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# Blackbody radiation for Edexcel A-level Physics 1. Brightness and apparent magnitude
2. Absolute magnitude
3. The Hertzsprung-Russel diagram

Apparent magnitude measures the brightness of a star as it appears from the Earth. The Hipparcos scale is a logarithmic scale that was originally used for classifying stars based on their apparent magnitude. The scale has a minimum value of 1, which represents the brightest stars, and a maximum value of 6, which represents the dimmest stars. The Hipparcos scale can be used to relate brightness and apparent magnitude. When the apparent magnitude increases by 1, the brightness increases by a factor of 2.51. An astronomical unit (AU) is the average distance between the Earth's and the Sun's centres. A parsec is the distance at which 1 AU subtends an angle of 1 arc second. A light year is the distance which an electromangetic wave travels in a year in a vacuum. Absolute magnitude of an object is the apparent magnitude if the object was at a distance of 10 parsecs from the observer. Absolute and apparent magnitude can be related by the following formula: m − M = 5log(d/10), where m is the apparent magnitude, M is the absoltue magnitude and d is the distance of the object in parsecs. The Hertzsprung-Russel diagram is a diagram which classifies stars. It has luminosity as compared to the Sun on the y-axis and temperature in Kelvin on the x axis. The dimmer stars are found at the bottom, whereas the brighter ones are found at the top. The hotter planets are found on the left, whereas the cooler ones are on the right. # 1

Use the Hertzsprung-Russel diagram to compare the brightness main sequence stars and supergiants.

Most supergiants are significiantly brighter than main sequence stars, with only a few of them having about the same luminosity. # 2

Explain what a value of 6 on the Hipparcos scale represents.

A value of 6 means that the star looks the dimmest it possibly could from Earth. # 3

State what happens to the brightness of a star when its apparent magnitude decreases by 2.

When the apparent magnitude decreases by 2, the brightness decreases by a factor of 2.51². # 4

Define what a light year is.

A light year is the distance which an electromangetic wave travels in a year in a vacuum. # 5

State what happens to the brightness of a star when its apparent magnitude increases by 1.

When the apparent magnitude increases by 1, the brightness increases by a factor of 2.51. End of page