When we look at the sky during a dark night, far from the city lights, we can see so many stars that we can feel overwhelmed because of the amount of stars. When we look towards specific areas we can see an almost continuous belt of dust, similar to the trace that someone leaves when a spills a bottle of milk. This trace of milk is our Galaxy, the Milky Way. However, the Milky Way does not cover everything that exists, the Universe extends beyond our galactic home.
The Milky Way is one in the hundreds of thousands of millions of galaxies that share the Universe with us, each one being an enormous set of stellar systems on their own.
From the Earth, to the naked eye and depending on the region of the sky we are observing, we can easily see three galaxies. It is the Local Group, which includes the Milky Way the Large Magellanic Cloud, and the Small Magellanic Cloud. The third member of the trio is the Andromeda galaxy in the homonymous constellation.
Magellanic Clouds and the Milky Way
In the 18th century, the French Charles Messier, who was a comet hunter, scrutinized the sky with his telescope (less than 20 cm of diameter) until he saw a blurry spot. When he found any, he took note of the position in a stellar map. The night after he aimed his telescope to the same point to see whether the spot was still there. If the spot had moved from there, it was a comet if not it was other thing. By that time, those spots were known as nebulae, latin word meaning ‘mist’ or ‘cloud’. In 1774, Messier had catalogued 45 nebulae together with their celestial coordinates and in 1784 the catalogue included 103 objects already.
A German born musician, William Herschel, who worked in the second half of his life in the construction of large telescopes together with his sister Caroline aimed his instruments towards the objects discovered by Messier, and given the ‘power’ of his telescope (4 times bigger than the one of Messier) discovered in seven years more than 2000 objects.
With this catalogue, Herschel tried to make a celestial map including all these objects. From the study of the nebulae, Herschel proposed that if the Milky Way was observed from a distance far enough it would look like a nebula itself.
Because of the power of Herschel telescope, he could resolve some of the blurry spots into globular clusters. In the decade of 1840, William Parsons started to build a telescope 16 m long with a mirror of 2 m of diameter, which was bigger than the one of Herschel.
Parsons aimed his telescope towards one of the objects of the Messier catalogue, concretely M51, and his surprise was immense when he saw a spiral structure, that later was named as the Whirpool galaxy because of this characteristic. He couldn’t find individual stars in it but discovered other nebulae with the same spiral structure.
M51. The Whirpool galaxy
At this point, the question arose: did these nebulae belong to the Milky Way? To answer it, it was necessary to know the size of the Milky Way and the distance to the nebulae.
Shortly before this discover, astronomers knew already the parallax method to measure the distance to close stars, but due to the large distances of the nebulae, this method was useless. After the initial development of methods in spectroscopy the English astronomer William Huggins aimed, in 1867, his telescope equipped with a spectroscope to the brightest star in the night sky, and applying the Doppler effect theory, developed by the Austrian Christian Doppler 20 years before, he found a slight red shift in the spectrum of the star. He calculated that Sirius was moving away from us at a speed of 50 km/s in the line of sight. In the same way, he calculated the speed of a large number of stars. It was only the beginning of the use of the Doppler effect technique in astronomy. Some years later it was found a way to use this method to calculate the distance of stars.
In the early 20th century, the observatory of the Harvard College was making tedious stellar observations from photographic and spectroscopic plates. Women who were deemed by that time appropriate for the tedious and repetitive work, showing machismo behaviour, did the measures and calculations earning less money than men. Several of these women made important contributions but, among all, stressed Henrietta Swan Leavitt.
Henrietta Swan Leavitt
In a number of photographic plates of the Small Magellanic Cloud, Leavitt observed multitude of stars, which changed their brightness periodically because they ‘pulse’, i.e. they expand and shrink regularly. These stars are known as Cepheid variables because the firsts Cepheid discovered is in the homonymous constellation.
Leavitt compiled more than one thousand of Cepheid in the Small Magellanic Cloud and, at least, 16 of them appeared in several photographic plates what enabled her calculating their periods.
She found that the stars were brighter when their periods where longer and established that the period and the brightness were related and that it was possible to graphically show the relation between the period and the luminosity, i.e. Leavitt had found a relation between the apparent magnitude of a variable star with a measure that was independent of the distance to the star: the change in the brightness. Leavitt had discovered a connexion between the period and the absolute magnitude, i.e. its actual magnitude.
Being these stars in the same region of the Small Magellanic Cloud, it could be assumed that all were almost at the same distance from Earth.
The difference between the absolute magnitude in the Cepheid of the Small Magellanic Cloud and their apparent magnitude could be then used to calculate the distance to the star using the inverse squared law: A star, as any other light source, will only show a quarter of its brightness if the distance to the observer is doubled, a sixteenth if the distance is increased fourfold, etcetera.
Since the relation discovered by Leavitt is applied to the Cepheid in general, the fact that determining the absolute magnitude would enable estimate the absolute magnitude of the others and it could be used the period-luminosity scale to find the absolute magnitude of any Cepheid variable star, and thus the distance to the star.
The problem was how to create a distances pattern from the behaviour of Cepheid because the closer Cepheid was farther enough so as to measure its distance using the parallax method.
