How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 10. Galaxies

10.1 The Milky Way and other galaxies

10.1.1 Island universes

Galaxies are massive collections of stars and other matter that are bound together by gravity. Al-Sufi made the first recorded observation of Andromeda - the closest spiral galaxy to our own - in about 964. He described it as a ‘little cloud’.[1]

In 1755, the German philosopher Immanuel Kant suggested that some nebulae - astronomical objects that are extended like clouds - could be ‘island universes’ that are separate from the rest of the Milky Way.[2]

10.1.2 Herschel’s map

William Herschel created one of the first maps of the Milky Way in 1785.[3] He did this by assuming that brighter stars are closer and duller stars are further away. He assumed that the Sun was near the centre, and saw that it’s moving towards the constellation of Hercules. Herschel suggested that the Galaxy has a central ‘Sun’ that the Solar System orbits.

A diagram of the Milky Way created in 1785.

Figure 10.1
Image credit

Herschel’s map of the Milky Way, 1785.

10.1.3 Spiral nebulae

In the 1840s and 1850s, the Irish astronomer William Parsons (also known as Lord Rosse) showed that some ‘nebulae’ have a spiral structure and some are elliptical.[4] He described Andromeda as having a spiral structure and this was evident in 1888 when the British astronomer Isaac Roberts became the first person to photograph it.[5]

A photograph of the Andromeda galaxy.

Figure 10.2
Image credit

Isaac Roberts’ photograph of Andromeda, 1888.

10.1.4 The ‘great debate’

In 1917, the American astronomer Heber Curtis showed that Andromeda is not just an ordinary cluster of stars because it contains more novas - particularly bright stars - than the Milky Way.[6]

The following year, the American astronomer Harlow Shapley attempted to measure the diameter of the Milky Way[7] using Henrietta Swan Leavitt’s Cepheid variable method[8] (discussed in Chapter 9).

Shapley showed that globular clusters - spherical clusters of old stars - are arranged in a sphere that encompasses the Earth. The Earth was not in the centre, and Shapley correctly concluded that the centre of the sphere represented the centre of the Galaxy.

Shapley argued that Andromeda is part of the Milky Way, and in 1920 Curtis and Shapley both presented evidence for their claims, in what was known as the ‘great debate’.[9,10]

Within four years, Edwin Hubble settled the matter by showing that Andromeda is about 860,000 light-years away, further than any other measured stars[11].

10.1.5 Galaxy categorisation

In 1926, Hubble became the first person to categorise galaxies according to their appearance.[12] Elliptical galaxies are shaped like ellipses, spiral galaxies are disc-shaped, and other galaxies are classified as irregular. The French astronomer Gérard de Vaucouleurs extended this system in 1959.[13]

10.2 Modern maps of the Milky Way

10.2.1 Kapteyn, Lindblad, and the speed of the Solar System

In 1922, Jacobus Kapteyn described the Milky Way as ‘lens-shaped’ and showed that it increases in density towards the centre.[14] Kapteyn had noted that stars appear to move in one of two directions, and in 1926 the Swedish astronomer Bertil Lindblad popularised Herschel’s suggestion that stars rotate around the centre of the Galaxy. Lindblad predicted that the Sun orbits at between 200 km/s and 300 km/s.[15] The Dutch astronomer Jan Oort confirmed this in 1927.[16]

We now know that the Solar System takes about 230 million years to orbit the Galaxy, travelling at about 250 km/s (just over half a million miles per hour).[17] It passes through the vertical gravitational centre of the Galactic disc once every 30 million years or so, about eight times with every Galactic orbit.[18] The Sun moves up and down because it is pulled by gravity. It is pulled towards the centre of gravity, overshoots slightly, and is then pulled back up or down. It moves about 200 light-years from the centre of the over 1000 light-year-thick disc at each maximum or minimum.

A diagram of the Milky Way showing the path of the Sun. The Sun moves up and down as it orbits the centre of the galaxy.

Figure 10.3
Image credit

The Sun moves above and below the plane of the Milky Way about eight times with every Galactic orbit (image is approximate and exaggerated for clarity).

