Object: eta Cassiopeiae

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C
: Cassiopeia
: 00:49:05 / +57.48
: 3.5 / 7.4
: 13”
: 319°
: G0V dM0
: Yellow / Orange-red
Detail sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 19/08/09 / 01:00
: Landgraaf
: 3 / 5
: Orion Optics UK 300 mm
: 12 mm Nagler type 4
: 133 / 37
Eta Cassiopeiae

Observing Report

Eta Cassiopeiae is a strikingly colourful pair of stars. Eta's primary is bright yellow, while its dim companion looks deep orange with a hint of red. This attractive couple lies within an asterism that, using the right magnification, looks like a small dolphin (see my sketch for details). The dolphin asterism consists of 13 stars, oriented southwest northeast, with eta Cassiopeiae situated at the tip of the tail fin. Eta Cassiopeiae was allready easily split in the 22mm Nagler. After trying all possible magnifications, I liked the view of both the double and the asterism best using the 12mm Nagler T4, giving a magnification of 133x and a field of view of 37'.


The common name for Eta Cassiopeiae is Achird, a name with no real meaning. William Herschel probably first discovered this double star in August 1779. According to his notes in his journal for august 1779 he came across Eta Cassiopeiae while selecting double stars he wanted to use for determining the stellar parallax.
Stacks Image 53
In the left margin you see that Herschel examined alpha, beta, gamma, delta, epsilon and eta Cassiopeiae (the parts about eta are marked in a light green color). He writes about these 6 stars: alpha is double but too far asunder. beta, gamma, delta, epsilon are single. eta is double and will do. Source: The Herschel Archive
Both Eta Cassiopeiae A and B are situated at a distance of 19 light-years. It is a physical binary with a period of 480 years and the separation varies between 5" and 16". At the moment Eta Cassiopeiae A and B are separated by 13" . As you can see on the image to the right, the position angle is 319 degrees. South is up, east is to the right, the same orientation as my sketch. (Source: Voyager by CarinaSoft)

Eta Cassiopeiae is a very interesting couple for visual observers, especially if you realise what you are actually looking at. The magnitude 3.5 A component is a star almost identical to our Sun, a G0 (yellow) main sequence dwarf star. Its radius is about 1.03 Suns, so its almost the same diameter as our Sun. The luminosity of eta Cassiopeiae A is 1.28 Suns. This star gives you an idea what our Sun would look like when placed at a distance of 19 light years. The magnitude 7.4 B component is a main sequence M0 Red dwarf. Because it is situated at the same distance as the A-component you can see how the brightness of stars drops dramatically as you go down to the cooler regions of the main sequence of the HR diagram.

On the "home-made" HR diagram below you see the the relative positions of eta Cassiopeiae A (yellow dot) and eta Cassiopeia B (red dot) on the main sequence, based on data from Voyager by CarinaSoft (Absolute magnitude and Temperature).
Stacks Image 104
HR Diagram Eta Cassiopeiae