Post by Andrei Tchentchik on Jun 24, 2019 16:12:59 GMT 2
(.#216).- We still do not know the exact mass of our galaxy.
We still do not know the exact mass of our galaxy.
By Tristan Vey - Updated on 15/03/2019 at 09:38
Artist's impression of our galaxy and globular clusters that revolve around it. NASA, ESA, and A. Feild (STScI)
How much does the Milky Way weigh exactly? Many studies are conducted to determine the mass of our galaxy, but the estimates are still very variable.
The news has been around the world this weekend: we finally have a clear idea of the mass of our galaxy, the Milky Way. According to a study published in The Astrophysical Journal, it would weigh exactly 1500 billion times the mass of our Sun, when the previous estimates oscillated between 500 and 3000 billion solar masses, adds the press release published on this occasion by the Hubble US-European space telescope site. What this document does not say, however, is that this new result is still tainted with great uncertainty. The mass of our galaxy would actually be between 1100 and 2300 solar masses.
If it is not totally uninteresting, this result is not as conclusive as announced. In fact, it illustrates rather well the difficulty that astronomy can have in weighing objects. Galaxies in particular are mainly composed of "dark matter", a mysterious and perfectly invisible substance of which we know almost nothing except that it manifests itself only by the gravitational influence it exerts on its environment. It is therefore quite difficult to quantify ...
"We have a pretty good idea of the amount of classical material, mostly stars, found in the Milky Way," says Françoise Combes, an astrophysicist at the Observatoire de Paris. "That's 40 to 50 billion solar masses. But there are dozens of times more black matter a priori. The question is exactly how many. "
Take the latest study, mentioned at the beginning of this article. Researchers at the Space Telescope Science Institute, Nasa's historical foundation for conducting research using the Hubble Space Telescope, have pooled their data with the European Gaia satellite, which has conducted the most comprehensive sky survey. nowadays. They looked at the exact way in which globular clusters, groups of a few hundred thousand stars, gravitate around the Milky Way (which contains about 200 billion stars, for comparison).
"The more massive a galaxy, the faster these clusters move under the action of gravity," summarizes Wyn Evans, of Cambridge University, who co-curates this work. "Most of the previous measurements looked only at the speed with which these clusters were approaching or retreating from us. We also measured their lateral movement, and therefore their actual actual speed. "This allowed them to refine the determination of the mass a little bit.
If these works are quite serious, they are still based on forty or so clusters, the farthest of which are located 130,000 light-years away. But the dark matter that makes up our galaxy extends well beyond, over 600,000 light-years. "So we must extrapolate the results obtained on a much larger volume," says Benedict Famaey, a specialist in the dynamics of galaxies at the Astronomical Observatory of Strasbourg. This induces a great uncertainty about the final result. Not to mention that researchers make the assumption that the "halo" of dark matter in which we find ourselves is spherical, which is not at all certain.
"With the publication of the second batch of Gaia data, studies have multiplied on the subject in recent months, with different methods and calculation models," says Benoît Famaey. "The results are quite varied and all have significant intrinsic margins of error. I myself have been working on this subject with colleagues in Germany using a method very different from that presented here and achieved about the same result. But that does not mean that we are right because there are disparities between the results of the different studies. "
Why want to measure at all costs the mass of our galaxy? By curiosity, already. It's our immediate environment. But there are also deeper reasons. If our galaxy is as massive as all these estimates suggest, then there should be thousands of satellite galaxies. "We only know a few dozen," adds Françoise Combes. "There is something here that we do not understand. Maybe there are galaxies of pure dark matter, without stars? Or maybe we are completely mistaken about the mass of our galaxy? "In this case, it remains to understand why ...
F I N .
We still do not know the exact mass of our galaxy.
By Tristan Vey - Updated on 15/03/2019 at 09:38
Artist's impression of our galaxy and globular clusters that revolve around it. NASA, ESA, and A. Feild (STScI)
How much does the Milky Way weigh exactly? Many studies are conducted to determine the mass of our galaxy, but the estimates are still very variable.
The news has been around the world this weekend: we finally have a clear idea of the mass of our galaxy, the Milky Way. According to a study published in The Astrophysical Journal, it would weigh exactly 1500 billion times the mass of our Sun, when the previous estimates oscillated between 500 and 3000 billion solar masses, adds the press release published on this occasion by the Hubble US-European space telescope site. What this document does not say, however, is that this new result is still tainted with great uncertainty. The mass of our galaxy would actually be between 1100 and 2300 solar masses.
If it is not totally uninteresting, this result is not as conclusive as announced. In fact, it illustrates rather well the difficulty that astronomy can have in weighing objects. Galaxies in particular are mainly composed of "dark matter", a mysterious and perfectly invisible substance of which we know almost nothing except that it manifests itself only by the gravitational influence it exerts on its environment. It is therefore quite difficult to quantify ...
"We have a pretty good idea of the amount of classical material, mostly stars, found in the Milky Way," says Françoise Combes, an astrophysicist at the Observatoire de Paris. "That's 40 to 50 billion solar masses. But there are dozens of times more black matter a priori. The question is exactly how many. "
Take the latest study, mentioned at the beginning of this article. Researchers at the Space Telescope Science Institute, Nasa's historical foundation for conducting research using the Hubble Space Telescope, have pooled their data with the European Gaia satellite, which has conducted the most comprehensive sky survey. nowadays. They looked at the exact way in which globular clusters, groups of a few hundred thousand stars, gravitate around the Milky Way (which contains about 200 billion stars, for comparison).
"The more massive a galaxy, the faster these clusters move under the action of gravity," summarizes Wyn Evans, of Cambridge University, who co-curates this work. "Most of the previous measurements looked only at the speed with which these clusters were approaching or retreating from us. We also measured their lateral movement, and therefore their actual actual speed. "This allowed them to refine the determination of the mass a little bit.
If these works are quite serious, they are still based on forty or so clusters, the farthest of which are located 130,000 light-years away. But the dark matter that makes up our galaxy extends well beyond, over 600,000 light-years. "So we must extrapolate the results obtained on a much larger volume," says Benedict Famaey, a specialist in the dynamics of galaxies at the Astronomical Observatory of Strasbourg. This induces a great uncertainty about the final result. Not to mention that researchers make the assumption that the "halo" of dark matter in which we find ourselves is spherical, which is not at all certain.
"With the publication of the second batch of Gaia data, studies have multiplied on the subject in recent months, with different methods and calculation models," says Benoît Famaey. "The results are quite varied and all have significant intrinsic margins of error. I myself have been working on this subject with colleagues in Germany using a method very different from that presented here and achieved about the same result. But that does not mean that we are right because there are disparities between the results of the different studies. "
Why want to measure at all costs the mass of our galaxy? By curiosity, already. It's our immediate environment. But there are also deeper reasons. If our galaxy is as massive as all these estimates suggest, then there should be thousands of satellite galaxies. "We only know a few dozen," adds Françoise Combes. "There is something here that we do not understand. Maybe there are galaxies of pure dark matter, without stars? Or maybe we are completely mistaken about the mass of our galaxy? "In this case, it remains to understand why ...
F I N .
