Photometric parallax: Difference between revisions

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{{wikinews|Huge star cluster discovered in neighbourhood of Milky Way}}

The '''photometric parallax method''' is a method of data analysis used in [[astronomy]] that uses the colours and apparent brightnesses of stars to infer their distances. It was used by the [[Sloan Digital Sky Survey]] to discover the [[Virgo super star cluster]].

Unlike the [[parallax#Stellar_parallax|stellar parallax]] method, photometric parallax can be used to estimate the distances of stars over 10 kpc away, at the expense of much more limited accuracy for individual measurements. Strictly speaking, it does not actually employ any measurements of parallax and can be considered a misnomer.

Parallax method is only good for nearby stars (< 50pc or ~163ly)

1 Parsec (pc) is the distance associated with a parallax angle of 1" (" = arcseconds)

To calculate distance of a nearby star using the Parallax Method:

d= 1/Parallax angle

d will turn out as a Parsec (pc)

To give the distance in Lightyears (ly), use d= 3.26/Parallax angle

== Principles of Measurement ==

As you move from one position to another objects change their relative positions. As far as you are concerned, near objects appear to move when compared with far objects. Objects that are very far away do not appear to move at all. You can demonstrate this effect by closing one eye and moving your head from side to side. An object that is near to you (for example the tip of your finger) will appear to move when compared with objects that are far away (for example a distant building).

This apparent movement is known as parallax and the effect can used to measure the distance to some of the stars in our galaxy. All stars appear to move over the period of a night, but some stars appear to move over the period of a night, but some stars appear to move in relation to other stars over the period of a year. A pair of measurements six months apart is the norm.

The reason for this apparent movement is that the Earth has moved over the period of a year. This change in observing position has meant that a close star will have an apparent movement when compared with a more distant set of stars. The closer a star is to the Earth, the greater will the parallax shift be.

Since all stars are very distant, this effect is a very small one and the parallax angle will be very small. It is usual to quote parallax angles not in degrees, but in seconds. An angle of 1 second of arc ('') is equal to one sixtieth of 1 minute of arc (') and 1 minute of arc is equal to one sixtieth of a degree.

== References ==