Life Cycles of Stars | Diffuse Nebula | Main-Sequence Stars | Red Giants after Main-Sequence | Death of a Low Mass Star | Death of a High-Mass Star | Star Families | Magnitude Scale | Measuring Stellar Distances | Classification of stars | Wien's Law and Stefan-Boltzmannn Law for a Blackbody | Stellar Spectra Measuring Stellar Distances There are a few units of measurement for measuring stellar distances, which astronomers have devised as the conventional units of measure would not be able handle as stellar distances are too vast. Light-year This is the most common unit of measure and what it means is the distance light travels in 1 year. Considering that light travels at 3 times 100 000 000 metres per second, The distance travelled in one year = 3 X 10 to the power of 8 metres times 3600 (no. of seconds in 1 minute) times 24 (no. of hours in one day) times 365 (no. of days in 1 year) and you get 9.460815 (this is about 946 080 000 000 000 metres) metres in a year! Imagine that distance! Yet it takes light approximately 4 years to reach the Earth from the closest star to the Sun, Alpha Centauri. Parsec Another favoured unit of measurement by astronomers, in simple terms, it means parallax second, where one parsec is equal to 3.26 light years. In a circle, the sum of the angles is 360 degrees, one degree is equal to 60 arc minute portions, and 1 arc minute portion is subdivided into 60 arc second portions. Therefore, one arc second (imagine the ‘smallness’ of this value) is equal to 1/60 arc minute or 1/3600 of a degree or 1/ (3600 times 360) of a circle. Similarly, a parsec is the distance at which a star is when it shows a parallax angle of 1 arc second, so can you imagine how small the star would appear? Astronomical Unit (AU) This unit of measure is equal approximately to the distance from the Earth to the Sun at about 93 million miles. Parallax Astronomers often use the parallax method of measurement to determine the distance to the stars. As Earth orbits the sun, the star to be measured in the foreground also seems to move with regard to the stars in the background. The angle through which the star moves over a period of 6 months is its parallax. The distance to the star can then be calculated by using simple geometry.
Knowing the parallax angle and the distance from Earth to the sun (1 AU) enables the distance (d) to be known using this formula, which indicates the relation between a star’s distance and its parallax:
Where d is the distance to the star in parsecs and p is the parallax angle of the star in arc seconds.
Worked Example:
Use the formula:
Substitute the value of p in:
Therefore, the distance from Earth to that star is 1/3 parsec, converting to light-years:
The distance to the star from Earth is 1. 087 light years. Life Cycles of Stars | Diffuse Nebula | Main-Sequence Stars | Red Giants after Main-Sequence | Death of a Low Mass Star | Death of a High-Mass Star | Star Families | Magnitude Scale | Measuring Stellar Distances | Classification of stars | Wien's Law and Stefan-Boltzmannn Law for a Blackbody | Stellar Spectra
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