OUR home galaxy – the Milky Way – has been “accurately weighed” for the first time, and it’s unsurprisingly heavy.
A groundbreaking study has revealed that it has roughly 1.5trillion (that’s 1,500,000,000,000) times more mass than the Sun.
Alamy The Milky Way can be seen from Earth on a clear night
Experts have been trying to measure the weight of the Milky Way for years, but it’s a difficult job.
The main sticking point has been “dark matter”, which is hypothetical matter that science tells us must exist – but there’s no way of seeing it directly.
That’s why previous estimates of the Milky Way’s weight have disagreed wildly – ranging from 500billion to 3trillion solar masses (the mass of the Sun).
“We just can’t detect dark matter directly,” said Laura Watkins, of the European Southern Observatory, who lead the team.
Alamy Measuring the Milky Way has always proved difficult, because of the significant amount of invisible “dark matter” lurking within
“That’s what leads to the present uncertainty in the Milky Way’s mass – you can’t measure accurately what you can’t see.”
But a revolutionary new weighing technique generates a total mass that takes account of the dark matter. That’s handy, as it makes up about 90% of the Milky Way’s mass.
A single solar mass weighs 2 x 10 to the power of 30 kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kilograms.
But the Milky Way weights 1.5trillion times that mass – that’s about 3,000,000,000,000,000,000,000,000,000,000,000,000,000,000 kilograms.
Scientists were able to work out this astronomically high number by measuring the speed of globular clusters – dense groups of stars orbiting the Milky Way’s spiral disc.
“The more massive a galaxy, the faster its clusters move under the pull of its gravity,” said N. Wyn Evans, from the University of Cambridge.
“Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight.
“However, we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated.”
Getty – Contributor Researchers used years of data from Nasa’s Hubble Space Telescope to make their measurements
By combining data from Nasa’s Hubble Space Telescope and the ESA’s Gaia space observatory.
This data revealed how objects moved through the Milky Way, allowing for very precise measurements to be made.
“Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy,” explained Tony Sohn, Space Telescope Science Institute, who led the Hubble measurements.
And Roeland P. van der Marel, who also works at the Space Telescope Science Institute, added: “We were lucky to have such a great combination of data.
“By combining Gaia’s measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way’s mass in a way that would be impossible without these two space telescopes.”
Now that scientists know the mass of the Milky Way, it will make it easier to make other precise cosmological measurements.
Deepest ever view of the wonders of Space
Late last year, scientists managed to successfully measure all of the starlight ever produced in the observable universe.
This not only includes current starlight, but all starlight “produced throughout the history of the observable universe” – so it’s quite a big number.
After all, our universe is estimated to be around 13.7billion years old, and began forming stars within a few hundred million years.
Science’s best guess is that there are roughly two trillion galaxies and a trillion-trillion stars.
Astrophysicists at Clemons College of Science used data from Nasa’s Fermi Gamma-ray Space Telescope to work out how many photons (particles of light) have ever been produced by stars.
Technically, the number is known as septenvigintillion – also know as this: 4,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.
Yes, that’s how many photons make up all the starlight ever produced in the observable universe, according to scientists writing in the journal Science.
For contrast, it’s estimated that the number of grains of sand on Earth totals around 7.5 x 10 the power of 18 – or 7,500,000,000,000,000,000.
And the amount of water molecules on Earth is estimated at 4.6 x 10 to the power of 43 – or 46,000,000,000,000,000,000,000,000,000,000,000,000,000,000 molecules.
Basically, there’s a whole lot of light out there.
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Are you surprised by the weight of the Milky Way? Let us know in the comments!
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