# How to measure distances in the universe?

How to measure the distance between stars in space? ? This is the question Jean-Louis Vayre for writing Science and the Future on our Facebook page. This is our Question of the Week, and here is our answer. Thank you all for your participation and loyalty.

## The light year as a unit

The nearest star to the Sun is Proxima Centauri, located 4.22 light-years away 39.9 trillion kilometers! Farther from us, the central black hole in the Milky Way, Sagittarius A *, lurking at 26,673 shelters, and the most distant galaxy ever discovered, HD1, is extraordinarily far away at. 13.5 billion light-years … A record that will forever remain an abstraction for our brains that cannot imagine such distances. But how do we measure them?

It all depends on the presumed proximity of the stars, whose distance we want to calculate. For those who are in our “suburbs” and up to a hundred al, astronomers use the annual parallax method. It consists of measuring twice at six-month intervals, the apparent orbit of a star, for example, against the background of distant stars. This displacement, which is in fact associated with the motion of our planet in its orbit, describes an ellipse whose size depends on the distance from the star. Thanks to the measurement of the angular displacement, we can geometrically calculate the distance of the star.

For more distant stars or galaxies, it is necessary to use other methods, as the apparent motion of the object to be studied is too weak. It is then necessary to resort to “standard candles”, a kind of headlights scattered throughout the Universe, and which always have the same absolute brightness. The distance that separates us from these candles is then calculated from their apparent brightness. The first to be used from the beginning of the 20th century are the Cepheids, large stars below 200 million years old, very clear and whose brightness varies regularly, between 1 and 135 days. The variations in their brightness are due to movements of their outer layers, which retreat and expand under the influence of the pressure and gravity acting in them. These stars are used, for example, to calculate the distance of the stars sheltered in the galaxies close to the Milky Way.

## Measure very long distances

To further map the universe, astronomers have searched for other standard lights. Type Ia supernovae fulfill this role: they occur during the explosion of stars called white dwarfs and located in binary systems. The brightness of these supernovae is very strong. All white dwarfs have a similar mass before exploding in a supernova, the difference between the absolute brightness and the apparent brightness makes it possible to determine distances and therefore use them as benchmarks. It was to calculate the slowdown in the expansion of the universe that the winners of the Nobel Prize in Physics in 2011 had launched huge observations of type Ia supernovae. Their work led to the opposite conclusion: the universe is in accelerated expansion! And this acceleration would be the result of a mysterious dark energy whose nature has still not been revealed despite intense research.

Finally, to calculate the distance of the most distant galaxies located billions of light-years away, astrophysicists rely on the Doppler-Fizeau effect, which generates a phenomenon called “redshift”. To put it simply, when a distant object moves away from us due to the expansion of the universe, its light shifts toward long wavelengths, corresponding to red and infrared at a distance. It’s exactly the same phenomenon that changes the sounds of firefighter sirens towards the bass or treble, depending on whether their vehicle is moving away from or approaching our ears. But astronomers still seek to refine their measurement, as the acceleration of the expansion of the universe can disrupt the calculations. One of the predicted paths is the use of gravitational waves, which would then be used as “standard sirens”.