Astronomical Terms

Concepts of Brightness

Magnitude The magnitude system offers one method for classifying the brightness of a star or other celestial object. In this scale, lower values correspond to brighter objects than higher magnitudes. Incrementing the magnitude by 1.0 relates to a decrease in the brightness by a factor of 2.5. Thus, a star with a magnitude of 1 is 100 times brighter than one with a magnitude of 6.0. In order to convert between changes in magnitude and brightness, use the following formulae:


The apparent brightness of an object in the sky as it appears to an observer on Earth and does not take into account how far away the object is.  Bright objects have a low apparent magnitude while dim objects will have a higher apparent magnitude. The apparent magnitude of the Sun is -26.7, which is a great deal brighter than its absolute magnitude, because it is located so nearby. The apparent magnitude system is arbitrary. The zero point goes back to Hipparchus' system; it is currently based on the definition that the star Vega has a magnitude equal to 0.0.

Absolute Magnitude

A scale for measuring the true brightness of a celestial body, correcting for changes in brightness due to the distance of the object.The absolute magnitude measures how bright an object would appear if it were located exactly 10 parsecs (about 32.6 light years) away from = Earth. On this scale, the Sun has an absolute magnitude of +4.8.

Time Standards

Julian day number and Julian Date

Astronomers use the Julian Date to measure time. The Julian day number is a count of the days elapsed since Greenwich mean noon on 1 January 4713 B.C., according to the Julian proleptic calendar. The Julian Date is the Julian day number followed by the fraction of the day elapsed since the preceding noon. Conveniently for astronomers, this avoids the date skip during an observation night. It also avoids the complexities of the calendar system.


In astronomy, an epoch is a moment in time for which celestial coordinates or orbital elements are specified. In the case of celestial coordinates, the position at other times can be computed by taking into account precession and proper motion. In the case of orbital elements, it is necessary to take account of perturbation by other bodies in order to calculate the orbital elements for a different time. You may encounter it in the ephemeris of a periodic star:

Time of Maximum = Epoch + (n´Period)

The currently used epoch is J2000.0, which corresponds to the situation at (Universal Time) 12:00, January 1, 2000, corresponding to the Julian day 2451545. Moving to a different epoch one must use a year length of exactly 365.25 days.


The time required for a repeating event, such as an orbit, pulsation, or rotation, to complete each cycle.


This is a means of classifying the star based on the nature of the periodic behaviour it exhibits. A regular variable is a star characterized by a consistent period, or length of time between successive brightness maxima. A Cepheid is a good example of a regular variable - its light follows a cyclical pattern governed by a period whose length remains constant from cycle to cycle.

By contrast, the period of an irregular variable is less consistent. The semi-regular class of variable stars are long period variables whose light curves exhibit additional complexities beyond those of the well-behaved regular variables. For instance, a semi-regular variable might have an average period of 100 days, meaning that on average, successive maxima are roughly 100 days apart. There are however, cycle-to-cycle variations; thus, the actual length of time between maxima might vary widely. Some maxima might be as little as 50 days apart, while others might be 150 days apart.

Irregular variables are stars that exhibit either no periodicity or very slight periodicity.

Measurements and Analysis


The measurement of light. Specifically refers to the procedure of highly accurate measuring of the apparent magnitudes of astronomical objects. In general, astronomers measure only a portion of the wavelength spectrum when they do photometry. Different types of photometry are defined by the portion of the wavelength that they examine. For example "UBV Photometry" measures the light within three standard regions defined by filters. These are Ultraviolet, Blue and Visual (hence UBV). There are many different photometry systems and standards. For precision, the apparent magnitude of a variable star is often measured with respect to a constant "comparison star". The difference in their magnitude values, (star – comp) is called "differential photometry".

Photometry may be done by eye (typically with errors from 0.1 to 0.2 mag), photograph (with a typical error of 0.1 mag) or photoelectric photometer or CCD camera (with errors on the order of 0.01 mag or better). (Photometry made with a photometer or CCD camera is commonly referred to as “photoelectric”.)

Light Curve

This is a plot of the amount of light detected from an object (i.e. the apparent magnitude) as a function of time. The vertical axis depicts the magnitude, which is inverted, so that the higher a value appears (i.e. more negative), the brighter the measurement is. Thus the maxima (highest points) correspond to the brightest magnitudes the star attains. The horizontal axis corresponds to the time of the measurement, given by the Julian Date. The time required for one complete oscillation of the light curve is known as the period. Light curves provide evidence of eclipsing binaries, variable stars, and track the progress of nova and supernova explosions.

Phase Diagram It is possible to compress a long stretch of data, such as a light curve, onto a small graph by converting the time variable to a related quantity, the phase. Phase is defined as the fractional portion of the number of periods, which have elapsed since a given epoch. It is given by subtracting an integral number of periods from the time of the measurement, and then expressing the remainder as a fraction of the period:

Expressing the time as a phase measurement allows one to wrap the light curve around the graph so that the horizontal axis depicts time as a fraction of an oscillation, ranging from 0.0 (start of a period) to 1.0 (end of a period). As in the light curve, the vertical axis (magnitude) is inverted so that the most negative value is actually at the top of the graph.


Astronomical Terminology courtesy of Akos Bakos.