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Earth & Space Science · Foundations

Milky Way

9 min read 5 practice questions
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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

Our , the Milky Way, is a : a flat, rotating disk of roughly 100 billion stars with spiral arms, a of older stars, and a vast spherical halo. The disk spans about 100,000 light-years. Our Sun sits inside the disk, about 26,000 light-years from the center, tucked into a small arm called the Orion Spur. At the center lies a supermassive black hole, . On dark nights, the pale band across the sky is simply the disk of our own galaxy, seen edge-on from within.

Why this matters

Every star you can see without a telescope belongs to the Milky Way, and so does the Sun. Knowing the galaxy's structure tells you where Earth sits in the cosmos and how the solar system moves through it. The same tools astronomers use to map the Milky Way - infrared surveys, radio telescopes, and the orbits of stars - are the tools used to study every other galaxy, so this lesson is also the foundation for understanding the universe at large. It turns an everyday sight, the band of the Milky Way in a dark sky, into something you can read and explain.

The college version

A barred spiral of a hundred billion stars

The Milky Way is the galaxy we live in - the home of the Sun, Earth, and every star visible without a telescope. Astronomers classify it as a barred spiral galaxy. 'Spiral' means its stars, gas, and dust form a flat, rotating disk with arms that wind around the center; 'barred' means the spiral arms attach to the ends of a short, straight bar of older stars that spans the middle. NASA's Spitzer Space Telescope surveys showed that the Milky Way's structure is dominated by two major arms - the Scutum-Centaurus and Perseus arms - which wrap off the ends of this central bar, with two thinner minor arms between them. Estimates place the galaxy's star count near 100 billion. The disk is about 100,000 light-years in diameter and only about 2,000 light-years thick, thin enough that astronomers compare it to a CD rather than a ball.

Disk, bulge, and halo: the three parts of the galaxy

The Milky Way's visible structure has three named parts. The disk is the flat, rotating layer of stars, gas, and dust where the spiral arms and nearly all young stars live. Toward the center, stars stop being confined to the disk: within about 10,000 light-years of the center they fill out a central bulge (also called the nuclear bulge), a concentration made mostly of older yellow and red stars. Surrounding everything is the halo, a roughly spherical region of very old, faint stars and globular clusters that extends at least 150,000 light-years from the center. Halo stars are older and poorer in heavy elements than disk stars, which is one reason astronomers read the galaxy's history in where stars are found. The point for this lesson is simple: the Milky Way is not a shapeless cloud of stars but an organized system with a disk, a bulge, and a halo.

Where we are: inside the disk, about halfway out

The Sun does not sit at the galaxy's center. It lies within the disk, about 26,000 light-years from the - roughly halfway from the center to the disk's edge, matching NASA's description of Earth as about halfway out. The Sun is tucked into a small, partial arm called the Orion Arm, or Orion Spur, located between the major Sagittarius and Perseus arms. Like everything in the disk, the Sun orbits the center: NASA puts one orbit of the solar system around the galaxy at about 240 million years. Because we live inside the disk, our view of the galaxy is edge-on. That is why the Milky Way appears in the night sky as a faint, milky band of light arcing across the sky: we are looking along the plane of the disk, through its crowded stars, gas, and dust. The galaxy's name comes from that pale band.

The center: Sagittarius A*

The true center of the galaxy lies in the direction of the constellation Sagittarius, hidden from visible light by thick dust. Radio and infrared observations see through that dust, and they reveal a bright radio source at the nucleus: Sagittarius A* (pronounced 'Sagittarius A-star'). Astronomers have shown that Sagittarius A* is a supermassive black hole - NASA gives its mass as about 4 million times the Sun's, while OpenStax cites about 4.6 million. The evidence came from tracking stars that whip around the center at enormous speeds: their orbits require a compact mass of millions of Suns inside a region smaller than the diameter of Mercury's orbit, and no cluster of ordinary stars could pack so much mass so tightly. NASA notes that this demonstration earned the 2020 Nobel Prize in Physics. The Milky Way is not unusual in this - most large galaxies harbor supermassive black holes at their centers - and it is just one galaxy among billions, a member of the Local Group of more than 50 galaxies whose nearest large member is Andromeda. The general study of galaxies, their types and structures, is its own topic; the point here is that our galaxy is one place within that larger picture.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

The Milky Way is the giant city of stars we live in. It is shaped like a flat, spinning pancake of stars called the disk, with a bulge of older stars in the middle and a big, round halo of ancient stars around everything. Our Sun is one of about a hundred billion stars in that disk, sitting about halfway out from the center. If you could float far above the pancake, you would see spiral arms winding around a straight bar of stars - that shape is why astronomers call it a barred spiral galaxy. On a very dark night, the pale ribbon of the Milky Way crossing the sky is simply the disk seen from inside, like standing in the middle of a crowd and seeing people in every direction. At the very center hides a supermassive black hole called Sagittarius A*, about four million times heavier than the Sun.

Picture it like this

Imagine a huge, thin vinyl record spinning slowly in space. The Sun is a tiny speck of dust sitting on the record, about halfway between the middle and the rim. When you stand on that speck, the record's surface stretches away from you in a flat plane, so you see it as a faint band across the sky - that band is the Milky Way of the night sky. The record's dense center hole is the bulge, and dust drifting far above and below the record is the halo.

Where the picture stops working

The record is too simple in a few ways. A record is solid and rigid, while the galaxy's disk is made of stars, gas, and dust that orbit the center at different speeds. A record has no spiral structure and nothing like a black hole at its center. And a record has a sharp edge you could walk off, while the galaxy's disk thins out gradually, with the halo extending far beyond the bright stars.

