While stellar parallax can only be used to measure distances to stars within hundreds of parsecs, Cepheid variable stars and supernovae can be used to measure larger distances such as the distances between galaxies and even galaxy clusters.Before we learn further about astronomy, there are some basic knowledges that we must know and understand.įirst, we will talk about measuring distance in astronomy.Īstronomical object lies in a very great distance from us. How far away is KNOX0325 in cm? How far away is it in light years? A light year is the distance light travels in a year and the speed of light is 3×10^10 cm/sec. The angle at December formed by red background star - December - blue background star you cna then see is 2p. Because of this large distance, the line of sight from the December position to the blue background star is parallel to the line of sight from the June position to the blue background star. To understand this, you have to realize the background stars are much, much further away than the Earth-Sun separation - much further than it looks in the diagram. Relative to the background stars the cluster of stars is shifting by 0.03 arc seconds. The observed shift is actually 2p, with p as indicated in the diagram. This exaggerated view shows how we can see the movement of nearby stars relative to the background of much more distant stars. A nearby star's apparent movement against the background of more distant stars is referred to as stellar parallax. This is distinguished from the annual apparent motion in the sky caused by the Earth's orbit around the Sun. The change in a star's position in the sky as a result of its true motion through space is called proper motion. These factors of 2, at least in the small-angle approximation, cancel out.) The angle we measure here is 2p, as p is defined below, but the distance we are using is twice the baseline, as the baseline distance is defined below. (Note the possibility here for making some mistakes with factors of two. So the distance is about 6cm / sin(six degrees) = 6cm/(6*3.14/180) = 60 cm where in the first equality we used the small angle approximation after converting from degrees to radians. I got about 3 thumb widths for the shift from viewing with one eye to the other. It's about 6 cm between one pupil and the other. Supernovae are much much brighter than Cepheids, allowing us to observe them to even greater distances. Cepheids, in turn, can be used to calibrate type Ia supernova explosions. Once calibrated, then by determining their period, we can determine their luminosity and use them as standard candles to measure even further distances. With distances determined to some of them, that relationship can be calibrated. These stars have a relationship between their period and average lumniosity. Some of these nearest stars are Cepheid variable stars with a luminoisty that varies over time. The first rung on this ladder is the use of trigonometric parallax to determine distances to the nearest stars. We refer to this sequence of distance determinations as the distance ladder. We have to do it in steps, getting distances to nearby objects and then using those objects to calibrate other objects that can be used to get to even further distances. Here we cover the basics of how that is done. Key to observing the consequences of this expansion is the ability to measure distances to things that are very far away. We have seen from the previous chapters, at least on very large scales, the Universe is the same everywhere and that it is expanding.
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