Space: Determining Astronomical Distances

The parallax method is used to measure distances within our galaxy, making it a crucial tool in astrophysics.
It makes use of the apparent change in position of a star when viewed from two different locations.
The principle behind the parallax method is simple: as Earth orbits the sun, nearby stars appear to shift against farther stars. This shift is the parallax.
The parallax angle (p) is measured in arcseconds, which is a unit of angle used in astronomy.
By using basic trigonometry, we can calculate the distance of the star: d = 1/p, where d is the distance in parsecs and p is the parallax angle in arcseconds.

Cepheid variables are a type of star used as ‘standard candles’ to measure astronomical distances due to their regular pattern of brightness.
The brightness of these stars changes over a period of time, ranging from a few days to several weeks.
The period of their pulsation relates directly to their absolute magnitude, or their true brightness.
Once the absolute magnitude is known, the apparent magnitude (how bright they look from Earth) can be used to calculate the distance.
The relationship between the Cepheid’s period of variation and its absolute magnitude is known as the period-luminosity relation.

Red shift is a method used for determining distances to galaxies beyond the Milky Way.
Due to the Doppler effect, light from galaxies moving away from us is stretched to longer, redder wavelengths, hence the term ‘red shift’.
The faster a galaxy is moving away from us, the greater its red shift.
By measuring the red shift, we can calculate the recessional velocity of the galaxy.
The Hubble Law states that the recessional velocity (v) of a galaxy is directly proportional to its distance (d) from us: v = Hd, where H is Hubble’s constant.
Determining an accurate value for Hubble’s constant is a crucial goal in astrophysics, since it can help to estimate the age and size of the universe.