Worked example

Astronomers measure the galaxy in light-years, the distance light travels in one year. The Milky Way's disk is about 100,000 light-years across, so its radius is about 50,000 light-years. The Sun is about 26,000 light-years from the center. Dividing 26,000 by 50,000 gives 0.52, so the Sun is 52 percent of the way from the center to the rim - about halfway out, exactly as NASA describes it. The same arithmetic gives a striking fact: light from the galactic center has to travel about 26,000 light-years to reach us, so the picture we get of the center tonight left there about 26,000 years ago.

Key takeaway

The Milky Way is a barred spiral galaxy about 100,000 light-years across, and our Sun sits in its disk about 26,000 light-years from a center that holds a supermassive black hole - the pale band we see at night is our own galaxy's disk seen from inside.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

What kind of galaxy is the Milky Way?

Choose an answer, then check it.
Question 2 of 3foundational

About how far from the galactic center is the Sun, and where is it located?

Choose an answer, then check it.
Question 3 of 3intermediate

On a dark night you see the Milky Way as a pale band of light stretching across the sky. Which statement best explains what you are looking at?

Choose an answer, then check it.
Practice all 5

Keep learning

Ready to build on this? Continue to the next lesson.

Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Describe the Milky Way's main parts - the disk, the central bulge, and the halo - and state what each contains.
  • Explain what it means to call the Milky Way a barred spiral galaxy.
  • State the galaxy's diameter and the Sun's location within it, using light-years.
  • Apply the inside-the-disk perspective to explain the band of the Milky Way in the night sky.
  • Analyze the evidence that a supermassive black hole, Sagittarius A*, sits at the galactic center.

Common mistakes

  • The band of the Milky Way in the night sky is a distant galaxy or a cloud in Earth's atmosphere.

    The band is the disk of our own galaxy seen edge-on from inside it; we are looking along the plane of the disk through its stars, gas, and dust.

  • The Sun is near the center of the galaxy.

    The Sun is about 26,000 light-years from the center, roughly halfway out in the disk, near the small Orion Spur arm.

  • The Milky Way contains all the stars in the universe.

    The Milky Way is one galaxy among billions, estimated to hold about 100 billion stars; every star we see without a telescope belongs to it, but the universe holds far more.

  • Sagittarius A* is a bright star at the galaxy's center.

    Sagittarius A* is a supermassive black hole of about 4 million solar masses; it is a radio source, not a star, and it was identified by tracking the orbits of stars around it.

  • The Milky Way is a few million light-years across.

    The disk spans about 100,000 light-years; it is the largest galaxies, not ours, that can exceed a million light-years - a point developed in the galaxies topic.

Easily confused

Galactic disk vs. Galactic halo

The disk is flat and rotating, holds most of the galaxy's young stars, gas, and dust; the halo is roughly spherical, holds very old, faint stars and globular clusters, and extends much farther from the center.

Sun's location vs. Galactic center

The Sun lies in the disk about 26,000 light-years from the center, near the Orion Spur; the center, hidden behind dust, holds the supermassive black hole Sagittarius A*.

Milky Way vs. A typical distant spiral galaxy

The Milky Way is the one galaxy we live inside, so we see it edge-on as a band and cannot photograph it whole; distant spirals are viewed from outside, which is why astronomers compare our galaxy's structure with theirs.

Key vocabulary

Galaxy
A large collection of stars, planets, gas, and dust bound together by gravity; the Milky Way is the galaxy that contains our solar system.
Barred spiral galaxy
A spiral galaxy whose arms wind off the ends of a central, bar-shaped concentration of older stars; the Milky Way is one.
Galactic disk
The flat, rotating layer of stars, gas, and dust that makes up most of the galaxy's visible material, about 100,000 light-years across.
Central bulge
The concentration of mostly older stars that fills the region within about 10,000 light-years of the galactic center, no longer confined to the disk.
Galactic halo
The roughly spherical region of very old, faint stars and globular clusters surrounding the disk and bulge, extending at least 150,000 light-years from the center.
Light-year
The distance light travels in one year, about 9.5 trillion kilometers; astronomers use it to measure the galaxy's size and the Sun's distance from the center.
Galactic center
The rotational center of the Milky Way, about 26,000 light-years from the Sun, in the direction of the constellation Sagittarius; it contains the supermassive black hole Sagittarius A*.
Sagittarius A*
The supermassive black hole at the center of the Milky Way, with a mass of about 4 million Suns, detected through radio emission and the orbits of stars around it.
Spiral arm
A curving concentration of stars, gas, and dust within the disk of a spiral galaxy; the Sun sits near a small arm called the Orion Spur.

Sources & references

  1. Galaxies (Universe) — NASA Science
  2. The Milky Way Galaxy (artist's concept) — NASA Science (NASA/JPL-Caltech/R. Hurt, SSC/Caltech)
  3. Black Holes (Overview) — NASA Science (science.nasa.gov)
  4. The Milky Way Galaxy (Imagine the Universe) — NASA Goddard Space Flight Center (Imagine the Universe)
  5. Astronomy 2e, Section 25.1 The Architecture of the Galaxy — OpenStax (Rice University)
  6. Astronomy 2e, Section 25.3 The Mass of the Galaxy — OpenStax (Rice University)
  7. Astronomy 2e, Section 25.4 The Center of the Galaxy — OpenStax (Rice University)

EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.

Researched 2026-08-21

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