Our Future Sun

Our Future Sun
By Judith E Braffman-Miller

Our Sun was born about 4.56 billion years ago from the jumbled fragments, left as lingering relics, of earlier generations of stars that perished long ago–their nuclear-fusing fires extinguished after consuming their necessary supply of fuel. Stars of all sizes fuse increasingly heavier and heavier atomic elements out of lighter ones, and when they reach the end of that long stellar road, they send their freshly fused batch of heavy atomic elements out into the space between stars, where they can then be incorporated into the freshly lit fires of younger stellar babies. The Big Bang birth of the Universe, that occurred about 14 billion years ago, produced only the lightest of atomic elements–hydrogen, helium, and traces of lithium. All of the atomic elements heavier than helium, called metals by astronomers, were manufactured in the searing-hot furnaces of the stars.

A red giant is a brightly shining crimson star of low to intermediate mass, and these elderly stars are in a late stage of stellar evolution, their outer gaseous atmospheres inflated and tenuous. These dying stars possess a large radius, and a surface temperature of “only” about 8,540 degrees Fahrenheit–or even lower–which is very cool for a star. In a research paper, published in the October 30, 2017 issue of the journal Nature Astronomy, a team of astronomers, led by Dr. Wouter Vlemmings, from Chalmers University of Technology in Sweden published, for the first time, observed details of the surface of an elderly red giant star with about the same mass as our own middle-aged Sun–providing a sneak preview of our Star’s fate.

The observations, revealing the aging, distant red giant, were acquired by the Chalmers astronomers from images that they had obtained from the Atacama Large Millimeter/submillimeter Array (ALMA), located in the Atacama desert of northern Chile. The elderly star proved to be a giant, with a diameter twice the size of Earth’s orbit around our Sun. In addition, the star’s atmosphere seemed to be affected by unexpected, strong shock waves.

The red giant star, dubbed W Hydrae, is 320 light-years from our Solar System–which practically places it in Earth’s cosmological backyard–and it is located in the constellation of Hydra, the Water Snake.

W Hydrae is an example of what is termed an asymptotic giant branch (AGB) star. AGB stars are bright, cool, elderly and in the process of losing mass by way of their strong and merciless stellar winds. This name is taken from the AGB star’s position on the Hertzsprung-Russell Diagram of Stellar Evolution, which classifies stars according to their brightness and temperature.

Dr. Vlemmings explained in a November 2017 Chalmers University Press Release that “For us it’s important to study not just what red giants look like, but how they change and how they seed the Galaxy with the elements that are the ingredients of life. Using the antennas of ALMA in their highest resolution configuration we can now make the most detailed observations ever of these cool and exciting stars.”

Stars that are like our Sun evolve over long timescales of many billions of years. Our middle aged Sun still has about 5 billion years to go before it meets its doom. However, when a star that is like our Sun reaches old age, these stellar senior citizens puff up and grow larger, cooler, and more vulnerable to losing their material as a result of powerful, rushing winds. Sunlike stars manufacture important elements–such as carbon, nitrogen, and oxygen–in their searing-hot, roiling nuclear-fusing furnaces. When these stars reach the red giant stage of their development, these heavy metals are released into interstellar space, available to be used by later generations of new baby stars.

ALMA’s images provide the best view yet of the surface of a red giant star that sports a mass that is similar to our own Sun. Earlier revealing images have shown details of considerably more massive red supergiant stars such as Betelgeuse and Antares.

Red giant stars show colors that range from yellow-orange to red, and the most abundant red giants are stars on the red giant branch (RGB) of the Hertzsprung-Russell Diagram. These red giants are still able to fuse hydrogen into helium in a shell that is encircling an inert core of helium. These crimson giants have radii tens to hundreds of times greater than that of our Sun, but their outer gaseous envelope has a much cooler temperature, providing them with a reddish-orange color. Even though red giants have a lower energy density in their envelopes than our Sun, they are many times more luminous because of their gigantic size. Indeed, RGB branch stars sport impressive luminosities that can be up to almost three thousand times that of our Star.

