Red Giants: 7 hot-to-handle facts about stars in the dying stages of their life

• Red giants are stars in the dying stages of their life.
• Their size and ultimate fate depend on their size and mass in "life."
• Red giants are one of the main processes for making heavier elements in the universe.

"The cosmos is within us. We are made of star stuff. We are a way for the universe to know itself," as the great Carl Sagan once said. Poignant and completely true.

But what star stuff? The answer? Red giants.

These enormous, bright stars can be seen as the heavier element factories of the universe. Without them, it can be argued Earth and all life upon it could never exist.

Even our own Sun is the child of one of these ex-stars. So, what exactly are they? Let's find out.

What is a red giant?

A red giant is a brilliant giant star with a low or intermediate mass (roughly 0.3 to 8 solar masses) that is in the late stage of a stellar lifecycle. The radius of these things is vast, and the surface temperature is generally between roughly 4,000 (2,204 degrees Celsius) and 5,800 degrees Fahrenheit (3,204 degrees Celsius), which is relatively cool because their energy is spread across a very large area.

The red giant can range in color from yellow-white to reddish-orange and includes stars of spectral types K and M, occasionally G, as well as Class S stars and the majority of carbon stars.

The means by which red giants produce energy depends on where in the cycle they are but include: -

• The majority of red giants are on the red-giant branch (RGB). These stars still fuse hydrogen into helium in a shell around an inert helium core.

• Red-clump stars, located in a clustering of red giants, that use the triple-alpha mechanism to fuse helium into carbon in their cores are located in the cool part of the horizontal branch - a later stage of stellar evolution.

• AGB stars, or asymptotic-giant-branch stars, are in a stage of stellar evolution undertaken by low to intermediate-mass stars. They have a degenerate carbon-oxygen core, a helium-burning shell, and a hydrogen-burning shell immediately beyond that.

Because they are luminous and very abundant, red giants make up a large number of well-known bright stars. For example, Arcturus, a K0 RGB star, is 36 light-years away, whereas Gamma Crucis (also called Gacrux) is an M-class giant about 88 light-years away.

In something like five billion years, our own Sun will turn into a red giant, expand, and engulf the inner planets, possibly even Earth.

How do red giants form?

Red giant stars tend to form from main-sequence stars that have masses between 0.3 and 8 times that of our Sun. In these stars, hydrogen and helium predominate when a star first develops from a collapsing molecular cloud in the interstellar medium, with trace levels of "metals" present (in stellar structure, this simply refers to any element that is not hydrogen or helium, i.e., an atomic number greater than two).

All of these components are generally evenly distributed across the star. When the core reaches a temperature of a few million kelvin, temperatures are then high enough to start fusing hydrogen and achieve hydrostatic equilibrium (the balance between gravity and outward pressure). At this point, the star enters the main sequence. During the main sequence, stability is maintained by a balance between the stars' own gravity and the outwards pressure from the thermonuclear fusion processes taking place at the core.

The star slowly fuses the hydrogen in its core during the course of its main sequence life, and when almost all of the hydrogen has been fused, that state of equilibrium is lost, and the core begins to collapse. The time this takes varies, but the main-sequence lifespan of our Sun will be roughly 10 billion years. The lifespan of more massive stars is shorter than that of less massive stars because they burn more quickly.

Nuclear processes can no longer take place in the star's core as it runs out of hydrogen fuel, which causes the core to start contracting under the star's gravity.

As the core collapses, the shell of plasma surrounding the core becomes hot enough to begin fusing hydrogen. The extra heat from this causes the outer layers of the star to expand greatly.

Burning hydrogen in the shell results in what has been called the mirror principle; when the core within the shell contracts, the layers of the star outside the shell must expand. Because the star's surface has expanded, the energy at the surface is dissipated, and the star's surface cools.

The stage where the star is in the process of cooling and expanding is often called the subgiant stage. Once the star cools sufficiently it stops expanding, its luminosity begins to rise, and it starts to ascend the red-giant branch of the Hertzsprung-Russell (H-R) diagram.

Why do stars become red giants?

The star's mass determines the course of its evolution as it progresses down the red-giant branch. When the core of the Sun and other stars less massive than about two times our Sun becomes sufficiently dense, electron degeneracy pressure will stop them from further collapsing. Once the core has degenerated, it will continue to heat up until it reaches a temperature of about 10⁸ K, which is hot enough to start the triple-alpha process of fusing helium to carbon, so-called because it involves three helium-4 isotopes or alpha particles.

The entire degenerate core will start helium fusion almost simultaneously when it reaches this temperature, which is known as a "helium flash." Helium fusion will start much more easily and without producing a helium flash because, in higher massive stars, the collapsing core will reach 10⁸ K before it is dense enough to degenerate.

