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Using Optical Observations to find the Diameter of HT Cas
Above is an optical image of the sky including HT Cas. Only the central
star in the HT Cas binary system is visible at optical wavelengths.
Optical images can be used to find the angular size of an object, such as a
galaxy or a supernova remnant. Then, if you have the distance to the star
from the Earth, you can calculate the actual diameter of the central star
from its angular size using the relationship:
 .
Because of this relationship, you can solve for one unknown (the actual
size, or diameter, for example) if you know the other two (the angular size
and the distance, in this case). In fact, that is exactly what you are
going to do to find the size of the central star in HT Cas and answer the
challenge question. Once you find or measure HT Cas's angular size and its
distance from Earth, you can plug the values into the equation above and
solve for its actual size. The 1.5 x 108 is a conversion factor
to account for the units used in this problem.
The distance from Earth to HT Cas (found using parallax) is 140 parsecs.
Find the angular size of HT Cas
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