10.2.2 Shapley and the size of the Milky Way

In 1930, Shapley predicted that the Galaxy is composed of a flattened sphere of globular clusters that is about 180,000 light-years in diameter, and about 90,000 light-years thick. He predicted that the disc of the Galaxy is about 90,000 light-years in diameter.[19]

That year, the Swiss-American astronomer Robert Julius Trumpler showed that the disc of the Galaxy is filled with interstellar dust.[20] This absorbs starlight, causing it to lose energy. This meant that the intrinsic luminosity of stars was higher than Shapley had estimated in the distance across the Galactic plane. It’s now thought that the Galactic disc has a diameter of about 100,000 light-years, with the halo extending for another 20,000-30,000 light-years.[17,21]

The American astronomer Joel Stebbins showed that Shapley’s vertical estimate was roughly correct, and globular clusters form a spherical halo around the Galactic centre.[19] In the 1930s, Shapley showed that the disc of the Galaxy is about 3000 light-years thick, with a central bulge that’s about 12,000 light-years thick.[19] This was also roughly correct. We now know that the Galactic disc is 1000-3000 light-years thick, increasing to about 16,000 light-years at the centre.[21,22]

10.2.3 Baade, Oort, and the shape of the Milky Way

In 1944, the German astronomer Walter Baade showed that most star formation occurs in open star clusters in the Galactic disc..[23] Open clusters were later shown to exist in the Galaxy’s spiral arms.

In 1951, Baade and the American astronomer Nicholas Ulrich Mayall showed that the spiral structure in Andromeda could be mapped using supergiant stars.[24] This is because supergiant stars have relatively short lifetimes (discussed in Chapter 11), and so they formed much more recently than the stars in globular clusters.

Later that year, the American astronomer William Wilson Morgan found the same pattern in the Milky Way and discovered the Orion and Perseus arms.[25] The Sagittarius arm was discovered shortly after by the astronomers Bart Bok, Michiel Bester, and Campbell Wade, who were working at Harvard College Observatory.[26]

In 1944, the Dutch astronomer Hendrik van de Hulst had predicted that hydrogen may produce an emission line at 21 cm, which can be detected with radio telescopes.[19] This was discovered by Harold Ewen and Edward Purcell while working at Harvard University in 1951.[27]

Oort and the Dutch-American astronomer Gart Westerhout and the Australian astronomer Frank John Kerr then created a map of hydrogen clouds in the Milky Way and found that the Sun is about 27,000 light-years from the centre.[28]

We now think that the Sun is about 28,000 light-years from the Galactic centre. It takes about 225 million years for the Solar System to complete one orbit around the Galaxy, and it has already travelled around at least 20 times.[17] In 2005, NASA’s Spitzer Space Telescope was used to show that the Milky Way is a barred spiral galaxy.[29]

A diagram of the Milky Way from the top and side, showing the disc is about 120,000 light-years in diameter. The Sun is about 28,000 light-years from the centre.

Figure 10.4
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A diagram of the Milky Way.

10.2.4 Dark matter

The mass of galaxies can be determined by finding the average stellar mass and multiplying this by the number of stars. It can also be determined by measuring how fast it’s rotating, using the equation for centripetal force (discussed in Chapter 5). In the 1970s, the astronomers Vera Rubin, Kent Ford, and Norbert Thonnard showed that these two values are not the same.[30]

The Milky Way contains about 400 billion stars[17] but it may be up to 1000 billion times as massive as the Sun, an average-sized star. Rubin showed that spiral galaxies like the Milky Way must contain a massive halo of dark matter for the edge of the disc to rotate as fast as it does while remaining gravitationally bound.

Fritz Zwicky had first predicted the existence of dark matter in the 1930s.[31] Zwicky found that the Coma cluster must be 400 times more massive than the sum of the mass of its stars in order to remain gravitationally bound.

Dark matter is thought to be composed of two types of matter: massive astrophysical compact halo objects (MACHOs) and weakly interacting massive particles (WIMPs).

The physicist Kim Greist devised the acronym MACHO in 1991 to describe objects like black holes and massive planets that do not emit much light.[32] Although a large amount of dark matter could be composed of MACHOs, they cannot account for it all.

