Galaxies

1. The Milky Way and other galaxies

1.1 Island universes

Galaxies are massive collections of stars and other matter that are bound together by gravity.

Iranian astronomer 'Abd al-Rahman 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, German philosopher Immanuel Kant suggested that some nebula - astronomical objects that are extended, like clouds - could be 'island universes' that are separate from the rest of the Milky Way[2].

Photograph of the LMC and SMC galaxies.

Other galaxies (the Large and Small Magellanic Clouds) as they seem when viewed with the naked eye. Image credit: Helen Klus/CC-NC-SA.

Animation showing how the plane of the galaxy moves across the night sky.

The night sky at SAAO, Sutherland, South Africa, showing the plane of the Milky Way. Image credit: Helen Klus/CC-NC-SA.

1.2 Herschel's map

British astronomer 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 is moving towards the constellation of Hercules.

Herschel suggested that the Galaxy has a central 'Sun', which the Solar System orbits.

Diagram of the Milky Way created in 1785.

Herschel's map of the Milky Way, 1785. Image credit: Caroline Herschel/Public domain.

1.3 Spiral nebula

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

Photograph of the Andromeda galaxy.

Isaac Roberts' photograph of Andromeda, 1888. Image credit: Isaac Roberts/Public domain.

1.4 The 'great debate'

In 1917, 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, American astronomer Harlow Shapley attempted to measure the diameter of the Milky Way[7], using a method devised by American astronomer Henrietta Swan Leavitt in 1908[8].

Certain types of stars regularly fluctuate in brightness, and Leavitt showed that you can determine the luminosity of some of these stars - known as Cepheid variables - by measuring how long each fluctuation lasts. The longer the time between fluctuations, the brighter they are.

This is the star's intrinsic brightness. The brightness that we observe on Earth depends on the star's distance. It will appear dimmer the further it is from Earth. Shapley worked out the distance to each Cepheid variable by comparing the two luminosities.

Shapley's results 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 was 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, American astronomer Edwin Hubble settled the matter by showing that Andromeda is about 860,000 light-years away, further than any other measured stars[11] (a light-year is the distance that light travels in 1 year, and is equal to about 9000 billion km).

Photograph of stars in a globular cluster.

47 Tuc, a globular cluster. Globular clusters are found in the Galactic halo. Image credit: ESO/CC-A.

Photograph of stars in an open cluster.

The Eagle Nebula, an open cluster. Open clusters are found in the Galaxy's spiral arms. Image credit: NASA/STScI/WikiSky/Public domain.

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. French astronomer Gérard de Vaucouleurs extended this system in 1959[13].

Hubble’s diagram showing galaxies classified by shape.

Hubble's system for classifying galaxies. Image credit: Cosmo0/Public domain.

Modern diagram showing galaxies classified by shape.

De Vaucouleurs' system for classifying galaxies. Image credit: Dr T.H. Jarrett (Caltech)/CC-SA.

2. Modern maps of the Milky Way

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

In 1922, Dutch astronomer 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, 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]. Dutch astronomer Jan Oort confirmed this in 1927[16]. We now know that the Solar System is travelling at about 250 km/s[17a].

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[18a].

That year, Swiss-American astronomer Julius Trumpler showed that the disc of the Galaxy is filled with interstellar dust[19]. This absorbs starlight, causing it to loose 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[17b][20a].

American astronomer Joel Stebbins showed that Shapley's vertical estimate was roughly correct, and globular clusters form a spherical halo around the Galactic centre[18b].

In the 1930s, Shapley showed that the disc of the Galaxy is about 3000 light-years thick, with a central bulge that is about 12,000 light-years thick[18c]. 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[20b][21].

Diagram of the Milky Way, showing the nucleus, disc, and halo.

Diagram of the Milky Way. Image credit: modified by Helen Klus, original image by ESA/CC-A.

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

German astronomer Walter Baade showed that most star formation occurs in open star clusters, in the Galactic disc, in 1944[22]. Open clusters were later shown to exist in the Galaxy's spiral arms.

In 1951, Baade and American astronomer Nicholas Ulrich Mayall showed that the spiral structure in Andromeda could be mapped using supergiant stars[23]. This is because supergiant stars have relatively short lifetimes, and so formed much more recently than the stars in globular clusters.

Later that year, American astronomer William Wilson Morgan found the same pattern in the Milky Way, and discovered the Orion and Perseus arms[24]. The Sagittarius arm was discovered shortly after by astronomers Bart Bok, Michiel Bester, and Campbell Wade, while working at Harvard College Observatory[25].

In 1944, Dutch astronomer Hendrik van Hulst had predicted that hydrogen may produce an emission line at 21 cm, which can be detected with radio telescopes[18d]. This was discovered by Harold Ewen and Edward Purcell while working at Harvard University in 1951[26].

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

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 twenty times[17c].

In 2005, NASA's Spitzer Space Telescope was used to show that the Milky Way is a barred spiral galaxy[28].

Photograph of a barred spiral galaxy.

Artist's concept of the Milky Way. Image credit: NASA/JPL-Caltech/R.Hurt(SSC-Caltech)/Public domain.

2.4 Dark matter

The Milky Way contains about 400 billion of stars[17d] but it may be up to 1000 billion times as massive as the Sun.

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. In the 1970s, astronomers Vera Rubin, Kent Ford, and Norbert Thonnard showed that these two values are not the same[29].

