Stars, like people, do not live forever. After a star has used up its necessary supply of hydrogen fuel in its searing-hot nuclear fusing heart, it has reached the tragic end of that long stellar road, and is about to meet its doom. Even though it is well-known that newborn baby stars, or protostars, are born surrounded by a swirling, whirling disk of planet-building gas and dust, new observations indicate that elderly stars may also be surrounded by similar disks–and may even get a second chance at having a new family of planets! In March 2016, astronomers announced that the Very Large Telescope Interferometer (VLTI) at the European Southern Observatory’s (ESO’s) Paranal Observatory in Chile has obtained the clearest view ever of the dusty disk swirling around an elderly, aging star. For the very first time such features can be compared to those surrounding baby stars–and they appear to be amazingly similar–perhaps giving the old star the opportunity to create a second generation of planet-children!
As stars come to the end of their “lives”, a large number of them form stable disks of gas and dust around them. This material is ejected by fierce stellar winds, while the elderly star is passing through the red giant phase of its evolution. These disks hauntingly resemble those that form planets orbiting young stars. But up until now, astronomers have not been able to compare the two types–one formed at the star’s birth, and the second at the end of its brilliant stellar “life”.
Even though there are numerous disks observed to be associated with baby stars that are sufficiently close to Earth for astronomers to study in depth, up until now there have been no corresponding elderly stars with disks that are close enough for detailed images to be obtained.
But this has now changed. A team of astronomers led by Dr. Michel Hillen and Dr. Hans Van Winckel of the Instituut voor Sterrenkunde in Leuven, Belgium, has used the power of the VLTI, armed with the Precision Integrated-optics Near-Infrared Imaging ExpeRiment (PIONIER) instrument, and the freshly upgraded Revolutionary Fast Low Noise Detector (RAPID) detector on PIONIER, to spy these elusive, whirling objects.
The Life And Death Of A Star
When an enormous, dark, frigid molecular cloud condenses, a rotating baby star forms within an especially dense blob embedded within the folds of the cloud. The dusty material encircling the neonatal star is also moving, and it will ultimately flatten into a pancake-like disk around the protostar’s equator. This dusty disk material is the stuff of planets, moons, asteroids, and comets!
Some astronomers suggest that planet birth can be compared to the way snowballs form. Over the passage of millions of years, tiny and inherently “sticky” dust motes merge together to create ever larger and larger objects–from boulder size, to mountain size, to moon size. At last, some of those colliding rocks grow to become planets, like our own Earth–or, alternatively, the cores of gaseous giant planets like Jupiter, Saturn, Uranus, and Neptune that reign majestically in our Solar System’s outer domain. Large objects that fail to contribute to the formation of baby planets frequently evolve into asteroids and comets.
Within the mysterious, dark, and blanketing folds of these enormous molecular clouds composed of gas and dust, fragile threads of material twist around one another and then merge together–continuing to grow ever larger and larger for hundreds of millions of years. The crush of relentless gravity, at long last, becomes so powerful that the hydrogen atoms–that are jumping around within these eerie, dense, and dark blobs–suddenly and dramatically fuse. This sets the baby star on fabulous fire, and these furious flames will lash and glare with great brilliance for as long as the star “lives”.
The process of nuclear fusion is what lights the baby star’s stellar fire. Brilliant and very, very hot protostars fight for their “lives” by balancing two eternally battling enemies in order to reach sparkling stellar adulthood. Indeed, all stars on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of stellar evolution, regardless of their age, must spend their “lives” maintaining a precious, precarious balance between the two enemies: gravity and radiation pressure. While the powerful, relentless pull of gravity tugs everything in, the push of radiation pressure forces everything out and away from the star! This necessary balance between the two enemies keeps the star “alive”, and on the main-sequence. Alas, stars, like people, grow old. When an elderly star has burned up its necessary supply of fuel in its nuclear-fusing heart, this core experiences an inevitable, tragic collapse–and the star perishes. Small stars, like our Sun, die with relative peacefulness–and great beauty. Small Sun-like stars gently toss their multicolored, shimmering outer gaseous layers into interstellar space. The lingering remnant core of a small Sun-like star evolves into a stellar ghost called a white dwarf. However, before the small star enters the white dwarf stage, it swells up to monstrous proportions, and becomes a red giant star. When our Sun enters its red giant stage, it will become a fiery red, enormous monstrosity, that engulfs some of its own planet-children in the seething furnace of its outer gaseous layers–first, Mercury, then Venus, and then–possibly–Earth. However, this will not happen for billions of years. Our Sun is about 4.56 billion years old, and is still a middle-aged star. Stars of our Sun’s mass “live” for about 10 billion years.
