- 4322 - MILKY WAY - How many stars in the galaxy? The Milky Way is the galaxy in which the sun and the Earth reside. Part of it is visible on a clear night (from sufficiently dark locations) as a thick opaque band of stars and dust stretching across the sky. We can see thousands of these stars with the naked eye, and many more with a telescope. But how many stars are in the Milky Way-
------------------- 4322 - MILKY WAY - How many stars in the galaxy?
- It's a
surprisingly difficult question to answer.
Even in the Andromeda galaxy, which is bright, large and at 2.3 million
light-years away relatively close to Earth we can distinguish only the largest
and brightest stars. A sun-size star would be too difficult for us to see.
-
- Astronomers know
that the Milky Way is a barred spiral galaxy that is about 100,000 light-years
across. A view outside the galaxy would reveal a central bulge surrounded by
four spiral arms, two major and two minor. The Milky Way's major arms are known
as Perseus and Sagittarius. Earth's sun is located in one of two minor spurs,
which is called the Orion Arm.
-
- The galaxy is
surrounded by a huge hot-gas halo that is several hundreds of thousands of
light-years in diameter. Astronomers estimate that the halo's mass is about the
same as that of all the stars in the Milky Way.
-
- Many of the Milky
Way's stars, however, are tough to view. That is because the center of the
galaxy has a galactic bulge filled with stars, gas and dust, as well as a
supermassive black hole. Stuff in this region is so densely packed that even
the most powerful telescopes cannot see through it.
-
- Astronomers used
to think that all of the stars in the universe were part of the Milky Way, but
that changed in the 1920s when Edwin Hubble, the American astronomer after whom
the famous telescope is named, managed to calculate the distance of the
Andromeda nebula (today known as the Andromeda galaxy). He found that it was
too far to be part of the Milky Way
-
- The primary way
astronomers estimate the number of stars in a galaxy is by determining the
galaxy's mass. This can be done by analyzing the rotation of the galaxy and the
spectrum of the light it emits.
-
- In general,
galaxies move away from each other as a result of the expansion of the universe
(unless they are on a collision course). As a result, we see the light of
distant galaxies shifted towards longer wavelengths due to the redshift (that
is essentially the Doppler Effect, the same effect that distorts the frequency
of a passing siren as it moves away from a bystander).
-
- In a rotating
galaxy, however, there will be a portion that is more blueshifted (stretched
towards the shorter wavelengths) because that portion is rotating toward Earth.
Astronomers must also know what the inclination or orientation of the galaxy is
before making an estimate, which is sometimes simply an "educated guess.
-
- A technique called
long-slit spectroscopy is best for performing this type of work. Here, an
elongated object such as a galaxy is viewed through an elongated slit, and the
light is refracted using a prism. This breaks down the colors of the stars into
the colors of the rainbow.
-
- Some of those
colors will be missing, displaying patterns of missing portions as elements of
the periodic table that absorbed parts of the light. This lets astronomers
figure out what elements are in the stars. Each type of star has a unique
chemical fingerprint that would show up in telescopes.
-
- Between different
galaxies of the same mass, there could be differences as to the types of stars
and the overall mass. Elliptical
galaxies tend to have more K- and M-type red dwarf stars than spiral galaxies.
Because elliptical galaxies are older, they will have less gas because that was
blown away during their evolution.
-
- Once a galaxy's
mass is determined, they can start figuring out how much of that mass is made
of stars. Most of the mass will be made up of dark matter, a type of matter
that emits no light but which is believed to make up most of the mass of the
universe.
-
- In a typical
galaxy, if you measure its mass by looking at the rotation curve, about 90% of
that is dark matter. With much of the
remaining stuff in the galaxy made up of diffuse gas and dust. About 3% of the galaxy's mass will be made up
of stars, but that could vary. Further, the size of the stars itself can
greatly vary from something the is the size of our sun, to something dozens of
times smaller or bigger.
-
- The galaxy-mapping
“Gaia mission”, the current estimate is between 100 to 400 billion stars. The Gaia mission, in orbit since 2013, has
managed to map positions of 1.7 billion stars in the sun's neighborhood up to
the distance of 326 light-years.
-
- The fundamental
problem is to measure the luminosity [distribution] for very faint red dwarfs
and then extrapolate to the brown-dwarf limit.
Red dwarfs are the most common stars in the universe and also the
longest-lived ones. However, because of their low luminosity they are sometimes
hard to spot.
-
- Brown dwarfs are
even dimmer. These are basically failed stars that didn't manage to accumulate
enough material to kick-start nuclear fusion in their core. They are therefore
something between a star and a planet and therefore even more difficult to spot
than faint red dwarfs, especially at long distances.
-
- A second
complication for the entire story are double stars, the frequency of which is
still not perfectly characterized. By
the end of Gaia's mission in 2025, scientists will have a somewhat better idea
about the number of stars in our galaxy but significant uncertainties will
likely remain.
-
January 20, 2023 MILKY
WAY -
How many stars in the galaxy? 4322
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