Stars - Observations

Andrew Conway

Dots of light

What can we ask about a dot of light?

Apparent magnitude

The Winter Hexagon

Winter hexagon

Source: Jomegat Copyright - see source.

Rigel and Capella


Parallax example

Source: Wikipedia CC-BY SA 3.0

Parallax and stars

Parallax of a star

Source: Wikipedia Public domain

Measuring parallax

  1. View a star on a particular date.
  2. Record its position - RA and declination.
  3. Repeat 6 months later when Earth is at the other side of its orbit.
  4. Record its position again.
  5. Calculate the angle between the old and new positions - this is the parallax.

Degrees, arcminutes and arcseconds

Parallax and distance

Distance to Capella

Distance to Rigel

Inverse square law

Inverse square law - diagram

Inverse square law Source: Borb CC-BY SA 2.5

You can understand the inverse square law by realising that light from a point source spreads out over larger areas.

Luminosity: Rigel vs Capella


Absolute magnitude


Star App. Mag. Distance (ly) Abs. Mag. Luminosity
Sun -26.7 0.000016 4.8 1
Sirius -1.4 8.6 1.5 22.5
Arcturus -0.1 36.7 0.3 114
Vega 0.0 25.3 0.6 50.1
Spica 1.0 262.1 -3.6 2250
Barnard's Star 9.5 5.9 13.2 0.00043
Proxima Centauri 11.0 4.2 15.5 0.000056

Colours of stars

Orion Source: Mouser CC-BY SA 3.0

Colour index

Absolute magnitude and colour index

The Hertzsprung Russell Diagram

HR Diagram

Source: Richard Powell CC-BY SA 2.5