![]() Recall that the first successful method for measuring the distance to stars was trigonometric parallax. Here is another place where astronomical jargon can be confusing. ![]() Even if all of the individual estimates are off by 10%, the measurement error associated with any one star becomes less important.Īstronomers often use a method of fitting the HR diagrams of clusters to a standard HR diagram in order to measure the clusters' distances more precisely, since this technique uses all of the stars to get a distance measurement. If you can measure many stars in the cluster and get many estimates of the distance, you can get a more precise estimate of the true distance by taking the average of all of the individual distance measurements. Since all measurements that you make have some error associated with them, though, if you measure the distance to one star, your estimate of its distance may be off by as much as 10% (the typical level of precision for many techniques). Similarly, you can measure the distance to the cluster if you can find some technique to measure the distance to any single star. For example, you can measure the age of the cluster by estimating the age of the Main Sequence Turn-Off, as we just saw. The megaparsec (Mpc) is commonly used to describe distances of one million parsecs.Because cluster stars are so homogeneous in their properties, we can often measure a few stars in the cluster and then generalize the result and apply it to the whole cluster. The centre of the Milky Way is over 8kpc from the earth.Īstronomy – despite the enormous distance it describes, the parsec is a relatively small unit in astronomical terms.Proxima Centauri – the nearest star to the Earth other than the Sun, is 1.29 parsecs away.Turner’s was accepted – parsec being derived from the definition of the unit as the distance from the sun to an astronomical object which has a parallax angle of one arcsecond. A unit of distance useful in astronomy had been defined but was without a name, and the Astronomer Royal appealed for suggestions. The term parsec was coined by British astronomer Herbert Hall Turner in 1913. With the parsec defined, deriving and describing huge distances became easy, since a distance i The distance from the Sun of a body with a parallax angle of 1 arcsecond was thus defined as a unit and, thanks to Turner, named the parsec. Once the parallax angle is established you can calculate the distance to a star using trigonometry, because we know Earth’s distance from the Sun. The parallax angle is obtained by halving the angular difference in measurements. The parallax angle is found by measuring the parallax motion (or apparent movement of a star relative to stable, more distant stars) when the star is observed from opposite sides of the Sun (an interval of six months on Earth). The parsec is a unit of length equivalent to around 20 trillion (20,000,000,000,000) miles, 31 trillion kilometres, or 206,264 times the distance from the earth to the sun.Ī parsec is also equivalent to approximately 3.26 light years (the journey distance if you travelled at the speed of light for three years and three months).Īstronomers used trigonometry to calculate the distance to stars long before the term parsec was coined, but the new unit made it easier to conceptualise unfathomable distances.Ī parsec is the distance from the sun to an astronomical object which has a parallax angle of one arcsecond (1/3600 of a degree).
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