U Cephei is an eclipsing binary system consisting of two stars that orbit their common centre of mass - AQA - A-Level Physics - Question 2 - 2022 - Paper 4

The average of the values in Table 1 differs from the laboratory value because the star's motion causes a Doppler shift in the observed wavelengths. This effect results in a slight increase or decrease in the wavelength as perceived from Earth.

To find the orbital speed, we can use the formula:

$v = \frac{2 \pi r}{T}$

where $r$ is the orbital radius and $T$ is the period of the star's orbit. From the data, if the period is known and we find an appropriate value for $r$, we substitute these values to verify the speed is approximately 250 km s^-1.

Using Kepler’s third law, we know that:

$T^2 = \frac{4 \pi^2 r^3}{G (m_1 + m_2)}$

Assuming typical values for mass and substituting into the equation will allow us to find the orbital radius of the primary star in meters.

The most prominent absorption lines would be those of hydrogen and helium, due to their abundance in stars of class B.

Astronomers could confirm the existence of a white dwarf and neutron star by studying the light spectrum emitted by the binary system. The characteristics of the spectral lines would indicate high temperature and density conditions consistent with a white dwarf, while the shorter wavelengths and potential pulsation observed would suggest the presence of a neutron star. Additionally, gravitational wave emissions or X-ray radiation signatures could provide supportive evidence.