Is the Milky Way the only one that exists in the Universe? Or does it extend far beyond it? We are now certain that the second answer is correct. However, it was first proven 100 years ago when astronomer Edwin Hubble read a paper on the distance to the Andromeda Nebula at a meeting of the American Astronomical Society.
Report which was read on 1 January
1 January is the day when everyone celebrates the beginning of the New Year, few people think about boring work and it seems that no one would think of having any meetings. However, on 1 January a hundred years ago, a meeting of the American Astronomical Society was held, at which Edwin Hubble, a scientist not very well known at the time, told how he had measured the distance to the Andromeda Nebula.
From the description it may seem that it was something uninteresting, but in fact, it was then that mankind finally became convinced that the Universe was much bigger than it seemed, and the modern picture of the world, in which the cosmos is filled with hundreds of billions of galaxies, began to take hold.
It was Hubble’s research that showed that the Universe was not limited to the Milky Way alone. But in order to understand how important that report that Hubble delivered on 1 January 1925 was, one needs to know how ideas about the cosmos have changed over the past centuries.
From the Solar System to the Galaxy
Six centuries ago, in the imagination of most astronomers, the Universe was extremely small by modern standards. At its center was the Earth, around it revolved the planets, the Moon and the Sun, and beyond them was a sphere on which hung the fixed stars.
When Nicolaus Copernicus proved in the first half of the 16th century that it was not the Earth but the Sun that was at the center of everything, for all the importance of this discovery, essentially little changed because the Universe remained confined to the Solar System.
A real revolution in the ideas about it imperceptibly took place already in the 18th century. In 1718, the English astronomer Edmond Halley meticulously studied astronomical records for several centuries and showed that the stars are not stationary, and in the following decades, scientists established that they can have different luminosity and be at different distances from us.
At first glance, it seemed that scientists had thrown humanity out of a stable and understandable Universe, which had the Sun as its center, into complete chaos with a bunch of incomprehensible objects, and the Sun was just one of them.
However, even in this gigantic chaos, they quickly found a pattern. Back in 1616, Galileo Galilei proved that the bright band of the Milky Way is a cluster of very small stars. And further observations showed that the number of luminaries increases near it.
All this allowed in the middle of the 18th century Thomas Wright and then Immanuel Kant to conclude that the stars around us form a single system in the form of a flat disc. And in 1784-1785 years William Herschel even was able to calculate the number of stars in different parts of the sky to establish the size of this disk and find out that the solar system is near its center. As we know now, he was wrong about both the first and the second, but the very concept of the “starry island” turned out to be correct and became fixed in people’s consciousness.
Nebulae and the problem of distances in space
However, there were enough objects of incomprehensible nature in space. And the most incomprehensible were light spots with not always clear boundaries, which were called nebulae. The one in the constellation of Andromeda stood out among them. Even Halley and Kant speculated that it might be similar to the Milky Way.
But, unlike the latter, it was not possible to identify individual stars in it for a long time. And this gave rise to the question of whether all nebulae are of the same nature, where they are located relative to the Milky Way: inside it, or are separate systems.
In 1864, scientists studied the spectra of nebulae and found that there are at least two types of nebulae: those whose spectrum is completely different from the stellar spectrum, and those that resemble luminaries. The Andromeda Nebula belonged to the latter.
In 1885, a supernova erupted in the Andromeda Nebula, which should have confirmed the extragalactic nature of this object, but astronomers misidentified its type. As a consequence, the distance to it was estimated to be only 17 thousand light years, which means that it was within the Milky Way.
In general, the problem of determining distances in space has remained one of the biggest obstacles in the development of ideas about our place in it. If you observe only a bunch of lights and darkness between them, how do you know which ones are near and small and which ones are far and big?
Scientists already knew that precise measurement of the stars’ motion could help. The same method that Halley used to “break” the “sphere of fixed stars”. However, this method was well suited for luminaries close to the Sun, which shifted by perceptibly large magnitudes. But for something very distant it gave too uncertain results.
That is why it was extremely difficult to estimate where nebulae with a spiral structure, such as the one in the Andromeda constellation, were located, even though scientists already knew that they consisted of stars. What was needed was a reliable method for determining distances, and it soon appeared.
Cepheids and the Great Debate
Cepheids became the key to measuring large distances in space. As early as the second half of the 16th century, astronomers discovered that some stars are variable. In 1784, John Goodricke and Edward Pigott discovered the variable nature of the star Delta Cephei.
Subsequently, it became clear that variables are of several different types. Stars whose brightness changes were similar to the Delta Cephei began to be called Cepheids. In 1908, the American astronomer Henrietta Leavitt, who had been studying variables all her life and had discovered an incredible number of them, noticed a regularity in the Cepheids. In general, they were all big old pulsating stars, clearly visible at great distances. However, their luminosity did vary. And the bigger it was, the bigger the pulsation period was.
