How exactly do they measure the distance from our galaxy to other galaxies?

Question

I understand (or at least think I do) that if galaxy ABC is X-million light years away, the light that we see left that galaxy X-million years ago ... but how do they know how long ago it left and how far away the galaxy is?

Or do we not know in distance how far galaxies are away and only know in time (and then again, how do we know the time)?

Thanks!

Answer 1

If something is one 'light-year' away it means that if we were traveling towards it at the speed of light, it would take one year to get there.

The speed of light is 299 792 458 m / s.

So to reach something that is one light-year away we would travel at 299 792 458 m / s for one year which is approximately 9,500,000,000,000 kilometers.

The distance of one galaxy from another is always measured in light years so if, for example, Galaxy A was 12 light-years away from Galaxy B... it would take 12 years, whilst travelling at the speed of light from Galaxy A, to reach Galaxy B.

12 x 9,500,000,000,000 kilometers.

'Light years' are used because the universe is so vast that if we were to measure distances in miles or kilometres we would end up with numbers with a billion zeros on the end.

I hope this helps your confusion!

Answer 2

One way is the use of Cephid variables (first link below).  The type of variable's period of brightness oscillation is dependent on its luminosity, so if two Cephids have a 50 day period, they are the same intrinsic brightness and the difference in relative brightness is due to distance alone.  This method is primarily used for nearer galaxies.

A second method is used for far away galaxies.  The Red Shift uses two properties of the light from a distant galaxy to determine its distance.  First, light isn't perfect, there are dark lines called absorption lines that occur at specific known frequencies.  Second, as an object moves away faster, the light from it (and the absorption lines) is shifted to the red end of the spectrum, and at a specific mathematical relationship.  See the second link.

Answer 3

A really long tape measure.

Answer 4

The main way astronomers gage the distance to galaxies is by finding type Ia supernovae in them. These are the most violent explosions known, and enough of them have been observed in order to be sure of many of their properties, such as spectral analysis (probably one of the most useful tools in an astronomer's arsenal after their own brains and after telescopes).

Once they were sure about the spectra of these supernovae, and their brightness, they became known as something called a "standard candle." If you know how bright something would be up close, you can tell how far away it is by how much dimmer than that it appears when you observe it.

The spectra, not just of the supernovae, but of the galaxies themselves started to show something interesting - red shift. This means that far away galaxies are flying away from us. The farther away, the more red-shift is apparent in their spectra. This led Edwin Hubble (after whom the space telescope is named) to realize that the universe is expanding.

Once we got telescopes good enough to see very deeply into space (such as the Hubble), galaxies were seen that were so far away, even the standard candle supernovae weren't accurate enough anymore. Light gets fainter on the square of the distance to the source, so after a while, the supernovae are so faint that it's hard to accurately tell /how/ much dimmer they are - even being off by a little means a BIG difference in distance at these extremes.

Fortunately, enough observations were made about this red-shift phenomenon to realize that the amount of red shift is a good measure of how far away a distant galaxy is. The more red shifted it is, the further away it is.

So here's a sumary:

  very nearby galaxies - the usual stuff that works on stars

  galaxies, not right next door or even pretty distant- supernova light's brightness

  distant galaxies - red shift measurement.

As for the time, the time and distance sort of go hand in hand thanks to our use of the "light year" as our standard unit. A light year is the distance (not time) that light travels in a vacuum in one year. Using miles to describe anything outside of our solar system would be insane - it would be like measuring in nanometers the distance between New York and Tokyo. Conveniently, if we know an object is one million light years away, then the light we're seeing from it, by definition is one million years old.

To use a more everyday analogy - I declare a new unit of distance: the Interstate-Hour. (70 miles unless you're speeding). My home city is about 6 interstate-hours from Atlanta, GA. That's convenient because you not only know how far away it is, but how long it takes to get there, all from the same unit.

Answer 5

We measure the distances to Galaxies in various ways depending on how far away they are.

Essentially we have ot build the Cosmic Distance Ladder:

http://en.wikipedia.org/wiki/Cosmic_dist...

So we use Parallax for close by stars, which lets us calibrate standard candles like Cepheid and Supernovae which are very bright, so can be detected at large distances. These are objects that come in known brightnesses. If we know how bright they really are and we measure how bright they seem to be, we can calculate how far away they are....

As for how long ago we are seeing them. The speed of light is well known. Once we know the distance we can work out how long it took light to get to us because speed = distance/time

ADDED: what's with the thumbs down?

If it's because I didn;t mention redshift - that's because the Hubble Law/redshift correlation method relies on using the standard candles like Cepheids and supernovae for calibration. Yes it's true that very very distant galaxies can now have distances determined by the Hubble law. There are other method used too - which I why I gave the link to the Cosmic Distance Ladder. In fact having more than one method gives us an important reality check!