Our Sun is doomed to eventually die after it has “lived” brilliantly and beautifully for about 10 billion years. The good news is that, because our Sun currently is a bit less than 5 billion years of age, it can still go on blissfully burning its supply of life-sustaining nuclear-fusing hydrogen fuel for another 5 billion years before the Grim Reaper comes to call. Solitary stars of our Sun’s mass all perish the same way–first ballooning in size to become enormous, swollen, crimson red giant stars before tossing their outer shimmering rainbow of multicolored gaseous layers into interstellar space–leaving behind only a dense Earth-sized relic core termed a white dwarf star to bear witness to the tragedy. In July 2019, an international team of astronomers announced that they had observed a rare dynamic event for the first time, when they witnessed the death of a distant red giant star. This observation provides a sneak preview of our Sun’s inevitable and sad demise.
Dr. Meredith Joyce, an astronomer based at The Australian National University (ANU) co-led the study with Dr. Laszlo Molnar and Dr. Laszlo Kiss from the Konkoly Observatory of the Hungarian Academy of Sciences. Dr. Joyce noted in a July 26, 2019 ANU Press Release that the star observed, dubbed T Ursae Minoris (T UMi), was similar to our Sun.
“This has been one of the rare opportunities when the signs of ageing could be directly observed in a star over human timescales. We anticipate our Sun and TMi will end their lives much more quietly and slowly compared with a supernova–a powerful and luminous explosion,” Dr. Joyce continued to explain.
The new findings strengthen the prediction that our dying Sun will become a red giant, before evolving into an expanding and glowing ring-shaped shell of gas in five billion years, leaving behind only a small white dwarf stellar corpse.
“It will become much bigger as it approaches death–eating Venus, Mercury and possibly the Earth in the process–before shrinking to become a white dwarf,” Dr. Joyce added.
The Life-Cycle Of Lonely Stars Like Our Sun
Stars are a lot like people. They are born, and then go on to enjoy a playful childhood and active youth–but, eventually, stars calm down when they evolve into glaring adults. However, the inevitable occurs when a star grows old and dies. Stars that are more massive than our Sun end their stellar lives when they explode in brilliant, violent supernova blasts. After the catastrophe, the erstwhile massive star leaves behind a dense, city-sized object termed a neutron star. Neutron stars are so dense that a teaspoon full of neutron star material can weigh as much as a herd of zebra. However, the most massive stars in the Universe have a different fate. When these especially massive stars run out of their necessary supply of life-sustaining nuclear-fusing fuel, they collapse into the oblivion of a black hole of stellar mass.
Less massive stars, like our Sun, come to the end of the stellar road more peacefully, without the brilliant grand finale fireworks display of their more massive stellar counterparts. When stars like our Sun live alone, without a binary companion, they first evolve into red giants that ultimately blow off their outer layers to become white dwarfs encircled by a beautiful shell of multicolored gases.
All stars, regardless of their mass, are kept brilliantly bouncy as a result of an enduring battle between gravity and radiation pressure. Gravity tries to pull all of the star’s material in, while radiation pressure tries to push everything out. This delicate balance between the two eternal foes goes on from stellar-birth to stellar-death. In the end, when the old star runs out of its necessary supply of nuclear-fusing fuel, it can no longer churn out radiation pressure to counteract the relentless and merciless pull of its own gravity As a result, gravity wins the war, and the star is doomed.
Today, our Sun is a lonely star, but it was probably not born that way. Our Star probably formed as a member of a heavily populated open cluster, along with thousands of other newborn, fiery sibling stars. Our Sun was either gravitationally evicted from its natal cluster due to interactions with others of its fiery kind or it simply peacefully floated away into the space between stars approximately 4.5 billion years ago. Likewise, our Sun’s long-lost siblings are thought to have migrated to more remote regions of our Milky Way Galaxy, never to return.
Our entire Solar System emerged from the tattered remains left over from the nuclear-fusing ovens of previous generations of dead ancient stars. Our Sun–like others of its kind–was born within a dense, cold blob tucked tenderly within the whirling, swirling, ruffling folds of a churning dark, giant molecular cloud. The dense blob ultimately collapsed under the intense pull of its own gravity, thus giving birth to a hot, glaring baby star (protostar). Within the secretive depths of these enormous and beautiful clouds, that float like lovely, eerie phantoms throughout our Galaxy in huge numbers, fragile threads of material tangle themselves up together, and the resulting clumps grow ever larger and larger for hundreds of thousands of years. Then, pulled inward by the relentless crush of gravity, the hydrogen atoms existing within the clumps rapidly and dramatically fuse. This process of nuclear fusion triggers a violent conflagration that will rage with brilliant fury for as long as the new star lives–for that is how a star is born.
Currently, our Sun is experiencing an active, nuclear-fusing midlife. It is classified as a main-sequence (hydrogen-burning) small star on the Hertzsprung-Russell Diagram of Stellar Evolution. As stars go, it isn’t special. There are eight major planets and an assortment of smaller bodies in orbit around our Sun, which is located in the distant suburbs of a typical, though magnificent, large spinning, starlit pinwheel in space–our barred-spiral Milky Way Galaxy.
