46 Fun Facts About Red Giants

Which of these nearby stars is too small ever to swell into a red giant?

Proxima is the Sun's nearest neighbour among the stars, yet it is so small and dim that it is too faint to see with the naked eye.

Why do red giants glow orange or red rather than white?

Their surfaces are fairly cool: the star gets brighter, but its size grows faster still, so each patch of its surface ends up cooler.

The brightest red giants in a galaxy all top out at about the same brightness, so astronomers use them to measure what?

The galaxy's distance: set against how bright those stars really are, their dimness gave an independent check on how fast the universe is expanding.

Which spectral classes do most red giants belong to?

K and M come from Annie Jump Cannon's O, B, A, F, G, K, M scheme, which was built on spectral lines but turned out to be sorting the stars by temperature.

Stars can fuse elements up to which metal, leaving red giants to build heavier ones by slowly capturing neutrons?

Iron: slow neutron capture in red giants builds elements as heavy as bismuth, but anything heavier needs the far faster neutron floods of violent cosmic events.

What is the sudden, runaway ignition of core helium fusion in a Sun-like red giant called?

Helium flash: for a few minutes the core pours out energy rivalling the whole Milky Way, yet almost none of it reaches the star's surface.

By what reaction do red-clump stars fuse helium into carbon?

Triple-alpha makes carbon efficiently thanks to a special energy level in carbon, which Fred Hoyle insisted must exist before a lab looked and found it.

Which element, with no stable isotopes, turned up in red giants' light in 1952, helping prove stars forge heavy elements?

Technetium halves in about 4 million years even in its longest-lived form, far too fast to be left over from the stars' birth, so the stars must have made it.

NASA's Kepler telescope found that deep inside red giants, the core spins how compared with the surface?

Much faster: Kepler's starquake readings confirmed theory, though the cores spin far slower than models had predicted.

How does a low- or intermediate-mass star's red-giant phase usually end?

Planetary nebula, then white dwarf: despite the name, the glowing shell has nothing to do with planets; it is gas the star cast off, lit up by the hot core left behind.

After a Sun-like red giant's core ignites helium, what happens to the star's size?

It shrinks sharply: the ignition's energy goes into expanding the dense core, and as that core cools, the star's bloated surface contracts.

Sun-like red giants can swell to about how many times the Sun's radius?

About 200 times: the Sun is expected to swell to a similar size, big enough to swallow the planets Mercury and Venus.

How does the surface of a red giant differ from the Sun's in its pattern of convection?

A few huge cells: the Sun's surface seethes with countless small ones, but a red giant's handful of vast cells makes its brightness wobble.

As a red giant's outer layers swell, what happens to its core?

It shrinks and heats up: with no fusion of its own the core squeezes inward, while fusion in a shell around it drives the outer layers outward.

On which classic astronomers' chart do red giants occupy the upper-right region?

Hertzsprung-Russell: plot a star cluster on it, and where its stars peel off the main sequence reveals how old the cluster is.

In Greek myth, the giant star Capella represented the goat that nursed which god?

Zeus: in one version the goat was Amalthea, and a horn Zeus broke off her became the cornucopia, the horn of plenty.

Long before the Sun becomes a red giant, what should it do to Earth?

Boil away its oceans: the Sun has been brightening slowly for eons, so surface life on Earth should end long before the Sun becomes a red giant.

How do very-high-mass stars typically end their lives instead of becoming red giants?

Type II supernovae: rather than red giants, such stars swell into red supergiants, whose cores eventually collapse and blow the star apart.

In roughly how many years will the Sun leave the main sequence and begin to become a red giant?

About 5 billion: that is when the hydrogen in its core should run out, ending the steady fusion that has kept it stable.

As the red-giant Sun sheds mass, Earth's orbit should widen, so what could still pull Earth in?

Tidal forces: the bloated Sun's tides, helped by drag from its thin outer gas, could sap Earth's orbit until the planet spirals in.

Why can red giants shed so much of their mass into space?

Their surface gravity is weak: their swollen outer layers are held so loosely that a Sun-like star should lose about a third of its mass.

Which of Saturn's moons could become habitable for a few hundred million years once the Sun is a red giant?

Titan: as the Sun's ultraviolet light fades, the haze in its thick atmosphere should thin, letting the surface warm enough for liquid water.

By one hypothesis, why are giant planets around red giants often unusually massive?

They soak up their star's gas: as the giant swells out toward a planet's orbit, gas can spill across onto the planet and bulk it up.

