NASA’s social media lead and James Webb Space Telescope mission website manager Maggie Masetti says questioning the how many stars are there in the Solar System is something that always results in an interesting, but possibly frustrating, answer. In practice, there is no 100% firm and solid answer, no matter how many estimates can be made.
In the case of Milky Way (which is the spiral galaxy of which the Solar System is a part), it would be very difficult to count its stars, especially from the position of planet Earth, within it and in its “suburbs”. For example, there are no photos of the galaxy as a whole, but only a few images of the so-called Galactic Plane, which is that bright band that can be photographed in the night sky.
Comparing the stars in the universe to the number of grains of sand on an Earth’s beach, the European Space Agency (ESA) website proposes measuring a small representative volume of sand and multiplying it by the proportion of the total area.
A hundred years ago, astronomers thought that all the stars in the universe were part of the Milky Way, but that changed in 1924, when Edwin Hubble, the American astronomer for whom the famous telescope was named, made a discovery that changed the understanding of the universe: he calculated the distance to the Andromeda Nebula — which came to be known as a galaxy. It was too distant to be part of the same galaxy as the Solar System.
Weighing the galaxy to estimate how many stars there are in it
To estimate the number of stars in the Milky Way, it is first necessary to know the mass of the galaxy. But “weighing” a star cluster of this magnitude is an indirect and complicated process.
Based on theoretical models and direct observations, the process involves analyzing the rotation of the galaxy and the spectrum of light it emits.
Measurement methods include: observe the orbital speed of stars and globular clusters around the center of the galaxy, the gravitational effects of the Milky Way in the movement of neighboring galaxies and the distribution of dark matter inferred.
After overcoming some challenges, such as defining the exact edges of the galaxy, the latest estimate suggests that the Milky Way has a total mass of approximately 1.5 trillion solar masses which includes not only stars, but also dust and dark matter. Since this, even without emitting, absorbing or reflecting light, represents 90% of the total, we have to assume that only about 10% of the galaxy’s mass is visible matter, that is, stars, gas and dust.
By astronomical standards, the Milky Way is considered a large galaxy, although not as massive as neighboring Andromeda, for example.
Counting the stars of the Milky Way
The next step involves isolating the stellar mass, which is a significant part of that 10% of matter determined in the “weighing”.
To arrive at a total number of stars, it is necessary to consider an average mass for these bright objects, which is not easy, as they vary enormously in size ranging from small brown dwarfs to supermassive giants.
Based on that calculation that the Milky Way has about 1.5 trillion solar masses in total, we conclude that 10% of that represents visible matter, which is therefore equivalent to 150 billion solar masses.
Assuming that most of this visible matter is made up of stars, we can use an average star mass close to that of the Sun and estimate that there are around 100 to 400 billion stars in the Milky Way . This is the current consensus used by astronomers.
It is important to highlight that these estimates are based on the best current understanding and are subject to revision as new methods of observation and analysis emerge.
Although indirect, this estimation method based on total mass can function as a kind of “second opinion” when compared to other ways of counting stars, such as direct observation of part of the galaxy, computer models of star formation and evolution, and analysis of the total light emitted.
Gaia: counting stars in the Milky Way
Among the most advanced methods of counting stars in the Milky Way, there are, since December 2013, ESA’s Gaia mission . This observation satellite has already managed to map the positions of 1.7 billion stars, from the interior of the Sun to a distance of 326 light years.
Capable of extremely precise measurements, which include the precise mapping of stars at distances of up to 30,000 light-years, the Gaia probe’s main objective is to create an extremely precise three-dimensional map of the Milky Way. In addition to measuring position, distance and movement, it also collects data on the brightness, temperature and composition of stars.
And although don’t literally count every star in the galaxy it has provided, and continues to provide, the most accurate and comprehensive data available on the stellar population of the Milky Way. This allows astronomers to make more accurate estimates of the total number of stars and to better understand the structure and composition of the galaxy.
According to Jos de Bruijne, a scientist working directly on the Gaia galaxy mapping mission, scientists are expected to have a more comprehensive view of the number of stars in the galaxy by the end of its mission next year, but he acknowledged to Space.com that “significant uncertainties will likely remain.”
To learn more about Gaia and how it is mapping stars in the Milky Way, you can visit the mission website.
Source: CNN Brasil
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