09 Observational Properties of Stars

1. Distance



then the distance becomes 1 parsec (= 206265 AU = 3.1 × 1016 km).

Nearest Neighbors

Stellar Motion

Proper Motion

2. Luminosity

Inverse Square Law

So, to determine L, first measure object's apparent brightness, then d can be calculated

Magnitude Scale - Apparent Magnitude


3. Stellar Temperatures

Color Index

Spectral Classification


➤  A, B, C ... (strongest to weakest)

➤  O, B, A, F, G, K, M (dropped the others)

4. Stellar Sizes

Direct Geometry

Indirect Method

F  ∝  T4 

L  ∝  R2  x  T4 

Radius-Luminosity Law

5. Hertzsprung - Russell Diagram (H-R)

A plot of luminosity against surface temperature (or spectral class), known as an H–R diagram, is a useful way to compare stars. 

H–R Diagram of Well-Known Stars

Plotted here are the data for some stars mentioned earlier in the text.

H–R Diagram of Nearby Stars

Most stars have properties within the long, thin, shaded region of the H–R diagram known as the main sequence (MS)

H–R Diagram of Brightest Stars

An H–R diagram for the 100 brightest stars in the sky is biased in favor of the most luminous stars

In summary

H-R Diagram for 20 000 stars

6. Extending the Cosmic Distance Scale

Spectroscopic Parallax

A measurement of the apparent brightness of a light source combined with some knowledge of its intrinsic properties can yield an estimate of the source's distance.

Luminosity Class

In this way, giants and supergiants can be distinguished from main-sequence stars:

7. Stellar Masses

Visual binaries can be measured directly. This is Kruger 60.

Spectroscopic Binary

Properties of binary stars can be determined indirectly by measuring the periodic Doppler shift of one star relative to the other as they move in their orbits.

Eclipsing Binary

If the two stars in a binary-star system happen to eclipse one another, additional information on their radii and masses can be obtained by observing the periodic decrease in starlight as one star passes in front of the other.

8. Other Properties

Distribution of stellar masses. The more massive stars are much rarer than the least massive 

Mass Dependence

R-M : directly proportional.

L-M : L is much faster

Life time

9. Summary