This blog post delves into the historical debate over whether nebulae are stellar birthplaces or other galaxies beyond our own, and details the astronomical discoveries that resolved this question.
The size of the universe has long been a subject of human fascination. To determine it, astronomers sought to measure the distances to distant stars. In the late 18th century, Herschel made the assumption that all stars have the same intrinsic brightness. He then used the fact that their apparent brightness, as observed from Earth, dims in proportion to the square of the distance to roughly estimate the distances to stars. This revealed that stars are not uniformly distributed throughout space. Instead, they cluster together in a structure that is generally flat like a disk, with a bulge protruding upward and downward at its center. Observing from Earth, located within this structure, one would see many stars in directions parallel to the disk’s flat surface, and relatively fewer stars in directions perpendicular to it. These observations precisely matched the characteristic band-like appearance of the Milky Way in the night sky. Inspired by this, astronomers began calling this vast collection of stars, which includes Earth, the ‘galaxy’.
After confirming the clustering of stars, curiosity grew about whether the space beyond was empty or another region containing celestial bodies, leading to a surge of interest in nebulae. When viewed through a telescope, nebulae appear as diffuse patches, unlike stars which show as distinct points of light. For a long time, it was debated whether nebulae were structures composed of dust and gas within our own galaxy, representing the early stages of star and planetary formation, or whether they were separate galaxies themselves, containing vast numbers of stars like our own Milky Way.
Scholars advocating the first hypothesis cited the fact that nebulae are frequently observed outside the flat plane of a galaxy but are rarely seen within it. According to them, since nebulae represent the early stages of star formation, they are scarcely observed within the flat plane of a galaxy where star formation has already concluded and numerous stars exist. Conversely, proponents of the other hypothesis noted that when viewed from afar at an angle, disk-shaped celestial bodies like our Milky Way appear elliptical. Since many nebulae actually exhibit elliptical shapes, they suggested these could be independent galaxies, similar to our own Milky Way. According to this view, the reason nebulae are scarcely observed within the flat plane of our galaxy, despite being evenly distributed throughout the universe, is that the stars, dust, and gas filling that region obscure the faint light of nebulae, preventing them from entering our field of view.
Ultimately, measuring the distance between Earth and a nebula would determine which of these two hypotheses is correct. The breakthrough for such distance measurements came from the study of variable stars, whose brightness fluctuates. Among variable stars are binary stars. In a binary star system, where two stars of different brightness orbit each other, the observed brightness changes at different times: when the stars do not eclipse each other, when the brighter star passes behind the dimmer star, and when the dimmer star passes behind the brighter star. In this case, the brightness changes symmetrically over time. However, another type of variable star also exists, whose brightness changes asymmetrically over time. This asymmetrical change is not caused by two stars eclipsing each other, but arises from fluctuations in the energy emitted as the star periodically expands and contracts due to an imbalance between gravitational and radiative pressures within the star. Such variable stars are called ‘Cepheid variables’.
In the 1910s, 25 Cepheid variables were discovered in the Magellanic Clouds. It was found that the brighter their maximum luminosity, the longer their period of brightness variation, and that a constant mathematical relationship exists between these two physical quantities. If this relationship applies to all Cepheid variables, then knowing the distance to one Cepheid variable allows the distance to another to be calculated. This is done by first determining its intrinsic brightness from its period, then comparing this to its apparent brightness. Based on this method, by finding a variable star belonging to a particular nebula and measuring its distance, the distance to the nebula itself could also be determined.
In the 1920s, Hubble discovered Cepheid variables in the Andromeda Nebula and calculated its distance, revealing that the distance between Earth and the Andromeda Nebula was ten times the diameter of our Milky Way galaxy. This became decisive evidence clearly demonstrating that the Andromeda Nebula was an independent galaxy existing outside our own Milky Way, thereby expanding the scope of the universe from a single galaxy to the external galactic world. This discovery marked a turning point in the study of cosmic structure in modern astronomy and subsequently became the foundation of external galaxy cosmology.
