannot match the detailed resolution of Hubble, because Spitzer is a much smaller telescope. The James Webb Space Telescope, the largest telescope ever launched into space, will have an infrared-capturing mirror 60 times larger in area than Spitzer’s. Webb’s sunshield protects the mirror from the heat of the Sun, Moon and Earth, which would otherwise interfere with the infrared light Webb is meant to capture.
These advantages will allow Webb to scrutinize the formation of Sun-like stars at a new level of detail across the whole of the Milky Way. The reach of previous infrared telescopes has been mostly limited to our own neighborhood.
Throughout the Milky Way Galaxy, relatively sparse hydrogen gas floats between the stars, awaiting ripples or waves that will compress the gas. These ripples -- something like a fish swimming through a placid pond -- could be caused by disturbances like stellar winds and jets from young stars within the cloud, by collisions with slower-moving material, or by an external force like a supernova blast wave. Whatever the cause, when the gas is squeezed, gravity takes over and a cloud of hydrogen molecules (and a small fraction of dust particles) condenses from the interstellar medium. That knot of gas is thicker than the surrounding material, and the center is shielded from the radiation that bakes the outer parts of the cloud. After a time, this insulated inner section cools and stops moving, allowing gravity to pull the gas closer together. Once this process starts, it continues until it creates cores that will further coalesce into stars.
Young stars like the Sun begin their lives by collapsing from these cores of gas, spinning in the same direction as the average motion of the parent cloud. The speed of the spin increases as the protostar forms, just as an ice skater twirls more quickly when bringing in his or her arms. In fact, so much momentum is brought into the system that the star would spin apart without a method to eject the momentum and slow down the spin. Fortunately, stars can channel their excess spin into a funnel of particles that leaves the star as twin jets, moving at tens to hundreds of miles per second and slamming into the slower-moving medium around it, leaving a spectacular light trail reminiscent of the contrails of a jet airplane across the sky.
As Webb examines the youngest protostars, it will see these majestic jets of shocked hydrogen gas ejected at high speeds from rapidly spinning cores into space, looking something like the wake of a boat racing through the ocean waves. As the shocks dissipate and cool, they alter the chemistry of the cloud around the star and radiate their energy away. In addition to capturing images of jets with its camera, Webb has instruments called spectrographs that will be able to detect the specific energy signatures left by these changes in the cloud around the star.
But how does this collapse happen? 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? Is the collapse helped or hindered by turbulent energy driven by other stars, winds, and radiation fields? With Webb’s help, we will further understand how to make a star … and from there, eventually, we will understand how to make a planetary system.
At the end of their lives, the most massive stars explode. Low-mass stars burn out slowly and fade away. But the in-between stars, those with masses near that of our Sun, turn into what we call “planetary nebulae.”
Planetary nebulae are so named because astronomers using the earliest telescopes saw them as vaguely planet-like smudges in the night sky. Hubble’s resolution has transformed our images and understanding of planetary nebulae by revealing intricate details in the bubbles of glowing gas created when a dying star casts its outer layers off into space.
All planetary nebulae form in approximately the same way. A medium-sized star becomes an inflated red giant star, then expels its outer layers to leave behind the core of the original star -- called a white dwarf -- whose radiation illuminates and energizes the discarded gas. But they don’t all look alike. The Helix Nebula is a giant eye staring at us through space. The Cat’s Eye Nebula looks like the cosmos was playing with a spirograph. The Red Rectangle’s appearance is exactly what it sounds like.
This variation does more than provide us with a wealth of beautiful images – it shows the diversity of the stars and processes that gave rise to these structures.
The Classical Ring
The Ring Nebula is one example of a common type of planetary nebula. Previous observations by several telescopes had revealed gaseous material in the ring’s central region, but Hubble showed its structure. In the Hubble image, the blue gas in the nebula’s center appears as a football-shaped structure that pierces through the red, doughnut shaped material. Hubble also revealed details of the dark, irregular knots of dense gas and dust embedded along the inner rim of the ring, which look like the spokes of a bicycle. In the center of the image, a white dot is all that remains of the former star’s hot core, now a white dwarf.
The Ring Nebula, created by its star’s death 4,000 years ago, is expanding at 43,000 miles per hour, based on changes seen in Hubble imagery over more than a decade. Its outer rings formed when fast-moving gas slammed into slow-moving material lost from the star at an earlier phase. The center of the nebula is moving faster than the expansion of the main ring. The nebula will expand for another 10,000 years, eventually fading from view as its material blends back into interstellar space.
The Beautiful Butterfly
The Bug or Butterfly Nebula, NGC 6302, is a bipolar nebula. Two tremendous lobes of material billow from its central star, which is shrouded in a dense, doughnut-shaped ring of dust. Hubble images show a complex history of ejections from the star. After evolving into a huge red giant, the star lost its extended outer layers. Some of this gas was cast off from its equator at a relatively slow speed, perhaps as low as 20,000 miles an hour, creating the doughnut-shaped ring. Other gas was ejected perpendicular to the ring at higher speeds, producing the elongated "wings" of the butterfly-shaped structure.
