- 4346 - MILKYWAY - Calculating galaxy mass? - We can measure the mass of the Milky Way by calculating how hard it is to escape the galaxy.
-------------------------- 4346 - MILKYWAY - Calculating galaxy mass
- If you want to determine your own mass, it's
pretty easy. Just step on a scale and look at the number it gives you. That
number tells you the gravitational pull of Earth upon you.
-
- The same scale could also be used to measure
the mass of Earth. If you place a kilogram mass on the scale, the weight it
gives is also the weight of Earth in the gravitational field of the kilogram.
With a bit of mass, you have the mass of Earth.
-
- Things aren't quite that simple. The Earth
is not a perfectly spherical, perfectly uniform mass, so its gravitational pull
varies slightly across the globe. But this method gives a reasonable ballpark
value, and we can use it to estimate the masses of other objects in the solar
system.
-
- But how can we determine the mass of
something larger, such as the Milky Way? One method is to estimate the number
of stars in the galaxy and their masses, then estimate the mass of all the
interstellar gas and dust, and then rough out the amount of dark matter… it all
gets very complicated.
-
- A better way is to look at how the orbital
speed of stars varies with distance from the galactic center. This is known as
the “rotation curve” and gives an upper mass limit on the Milky Way, which
seems to be around 600 billion to a trillion solar masses. The wide uncertainty
gives you an idea of just how difficult it is to measure our galaxy's mass.
-
- But a new study could help astronomers pin
things down. The method looks at the
escape velocity of stars in our galaxy. If a star is moving fast enough, it can
overcome the gravitational pull of the Milky Way and escape into interstellar
space. The minimum speed necessary to escape depends upon our galaxy's mass, so
measuring one gives you the other.
-
- Unfortunately, only a handful of stars are
known to be escaping, which is not enough to get a good handle on galactic
mass. So they looked at the statistical distribution of stellar speeds as
measured by the “Gaia spacecraft”.
-
- The method is similar to weighing the moon
with a handful of dust. If you were standing on the moon and tossed dust
upward, the slower-moving dust particles would reach a lower height than faster
particles. If you measured the speeds and positions of the dust particles, the
statistical relation between speed and height would tell you how strongly the
moon pulls on the motes, and thus the mass of the moon. It would be easier just
to bring our kilogram and scale to measure lunar mass, but the dust method
could work.
-
- In the Milky Way, the stars are like dust
motes, swirling around in the gravitational field of the galaxy. The team used
the speeds and positions of a billion stars to estimate the escape velocity at
different distances from the galactic center. From that, they could determine
the overall mass of the Milky Way. They calculated a mass of 640 billion
suns. That is a lot of suns.
-
- This is on the lower end of earlier
estimates, and if accurate it means that the Milky Way has a bit less dark
matter than we thought.
-
-
February 10, 2023 MILKYWAY
- Calculating galaxy mass 4346
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--------------------- --- Friday, February 9,
2024
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