How do astronomers know how far away that galaxy is? For starters, it would help if a star would explode.
Some cosmic distances, such as the distance to the nearest asteroid, can be measured simply by using radar.
- Larger distances depend on spotting an exploding star in a distant galaxy, or even measuring the expansion of space-time itself.
The astronomical community is buzzing over the discovery of a Type 1a supernova in a nearby galaxy called M101 -- also known as the Pinwheel Galaxy.
There are actually several types of supernova but those of the Type 1a variety are not the result of the explosion of a massive star, but the destruction of a white dwarf orbiting a younger binary partner, or two white dwarfs slamming into one another. In the former case, material is stripped from the binary partner until the material reaches "critical mass" around the white dwarf. At that point, the supernova ignites.
Supernovae of this type have been observed for decades and we have learned that they are very predictable in the way they evolve. For example, it's easy to calculate how much energy they are pouring into the Universe by studying the way their output of energy changes with time. This makes them very useful as a tool to measure cosmic distances.
But they are only good for calculating distances to the galaxies in which they occur. Fortunately, there are a number of other ways that we can measure distances in space and Type 1a supernovae are just one of them.
Radar and Parallax
Taking a step back, it's easy to measure distances within our solar system as we can bounce radar signals off the nearest planets and, knowing the speed an electromagnetic signal travels (the speed of light) and timing how long it takes for the signal to get to the object and back again, we can get a very good measure of these interplanetary distances.
Measuring the distance to the nearest star, on the other hand, can be a little tricky.
If we were to send a radar signal to the nearest star, Proxima Centauri, it would take about 4 years to get there and 4 years to get back and even then, the signal would be incredibly weak and impossible detect. Instead we use a technique called "parallax."
The concept of parallax measurements of the stars is simple. It takes a year for the Earth to complete one orbit around the sun, so we take a measurement of the position of a nearby star in the sky in January for example, and then repeat the exercise six months later in July.
The Earth will be at extremes of its orbit and so any nearby stars will seem to have shifted a tiny bit in the sky. From knowing the diameter of the Earth's orbit and the apparent angular movement of the star, we can calculate the distance to it. Easy.
In the same way, it's possible to measure the length of your arm! Extend your arm and point one finger up toward the sky, now shut one eye and notice where your finger is with respect to background objects, open that eye and then close the other and notice its position again. Your eyes are simulating the position of the Earth at the extremes of its orbit and your finger, the nearby star.
Knowing the distance between your eyes and the angular shift of your finger, you could calculate the length of your arm. Useful! Of course it's easier to just measure it with a rule but we don't have that luxury with nearby stars.
For more distant objects, using the parallax method becomes more and more difficult the further the objects are. For example, distances to some of the nearby galaxies cannot be deciphered using the parallax method, but it is possible to measure their distances using 'standard candles.'
Type 1a supernova, like the one in M101, is one type of standard candle -- the amount of energy released by this type of supernova is assumed to be the same, no matter where they are located in the Cosmos. Another type of standard candle is a type of star called a Cepheid variable. Cepheids are a special type of star that pulsates in a predictable way, and like the Type 1a supernova, they are assumed to be the same, no matter where they are located.
Both types of objects can be easily identified by the way their output of light varies over time and once they've been identified, it's easy to calculate the actual amount of energy, or light, that it should be giving out.
Now it's a simple step to compare the actual energy output against the observed (or "apparent") output from measuring it in the sky using something called the "Inverse-Square Law." Using this mathematical formula, you can calculate how far away the standard candle is. The distance to many nearby galaxies have been measured using Cepheid variable stars, whereas more distant galaxies have been measured using Type 1a supernovae.
One final way that cosmic distances are calculated is to employ a technique that relies on the way that light propagates throughout the Universe.
If light from an object is looked at through a spectroscope, not surprisingly, its spectrum can be seen, and if certain gasses are present, then they will show themselves by dark absorption lines against the spectrum.
Specific gasses have very specific absorption line patterns making their presence very easy to detect. If the object, and indeed the observer, was stationary then the absorption lines will be in their rightful, predicted place in the spectrum, but if they are moving relative to each other, then the absorption lines become shifted.
If the distance between the objects is increasing, then the lines will have moved toward the red end of the spectrum (red-shifted) and if the distance was decreasing then the lines would be moving toward the blue end (blue-shifted).
However, most stars and galaxies are red-shifted from our perspective. The mechanism that makes this happen is related to the way the Universe is expanding. When the light was emitted, the absorption lines were in their rightful place in the spectrum, but by the time the light got to the observer, the Universe had expanded, and the spectrum of the light had shifted.
By measuring the amount of red-shifting, it's possible to calculate how fast the galaxies are moving apart and also how far away they are. The formula for this is pretty tried and tested, unfortunately it relies on one of sciences only variable constant, the Hubble Constant. We are narrowing down the exact value of this but until then, the distant estimates using this technique are often have some big error margins!
We often see reports in newspapers and scientific journals that refer to distances of tens, to hundreds to millions -- even billions -- of light-years. But as you can see, astronomers have some pretty tried and tested methods for measuring distances in the Universe. They have been developed using some ingenious techniques but all have helped us build a pretty good understanding of the scale of the Universe in which we live.