Astronomical Glossary

Two astronomical objects might look like they have the same brightness when viewed from Earth, but they could be at completely different distances. To account for this, absolute magnitude is used to describe the intrinsic brightness of an object, regardless of how far away or close it is to Earth. So the absolute magnitude of an object is how bright it is if it were at a fixed distance of 10 parsecs from Earth.

The point in an object’s orbit where it is furthest from the Sun

How bright an object appears to be when viewed from Earth. It runs on a reverse scale, so that the Sun has an apparent magnitude of -26.5, and the apparent magnitude of Alpha Centuari is -0.28. This means the Sun appears to be much brighter than Alpha Centuari when viewed from Earth.  The faintest object the human eye can see has a magnitude of about +7 (if well away from major cities and bright lights).

In astronomy, angles are used to describe apparent sizes and positions of objects. For example, if we were to draw two lines from ourselves to the ‘top’ and ‘bottom’ of the moon, the angle between those lines would be the angular diameter, being 31 arcminutes. There are 60 arcminutes in one degree, and 60 arcseconds in each arcminute. Which one is used (degrees, arcminutes, or arcseconds) depends on the size of the object.

It’s possible to work out the angular diameters of objects by just using your hand held at arm’s length. You can find out what to do here:

A unit of length, defined as the distance between the Earth and the Sun. However, the Earth is not the same distance from the sun at all points in the orbit. For this reason, an AU is now defined exactly as 149,597,870,700 meters.

A list of all the objects included in a survey. Each object can be given a catalogue name based on where it is located and what instrument was used in the survey.

The AAO is currently working to develop a database of surveys that will be made available to the public and Australian astronomical community, called the All-Sky Virtual Observatory (ASVO).  The database will be contributed to by a number of organisations and institutions across Australia, which will make the database a valuable research tool. You can find out more about the ASVO here:

Charged-Coupled Device is a device with regions that are sensitive to incoming light. CCD are used in digital cameras (including phone cameras) as a way to convert the light from an object into a digital signal.

The range of all wavelengths of electromagnetic radiation. Optical light is only a tiny part of the electromagnetic spectrum: this is the light by which humans see. The electromagnetic spectrum also includes infrared, microwaves, radio waves, ultraviolet wavelengths, X-rays and more. These names are a human division of the continuous electromagnetic spectrum, which runs from Gamma rays (short wavelengths) to Radio waves (long wavelengths) and beyond. Conducting astronomical observations in different parts of the spectrum can yield more information about an object: click here to see what the Milky Way looks like when viewed in different parts of the spectrum.

Elliptical galaxies appear to be smooth and featureless, and have a ellipsoidal shape.  The stars move around in random orbits, as opposed to the ordered motions that produce the disks of spiral galaxies.  You can see an example of an elliptical galaxy, M87, here.


The area of sky that a telescope can view at any one time, usually expressed in square degrees or arcminutes. The Two Degree Field (2dF) popular instrument on the AAT has a very descriptive name, as its field of view is a very large 2 square degrees.

Some instruments (like SAMI) can take spectra of multiple objects at once, which means light from the objects needs to be collected and analysed without mixing the light together. A field plate is a surface that has been specially configured so that optic fibres are in the right place to receive light from specific targets, when the telescope is pointing at the right point in the sky. 2dF is a robot that configures field plates.

An accessory to the telescope instrument, used to block certain wavelengths: this enhances the signal-to-noise ratio of the remaining wavelengths, and allows astronomers to focus in on certain details. Some filters can also block out light pollution and noise from the Earth’s atmosphere.

The total amount of energy that passes through a unit area per unit time.

The pancake shaped region around the central bulge of a galaxy. The Milky Way’s disk is 100,000 light years across and 1,000 light years thick. The spiral arms of a galaxy live in the flat planes of the disk.

A region in space where a planet is at just the right distance from its host star to support liquid water. The search for other life in our universe is focused on discovering exoplanets in this zone because as far as we know, life requires the presence of liquid water to exist.