That is, as the young Danish astronomer Ejnar Hertzsprung soon established, knowing the period of a Cepheid, it is possible to determine its luminosity, and hence the distance to it. In 1915-1916, this method was used to determine distances to Cepheids by the American astronomer Harlow Shapley.
The situation of what our Universe looked like became more and more confusing, and finally, in 1920, a discussion of the problem was organized, which became known as the Great Debate. The participants were Shapley and another American scientist, Heber Curtis.
Interestingly, in this discussion, Shapley himself defended the view that nebulae with spiral structure are inside the Milky Way, it has a gigantic size – about 300 thousand light years in diameter, and the Solar System is far from its center.
Curtis, on the other hand, insisted that the Andromeda Nebula was a separate galaxy, similar to the Milky Way, but that our star system was much smaller than it is, and that the Sun was near the center. Scientists argued about just about everything: the reliability of cepheids as a way of estimating distance, the nature of new and supernovae stars, and the dust and gas inside galaxies.
In the end, both agreed with some of their opponent’s arguments, but in general, each considered himself the winner. It seemed that the problem of what the Universe looks like was again far from being solved because each of the two scientists had created his own, relatively consistent model of it.
Edwin Hubble
But at the same time Edwin Hubble, about whom few people in the scientific world had heard before, was working on the problem. At his disposal was the most powerful telescope at that time with a mirror diameter of 254 cm. And it was thanks to it that he discovered a cepheid in the Andromeda Nebula, which everyone was tired of arguing about.
The beauty of his discovery was that he did not add anything new to the method previously used by Shapley. But he used it to refute the latter’s assertion.
From the measurements, it turned out that the distance to the Andromeda Nebula was 275 kpc, and therefore it is correct to call it the Andromeda Galaxy because it is a separate star system from the Milky Way. True, in fact, at that time this fact was not so sensational. In 1922, Ernst Öpik suggested that its flattened shape is caused by rotation, and calculated that the distance to it is about 450 kpc. And in 1923 Knut Lundmark determined from the luminosity of new stars that the distance to this star system is 1 mpc.
However, it was Hubble’s measurement that was the final point in the dispute. Shapley was wrong about the place of other galaxies in the overall picture of the Universe. Although he was closer to the truth when it came to the size of the Milky Way and the position of the Solar System within it. This is often the case when scientists talk about the big picture of the world: they may be wrong about one thing, but right about something else.
By the way, Hubble himself made a mistake in estimating the distance to the Andromeda Galaxy, underestimating it by almost three times. Because it turned out that there are two different kinds of Cepheids, and each of them has its dependence of period on luminosity. However, he was right in the main: the Universe is indeed much larger than our Galaxy.
A modern view of the Universe
The method proposed by Hubble made it possible to estimate distances to other galaxies as well. Gradually, scientists began to understand the structure of the Universe beyond the Milky Way. And, as with the stars two centuries earlier, it turned out that it was not a mess.
Galaxies unite into clusters, those into superclusters, and the latter into even larger structures – walls and sheets. At the global level, the structure resembles a spider’s web or a bee honeycomb.
It was also possible to determine where it all came from. Hubble and his Belgian colleague Georges Lemaître found that galaxies move away from each other faster the further apart they are. This is visible because the Doppler effect shifts the lines in their spectra to the red side.
This phenomenon was called redshift, and the constant underlying the mathematical law describing it was called Hubble’s constant. It was from this pattern that the Big Bang theory was born. The latter occurred about 13.7 billion years ago, gave birth to space and time, then matter and energy separated. The former formed clouds of gas, from which the first galaxies were formed a few hundred million years later.
It would seem that for the first time in several centuries, scientists have a consistent picture of the Universe, in which everything is clear and there is no room for disputes. But this impression is false because there are still many questions, only they concern not so much what we see, but what we do not see.
For example, back in the early 20th century, scientists noticed that the motion of stars in the Galaxy is different from that which can be calculated based on the gravitational influence of only those objects we can see. There appeared to be some hidden mass lurking within it. In the mid-1960s, this assumption grew into the modern concept of dark matter.
What it is, no one has seen, but it is claimed that there is much more of it in the Universe than there are stars and nebulae. The debate between those who argue that it does not exist, and supporters of several versions of what it is, has no less magnitude than the discussion between Shapley and Curtis.
In the late 1990s, in order to explain the acceleration of galaxy expansion, we had to assume the existence of dark energy. It should be even more widespread than dark matter, but even less understandable and cognisable.
The launch of the James Webb Space Telescope in 2021 has led to a new debate about the present and past of the Universe. Because it turned out that the early galaxies and black holes in them look much more mature than it should be according to current ideas. And now it is not clear whether the Big Bang scenario is false, or whether there are some much less radical ways to solve this contradiction.
Be that as it may, the paper read by Edwin Hubble on 1 January 1925 was one of the most important steps taken by mankind in understanding the Universe. But it seems that it was not the last.
