There’s a gap between the way scientists usually describe astronomical distances, and the units that make sense to non-astronomers. Space, as Douglas Adams so authoritatively told us, is really big. This means we need big units to describe it. You can measure the distance from Earth to the Andromeda Galaxy in centimeters if you really want to, but you end up with a lot of pointless and confusing zeros at the end.
The units for cosmic space that the public are most familiar with are light years. This, as the name suggests, is the distance light will travel through a vacuum in the time it takes for the Earth to orbit the Sun. True, it is well beyond the range of most people’s capacity to envisage, but at least the name is familiar. Moreover, the nearest star to us, other than the Sun, is a little over four light years away, so it gives a sense of scale when talking about more distant stars.
Light years as a unit also got a rather comprehension-handy boost from Monty Python's The Meaning of Life, where the galaxy is described as being; “A hundred thousand light years side to side.” That figure was not only quite accurate at the time, but hasn’t been changed much by subsequent research, despite the discovery of some rare outlying stars.
Light years also instantly tell us something intriguing, if not always useful – how long the light has taken to reach us. Thus, when we say that a supernova we just saw is 21 million light years away we don’t need to do any recalculations to know it exploded 21 million years ago.
Indeed, light years are such useful units that sometimes it’s handy to use their derivatives, measuring the distance between stars in light months, or the distance to the Voyager spacecraft in light hours.
It’s the unit we at IFLScience use most often for astronomical stories. If you read the original scientific papers we’re reporting on, however, light years seldom get a mention.
Instead, the three most referenced units for astronomical distances are astronomical units (AU), parsecs (pc), and redshift (z). Now and then, we and other popular science sites will use these as well, particularly AU, so it’s useful to know what each means.
Before we start, however, it’s worth noting how Earth-based all these are. No alien civilization would use these units (redshift aside), because their planet would be a different distance from its star and take some other time to complete an orbit. In other words, our local conditions are imprinted in the way we measure the universe. Our view of the universe is seldom objective, it’s shaped by where we come from.
Astronomical units are used for measuring distances within the Solar system, and sometimes within other star systems. An AU is equal to the average distance between the Earth and the Sun: 150 million km, 93 million miles, or 8.3 light minutes. This has several advantages. Most of us may not know exactly how far we are from the Sun, but it’s still a useful measuring stick for assessing whether a passing object is a threat, or understanding why other planets are so much hotter or colder. Meanwhile, light years are too large to describe these sorts of distances well, like trying to use miles to measure the width of your finger.
Parsecs are a much less intuitive unit. A parsec is the distance a star must be to show a parallax of one arcsecond. That is, it appears to move one 3,600th of a degree against more distant stars, as the Earth switches between opposite ends of its orbit.
That’s hardly a snappy description, and it’s also not obvious why it is a better unit than the light year. One parsec is equal to 3.26 light years, so it’s not as though it is sufficiently bigger or smaller to be a much better way of describing certain distances. Indeed, parsecs are so poorly understood that the original Star Wars film appeared to use them as a measure of time, not distance – although a very clever explanation has since been invented to save face.
Despite this, most papers refer to the nearby stars in parsecs, distances within the galaxy in kiloparsecs, and nearby galaxies’ distances in megaparsecs. Converting to light years just involves dividing by 3.26.
The reason astronomers prefer parsecs seems to be more out of force of habit than any advantage over light years. Some might cynically argue that it’s a holdover from the days when using jargon that excluded non-specialists was considered desirable, not a hindrance to public appreciation of science.
The reasons for using redshift are more evident, but it’s still the most confusing unit of all.
Redshift is a product of the expansion of the universe. As the universe grows, galaxies move apart from each other, which leads to light from them being shifted to longer wavelengths. For nearby galaxies, this shift is small and can be overwhelmed by localized factors so that some galaxies are even moving towards us (or, more accurately, us towards them).
However, at greater distances, the further away a galaxy is the more the wavelength will be lengthened, or redshifted. The relationship between the redshift and the distance is not known perfectly, depending on a still uncertain measure known as the Hubble Constant. Since in most cases it is the redshift we can measure, it makes sense to describe a galaxy this way, rather than its distance in light years or parsecs. Any translation would be imperfect.
More confusingly still, converting redshift into light years is not straightforward. It depends on a variety of assumptions about the universe, which are not universally agreed on. Different online calculators exist, and they don’t always produce the same answers, because they see these answers differently.
As a result, when popular science articles talk about the distances to galaxies from the early universe, there are several layers of uncertainty to the figures.
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