State what is meant by a binary star.

The peak spectral line of Sirius B has a measured wavelength of 115 nm. Show that the surface temperature of Sirius B is about 25 000 K.

The mass of Sirius B is about the same mass as the Sun. The luminosity of Sirius B is 2.5 % of the luminosity of the Sun. Show, with a calculation, that Sirius B is not a main sequence star.

Determine the radius of Sirius B in terms of the radius of the Sun.

Identify the star type of Sirius B.

draw the approximate positions of Sirius A, labelled A and Sirius B, labelled B.

sketch the expected evolutionary path for Sirius A.

two stars orbiting a common centre «of mass»

Do not accept “stars which orbit each other”

«\(\lambda \) x T = 2.9 x 10^–3»

T = \(\frac{{2.9 \times {{10}^{ - 3}}}}{{115 \times {{10}^{ - 9}}}}\) = 25217 «K»

use of the mass-luminosity relationship or \({\left( {\frac{{{M_{{\text{Sirius}}}}}}{{{M_{{\text{Sun}}}}}}} \right)^{3.5}}\) = 1

if Sirius B is on the main sequence then \(\left( {\frac{{{L_{\,{\text{Sirius}}\,{\text{B}}}}}}{{L{\,_{{\text{Sun}}}}}}} \right)\) = 1 «which it is not»

Conclusion is given, justification must be stated

Allow reverse argument beginning with luminosity

\(\left( {\frac{{{L_{\,{\text{Sirius}}\,{\text{B}}}}}}{{L{\,_{{\text{Sun}}}}}}} \right)\) = 0.025

r _Sirius = «\(\sqrt {0.025 \times {{\left( {\frac{{5800}}{{25000}}} \right)}^4}} \) =» 0.0085 r _Sun

white dwarf

Sirius A on the main sequence above and to the left of the Sun AND Sirius B on white dwarf area as shown

Both positions must be labelled 

Allow the position anywhere within the limits shown.

arrow goes up and right and then loops to white dwarf area