# how is distance measured in space?

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i know its measured in lightyears but im watching a video on the james webb telescope and the discovery of huge galaxies and how we have to rethink the beginning but how do they know how old the galaxies are by just pictures? i imagine they have a lot of other things that tell them more information but still. it seems damn near impossible to even tell what anything is especially with how small and warped it is too. thank you i appreciate any info on this. im just throwing the math flair since theres no space

In: 8

The general approach is called a “distance ladder”, because there are several techniques used for different distances that are used to calibrate and verify each other. For very nearby objects, like in the solar system, you can simply bounce radar signals off them and see how long it takes them to come back. For nearby stars, you can use the parallax method, which relies on the fact that the earth’s motion around the sun causes nearby stars to look as if they move around slightly relative to distant stars (this is like how an analogue clock seems to show a slightly different time if you move around it due to the gap between the hands and the clock face).

Next, there are certain kinds of objects in the sky, known as “standard candles”, that have a predictable absolute brightness. For example, Cepheid variable stars have a brightness that oscillates over time, and by looking at nearby Cepheid variables, you can tell that there is a consistent relationship between their absolute brightness and the rate at which their brightness changes. You can use this to tell how bright a distant Cepheid variable “should” be, and then infer how far away it is based on how bright it appears.

This then led to an extremely important observation that, for distant stars, there is a straightforward relationship between how far away a star is, and how fast it is moving away from us (this is Hubble’s law). It is easy to measure how fast a star is moving away from us because if you split its light into frequencies, you see characteristic patterns related to the atoms and molecules that make up the star (e.g., there are certain frequencies that hydrogen is very good at emitting/absorbing). But for a star that is moving away from us, these frequencies are shifted downwards by the Doppler effect, and the faster they are moving away, the bigger that redshift is.

> i know its measured in lightyears but im watching a video on the james webb telescope and the discovery of huge galaxies and how we have to rethink the beginning but how do they know how old the galaxies are by just pictures?

Unless there has been some recent news, my understanding is that this was based on a very preliminary analysis, and at this stage there are several much more mundane factors that could explain the results.

There are a number of ways to measure long astronomical distances but one you may be interested in comes from the discovery of something called [cepheid variables](https://en.wikipedia.org/wiki/Cepheid_variable).

This is a type of star that pulses in brightness. There is a constant relationship between how bright it is and how fast it’s brightness pulses. So by measuring the frequency of its pulsing and it’s apparent brightness from earth, you can calculate how far away it is. So when we spot a Cepheid in some distant Galaxy, we immediately have a way of approximating the distance to that galaxy.

The general approach is called a “distance ladder”, because there are several techniques used for different distances that are used to calibrate and verify each other. For very nearby objects, like in the solar system, you can simply bounce radar signals off them and see how long it takes them to come back. For nearby stars, you can use the parallax method, which relies on the fact that the earth’s motion around the sun causes nearby stars to look as if they move around slightly relative to distant stars (this is like how an analogue clock seems to show a slightly different time if you move around it due to the gap between the hands and the clock face).

Next, there are certain kinds of objects in the sky, known as “standard candles”, that have a predictable absolute brightness. For example, Cepheid variable stars have a brightness that oscillates over time, and by looking at nearby Cepheid variables, you can tell that there is a consistent relationship between their absolute brightness and the rate at which their brightness changes. You can use this to tell how bright a distant Cepheid variable “should” be, and then infer how far away it is based on how bright it appears.

This then led to an extremely important observation that, for distant stars, there is a straightforward relationship between how far away a star is, and how fast it is moving away from us (this is Hubble’s law). It is easy to measure how fast a star is moving away from us because if you split its light into frequencies, you see characteristic patterns related to the atoms and molecules that make up the star (e.g., there are certain frequencies that hydrogen is very good at emitting/absorbing). But for a star that is moving away from us, these frequencies are shifted downwards by the Doppler effect, and the faster they are moving away, the bigger that redshift is.

> i know its measured in lightyears but im watching a video on the james webb telescope and the discovery of huge galaxies and how we have to rethink the beginning but how do they know how old the galaxies are by just pictures?

Unless there has been some recent news, my understanding is that this was based on a very preliminary analysis, and at this stage there are several much more mundane factors that could explain the results.

There are a number of ways to measure long astronomical distances but one you may be interested in comes from the discovery of something called [cepheid variables](https://en.wikipedia.org/wiki/Cepheid_variable).

This is a type of star that pulses in brightness. There is a constant relationship between how bright it is and how fast it’s brightness pulses. So by measuring the frequency of its pulsing and it’s apparent brightness from earth, you can calculate how far away it is. So when we spot a Cepheid in some distant Galaxy, we immediately have a way of approximating the distance to that galaxy.

A bunch of different ways: Cepheids as koolaid said,

Parallax can be used to measure objects in our galaxy just be measuring angles at different places in Earth’s orbit.

Type I supernovae, supernova from binary stars where a white dwarf is siphoning matter from another star, always explode at 1.44 solar masses. Which means their luminosity is constant and changes in perceived luminosity becomes a range-finder.

The last is red-shift. We see a fairly predictable relationship between relative velocity (which causes redshift) and distance, so if we see how much a hydrogen line is red-shifted we can estimate the distance.

Stars look white but that white light is made up of lots of different colors. If you put that light through a prism, you get a spectrum of all the colors, but because of the elements in the stars, there are specific lines of that spectrum.

If we look at the light from a distance galaxy, those same lines show up, but they are shifted based on the motion of the galaxy in relation to us, and the amount of the shift depends on their velocity.

We know that the larger the red shift is, the farther away the galaxy is, and therefore the older it is since it took the light a long time to reach us.

A bunch of different ways: Cepheids as koolaid said,

Parallax can be used to measure objects in our galaxy just be measuring angles at different places in Earth’s orbit.

Type I supernovae, supernova from binary stars where a white dwarf is siphoning matter from another star, always explode at 1.44 solar masses. Which means their luminosity is constant and changes in perceived luminosity becomes a range-finder.

The last is red-shift. We see a fairly predictable relationship between relative velocity (which causes redshift) and distance, so if we see how much a hydrogen line is red-shifted we can estimate the distance.