Red giant stars

Red giant and supergiant stars are large stars with relatively cool surfaces that are in the last stages of their lives. Earth and the life on it are composed of chemical elements synthesized by nuclear fusion reactions occurring in such stars and ejected into space as the stars die.

Overview

Stars are not everlasting; they change through a series of stages analogous to the life cycle of a living organism. Stars are formed, or “born,” from clouds of gas and dust called nebulae. They generate energy for most of their lives by fusing lighter atomic nuclei into heavier atomic nuclei in their interiors. They stop generating energy or “die” when they exhaust their nuclear fuel. The red giant or red supergiant stage is a part of the stellar life cycle that a star goes through near the end of its energy-producing life.

Nebulae are large interstellar clouds of gas (mostly hydrogen, some helium, and traces of other elements) and dust grains containing enough matter to form hundreds to thousands of stars. If the density in some region of the nebula is great enough, the gravitational pull between the particles will make the surrounding part of the nebula—perhaps about ten trillion kilometers, or one light-year across—begin to contract. As the individual atoms are pulled inward, they gain speed, and their energy increases, heating the gas. As the temperature rises, the contracting material begins to shine as an embryonic star or protostar. The central part of the protostar gets hot enough that collisions between atoms occur with sufficient energy to strip away electrons from the atoms, and the gas (at least near the center) becomes ionized, consisting mostly of protons (bare hydrogen nuclei), much smaller amounts of other nuclei, and lots of free electrons.

When the core temperature of the protostar reaches a few million kelvins, the bare hydrogen nuclei (protons) move fast enough to overcome their electrical repulsion (known as the Coulomb barrier) and, via a mechanism known as quantum mechanical tunneling, get close enough to fuse together. This first nuclear fusion process, through a series of intermediate steps, fuses four hydrogen nuclei into one helium nucleus, releasing energy. The four hydrogen nuclei have slightly more total mass than the one helium nucleus produced, and this slight excess in mass is converted into energy according to Einstein’s famous equation E = mc², which states that energy, E, and mass, m, are related by a physical constant (the speed of light squared, or c²). The nuclear energy produced as a result of this fusion stops the gravitational contraction of the protostar and turns it into a stable main sequence star, like the Sun. The energy is radiated away from the star’s surface as light and other forms of electromagnetic radiation at the same rate it is being produced by hydrogen fusion in the star’s core.

Because a typical star is composed initially mostly of hydrogen, this main sequence stage of fusing hydrogen into helium in its core is the longest part of a star’s energy-producing life. As hydrogen in the star’s core is exhausted, the core, now mostly helium, begins to shrink again and grow hotter, as it did when it was a protostar. Hydrogen fusion is transferred to a hydrogen-rich shell surrounding the shrinking helium core. The star’s outer layers expand, and the expanding stellar surface cools and turns red in color. (It is important to note the opposite but simultaneous processes that occur in the two regions: the star’s core shrinks and heats up while its outer layers expand and cool off.) The star becomes a red giant or red supergiant, so called because of its large size and red color.

The large surface area with accompanying low surface gravity results in an extremely powerful stellar wind, which blows gas outward. The low surface temperature allows dust grains to form, which are blown outward by the intense flow of photons. Both processes contribute to a significant loss of mass from the outer layers of red giants and supergiants. Perhaps as much as 10^-4 solar masses are lost each year, forced outward at speeds of tens of kilometers per second.

Stars with about the mass of the Sun become red giants. The Sun is a star about midway through its main sequence stage. It formed about 4.5 billion years ago and probably has another three to six billion years before it exhausts the hydrogen in its core and expands to become a red giant. It will grow until its diameter increases to about one hundred times its size when the Sun will engulf the orbit of Mercury and perhaps Venus. Even if the Sun does not expand enough to swallow Earth, the Sun will be about one thousand times brighter than it is now, and Earth’s surface temperature will climb to between 1,000 and 2,000 kelvins. Earth’s atmosphere will escape into space, the oceans will boil away, and the surface will become a sea of hot, at least partly molten, rock.

