Known to all astronomers, the Cepheids, of variable stars, are used as benchmarks to measure distances and made it possible for Hubble to discover its famous law. But they become unusable beyond a hundred million light years. However, we can go much further with a particular class of Cepheids from a group of American astronomers.

At the beginning of last century, despite the arguments of Wright and Kant, most astronomers believed that galaxies were just objects within our own Milky Way. Everything changed with the discovery in 1912 by Henrietta Leavitt of a precise mathematical relationship between the brightness of stars variables of Cepheids, their period of pulsation, from the stars which they had identified in the two Magellanic Clouds.

We now know that Cepheids are giant stars of class I merge their heart helium in carbon. The star itself is enriched in helium. However, when the temperature of the star increases, its helium ionizes upper layers which increases the opacity of the star. The radiation pressure becomes stronger, it can counteract the forces of gravity and the star expands, becoming brighter as its surface increases. In doing so, its temperature decreases and the helium ions eventually capture electrons. The opacity of neutral helium lowering the radiation pressure drops and gravity of the star contracts. Its surface thus decrease its luminosity and the star is at the beginning of a new cycle of pulsation.

Four to fifteen times more massive than our Sun, the Cepheids are bright, 100 to 300,000 times more than our star. The relationship found by Henrietta Leavitt provides a powerful means to determine the distances of galaxies with Cepheids. Indeed, the precise relationship between brightness and pulsation period gives an estimate of the absolute magnitude of these stars. Therefore, by comparing their apparent magnitude with absolute, obtained by the Leavitt relationship, we can estimate the distance of the star. It is the same principle which makes it possible to know the distance from a candle according to its luminosity, it will be all the more weak as the candle is far.

Using the relationship of Henrietta Leavitt, Hubble showed in 1923 that the Andromeda galaxy was located more than a million light years (it is estimated this distance to at least 2.4 million light years). Given its apparent size, it should be of a size comparable to that of the Milky Way. The realm of galaxies and the universe Islands of Kant-Wright needed now to humanity.

Henrietta Leavitt.

The relationship of Henrietta Leavitt is calibrated according to the distances of Cepheids determined by means such as parallax and is therefore not free of errors. It in turn serves to calibrate the Hubble law at the cost of uncertainty. For astronomers, there is a range of distances that can be determined through a succession of tools operating at increasing scales. Propagating errors, the estimated distance becomes less and less precise as we delve into the depths of the observable universe. In particular, over a hundred million light-years, the Cepheids are bright enough to be easily usable. Their brightness drowns in the galaxies observed.

Editor Krzysztof Stanek, a professor of astronomy at Ohio University, Jonathan Bird, a PhD in astronomy, became interested in a class of Cepheids little studied, the ultra-long periods (ULP).

Very rare, it was thought that they behaved differently from normal Cepheids and could not be used to estimate distances. In 2006, however, Stanek had shown that one could nevertheless do it by measuring with one of it the distance from the M33 galaxy starting from a ULP contained in a binary system.  The distance found was 3 million light years which was determined by other means.

By studying 19 ULP Cepheids, Bird has confirmed that they could be used to estimate distances with an error of 10 to 20%. Because they are much more brilliant than the classical Cepheids, they should make it possible to probe the Universe up to larger distances, of about 300 million light-years which is three times farther than usual Cepheids.

This is one more tool for cosmologists, and it will perhaps learn more about dark energy and the fate of the universe.

ULP Cepheids in the galaxy M81

Credit: Ohio State University