- 4427 - MILKYWAY GALAXY - how big is it? - Exactly how massive is the Milky Way? How do you weigh a galaxy? It's an astronomical challenge, particularly if it's the galaxy you call home. It turns out there are several ways to get a handle on the mass of the Milky Way.
------------------------- 4427 - MILKYWAY GALAXY - how big is it?
- One method is to look at the motion of
stars in the galaxy. Most Milky Way stars follow a roughly circular path around
the galactic center. Just as planets orbit the Sun, stars orbit the galaxy.
Since gravity is the force holding stars in their orbit, you can use a star's
speed and distance from the center to determine the mass within its orbit.
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- Not all stars have circular orbits, but
they do on average. So you can plot the speed vs. distance from the center for
known stars, and get what is known as the rotation curve. Measurements of this
curve in the Milky Way and other galaxies were the first evidence that galaxies
had much more mass than could be accounted for by visible stars, leading to the
idea of dark matter.
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- One of the problems with the “rotation
curve method” is that we can only measure stars to a certain distance. We now
know that most of our galaxy's mass isn't concentrated at the center, but
rather extends outward into a galactic halo. We can estimate the mass of the
halo from the rotation curve, but we can also look at the motion of globular
clusters.
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- Globular clusters are bright dense clusters
of stars. Since stars within a globular cluster are gravitationally bound,
these clusters move around the galaxy like a single object. They are found in a
sphere surrounding the Milky Way, so measuring their motion helps us measure
the mass of the galactic halo.
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- To measure the outer region of the galactic
halo, we can look at the motion of satellite galaxies such as the Magellanic
Clouds. There are about 60 small galaxies within about 1.4 million light-years
of the Milky Way. Not all of them are in orbit around our galaxy, but many of
them are. Since they lay outside our galactic halo, their orbital motions are
determined by all of our galaxy's mass. The only downside of this approach is
that with only a few dozen orbiting galaxies, the result isn't particularly
accurate.
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- All of these approaches calculate the mass
of the Milky Way from orbital motion. There are a few methods that don't rely
on the orbital motion. One of these is to look at the tidal plumes of dwarf
galaxies. In the history of our galaxy, there are some globular clusters and
dwarf galaxies that strayed too close to the central region of the Milky Way
and were ripped apart by tidal forces. The remnants of these galaxies form a
stream of stars, such as the Sagittarius stream. By calculating the motion of
these streams we can estimate galactic mass.
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- Another approach is to look at stars leaving
our galaxy. Occasionally a star will have a near miss with another star and
gain enough velocity to escape our galaxy. Since the escape velocity depends on
galactic mass, a statistical measure of escaping stars gives a mass for the
galaxy.
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- We can look at the local group of galaxies.
This includes the Andromeda galaxy and its satellite galaxies. Our local group
is gravitationally isolated from more distant galaxy clusters, so looking at
the equilibrium state of the local group gives us a handle on its overall mass
and the mass of the Milky Way.
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- Each of these approaches has its own
advantages and levels of accuracy. None of them are the final say on their own.
In this latest work, the team took a statistical average of various methods and
derived what we might call the best value for the mass of our galaxy. The value
they determined was a “trillion solar masses”, give or take a few hundred
billion solar masses.
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April 10, 2023 MILKYWAY
GALAXY - how big is it? 4427
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--------------------- --- Wednesday, April 10,
2024
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