Scientists believe that later, as the central star heated up, a much faster stellar wind, a stream of charged particles traveling at more than 2 million miles an hour, plowed through the existing wing-shaped structure, further modifying its shape and appearance. Hubble also shows numerous finger-like projections pointing back to the star, which may mark denser blobs of material that have resisted the pressure from the stellar wind.
The Complex Cat’s Eye
Hubble images of the Cat’s Eye Nebula, which exhibits one of the most complex structures known, suggest that the star ejected its mass in a series of pulses at 1,500-year intervals, creating dust shells that formed its complex onion-like structure. Its symmetrical structure is likely the result of two wobbling jets of gas emanating from the vicinity of the core.
Hubble’s Observations Unveil New Mysteries
While most Hubble observations of planetary nebulae clarify the behavior of their expiring stars, others have raised new questions about how the nebulae form and the conditions of the universe around them.
In one example, astronomers using Hubble as part of a survey of more than 100 planetary nebulae in the central bulge of the Milky Way galaxy found that butterfly- or hourglass-shaped planetary nebulae tend to be mysteriously aligned to point the same way – that is, their rotation axes tend to be perpendicular to the plane of our galaxy. This is a puzzling result because each nebula formed in a different place, at different times, and under differing conditions
The current hypothesis is that the alignment was caused by strong magnetic fields that were present when the galactic bulge formed billions of years ago. And since nebulae in the outer galaxy do not line up in the same orderly way, these fields would have to have been many times stronger than they are in our present-day region of the galaxy.
Planetary nebulae provide some of Hubble’s most striking images, glowing, ethereal-seeming, abstract structures that would be at home in a vast art exhibit. But they aren’t just pretty pictures. Their morphologies, captured by Hubble, tell us about the process of star death, the steps in the process, and the conditions that eventually create the varied nebulae we see.
When massive stars – eight or more times the mass of our Sun – end their lives, they do so in titanic explosions that leave behind neutron stars or black holes. As these stars run out of fuel at their cores, they collapse and then explode, spewing stellar material out into space at millions of miles per hour. Among those materials are heavy elements that are only created in supernovae, as well as iron and nickel. We carry the products of supernova explosions in our own bodies; our planets and universe are rich with them. Further, supernovae create the conditions for the birth of new stars, their shock waves compressing clouds of gas and dust and triggering waves of stellar formation.
So understanding supernovae is key to many of our questions about the universe, and how we fit into it. Hubble, using its sophisticated suite of both cameras and spectrographs, has brought us some of our most intimate views of the supernovae process, allowing us to paint a clearer and more detailed picture of just how stars explode. Over the years, Hubble has witnessed the before, during and after stages of supernovae, puzzle pieces that can be assembled to form a coherent image of these stellar deaths.
One of our first exhibits is the unstable star Eta Carinae. Images Hubble has captured of Eta Carinae, 150 times the mass of the Sun, make it look like an explosion in progress – a giant, double-lobed cloud with a glowing center. Eta Carinae has been having violent outbursts for 200 years, brightening and dimming repeatedly, and will probably explode within a million years. That may seem like a long lifespan, but it’s short for stars, many of which live for billions of years.
Hubble has studied chemical elements being ejected by Eta Carinae and estimated the amount of material being carried away from the doomed star by its stellar wind. The telescope has allowed astronomers to analyze Eta Carinae’s billowing structure, revealing a different shape than previous models predicted. Hubble observations of Eta Carinae show how material is emitted from a star even in advance of the eventual explosion, something we see again when we move on to an actual supernova explosion essentially witnessed in real-time: Supernova 1987A.
In February of 1987, astronomers saw one of the brightest stellar explosions in over 400 years. The supernova, named SN 1987A, blazed with the power of 100 million suns for several months. Hubble quickly captured observations of the explosion upon its launch three years later. Since then, Hubble has taken hundreds of pictures of the star’s violent demise, observing rings and knots around the star lighting up as the shock wave from the explosion crashes into previously expelled gas around the star at tens of millions of miles per hour. The changes shed light on the effects a supernova can have in the surrounding galaxy, including how the released energy affects the chemistry of the environment.
Hubble observations of the supernova have included details of two outer loops of glowing gas; a dumbbell-shaped central structure consisting of a pair of debris blobs in the center of the supernova, racing away from each other at roughly 20 million miles an hour; and a distinctive, glowing ring, about a light-year in diameter, around the supernova. The ring existed at least 20,000 years before the star exploded, but x-rays from the explosion energized its gas, making it glow for two decades now. The bulk of the explosion is currently hitting the gas ring, and will eventually shred it, perhaps over the course of decades. The central part of the explosion, the dumbbell, will become something called a supernova remnant. This is the first time a telescope has witnessed the transition from supernova explosion to supernova remnant.
