For many ancient cultures, the stars represented the abode of the divine and were thought to be unchanging, ever-lasting, and eternal. Through science, we’ve learned that stars are celestial objects instead which, like people, are born, live, and die. The scientific study of their life cycles through cosmic time, which is one of the main research areas of astronomy and astrophysics, also gave us great insights to the origin and eventual fate of our home star, the Sun, to which all of life on Earth is intricately bound. This area of astronomical study has been one of the many to be fundamentally changed during the last 25 years with the launch of NASA’s Hubble Space Telescope, which has provided scientists with spectacular and unprecedented views of the perpetual cosmic cycle of stellar life and death.
Of Stellar Nurseries and Stellar Remnants
Stars are born from the gravitational collapse of cold and massive clouds of gas and dust that lie in the interstellar medium. Dynamical instabilities that are often caused by the destructive supernova explosions of other nearby stars that have reached the end of their lives cause these molecular clouds to start collapsing under their own weight. The areas of most density within the clouds then begin to accrete even more matter from their surroundings, forming smaller and even more massive clumps, called protostars, whose temperature and pressure begins to rise steadily. When the temperature at the center of the protostars reaches a critical point of approximately 10 million degrees, they undergo outward nuclear fusion reactions by transforming hydrogen into helium, which stops the initial gravitational contraction. It is this moment of ignition of thermonuclear fusion reactions which marks the birth of a new star.
A scientific visualization which takes the viewer across interstellar space and into a 3D model of the Orion Nebula. The 3D model combines astronomical knowledge, scientific intuition, and artistic interpretation to create an awe-inspiring journey into the star forming cloud.Video Credit: NASA, ESA, F. Summers, G. Bacon, L. Frattare, Z. Levay, and K. Litaker (STScI) Acknowledgment: A. Mellinger, R. Gendler, and R. Andreo
The exact mechanisms that govern the process of stellar formation were very difficult in the past to be studied with ground-based telescopes, not least because of the fact that the distortions of the Earth’s atmosphere prevented astronomers from gaining any detailed views deep inside the stellar nurseries of molecular interstellar clouds. Hubble’s sharp resolution above the veil of our planet’s atmosphere has provided scientists with breath-taking, very high-definition images of these star formation factories in the galaxy, allowing them to glimpse stars and planets in the making.
One of the most popular and well-known of these lies in the iconic Orion Nebula, or M42, a massive 20 light-years-wide diffuse nebula, which has been studied extensively for decades by amateur and professional astronomers alike. Beginning in the early 1990s, scientists have targeted Hubble many times toward this region of the sky, while also taking advantage of the orbiting observatory’s advancing capabilities with time with each shuttle servicing mission. Throughout this period, Hubble has peered deep into the Orion nebula, revealing the presence of hundreds of protoplanetary disks around newly born stars, called “proplyds,” which mark the earliest stages of planetary formation.
Furthermore, the space telescope has enable scientists to conduct photometric and spectroscopic studies of more than 3,000 young stars inside the nebula and observe the way their evolution is influenced by the intense ionising radiation from Orion’s four bigger and brighter stars. The latter, better known as the Trapezium, lie at the heart of the nebula and bathe their surroundings with energetic ultraviolet radiation which slowly lights up and erodes the surrounding gas and dust, leading to the formation of large cavities that give Orion its characteristic ethereal shape. Overall, this tumultuous activity constantly reshapes the nebula’s interstellar material in a constant battle between creation and destruction, one that leads to the formation of new generations of stars through the collapse of neighboring gas clouds in some parts of the nebula, or eating away at the latter’s star-forming material in others. “Orion may seem very peaceful on a cold winter night, but in reality it holds very massive, luminous stars that are destroying the dusty gas cloud from which they formed,” says Dr. Tom Megeath, a professor of astrophysics at the University of Toledo, in Ohio. “Eventually, the cloud of material will disperse and the Orion Nebula will fade from our sky.” Before that happens, however, the Orion nebula will keep providing scientists with great insights not only about the origins of other stellar and planetary systems, but for our own as well. “Images of photo-evaporating circumstellar disks taken with the Hubble Space Telescope have shown that the canonical scenario for star formation, valid for isolated low-mass stars quietly forming in sparse low-mass clusters may not adequately account for typical star formation in rich clusters [like Orion],” writes the research team of the The Hubble Space Telescope Treasury Program on the Orion Nebula Cluster, which has studied the latter extensively in recent years with the help of Hubble. “As the Sun appears to have formed in a similar environment, understanding the Orion Nebula Cluster may shed light not only on key passages of the star and planet formation in general, but also on the origin of our own planetary system.”
Another well-known site of star formation in the galaxy lies in the Eagle Nebula, or M16, at a distance of approximately 7,000 light-years away, toward the direction of the constellation Serpens. A region inside M16, better known as the “Pillars of Creation,” was first imaged by Hubble in 1995 in stunning resolution, capturing the dramatic view of three giant columns of cold molecular gas and dust, which are shaped, and at the same time slowly eroded by, the intense blue light and strong stellar winds of neighboring massive O-type stars. “These pillars represent a very dynamic, active process,” says Paul Scowen, a researcher at the Arizona State University, who participated in the team that had taken the famous Hubble photo. “The gas is not being passively heated up and gently wafting away into space. The gaseous pillars are actually getting ionized, a process by which electrons are stripped off of atoms, and heated up by radiation from the massive stars. And then they are being eroded by the stars’ strong winds and barrage of charged particles, which are literally sandblasting away the tops of these pillars.” Cocooned inside the pillars themselves, astronomers have also discovered more than 70 knots of denser gas, called “Evaporating Gaseous Globules,” or EGGs, where new stars are born. The evolution of these baby stellar systems is uncertain, however, due to the harsh radiation environment from the nearby massive stars, as well as the presence of a supernova that exploded in the vicinity approximately 8,000 years ago. Observations in recent years by NASA’s Spitzer Space Telescope have provided evidence that a series of shock waves from that explosion reached the Pillars of Creation a couple of thousand years later and may have destroyed them. Yet, since the entire region is located about 7,000 light-years away, astronomers can’t be sure of the outcome, for the light from this event will only reach Earth in a thousand years or so.