Red giants evolved from main-sequence stars–like our own Sun–that sport masses from about 0.3 solar-masses to around 8 solar-masses. When a star is born from a collapsing cold, dark, giant molecular cloud, lurking with great loveliness, in the space between stars, it is primarily composed of only hydrogen and helium–with relatively scanty quantities of metals. These elements are usually mixed up and scattered throughout the entire star. A newborn star reaches the main-sequence of its stellar “life” when its core skyrockets to a temperature high enough for it to start fusing hydrogen (a few million Kelvin). When this temperature is reached, the young star establishes hydrostatic equilibrium, whereby gravity seeks to pull all of its stellar material inward, while–at the same time–radiation pressure tries to push everything outward. This necessary and delicate balance between the two eternally battling forces causes the star to stay bouncy and fluffy during its entire time on the main-sequence. Over its entire “life” on the main-sequence, the star gradually fuses the hydrogen in its core into helium. However, the aging star finally is forced to face its inevitable doom after it has managed to fuse nearly all of its supply of hydrogen fuel in its seething-hot core. Relatively small stars, like our Sun, stay on the hydrogen-burning main-sequence for a about 10 billion years before they perish. Alas, more massive stars “live” fast and pay for it by “dying” young. These massive stars burn their supply of hydrogen fuel much faster than their smaller stellar kin, and therefore have shorter “lifetimes”. Red dwarf stars, the smallest true stars in the Universe, can live for trillions of years. Because our Universe is “only” about 14 billion years old, there are no red dwarf relics inhabiting our Cosmos–at least, not yet.

When a main-sequence star has finally managed to burn the hydrogen fuel in its core, nuclear reactions can no longer continue. At this point, the core starts to shrivel due to the pull of its own gravity. This contraction brings additional hydrogen into a zone where the temperature and pressure are high enough to cause nuclear fusion to resume in a shell that is surrounding the core. The outer gaseous layers of the star then start to expand greatly–and this triggers the red giant stage of the former Sun-like star’s existence. As the star continues to expand, the energy that is manufactured as a result of the burning shell is spread over a much greater surface area. This results in a lower surface temperature, as well as a shift in the star’s visible light output towards the red–thus becoming a red giant.

Our Star’s Life-Cycle

Our Sun was born a member of a dense open cluster along with thousands of other sparkling sibling stars. Many astronomers think that our young Sun was hurled out of its birth cluster as a result of gravitational interactions with sibling stars, or that it simply ran away from home about 4.5 billion years ago. Our Star’s long-lost siblings have also wandered off to more remote regions of our Milky Way Galaxy–and there could very well be as many as 3,500 of these stellar vagabonds existing in the space between stars.

Like its sibling stars, our Sun was born in a frigidly cold and extremely dense blob, tucked within the swirling, undulating folds of one of the many giant, dark and frigid molecular clouds that can be found scattered throughout our entire Galaxy. The dense blob eventually collapsed under the pull of its own gravity, giving rise to a new star. In the hidden folds of these vast and beautiful clouds, composed of dust and gas, fragile tendrils of material gradually merge and then clump together and grow for hundreds of thousands of years. Then, squeezed together tightly by the crushing pull of gravity, hydrogen atoms within this clump suddenly fuse. This lights the baby star’s stellar fires that will flame for as long as the new star lives, because this is how a star is born. Although it may seem counter-intuitive, things have to get very cold in order for a searing-hot and fiery baby star to be born.

All of our Milky Way’s billions of stars were born this way–as the result of the collapse of a frigid pocket within a cold molecular cloud composed mostly of hydrogen gas, but also containing a smaller amount of dust. These huge star-birthing dark clouds have a tendency to combine together, but stars of similar chemistry commonly wind up tucked within the same clouds at about the same time.

As stars go, our relatively small Sun does not stand out in the crowd. There are eight known major planets, moons, and an assortment of smaller objects in the family of our Star, which is located in the distant suburbs of an ordinary–but, nevertheless, majestic–large, and very ancient, barred spiral Galaxy, in one of its pinwheel-like arms. If it were possible to trace the history of atoms found on our own planet today all the way back about 7 billion years, we would probably find them scattered widely throughout our Milky Way. Some of these scattered atoms are now located in a single strand of your genetic matrial (DNA), even though long ago they were forming deep within ancient, alien stars dwelling in our then-young Galaxy.