Leavitt was set aside from the tasks she was doing because the boss in the observatory thought that her job was to gather data and not making calculations, but Ejnar Hertzsprung, in the observatory close to Berlin, took charge of it.
Hertzsprung studied the proper motion of stars, the motion in the space of the star and our Sun, of thirteen Cepheid close to the Sun, and using statistics calculated the ‘average’ distance to the local Cepheid, as well as an ‘average’ apparent magnitude. With these values he was able to calculate an ‘average’ absolute magnitude for an ‘average’ period Cepheid.
Perhaps there were too many ‘averages’, but what Hertzsprung did next was to choose a Cepheid in the Small Magellanic Cloud with the same period as his ‘average’ star. He compared the photographic brightness of the Cepheid in the Cloud with absolute magnitude it should have and calculated the distance: 3000 light-years. This distance put the Small Magellanic Cloud within the Milky Way. It is thought that it was a typo and that the distance should have been 30000 light-years. Even though the distance was well below the actual one.
Why this discrepancy? Actually it was an experimental error. The Cepheid in the Small Magellanic Cloud had been photographed using plates sensitive to the blue light while the local Cepheid had been photographed using plates sensitive to red light. This provoked a difference in the apparent brightness making that the Cepheid in the Small Magellanic Cloud look brighter and thus closer.
The North American astronomer Harlow Shapley knew how to understand the astronomical meaning of Cepheid. Working in the Mount Wilson observatory in Los Angeles, with the 1.5 m telescope, Shapley studied the globular clusters and discovered Cepheid in them. Using the Hertzsprung’s technique, and refining it, he determined the distance to the clusters, being between 50000 and 220000 light-years. It was thought that the clusters were within the Milky Way, but it also was thought that the Milky Way had a diameter of 30000 light-years so the real diameter should be larger than thought. Shapley estimated a diameter of the Milky Way of 300000 light-years being the galactic centre in the direction of Sagittarius.
Astronomers were cautious about this result, in part because they considered as unreliable the Hertzsprung’s method.
At the same time, telescopes were aiming at spiral nebulae and many astronomers suggested that they were galaxies comparable to the Milky Way full of stars because, when the light was passed through a spectroscope it was similar to the light of the stars and not to the one of a gas cloud.
In 1912, Vesto Slipher, in the Lowell observatory, took a detail look at a spiral galaxy in the Andromeda constellation and could measure its Doppler shift. The value he found impressed everybody: it was approaching at a speed of 300 km/s. Later, Slipher observed 15 more spiral galaxies and discovered that 13 of them were moving away from the Earth even faster than the approaching speed of Andromeda.
M31. Andromeda galaxy
In 1919, after having received education as lawyer and after getting a Ph.D. in astronomy and return from war, Edwin Hubble started trying to classify nebulae. Using the new 2,5 m telescope in Mount Wilson he hoped to resolve stars in the spiral galaxies, concretely in Andromeda.
Hubble focused his attention in a points of light known as novae, stars that suffer recurrent mass explosions provoking their luminosity to change (don’t mix it up with supernovae where the explosion of the full star occurs)
Through the comparison of photographic plates showing the same region of the sky, what he initially thought to be a nova he later realised that a star was increasing and decreasing its brightness periodically. It wasn’t a nova but a Cepheid!
Plate where Hubble annotated that it was a variable Cepheid and not a nova
Using the Hertzsprung’s technique, as refined by Shapley, he calculated the distance to Andromeda and got a value of 900000 light-years, what was larger that the size of the Milky Way as Shapley calculated. Andromeda was itself a galaxy!
Because he found Cepheid in spiral galaxies, Hubble made the Universe size to considerably increase. Hubble used the Cepheid to develop distance indicators for galaxies in the same way Shapley did for globular clusters.
While this was occurring in Mount Wilson, in Lowell, Slipher was still measuring Doppler shifts for spiral galaxies, including the ones where Hubble used his technique to calculate the distance.
Milton Humason joint Mount Wilson to work as assistant because of his father-in-law. When one night the telescope operator was ill he worked in his position in such a successful way that he was appointed as permanent operator as well as assistant to Hubble permanently. Humason got enough information about additional Doppler shifts from other galaxies. Hubble gathered all these data to establish a relation between the red shifts and the distances. The relation was simple: except for the closest galaxies, the farther a galaxy the faster it moved away. The rate is now known as the Hubble’s constant.
Although the values that Shapley or Hubble found by their time were rough, the precision has been increased now and currently we know that the Milky Way has a diameter of 100000 light-years and that the Andromeda galaxy is 2.5 million light-years away from us. Though the values are slightly different, the important thing is to remember that the efforts to understand the Universe made possible the development of techniques and methods that, even nowadays, are being used by the modern astronomers and astrophysicists.
As Hubble said:
But with increasing distance our knowledge fades, and fades rapidly, until at the last dim horizon we search among ghostly errors of observations for landmarks that are scarcely more substantial. The search will continue. The urge is older than history. It is not satisfied and it will not be suppressed.
References:
Galaxias. Time Life Folio
Astrofísica. Manuel Rego, María José Fernández
Acelerando la Ciencia 
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