The core of a red giant continues to shrink and heat until it reaches a temperature of about 100 million kelvins. At this temperature, the helium nuclei composing the core begin to fuse into carbon nuclei; three helium nuclei fuse into one carbon nucleus. As with hydrogen fusion, the total input mass of the three helium nuclei is slightly greater than the output mass of the carbon nucleus, and the mass difference is converted into energy. The high density of the contracting core has made the free electrons there degenerate, meaning they no longer behave as an ideal gas. As a result, helium fusion ignites explosively in a runaway process called the helium flash. However, this explosion in the core is not seen externally because the outer layers act like a blanket.

The energy released by the helium flash expands the core and lowers its temperature enough so helium fusion continues there at a more sedate rate. The outer layers shrink some and grow hotter, changing from red to orange or yellow. Although still a giant star, it is not as large as it was as a red giant. This is another period of stability called the horizontal branch stage. It is somewhat like the main sequence stage, except now the star has two nuclear energy sources—helium fusion in its core and hydrogen fusion in a shell surrounding the core—and this stage is of much shorter duration than the main sequence stage, lasting only about one-tenth as long.

When the core has exhausted all its helium, it once again begins to shrink and heat up, trying to tap a new nuclear fuel. Helium fusion is transferred to a helium-rich shell surrounding the shrinking core, and hydrogen fusion continues in a hydrogen-rich shell still farther out. The outer layers again expand to accommodate the increased energy flow, and the stellar surface cools and once again turns red in color. The star becomes a red giant a second time, even brighter than before.

Stars like the Sun are not massive enough for their cores to gravitationally shrink enough to grow hot enough to initiate any more fusion reactions. A strong stellar wind and thermal pulsations puff off the outer layers as expanding bubbles of gas called planetary nebulae. (Planetary nebulae have nothing to do with planets. The name dates back to the 1700s, when, viewed through telescopes of that time, they looked round, like planets, and fuzzy, like nebulae.)

All that remains of the star itself is the exposed core, composed mostly of carbon nuclei and degenerate free electrons. This means the electrons are packed together as tightly as quantum mechanics allows. Consequently, the star cannot shrink further, so it cannot generate more energy by gravitational contraction to get hot enough to start new nuclear fusion reactions. The star shines only because it is very hot, but as it shines, it radiates its energy away and cools off. Such a star has about the mass of the Sun packed into a sphere about the size of Earth, giving it an average density of approximately one metric ton per cubic centimeter. Since it was formerly the core of a red giant, at first, it is very hot and shines with a white or bluish-white glow; then, it is called a white dwarf. As it cools and fades, eventually, it turns into a black dwarf.

When stars much more massive than the Sun exhaust the hydrogen in their cores, they become red supergiants. In stars exceeding at least eight times the Sun’s mass, the core gravitationally contracts sufficiently to get hot enough to undergo a whole series of nuclear fusion reactions, each one proceeding more rapidly, that synthesize progressively heavier nuclei up to the element iron. Such a massive star quickly (on an astronomical timescale) develops an “onion-skin” interior, with an iron core surrounded by layers (like an onion) of lighter nuclei. The outer part of the star, still hydrogen-rich, has expanded to become so large that the star is called a supergiant, with a diameter hundreds and maybe up to a thousand times the diameter of Earth's Sun.

Iron is the heaviest nucleus that can be formed in fusion reactions that release energy; to form heavier nuclei through fusion requires the input of energy. Since the iron core of a supergiant cannot tap any further fusion reactions to produce energy, it is unable to support itself against gravity, and it collapses. The outer layers collapse, too, and rebound off the core, sending shock waves through the star. The star explodes as a core-collapse (or Type II) supernova, becoming billions of times more luminous than the Sun. The stellar explosion is so violent that energy is available to synthesize the elements heavier than iron, but only in relatively small amounts. These heavy elements, along with those formed in the star’s interior before it exploded as a supernova, are dispersed into space, there to enrich the nebulae (clouds of gas) from which new stars and planetary systems will form. The atoms on Earth and in people's bodies that are heavier than hydrogen and helium were made in massive supergiant stars that exploded as Type II supernovae before this solar system formed. A collapsed, compact remnant of the star itself may survive the explosion as either a neutron star or a black hole, depending on whether its mass is less than or greater than about two to three times the Sun’s mass.

