The Hidden Hearts of Red Giant Stars

When small stars like our Sun grow old, they experience a dramatic sea-change into swollen, crimson giant stars that are appropriately named red giants. These bloated stars are in a late phase of stellar evolution, and they are very luminous. They are also of low to intermediate mass that amounts to approximately 0.3 to 8 times that of our Sun. Because the outer atmosphere of a red giant is swollen and tenuous, its radius becomes large, while its surface temperature reaches about 8.500 degrees Fahrenheit, or even lower–which is relatively cool for a star. By comparison, our Sun has a surface temperature of approximately 10,000 degrees Fahrenheit. Recently, a strange collection of several red giant stars, that were discovered in 2015, became an intriguing mystery for astronomers to solve. This is because they seemed to be both young and old at the same time. In June 2019, a team of astronomers were able to prove that these tricky giant stars are not as young as they look. They are, indeed, elderly stars that benefited from stellar mergers that made them appear superficially young.

Scientists from the Max Planck Institute for Solar System Research (Germany), Aarhus University (Denmark), and Ohio State University (U.S.) have now announced that they have solved this intriguing contradiction. For the first time, they studied the abundances of carbon, nitrogen, and oxygen that whirled up from the nuclei of these stars to their surfaces. This enabled the astronomers to take an indirect peek at processes that were occurring deep within the hidden hearts of these stars. From their findings, the team concluded that these stars must have merged with others during an advanced stage of stellar evolution. Therefore, in these particular cases, mass is not a reliable criterion for age determination. The stars are indeed old.

Over the passage of millions or billions of years, all stars pass through different stages of stellar evolution–and all of these stages cause the star to differ dramatically in appearance. Nevertheless, stars do not readily reveal their ages–at least not at first glance. The duration of each phase differs greatly from one star to another. However, when astronomers peer deeper and deeper into a star’s deceitful heart, they can reconstruct the secretive star’s life story. There are several methods that make it possible now for astronomers to reliably determine the age of a star.

For small stars, like our Sun, the life cycle progresses from a hydrogen-burning main-sequence star, to a bloated red giant, to a dense little stellar corpse called a white dwarf. Our own Sun is a middle-aged star. It has “lived” for about 4.56 billion years, and it has approximately 5 billion years more to go before it perishes.

The Future Of Our Star

As stars go, our Sun isn’t particularly special. There are eight major planets and an assortment of other objects in orbit around our Star, which is located in the outer limits of our large pin-wheel shaped Milky Way Galaxy, in one of its twirling spiral arms.

A star of our Sun’s relatively small mass “lives” for approximately 10 billion years, but since our Star is only half way there, it is still enjoying a brilliant middle age–and it remains bouncy enough to go on happily burning hydrogen in its heart by way of nuclear fusion. Nuclear fusion is the process that enables a star to form increasingly heavier and heavier atomic elements out of lighter ones (stellar nucleosynthesis). When our Sun, and other stars of similar mass, have managed to burn up their necessary supply of hydrogen fuel for energy, they start to show their age. They are now old stars. In the heart of an old sunlike star, 바카라사이트 there is a hidden heart of helium, surrounded by a shell in which hydrogen is still being fused into helium. As the shell begins to expand outward, the core grows larger as the star grows older. The helium core itself eventually shrivels under the pull of its own relentless gravity, and ultimately this hot stellar heart becomes so hot that it triggers a new stage of nuclear fusion. At this point, the helium is burned to form the heavier atomic element, carbon. About five billion years from now, our Star will contain a very hot small heart that will emit more energy than our Sun does now. The outer gaseous layers of our Sun will have bloated up to ghastly proportions, and it will no longer be the small sparkling star that we are familiar with. Alas, our Star will now be a crimson swollen red giant. As such, it will proceed to engulf Mercury, then Venus, and then (probably) Earth, in its merciless flames. The temperature at our future Star’s surface will be considerably cooler than it is now, which accounts for its red hue. However, even if our Sun has cooled off considerably by star-standards, it will still be sufficiently hot and glaring enough to convert the currently icy, frigid inhabitants of the remote Kuiper Belt (such as Pluto) into pleasantly toasty havens of refuge for any life that may have managed to survive on the broiled wreck that was once our Earth. The heart of our dying Star will continue to shrivel, and because it will no longer have the ability to produce radiation by way of the process of nuclear fusion, all further evolution will result from the relentless influence of its own gravity. In the end, our Sun will hurl its outer gaseous layers into space, and the remnant core of our dying Star will be all that is left behind to tell the sad story of how once our Sun existed.

The relic core left behind by our doomed Sun will remain intact, and all of its matter will finally collapse into this small remnant body that will only be about the same size as Earth. In this way, our Sun will undergo a metamorphosis into a type of stellar ghost called a white dwarf star. The new white dwarf will be encircled by a beautiful multicolored shimmering cloud of gas termed a planetary nebula. These lovely objects–sometimes referred to as the “butterflies of the Universe”–got their name because early astronomers believed that they resembled the planets Uranus and Neptune.

A white dwarf star is an extremely dense body that radiates away the energy of its collapse, and is primarily made up of carbon and oxygen nuclei swimming in a sea of degenerate electrons. The equation of state for degenerate matter is “soft”. This means that any additional contribution of mass will ultimately cause the white dwarf to grow increasingly smaller–even as its central density grows ever larger. The dead star’s radius will ultimately shrink to a mere few thousand kilometers. Our Sun, and other sunlike stars, are doomed to grow cooler and cooler with the passage of time.


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