What is the end of the galaxy
"At the other end of the Milky Way"
With a network of radio telescopes, astronomers have looked into the spiral arms of the Milky Way and measured the distance to hundreds of stars. From our point of view, one of the objects examined is at the other end of the Milky Way - around 66,000 light years away, as the team now reports in the journal Science. In an interview, first author Alberto Sanna from the Max Planck Institute for Radio Astronomy explains how this extraordinary find was made and what the observations reveal about the structure of our galaxy.
World of physics: You measure the structure of the Milky Way with the help of parallaxes. How exactly does it work?
Alberto Sanna: This is one of the oldest and most precise methods of determining distances in our Milky Way. We consider the position of a star at different times of the year. As the earth rotates around the sun, the angle at which we can see the distant star also changes. We know the distance between the earth and the sun. From the angle differences, we can then use elementary geometry to determine the length of the legs in this triangle. The crucial thing about this trigonometric determination is that we do not need any further physical assumptions. Other methods of determining distance are based on additional hypotheses and therefore bring additional uncertainties into the distance measurements. Or they only work in certain areas of the Milky Way.
Distance measurement principle
How exactly can you determine the distances?
That depends on the distance and on the observation conditions. The further away an object is from us, the smaller the angle at which we can observe it at different times of the year. This means that the error also increases disproportionately. In order to be able to measure very distant stars in tens of thousands of light years, you need an extremely precise angle measurement. Fortunately, the Very Long Baseline Array - a network of large radio telescopes - delivers such a high angular resolution. With this instrument, we see so clearly that we could distinguish two objects on the moon that are only half the distance between them as a cent coin. Of course, these objects would still have to glow in the radio range so that we can measure them.
And what did you find?
We examined some of the spiral arms of our galaxy and were able to precisely measure several hundred heavy, young stars. With these measurements, we have now caught a special catch: We have succeeded in determining the furthest exact distance to a star in the Milky Way so far. From our point of view, one of the objects examined is at the other end of the Milky Way. This object lies at a distance of around 66,000 light years, with the uncertainty in determining the distance being around ten percent. For comparison: the galactic center is about 27,000 light years away from us.
Position of the earth and the star under study in the Milky Way system
The opposite side of the Milky Way can hardly be observed with ordinary telescopes because it is hidden behind a thick veil of dust. How did you manage to take a look at this region?
In fact, the center of the Milky Way is permeated with huge clouds of gas and dust that swallow normal light. As a result, we know our own galaxy much worse than other galaxies that we can analyze from above. Our research group does not work with optical, but with radio telescopes. Radio radiation is only slightly disturbed by the galactic dust clouds. We have also exploited a special effect: young, heavy stars - ten times the mass of the Sun or even heavier - shine particularly strongly and are often surrounded by a dense cloud of molecules. Later, in the course of their evolution, these stars blow these molecular clouds out into space. But in the early phase after its formation, this cloud is still dense and absorbs the light from its star. In particular, water and methanol molecules emit intense radiation, so-called maser light.
What is this burl light made of?
In principle, this is the same as laser radiation, except that the frequency of the radiation is in the microwave range. If a substance receives enough energy to be excited to glow at a characteristic frequency, this can lead to a self-amplified emission. This is exactly what happens with laser radiation. In the molecular clouds around highly active young stars, however, this also happens in the microwave range. As a result, these clouds shine particularly brightly in the microwave range at a certain frequency. With our method, we actually don't see the stars themselves, but the gas clouds in their immediate vicinity.
What do these measurements reveal about the structure of the Milky Way?
We can study regions with high star formation rates where young and heavy stars are forming. These regions are mostly found in the main arms of our galaxy. We have only known for a few years that our own solar system is also located in a main arm. This shows that it is much more difficult to determine the structure of our own galaxy than it is for other galaxies, the shape of which we can recognize immediately. The star at the other end of the galaxy that we have now been able to measure is also located in a main arm, the so-called Scutum Centaurus arm. This is named after the constellations on which we can determine its extent. We now hope to use our method to discover many more objects at the other end of the Milky Way in the future, including with other telescope networks.
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