The sun’s sibling stars, astronomical lecture

On Saturday, 10 May, there was a lecture in the old astronomical observatory in Leiden, the Netherlands. Its subject was the “siblings” of the sun, the thousands of stars which formed about 4.6 billion years ago along with the sun. The photos in this blog post were made with a smartphone.

The lecturer was Professor Simon Portegies Zwart.

Stars were born and are born throughout the universe, he said. Sometimes, 100 together. Sometimes 100 million together. Usually, somewhere between 1000 and 10,000 together, as probably happened when the sun arose.

These many stars very probably have many planets; called ‘exoplanets‘, as they are outside our solar system. So far, about 1000 exoplanets are known, thanks to the Hubble telescope and other recent research. It is far easier to discover a new star than a new exoplanet; as a planet’s light, compared to a star, is like a firefly’s light near a lighthouse. At least some exoplanets have exomoons.

So, new dots in the sky get planet status. On the other hand, a solar planet recently lost planet status: Pluto. Most astronomers were in favour of changing Pluto from planet to dwarf planet. Many United States astronomers were against; probably, Professor Portegies Zwart said, because Pluto was the only planet discovered in the USA. The congress deciding on Pluto was in Prague, a relatively easy place to get the demotion of Pluto accepted.

Why did astronomers no longer want to accept Pluto as a planet? Mainly, the lecturer said, because of its eccentric and highly inclined orbit. While “real” planets go around the sun in similar orbits, close to a flat reference plane called the ecliptic, Pluto clearly differs.

The eccentric orbit of Pluto may have been caused by a sibling star approaching the sun, creating chaos.

This may have happened when ten stars were together within a circle of 1,6. parsec.

On a much bigger scale than planetary systems, there are galaxies. Like the Andromeda galaxy, with probably over a trillion stars; more than ‘our’ Milky Way, which together with Andromeda and about 50 smaller galaxies forms the Local Group. The Local Group, again, is part of the Virgin Supercluster; or Local Supercluster.

Earth's position in the universe

The lecturer showed an animation about how the sun and its sibling stars were close together shortly after their origin.

Then, they started orbiting through the Milky Way. Differences in their speed, orbits, etc. made them drift further and further apart.

Which stars are siblings of the sun? That is not an easy question. Astronomers need complex calculations with complex computers for that. Star HD 28676 may be a good candidate. The solar sibling stars, according to recent research, are still fairly close together. “Close” in a cosmic sense, with, measured by planet Earth standards, still enormous distances between them.