50 Fun Facts About White Dwarfs

A white dwarf packs a mass comparable to the Sun's into a volume roughly the size of what?

Earth is the usual yardstick: with a Sun's worth of matter in a planet-sized ball, a white dwarf's surface gravity runs to 100,000 times ours or more.

What makes a white dwarf shine?

Leftover heat from its past is all it has: fusion in its core has stopped, so a white dwarf simply glows like an ember, fading and reddening over billions of years.

Sirius B now weighs less than Sirius A; what about at birth?

B was heavier, perhaps five Suns or more, so it burned through its fuel first and then shed most of its bulk, leaving a faint cinder beside the star that outlived it.

Which astronomer's famous 'new star' of 1572 is now known to have been an exploding white dwarf?

Tycho Brahe's star is long gone, but in 2008 astronomers caught its light still echoing off dust clouds and read in it the signature of a white dwarf blown apart.

Which NASA space telescope's data revealed, in 2015, a white dwarf tearing apart a small rocky world?

Kepler recorded the star dimming every 4.5 hours as a crumbling mini planet crossed its face, the first time anyone had watched a dead star tear one of its own worlds apart.

What holds a white dwarf up against gravitational collapse?

Electron degeneracy pressure is a quantum effect rather than heat: electrons refuse to be squeezed into the same state, so the star stays propped up even after it has gone cold.

What is the approximate maximum mass of a non-rotating white dwarf, the Chandrasekhar limit?

1.44 solar masses is the ceiling: every white dwarf weighed so far comes in under it, and a dead star left any heavier cannot stay a white dwarf at all.

The 19-year-old Chandrasekhar did the key calculation behind his mass limit on a 1930 voyage to study at which university?

Cambridge was his destination: on the ship from India he added relativity to the theory of white dwarfs and found that it set a ceiling on their mass.

Chandrasekhar's 1983 Nobel Prize honoured his studies of what?

The evolution of stars and their structure: the citation reached back to work he began as a student, though by 1983 he had long since moved on to the mathematics of black holes.

Which city, now in Pakistan, was Chandrasekhar's birthplace?

Lahore was only a posting: his father was auditing the railways there, and the Tamil family later settled in Madras, where Chandrasekhar went on to study physics.

R. H. Fowler solved the white dwarf density puzzle in 1926 using the exclusion principle named after which physicist?

Wolfgang Pauli's exclusion principle let Fowler explain how so dense a star resists collapse; four years later Fowler took on a research student named Chandrasekhar.

Which astronomer deduced in 1844, from a wobble in Sirius's path, that it had an unseen companion?

Friedrich Bessel worked from the bright star's wavering track alone; nearly two decades passed before anyone actually set eyes on the faint companion he had predicted.

Which American telescope maker first spotted the faint companion of Sirius?

Alvan Graham Clark was not hunting for it: he chose Sirius only as a bright point of light for checking a large new lens his family firm had made, and saw a speck beside it.

Which Dutch-American astronomer discovered the first isolated white dwarf, now named after him, in 1917?

Adriaan van Maanen's find at first passed for an ordinary star a little hotter than the Sun; only later was it recognised as a dead star.

A 1917 spectrum of a nearby white dwarf is now seen as the earliest evidence of what?

A planetary system beyond ours: the plate shows calcium, magnesium and iron that should have sunk from view long ago, so rocky debris must still be falling onto the star.

Gas pulled from a companion star usually gathers into what around a white dwarf before falling in?

An accretion disc: the stolen gas carries too much sideways motion to fall straight in, so it circles the white dwarf, where friction heats it and lets it spiral down to the surface.

Sirius is the Dog Star; what nickname do astronomers give its white dwarf companion, Sirius B?

The Pup sounds young, but the name misleads: Sirius B is a dead star, a dim ember that takes a telescope to pick out of its brilliant partner's glare.

In a famous 1927 image, a ton of Sirius B's material would be a nugget that fits in what?

A matchbox: the lecturer who coined the image admitted that when the star's light first carried this message in 1914, most astronomers answered 'Shut up. Don't talk nonsense.'

What happens to a white dwarf's size as its mass goes up?

It shrinks: the extra gravity crushes the matter tighter, so the heaviest white dwarfs are the smallest, the reverse of how ordinary stars behave.

Before it becomes a white dwarf, a star like the Sun first swells into what?

A red giant: in about five billion years the Sun will balloon until it swallows Mercury and Venus, then puff its outer layers away and leave only its hot core behind.

About 97% of our galaxy's stars end as white dwarfs; which are the exception?

The most massive ones, born with more than about eight to ten Suns of material: their cores grow too heavy to settle as white dwarfs, so they end in collapse and explosion instead.

What do astronomers call the flare-up when gas stolen from a companion ignites on a white dwarf's surface?

A nova: the stolen gas piles up until it ignites in a runaway blast on the surface, yet the dead star underneath survives, and some systems have been seen to erupt again and again.

Which of these objects is even denser than a white dwarf?

A neutron star is denser because the squeeze has gone a step further: protons and electrons are crushed together into neutrons, packing a Sun's worth of matter into a ball the width of a city.

A white dwarf pushed toward the Chandrasekhar limit, by a companion's gas or a merger, can ignite as which event?

A Type Ia supernova destroys the star outright, and because these blasts peak at nearly the same brightness, astronomers use them to gauge distances to far-off galaxies.

What name is given to a white dwarf that has cooled until it no longer glows?

A black dwarf is still only a prediction: cooling that far takes much longer than the 13.8 billion years the universe has existed, so not one is thought to have formed yet.

BPM 37093, a white dwarf thought to have a crystallised carbon core, has which Beatles-inspired nickname?

Lucy, from 'Lucy in the Sky with Diamonds': its discoverers joked that grading this gem would take a jeweller's loupe the size of the Sun, whose own core is expected to crystallise one day.

What is the ejected shell of gas that surrounds a newly formed white dwarf called?

A planetary nebula has nothing to do with planets: the name stuck because the round, glowing shells looked like planets' discs to observers using early telescopes.

Asteroseismology probes the inside of a white dwarf by measuring what?

Its rhythmic flickering: some white dwarfs ring like gongs, and the pattern of beats reveals what lies hidden inside them, much as tremors reveal the interior of our own planet.

Why are white dwarfs called 'white'?

The first few found were white, and the name stuck, but they are not all that colour: as the stars cool over billions of years their glow slides from blue-white toward yellow and orange.

Because white dwarfs cool at a known rate, the faintest ones in a star cluster reveal what about it?

Its age: the coolest white dwarfs have been fading the longest, so they work as clocks, and even the oldest still glow faintly, which sets a floor under the age of the universe.

Light leaving Sirius B is stretched to redder wavelengths by the white dwarf's intense gravity, as predicted by which theory?

General relativity: Einstein predicted that light climbing out of strong gravity loses energy, and Hubble has since used the size of that shift to weigh Sirius B at about the mass of the Sun.

Sirius A far outshines Sirius B in visible light; in which light is B brighter?

X-rays: Sirius B's surface is about 25,000 degrees, far hotter than its partner's, so in a NASA X-ray image the tiny white dwarf blazes while brilliant Sirius A all but vanishes.

WD 1856 b, a planet found transiting a white dwarf in 2020, outsizes its star; it matches which planet in size?

Jupiter: the planet is several times wider than the white dwarf it circles every 1.4 days, so close that it must have moved in after the star died, or it would have been swallowed.

The white dwarf ZTF J1901+1458 packs more than the Sun's mass into a body about the size of what?

The Moon: the star is thought to be two white dwarfs merged into one, which would explain why it spins once every seven minutes and carries a magnetic field a billion times the Sun's.

Which fictional planet orbits 40 Eridani A, the primary of the system with the first known white dwarf?

Vulcan, Mr Spock's home world: a real planet reported around the star in 2018 was nicknamed after it, until closer study in 2024 showed the signal was only the star's own jitter.

A typical white dwarf is made almost entirely of which two elements?

Carbon and oxygen are the ash of the helium the star burned late in life; it never grew hot enough to fuse them any further, so the spent core was simply left behind.

Exploding white dwarfs served as the 'standard candles' behind which Nobel-winning discovery of 1998?

The universe's accelerating expansion: two teams found distant exploding white dwarfs fainter than expected, a surprise now blamed on a mysterious 'dark energy' pushing space apart.

In 2017 Hubble weighed the white dwarf Stein 2051 B by watching it do what to a distant star's light?

Bend it: the white dwarf's gravity nudged the background star's apparent position, repeating around a dead star the famous starlight test of the 1919 solar eclipse.

Which astronomer first recorded the faint pair of stars 40 Eridani B and C, in 1783?

William Herschel logged them as just another double star; well over a century passed before anyone realised that one of the pair was a wholly new kind of object.

Which Mount Wilson astronomer found that the faint companion of Sirius was white-hot rather than a cool red star?

Walter Adams found its spectrum matched brilliant Sirius itself; a star that hot yet that dim had to be tiny, and the puzzle of its impossible-seeming density began.

Which Soviet physicist's 1932 paper on the maximum mass of stars imagined a dense star like one giant atomic nucleus?

Lev Landau wrote it in 1931, a year before it appeared in print, and his picture of a star crushed into one gigantic nucleus anticipated a kind of star that was later found.

At which American university did Chandrasekhar teach from 1937 until his death in 1995?

Chicago kept him for nearly six decades: he edited the Astrophysical Journal there for 19 years and guided more than fifty students to their doctorates.

Which bright star in Canis Minor, like Sirius, turned out to have a white dwarf companion?

Procyon's companion was inferred from the bright star's wobble in 1844 but not actually seen until 1896, hidden in a glare some 15,000 times brighter than itself.

Which nearby shell of gas in Aquarius, cast off by a dying star, has a white dwarf at its centre?

The Helix is one of the closest objects of its kind, yet its glow is spread so thinly across the sky that it is hard to see by eye.

Which ESA star-mapping spacecraft revealed in 2019 how white dwarfs turn solid as they cool?

Gaia's census revealed a pile-up of white dwarfs at certain colours and brightnesses: heat released as their cores freeze stalls their fading, so some look up to two billion years younger than they are.

Chandrasekhar was the nephew of which earlier Indian winner of the Nobel Prize in Physics?

C. V. Raman won in 1930 for discovering that scattered light can change its wavelength; his nephew's prize, 53 years later, gave the family its second Nobel in physics.

Which project's 2006 catalogue listed over 9,000 white dwarfs, most of them newly found?

The Sloan Digital Sky Survey confirmed 9,316 of them, about 6,000 never seen before, which roughly doubled the number of confirmed white dwarfs at a stroke.

Which famous British astronomer publicly attacked Chandrasekhar's mass limit at a Royal Astronomical Society meeting in 1935?

Arthur Eddington declared that some law of nature must stop a star from behaving so absurdly; he was wrong, but his standing meant the limit took years to be accepted.

White dwarfs of spectral class DA, by far the commonest kind, have atmospheres dominated by which element?

Hydrogen floats to the top because a white dwarf's fierce gravity sorts its ingredients by weight, sinking everything heavier and leaving the lightest element as a thin skin.

AR Scorpii was identified in 2016 as the first white dwarf to behave like which kind of rapidly spinning dead star?

A pulsar: the white dwarf spins so fast that it lashes its red dwarf companion with a beam of radiation, making the whole system brighten and fade every two minutes.