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: The Sun Rotates Round the Galaxy

The galactocentric universe

Galileo’s observation that the Milky Way consisted of many stars was the first clue that the universe was bigger than a fixed sphere. It took over 100 years, into the 1750s, before it was shown that the Sun existed as one star within the many stars making up the Milky Way, and a further 175 years for it to be accepted that the Sun went round the centre of the Milky Way.

The English astronomer, Thomas Wright, was looking for an explanation as to why the Milky Way had a circular distribution across the sky. In 1750, he published An original theory or new hypothesis of the universe in which he proposed that the Solar System (the Sun and planets) was part of the Milky Way which was the shape of a flattened disc of stars. Wright also speculated that the faint cloudy patches seen in the sky, called nebulae, are other galaxies that are a great distance away. This built on the observational work of Edmund Halley (of Halley’s comet fame) who studied nebulae and suggested that they could be ‘light created before the Sun’. The philosopher Immanuel Kant published in 1755 his work on putting these ideas into the mathematical framework of Newton’s laws and hypothesised that the nebulae were ‘island universes’. More evidence was provided in 1785 by the Herschel siblings, Caroline and William, who counted the number of stars in 683 regions of the sky and concluded that the Milky Way is indeed a disc shaped galaxy that rotated and the Sun is at it’s centre. The model remained a heliocentric one, although now the universe was at least the size of the Milky Way, and with the possibility of other galaxies existing, the universe could be much bigger. More evidence was needed.

Over 100 years later, in the early 1900s, the size and shape of the Milky Way was still a topic of debate. Jacobus Kapteyn, a Dutch astronomer, observed the positions and movements of the stars in the Milky Way and estimated that it was an oblate spheroid shape with approximately 8.5 kpc

The Milky Way Galaxy

Figure 1.3: The Milky Way Galaxy. This artist’s picture shows what we think the Milky Way looks like. Two major spiral arms can be seen attached to the ends of a thick central bar. "The position of the Sun is shown. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech).

diameter and 2 kpc thick with the Sun at 0.6 kpc from the centre. (A kpc is a kiloparsec, a unit of distance that astronomers use.) This is called the 'Kapteyn universe’. A few years later, in 1918, the American astronomer Harlow Shapley used his observations of pulsating stars (called RR Lyrae) to estimate the distance to globular clusters, which are groups of stars (there are about 150 to 200 in the Milky Way). From this he determined that the Milky Way has a diameter of about 100 kpc and the Sun is at 15 kpc from the centre. This was much bigger than the measurements of Kapteyn and put the Sun away from the centre, meaning that the Sun rotates about the Galaxy. This was evidence for the galactocentric universe. In fact, Kapteyn’s estimate was too small and Shapley’s was too large. Today, our estimate of the Milky Way is that it has a diameter of 50 kpc and the Sun is 8 kpc from the centre of the Galaxy. It took a few more years for the galactocentric model to be accepted, by about 1925, but it was a short-lived model and was quickly superseded by the work of the great astronomer Edwin Hubble.

: The Universe is Infinite with No Centre

The extragalactic universe

The next paradigm shift came from the clue that the Milky Way is not the only galaxy in the universe, there are billions (or possibly trillions) of other galaxies. We call any galaxies that are not the Milky Way ‘extragalactic’.

It starts with the American astronomer Henrietta Leavitt who, in 1912 while working as a ‘computer’ at the Harvard College Observatory, developed a technique to measure greater distances to the stars. Leavitt was working on pulsating stars called Cepheid variables and noticed that the brighter Cepheids took longer to pulse than dimmer ones. She established a relationship between the period of the pulsations and their brightness which allowed Cepheids to be used as what is known as a ‘standard candle’. This is now known as Leavitt’s Law and allowed astronomers to measure large distances that is still used today. Shapley used Leavitt’s discovery for his measurements of the distances to globular clusters to provide evidence for the galactocentric universe.

Another American astronomer, Heber Curtis, was studying nebulae which are clouds of dust and gas. His distance measurements were much larger than those of Shapley’s globular clusters, and Curtis proposed that the nebulae were big and far away and exist in other galaxies. Shapley maintained that the nebulae were small and lay in the Milky Way, and concluded that there were no galaxies beyond the Milky Way. The disagreement between the two scientists resulted in what is now called ‘The Great Debate’. The debate took place on the 26th April 1920 at the Smithsonian Institute of Natural History. The debate did not come to any conclusion. Again, more clues were needed.

