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Home | History | Science | Observations | Atlases | Maps | Glossary
The Small Magellanic Cloud (SMC) - Glossary of Terms
Light Year
A light year is a measurement of distance not time. It is the distance that light travels in one year. This distance is 9.46 x 1012km. The nearest star is 4.3 light years distant (Proxima Centauri).
The Small Magellanic cloud is 195,000 light years distant. This means that the light we see from the galaxy is 195,000 years old. The main body of the cloud is only 10,000 light years across, so it is easy to say that the stars in the cloud are all at the same distance give or take 5%. This is why the clouds are recognized good testing grounds for stellar models.
Most extragalatic objects are measured in distances of millions or even billions of light years.
Visual Magnitude
Is a system of measuring the amount of light being received from a star or other luminous object. It was invented by an ancient Greek Astronomer, Hipparchus. The brightest star was given the place of first magnitude, stars half as bright were called second magnitude and so on till the faintest stars seen to the naked eye were made sixth magnitude. Hipparchus's records no longer exist, but his observations were to form the basis of Ptolemy's catalogue of naked eye stars.
The invention of the telescope and more accurate ways of measuring the light of the stars allowed for a refinement of the magnitude scale which was turned logarithmic in 1854 by Norman Pogson.
A first magnitude star is 100 times as bright as a sixth magnitude one. Therefore the difference between one magnitude and the next is a change of 2.512. To anchor this new system Pogson used the star Lambda Ursae Minoris, assigning it the magnitude of +6.55. By measuring this star against any other, astronomers are able to establish the magnitude of any give object. Because the constant is negative, fainter stars have larger magnitudes than brighter ones. The brightest star in the sky is Sirius at a magnitude of -1.4.
Absolute Magnitude
Absolute magnitude is an estimate of the stars brightness if it were placed a fixed distance from the Earth. This distance is 10 Parsecs. If the sun were place at 10pc distance it would appear as a +4.8 magnitude star. This magnitude scale allows astronomers to estimate the total energy output of all stars. A star with an absolute magnitude of +4.8 is not as luminous as a star with an absolute magnitude of -1.8.
Radial Velocity
Radial velocity, or velocity along the line of sight, is revealed by looking at the spectrum of a star. The absorption lines in a stars spectra are a result of specific elements. Hydrogen for instance will always absorb the same area of the spectrum. If the star is approaching the Earth the Hydrogen line will be shifted into the blue area of the spectrum, conversely if the line is moved toward the red end of the spectrum the star would be receding. The amount that the absorption line is moved will indicate the velocity as observed from the Earth. This measurement is not the real stars velocity, just what it appears to be to the observer. Stars in the galaxy, local to the sun appear to have a radial velocity below 30 km per second. The radial velocity of stars in the SMC appear to about 150 km per second.
John Herschel
- 1792 Born 7th March 1792 in Buckinghamshire England.
- John went to various schools during his early years. Dr Gretton's school in Hitcham and Eton College. He was also privatly tutored at home by Mr Rogers in mathamatics.
- 1809 John entered St John's college in Cambridge.
- 1812 He assisted with the formation of the Analytical Society to introduce new international methods of mathematics.
- 1813 Herschell graduated college,and was accepted into the Royal Society.
- 1816 John turned toward astronomy to carry one his fathers work.
- 1820 He was elected to the position of vice-president in the Astronomical Society.
- 1821 He was awarded the Copley Medal of the Royal Society for his work in mathametics.
- 1824 Published his first astronomical paper on double stars.
- 1825 Recieved the gold medal from the Astronomical society
- 1827 Elected President of the Astronomical society.
- 1829 Married Margaret Brodie Stewart.
- 1831 Knighted, and proposed as President for the Royal society which he lost.
- 1833 Sailed to South Africa to observe the souther skies.
- 1835 Observed the return of Haleys comet.
- 1838 Returned to England.
- 1839 Does pioneering work in photography.
James Dunlop
- 1793 Born 31st October 1793
- 1810 Began telescope building
- 1821 Appointed as assistant astronomer at the Parramatta observatory.
- 1827-31 Worked in Sir Thomas Brisbane's private observatory in Scotland.
- 1831-47 Appointed as the superintendent of Parramatta observatory.
- Remembered for his work "Catalogue of 7835 stars from Observations made at the Observatory at Parramatta".
- 1828 Awarded the gold medal from the Royal Astronomical Society
- 1848 Died 22nd September 1848.
Ferdinand Magellan