Gravitational lensing refers to the distortion of light of a distant source due to the gravitational field produced from a closer massive object. This effect often causes distant galaxies to appear as long curved stripes in a deep astronomical image due to a foreground galaxy cluster.

You can see examples of gravitational lensing here.

Integral Field Spectroscopy uses instruments called Integral Field Units (IFU) to gather multiple spectra across an object in 2D. This is typically performed using multiple optic fibres that have been fused together. This allows astronomers to measure differences in spectra between regions of a single object.

AAO instruments that offer IFS capabilties: KOALA, Spiral, SAMI, TAIPAN, Hector

Two atoms are isotopes of each other if they have the same number of protons in their nucleus but have a different number of neutrons.

For example: Hydrogen has 1 proton and 0 neutrons in its nucleus. Deuterium has 1 proton and 1 neutron in its nucleus. This means that Deuterium is an isotope of Hydrogen.

The total energy that is emitted by an astronomical object per unit time.

The metallicity, denoted Z, of an object (like a galaxy or star) is the fraction of mass composed of metals, where in astronomy metals are classed as everything that is not Hydrogen or Helium.

This is a term used to describe the appearance and structural features of a galaxy. Features considered include things like spiral arms, disks, bulges, bars, and for more complex systems, things like tidal tails and stellar halos. The most famous morphology classification system is the Hubble sequence, which was made by Edwin Hubble. In this system, galaxies can be classified to have morphological types such as Elliptical or Spiral.

Optical fibres are thin glass threads that allow light to internally reflect along their length. They efficiently transport light from one place to another. The AAT uses optical fibre technology to transport light at its focal plane to spectrographs located 50 metres away below the telescope.  

Parallax is the apparent change in a position of an object when it is viewed from two different locations. This is like when you try and read an analogue clock from opposite sides of the room – the times you read will be slightly different based on from what angle the hands are viewed. In astronomy, parallax can be used to determine distance.

A parsec is a measure of large distances. 1 parsec is equal to 3.26 light years.

The point in an object’s orbit where it is closest to the Sun.

A technique used to measure the intensity of light coming from an astronomical object.

A material that responds to an electric field by contraction or stress. This also works in reverse, as in the material will generate an electric charge in response to being placed under mechanical stress (eg, squeezed). Piezoelectric materials include some ceramics and quartz. The piezoelectric property is used in Starbugs.

Field plates used to be made of metal, and holes were cut into them so the optical fibres could be fed in. Plate plugs secured the optical fibres to the plate. Plate plugs are being replaced by technologies such as Starbugs.

A technique used to measure the polarisation of the light coming from an object.  

Light travels in a wave, which means it oscillates through space. There is no set orientation for this oscillation, but when the light is polarised, all the vibrations are restricted to a single plane. Light can be polarised using filters.

A telescope receives light not only from the astronomical target (the desired signal), but also from Earth’s atmosphere, light pollution, and even radiation from the telescope’s own instruments: this is all ‘noise’. The signal-to-noise ratio is just the ratio of how much desired signal the telescope receives, compared to how much noise it receives. It is an indication of how good the data are.

A unit defined as the mass of our Sun.

1 solar mass = 1 x mass of our sun
= 2,000,000,000,000,000,000,000,000,000,000kg

A technique where short exposure images of objects are taken and then processed to remove the effects of the Earth’s atmosphere. These short exposures are then combined into one image. This technique dramatically increases the resolution of telescopes that are based on Earth.

Spiral galaxies are galaxies that are made of a flat, rotating disk, usually with a central bulge and spiral arms- like our Galaxy, the Milky Way. NGC 1566 is also an example of a spiral galaxy.

A project where a telescope is not observing a single object, but is generally imaging a large region of the sky, or to amass observations of many similar objects: like a survey of stars or galaxies. GAMA is an astronomical survey.

The wavelength of a wave is the distance between two successive peaks or troughs. For example, light is a wave, and the wavelength of the light is what determines its colour.