How many stars are there in the Milky Way? See interesting facts about the galaxy

NASA’s social media leader and manager of the James Webb Space Telescope mission website, Maggie Masetti says that 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 part), it would be extremely difficult to count its stars, especially from the position of planet Earth, within it and in its “suburbs”. At least for one reason, for example, there are no photos of the galaxy as a whole, but only some images of the so-called Galactic Plane, which is that bright stripe that is possible to photograph in the night sky.

Comparing the stars in the universe to the number of grains of sand on an Earth 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 after whom the famous telescope was named, made the discovery that changed one’s 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 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 orbital speed of stars and globular clusters around the center of the galaxy, 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 most recent estimate suggests that the Milky Way has a total mass of approximately 1.5 trillion solar masseswhich includes not only stars but also dust and dark matter. As this, even without emitting, absorbing or reflecting light, represents 90% of the total, we have that only around 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, because they vary enormously in sizeranging from small brown dwarfs to supermassive giants.

Based on that calculation that the Milky Way has around 1.5 trillion solar masses in total, we conclude that 10% of this represents visible matter, which is therefore equivalent to 150 billion solar masses.

Assuming that the majority 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 method of estimating the total mass can act as a kind of “second opinion” when compared to other ways of counting stars, such as direct observation of part of the galaxy, computational models of star formation and evolution and analysis of the total light emitted.

Gaia: counting stars in the Milky Way

Gaia Early Data Release 3 – Gaia Catalog of Nearby Stars – fly-through

Among the most advanced methods of counting stars in the Milky Way, there are, as of 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 reach the precise mapping of stars at distances of up to 30 thousand 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 galaxyit has provided, and continues to provide, the most accurate and comprehensive data on the Milky Way’s stellar population that exists. This allows astronomers to make more accurate estimates of the total number of stars and better understand the structure and composition of the galaxy.

According to Jos de Bruijne, a scientist who works directly on the Gaia galaxy mapping mission, the expectation is that, by the end of their mission, next year, scientists will have a more comprehensive view of the number of stars in the galaxy, but acknowledged to Space.com that “significant uncertainties are likely to remain.”

To learn more about Gaia and how she is mapping stars in the Milky Way, you can visit the mission website.