50 Fun Facts About Red Giants

Roughly what range of initial masses, in solar masses, produces a red giant?

Anything more massive becomes a supergiant instead, and the very smallest red dwarfs never puff up at all.

What is the surface temperature of a typical red giant?

The Sun's photosphere is nearly 6,000 K; a red giant's inflated envelope is cooler but so vast that the star is far more luminous.

A star becomes a red giant when it has exhausted which fuel in its core?

Fusion then continues in a shell around the inert core, and the outer layers balloon outward.

Which spectral classes do most red giants belong to?

Some are class G, and the carbon stars and class S stars also count; colours run from yellow-white to reddish-orange.

What are stars on the red-giant branch fusing, and where?

Red-clump stars have moved on to core helium burning; AGB stars burn helium and hydrogen in two shells.

What is the sudden ignition of the second-lightest element in the degenerate core of a Sun-like red giant called?

It happens at roughly 100 million kelvin; stars above about 2 solar masses start helium fusion smoothly and skip it.

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

Three helium nuclei (alpha particles) combine to make one carbon nucleus.

What is the second swollen phase, after core helium is exhausted, called?

AGB stars have a helium-burning shell around a degenerate carbon-oxygen core, with a hydrogen shell farther out.

What are the convective episodes that carry carbon to a red giant's surface called?

The first happens on the red-giant branch; the second and third, on the AGB, are what make a star visibly carbon-rich.

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

Stars below about 8 solar masses never ignite their carbon-oxygen cores.

How long does the whole red-giant phase last for a star of one solar mass?

Almost all of it is spent on the red-giant branch; the horizontal and asymptotic branches go by tens of times faster.

How much larger than the Sun can the radius of a red-giant-branch star get?

Their luminosities reach nearly 3,000 times the Sun's despite surface temperatures of only 3,000-4,000 K.

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

Those giant cells are what make the brightness of red giants and red supergiants vary so much.

What is the 'mirror principle' in red-giant evolution?

The hydrogen-burning shell divides the star; the physics is complex but follows from conserving gravitational and thermal energy.

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

It was created independently by Ejnar Hertzsprung in 1911 and Henry Norris Russell in 1913.

Which stars, in the 0.2-0.5 solar mass range, become red giants but never manage to ignite helium?

They never reach the tip of the red-giant branch; they simply puff off their outer layers and become white dwarfs.

What do the very lowest-mass stars eventually become instead of red giants?

Being fully convective, they burn nearly all their hydrogen and grow hotter and up to ten times brighter before fading.

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

The most massive of all can become Wolf-Rayet stars without ever becoming giants or supergiants.

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

Its main-sequence lifetime is about 10 billion years in total.

When the Sun becomes a red giant, roughly how large will it grow?

That is about 1.2 AU, enough to engulf Mercury, Venus and probably Earth; it will lose 38% of its mass in the process.

What fraction of its mass will the Sun lose as it swells into a red giant?

It will then contract into a white dwarf; some studies suggest a habitable zone at 7-22 AU could persist for a billion years afterwards.

For how long might a planet at Jupiter's distance sit in a Sun-like star's habitable zone during its red-giant phase?

For a 0.5-solar-mass star the same orbit stays habitable for 5.8 billion years.

By one hypothesis, why might giant planets around red giants be more massive?

Several hundred giant planets had been found around giant stars by 2023, and their masses do not track their stars' masses.

Which red giant in Bootes is the brightest star in the northern celestial hemisphere?

It is the fourth-brightest star in the whole night sky, and you find it by following the arc of the Big Dipper's handle.

Light from which red giant was used to switch on the 1933 Chicago World's Fair?

Organisers thought the light had left the star during the previous Chicago fair in 1893; at 36.7 light-years it actually set off in 1896.

Which red giant is the 'eye' of Taurus, with a name meaning 'the follower' in Arabic?

It seems to follow the Pleiades across the sky, and merely lies in front of the Hyades cluster, which is more than twice as far away.

