Transcript 1B11 Foundations of Astronomy Star names and magnitudes

1B11
Foundations of Astronomy
Glossary of terms
Liz Puchnarewicz
[email protected]
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