Hubble has plenty of experience with supernovae remnants, observing them in all their striking diversity. One of the most famous is the Crab Nebula. Japanese and Chinese astronomers, and likely Native Americans, recorded its star’s explosion in 1054. Hubble images show a vast network of filaments of gas, mostly hydrogen, illuminated by the eerie bluish glow of the stellar remnant in the center. Another supernova remnant, Cassiopeia A, the youngest-known in our galaxy, displays shredded streams of debris arranged into small, cooling knots of gas and tiny clumps of matter that are tens of times larger than the diameter of our own solar system. Finally, a delicate bubble of gas, the supernova remnant called SN 0509.67.5, is all that remains of a powerful stellar explosion in the nearby Large Magellanic Cloud. The shell’s rippled surface may be caused by variations in the density of nearby gas, or driven by the explosion itself.
Hubble’s supernova remnant observations are often combined with observations by another of NASA’s Great Observatories, the Chandra X-ray Observatory. When young, the gas of supernovae remnants radiate the most in ultraviolet and x-ray light, then in visible light as they age and cool. Chandra captures the view of gas at millions of degrees, while Hubble captures the gas at 10,000 degrees and below. Hubble’s ability to work in conjunction with other observatories has given astronomers a much more comprehensive view of cosmic objects than any telescope could provide on its own.
Supernovae are important for more than their mechanics. Certain types of supernovae, those which occur in binary systems involving at least one white dwarf star, can be used as “standard candles” to help us measure distances to tremendously far away objects. It’s critical that astronomers understand as much as they can about supernovae to refine their models of how stars function. Supernovae have much to teach us about the workings of the universe. As Hubble continues its exploration of the cosmos, we rely on the information it gleans about these massive explosions to help us learn how we – and the universe – came to be.
Stars are born from the gas and dust of interstellar space. When they eventually burn out and die, they bequeath their legacy of elements created within their cores back to the interstellar medium from which they formed. The signposts marking this ongoing cycle of birth, death and renewal would be easily visible to any casual observer who had a bird's-eye view of our pinwheel-shaped galaxy.
One star-birth region Hubble has studied extensively is the Eagle Nebula, also called M16, about 7,000 light-years from Earth. Most of this molecular cloud is so dense and cool that its hydrogen atoms are bound as molecules. This "molecular hydrogen" is the raw material for building new stars. The cloud contains microscopic dust particles of carbon (in the form of graphite), silicates and other compounds. Though this trace dust accounts for only a fraction of the nebula's mass, it's enough dust to absorb visible light — cloaking some of the visual details of star birth.
A cluster of about 100 newborn stars glitters inside the nebula. These young stars emit intense ultraviolet radiation. When this ultraviolet light hits the surface of the cloud, it heats that gas, causing it to "evaporate" and stream away from the surface. Unlike other stellar nebulae that we see face-on — like the great Orion Nebula — M16 presents astronomers with a unique side view of the structure of a typical star-birth region: the cluster of hot, young stars in the center of the cavity, the evaporating surface of the cloud and finally the great cold mass of the cloud itself.
Inside the gaseous towers of M16, which are light-years long, the interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings. Because the columns are denser than their surroundings, they are not evaporating as rapidly as the surrounding gas and so remain. As more and more gas and dust falls onto these growing clumps they get further compressed by their own weight, until finally they trigger nuclear fusion reactions in their cores and "turn on" as stars.
Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space — a process called "photoevaporation." The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The regions that are denser than their surroundings are left behind after the gas around them is gone.
As the pillars themselves are slowly eroded away by the ultraviolet light, small globules of even denser gas and dust buried within the pillars are uncovered. Forming inside at least some of the globules are embryonic stars. The stars in M16 continue to grow as more and more gas falls onto them, but photoevaporation ultimately inhibits the further growth of the embryonic stars by dispersing the cloud of gas and dust they were "feeding" from.
Astronomers have gained new perspectives on star formation thanks to Hubble observations of the pillars in the Eagle Nebula:
- We better understand the processes that control the sizes of stars forming in a molecular cloud versus those forming in isolation.
- We now know a star will continue to grow until it nears the point where nuclear fusion begins in its interior. When this happens, the star begins to blow a strong "wind" that clears away the residual material. Hubble has imaged this process in detail.
- M16 provides glimpses of stars at different stages of being uncovered. Hubble captured an unprecedented look at what stars and their surroundings look like before they are truly stars.
- When the 1995 images of M16 were taken, it was the first time that astronomers actually saw the process of forming stars being uncovered by photoevaporation.
In the coming decade, Hubble will continue to explore nebulae across our skies, while the James Webb Space Telescope will use its sensitivity to infrared light to peer through dust in nebulae and reveal greater details about stellar nurseries.
Watch throughout the year for more articles on Hubble's 25 years of discovery.