Rubin showed that spiral galaxies like the Milky Way must contain a massive halo of dark matter in order for the edge of the disc to rotate as fast as it does while remaining gravitationally bound.

Swiss astrophysicist Fritz Zwicky first predicted the existence of dark matter in the 1930s[30]. 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).

Physicist Kim Greist devised the acronym MACHO in 1991 in order to describe objects like black holes and massive planets that do not emit much light[31]. 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 astronomers Gary Steigman and Michael Turner in 1985[32]. The most popular candidates for WIMPs are hypothetical particles called neutralinos[33].

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[34]. Physicist Rainer Schödel discovered a supermassive black hole within the central bulge our own galaxy in 2002[35]. This is known as Sagittarius A*, and it is at least 4 million times as massive as the Sun.

The first hypervelocity stars were discovered in 2005[36]. Hypervelocity stars are thought to have once been part of a binary star system that broke apart as it approached Sagittarius A*. While one star was captured, the other was catapulted away at a velocity exceeding the escape velocity of the Galaxy.

Hypervelocity stars challenge the idea that intergalactic space is empty, and by studying the path they have taken, information can be obtained about the distribution of dark matter within the Galaxy.

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*[37].

Fermi bubbles extend for a height that is comparable to the radius of the disc of the Milky Way, and may be millions of years old, although we still don't know exactly how they're produced.

Artist’s impression of gamma ray bubbles emanating from the centre of the Milky Way.

An illustration of gamma ray bubbles in the Milky Way. Image credit: NASA's Goddard Space Flight Center/Public domain.

3. Galaxy clusters and superclusters

There are thought to be over 100 billion galaxies in the observable universe[38]. Although most galaxies are separated by distances thousands of times their own diameter, they do sometimes interact and even collide.

Photograph of six galaxies.

Seyfert's Sextet contains 6 galaxies, 4 of which will eventually combine. Image credit: NASA/Public domain.

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[39].

The Local Group is one of at least 100 clusters within the Laniakea Supercluster[40], 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[41].

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

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[42]. It was found to be about one billion light-years long, and about one billion light-years from Earth.

Astronomers Istvan Horvath, Jon Hakkila, and Zsolt Bagoly discovered the Hercules-Corona Borealis Great Wall in 2013, using data from NASA's Swift satellite[43]. The Hercules-Corona Borealis Great Wall is almost 10 times the size of the Sloan Great Wall, and about 10 billion light-years from Earth.

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

The Local Group galaxy cluster. Image credit: Andrew Z. Colvin/CC-SA.

Diagram showing the position of clusters surrounding the Local Group.

Clusters surrounding the Local Group. Image credit: Andrew Z. Colvin/CC-SA.

Diagram showing the position of the Laniakea Supercluster relative to other superclusters.

Local superclusters, showing the position of the Laniakea Supercluster. Image credit: modified by Helen Klus, original image by Andrew Z. Colvin/CC-SA.

4. References

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  2. Kant, I. and Johnston, I. (trans), 2008 (1755), 'Universal Natural History and Theory of the Heavens', California State University.

  3. Herschel, W., 1785, 'On the Construction of the Heavens', Philosophical Transactions of the Royal Society of London, 75, pp.213-266.

  4. Mollan, G. R., 2014, 'William Parsons, 3rd Earl of Rosse: Astronomy and the Castle in Nineteenth-Century Ireland', Oxford University Press.

  5. Roberts, I., 2010 (1893), 'Photographs of Stars, Star-Clusters and Nebulae', Cambridge University Press.

  6. Curtis, H. D., 1917, 'Novae in spiral nebulae and the island universe theory', Publications of the Astronomical Society of the Pacific, 29, pp.206-207.

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  8. Leavitt, H. S., 1908, '1777 variables in the Magellanic Clouds', Annals of Harvard College Observatory, 60, pp.87-108.

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  11. Hubble, E., 'Finds Spiral Nebulae are Stellar Systems; Dr. Hubbell Confirms View That They Are 'Island Universes' Similar to Our Own.', New York Times, November 23, 1924.

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  34. Hubble News Release, 'Massive Black Holes Dwell in Most Galaxies, According to Hubble Census', last accessed 15-02-16.

  35. Schödel, R., et al, 2002, 'A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way', Nature, 419, pp.694-696.

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  37. Su, M., Slatyer, T. R., and Finkbeiner, D. P., 2010, 'Giant gamma-ray bubbles from Fermi-LAT: active galactic nucleus activity or bipolar galactic wind?', The Astrophysical Journal, 724, pp.1044-1082.

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  40. Tully, R. B., Courtois, H., Hoffman, Y., and Pomarède, D., 2014, 'The Laniakea supercluster of galaxies', Nature, 513, pp.71-73.

  41. Tully, R. B., 1987, 'More about clustering on a scale of 0.1 c', The Astrophysical Journal, 323, pp.1-18.

  42. Gott III, J. R., Schlegel, D., Hoyle, F., Vogeley, M., Tegmark, M., Bahcall, N. and Brinkmann, J., 2005, 'A Map of the Universe', The Astrophysical Journal, 624, pp.463-484.

  43. Horvath I., Hakkila J., and Bagoly Z., 2013, 'The largest structure of the Universe, defined by Gamma-Ray Bursts', 7th Huntsville Gamma-Ray Burst Symposium, Proceedings, 1.

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