The existence of celestial bodies outside the Milky Way
Prof. Dr. Karl-Heinz Kampert / Astrophysics
Photo: UniService Transfer

The discovery of a new galaxy

Astrophysicist Karl-Heinz Kampert on Edwin Powell Hubble's calculation of the existence of celestial bodies outside the Milky Way Edwin Powell Hubble revolutionized the world in 1923 with a groundbreaking calculation. Who is this man?

Kampert: Hubble was an extraordinary personality and founded the field of extragalactic astronomy and observational cosmology. Born in 1889 in Marshfield, Missouri, the son of a successful insurance salesman, he initially studied mathematics and astronomy at the University of Chicago, graduating with a Bachelor of Science degree. At the request of his dying father, he then went on to study law at Oxford (UK), graduating with a Master's degree just three years later. After the death of his father, he continued his astronomy studies in Chicago in 1914 and received his doctorate there in 1917 with the topic "Photographic investigation of faint nebulae". In the same year, Germany declared war on the USA, which prompted Hubble to finish his dissertation early in order to go into military service. After the end of the war, he spent another year in Cambridge (UK) and at the age of 29 he finally accepted a position at the Mount Wilson Observatory near Pasadena. At this time, the world's largest telescope with a diameter of 2.5 m was put into operation there, with which Hubble was soon to shatter the concept of the expansion of the universe. This first groundbreaking discovery was to be followed by others. He remained loyal to the Mount Wilson Observatory until his death at the age of just 63.

Despite his groundbreaking work, Hubble did not receive a Nobel Prize, as astronomy was not considered by the Nobel Prize committee at the time. During his advanced career, he campaigned hard for astronomy to be considered as a sub-discipline of physics when the Nobel Prize was awarded. However, this commitment was only crowned with success shortly after his death. Hubble's work was posthumously recognized as worthy of the Nobel Prize.

In addition to his scientific talent, Hubble was also considered a gifted athlete in many sports and led the University of Chicago basketball team to their first national title. He is also said to have been an excellent boxer. However, no evidence of this can be found.

The universe with 100,000 light years is the Milky Way. But that was thought to be the end of it, until Edwin Powell Hubble proved on December 7, 1923 that the Andromeda Nebula lies outside the Milky Way. How did he manage that?

Kampert: As is so often the case in science, it was a chance observation with far-reaching consequences: Hubble had been studying so-called nova stars, which - as we know today - suddenly shine much brighter due to thermonuclear explosions and then die down again. Through repeated observations, he noticed that the brightness of one of his suspected nova stars changed periodically every 31 days and that it was therefore a Cepheid-type star. Stars of this type had already been discovered 10 years earlier by the astronomer Henrietta Leavitt and were suitable for measuring distances in space due to a fixed correlation between period and luminosity. By applying this relationship found by Leavitt, Hubble was able to calculate that the star must be almost a million light years away, i.e. 10 times further away than the extent of our Milky Way. Since the star was in the direction of the Andromeda Nebula, Hubble was able to conclude that this nebula was not a nebula on the edge of the Milky Way, but rather a galaxy in its own right.

What exactly is the Andromeda Nebula?

Kampert: The Andromeda galaxy, also known outdatedly as the Andromeda Nebula, is the closest spiral galaxy and at the same time the most distant object that can be seen with the naked eye as an extended elliptical object under good conditions. With good binoculars you can even see the structures, which impressed me personally as a chance observation during a night in a mountain hut when I was a student. The Andromeda galaxy is home to a similar number of stars as our Milky Way and together they form the center of mass of the local group, a collection of about 70 dwarf galaxies within a diameter of about 5-10 million light years. The Milky Way and Andromeda galaxy are on a collision course due to their mass attraction. However, this lasting event is not expected to occur for several billion years and the two galaxies will merge to form an elliptical galaxy.

Edwin Hubble and the Andromeda galaxy, © Giuseppe Donatiello /
Photo: public domain, historical

This is the first time that Hubble has demonstrated that the universe consists not only of our galaxy, but...?

Kampert: The discovery of the Andromeda galaxy as a neighboring galaxy to the Milky Way not only increased the size of the known universe at the time many times over, but literally degraded the Milky Way overnight to an ordinary galaxy among several hundred billion other galaxies in the universe. In the course of history, this was a further step towards modesty: first the Earth was considered the center of the universe, then the sun, then the Milky Way, and with Hubble's discovery, the Milky Way finally had to relegate itself to the mediocrity of the universe. The assumption that the spiral-shaped nebulae in the night sky could be "islands of worlds" like our galaxy had already been published anonymously by the philosopher Immanuel Kant in 1755. Hubble's observation finally proved it.

However, the process of acceptance of these groundbreaking results in the scientific community was to take years: Hubble first published his findings in the New York Times in November 1924, reported on them for the first time to the American Astronomical Society in January 1925, and it would be another 4 years before they were published in a scientific journal.

Six years later, in 1929, he also calculated that the universe was expanding and how fast. So the cosmos must have been closer together earlier. What is still happening today?

