[ETL]

Redshift refers to the displacement of a set of spectral lines which identify a particular chemical compound.

The chemical "ID" is like a bar code for merchandise at the store, only, that with chemicals, each line represents a particular chemical element with a known frequency and wavelength, such that, if the elements appear at a different frequency/wavelength, they know that the object is either moving away from us or towards us. If the sequence is shifted to a lower frequency/longer wavelength, then the host object is assumed to be moving away. ie, redshifted. Vice-versa for objects with lines shifted to higher frequencies/shorter wavelengths. ie, blueshifted

[ETL]

“The cosmic distance ladder (also known as the extragalactic distance scale) is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are “close enough” (within about a thousand parsecs) to Earth.

The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at close distances and methods that work at larger distances. Several methods rely on a standard candle, which is an astronomical object that has a known luminosity.

The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy. Instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung. ...”

Contents

1 Direct measurement
1.1 Astronomical unit
1.2 Parallax
2 Standard candles
2.1 Problems
3 Standard siren
4 Standard ruler
5 Galactic distance indicators
5.1 Main sequence fitting
6 Extragalactic distance scale
6.1 Wilson–Bappu effect
6.2 Classical Cepheids
6.3 Supernovae
6.3.1 Measuring a supernova’s photosphere
6.3.2 Type Ia light curves
6.3.3 Novae in distance determinations
6.4 Globular cluster luminosity function
6.5 Planetary nebula luminosity function
6.6 Surface brightness fluctuation method
6.7 D–[omega]; relation
7 Overlap and scaling
8 See also
9 References
10 Bibliography
11 External links

https://en.wikipedia.org/wiki/Cosmic_distance_ladder