Review

#01

Parallax is a geometric method. The apparent shift in position of a star in the sky due to the Earth's motion around the Sun is related to its distance.

#02

A parsec is a distance. It is defined by a parallax of 1 arcsec, relating to a triangle with base 1 AU and height 1 parsec.

#07

Absolute brightness relates to luminosity, but apparent brightness relates to flux, involving both luminosity and distance.

#09

The classification has to do with the appearance and strength of different kinds of spectral lines, such as those of hydrogen, and these relate to the temperature of the photospheres of stars.

#10

One reason is that the stars are so hot that hydrogen is ionized. Without a bound electron, there can be no lines.

#11

Luminosity and temperature (or spectral class).

#12

The main sequence is a location in the H-R Diagram where one finds most stars. It refers to the period of time when stars are burning hydrogen in their cores. Where a star appears on the main sequence is governed mainly by its mass.

#13

In spectroscopic parallax, one uses spectral class to infer a luminosity for a star. Combining this with the apparent brightness gives the star's distance.

#17

Stars in binaries orbit one another because of gravity. The strength of gravity determines how fast stars orbit each other, and this strength is set by their mass. So by observing binary orbits (period or speed), one can get an idea of the masses of the two stars.