Nineteenth-Century Developments in Measuring the Locations and Distances of Celestial Bodies

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Nineteenth-Century Developments in Measuring the Locations and Distances of Celestial Bodies

Overview

At the beginning of the nineteenth century, astronomers knew how far away some objects in the solar system are, but not how far away any of the stars are. During the nineteenth century they measured accurate positions for thousands of stars and gathered their data in catalogs. Their measurements were precise enough to reveal tiny shifts in stellar positions. These shifts resulted not from the motions of the stars but from the motions of Earth in its orbit, and this knowledge allowed several astronomers in the 1830s to measure the distances to several of the closest stars. These distance measurements were the beginning of the work that is at the heart of modern cosmology: the discovery of the size and shape of the universe.

Background

During the nineteenth century, universities and observatories became important resources that allowed scientists to carry out large projects like star catalogs. The first three people to measure the distance to a star—Friedrich Bessel, Thomas Henderson, and F. G. W. Struve—all worked for organizations that supported their work and supplied the fine instrumentation they used. Astronomers were able to use the resources of their schools and observatories to work together to produce accurate and complete catalogs of the stars. All of this astronomical activity led to several important discoveries of how the positions of the stars in the sky change as a result of the motion of Earth around the Sun.

One of the effects of the way that Earth orbits the Sun is that over the course of a year nearby stars show a tiny annual back-and-forth motion called parallax. Parallax provides the most direct way to find the distance to a star. You see something like parallax when you walk from one place to another and notice that a nearby object like a tree seems to be in a different position against more distant background objects, say to the right of a building rather than lined up with the building. The farther you walk and the closer the tree is to you, the more it will seem to shift against the background. For many years astronomers knew that stars should show the same kind of change in position, and if they could measure a star's parallax they could calculate its distance. The distance we move (from one end of Earth's orbit to the other) is the same for all stars, and the closer a star is to us, the more parallax we should see compared to more distant stars. We see parallax not because the stars themselves are moving, but because our view of them is. The apparent changes in a star's position are far too small to be visible to the naked eye, or even to be easily measurable. Astronomers from Galileo onward tried to measure parallax, but it wasn't until the 1830s that astronomers had the tools that could do the job.

Before the apparent motion due to parallax could be measured, astronomers had to identify and sort out other effects of Earth's motion. James Bradley (1693-1762) discovered small effects called aberration and nutation that other astronomers then took into account in making their parallax measurements. Without taking these tiny effects precisely into account, astronomers would have made inaccurate measurements. Furthermore, many star catalogs recorded the motion of a star across the sky, called its proper motion. Proper motion is due to the motion of the star relative to the Sun. Astronomers measuring the distance to stars chose to observe stars with a large proper motion, guessing that these stars might be closer and show a larger (and easier to measure) parallax. Without the catalogs to point them toward likely targets, their work would have been much more time consuming and difficult.

Astronomers in the nineteenth century also investigated the Sun. Joseph von Fraunhofer (1787-1826) studied the Sun and developed the heliometer, an instrument that measures the width of the Sun on the sky very precisely. Friedrich Bessel (1784-1846) used this instrument to measure a star's parallax and calculate its distance.

Bessel used the heliometer to measure the position of a star called 61 Cygni relative to a dimmer star nearby. His own catalog showed 61 Cygni to have a large proper motion. He guessed (correctly, as it turned out) that the dimmer star was much further away and would show no measurable parallax, and so could be considered an unmoving reference point for measuring 61 Cygni's parallax. He compared the positions of the two stars at different times of the year, six months apart (when Earth was at opposite ends of its orbit). He was able to measure 61 Cygni's parallax and calculate its distance, and he found that 61 Cygni was about ten light years away (a figure only about 6% higher than the currently accepted distance). At around the same time, two other astronomers announced that they had used parallax to calculate stellar distances as well. Thomas Henderson measured the distance to Alpha Centauri (the closest star to the Sun), and F. G. W. Struve (17931864) did the same for Vega. By the end of the century astronomers had used parallax to measure the distance to approximately thirty stars.

Impact

Bessel's work, along with that of Struve and Henderson, proved that astronomers could measure the distances to stars using parallax. Other parallax measurements followed, first a trickle and later a flood. Struve, like Bessel, used a relatively sophisticated instrument that was not widely available at other observatories, but Henderson used a fairly common piece of equipment, so other observatories were able to begin making their own parallax measurements. In the late 1880s astronomers in England began to use photography to measure parallax and found that it took less work and was more accurate. In the twentieth century a satellite measured the parallax of thousands to stars.

As scientists measured parallaxes and calculated distances, they realized that the brightest stars were not, as you might expect, always the closest. This was a clue to them that not all stars are the same. If a star has no measurable parallax, it must be relatively far away. If it still appears very bright to us despite the distance, it must be a true powerhouse of a star. Later astronomers used other tools to classify stars based on their brightness and other properties. Their work relied upon this discovery about differences between stars.

Distance measurements, along with the positional information contained in the great catalogs of the nineteenth century, enabled astronomers by the end of the century to begin to try to map out the parts of the universe that we could see. Although astronomers were not yet sure that the Milky Way was a stellar system separate from other systems (which we call galaxies today), they were able, in the 1870s, to see that the Milky Way rotated.

Once astronomers began to measure the distances to stars, they could start to use a technique known as the standard candle. Sometimes all the stars (or galaxies) of a particular type are of roughly the same brightness, and that type of star or galaxy can be used as a standard candle. Any time you see a star of that type, you know how bright it really is. This is different from how bright it appears from Earth. The brightness that we see in the night sky is a combination of the star's true brightness and its distance from us. The true brightness, the apparent brightness in the sky, and the distance are like three pieces of a puzzle; if you have any two of them, you know what the third piece is. So if you know what type a star is, and that type has a constant true brightness, you can measure the apparent brightness that we see and calculate its distance. This method is a powerful one for determining cosmic distances, and it is still being refined today. But the first step in getting it to work is to find the distance of at least a few "standard candle" stars so that you can use the distance and the apparent brightness to find the true brightness. Thus, distances found through parallax are the first step in figuring out distances to much more faraway stars and eventually galaxies.

In the late twentieth century a European satellite named Hipparcos mapped the skies with unprecedented accuracy and measured the parallaxes of thousands of stars. Since Hipparcos was above the distorting effects of Earth's atmosphere, it could measure very tiny changes in position. The data from Hipparcos are being used to answer some important questions in cosmology by recalculating the distance to some of the known "standard candle" stars, which are crucial links to determining the age and size of the universe.

Using various standard candles, astronomers in the twentieth century extended their distance scale until now we measure distances to galaxies that are billions of light years away. The rate at which these galaxies are moving away from us is relatively easy to measure. The distances are harder to measure but are very valuable to know because they help astronomers determine how fast the universe is expanding and how old it is, both necessary to explaining the origin and life history of the universe.

MARY HROVAT