Our galaxy gives birth to one sun per year

Using the Spitzer space telescope, astronomers have determined the rate at which new stars appear in the Milky Way.

 

The Spitzer space telescope (artist’s drawing). The study linked to the birth rate in our galaxy was carried out before this orbital observatory ran out of its instrument cooling helium in May 2009. Since then, Spitzer has seen its infrared sensitivity greatly diminish. This progressive loss of the coolant is perfectly normal. Credit: NASA

Our galaxy, the Milky Way, like its peers, has numerous star formation regions. Some of them like the Orion nebula (see this portfolio) have even been studied in detail.

How are new suns counted?
It has been noted that clouds of gas in these celestial nurseries cave in on themselves. The tremendous forces at work end up by starting nuclear reactions which constitute the “engine” of the stars: new suns, more or less massive than ours, are then born, accompanied by a disc of dust which will then become a series of planets.
Astronomers used to have great difficulty in quantifying the number of stars that could come to life in this way over a given lapse of time and they had to content themselves with indirect methods, such as measuring the radio waves generated by gas clouds where new stars were forming. However, a team of scientists led by Thomas Robitaille from the Harvard-Smithsonian Center for Astrophysics undertook the surveying of our galaxy using NASA’s infrared space telescope, Spitzer. To be exact, they examined 100 million stars and 20,000 “YSO”. These Young Stellar Objects are gestating stars which are difficult to detect in visible light as they are still opaque due to the gas cloud surrounding them. But in infrared light, Spitzer’s favourite wavelength range, these “YSO” shine. This much more direct way of counting has made it possible to assess our Milky Way’s birth rate: about one star the size of the Sun per year (the exact range goes from two thirds to one and a half).

This infrared photograph of our galaxy (from Earth we obviously see it edge-on as we are part of it) only covers a small part of the survey carried out by Thomas Robitaille and his colleagues using Spitzer. The white circles are “YSO”, Young Stellar Objects, stars in the making. The complete survey is 34 times bigger! Credit: NASA/JPL-Caltech/T. Robitaille (Harvard-Smithsonian)/GLIMPSE Team

A reasonable rate
Our galaxy is home to a hundred billion stars, this rate might seem low, even positively slow. But we must not forget that, first of all, this does not mean that there is only one gestating star per year. Quite the contrary, we know that the Milky Way is abounding in very active stellar nurseries. It simply means that an average of one sun reaches maturity every year whilst the others await their turn. We have to take into account the fact that the processes that result in a star coming to life take time! And lastly, astronomers reiterate that the galaxy in which our Sun evolves has been in existence for about 11 billion years. The birth rate there was previously much more active and it is, in fact, a good thing that it has eased up! The reason for this being that new stars are formed from clouds of gas released by those that are dying, thus creating a cycle; but, if the rate at which the young suns form was to speed up, the gas provided by the dying stars would no longer be sufficient and the cycle could then start to run down, sign of a declining galaxy... With a new sun on average each year*, the Milky Way is therefore maintaining a reasonable rate.

(*) One solar mass each year on average to be exact.

Published on 12 March 2010