# Further Physics - Observing the Universe - Parallax, Cepheids, The Curtis-Shapely Debate and The Hubble Constant

Further Physics - Observing the Universe - Parallax, Cepheids, The Curtis-Shapely Debate and The Hubble Constant

Measuring Distance Using Parallax

Parallax can be thought of as the apparent motion of an object against a background. However, it is actually the motion of the of the observer that causes the parallax motion of an object.

The distance to the stars is so great, that we cannot observe the parallax motion with the naked eye. However, a simple way to observe parallax is if you hold you hand out in front of you with you thumb sticking up and alternately close one eye then the other. Although your thumb appears to move, in reality you are just looking at it from a different angle.

The parallax angle of a star is half the angle moved against a background of distant stars in 6 months. An object that is further away from the Earth will have a smaller parallax angle than a closer object.

Using Parallax

Astronomers use parallax to measure interstellar distances using the unit parsec (pc). A parsec is of a similar magnitude to a light year, with 1 parsec equalling roughly 3 and 1/4 light years.

Angles are measured in degrees, minutes and seconds. A star that is one parsec away has a parallax angle of one second of arc. The distance in parsecs can be found by dividing 1 by the parallax angle, as shown in the following formula:

Distance (Parsecs) = 1/ Parallax Angle (Arcseconds)

Parallax is very useful for measuring the distances of relatively close objects. For example the typical interstellar distance between stars is a few parsecs. Astronomers can also use the parsec or MEGAparsec. (Mpc) to describe much bigger intergalactic distances even thought these objects are so far away that the parallax angle is too small to measure.

Measuring Distance using Brightness

Another method that astronomers use to measure the distance to stars is to observe…

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