Measuring the size of the universe is difficult. The exact rate of the universe's expansion is not yet known. Calculating how far away objects are is done by using the Cosmic Distance Ladder. The idea is that different objects can be rungs on a ladder to measure different distances, from looking at the movements of stars to observing cepheid variables.
The objects need to be adjusted to each other in order to be accurate. The speed of the changes in brightness is related to the true brightness of the cepheid variables. We can work out the true brightness of a star when we compare it to its apparent brightness.
We can compare the apparent brightness to the true brightness of the explosions because they always have the same level of brightness. To measure distance accurately, we need a way to check that the distances are accurate.
UGC 9391 contains cepheid variable stars and hosted a type Ia supernova. This week's image from the Hubble Space Telescope shows a useful tool.
Hubble scientists explained that theUGC 9391 provided a natural laboratory in which to compare two measuring techniques. Hubble wants to quantify how quickly the universe is expanding.
There is a recommended video.
The best pictures from the telescope.
The Hubble Space Telescope was launched 30 years ago on April 24th.
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