Parallax

:This article is about parallax, the perspective shift. For the DC comic book character see Parallax (comics).

The scale of the Universe

All these various astronomical parallax methods allow us to establish the first rungs on the cosmic scale ladder, out to a few hundred light years. Beyond that, other methods must be taken into use: e.g., "spectroscopic parallaxes" — not really parallaxes at all. It is a prototype of a "standard candle" method, where we observe the apparent brightness of an object we know, based on some physical theory, the true brightness of. For groups of stars we have the Hertzsprung-Russell diagram which allows us to derive a star's absolute brightness or magnitude M from its spectral type. The observed (apparent) brightness or magnitude being m, we can then derive its parallax p by

Related Topics:
Light years - Spectroscopic - Hertzsprung-Russell diagram - Absolute brightness - (apparent) brightness

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

5 log p + 5 = M - m ,

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

called "spectroscopic parallax".

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Further methods, mostly of the standard candle variety, are the variable stars called Cepheids — the

Related Topics:
Standard candle - Cepheids

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

absolute brightness of which depends on their observed period of variation —, supernova brightnesses, globular cluster sizes and brightnesses, complete galaxy brightnesses etc. These are all much more uncertain as they are not based on simple geometry. Yet, parallaxes are the basis of everything, as they allow the calibration of these more uncertain methods in the Solar neighbourhood.

Related Topics:
Supernova - Globular cluster - Galaxy

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A very modern method which is not a traditional parallax method but also geometric in nature, is "gravitational lensing parallax". It depends on observing the differential time delay of brightness variations from a remote quasar reaching us by two different paths through the gravitational field or "lens" of a foreground galaxy. If the redshifts of both the quasar and the foreground galaxy are known, one can show that the absolute distances of both are proportional to the differential delay, and can in fact be calculated given also the geometry of the gravitational lens on the celestial sphere.

Related Topics:
Gravitational lensing - Quasar

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

All these independent techniques aim at determining Hubble's constant, the constant describing how the redshift of galaxies, due to the Universe's expansion, depends on these galaxies' distance from us. Knowing Hubble's constant again allows us to determine, by simply running the film of the cosmic expansion backwards, how long ago it was when all these galaxies were collected in a single point -- the Big Bang. Current knowledge puts this at some 14.7 billion years ago, but with considerable uncertainty and dependence on various model assumptions.

Related Topics:
Hubble's constant - Redshift - Big Bang

~ ~ ~ ~ ~ ~ ~ ~ ~ ~