Clusters Glossary


A techique used in astronomy to precisely measure the positions of stars and other celestial objects.

CCD (Charged-Couple Device)

A CCD measures electrical charge accurately and efficiently, allowing the photons that arrive at a telescope to be converted into electrons and stored as digital values for each pixel.

CMD (Colour Magnitude Diagram)

 A CMD is a key diagram in astronomy which uses observational data to examine the properties of a population of stars. These properties include 'surface' temperature (or equivalently spectral type) on the x-axis and brightness or luminosity on the y-axis.

Doppler Effect

Named after the Austrian physicist Christian Doppler, the Doppler effect is the change in frequency of a wave for an observer moving relative to the light emitting object. This is most commonly seen as a redshift, particularly in galaxies, which provided one of the key pieces of evidence of the Big Bang Theory. The Doppler effect also manifests itself in sound as the change in pitch of a siren as it passes a static observer.

FITS file

Standing for File Image Transfer System, a FITS file is the standard file format for astronomical images

FWHM (Full Width Half Maximum)

A measure of the quality of an astronomical image based on how much the telescope and atmosphere have smeared a point source over the image.

HRD (Hertzsprung-Russell Diagram)

A HRD is a theoretical diagram (cf. the CMD which uses observational data) produced in the early 20th century which proposed how a population of stars might be linked in terms of their properties.


A star's intensity is a measure of how much light we receive from it into our detector (eye, telescope, CCD, etc.) which is determined by several factors including how bright the star actually is and its distance from us.


A measure of the brightness (as seen from Earth) of an astronomical object such as a star or planet.

It is often important to differentiate which type of magnitude we mean:

  • Absolute - astronomers use the absolute magnitude (with a capital M) to show how bright a star would appear if it was at a standard distance of 10 parsecs (1 parsec = 3.09 * 1016 m or 3.26 light years). For example, since our Sun is very close, it has an apparent magnitude of  -26.72. However, if it were at a distance of 10 parsecs it would have an apparent magnitude of +4.8. This value of 4.8 is known as its absolute magnitude.
  • Apparent - initially, when we look at or measure a star, we think about how bright it is as seen from Earth (i.e. its intensity). This is the star’s apparent magnitude (with a lower case m) and does not take into account how far away the star is.
  • Instrumental - photometry packages such as Makali'i and LT Image return values which are quoted as either counts (the number of photons hitting the CCD during the exposure) but sometimes also as a value quoted in magnitudes. This 'magnitude' is yet to be calibrated as it has not taken into account the performance of the telescope and CCD (i.e. the instrumentation), the exposure time and the parameters used in the program itself. While calibration can be achieved, these magnitudes are considered as 'instrumental' at this stage. For the purposes of activities such as the NSO clusters and variable star research programs, instrumental magnitudes are perfectly acceptable. It is only when wanting to compare these results with previous research that calibration (or standardisation) is required.


Photometry is the science (some would say art!) of using astronomical data in FITS format to accurately measure the brightness of stars and other celestial objects. Data collected by the National Schools' Obervatory (NSO) in conjunction with programs such as Makali'i make it possible to this.


Polarimetry is the study of how light has been polarised - an effect seen when light passes through strong magnetic fields.


Spectroscopy allows scientists to examine an object’s brightness as a function of its wavelength. 

Wien's Law

Wien's Law allows a calculation to be made of a black-body's temperature by measuring the peak wavelength of the object's emission. Fortunately, star's surfaces can be considered as black-bodies so we can apply this law to get a good idea of a star's temperature.