50 Fun Facts About Galaxies

The word galaxy comes from a Greek word meaning what?

It referred to the pale band of our own galaxy across the night sky.

What type of galaxy is the Milky Way?

A bar of stars runs through its core and merges into the spiral arms.

What are the three main types of galaxy?

The Hubble sequence refines these by shape.

Most of the mass in a typical galaxy is in what form?

Only a few percent is visible as stars and nebulae.

What is commonly found at the centre of a galaxy?

Its existence in galactic cores was confirmed through X-ray astronomy.

A 2016 study estimated the observable universe holds at least how many galaxies?

That was ten times more than are directly seen in Hubble images.

Roughly how far away is the Andromeda Galaxy?

It is the Milky Way's nearest large neighbour.

The Milky Way belongs to which small association of galaxies?

It dominates the group together with Andromeda.

The Milky Way's home group is part of which supercluster?

That supercluster is itself a portion of the larger Laniakea Supercluster.

Before their nature was understood, galaxies were popularly called what?

Astronomers also called them spiral nebulae.

Which astronomer championed the word "galaxy" while Edwin Hubble stuck with "nebulae"?

The nomenclature only fully changed after Hubble's death in 1953.

Which Greek philosopher proposed that the Milky Way might be made of distant stars?

Aristotle instead thought it was burning vapour in the upper atmosphere.

Which astronomer first tried to measure the Milky Way's parallax and concluded it was remote?

No parallax meant it could not be part of the atmosphere.

Who proved in 1610 that the Milky Way is made of countless faint stars?

He turned his new telescope on the pale band and resolved it.

In 1750 who speculated that the Milky Way was a flattened, rotating disk of stars?

Kant elaborated on his idea five years later.

Who first mapped the Milky Way's shape in 1785 by counting stars in different directions?

His diagram wrongly put the Solar System near the centre.

Which of these galaxies can be seen with the naked eye on a dark night?

Andromeda and the two Magellanic Clouds are the others.

Which 10th-century Persian astronomer described Andromeda as a "small cloud"?

His Book of Fixed Stars may also mention the Large Magellanic Cloud.

How many nebulous objects did Charles Messier's 18th-century catalogue list?

Herschel later assembled a catalogue of 5,000 nebulae.

Who first observed spiral structure in a nebula, M51, in 1845?

M51 is now known as the Whirlpool.

Who discovered in 1912 that spiral nebulae showed large Doppler shifts, mostly moving away?

His spectrographic work predated Hubble's distance measurements.

In which year did the "Great Debate" over the nature of spiral nebulae take place?

Harlow Shapley argued against Heber Curtis's island-universe view.

Which variable stars let Edwin Hubble measure distances to the spiral nebulae?

He used the 100-inch telescope on Mount Wilson.

Hendrik van de Hulst predicted in 1944 that interstellar hydrogen would emit at which wavelength?

It was observed in 1951 and is unaffected by dust.

Whose 1970s work on galaxy rotation speeds pointed to large amounts of unseen mass?

The galaxy rotation problem is now explained by dark matter.

The Hubble Deep Field suggested about how many galaxies in the observable universe?

It was a very long exposure of a nearly empty patch of sky.

In the Hubble scheme, an E7 elliptical galaxy is what?

E0 is the nearly spherical end of the scale.

The largest and most luminous galaxies known belong to which class?

These supergiant ellipticals have faint halos stretching for megaparsecs.

About what fraction of elliptical galaxies have a shell-like halo structure?

NGC 3923 has more than 20 shells; the structure never appears in spirals.

A spiral galaxy with poorly defined, patchy arms is called what?

Grand design spirals have prominent, well-defined arms.

Spiral arms are thought to be what kind of phenomenon?

Stars slow as they pass through, like cars bunching on a highway.

Roughly how many stars does the Milky Way contain?

Its total mass is about 600 billion times that of the Sun.

A ring galaxy is thought to form when what happens?

Andromeda shows a multi-ring structure in infrared, possibly from such an event.

Which Hubble type denotes a lenticular galaxy?

Lenticulars sit between ellipticals and spirals, with ill-defined arms.

Which is the most common kind of galaxy?

They are about a hundredth the size of the Milky Way, with only a few billion stars.

Which small satellite galaxy is the Milky Way in the process of cannibalising?

The Canis Major Dwarf is also being absorbed.

About how long does a starburst episode typically last?

The galaxy would otherwise exhaust its gas long before the end of its life.

Which galaxy is the prototype starburst triggered by a close encounter with M81?

Irregular galaxies often show scattered knots of starburst activity.

Radio galaxies are classified using which scheme?

FR I sources are fainter and more elongated; FR II are more luminous.

Which galaxy, reported in 2022, has radio lobes spanning a then-record 5 megaparsecs?

That is about 16 million light-years of radio structure; Porphyrion, reported in 2024, has since been measured at roughly 7 megaparsecs.

Lower-luminosity active galactic nuclei are known as what kind of galaxies?

Through a telescope they look like an ordinary galaxy with a bright star on the core.

A blazar is an active galaxy whose relativistic jet is doing what?

Seyferts, quasars and blazars may all be the same thing seen from different angles.

Which structure, discovered in 2013, was hailed as the largest known in the universe?

It is about 10 billion light-years long and was identified from a clustering of gamma-ray bursts.

The Virgo Supercluster is itself part of which larger supercluster?

The name is Hawaiian for "immense heaven".

About how fast are the Milky Way and Andromeda approaching each other?

They may collide in five to six billion years.

Simulations suggest the Milky Way merged long ago with a large galaxy nicknamed what?

It has never collided with anything as big as Andromeda.

The first stars to form in protogalaxies are known as which group?

They were almost entirely hydrogen and helium.

The period after recombination and before the first stars is called what?

Neutral hydrogen absorbed light and nothing yet shone.

Which long-lived stars will be the last to fade at the end of the stellar age?

The stellar age is expected to wind down after ten to a hundred trillion years.

The Antennae are a famous example of galaxies showing what feature?

Gravitational stripping during a collision draws out long trains of stars.