How big is the Milky Way Galaxy?

Reading and Assessment

When you look up at the night sky at night, every star you see is part of the Milky Way galaxy. On a clear night you can see a narrow band of stars stretching across the sky. Through a telescope or pair of binoculars it becomes clear that this band is a dense region of the Milky Way. The band of light we see as the Milky Way is the light that originates from stars and other material that lie within the galactic plane (Note: the galactic plane is the actual mid-plane – the flat part – of our galaxy’s huge spinning disk of stars). The disk of the Milky Way galaxy spans a distance of about 100,000 light years (one light year is about 9.5 x 1015 meters). The Milky Way galaxy though is only about 2000 light years thick at the edge of a spiral arm where the sun is located.

Our Galaxy contains about 200 billion stars. Most of the stars are located in the disk of our galaxy, which is the site of most of the star formation because it contains lots of gas and dust.


The Andromeda galaxy is a spiral galaxy much like our own Milky Way galaxy but is approximately 2.5 million light-years away.

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies belonging to a group of 50 closely bound galaxies known as the Local Group.

Andromeda is one of the few blue-shifted galaxies (moving toward us) at a speed of 100 to 140 kilometers per second and is expected to collide in about 4.5 billion years with our own Milky Way galaxy.

An object moving toward us will have light shifted to the blue end of the spectrum. Objects moving away will be red-shifted. This is attributable to the Doppler effect.
Photo: NASA
Where is the earth in the Milky Way galaxy?

On a clear night you can see about 3,000 stars. This is only a fraction of the stars in the Milky Way galaxy -- there are approximately 200-400 billion stars in the Milky Way galaxy.


Our sun and solar system is at the edge of a spiral arm called the Orion Arm, and is about two-thirds of the way from the center of our galaxy to the edge of the starlight.

See image to left -- sun is in the cross-hairs


Image from Wikipedia

Just as the earth orbits around the sun, our solar system orbits the galaxy's center of mass -- but each rotation takes 225 million Earth years. This period of time is called a "cosmic year".

What are the parts of the Milky Way galaxy?

A spiral galaxy like the Milky Way has 3 basic components to its visible matter: (1) the disk (containing the spiral arms), (2) the halo, which extends beyond the disk, and (3) the nucleus or central bulge. The halo of the galaxy is rather spherical in shape and contains little gas, dust, or star formation. The clusters found in the halo are globular clusters, so the halo contains very old stars. (See reference for more information).

What is in the center of the Milky Way galaxy?

The center of the Milky Way galaxy lies in the direction of the constellation Sagittarius, and it is here that the Milky Way looks brightest. From Sagittarius, the Milky Way appears to pass westward through the constellations of Scorpius, Ara, Norma, Triangulum Australe, Circinus, Centaurus, Musca, Crux, Carina, Vela, Puppis, Canis Major, Monoceros, Orion and Gemini, Taurus, Auriga, Perseus, Andromeda, Cassiopeia, Cepheus and Lacerta, Cygnus, Vulpecula, Sagitta, Aquila, Ophiuchus, Scutum, and back to Sagittarius. The fact that the Milky Way divides the night sky into two roughly equal hemispheres indicates that the Solar System lies close to the galactic plane. (See Reference from Wikipedia)

The center of the galaxy looks awfully bright from the image above. Perhaps millions of stars are in the center. But, radio telescopes tell us more. Observational data suggests that a massive black hole exists at the center of the central star cluster.

The galactic center harbors a compact object of very large mass as determined by the motion of material around the center. The intense radio source named Sagittarius A*, thought to mark the center of the Milky Way, is newly confirmed to be a supermassive black hole.[See reference below] Most galaxies are believed to have a supermassive black hole at their center.

How old is the Milky Way galaxy?

Astronomers have known that the Milky Way is among the oldest of galaxies. A recent study puts the age of the Milky Way at approximately 14.5 billion years, plus or minus more than 2 billion years. (See reference below)


You can see an example of a Hertzsprung-Russell diagram to the right. This diagram is based on measurements from 23,000 stars in our Milky Way galaxy. The Hertzsprung-Russell diagram is named after its creators, astronomers Ejnar Hertzsprung and Henry Norris Russell.

These collections of stars by brightness and temperature are important when talking about stellar evolution. In general, stars are created in the main sequence. (Of course, when we say "in the main sequence" we really mean "having a brightness and a temperature that causes them to be plotted within the main sequence on a Hertzsprung-Russell diagram.") After billions of years, they expand into red giants. Then, after another one or two billion years, they shrink into white dwarfs.

See: Wikipedia -- Hertzsprung-Russell Diagram


What's beyond the Milky Way galaxy?


The Milky Way galaxy is just one of probably several hundred billion galaxies in the observable universe. The image on the left shows the part of the universe that is within 5 million light years of earth. The Milky Way is shown as one of three large galaxies that belong to the Local Group (Milky Way galaxy, Andromeda galaxy and Triangulum galaxy).

A visit to the site atlasoftheuniverse will let you zoom in and out and give you a better feel for where earth lies in the universe.



1) How many stars does our galaxy contain?

billion stars
2) How thick is the Milky Way galaxy?

light years
3) How long is a cosmic year?

million years

Multiple Choice Questions

1. If one light year is about 9.5 x 1015 meters, what is the approximate diameter of the Milky Way galaxy?
a) 950,000,000,000,000,000,000 meters
b) 950,000,000,000,000 meters
c) 9.5 x 1020 meters
d) both a and c are correct

2. The Milky Way galaxy is:
a) considered a small cluster of stars
b) a spiral galaxy similar to the Andromeda galaxy
c) composed of between 100 million and 200 million stars
d) the largest galaxy in the universe

3. The center our Milky Way galaxy
a) contains a very large sun
b) is believed to contain a massive black hole
c) contains hundreds of stars similar to our sun
d) is
a large empty hole

4. The age of the Milky Way galaxy
a) is the same as the age of our sun
b) about 5 billion years old

c) about 13.5 billion years old
d) about 25 billion years old

5. If we could be here in about 4.5 billion years
a) the sun may no longer be here
b) the earth may no longer be here
c) our galaxy and the Andromeda galaxy will collide
d) all of the above

Further Readings

When Galaxies Collide - In 4 billion years the Milky Way and Andromeda Galaxy will pass through each other.

Wikipedia -- Milky Way galaxy

Milky Way galaxy lecture from University of Oregon -- excellent read for more advanced students. This lecture page also has a quiz attached at the end of the text.

(4 September 2008). "Astrophysics: Bringing black holes into focus". Nature 455 (7209:39–40.)

Scientist refines cosmic clock to determine age of Milky Way --University of Chicago

Where is M13? — A Three Dimensional Galactic Atlas -- This requires a download

An Atlas of the Universe -- This web page is designed to give everyone an idea of what our universe actually looks like -- It's a great piece of work!

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