- 4210 - MILKY WAY - rotation tells us size? The Moon rotates around the Earth. The Earth rotates around the Sun. The Sun rotates around the Galaxy. What is the rotation of the galaxy around? Well if we know it we can calculate the mass of the Galaxy.
--------------------- 4210 - MILKY WAY - rotation tells us size?
- The Milky Way's
Mass is much lower than we thought? How
massive is the Milky Way? It’s an easy question to ask, but a difficult one to
answer. Despite the challenges, a new
study has calculated an accurate mass of our galaxy, and it’s smaller than we
thought.
-
- One way to
determine a galaxy’s mass is by looking at what’s known as its rotation curve.
Measure the speed of stars in a galaxy versus their distance from the galactic
center. The speed at which a star orbits is proportional to the amount of mass
within its orbit, so from a galaxy’s rotation curve you can map the function of
mass per radius and get a good idea of its total mass.
-
- We’ve measured the
rotation curves for several nearby galaxies such as Andromeda, so we know the
masses of many galaxies quite accurately.
-
- But since we are
in the Milky Way itself, we don’t have a great view of stars throughout the
galaxy. Toward the center of the galaxy, there is so much gas and dust we can’t
even see stars on the far side.
-
- So instead we
measure the rotation curve using neutral hydrogen, which emits faint light with
a wavelength of about 21 centimeters. This isn’t as accurate as stellar
measurements, but it has given us a rough idea of our galaxy’s mass. We’ve also
looked at the motions of the globular clusters that orbit in the halo of the
Milky Way. From these observations, our best estimate of the mass of the Milky
Way is about a trillion solar masses.
-
- The third data
release of the Gaia spacecraft contains the positions of more than 1.8 billion
stars and the motions of more than 1.5 billion stars. While this is only a
fraction of the estimated 100-400 billion stars in our galaxy, it is a large
enough number to calculate an accurate rotation curve.
-
- There resulting
rotation curve is known as the Keplerian decline. This is the outer region of
the Milky Way where stellar speeds start to drop off roughly in accordance with
Kepler’s laws since almost all of the galaxy’s mass is closer to the galactic
center.
-
- The Keplerian
decline allows the team to place a clear upper limit on the mass of the Milky
Way. What they found was surprising. The best fit to their data placed the mass
at about 200 billion solar masses, which is a fifth of previous estimates.
-
- The absolute upper
mass limit for the Milky Way is 540 billion, meaning that the Milky Way is at
least half as massive as we thought. Given the amount of known regular matter
in the galaxy, this means the Milky Way has significantly less dark matter than
we thought.
-
-
November 5, 2023
MILKY WAY - rotation tells us size? 4210
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