Structure of the Milky Way

We live inside a vast barred spiral galaxy containing hundreds of billions of stars. But because we are embedded inside it, mapping its true shape has required centuries of clever astronomy. Explore the grand architecture of our galactic home.

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11

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5–10 min

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Q1 Question 1 of 11

What type of galaxy is the Milky Way?

An elliptical galaxy A barred spiral galaxy An irregular galaxy A lenticular galaxy

Q2 Question 2 of 11

Approximately how wide is the Milky Way's disk?

About 1,000 light-years in diameter About 10,000 light-years in diameter About 100,000 light-years in diameter About 1 million light-years in diameter

Q3 Question 3 of 11

Where in the Milky Way does our Sun reside?

At the very center of the galaxy, near the central bulge In the Perseus Arm, the outermost major spiral arm In the stellar halo, above the galactic disk In a minor arm called the Orion Spur, about 26,000 light-years from the galactic center

Q4 Question 4 of 11

What are the four major spiral arms of the Milky Way?

Scutum-Centaurus, Sagittarius, Perseus, and Outer Orion, Andromeda, Cygnus, and Aquila Alpha, Beta, Gamma, and Delta Hercules, Leo, Virgo, and Sculptor

Q5 Question 5 of 11

At what speed does the Sun orbit the center of the Milky Way?

About 30 km/s — the same as Earth's orbital speed around the Sun About 220 km/s About 1,000 km/s — close to 0.3% of the speed of light About 0.5 km/s — barely moving compared to stars near the galactic center

Q6 Question 6 of 11

How long does it take the Sun to complete one orbit around the galactic center?

About 1 million years About 4.6 billion years — the same as the age of the Solar System About 225-250 million years, sometimes called a galactic year About 13.8 billion years — exactly the age of the universe

Q7 Question 7 of 11

What is the central bulge of the Milky Way?

A region of gas and dust where new stars are currently forming rapidly A dense, roughly spherical region of mostly older stars surrounding the galactic center The disk of the Milky Way seen edge-on from our position A cluster of globular clusters orbiting just outside the galaxy

Q8 Question 8 of 11

What is the stellar halo of the Milky Way?

The layer of hot ionized gas above and below the disk The glow of light from the Milky Way visible on dark nights A sparse, roughly spherical region surrounding the disk that contains old stars and globular clusters A ring of gas that is currently being accreted onto the disk

Q9 Question 9 of 11

Why can we not see the full structure of the Milky Way in visible light?

The Milky Way rotates so fast that its light is Doppler-shifted out of the visible spectrum Dust particles in the disk absorb and scatter visible light, blocking our view of distant regions — especially the galactic center The Milky Way is too large for any telescope to capture in a single image Stars in distant parts of the galaxy have burned out before their light reaches us

Q10 Question 10 of 11

Which types of astronomy have been most useful for mapping the Milky Way's true structure?

Visible-light photography, which provides the sharpest images of star positions Ultraviolet astronomy, because UV passes through dust more easily than visible light Infrared and radio astronomy, because these wavelengths pass through the dust-filled disk X-ray astronomy, which is best suited for studying spiral arm structure

Q11 Question 11 of 11

Approximately how many stars are in the Milky Way?

About 1 million stars — the Milky Way is actually quite small Between 200 and 400 billion stars Exactly 100 billion stars — this is the accepted scientific consensus About 10 trillion stars — the Milky Way is one of the largest galaxies in the universe

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