Five times lighter than expected

Five times lighter than expected

Astronomers have come up with a new estimate for the total weight of the Milky Way: the galaxy that houses the Sun. It weighs about 200 billion suns. This makes the Milky Way four to five times lighter than expected.

Scientists regularly try to determine the weight of the Milky Way. This is difficult, because in addition to all the stars, black holes, nebulae and piles of gas, they must also include dark matter in the calculations. Canadian scientists came out in 2017 With the weight of 700 billion suns. In 2019, another research team came up with a different weight: 1.54 trillion (!!) suns.

Scientists have now used data from Gaia to accurately estimate the weight of our host galaxy. The Gaia catalog contains 1.8 billion stars. Thanks to this data, the researchers got a good picture of the so-called Rotation curve From our galaxy. The rotation curve is a representation of the speeds at which matter – such as stars – rotates around the center of the galaxy at different distances. This is often illustrated in a chart, as you can see below.

In this graph of the rotation curve you see the speed (y-axis) and distance from the center (x-axis). White dots and error bars show Gaia measurements. The blue curve connects these points. It is noted that in the outer regions of the Milky Way the speed decreases rapidly. This dip is also called the Kepler dip. This may be because there is little dark matter present.

Scientists have always assumed that dark matter contains six times the regular matter in the Milky Way. However, the strong decrease in Kepler’s level is clear evidence that our galaxies contain a much smaller amount of dark matter. The researchers write In the new paper About a third of the matter in the Milky Way is ordinary matter. The rest is dark matter.

This seems obvious, but it is not. Other spiral galaxies in the region do not suffer from Kepler decay. Therefore, our Milky Way Galaxy appears unique. How is that? One possible explanation is that the Milky Way has not experienced many collisions with other galaxies in the past. The last major merger with another galaxy occurred nine billion years ago, more than four billion years before the birth of our solar system. Another explanation could be different measurement methods. To determine the rotation curve of other galaxies, scientists look at neutral hydrogen gas. In the case of the Milky Way, a six-dimensional data set was used. Is it a matter of comparing apples and oranges?

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There is no doubt that the search for the true weight of the Milky Way will continue. Fortunately, it is thanks to this kind of research that we learn more about our galaxy and the distribution of dark and visible matter.

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