# How do they know distance of the original standard candle?

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1. Jul 23, 2015

### james gander

First thing i want to do is tell you what i do know.

I know to that astronomers use a standard candle such as a Type 1a supernova or a cephied varibale because they are always the same brightness.

I know that a light source is four times less bright when it is twice as distant.

I know that they use supernovae to calculate the distances of galaxies using this information.

The problem is for this method to work then surely they needed to know the distance to the original supernova.

How do they measure the distance of the original original light source they are using as a standard candle?

2. Jul 23, 2015

### Bandersnatch

Last edited by a moderator: May 7, 2017
3. Jul 23, 2015

### marcus

In some treatments of the "distance ladder" there is a stage or "rung" in the ladder that comes between parallax and the H-R diagram comparison
It is called "moving cluster" method.
You use doppler shift to tell radial speed. For example if the cluster is moving away then the stars should be be making a smaller and smaller angle to each other. Depending on how far. So you watch the cluster for some tens of years. Then you use TRIGONOMETRY to calculate the distance to the cluster.
This is for small nearby clusters in our galaxy. The doppler shift is for ordinary motion, not cosmological expansion. The Hyades are a good example of a moving cluster that helps to calibrate distance scale.
===quote https://en.wikipedia.org/wiki/Open_cluster#Astronomical_distance_scale ==
Determining the distances to astronomical objects is crucial to understanding them, but the vast majority of objects are too far away for their distances to be directly determined. Calibration of the astronomical distance scale relies on a sequence of indirect and sometimes uncertain measurements relating the closest objects, for which distances can be directly measured, to increasingly distant objects.[55] Open clusters are a crucial step in this sequence.

The closest open clusters can have their distance measured directly by one of two methods. First, the parallax (the small change in apparent position over the course of a year caused by the Earth moving from one side of its orbit around the Sun to the other) of stars in close open clusters can be measured, like other individual stars. Clusters such as the Pleiades, Hyades and a few others within about 500 light years are close enough for this method to be viable, and results from the Hipparcos position-measuring satellite yielded accurate distances for several clusters.[56][57]

The other direct method is the so-called moving cluster method. This relies on the fact that the stars of a cluster share a common motion through space. Measuring the proper motions of cluster members and plotting their apparent motions across the sky will reveal that they converge on a vanishing point. The radial velocity of cluster members can be determined from Doppler shiftmeasurements of their spectra, and once the radial velocity, proper motion and angular distance from the cluster to its vanishing point are known, simple trigonometry will reveal the distance to the cluster. The Hyades are the best known application of this method, which reveals their distance to be 46.3 parsecs.[58]

Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching the main sequence on the Hertzsprung-Russell diagram for a cluster at a known distance with that of a more distant cluster, the distance to the more distant cluster can be estimated.
==endquote==

4. Jul 24, 2015

### Bandersnatch

Cheers marcus, I wasn't aware of this one.