Observing and
Measuring Stars

Part B (Stars & Extremes)
➤ Week-4: The Sun & Stellar Properties
➤ Topic-2: Observing and Measuring Stars

1. Distance

Direct Method: Parallax

Parallax is the object's apparent shift relative to more distance background as the observer's point of view (defined as baseline) changes. The shifts are very small for even the closest stars. Thus, they are in arc seconds rather than degrees.

A New Distance Unit: Parsec

The unit name is defined as "Parsec = Parallax in arcseconds". Earth is at 1 AU (150 million km) away from Sun creating a baseline (see picture). If an object subtends 1 arcsecond from this baseline then the distance becomes 1 parsec (= 206265 AU = 3.1 × 1016 km).

Nearest Neighbors

To comprehend the large distances let's create a new scale using Sun-Earth distance; take "Sun - Earth" distance as "1 m" (note that end of Solar System becomes 50 m in this scale).

Proxima Centauri (alpha Centauri)

p = 0.76 " (the largest) = 1.3 pc = 270 000 AU = 4.3 ly
In new scale: 270 km

Barnard's Star (next nearest)

p = 0.55 " = 1.8 pc = 6.0 ly
In new scale: 370 km

Stellar Motion

Stars have "true space motion" which has two components:

Transverse : across the sky (location changes)
Radial : along the line of sight (using Doppler shift)

Proper Motion is defined from the "transverse" component: Annual motion of a star across sky as seen from Earth. Its value is then corrected for parallax:

Take the transverse component
Express the value in arcseconds per year = " / yr

For example, Barnard Star: moved 227 " during 22 years = 10.3 " / yr (largest)

2. Luminosity

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3. Stellar Temprature

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4. Stellar Sizes

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