Most dark matter is thought to be composed of massive particles that do not interact with the electromagnetic force. These are WIMPs, an acronym coined by the astronomers Gary Steigman and Michael Turner in 1985.[33] The most popular candidates for WIMPs are hypothetical particles called neutralinos.[34]

A photograph of the Bullet Cluster, showing a central mass of gas surrounded on either side by two lumps of dark matter.

Figure 10.5
Image credit

The Bullet Cluster formed from a collision between two galaxy clusters. The gas (centre, pink) and dark matter (blue) collided, but most of the stars did not (white and orange).

Some of the best evidence for dark matter comes from the Bullet Cluster.[35] This was formed from a collision between two galaxy clusters. Individual galaxies are so far apart that most did not collide when this happened, but the gas between the galaxies did. This meant that the galaxies and the gas were separated. The gas emits X-rays, which are shown in pink in Figure 10.10. The stars within galaxies are visible in optical light, which is shown in white and orange.

Dark matter was also separated from the gas during the collision. This is because it was not slowed by interactions with the light around it, and so moved ahead of the gas as each cluster passed through the other. In Figure 10.10, the dark matter is mostly in the blue coloured region. This was mapped using gravitational lensing (discussed in Chapter 8).

10.2.5 Supermassive black holes

In 1997, the Hubble Space Telescope was used to show that most galaxies contain supermassive black holes at their centre.[36] Physicist Rainer Schödel discovered a supermassive black hole within the central bulge our own galaxy in 2002.[37] This is known as Sagittarius A*, and it is at least 4 million times as massive as the Sun (discussed in Chapter 14).

In 2010, data from NASA’s Fermi Gamma-ray Space Telescope was used to show that the Milky Way contains two giant structures known as Fermi, or gamma-ray, bubbles, which are thought to emanate from Sagittarius A*.[38] Fermi bubbles extend for a height that is comparable to the radius of the disc of the Milky Way. They could be millions of years old, although we still don’t know exactly how they’re produced.

An artist’s impression of gamma-ray bubbles emanating from the centre of the Milky Way.

Figure 10.6
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Gamma-ray bubbles emanate from Sagittarius A*, the supermassive black hole at the centre of the Milky Way.

10.3 Galaxy clusters and superclusters

There are thought to be over 100 billion galaxies in the observable universe.[39] Although most galaxies are separated by distances thousands of times their own diameter, they do sometimes interact and even collide.

Galaxies are usually gravitationally bound to other galaxies in a cluster, and clusters are bound in superclusters. Superclusters form structures known as galaxy filaments, and these are the largest structures in the observable universe.

The Milky Way and Andromeda galaxies are the two largest spiral galaxies in the Local Group cluster, which contains over 30 galaxies, many of which orbit the Milky Way.[40]

The Local Group is one of at least 100 clusters within the Laniakea Supercluster,[41] and the Laniakea Supercluster is one of at least 20 superclusters in the Pisces-Cetus Supercluster Complex, a galaxy filament that was discovered in 1987.[42]

The largest known gravitationally bound structures include the Sloan Great Wall galaxy filament and the Hercules-Corona Borealis Great Wall galaxy filament.

The American astronomer John Richard Gott III and his colleagues discovered the Sloan Great Wall in 2003 using information from the Sloan Digital Sky Survey in New Mexico.[43] It was found to be about one billion light-years long and it is about one billion light-years from Earth.

The astronomers Istvan Horvath, Jon Hakkila, and Zsolt Bagoly discovered the Hercules-Corona Borealis Great Wall in 2013 using data from NASA’s Swift satellite.[44] The Hercules-Corona Borealis Great Wall is almost 10 times the size of the Sloan Great Wall and it is about 10 times the distance from Earth.

A diagram showing the position of the Milky Way relative to the closest galaxies - the Local Galactic Group.

Figure 10.7
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The Local Group galaxy cluster.

A diagram showing clusters around the Local Group.

Figure 10.8
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Clusters surrounding the Local Group.

A diagram showing the position of the Laniakea Supercluster relative to other superclusters.

Figure 10.9
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Local superclusters, showing the position of the Laniakea Supercluster.

Computer simulated image showing galaxy filaments.

Figure 10.10
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A map showing galaxy filaments within the Laniakea Supercluster.

10.4 References

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