Because these stars are located on a roughly horizontal line in the H-R diagram of many star clusters, the core helium-fusing phase of their lives is known as the "horizontal branch" in metal-poor stars. Alternatively, on the so-called red clump in the H-R diagram are metal-rich helium-fusing stars.

When the star's helium runs out and it collapses again, helium in a shell starts to fuse in an analogous process. Hydrogen could start fusing in a shell nearby the helium shell that is burning at the same moment.

As a result, the star enters a second red-giant phase, the asymptotic giant branch. This process of fusing helium tends to result in the formation of a carbon-oxygen core. However, fusion in a star's degenerate carbon-oxygen core will never begin in a star with a mass of fewer than eight times that of our Sun.

Instead, the star will expel its outer layers near the conclusion of the asymptotic-giant-branch phase, leaving the star's core exposed and generating a planetary nebula before eventually becoming a white dwarf. The red-giant stage of the star's evolution is finally completed by the ejection of the outer mass and the formation of a planetary nebula.

For a solar mass star, the red-giant phase normally lasts just around a billion years overall, with practically all of that time being spent on the red-giant branch. The phases of the horizontal branch and asymptotic huge branch move ten times more quickly.

The star is large enough to develop into a red giant at a mass of between 0.2 and 0.5 M, but not enough to start the fusion of helium.

These "intermediate" stars gradually cool and brighten, but they never reach the red-giant branch's tip or the helium core flash. The red-giant branch puffs off its outer layers when its ascension is complete, turning into a white dwarf after resembling a post-asymptotic giant branch star.

Not all stars become red giants

Fully convective very-low-mass stars can continue fusing hydrogen into helium for up to a trillion years until just a small portion of the star is composed of hydrogen. Similar to more massive main-sequence stars, temperature and luminosity gradually rise over this period. Still, because of the length of time involved, the temperature finally rises by roughly 50% and the luminosity by about ten times.

These kinds of stars eventually reach a level of helium where it stops being convective, and the remaining hydrogen locked inside the core is oxidized in just a few billion years. Depending on their mass, these stars can grow hotter than the Sun and tens of times more luminous than when they originated, though still not as bright as the Sun, for a period of time during hydrogen shell burning, depending on mass.

They continue to burn hydrogen even after a few more billion years, although they begin to dim and cool and form a white dwarf.

Very high-mass stars evolve into red supergiants at the end of an evolutionary path that takes them back and forth horizontally over the H-R diagram. Eventually, much of the time, these will explode as type II supernovae.

Is a red giant hotter than the Sun?

In short, yes and no. It all depends on where you measure the temperature.

Although our Sun is extremely hot, it is not the universe's hottest star. Our Sun will go through various phases of its "life," just like all other stars. In each stage, the star's physical characteristics, including temperature and nuclear processes, vary.

The Sun is currently in the main sequence stage when its temperature is roughly 10 million degrees Fahrenheit (roughly 5,500 degrees Celsius). As we've previously discussed, stars with the right mass and size will then develop into a red giant later in their existence.

Although a red giant's surface is not particularly hot, between 4,000 (2,204 degrees Celsius) and 5,800 degrees Fahrenheit (3,204 degrees Celsius), its interior could be vastly hotter than the Sun. While many other stars in the universe are blazing red giants, the Sun has not yet reached its red giant stage and most likely won't for billions of years.

So, depending on where you measure the temperature of a red giant, it can either be colder than the Sun or much, much hotter.

The sizes of stars vary widely throughout the cosmos. Some stars have a mass several times greater than the Sun.

Supernovae can form in stars that are several times as massive as the Sun too. Supernovae are stars that eventually explode after experiencing extreme contraction. Prior to exploding, they may reach temperatures of 3 billion degrees Celsius, which is something like 300 times hotter than the Sun.

What are some too-hot-to-handle facts about red giants?

Congratulations on making it this far. We hope, by now, you have a good understanding of what these incredible feats of nature are all about.

But, if you want to know some more about them, here is handpicked selection of great facts about them.

1. Red giants should be able to support nearby life

New research indicates that a Sun-like (one solar mass) star may be able to sustain a habitable zone for several billion years at a distance of about two astronomical units, out to about nine astronomical units, while it is evolving into a red giant. This is despite the fact that a one solar mass star may destroy a planetary system during its evolution into a red giant.

In addition, such a hypothetical one solar mass star might sustain a habitable zone at between 7 and 22 astronomical units for an additional one billion years after it has fully evolved into a red giant.

