school energy&environment

Star groups: reality or fiction?

Constellations are groups of stars which form certain familiar shapes in the sky. Nevertheless, the celestial sphere is simply a two-dimensional projection of the universe that surrounds us centred on our planet. Thus when considering the third dimension, which is depth, stars belonging to the same constellation are not bound together in any way, in fact, they are often considerably far from one another. Therefore the belief that certain constellations may influence people’s lives is apparently groundless.
Yet this doesn’t mean that stars live alone, on the contrary. Often stars form complex systems with two, three or more components bound together by their mutual gravitational attraction.
The most common configuration are double stars, which are pairs of stars that orbit around a single centre of gravity. In particularly tight systems, the two components can exchange masses, sometimes consistently. In fact, the gravitation force of the most dense and compact star draws in matter from the more expanded mate, even if their masses should be the same size.
In addition to doubles, one can find multiple systems and real masses, groups of stars bound together by gravity. The latter are divided into two families: open masses and globular ones. The first ones are made up by a number of newly formed stars ranging from ten to a thousand and are all gathered in an area with an approximately ten light year diameter. The open masses are in fact proof of the stars’ young age because they tend to be born in clusters within our galaxy’s disk. Within scale times of two to three billion years open masses will break up: gravitational interactions act as slingshots and end up expelling all the massed stars. Numerous open masses are sufficiently close for us to see them with the naked eye: the brightest one of them all is the one of the Seven Sisters in the Taurus constellation, only 425 light years away from Earth.
Globular masses instead are totally different. They are more unusual than the open ones, they can be made up of over a million of stars gathered in no more than about one hundred light years: they are so dense that they can survive much longer against gravitation attacks which instead break up all the young open masses. In fact globular masses are made of very old stars, that were born when the galaxy was still being formed, and they are spread in a spherical halo around the centre. Scientists study them just because they can foster secrets on the formation of the Milky Way itself.