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How Much Does the Milky Way Weigh?

By David Reneke

How much does the Milky Way weigh?

Our home galaxy is a massive star-studded conglomeration more than 100,000 light years across. Calculating the weight of something so colossal seems beyond the realm of possibility, especially when peering from the inside out. However, Canadian astronomers at McMaster University have calculated that the Milky Way is a little lighter than previously believed, with a mass equivalent to about 700 billion suns!

Previous estimates about the Milky Way’s mass have ranged from 100 billion to 1.6 trillion times that of our own Sun, but Prof William Harris and PhD student Gwendolyn Eadie devised a novel way to measure the galaxy’s dark matter – via the invisible and undetectable.

To arrive at their current estimate, the team studied the positions and velocities of numerous globular star clusters that orbit the Milky Way. As the authors note, “the orbits of globular clusters are determined by the galaxy’s gravity, which is dictated by its massive dark matter component”.

Based on early reviews, their approach is one of the most thorough analyses to date. “With our estimate, it seems that dark matter makes up about 88% of the Milky Way’s mass,” Eadie said. “The rest of the weight are in the things you might expect like stars, planets, gas, moons and dust.”

The ultimate determination of the Milky Way’s total mass could provide scientists with clues about its long and mysterious history. Scientists who study the evolution of galaxies look at how the mass relates to its evolution. If we have a better idea of what the mass of the Milky Way is, we can understand how it and other galaxies form and evolve.

Until then we can all feel content knowing that the Sun has a mass roughly 330,000 times that of the Earth, and the Milky Way has a mass some 700 billion times that of the Sun. So, in a nutshell mere Earthling, you are not the centre of the Universe!


Hubble Spies Big Bang Frontiers

Observations by the Hubble Space Telescope have taken advantage of gravitational lensing to reveal the largest sample of the faintest and earliest known galaxies in the universe.

Some of these galaxies formed just 600 million years after the Big Bang, and are fainter than any other galaxy yet uncovered by Hubble. The team has determined that these small galaxies were vital to creating the universe that we see today.

They have discovered more than 250 tiny galaxies that existed only 600–900 million years after the Big Bang, one of the largest samples of dwarf galaxies yet to be discovered at these epochs. The light from these galaxies took more than 12 billion years to reach the telescope, allowing the astronomers to look back in time when the universe was still very young.

Although impressive, the number of galaxies found at this early period is not the team’s only remarkable breakthrough, The faintest galaxies detected in these Hubble observations are fainter than any other yet uncovered in the deepest Hubble observations..

The accumulated light emitted by these galaxies could have played a major role in one of the most mysterious periods of the universe’s early history – the epoch of reionisation. Reionisation started when the thick fog of hydrogen gas that cloaked the early universe began to clear. This enabled ultraviolet light to travel over larger distances without being blocked, and the universe became transparent to ultraviolet light.

By observing the ultraviolet light from the galaxies found in this study, astronomers were able to determine that the smallest and most abundant of the galaxies could be the major components in maintaining the transparency. They basically established that the epoch of reionisation, which ends at the point when the universe is fully transparent, came to a close about 700 million years after the Big Bang.

To make these discoveries the team utilised the deepest images of gravitational lensing made so far in three galaxy clusters, which were taken by Hubble. These clusters generate immense gravitational fields capable of magnifying the light from the faint galaxies that lie far behind the clusters themselves. This makes it possible to search for and study the first generation of galaxies in the universe.

The galaxy clusters act as powerful natural telescopes, and unveil these faint dwarf galaxies that would otherwise be invisible. More galaxies, at even earlier times, are likely to be revealed when Hubble peers at three more of these galaxy clusters in the near future.


David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com