Measuring distance in space

As you know, space is quite large. Picture the biggest skyscraper you have ever seen. Space is at least seven times bigger than that. Considering we are for the most part stuck here on earth, I’ve always been curious how distance is measured in space. So I am going to look it up, and write it down.

The Cosmic Distance Ladder

The cosmic distance ladder is a set of techniques astronomers use to measure distance in space. There are a few steps on that ladder, each representing a different technique that has the ability to measure objects further out in space then the one prior.

Rung 1: Stellar Parallax

Someone figured out that you can measure distance between stars by measuring the change in an angle (called parallax angle) between two points of the earths orbit. The first thing we need is two different points of orbit for the earth. So perhaps earths position on January, and earths position in July which are a known distance apart. Then we look at a nearby star, and see how the angle between that star and stars in the distance behind it change. Using some presumably basic and easily calculable for the layman trigonometry, scientists can determine the distance between stars for up to 10,000 lightyears away. Beyond 10,000 lightyears, the change in the angle becomes too small for our current technology to pick up on.

If that seemed simple enough to understand conceptually then I hope you enjoyed that feeling, because its about to disappear. The other rungs are a lot more difficult to understand.

Rung 2: Standard Candle

The amount of light a star emits is referred to as luminosity, and the brightness of that light is referred to as magnitude. There are two types of magnitude, apparent magnitude and absolute magnitude. Apparent magnitude refers to the brightness that can be seen from earth. The brighter the star, the lower magnitude it has. For example, Venus, which can be seen from earth even during the day at times, has a negative apparent magnitude.

It’s apparently “easy” to calculate apparent magnitude. But calculating absolute magnitude is a lot harder. Absolute magnitude is the brightness of a star when viewed from the standardized distance of 10 parsecs (32.6 lightyears) away.

So to measure absolute magnitude, astronomers use tools they call “standard candles”. These standard candles are Cepheid variable stars and Type 1a supernovae.

Scientists discovered how to calculate the absolute magnitude of Type 1a supernovae and Cepheid variable stars, so these “candles” are used to measure distance. The technique is to compare apparent and absolute magnitude. If absolute magnitude is known, then the apparent magnitude can be used to figure out the objects distance from earth.

Cepheid Variable stars:

Cepheid Variable stars are pulsating stars that brighten and dim at certain intervals. Astronomers learned that there is a relationship between the time between pulsation of the star from bright to dim and its average apparent magnitude. If the pulsation cycle is longer, it means the star is brighter.

They used this knowledge to infer that differences in apparent magnitude were related to absolute magnitude. Ultimately, astronomers were able to create a graph that shows the period of pulsation of a Cepheid, and the corresponding absolute magnitude.

Once astronomers know the absolute magnitude, the unknown distance of a cepheid can be calculated by using the chart for absolute magnitude, and then comparing absolute magnitude with its apparent magnitude to calculate its distance from earth. Hubble has measured the distance of a star using this technique at around 100,000,000 light years which is significantly further than stellar parallax.

Rung 3: Type 1a supernovae

Once we move past 100,000,000 light years away, the distance is too far for astronomers to make out individual stars, so Cepheid variables are no longer useful. Instead, supernovas are used.

Supernovas are a result of a stellar explosion, which creates a huge amount of light. There are different types of supernovae, but only type 1a are used for measuring distance. A type 1a occurs when a white dwarf star explodes. Astronomers know the limit point for these stars that triggers the explosion based on principles of nuclear physics. Knowing the limit is important as it makes these events predictable and unchanging.

All type 1a’s emit the same amount of light and produce the same light curve after exploding. Since this event is consistent, astronomers have a known absolute magnitude at various stages of the explosion. This means the amount of light produced, the shape of the light curve created, and the decrease in brightness all follow the same curve.

So all astronomers need to do to measure distance beyond 100,000,000 light years is find a type 1a supernova by examining its light curve and ensuring it truly is a type 1a. Once it is determined it is a type 1a, they measure its maximum apparent magnitude. Since they know the maximum absolute magnitude of a type 1a, they can then compare the maximum apparent magnitude they just measured with the absolute magnitude to determine its distance from earth.

This is how astronomers have been able to measure the distance of galaxies that are up to 1,000,000,000 light years away.

Rung 4: Redshift

This rung relies on the use of the expanding universe theory.

Objects in space have characteristic lines on the light spectrum that tend towards the red end of the spectrum. The amount of redshift changes between different objects and galaxies.

It was observed that objects in space moving towards us have a shortening display of wavelength, which means it tends towards the blue end of the light spectrum. Objects moving away from us tend towards the red side of the spectrum. Astronomers noticed that pretty much all galaxies were moving away from us, and that the further away they were, the more redshift (longer light wavelengths shifting it to the red end of the spectrum) they displayed. Interestingly, the Andromeda galaxy is moving closer to us by about 300km/s. Luckily, it is 2.2 million light years away still.

A formula was developed that shows that the velocity in which galaxies move away from us, is proportional to their distance to us. This means, if we can measure the velocity they are moving away from us using redshift, we should be able to calculate their distance to us.

Red shift needs to account for the expanding universe theory, so it is more complicated than just a certain redshift = the velocity of the galaxy. Since the universe is constantly stretching, light travelling across it is stretched which makes the wavelengths of that light longer, thus shifting it more toward the red end of the spectrum.

Redshift has been used to measure the distance of objects 1.3×10^10 light years away from us.

So there you have it…

Now you know how to measure the distance of objects millions of kilometres away.