Knowledge Gained

A well-known example of a red supergiant is Betelgeuse (Alpha Orionis). Anyone familiar with the night sky probably can pick out this star in the constellation of Orion the hunter. It appears as a bright red star in Orion’s shoulder, one of the ten brightest-appearing stars in the night sky. Its red color illustrates that its surface is relatively cool, about 3,000 kelvins (only about half the Sun’s surface temperature). It is located about 130 parsecs, or 425 light-years, away from Earth. Its distance, together with its apparent brightness, can be used to calculate its real brightness; it is about 40,000 times more luminous than the Sun. (That is why it appears so bright in the night sky, even though it is so far away.) The angular diameter of Betelgeuse has been measured to be almost 0.05 arc second, using a technique called speckle interferometry. Its angular size combined with its distance can be used to calculate its actual size, which is about six hundred to seven hundred times larger than the Sun; if placed at the center of the solar system, it would be about twice as big as the orbit of Mars and extend out into the asteroid belt.

Betelgeuse’s large size makes its outer atmosphere unstable. Its brightness changes over timescales of weeks to years. The slower variations probably result from its outer layers expanding and shrinking, its size varying by up to 60 percent. The flickering observed over a few weeks is most likely caused by rising bubbles of very hot gases thousands of kilometers across. Poorly defined bright spots have been detected by speckle interferometry and by the Hubble Space Telescope; they may be these hypothesized hot bubbles or possibly giant storms like those that occur in active regions on our Sun. The star has a strong stellar wind and is surrounded by a shell of gas and dust that has been blown outward to a distance of several thousand astronomical units. (For comparison, Earth is one astronomical unit and Neptune is thirty astronomical units from the Sun.)

The future of Betelgeuse remained unclear. Its original mass may have been about twelve to seventeen times the Sun’s mass, but how much mass it has lost as a red supergiant is uncertain. It may be nearing the end of its life. If enough mass is expelled, Betelgeuse could become a white dwarf. If not, it will explode as a Type II supernova. Continued study of Betelgeuse could reveal more clues to the nature of this red supergiant and its future. The star became a focus of great attention again when efforts were made to comprehend an approximately 40 percent reduction in brightness over a period between 2019 and 2020. In 2022, the National Aeronautics and Space Administration (NASA) reported that a data analysis provided evidence of an eruption of part of Betelgeuse that occurred in 2019. According to the observing scientists, a surface mass ejection (SME) of a particularly significant size was produced, impacting the photosphere, following this eruption that the star then had to recover from.

Above and to the right of Orion is the constellation of Taurus the bull. The bright orange star Aldebaran (Alpha Tauri), at one end of the V-shaped pattern of stars that makes up the bull’s head, is an orange giant. (Orange giants are not quite as cool or as large as red giants, but Aldebaran is easy to spot, not far from Orion and Betelgeuse.) Aldebaran is about twenty parsecs, or sixty-five light-years, away. It is about 370 times more luminous than and about 60 times larger than the Sun; if placed at the center of the solar system, it would fill more than half of Mercury’s orbit around the Sun.

Further studies of both red giants and red supergiants were carried out into the 2020s, with information gleaned from telescopic research, including everything from cellular makeup to bright spot behavior. For example, in 2022, scientists made headway into insights about the true nature of the core of red giants.

Context

Without the formation of heavier chemical elements by nuclear fusion reactions in the interiors of massive red supergiants and the dispersal of these heavy elements into interstellar space when they explode as Type II supernovae, planets like Earth and life as humans know it could not exist. The first stars and any accompanying planets to form in the early days of the Milky Way galaxy would have been composed of not much but hydrogen and helium. Planets similar to Jupiter could have formed, but not rocky/metallic planets like Earth.

The nebula from which the Sun and solar system formed about 4.5 billion years ago was composed about 2 percent (by mass) of atoms heavier than helium, having been enriched in these heavier elements by earlier massive stars that had exploded as Type II supernovae. This provided the raw materials for rocky planets like Earth and for life itself. The process of enrichment of interstellar gas continued as massive stars quickly ran through their life cycles and exploded.

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