In 1917, Heber Curtis observed a stellar nova in what was then called the Great Andromeda Nebula, leading to the discovery of the Andromeda Galaxy. Edwin Hubble later measured its distance to be 2.5 million light years away, beginning the era of extragalactic astronomy. The discovery and investigation of quasars also contributed to the field, revealing them to be active galactic nuclei. Today, millions of galaxies have been photographed and classified, with an estimated 100 billion in the observable universe.
The dawn of extragalactic astronomy was in 1917, when the American astronomer Heber Curtis observed a stellar nova within M31, the formal name for what was then called the Great Andromeda Nebula. At the time, spiral nebulae such as Andromeda were thought to lie within our own galaxy, only several times the size of our solar system and less than 50,000 light-years away. They thought the Milky Way represented the entire universe.
After observing the nova in M31, Curtis searched the photographic record, noting 11 other novae in the region. If M31 was just a stellar nebula, why were there so many novae in it, and why were these characteristically dimmer than other novae? Reasoning on the observation that these novae were about 10 magnitudes fainter than known novae in our galaxy, Curtis argued that the Great Andromeda Nebula was in fact an “island universe,” distinct from the Milky Way and located 500,000 light-years away. . At first, astronomers did not accept his hypothesis and a scientific debate began.
In 1920, Harlow Shapley, another American astronomer, challenged Curtis to a big debate on important astronomical questions of the day, including whether spiral nebulae like Andromeda were really outside our galaxy. Many fellow astronomers have followed the debate, but the final results have been inconclusive. It was not until 1925, when Edwin Hubble (for whom the Hubble Space Telescope is named) published observations from the 100-inch Hooker Telescope, then the largest in the world, that he had discovered Cepheid variable stars in the Andromeda nebulae and used them to measure its distance, it turned out to be a whopping 2.5 million light years. The era of extragalactic astronomy had begun and the Andromeda Nebula was renamed the Andromeda Galaxy.
For the past 80 years, extragalactic astronomy has been an active area of research. By measuring the relative speed of galaxies using their optical signature, it has been found that all galaxies are moving away from each other and the entire universe is expanding. In 1998, observations of the type Ia supernova even suggested that the expansion is accelerating. Cosmologists now think it is likely that the universe will end up in a “heat death” in which the accelerating expansion causes all matter to scatter and freeze.
An important episode in extragalactic astronomy is the discovery and investigation of quasars, QUAsi-star radio sources. These point sources of light were known to be very bright and very remote, among the most distant objects known, some as far as 13 billion light-years away. Although quasars were first observed in the 1950s, it wasn’t until the 1970s that a scientific consensus began to emerge about the nature of quasars: they were active galactic nuclei, made up of supermassive black holes sucking in several solar masses worth of material per century and releasing massive amounts of radiation in the process. Formal models have been built to describe it, and one of the greatest mysteries of extragalactic astronomy has been solved.
Today millions of galaxies have been photographed and classified by scientists, sometimes even with the help of the public (as in GalaxyZoo). Galaxies are spiral or elliptical. It is estimated that there are about one hundred billion galaxies in the observable universe. Interestingly, this is about the same as the number of neurons in a human brain.
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