The Pillars of Creation were imaged again by Hubble earlier this year, in celebration of the telescope’s 25th anniversary. This time, astronomers took advantage of the infrared observing capabilities of the Wide Field Camera 3 which was installed during the last Hubble servicing mission in 2009. This latest image shows the iconic star formation site in a completely new light, allowing scientists to lift the veil of the Pillar’s obscuring gas and dust in order to glimpse its interior. “I’m impressed by how transitory these structures are,” says Scowen. “They are actively being ablated away before our very eyes. The ghostly bluish haze around the dense edges of the pillars is material getting heated up and evaporating away into space. We have caught these pillars at a very unique and short-lived moment in their evolution.”
Hubble has also been instrumental in providing astronomers with great views of the other end of stellar evolution, while returning breath-taking images of the stellar remnants of high-mass stars that have ended their lives in spectacular supernova explosions, as well as the gentle wisps of planetary nebulae which are the end products of lower-mass stars like our Sun that have ended their lives by slowly dissipating their outer layers into interstellar space. One of the earliest such studies by Hubble was that of SN 1987A, a supernova that suddenly exploded in early 1987 in the Large Magellanic Cloud which is a satellite dwarf galaxy to our own. The orbiting observatory has provided astronomers with spectacular views of the supernova remnant’s structure, allowing them to observe in detail for the first time the expansion of the supernova material directly, for more than 20 years. “The Hubble observations have helped us rewrite the textbooks on exploding stars,” says Dr. Robert Kirshner, a professor of astronomy at Harvard University. “We found that the actual world is more complicated and interesting than anyone dared to imagine. There are mysterious triple rings of glowing gas and powerful blasts sent out from the explosion that are just having an impact now, 20 years later. The sharp pictures from the Hubble telescope helped us to ask and answer new questions about Supernova 1987A. In fact, without Hubble we wouldn’t even know what to ask.”
The orbiting space telescope has also studied in detail such phenomena in our own galaxy, like the Crab Nebula, a 6-light-year-wide remnant of a star that exploded in 1054, and Tycho’s Supernova, or SN 1572, among others. In addition, it has provided stunning images of our galaxy’s ashortage of planetary nebulae, changing our views of the evolution of low-mass stars like our Sun. “[These] HST images allow us to preview our own future by looking at what happens to stars similar to ours as they approach their deaths,” writes Bruce Balick, an astronomer at the University of Washington. “A dozen years ago we believed that old, round stars just “huffed and puffed” a few times and threw off a spherical bubble nebula. Images such as those below show that our future is likely to be far more interesting, complex, and handsomely striking than we could have possibly imagined just a few years ago.”
Worlds Beyond Our Own
For centuries, the notion of planets orbiting other stars was well inside the realm of science fiction and fantasy. All this changed a little over 20 years ago, with the discovery of the first extrasolar planet around the pulsar PSR B1257+12. Ever since, the field of exoplanetary research has been one of the most flourishing and productive in astronomy, with new discoveries coming at a regular basis, most of which have been courtesy of NASA’s Kepler space telescope, which has discovered many thousands of exoplanets and exoplanet candidates to date within our galaxy.
As detailed in a previous AmericaSpace article, the Hubble Space Telescope has had a leading role in the emerging field of exoplanet study, by allowing astronomers to obtain the transmission spectrum of dozens of extrasolar planets, ranging from “Super-Earth”-type planets slightly larger than our own, to “hot Jupiter” gas giants many times more massive than Jupiter in our own Solar System. These observations have helped researchers to put constrains on the elemental abundances of the planetary atmospheres for many of these worlds, providing valuable insights about their internal processes and formation conditions.
Other important Hubble contributions include the direct imaging of an exoplanet candidate, like Fomalhaut b, a massive planet-like object that orbits the nearby star Fomalhaut at a distance of 25 light-years away, the detection of a new class of steamy, waterworlds, the possible detection of terrestrial planets around Alpha Centauri, which is the nearest star system to Earth, the discovery of ancient terrestrial exoplanets more than 10 billion years old, as well as the detection of water vapor in the atmospheres of hot Jupiters, just to name a few.
Hubble’s importance in the field of exoplanetary research is a testament to the ingenuity of thousands of scientists and engineers who envisioned ways for improving the observatory’s capabilities through the years, as well as the astronauts who serviced it. “Even the most optimistic person to whom you could have spoken back in 1990, couldn’t have predicted the degree to which Hubble would rewrite our astrophysics and planetary science textbooks,” remarked NASA Administrator Charlie Bolden, during a recent celebration of the Hubble space telescope’s 25th anniversary at the Newseum in Washington, D.C. “A quarter century later, Hubble has fundamentally changed our human understanding of our Universe and our place in it.” Perhaps equally important is that fact that Hubble has proved the worth of space-based observatories in the study of worlds beyond our own, helping to pave the way for the next generation of space telescopes, like the James Webb Space Telescope, in the hunt for planets that might be similar to Earth.
As the next wave of bigger and much more advanced space observatories prepare to open their eyes in the coming years to gaze even deeper into the Cosmos, they will rest on the shoulders of the now comparatively archaic but ever so powerful Hubble.
You can read Part 4, here.
Below are more images from the Hubble Space Telescope: