50 Fun Facts About Milky Way

What type of galaxy is the Milky Way?

Astronomers began suspecting a central bar in the 1960s, and infrared observations in 2005 confirmed it was bigger than expected.

Roughly how many stars is the Milky Way estimated to contain?

The uncertainty comes from very-low-mass stars that are hard to detect; there are thought to be at least as many planets as stars.

The Sun sits on the inner edge of which spiral arm?

We are about 27,000 light-years from the Galactic Center; the next arm out, the Perseus Arm, is around 6,500 light-years away.

What is the intense radio source at the Galactic Center called?

It marks a supermassive black hole; the '*' is pronounced 'star'.

Roughly how massive is the black hole at the centre of the Milky Way?

It is a quiet giant, accreting only about a hundred-thousandth of a solar mass per year, typical of an inactive galactic nucleus.

Who first resolved the band of the Milky Way into individual stars with a telescope, in 1610?

Until the early 1920s most astronomers still thought the Milky Way contained every star in the universe.

Whose 1923 observations showed the Milky Way was just one galaxy among many?

He used Cepheid variables and the 100-inch Hooker telescope to put the Andromeda Nebula far outside the Milky Way.

The 1920 'Great Debate' about the scale of the universe was between Harlow Shapley and which astronomer?

Curtis had noticed that novae in the Andromeda Nebula were about 10 magnitudes fainter than those in the Milky Way, and argued it was a separate 'island universe'.

Which instrument did Hubble use to settle the debate over spiral nebulae?

Its light-gathering power let him photograph individual stars in the outer parts of spiral nebulae.

In Greek myth, whose spilled milk formed the Milky Way?

Zeus put the infant Heracles to her breast while she slept; when she woke and pushed him away, the milk sprayed across the sky.

The word 'galaxy' comes from a Greek root meaning what?

Galaxias kyklos meant 'milky circle', one of 11 circles the Greeks drew on the sky alongside the zodiac and the tropics.

Which English poet's work from around 1380 gives the earliest English name for the galaxy?

He wrote of 'the Galaxye which men clepeth the Milky Wey, for hit is whyt' in The House of Fame.

In Finnish, Estonian and several Turkic languages, the galaxy is named after what?

Northern peoples noticed that migrating birds follow the band; Chuvash and Tatar call it the 'Way of the grey goose'.

What does the Chinese name for the Milky Way, used across East Asia, translate to?

In Japan the band is also called the 'River of Heaven' (Ama no gawa), and in Japanese and Korean the 'silver river' word means any galaxy.

Pilgrims to which holy site traditionally used the Milky Way as a guide?

Oddly, the pilgrimage road itself was also nicknamed 'La Voje Ladee', the Milky Way.

Swedes call the galaxy 'Vintergatan'. What does it mean?

At high latitudes the summer sky never gets fully dark, so the band shows best in winter.

In Arabic and other West Asian languages, the galaxy's name refers to which material?

Darb at-Tabbanah means 'Pathway of the Straw'; medieval Arabs may have borrowed the image from Armenians.

The Gomeroi people of Australia see the dark dust lanes of the band as which giant animal?

Southern peoples including the Inca built 'dark cloud constellations' out of the dust regions rather than the stars.

In which constellation does the Galactic Center lie, where the band appears brightest?

The band runs from there around the whole sky to the anticentre in Auriga, splitting the sky into two near-equal halves.

What is the dark region of the band where interstellar dust hides the stars behind called?

The Zone of Avoidance is different: it is the part of the sky where the Milky Way's own dust blocks our view of galaxies beyond.

Roughly what fraction of humanity cannot see the Milky Way from home due to light pollution?

The sky needs to be darker than about 20.2 magnitudes per square arcsecond for the band to appear at all.

Which philosopher wrote in Meteorologica that the Milky Way was part of Earth's upper atmosphere?

Olympiodorus the Younger later objected that a sublunary band would show parallax and look different from different places, which it does not.

Which Persian scholar wrote c.1000 AD that the Milky Way was 'countless fragments' of nebulous stars?

Nasir al-Din al-Tusi later wrote that it was made of small, tightly clustered stars that only 'seem to be cloudy patches'.

Which philosopher speculated in 1755 that the Milky Way was a rotating disc of stars and coined 'island universes'?

He was building on, and misreading, Thomas Wright's suggestion that the galaxy was shaped like a balloon.

Who first tried to map the Milky Way's shape in 1785 by counting stars across the sky?

His diagram put the Solar System near the centre, an error that would not be corrected for well over a century.

Which ESA mission launched in 2013 measured parallaxes of about a billion Milky Way stars?

Gaia, which ended science observations in January 2025, expanded the number of precisely measured stars from about 2 million in the 1990s to 2 billion and detected a wobble in the galactic disc.

Roughly how long does the Solar System take to complete one orbit of the galaxy, a 'galactic year'?

The Sun has managed roughly 18-20 laps in its lifetime and about 1/1250 of a lap since humans appeared.

At roughly what speed does the Solar System orbit the centre of the galaxy?

That is about 490,000 mph, or 0.073% of the speed of light.

The Sun's general motion through the galaxy is towards which bright star?

This 'solar apex' is roughly 90 degrees away from the direction of the Galactic Center.

What is the other giant spiral in the Local Group besides the Milky Way?

The two are effectively a binary system among about 50 gravitationally bound galaxies.

What is the largest satellite galaxy of the Milky Way?

It is about 32,200 light-years across, and with its small companion it trails a stream of neutral hydrogen across 100 degrees of sky.

Which is the closest known satellite galaxy of the Milky Way?

Some astronomers argue the 'dwarf' is really just an overdensity produced by the warp of the Milky Way's own disc.

What is the nickname of the large galaxy the Milky Way merged with 11 billion years ago?

Since then the galaxy appears to have avoided any major mergers for 10 billion years, which is unusual for a spiral of its type.

Roughly when is the Andromeda-Milky Way collision expected?

Individual stars are very unlikely to hit each other; over about six billion years the pair should merge into one elliptical or large disc galaxy.

How fast is Andromeda approaching us?

That is roughly 220,000-310,000 mph, but the galaxies are so far apart that the meeting is billions of years off.

What is the best evidence for dark matter in the Milky Way?

If only visible matter were present, orbital speeds should fall off with distance; instead stars keep moving at 200-220 km/s.

The giant gamma-ray lobes above and below the galactic core, found in 2010, are named after which space telescope?

Each bubble is about 25,000 light-years across, roughly a quarter of the galaxy's diameter.

Which two arms are thought to be the Milky Way's only major arms as traced by old stars?

Gas and young stars trace four arms, old stars only two, and nobody is quite sure why the counts disagree.

Which spacecraft's 2005 data showed the Milky Way's central bar was larger than thought?

Spitzer worked in the infrared, which cuts through the dust that hides the galactic centre from optical telescopes.

Roughly what proportion of the Milky Way's mass is thought to be dark matter?

A 2019 estimate put the virial mass at 1.54 trillion solar masses, of which all the stars together make up only a few percent.

Interstellar gas in the Milky Way is roughly 90% hydrogen and 10% what, by mass?

Two-thirds of the hydrogen is atomic and one-third molecular; the gas altogether weighs 10-15% as much as the stars.

Roughly how thick is the Milky Way's disc at the spiral arms, compared with its width of some 87,000 ly?

Seen edge-on it is a thin pancake with a fatter bar in the middle; the dark matter halo may reach almost 2 million light-years across.

If the Solar System out to Neptune were the size of a US quarter, how big would the Milky Way be?

The 1997 study that made the comparison also concluded the Milky Way is a fairly ordinary spiral, not one of the largest known.

Which supercluster, named with a Hawaiian word, contains the Virgo Supercluster and thus the Milky Way?

The Local Group sits in the Virgo Supercluster, which is being drawn towards the Great Attractor within Laniakea.

Which country's Very Long Baseline Array clocked outer-galaxy stars at 254 km/s in 2009?

Those high speeds implied the Milky Way is roughly as massive as Andromeda, at about 700 billion solar masses within 160,000 light-years.

In the Babylonian epic Enuma Elish, the Milky Way is made from the severed tail of which primeval dragon?

Marduk slays her and sets the tail in the sky; the story was long thought to have a Sumerian original but is now seen as Babylonian propaganda.

Roughly what proportion of the Milky Way's globular clusters travel on retrograde orbits?

Globular clusters can follow rosette-shaped paths through the halo rather than the neat ellipses planets trace around a star.

Per 2013 Kepler data, how many Earth-sized planets may sit in habitable zones in the Milky Way?

The November 2013 estimate covered Sun-like stars and red dwarfs; about 11 billion of those planets may circle Sun-like stars.

Collisions with which small galaxy may have shaped the Milky Way's arms, per a 2011 simulation?

The Milky Way is still accreting material from several small galaxies today.

The 2007 halo star HE 1523-0901 was estimated to be roughly how old?

Such ages are measured from the abundance of long-lived radioactive elements like thorium-232 and uranium-238.