A white dwarf destroys itself in a special class of stellar explosion that is termed a Type Ia supernova. However, there is a long-standing debate among astronomers about what exactly triggers these violent white dwarf blasts. Two main theories exist. The first is that Type Ia supernovae are the result of an accumulation of material onto a white dwarf that it has stolen from a companion star (and victim). The second proposal suggests that Type Ia blasts are the tragic result of a horrendous and violent merger of a duo of white dwarfs. It is very important for scientists to determine precisely what triggers Type Ia supernovae because these mysterious blasts are what astronomers use to calculate the expansion rate of the Universe.
"Astronomers use Type Ia supernovae as distant markers across the Universe, which helped us discover that its expansion was accelerating. If there are any differences in how these supernovae explode and the amount of light they produce, that could have an impact on our understanding of this expansion," explained Dr. Sayan Chakraborti in a March 30, 2016 Chandra X-ray Observatory Press Release. Dr. Chakraborti, who led the study, is at Harvard University in Cambridge, Massachusetts.
It is thought that Type Ia supernovae explode in consistent ways. This makes them excellent "standard candles" for astronomers to use when determining cosmological distances. As a result, they have been used for decades to help astronomers study the expansion and size of the Universe.
The object that the astronomers used for this study had been previously identified, and then bestowed, with the bland name of G1.9+0.3. G1.9+0.3 is the remnant of the most recent supernova in our Milky Way, and it has been estimated that the stellar explosion occurred about 110 years ago within a dusty region of our Galaxy that veiled its visible light from reaching Earth.
Ghosts Of Small Sun-Like Stars
When a small star like our Sun has reached the end of that long stellar road, after it has used up its necessary supply of hydrogen fuel in its nuclear-fusing heart, it tosses most of its multicolored, shimmering outer layers of gas into the space between stars. These beautiful objects are called planetary nebulae, and they are frequently referred to as the "butterflies" of the Cosmos because of their great beauty. Only the searing-hot core of the former Sun-like star remains to tell the tragic tale of the star that was--and this core is the white dwarf, with a truly roasting temperature that exceeds 100,000 Kelvin.
Since a white dwarf cannot create internal pressure derived from the release of energy resulting from nuclear fusion--because it no longer has fuel to fuse--gravity crushes the stellar matter inward until even the electrons that make up the white dwarf's atoms are smashed together.
With a surface gravity of about 100,000 times that of Earth, the atmosphere of a white dwarf is also weird. The heavier atoms in its atmosphere tumble downward, while the lighter ones remain where they are at the surface. A percentage of white dwarfs possess almost pure hydrogen or helium atmospheres--which are, of course, very light. In addition, the gravity of a white dwarf pulls the atmosphere close around it into a very slender layer. If this sort of thing happened on Earth, the top of the atmosphere would be below the tops of skyscrapers.
Astronomers also hypothesize that there is a crust about 50 kilometers thick beneath the atmosphere of many white dwarfs. At the bottom of this strange crust is a crystalline lattice of carbon and oxygen atoms. Because diamond is really just crystallized carbon, it may be possible to make a comparison between a cool carbon/oxygen white dwarf and a diamond!
Stars of all masses "live" out their brilliant, fiery lives on the hydrogen-burning main-sequence of the Hertzsprung-Russell Diagram of stellar evolution. Stars keep themselves healthy and fluffy by maintaining a very precious, delicate, and absolutely necessary balance between two antagonistic forces--gravity and radiation pressure. The gravity of a star tends to pull all of the stellar material towards the star--it is the squeezing force that crushes everything inward. Conversely, the star's radiation pressure keeps it bouncy against the relentless and potentially fatal squeeze of its own powerful gravity, by pushing its stellar material outward and away from the star. A star's radiation pressure results from nuclear fusion, which starts off with the burning of hydrogen--the lightest and most abundant atomic element in the Universe--into helium. Helium is the second-lightest atomic element in the Cosmos, and this process continues on to fuse increasingly heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis).
When a massive main-sequence star, that weighs-in at a hefty eight solar-masses, has at last succeeded in fusing its entire necessary supply of nuclear fuel, it destroys itself in the violent and fiery tantrum of a Type II, core-collapse supernova. The massive star-that-was can no longer keep itself bouncy against the powerful and merciless crush of its own gravitational embrace, and gravity at last tragically wins the struggle against radiation pressure.
