Tech UPTechnologyHow do we measure the Milky Way from within?

How do we measure the Milky Way from within?


A galaxy is an inconceivably huge structure. A disorderly grouping of stars, gases, dust and other more exotic objects, isolated in space and held together by their own gravity . Our own is about 180,000 light-years across , contains more than a hundred billion stars, and packs a mass of about a million million times the mass of the Sun. It receives the name of Milky Way. Today we also know millions of other galaxies beyond our own, some just a fraction of the size and others several times larger.

The Sun, Earth, and the entire solar system are located in a region of the Milky Way galaxy known as the galactic disk , a relatively flat, circular region that contains the vast majority of the galaxy’s stars and interstellar gas . We know this from what we can observe from Earth. If we look at the night sky we will see a fairly uniform distribution of stars, except for a kind of band that crosses the sky and in which more stars are concentrated. This band is nothing more than what the Milky Way looks like from within. This position, however, makes it extremely difficult for us to see the rest of the galaxy . It is as if we were trying to figure out the distribution of paths, ponds and gardens within a large park without being able to leave a bench located near one of its ends. In some directions we will be able to see without too much hindrance , but in others the nearby trees or bushes will block our vision completely .

This is why, in order to try to elucidate the appearance of our galaxy, we have resorted to observing galaxies that we think should have a similar shape. We think that the Milky Way should be a spiral galaxy , with a disc formed by different arms, which rotates around a central region , more spherical and bulging, the galactic bulge or nucleus . This would be the region with the highest density of stars and which would contain at its center the supermassive black hole known as Sagittarius A*. Both the disk and the bulge would be surrounded by the galactic halo , a much more diffuse spherical region, populated by old and faint stars.

But all this has not always been so clear. It was not until the second quarter of the 20th century that we were able to appreciate that ours was one of countless galaxies that exist in the universe . Even before this and before we even knew how to distinguish between “our galaxy” and “the rest of the universe”, there were attempts to establish the shape and size of the galaxy. English astronomer William Herschel counted the number of stars he saw in each direction to try to create a map of the Milky Way. This was before the distance to a star was measured for the first time, so he assumed that all stars must be approximately equally bright and that observed differences in brightness would be due to distance rather than differences in the stars themselves. stars.

Herschel obtained that the Sun should be in the center of the galaxy and that it should have a size of about 30,000 light years , when in fact it is about six times larger and we are rather towards the outskirts. What Herschel did not take into account is that interstellar space is full of gas and dust that, at a sufficient distance, can completely block the light of distant stars. So he looked in the direction away from the galactic center, where there is less of this dust, without too much obstruction, but when he tried to look in the direction of the Milky Way’s core , where he should be looking at a lot more stars, his view was blocked . by this dust and this gas . We have now been able to see beyond the dust using wavelengths of light that Herschel did not have access to . He made his observations in visible light, but today we can observe in other wavelengths, such as X-rays, ultraviolet, infrared, and especially radio waves. The latter are able to pass through large clouds of gas and dust without too much hindrance , showing us what is hidden behind them.

Combining observations in new regions of the electromagnetic spectrum with different methods to measure the distance to stars and other objects in the universe, we were finally able to establish the true shape of our galaxy . Although we are now very certain that our galaxy is a spiral galaxy and of the approximate size, mass and number of stars it contains, the reality is that we do not know its exact shape and precise appearance , so the best we have are comparisons with galaxies. acquaintances that should have similar appearances and sizes.


Rix, Hans-Walter et al, 2013, The Milky Way’s Stellar Disk, The Astronomy and Astrophysics Review. 21: 61., doi:10.1007/s00159-013-0061-8



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