Following the Big Dipper's handle past Arcturus leads to which bright star?

Spica: it is really two stars orbiting so closely that their gravity pulls each other into egg shapes.

At the 1933 Chicago World's Fair, what was the red giant Arcturus used to do?

Switch on the lights: its light, focused onto photocells, was picked because it was thought to have left the star during Chicago's 1893 fair.

Aldebaran's Arabic name is usually translated 'the follower', as it seems to trail which star cluster?

The Pleiades: nicknamed the Seven Sisters, the cluster actually holds more than a thousand stars, though only a handful stand out to the eye.

Which probe is drifting toward the red giant Aldebaran, due for its closest approach in about 2 million years?

Pioneer 10: in case intelligent life ever intercepts it, the probe carries a plaque showing a man, a woman and where the Sun lies.

Which red giant in Gemini hosts an exoplanet named Thestias by the IAU in 2015?

Pollux: despite its Beta Geminorum label, it shines slightly brighter than its twin Castor, making it Gemini's brightest star.

Which red giant marks the top of the Southern Cross, about 88 light-years away?

Gacrux: the ancient Greeks could see it, but the slow wobble of Earth's axis has since sunk the whole cross below most of Europe's horizon.

Which red giant in Cetus was recorded as variable in 1596?

Mira: David Fabricius took it for a nova when it faded from view, but then saw it again years later.

What did NASA's GALEX telescope reveal around a red giant in Cetus?

A comet-like tail: the star is racing through space, so the gas it keeps losing trails out behind it.

When Betelgeuse finally dies, what could its light do as seen from Earth?

Be visible even by day: it could stay that bright for months, yet at hundreds of light-years it is too far away to harm life on Earth.

In 1920, Orion's red supergiant became the first star besides the Sun to have what measured?

Its apparent diameter is exceeded only by R Doradus and the Sun.

What does the runaway red supergiant in Orion create as it ploughs through interstellar gas at 30 km/s?

A bow shock: the star's powerful winds slam into the gas it is plowing through, heating it enough to glow in infrared light.

The red supergiant Antares is known as the heart of which constellation?

Scorpius: in one Greek myth a scorpion killed Orion the hunter, and the two were set on opposite sides of the sky.

Which term is sometimes used for the most extended, unstable stars like VY Canis Majoris?

Red hypergiant: 19th-century observers took VY Canis Majoris for a multiple star, but its 'companions' turned out to be bright zones in the material around it.

What is the expanding stage between the main sequence and the red-giant branch called?

Subgiant: in more massive stars this stage passes so quickly that plots of star clusters show a gap, with few stars caught in it.

In old, metal-poor star clusters, which group do red giants join once they begin fusing helium in their cores?

Horizontal-branch: how hot each star gets there depends on how much gas it lost as a red giant, with those that lost the most ending up the bluest.

Which giant star in the Big Dipper's bowl is one of the two 'Pointers' to the North Star?

Dubhe: Earth's axis points almost at the North Star, so as the sky wheels around it all night, the Pointers keep aiming at it however the Dipper is turned.

Illustrations depict red giants with crisp outlines, but what does a real red giant's outer boundary look like?

A gradual fade: the outer gas is so thin that the star has no well-defined surface, and its light dims toward the limb.

The name Arcturus is usually traced to Greek words meaning 'guardian' of which animal?

The bear: probably because the star seems to follow the Great Bear, Ursa Major, across the sky.

The red giant Arcturus is the brightest star in which constellation?

Boötes: Arcturus travels with a stream of old stars that some astronomers think came from a small galaxy the Milky Way swallowed.

Aldebaran looks like the brightest member of the Hyades star cluster, but where does it really lie?

Far in front of it: it just happens to sit along our line of sight, less than half as far away as the cluster and unrelated to it.

In 1718, which astronomer concluded that Aldebaran had shifted position since ancient times?

Edmond Halley: these were supposedly 'fixed stars', yet Sirius and Arcturus had also drifted from where ancient Greek astronomers charted them.

The name Betelgeuse is most likely derived from an Arabic phrase meaning which part of Orion?

The hand: the old gloss 'armpit' came from a medieval misreading of one Arabic letter in Yad al-Jawza, 'the hand of Orion'.

Which astronomer noted Betelgeuse's variable brightness in 1836?

John Herschel: in October 1837 he saw Betelgeuse outshine Rigel, normally the brightest star in Orion.