1B11 Foundations of Astronomy Glossary of terms Liz Puchnarewicz emp@mssl.ucl.ac.uk www.ucl.ac.uk/webct www.mssl.ucl.ac.uk/

1B11 Glossary of terms - A apparent magnitude – the brightness that a star has on the sky as seen by an observer from Earth. Logarithmic scale defined by Pogson.

1B11 Glossary of terms - B Black hole – a compact object whose mass is contained within a radius which is smaller than its Schwarzschild radius

1B11 Glossary of terms - C Compact object – an exotic star such as a neutron star or black hole made up of extremely dense material. Usual formed after the normal hydrogen-burning phase of the most massive stars.

1B11 Glossary of terms - D Declination – celestial co-ordinates in the North-South direction. Dec is measured in degrees – 360 degrees in the circle (measured from –90O in the south to +90O in the north), 60 arcminutes in one degree and 60 arcseconds in one arcminute.

1B11 Glossary of terms - E Escape velocity – the velocity which must be reached to escape the gravitational pull of a body

1B11 Glossary of terms - H Hour angle – the angle between the observer’s meridian and the great circle joining the celestial pole(s) to the star. It is measured in hours, westwards from the observer’s meridian. So if HA=3h, the star made a transit of the meridian 3 hours before.

1B11 Glossary of terms - M Meridian – the plane passing north-south through the observer and the zenith (draws a great circle on the sky)

1B11 Glossary of terms - N Neutron star – a compact, highly dense star which is formed from the collapse of massive stars and is mostly made up of a material which is like a “neutron soup”

1B11 Glossary of terms - P Parsec – One parsec is equivalent to 3.26 light years, or 3.1x1013 km. A star at a distance of 1 parsec has a parallax of 1 arcsecond. Parallax – Nearby objects appear to move faster with respect to more distant objects as you go past them. This effect is called parallax and is used to measure the distances to nearby stars.

1B11 Glossary of terms - R Right Ascension – celestial co-ordinates in the East-West direction. RA is measured in hours – there are 24 hours equivalent to 360 degrees, 60 minutes in an hour and 60 seconds in one minute.

1B11 Glossary of terms - S Schwarzschild radius – the radius of a body at which the escape velocity exceeds the speed of light Supernova – when a massive star at the end of its normal stellar evolution explodes to form a neutron star or black hole

1B11 Glossary of terms - T Transient – a transient source is one whose emission is not steady or stable, eg a nova or supernova which may suddenly go into outburst

1B11 Glossary of terms - V Vernal Equinox – The zero point for RA which is defined as the position of the Sun in the sky at the Vernal Equinox (~21 March), the point at which the Sun crosses the equator from South to North. It is also known as the “First Point of Aries” (although it is now in Pisces) and it is measured eastwards.

1B11 Glossary of terms - Z Zenith – the point on the celestial sphere which is directly above the observer