Stars are not eternal. When our Sun, and similar stars, have finally burned up their necessary supply of hydrogen fuel, their looks change. At this point, the star is elderly. In the hot heart of an elderly Sun-like star, there lurks a core of helium. The helium core of our Sun will be encased within a shell in which hydrogen is still being converted into helium. The shell will eventually expand outward, as our Star’s hot core grows larger, as it grows older. The helium core itself will shrivel under its own relentless weight, and it will grow ever hotter and hotter until it becomes broiling enough at the center for yet another stage of nuclear burning to begin. At this new stage, the helium will be fused to manufacture the heavier atomic element, carbon. About 5 billion years from now, our Star will have only an extremely hot and small core that will be emitting more energy than our still-“living” Sun is at present. The outer gaseous layers of our Sun will swell up to monstrous proportions, and it will bear little resemblance to the Sun that we see today. Our Star will have experienced a sea-change into a bloated red giant. Our swollen, hot Sun at this stage will swallow Mercury first, and then proceed to devour Venus. Our Earth may be next, as our Sun proceeds to consume its inner planets. The temperature at the surface of this bloated sphere of crimson gas will be significantly cooler than that of our Sun’s surface today. This explains its new–and comparatively cool–red hue. Nevertheless, our Star will still be hot enough to convert the icy inhabitants of the remote Kuiper Belt, such as Pluto and its large moon, Charon, into tropic paradises–at least for a while. Our Sun is doomed, and the core of our dying Star will continue to shrivel. Because it can no longer manufacture radiation by way of the process of nuclear fusion, all further evolution will be governed by gravity alone. In the end, our Sun will throw its outer gaseous layers into interstellar space–while its core remains intact. All of our Star’s material will ultimately collapse into this small relic core that is only approximately the size of our Earth. Our Sun will now become a type of stellar ghost called a white dwarf. In our Star’s afterlife, its remaining white dwarf ghost will be encircled by a beautiful expanding shell of gas called a planetary nebula. These lovely “butterflies of the Cosmos” got this designation because early astronomers thought that they resembled the ice-giant planets Uranus and Neptune. A white dwarf is a dense object that radiates away the energy derived from its collapse, and it is usually composed of carbon and oxygen nuclei swimming in a dismal sea of degenerate electrons. If any additional mass is somehow contributed to this tiny body, it will only result in a smaller white dwarf. This is because additional mass will cause the white dwarf to shrink even further, even as its central density becomes larger. Our now-dead Sun’s radius will diminish to a mere few thousand kilometers. A white dwarf is doomed to grow progressively cooler over time.

The inevitable end will come when our Sun becomes an object called a black dwarf. Black dwarf stars are hypothetical objects. This is because it is generally thought that none, as yet, inhabit our Universe. This is because it takes hundreds of billions of years for a white dwarf to cool down to the black dwarf phase–and our Universe is less than 14 billion years old. The cooling white dwarf Sun will first radiate yellow light and then red light, drawing from what is left of its reservoir of thermal energy. Its atomic nuclei will be crushed together as tightly as physically possible. At this point, no further collapse can occur. Our Sun–and stars like it–will cool down, becoming precisely the same temperature as the extremely cold interstellar environment. A black dwarf radiates no light at all. In this final, devastating phase of stellar evolution, as a carbon-oxygen-rich black dwarf, our Sun will roam the Milky Way. Eventually, during this long journey, it may manage to meet up with another molecular cloud–just like the one from which it, and its sparkling sibling stars, had been born from so long ago. If this happens, the Sun will again become a part of the process that will give birth to a brand new baby star, with all of its enchanting and beautiful promise of wonders to be.

Sneak Preview

The new observations of W Hydrae surprised the Chalmers University astronomers. This is because of the appearance of an unexpectedly compact and bright spot. This bright spot indicates that the star has hot gas in a layer above the star’s surface: a chromosphere.

Measurements of W Hydrae’s bright spot indicate that there are extremely strong shock waves within the star’s atmosphere that attain higher temperatures than those predicted by current theoretical models for AGB stars, explained Dr. Theo Khouri to the press. Dr. Khouri is an astronomer at Chalmers University and a member of the team.

However, there is an alternative possibility that is at least as surprising. This possibility suggests that the star was in the process of experiencing a giant flare when the observations were being conducted.

The astronomers are currently carrying out additional observations, with ALMA and other instruments, in their quest to understand W Hydrae’s surprising atmosphere. Even though observations with ALMA’s highest resolution configuration are challenging, they are also rewarding. As team member Dr. Elvire De Beck, also an astronomer at Chalmers, told the press:

“It is humbling to look at our image of W Hydrae and see its size compared to the orbit of the Earth. We are born from material created in stars like this, so for us it is exciting to have the challenge of understanding something which tells us both about our origins and our future.”

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. 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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