Kampert: During these 6 years until the final publication of his results, Hubble determined the distances to a further 24 extragalactic nebulae in countless additional nights of observation using various methods. In addition to their distances, spectroscopic data, i.e. atomic absorption spectra, were also available for these nebulae. From the observed shift of the characteristic absorption lines relative to laboratory measurements, the velocity of the star systems relative to the Earth could be determined in analogy to the Doppler shift in sound. Hubble plotted the relative velocity of these nebulae as a function of their distance and established a linear relationship: the further away the nebulae were, regardless of the direction, the faster they appeared to move away from us, i.e. escape velocity = H * distance! The quantity H denotes the Hubble constant and the equation itself is known as the Hubble law. The result is astonishing: Are we at the center of the universe after all? And why are all galaxies pointing away from us? Moreover, if the galaxies will be further away tomorrow than they are today, they must have been closer yesterday, i.e. everything must have been created a long time ago in one point - the Big Bang! The consequences were once again revolutionary. The assumption of a dynamically expanding universe made by the Belgian physicist and theologian Georges Lemaitre in 1927 was thus confirmed by Hubble and his observation was the first pillar of today's Big Bang model. In retrospect, Hubble's measurements were wrong by a factor of almost 10, but the law as such was nevertheless correct. Also, of course, we are not at the center of the universe, but it is space itself that is continuously expanding for all eternity, so we would make the same expansion observation everywhere in the universe.

About 10 years ago, historians of science discovered that the Swedish astronomer Knut Lundmark had discovered the same evidence of an expanding universe as early as 1924, and with much greater precision than Edwin Hubble. However, as Lundmark's methods of measuring distances were not generally accepted at the time, his measurements were virtually ignored.

And yet the universe is finite. As a human being, you can hardly comprehend this, much less imagine it. Can you explain that?

Kampert: It is indeed hard to comprehend, because our limited horizon of experience means that we can neither imagine the dimensions of the universe nor a dynamic space. Instead, we are used to moving in a static three-dimensional space, determining locations and recording the temporal sequence of events in this space. While in Albert Einstein's special theory of relativity, space is still considered separately from time, in the general theory of relativity they merge to form what is known as four-dimensional space-time; time becomes an inseparable part of space. The Dutch astronomer Willem de Sitter finally showed that Einstein's equations also allowed for the general case of curved dynamic spaces. This insight in turn prompted Einstein to describe a quantity he had artificially introduced to maintain a static universe as his "greatest folly". So we are in good company with our limited imagination about space and time!

The finite nature of the universe is nevertheless an inevitable consequence of the finite age of the universe of around 14 billion years. In the Big Bang, time and space were created simultaneously and since then this space has been expanding with us somewhere in it. Nevertheless, there is no "edge" of the universe, simply because there is no space outside the universe in which this edge could be perceived.

Younger generations know its name mainly because of the Hubble Space Telescope named after it, which is to be used until 2026. Where does it move and what does it measure?

Kampert: The Hubble Space Telescope has been observing space for 33 years and, with a length of more than 13 meters and a weight of around 11 tons, is about the size of a school bus. It orbits the earth every 96 minutes at an altitude of around 550 km. Its centerpiece is a mirror with a diameter of 2.4 m, on which 5 instruments detect light from the ultraviolet to the near infrared range with the highest resolution. A special feature of the Hubble telescope is that its orbit allows it to be flown to with the NASA space shuttle in order to carry out service work. This was also necessary right at the beginning, as the telescope - as was only discovered in orbit - was "course-sighted" due to adjustment and grinding errors in the primary mirror and did not provide sharp images. Fortunately, these were quickly rectified and since then the Hubble telescope has provided us with an incredible number of discoveries and insights, the list of which would go beyond the scope of this interview. However, in connection with Edwin Hubble's observations, I would like to mention the measurements made by an American team that led to the realization that the universe has been expanding at an accelerated rate for several billion years. The Nobel Prize in Physics was awarded for this in 2011.

Like a 33-year-old school bus, the Hubble telescope is now showing signs of wear and tear, which has led to the failure of some components. However, Hubble is expected to remain in operation until at least 2026, and perhaps even until the mid-2030s if the orbit, which has since dropped, can be raised again. The Hubble telescope is to be replaced in the near future by the Nancy Grace Roman Space Telescope, which is provisionally scheduled to be launched in May 2027 and also has a 2.4-meter primary mirror. The James Webb Space Telescope was already launched in December 2021 and extends Hubble's optical measurements into the infrared range, which is particularly important for observations of the early universe. We can therefore continue to hope for many exciting results and new findings.

Uwe Blass

Prof. Dr. Karl-Heinz Kampert studied physics at the Westfälische Wilhelms-Universität Münster from 1977 to 1983. From 1983 to 1986, Kampert was a research assistant at the Westfälische Wilhelms-Universität and received his doctorate in 1986. He then spent three years as a postdoctoral research fellow at the large-scale research facility CERN in Switzerland. From 1989 to 1995, he was an assistant professor of physics at the University of Münster, during which time he qualified as a professor in 1993. He then taught as a professor of physics at the University of Karlsruhe and the Karlsruhe Research Center, which both merged to form the Karlsruhe Institute of Technology in 2009. He has been teaching experimental physics at the University of Wuppertal since 2003.



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