Nicholas Bakalar and Kenneth Chang Jan 24 , 2017, The New York Times
Absolutely every bit of our galaxy
Absolutely every bit of our galaxy
Astronomers have arrived at what they believe to be the most accurate measure yet of the mass of the Milky Way: about 4.8 x 10(11) times the mass of the sun, or ‘solar masses’, to use a standard unit of mass in astronomy. This comes to about 9.5 x 10(41) kg — that is, 95 followed by 40 zeros. The number, of course, is inexact, as obviously no direct measure of all the billions of stars and other objects in the Milky Way could be taken.
But in a paper to be published in The Astrophysical Journal, scientists used methods of measurement that involve complex mathematical and statistical techniques called hierarchical Bayesian analysis, as well as direct measurements of the velocity of globular clusters, the tightly packed spherical groups of 10,000 to 1,00,000 old stars that move through the galaxy. Just as the mass of the sun can be calculated by measuring its gravitational pull on Earth, the mass of the Milky Way can be calculated by measuring its gravitational pull on the globular clusters.
The estimate includes everything within 125 kiloparsecs of the centre of the galaxy — that is, within 3.9 x 10(18) km. And ‘everything’ is not just stars: There are planets, moons, gases, dust and other objects, not to mention the immense amount of dark matter. It cannot be detected directly, but its mass can be inferred from its gravitational effect on other objects.
“The biggest thing is that we’re including measurement uncertainties that are carried through the analysis,” said the lead author, Gwendolyn M Eadie, a doctoral candidate at McMaster University in Hamilton, Ontario, Canada. “So we have a good handle on the uncertainty in our mass estimate. The low end is 4.0 x 10(11) solar masses, and the high end is 5.8 x 10(11).”
Gwendolyn said that the findings were important from an astronomer’s perspective. “The methods we’ve developed could be important in other studies that do other kinds of research,” she said. “These methods have been used in other fields, but they’re starting to become more useful in astronomy now that we have computers that can do these complex calculations.” What does it mean for the rest of us? “It just satisfies curiosity about the world we live in,” she said.
When Venus smiled for a few days
For a few days, Venus smiled — sideways. When Japan’s Akatsuki spacecraft pulled into orbit around Venus in December 2015 and turned on its instruments, it almost immediately discovered a bow-shape feature in the atmosphere stretching 6,000 miles, almost pole to pole — a sideways smile.
More remarkably, while Venus’ winds blow at speeds up to 250 mph and clouds whip around the planet every four days, this gargantuan sideways smile did not move, but remained fixed above the ground for four days. Because of Akatsuki’s large looping orbit, the spacecraft could not make more observations for a month.
When the spacecraft looked at the same region again, the smile had disappeared. Except for a few brief glimmers in April and May last year, the smile has not returned. In a recent paper published in the journal Nature Geoscience, scientists working on the mission describe their observations in detail and suggest it was a “gravity wave” — a disturbance in the winds caused by the underlying topography that propagated upward. The bow-shape arc appeared above Aphrodite Terra, a highland region about the size of Africa that rises up to three miles from the surface.
Scientists working on data from the European Space Agency’s Venus Express reported finding a similar disturbance in the atmosphere. The authors of the new paper said that numerical simulations provided preliminary support for the idea, but that they still could not explain how the gravity wave forms and propagates in the lower atmosphere. Or why the prominent smile was seen in December 2015 and not since. Scientists also cannot yet answer the big question Akatsuki was sent to investigate: Why do the winds blow so fast on Venus to begin with?
Astronomers have arrived at what they believe to be the most accurate measure yet of the mass of the Milky Way: about 4.8 x 10(11) times the mass of the sun, or ‘solar masses’, to use a standard unit of mass in astronomy. This comes to about 9.5 x 10(41) kg — that is, 95 followed by 40 zeros. The number, of course, is inexact, as obviously no direct measure of all the billions of stars and other objects in the Milky Way could be taken.
But in a paper to be published in The Astrophysical Journal, scientists used methods of measurement that involve complex mathematical and statistical techniques called hierarchical Bayesian analysis, as well as direct measurements of the velocity of globular clusters, the tightly packed spherical groups of 10,000 to 1,00,000 old stars that move through the galaxy. Just as the mass of the sun can be calculated by measuring its gravitational pull on Earth, the mass of the Milky Way can be calculated by measuring its gravitational pull on the globular clusters.
The estimate includes everything within 125 kiloparsecs of the centre of the galaxy — that is, within 3.9 x 10(18) km. And ‘everything’ is not just stars: There are planets, moons, gases, dust and other objects, not to mention the immense amount of dark matter. It cannot be detected directly, but its mass can be inferred from its gravitational effect on other objects.
“The biggest thing is that we’re including measurement uncertainties that are carried through the analysis,” said the lead author, Gwendolyn M Eadie, a doctoral candidate at McMaster University in Hamilton, Ontario, Canada. “So we have a good handle on the uncertainty in our mass estimate. The low end is 4.0 x 10(11) solar masses, and the high end is 5.8 x 10(11).”
Gwendolyn said that the findings were important from an astronomer’s perspective. “The methods we’ve developed could be important in other studies that do other kinds of research,” she said. “These methods have been used in other fields, but they’re starting to become more useful in astronomy now that we have computers that can do these complex calculations.” What does it mean for the rest of us? “It just satisfies curiosity about the world we live in,” she said.
When Venus smiled for a few days
For a few days, Venus smiled — sideways. When Japan’s Akatsuki spacecraft pulled into orbit around Venus in December 2015 and turned on its instruments, it almost immediately discovered a bow-shape feature in the atmosphere stretching 6,000 miles, almost pole to pole — a sideways smile.
More remarkably, while Venus’ winds blow at speeds up to 250 mph and clouds whip around the planet every four days, this gargantuan sideways smile did not move, but remained fixed above the ground for four days. Because of Akatsuki’s large looping orbit, the spacecraft could not make more observations for a month.
When the spacecraft looked at the same region again, the smile had disappeared. Except for a few brief glimmers in April and May last year, the smile has not returned. In a recent paper published in the journal Nature Geoscience, scientists working on the mission describe their observations in detail and suggest it was a “gravity wave” — a disturbance in the winds caused by the underlying topography that propagated upward. The bow-shape arc appeared above Aphrodite Terra, a highland region about the size of Africa that rises up to three miles from the surface.
Scientists working on data from the European Space Agency’s Venus Express reported finding a similar disturbance in the atmosphere. The authors of the new paper said that numerical simulations provided preliminary support for the idea, but that they still could not explain how the gravity wave forms and propagates in the lower atmosphere. Or why the prominent smile was seen in December 2015 and not since. Scientists also cannot yet answer the big question Akatsuki was sent to investigate: Why do the winds blow so fast on Venus to begin with?
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