It is easier for astronomers to observe dust surrounding a star than it is for them to spot rocks or planets. Swirling dust, dancing around a distant star, blankets more area than a planet, asteroid, or comet. As the tiny dust motes absorb heat from their stellar-parent, they re-emit most of their light at long infrared wavelengths. In a way that is similar to burners on an electric stove top that turn from “red” to “white” as they grow increasingly hotter and hotter, the distant dust in a disk will possess different temperatures depending on its separation from its parent-star. This information can then be used to determine the structure and age of the disk, and provide valuable information about whether or not planets are in the process of forming–or if they have already been born. Astronomers quantify the color of the dust by measuring its spectrum, or brightness, with the heat-sensitive, infrared vision of telescopes like NASA’s Spitzer Space Telescope. Infrared detecting technologies are perfect for observing distant planet-birthing disks around distant stars, and characterizing exoplanets, which are planets belonging to a star beyond our own Sun.
Terrestrial planets–which are rocky planets like our Earth–form around many, if not most of the nearby Sun-like stars in our Milky Way Galaxy. This indicates that the potential for life might be considerably more common than once thought. Indeed, the Spitzer Space Telescope has found observational hints of planet formation around dead stars, and stars as young as a “mere” one million years old–which is very young for a star. Future missions will follow-up on many of Spitzer’s observations to determine whether planets like our own Earth exist in any of these systems.
Dusty Disks Around Old Stars May Give Them A Second Chance
The team of astronomers, who discovered the disk circling an elderly star, had targeted an old binary star system dubbed IRAS 08544-4431, which resides about 4,000 light-years from Earth in the southern constellation Vela (The Sails). This binary star is composed of a bloated red giant star, which was responsible for shedding the material in the surrounding dusty disk, and a less-evolved main-sequence, normal star orbiting close to it. 바카라사이트
Dr. Jacques Kluska explained in a March 9, 2016 ESO Press Release that “By combining light from several telescopes of the Very Large Telescope Interferometer, we obtained an image of stunning sharpness–equivalent to what a telescope with a diameter of 150 meters would see. The resolution is so high that, for comparison, we could determine the size and shape of a one euro coin seen from a distance of two thousand kilometers.” Dr. Kluska is a team member from the University of Exeter in the United Kingdom.
Because of the unprecedented sharpness of the images derived from the VLTI, and a new imaging technique that is able to eliminate central stars from the image to unveil what lies around them, the team could determine all of the building blocks of the IRAS 08544-4431 system for the first time.
The most prominent feature seen on the images is the clearly resolved ring. The inner edge of the dusty ring, observed for the first time, corresponds very well with the predicted beginning of the dusty disk: closer to the stellar duo, the dust would evaporate in the shower of ferocious radiation rushing out from the stars.
“We were also surprised to find a fainter glow that is probably coming from a small accretion disk around the companion star. We knew the star was double, but were’nt expecting to see the companion directly. It is really thanks to the jump in performance now provided by the new detector in PIONIER,” explained study lead author, Dr. Hillen, in the March 9, 2016 ESO Press Release.
The team discovered that the disks swirling around elderly stars are indeed very similar to the planet-birthing ones that surround young stars. Could a second generation of baby planets really be born around these old stars? That is the question, and the answer is yet to be determined. Nevertheless, it is certainly an interesting possibility.