2. Our parent Sun may one day kill us all

Our own Sun will enter the red giant phase in about five billion years. Its outer layers will swallow Mercury and Venus and possibly Earth as it expands. Whether or not our planet will be swallowed by the red giant sun or whether it will orbit too closely is still up for debate among scientists.

In any case, Earth's current state of life will end.

In fact, it is conceivable that surface life will disappear from our planet long before the Sun evolves into a red giant. As main-sequence stars of its mass do, our star has been growing warmer over the ages, and in a few hundred million years, it will be hot enough to begin evaporating the seas. Therefore, there may not be much left on Earth for our bloated, red-giant sun to destroy.

According to astronomer Don Kurtz of the University of Lancashire, "the Earth will eventually perish as the Sun boils the oceans, but the hot rock will endure."

However, other planets may be luckier. The "habitable zone" of a star's home system, or the range of orbital distances where liquid water can exist on a planet's surface, is altered as a star transforms into a red giant. A star's red giant phase lasts for around a billion years, which may be enough time for life to emerge on distant planets and moons that may finally receive some warmth.

Exoplanet scientist Ramses M. Ramirez, a researcher at Cornell University's Carl Sagan Institute, said in a statement: "When a star ages and brightens, the habitable zone goes outward, and you're essentially providing a second wind to a planetary system."

"At the moment, objects in these outer regions, such as Europa and Enceladus, moons orbiting Jupiter and Saturn, are locked within our own solar system," he added.

However, the window of opportunity will only be available for a short time. The brightness of life will fade as the Sun and other smaller stars contract back to white dwarfs. Additionally, supernovae from bigger stars may cause additional problems for habitability.

3. Red "supergiant" stars barely rotate

All red supergiant stars that have been detected so far rotate either slowly or extremely slowly, and in certain circumstances, it can be challenging to tell if the star is spinning at all. Causes for this include loss of mass, which effectively slows a star's rotation.

A few stars, like Betelgeuse, which rotates at just approximately 5 km/sec, may have binary interactions with other stars that cause them to revolve. Red giants still revolve, and their cores rotate at rates that are much faster than the star's periphery.

4. There is one red giant that is absolutely huge

Believe it or not, one of the biggest red giants ever discovered is VY Canis Majoris. This red giant is absolutely huge and is roughly 1,400 times bigger than our Sun.

Around 4,900 light-years separate VY Canis Majoris from the Sun. It is also one of the largest stars in the Milky Way galaxy that has ever been seen or detected.

It is thought that VY Canis Majoris has lost more than half of its mass, which was previously estimated to be roughly 17 solar masses. The radius of VY Canis Majoris has been calculated to be around 1,420 times larger than the radius of the Sun.

The Sun's diameter is 1,750 times smaller than VY Canis Majoris' diameter, which is roughly 1.5 billion miles (2.4 billion kilometers). If this hypergiant were to ever appear in the center of the Solar System, its surface would extend beyond the orbit of Jupiter, and some estimate it would extend up to the orbit of Saturn.

5. Some red giants are invisible in visible light

Special red giants, called asymptotic red giants, are a particular type that is so active and unstable that the rate at which they expel enormous amounts of their own matter quickly hides them from prying eyes.

The expelled material enshrouds them in a dense cocoon of dust and condensed stellar detritus. This effectively makes these stars invisible to the wavelengths of light that human vision can perceive.

This cocoon is not to be confused with a planetary nebula, however. While the primary causes of the enormous rate of mass loss in these stars are still not fully understood, it is believed that radiation pressure and their intense pulsations are the primary culprits.

6. From what we can tell, some red giants also have giant planets

Although huge planets are predicted to orbit massive stars, there is no correlation between the masses of the roughly 50 giant planets that have been identified so far orbiting red giant stars.

One explanation for this phenomenon is the "Wind Roche Lobe Overflow Mechanism," which states essentially that as red giants grow, their planets may accrete some star material as they enter its gravitational field.

7. Depending on the original star's size, it will end its days very differently

The final fate of a red giant depends entirely on the amount of mass it had during its main sequence. For average stars, like our Sun, it will end its days as a shadow of its former self as a white dwarf.

For larger, massive stars, following a dramatic supernova, the star can end its days as either a neutron star or, if massive enough, birth a black hole!

And that, red giant fans, is your lot for today.

These giant veteran stars are in the twilight years of their life. With their primary hydrogen fuel all but spent, they continue to fuse other elements until they inevitably either go out in a blaze of glory or shrivel away into relatively cold, dead bodies long past their prime.

But don't feel sad for these stars. It is a natural and necessary part of the life cycle of a star. Without them, most of the more complex (heavier) elements would not exist. Even our planet, and you, are the product of this never-ending cycle of birth and death.