Stars do not live forever–they send their brilliant stellar fires out into space for millions to billions of years, but they are doomed roiling, glaring spheres of gas. When stars run out of their necessary supply of nuclear-fusing fuel, they are ready for their inevitable meeting with the Grim Reaper. Massive stars blast themselves to pieces in violent and brilliant core-collapse (Type II) supernovae, while small stars like our Sun may–or may not–pass away in relative peace. Small stars, like our Sun, can also go supernova–just like the big guys–but the conditions must be right in order for them to give this sort of brilliant farewell performance to the Cosmos. When a small sun-like star perishes in the noisy blast of a brilliant Type Ia supernova, it leaves behind a lingering ghost, which is termed a white dwarf star- the erstwhile small star’s relic core. A new study published in the August 10, 2019 issue of The Astrophysical Journal proposes that these ghostly remains of stars long gone still haunt stars that are living today.
A Type Ia supernova is an explosion that ignites in binary stellar systems in which a duo of sister stars are in orbit around one another–and one of the stars is a white dwarf. The other star can by anything from a giant star to an even smaller white dwarf. The fatal blast in this type of binary system occurs because the stellar ghost has been gravitationally sipping up its sister star’s material, causing it to acquire enough mass to go critical. A second explosive scenario, also taking place in a binary system, occurs when a duo of white dwarf stars crash into one another.
White dwarfs are not all the same. These dense stellar ghosts can range from 50% of the mass of our Sun, to almost 50 times its mass. Solitary small stars like our Sun, die a quiet death. After a lonely small sun-like star has run out of its necessary hydrogen fuel to fuse, it puffs its beautiful multicolored shimmering layers of gas into the space between stars. However, its lingering core (the white dwarf) has remained intact, and it rests in peace at the heart of this lovely shroud of glimmering gas.
By studying the “fossil” relics of long-dead exploded white dwarfs, a team of astronomers, led by Dr. Evan Kirby of the California Institute of Technology (Caltech) in Pasadena, have found that white dwarfs in the early Universe frequently blasted themselves to pieces at lower masses than they do today. This discovery suggests that a white dwarf can explode as the result of a variety of triggers, and it does not necessarily have to reach critical mass before the fatal blast.
Fatal Attractions Among The Stars
All of the stars in the observable Universe, both large and small, live out their entire nuclear-fusing main-sequence “lives” by keeping a very delicate and necessary balance between two ancient foes–gravity and radiation pressure. The main-sequence refers to hydrogen-burning stars on the Hertzsprung-Russell Diagram of Stellar Evolution. Main-sequence stars still have enough nuclear-fusing hydrogen fuel to keep themselves bouncy against the crush of their own gravity.
The outward push of a star’s radiation pressure forces everything away from the star, while simultaneously gravity tries to mercilessly pull everything inward. The radiation pressure of a star is the result of the process of nuclear-fusion which commences with the burning of hydrogen, the lightest and most abundant atomic element in the Cosmos, into helium–which is the second lightest. This process of stellar nucleosynthesis continually fuses increasingly heavier and heavier atomic elements out of lighter ones. Indeed, all of the atomic elements heavier than helium–termed metals in the jargon of astronomers–formed within the searing-hot nuclear-fusing cores of the billions of stars inhabiting our vast Universe. Alternatively, the heaviest metals of all–such as gold and uranium–form in the spectacular supernova explosions heralding the death of a star.
Many supernovae are triggered when a lone, especially massive star runs out of its necessary supply of nuclear-fusing fuel, and rips itself apart in a spectacular core-collapse explosion. The progenitor of a core-collapse (Type II) supernova is usually a massive star that contains an extremely heavy core that weighs-in at about 1.4 times solar mass. Smaller stars normally do not die this way. In fact, smaller stars live much longer than more massive stars. This is because less massive stars are not as hot, and hence burn their fuel more slowly, than their heavier stellar kin. Small stars of our Sun’s mass generally last for about 10 billion years. More massive stars, however, live fast and die young–frequently living for millions (as opposed to billions) of years.
Like all stars, our Sun is doomed to run out of its necessary hydrogen fuel. It is a middle-aged star of about 4.56 billion years of age, and it can continue to fuse hydrogen in its core for another 5 billion years, or so.
