Classification of stars

1. Classification of stars:

When you go outside on a dark moonless night, it is a wonderful sight to see the sky illuminated by thousands of stars. In total, there are about 6000 stars that it is possible to see with the unaided eye.
⇒Brightness:
The brightness of a star is a measure of how much visible light from the star reaches our eyes.
⇒Luminosity:
The luminosity of a star is the energy it emits per second, in all wavelengths.

Apparent magnitude (m) is a measure of the brightness of a celestial object as seen from Earth. It’s a logarithmic scale that decreases as the object appears brighter.

Figure 1 Apparent magnitude
Apparent magnitude is defined as:
[math] m = -2.5 \, log10 \, (F) [/math]
where F is the flux (energy per unit area per unit time) of the object.
Apparent magnitude is important in astronomy because it allows us to:
– Compare the brightness of different objects
– Estimate the distance to nearby stars (using parallax method)
Study the properties of celestial objects (e.g., luminosity, size, temperature)
⇒Classification of brightness:
Hipparchus was a Greek astronomer who lived some 2200 years ago. He was the first person to begin to categories stars according to their visual brightness in the sky.
Hipparchus began by cataloguing all the brightest stars, and these he called first-magnitude stars. Then he listed the next brightest, and called them second-magnitude stars, and so on until he reached sixth-magnitude stars.

The Hipparcos scale is a system used to measure the brightness of stars, specifically their apparent magnitude. It was developed from the data collected by the Hipparcos satellite, which was launched in 1989 and measured the positions, distances, and brightness of hundreds of thousands of stars.
The Hipparcos scale is based on the visual magnitude (V) of stars, which is a measure of their brightness as seen from Earth. The scale ranges from 0 (very bright) to 20 (very faint), with each step representing a change in brightness of about 2.5 times.
Figure 2 System of Hipparcos
A first-magnitude star turns out to be about 24 times the brightness of a second-magnitude star; and a second-magnitude star is about 2 times the brightness of a third-magnitude star.
Astronomers settled on the convention that a first-magnitude star is 100 times brighter than a sixth-magnitude star. This led to a modern, more precise, classification of a star’s brightness, or apparent magnitude, given the symbol m.
The modern scale extends below I for the very bright stars, and above 6 for dull stars, which we can see using binoculars or telescopes. Table 1 shows a list of some bright stars, seen in the night sky.
Table 1 Apparent magnitudes of some bright stars visible in the night sky.

Earlier, you learnt that the ratio of the brightness of a first-magnitude star to a sixth-magnitude star is 100, and that there is a constant ratio (which we shall call r) between each successive magnitude of brightness (about 2). This leads to two equations:
[math] \frac{I_1}{I_6} = 100 [/math]
Defining the ratio in brightness between first and sixth-magnitude apart in brightness is
[math] I^5 = 100 [/math]
Therefore, the ratio of brightness between stars that are one magnitude apart in brightness is
[math] r = 100^{\frac{1}{5}} = 2.51 [/math]
Referring to table 1 can see that Vega has an apparent magnitude of 0.0 and spica an apparent magnitude of 1.0. This means that Vega is [math] 2.51 × 2.51 ≈ 6.3[/math]times brighter than spica.
Since, Polaris has an apparent magnitude of 2.0, it means that Vega is times brighter than Polaris.
Brightness is a subjective measurement that can vary from person to person. The perception of brightness can be influenced by various factors such as:
– Individual visual acuity and sensitivity
– Adaptation to lighting conditions
– Surrounding environment and context
– Psychological and physiological factors