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Lecture 3

# AS101 Lecture Notes - Lecture 3: Spectral Sequence, Absolute Magnitude

Department
Astronomy
Course Code
AS101
Professor
Ioannis John Haranas
Lecture
3

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Lesson 3: Stellar classifications and spectral sequence
-Stars like our sun have been around for billions of years
-humans have been around for 10,000 years
Brightness and luminosity
-stars are classified according to their luminosity and surface temperature
-luminosity: the total amount of power a star radiates into space
-our sun has a luminosity of 3.8x10^26 watts
-this means that every second, the sun radiates 3.8x10^26 joules of energy into space in all directions in
the form of light
Apparent brightness
-luminosity cannot be measured directly because its brightness
-the energy of a star depends on its distance away from us as well as its true luminosity
Apparent magnitude: the brightness of a star as perceived from Earth. Originally divided into 6
categories – the lower the number the brighter the star. Some stars only appear dimmer because
they are farther away
Absolute magnitude: intrinsic property of a star, true brightness, stars of the same apparent
magnitude do not usually have the same brightness because they are at different distances.
Calculate absolute magnitude by putting stars at the same distance from earth – 32.6 lightyears,
the distance of our sun from earth. Sun has absolute magnitude of +5. Stars with absolute
magnitudes greater than 5 (smaller numerical value) are brighter than the sun but only appear
dimmer because of their distance. Absolute magnitude allows us to figure out the distance away
from earth it is
How to measure apparent brightness:
Apparent brightness = luminosity/ 4(3.14)r^2
If we were closer to the star we could re-arrange the formula to get
Luminosity= 4(3.14) r^2 (apparent brightness)
Magnitude
Hipparchus:
-developed a magnitude system to classify stars according to their brightness
-system is still in use today
-today the system is more refined and some of the brightest stars have a negative magnitude
-Sirius has a magnitude of- 1.46.
The spectral sequence
-by studying the emission and absorption lines in a star’s spectrum astronomers are able to distinguish
differences in temperatures of different coloured stars
-the hottest stars are blue
-the coldest stars are red
-the sun is an in-between star
-blue stars are spectral type 0, the next hottest B, A, F, G, K, and M (red in colour)