When small stars, like our sun, finally have managed to fuse most of their necessary hydrogen fuel into heavier things, they first swell into glaring, bloated red giant stars. This evolved sun-like star at this late stage of development contains a worn-out heart composed of helium, surrounded by a shell in which there is still a small amount of hydrogen left to be burned into helium. This shell begins to travel outward, and the dying heart of the small star grows ever larger, as the star ages. At last, the helium heart begins to shrink. As it does so, its temperature soars at its center to the point that the helium is fused into the even heavier metal carbon. The star ends up with a very small, but extremely hot heart, that produces more energy that it did when it was still a hydrogen-burning star on the main-sequence. The star is now doomed, and its outer layers of gas are now swollen and red. The temperature at the glaring surface of this bloated red giant is cooler than it was when it was still a young star.
When small stars are like our Sun, and live alone, they die gently and beautifully–leaving their relic cores behind as white dwarf stars. However, when the star has company, explosive things can happen in the form of a Type Ia blast. These brilliant explosions hurl the former star’s newly forged elements out into space where they may be incorporated into later generations of stars.
Nevertheless, it is still uncertain why some white dwarfs go supernova while others of their kind do not. In the early decades of the 20th-century, the great Indian-American astrophysicist Subramanyan Chandrasekhar (1910-1995) determined that if a white dwarf possessed more than 1.4 times solar-mass, it would explode in a Type Ia supernova. This particular mass is called the Chandrasekhar mass, and although Chandrasekhar’s calculations explained one means by which some of the more massive white dwarfs explode, it did not explain why other white dwarfs of smaller mass also blow themselves up.
Ancient Stellar Ghosts Haunt Stars Today
Observing Type Ia supernovae must be accomplished quickly. This is because they flare into existence, and then dim to darkness, within only a few months. In order to study supernovae that have long since faded, and the stellar white dwarf ghosts that created them, Dr. Kirby and his colleagues used a technique informally named galactic archaeology.
The term galactic archaeology refers to the search for lingering chemical signatures of ancient explosions in other, more youthful stars. When a white dwarf explodes in a Type Ia supernova, it “pollutes” its galactic surroundings with the elements forged during the blast–metals like nickel and iron. Then those heavy atomic elements wander through space, eventually to be incorporated into any newly forming baby stars located in that region. This is comparable to the way that fossils currently provide scientists with important clues about animals that have long been extinct. The quantities of nickel and iron show how massive their long-gone predecessors must have been.
Using the Keck II telescope in Hawaii, Dr. Kirby and his team first went on the hunt for certain ancient galaxies, that had run out of material to form baby stars during the first billion years of the Universe’s existence. The astronomers discovered that most of the stars within these galaxies had relatively low nickel content. This was a tattle-tale clue that the white dwarf ghosts that provided them with nickel must have been of relatively low mass–likely lower than the Chandrasekhar mass.
However, the team of galactic archaeologists found that the nickel content was higher in more recently formed gas. This means that, as time passed after the Big Bang, white dwarfs started to explode with higher masses.
It is important for scientists to understand the mysterious processes that trigger Type Ia supernovae. This is because the explosions themselves have been successfully used as tools for making cosmological measurements. No matter how they exploded, most Type Ia supernovae display a well-known relationship between luminosity and the time it takes for them to dim.
“We call Type Ia supernovae ‘standardizable candles’,” Dr. Kirby commented in an August 6, 2010 Caltech Press release.
“If you look at a candle at a distance it will look dimmer than when it is up close. If you know how bright it is supposed to be up close, and measure how bright it is at a distance, you can calculate that distance. Type Ia supernovae have been very useful in calculating things like the rate of expansion of the Universe. We use them all the time in cosmology. So, it’s important to understand where they come from, and characterize the white dwarfs that make these explosions,” he added.
The scientists plan to study elements other than nickel next, and are planning to target manganese in particular. Manganese production is extremely sensitive to the mass of the supernova that churns it out, and therefore provides a precise way to validate conclusions derived from the nickel content.
The research paper is published under the title “Evidence for Sub-Chandrasekhar Type Ia Supernovae from Stellar Abundances in Dwarf Galaxies”.