Today, our Sun is still sufficiently youthful and bouncy to go on burning hydrogen in its heart by way of nuclear fusion–which continually creates heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). But our Sun’s looks will change when it finally begins to run out of its necessary supply of hydrogen fuel. At this point, our Sun will evolve into an elderly star. In the dying heart of our Sun, there will exist a core of helium that is encased within a shell composed of hydrogen that is still in the process of being fused into helium. Eventually, the shell will begin to travel outward, and the core will grow larger as our Sun grows older. The helium core itself will ultimately shrivel under the intense pull of its own weight–until, finally, it grows seething-hot enough to trigger a new stage of nuclear-fusion. At this point, the atoms existing in our dying Star’s helium core will begin to be fused into the heavier atomic element, carbon. In another five billion years, our Star will possess a small and searing-hot core that will be emitting more energy than it does now. The outer gaseous layers of our doomed Sun will have ballooned to hideous proportions, and it will no longer be the same Star we are familiar with today. Alas, it will have experienced a sea-change into a ghastly red giant that will go on to engulf and incinerate Mercury, then Venus, and then (possibly) Earth. The temperature at the surface of our Star, in its future red giant phase, will be quite a bit cooler than it is now–which will account for its comparatively cool red hue. Nonetheless, our evolved Star will still be hot enough to convert the currently frigid, icy inhabitants of the distant Kuiper Belt–such as the dwarf planet Pluto–into delightful tropical havens of refuge for what may (or may not) be left of humanity. The dying hot heart of our Sun will continue to shrivel, and since it can no longer produce radiation by way of nuclear fusion, all further evolution will be governed only by the force of gravity. Our Star will then finally hurl off its outer layers–but its core will stay in one piece. All of our Sun’s material will finally collapse into this small relic body–the newly-formed white dwarf. The baby white dwarf will be encircled by a lovely expanding shell of multicolored gases termed a planetary nebula. White dwarfs radiate away the energy of their collapse, and our future white dwarf Sun will likely be composed of carbon and oxygen atomic nuclei swimming around in a swirling sea of degenerate electrons. The equation of state for degenerate matter is “soft”. This means that any mass added to the body will cause it to grow smaller in size. Continuing to add mass to a white dwarf causes it to shrink further, even as its central density grows larger. Our Sun’s radius will finally shrink to only a few thousand kilometers. Our Sun, and other stars like it, will grow progressively cooler over time when they evolve into white dwarfs.
Red Giants In General
Red giants are luminous giant stars of low to intermediate mass that weigh-in at the approximate equivalent of 0.3 to 8 times that of our Sun. These swollen stars represent a late phase of stellar evolution, and their outer atmospheres are both inflated and tenuous. This makes their radii large. The surface temperature of a red giant is about 8,500 degrees Fahrenheit–or lower, and their true colors are anywhere from yellow-orange to red. Many of the observed bright stars are red giants. This is because they are rather common denizens of our Galaxy, and they are also very luminous. For example, the red giant Arcturus is 36 light-years away, and Gamma Crucis is 88 light-years from Earth.
In general, red giants sport radii that are tens to hundreds of times greater than that of our Star. Despite the lower energy density of their envelope, red giants are much more luminous than our Sun because of their enormous size. Indeed, red giants have luminosities that can be as much as almost three thousand times that of our Star.
Another important feature that red giants have is that, unlike still-living sun-like stars whose photospheres display numerous small convection cells (solar granules), red giant photospheres have only a few large cells. This feature is responsible for causing the variations in brightness so common in these swollen stars.
Over the main-sequence “life” of a sun-like star, its supply of hydrogen in its core is slowly fused into helium. The doomed star’s main-sequence “life” comes to a tragtic end when almost all of the hydrogen in its core has been fused. Stars that are more massive than our Sun burn disproportionately faster, and thus spend less time on the main-sequence than their smaller stellar kin. The larger the star, the shorter its hydrogen-burning “life”.
Sneak Preview Of Our Sun’s Demise
The dying sun-like star T UMi was born about 1.2 billion years ago, sporting a mass approximately twice that of our Sun. It is situated in the Ursae Minoris (Little Bear) Constellation over 3000 light-years from Earth.
The international team of astronomers found that over the past few million years–as T UMi reached the last phase of “life” before its sea-change into a white dwarf–it has been experiencing a series of pulses, whereby its temperature, brightness, and size have fluctuated dramatically.
“Energy production in T UMi has become unstable. During this phase, nuclear fusion flares up deep inside, causing ‘hiccups’ that we call thermal pulses. These pulses cause drastic, rapid changes in the size and brightness of the star, which are detectable over centuries. The pulses of old stars like T UMi also enrich the entire Universe with elements including carbon, nitrogen, tin and lead,” Dr. Joyce explained in the July 26, 2019 ANU Press Release.
The astronomers have observed the star diminishing in temperature, size, and brightness over the past three deades.
Dr. Joyce continued to note that “We believe the star is entering one of its last remaining pulses, and we’d expect to see it expanding again in our lifetimes. The star will eventually become a white dwarf within a few hundred thousand years. Both amateur and professional astronomers will continue to observe the evolution of the star in the coming decades, which will provide a direct test of our predictions within the next 30 to 50 years.”
This research is published in the July 5, 2019 issue of The Astrophysical Journal.