The Milky Way as seen from the Isle of Wight in the UK. Photograph: Chad Powell/Barcroft Images
Our galaxy has a collective mass 700 billion times that of the sun, according to the most accurate measurement yet by scientists.
The estimate covers the mass of all the stars, black holes, gas clouds, dust, dark matter and other unidentified flying objects in the Milky Way. The previous rough ballpark figure was around a trillion solar masses - the standard measure for big astronomical objects.
Scientists began trying to “weigh” the Earth two centuries ago, and astronomers eventually established the distance to the sun. Later, using Newton’s equations, they arrived at a mass for the sun (it is 330,000 times the mass of the Earth).
But the galaxy had proved more intractable. One big problem is that to arrive at a good estimate, astronomers have to be sure of the speeds of very distant objects. The other is that – because Earth is where it is – only a fraction of the galaxy is visible to telescopes.
“The fact that we sit inside the galaxy does introduce some difficulties,” said Gwendolyn Eadie, a PhD student at McMaster University in Ottawa, who led the study which was presented at the Canadian Astronomical Society’s annual conference in Winnipeg on Tuesday.
“We have a heliocentric perspective: we see everything from the perspective of our sun’s position (and movement) through the galaxy. It’s important that we take the movement and position of the sun into account when we measure the motions and positions of other objects in the Milky Way. Luckily, in this regard I can stand on the shoulders of giants. Over the years, many astronomers have put in an enormous effort to figure out how to take these things into account.”
She and her supervisor, Professor William Harris, an astronomer and physicist at McMaster, made the best of the incomplete data and devised a “galactic mass estimator” to make calculations that they believe are more plausible than any so far. They have submitted a paper to the Astrophysical Journal. The ultimate prize, in research of this kind, is a better understanding of dark matter – the cold, invisible, untouchable but massive material that must act as gravitational glue in every galaxy, but which so far has not been identified. But better estimates of mass will deliver better understanding.
“The mass of a galaxy’s dark matter halo plays a large role in the formation and evolution of that galaxy. Certain properties such as star formation rates and the size of supermassive black holes are known to depend on the mass of the galaxy,” Eadie said.
“So, pinning down the mass of our own galaxy, the Milky Way, is incredibly important both for our understanding of our own galaxy, and putting the Milky Way into the context of other galaxies in the universe.”
The new approach, she says, provides an estimate for the total mass held within any distance from the galactic centre. The galaxy is roughly 100 to 120 million light years in length so the calculations are a work in progress. She believes that comparisons of measurements and results will lead to better calculations and more accurate models of the galaxy.
“We can also compare the total mass estimate to the amount of visible matter that we see in the Milky Way and then get a prediction for the amount of dark matter,” said Eadie.
“With our estimate, it seems that dark matter makes up about 88% of the Milky Way’s mass.”