The defining clue came in 1924, by the American astronomer Edwin Hubble. A new 2.5 m diameter telescope, the Hooker Telescope, had just been completed at the Mount Wilson Observatory and was the largest ever built at that time. Hubble was working on this telescope to observe Cepheid variables in nebulae. By using the power of the Hooker Telescope he could improve the accuracy of the measurements of distances to the nebulae. He found that several of the nebulae had significantly greater distances than the size of the Milky Way so they had to exist outside of it. This was the evidence needed that galaxies existed outside of the Milky Way, and the extragalactic view of the universe became accepted when he presented his findings in 1925 to a meeting of the American Astronomical Society. This was a revolutionary observation on a par with Galileo’s discoveries when looking through the first telescopes. Hubble’s clue led to the view that the universe is infinite and that our Milky Way is not unique.

: The Universe Had a Beginning and Evolves

The expanding universe

Discovering galaxies outside of the Milky Way, although important, was not Edwin Hubble’s greatest achievement. In 1929, Hubble had what is now considered to be his greatest discovery, he showed that the galaxies are moving away from us and from this he concluded that the universe is expanding. This is called "Hubble’s Law’ and is extremely important in modern cosmology.

This was another paradigm shift in our thinking about the universe. Until then, the universe had been assumed to be unchanging; it’s always been here and will continue to be here for infinity. Now, Hubble was proposing that the universe is changing, it is expanding. If the universe is expanding going forwards in time then, if we go backwards in time, it must have been contracting until ultimately there was a single point that the universe began from. This is the point at which the Big Bang happened and the universe started. A universe that started from an explosion, from a single point, is called the "Big Bang Model’ and is a key part of the current ‘Standard Model of Cosmology’. This was the start of cosmology as a topic in it’s own right, because now the universe is seen as changing in time and we can investigate these changes.

Since the universe is expanding, and had a beginning, it may also have an end. Looking into the future to determine how the universe will end is difficult and it depends on the accuracy of our detailed measurements.

There was much debate about the Big Bang model and it was not accepted by all scientists. A key proponent against it was the British astronomer, Fred Hoyle, who supported an unchanging, steady state universe and argued that matter is created between the galaxies allowing new galaxies to form as the universe expands. Hoyle rejected the Big Bang model right up to his death in 2001. Ironically, it was Hoyle who coined the phrase ‘The Big Bang’ when arguing against it on BBC radio in 1949. The name stuck even though his ideas did not. The general acceptance of the Big Bang model came in 1964 when the Cosmic Microwave Background was observed, providing a clue that the Big Bang model explains very well.

When Did the Standard Model Become Accepted?

With each new set of clues that have been discovered, our understanding of the size of the universe has changed. The Ancient Greeks believed that the stars were fixed on a sphere around the Earth. Then Copernicus showed that the stars must be at a much greater distance away because no parallax is observed when the Earth rotates about the Sun, rapidly followed by Galileo observing that the Milky Way consisted of stars. In 1785, Herschel showed that the Sun is one star within the Galaxy and the size of the universe became at least as big as the Milky Way and maybe there were other ‘island universes’ making it much bigger. Then in 1924, Hubble ended the Great Debate by providing evidence that other galaxies exist, and the size of the universe was considered to be infinite. In 1929, Hubble went on to provide the clue that the universe was expanding and the size of the universe became finite again. For the first time the universe had an age, the current estimate is 13.8 billion years. Today, we do not know if the universe is infinite or finite, but we consider that the finite size of the universe is the amount we are ever able to see, given that light travels at a fixed speed. The current estimate of the size of the observable universe is 93 billion light-years (9 x 1023 km). (A light-year is the distance light can travel in one year; 9.5 million million kilometres.)

We can see from looking at the history of cosmological theories that changing an accepted theory is a difficult process even when the evidence is there. When a major theory change occurred there has been debate, disagreements, and even threats of torture. We can also see that each theory change required new clues that either came from new technology or from a new way of thinking. It took three decades for Hubble’s Big Bang theory of an expanding universe to be accepted by the general population of scientists. It took over 50 years from the first evidence of the existence of dark matter to it being added to the Big Bang model in the 1980s. The development of a standard model took a few more years. In 1995, Jerry Ostriker and Paul Steinhardt [1] published a proposal in the journal Nature where they used the phrase the ‘concordance model’ because it was in accordance with the best cosmological measurements that existed at that time.

In 1998, the discovery of the accelerating expansion of the galaxies provided the evidence for the existence of dark energy and it was accepted as part of the model. The model became known as ‘The Standard Model of Cosmology’ and is now the leading model, although, as you will see in the rest of this book; it is not the only model and some scientists continue to look for alternative explanations to the clues. If changes to the accepted model are required, it will not be easy and it will be necessary to have new clues.

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