Which defunct probe is drifting toward Taurus's bright red giant, due for its closest approach in about 2,000,000 years?

Aldebaran itself is about 67 light-years away with a radius roughly 45 times the Sun's but about the same mass.

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

The planet had been announced in 2006 as Pollux b; the star is the brightest in its constellation despite being Beta Geminorum.

Which star in the Southern Cross is the nearest M-type red giant to Earth, about 88 light-years away?

It is the third-brightest star in Crux; Arcturus, a K-type giant, is closer at 36 light-years.

Which red giant in Cetus, recorded as variable in 1596, has a Latin name meaning 'wonderful'?

David Fabricius first noted its variability; Johannes Hevelius named it in 1662, and it is now the prototype of the Mira variables.

How long is the tail of shed material that NASA's GALEX telescope found trailing behind Mira?

Mira is tearing through interstellar gas at 130 km/s, and the tail formed over tens of thousands of years.

Which red supergiant in Orion is usually the tenth-brightest star in the night sky?

It is second in its constellation to Rigel, and its distance is oddly uncertain at somewhere between 400 and 600 light-years.

By roughly what factor did Orion's red supergiant dim in its 2019-20 'Great Dimming'?

It fell from magnitude 0.5 to 1.7; Hubble data suggest a surface mass ejection cooled into dust that blocked about a quarter of the star's face.

When Orion's shoulder star eventually explodes as a supernova, how is it expected to appear from Earth?

It is less than 10 million years old, expected to blow within about 100,000 years, and life on Earth will be unharmed.

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?

It was ejected from the Orion OB1 association, which includes the stars of Orion's Belt.

The name of the red supergiant Antares means 'rival of' which god?

Its reddish hue resembles Mars, the planet named for the Roman equivalent; it marks the heart of Scorpius.

Which are the only two first-magnitude red supergiant stars in the sky?

Red supergiants are the largest stars by volume, though not the most massive or luminous.

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

NML Cygni is another example; formal hypergiant spectral classes are rarely applied to red supergiants.

What is the main-sequence lifetime of the Sun, before it starts on the road to becoming a red giant?

More massive stars burn their fuel disproportionately faster and live shorter lives.

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

Once the envelope becomes convective the star stops expanding, brightens, and starts climbing the red-giant branch.

Which class of core helium-fusing stars forms a flat line across the H-R diagram in metal-poor clusters?

Their metal-rich cousins pile up in the 'red clump' instead; both have luminosities near 75 times the Sun's.

Which two red-clump giants are the 'twins' and the 'she-goat' of the sky, respectively?

Aldebaran, Arcturus and Gacrux are red-giant-branch stars, while Mira and Rasalgethi are on the asymptotic giant branch.

Unlike the crisp edge shown in illustrations, what does the outer boundary of a real red giant look like?

The envelope's density is so low that there is no well-defined photosphere; the coolest giants even show masers in their spectra.

The name Arcturus comes from Greek words meaning 'guardian' of which animal?

The constellation Boötes was called Arctophylax, 'bear guardian', for following Ursa Major around the pole.

How far from the Sun is the red giant Arcturus?

That proximity gives it a proper motion of two arcseconds a year, and its light used to open the 1933 Chicago World's Fair left in 1896.

Although it appears to lead the Hyades, Aldebaran is roughly how far from Earth, well in front of the cluster?

The bull's-head cluster behind it is more than twice as distant, at about 150 light-years.

Which astronomer used a 509 AD lunar occultation of Aldebaran to help discover proper motion in 1718?

He realised the star had shifted several arcminutes north since the Athens observation; Sirius and Arcturus showed similar drift.

The name Betelgeuse derives from an Arabic phrase meaning which part of Orion?

A 13th-century misreading of one Arabic letter turned Yad al-Jawza into the form Europeans adopted.

Which astronomer first described Betelgeuse's variable brightness in his 1836 Outlines of Astronomy?

He watched it outshine Rigel in October 1837 and again in November 1839, then fall quiet for a decade.