Astronomical distances, light speed, and uncertainty

[DreamGypsy]

10. Distance Measurement in Astronomy

You are correct in your understanding that "The longer ago the light was cast from a star, the less we should know about its current state." In fact, if the moon were to suddenly, magically vanish from space, then you could look up and see it, and 1.2 seconds later it would disappear. Or, if there is a mountain 10 miles away visible from your house and it suddenly, magically vanished, then you could see it and about 52 microseconds later it would disappear.

Sometimes the press will report something like: "Yesterday astronomers discovered a supernova of star xyzzy-23 in the Foo galaxy 200,000 light years distant.". What this really means is "Yesterday the first observation was made of a supernova of star xyzzy-23 which occurred 200,000 years ago in the Foo galaxy.".

Of course, scientists assume that the laws of physics as we currently understand them are the same throughout the universe. So, just as physics tells us that the moon or mountains won't 'magically' vanish, we are able to extrapolate our knowledge of stars and galaxies and the universe based on our current understanding of physics, astronomy, and cosmology. We have classifications of stars and understand the typical life-cycles that various star types go through. Thus , just as we can predict that the sun will consume its hydrogen and become a red giant in 4 to 5 billion years, so can we predict, based on our estimates of age, mass, and other parameters for a distant star, predict its current state and possible likely outcomes of its stellar evolution.

Note that all estimates of the mass, age, distance, and other properties of distant astronomical features have inherent uncertainties in them.

There is a reasonably good summary of various techniques for measuring astronomical distances here Distance Measurement in Astronomy:

Since all stars appear as points of light, even with the largest telescopes, and since geometrical distance measurement by parallax is possible only for the closest stars, an overlapping chain of distance measurement techniques has been developed.

The distance indicators include:

Parallax
Cepheid Variables
Planetary Nebulae
Most luminous supergiants
Most luminous globular clusters
Most luminous H II regions
Supernovae
Hubble constant and red shifts

A supporting idea for distance measurement is that if a specific kind of light source is known to have a constant and dependable absolute luminosity, then the measured intensity at the detector can be used to calculate its distance. Light from a point source diminishes according to the purely geometrical inverse square law, so the number of photons into a standard area detector can be used as a distance measurement. This is often referred to as the "standard candle" approach.

Each entry in the list of distance indicators has a link to an explanation of the method used, for example Hubble constant and red shifts will take you here.

Hubble's Law

Hubble's law is a statement of a direct correlation between the distance to a galaxy and its recessional velocity as determined by the red shift. It can be stated as

The reported value of the Hubble parameter has varied widely over the years, testament to the difficulty of astronomical distance measurement. But with high precision experiments after 1990 the range of the reported values has narrowed greatly to values in the range.

Hope this information helps.



BTW, Prince Myshkin, I make no claim to being wiser or better educated than anyone. I know that we are all naive idiots.