A team of astronomers recently began to accurately locate the edges of our Galaxy. These new results suggest that the Milky Way has a radius of 520,000 light-years.
It is very complicated in astronomy to define real limits. Take for example the solar system. Some will say that its limit is at the level of the heliosphere, 19 billion kilometers from the Sun. It is indeed from this line that we enter the interstellar medium. But for others, to truly measure the limit of our system, we must take into account the true zone of influence of our star. If this is the case, then the solar system extends to the outer edge of the Oort cloud, one light year from the Sun. So we have two different readings. For the Milky Way, it’s a bit the same principle.
The edges of our Galaxy
The diameter of our galaxy is generally estimated at about 100,000 to 120,000 light-years. But there are beyond this limit other stars that could be considered part of the Milky Way, even if the volume of objects is much lower.
According to a group of scientists, we should take them into account, since they remain under the influence of the galactic center. Starting from this principle, where exactly is the edge of our galaxy? Finding out is not easy. It’s like trying to determine the outer limit of a forest from its center. Just by observing the trees.
In an attempt to determine it, astronomers have focused on so-called blue horizontal-branch (BHB) stars. These objects are naturally sufficiently old and luminous to determine the distance that separates them from the Earth. For the calculations, the researchers had to rely on the Hyper Suprime-Cam (HSC) digital camera, installed on the 8.2-meter-diameter Subaru telescope in Hawaii.
520,000 light years from radius
By calculating the population of BHB stars in the galaxy’s halo, they then noticed a sharp drop in density about 520,000 light-years from the galactic center. In other words, they identified the outermost edge of the Galaxy. To give you a more precise idea, 520,000 light-years of radius represent about 20 times the distance between the galactic center and our solar system (26,000 light-years).
The researchers were also able to determine that the stars in these outermost regions (about a billion only) were about 12 billion years old. Very, very old objects therefore, formed during the first ages of the Milky Way.
These new results allow us today to better “know”, once again, our Galaxy, and to imagine its history with more precision. Our dear Milky Way, after the recent calculations, still remains a little smaller than its neighbor Andromeda. The researchers have estimated — with the same method — that the largest galaxy of the local Group has an expanded radius of about 538 000 light-years.