Expanding Horizons: From Spiral Nebulae to Galaxies


Astronomy and Cosmology have for a long time, been one of the most debated realms of science. Observations paved the way to theories and arguments through the origins of the universe, up to the classification of stars. Until a not-so-distant past, galaxies such as “Andromeda” were simply termed nebulas. With the existence of other galaxies put in question; “Whether our galaxy the Milky Way, was in fact, the only existing galaxy in the universe?” was one such topic. 

After the observations made by William Herschel where he cataloged nebulae, it was in speculation as to whether these could exist independent of the Milky Way due to their size. The study of nebulae using high-powered telescopes was then spearheaded into the interests of astronomers over the globe. However, laying the foundation stone to clear this debate took place in the lonely deserts of Arizona.    

Shifts in the Spectrum

The 19th century saw a revolution in how we analyze astronomy with the theorem of the relative shift of frequency outlined by the Austrian physicist Christian Doppler. Now known as Doppler’s effect, we can represent this mathematically as follows (in a non-relativistic sense).

Doppler's Frequency Shift Formula
Doppler’s Equation in a non-relativistic sense

Here “ce is the speed of propagation of waves in the considered medium while “vi denotes the speed of the preceptor. The preceptor mentioned here is either the observer or the emitter. Here, when the observer is moving towards the emitter the numerator takes the value “(ce + vobserver)”. When the emitter is moving towards the observer the denominator takes the value and “(cevemitter)” and vice-versa.   

Assuming that the Earth is in an inertial frame, we can observe that if a star is moving away from the earth, then the frequency observed is lower. That is, the wavelength is shifted towards the red wavelengths. Conversely, if a star is moving towards Earth it can be seen to be shifted towards the blue wavelengths.

Representation of Doppler shift by considering absorption spectra

Later on, by analyzing the absorption spectra of stars and applying Doppler’s theorem, William Huggins was able to determine the velocity of a star moving away from Earth.

Spiral Spectographs

In the late 19th century, American astronomer Percival Lowell took in a young Vesto Slipher as his assistant in his observatory. A dedicated mathematician and budding astronomer, Slipher was tasked with examining the absorption spectra of stars obtained from the outer spirals of then observable nebulae. By considering the speeds and directions of motion of the stars to earth, a picture of the relative motion of the solar system and the nebulae could be estimated.

Upon examination, it was a surprise to find that certain nebulas moved at unimaginably high speeds towards the earth. According to Slipher’s calculations, the Andromeda nebula was on a collision course with the solar system at a speed of 300 km s-1. With the estimated size of the Andromeda nebula, panic at this point rose as a seemingly catastrophic event was upon us.

By the 20th century, Slipher’s analysis of spectrographs obtained by numerous other nebulae showed that only a few moved towards the solar system while most nebulae moved away from it. This and the speeds with which they were moving led to the spark of a scientific debate.

The Newest Big Debate

In 1920, the Smithsonian Museum, one of the most historically significant buildings to science, saw a massive gathering of brilliant minds. Harlow Shapley was seen debating for the universe being limited to just our Milky Way. His opposition was led by Heber D. Curtis; who used Slipher’s findings to debate that the so-called nebulae were in fact, systems of stars independent of the Milky Way.

A paper in 1929 titled, “A relationship between the distance and radial velocity among extra-galactic nebulae” published by one of the most world-renowned astronomers, Edwin Hubble, settled this debate. Using the then-new Hooker telescope at the Mount Wilson Observatory in the rocky hills of California, he proceeded to show that by the use of Cepheid stars, certain receding nebulae were moving at speeds too fast for them to be contained within the known boundaries of the Milky Way galaxy. Thus, the scientific community agreed on the hypothesis that extra-galactic nebulae, or “galaxies” do in fact exist.

Thus, a terminology switch from nebulae to “galaxies” took place from then onwards. This was the first step for Hubble to show that the whole universe was in fact expanding; and that it was expanding at an unprecedented rate.


Hughes, D. W., Dinwiddie, R., Johnson, P. & Jackson, T., 2017. In: The Astronomy Book. New York: Dorling Kindersley Limited, pp. 156-161, 172-177.

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01. Featured Image: https://bit.ly/35UB9Wr
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