Astronomy Dictionary

Browse through Sky at Night Magazine’s comprehensive astronomical dictionary, with over 300 entries.

Barred Irregular Galaxy

This is an irregular galaxy which exhibits a bar structure.


Basins or impact basins are the results of massive impacts on a planetary surface. They appear as vast craters. Some lunar basins have been covered or partially filled by lava, which has later erupted from beneath the surface.

Bayer designation

This is given to stars in a constellation to indicate their order of brightness using the Greek alphabet; where Alpha (a) denotes the brightest star in a constellations and Beta (b) denotes the second brightest and so on.


This is an optical component of a bino viewer that splits the light coming into the eyepiece holder into two beams for viewing.

Bi-polar outflow

When stars are born they begin to form jets at their poles known as bi-polar outflows. These jets are thought to originate from material that collapsed onto the star as it was forming. They may, in fact, be crucial to the process.

Bias frames

An extremely short exposure from a CCD camera or DSLR. It’s used to subtract the noise generated by the sensor when it reads the light it has recorded, together with noise from other sources, such as a computer.

Binary star

A binary star is a system of two stars that are gravitationally bound by their own gravities orbiting around a common centre of mass.

Black hole

The object that is formed after the death of a very massive star. Matter collapses in on the dying star to such an extent that its gravitational field increases to a point where not even light can escape from it.

Black hole stars

Astronomers know that in the early Universe, black holes with masses many times that of the Sun roamed space. But they’re not sure where these monsters came from. Enter a new theory – black hole stars. Black hole stars were supermassive stars in the early Universe. At their centres were massive black holes, formed by crushing gravity. As a supermassive star became more massive and grew, the black hole at its centre would have gorged on the star’s gas and would also have grown. Eventually the star would begin to die as it cooled down and, in doing so, would expose the massive black hole at its heart.


These are powerful jets of radiation coming from the heart of a quasar (see ‘Quasar’) that we are viewing ‘head on’.

Bok Globule

A Bok Globule is a dense cloud of dust and gas in space thought to be important in the role of star formation, named after the astronomer Bart Bok.


Bracketing is an imaging method where you take several exposures (of the same object) with slightly different settings either side of the planned settings, to see what works best.

Bulb (or B setting)

This is a setting usually seen on old SLR cameras (and now seen in a different guise in the manual mode on some DSLRs), which allows the shutter of the camera to be left open for an extended period of time. This is especially crucial for long-exposure work where the shutter may have to be open for several minutes to capture enough light from a celestial object.

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