Type II supernovae normally blast an elderly, massive star to smithereens in the rage of a brilliant stellar funeral pyre. The tragic, violent, but nevertheless rather beautiful event, occurs when the iron core of a massive star gains so much weight that it reaches 1.4 solar-masses. This sets off the Type II core-collapse event. The most massive stellar denizens of the Universe collapse and blow themselves up into the oblivion of a black hole. Massive stars--that are not that massive--also blast themselves into fragments, but they leave behind, as a kind of souvenir to the Universe, their extremely dense relic cores. This remnant core is termed a neutron star, and it is essentially one big Chicago-sized atomic nucleus. One teaspoon of neutron star material would weigh as much as a family of water buffalo.
But small, less massive stars, like our own Sun, die in relative peace. When a small star goes gentle into that good night, it has at last fused its necessary supply of life-sustaining hydrogen fuel--and it becomes a bloated red giant star, that eventually tosses its outer gaseous layers into interstellar space, leaving behind the white dwarf relic core of the now deceased star. Alas, our Sun will take this fatal route in about 5 billion years, first swelling up into a crimson stellar monster of a voracious red giant, that will incinerate first Mercury and then Venus in its terrible, furious fires--before it goes on to possibly vaporize our Earth, as well. But, at last, our Sun will evolve into a small, dense white dwarf, beautifully surrounded by an exquisite shell of glowing varicolored gases.
Although our Sun is a lonely, solitary little star, many stars like it enjoy the company of others. A large number of Sun-like stars are members of binary systems, where they dwell very close to a neighboring stellar sibling. The sibling star may still be thriving on the hydrogen-burning main-sequence, long after the progenitor star of its white dwarf companion has perished. The white dwarf, in this particular case, may become somewhat sinister--and gulp down the stellar material of its sibling's gases until, at long last, it can swallow no more. The vampire-like white dwarf devours all that it can of its stellar companion's nourishing gases--but this terrible buffet backfires on the evil, dense stellar ghost. The white dwarf finally feasts on so much of its sibling star's material that it reaches "critical mass" and blows itself up in a supernova blast--just like the big guys. The white dwarf pays for its cruelty, and meets its fate as a brilliant member of that special class of supernova called a Type Ia.
Our Galaxy's Youngest Stellar Blast
The new study of the most recent Type Ia supernova, to explode in our Milky Way Galaxy, used archival data derived from both Chandra and the VLA. This research examines how the expanding supernova remnant G1.0+0.3 interacts with the gas and dust swirling around in the region surrounding the blast. The resulting X-ray and radio emission provides a treasure chest of intriguing clues revealing long-hidden secrets about the true cause of this stellar explosion--and others like it. Of special importance is the observation that there was an increase in X-ray and radio brightness of this supernova remnant as time went by.
"We observed that the X-ray and radio brightness increased with time, so the data point strongly to a collision between two white dwarfs as being the trigger for the supernova explosion in G1.0+0.3," explained study co-author Dr. Francesca Childs in the March 30, 2016 Chandra Press Release. Dr. Childs is also of Harvard University.
Indeed, merging carbon-oxygen-rich white dwarfs, crashing into each other at the end of a catastrophic gravitational dance, are promising progenitor systems explaining the origins of Type Ia supernovae. Therefore, the result of these observations suggest that Type Ia supernovae are either all caused by white dwarf collisions, or are triggered by a combination of white dwarf collisions and the mechanism whereby a sinister vampire-like white dwarf sips up material from a badly victimized main-sequence companion star.
"It is important to identify the trigger mechanism for Type Ia supernovas because if there is more than one cause, then the contribution from each may change over time," explained Harvard's Dr. Alicia Soderberg, another study co-author, in the same Press Release. "This means astronomers might have to recalibrate some of the ways we use them as 'standard candles' in cosmology," she added.
The team also determined a new estimate for the age of the supernova remnant of approximately 110 years--which makes it younger than previous estimates of about 150 years.
This article is dedicated to Kathy and John Braffman.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
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