The Size of Our Galaxy Stretches Tens of Thousands of Light Years Across, But It's Height Is Rather Small 

Key Takeaways on the Size of Our Galaxy

• Understanding the size of our galaxy depends on the definition used for our galaxy. There are a range of definitions, some very specific and some that delve into the realm of dark matter.
• The Milky Way Galaxy stretches about 80,000 to 100,000 light-years across. However, it’s only about 1,000 light-years tall, which is pretty skinny for a galaxy.
• The distance from Earth to the center of the Milky Way — a supermassive black hole known as Sagittarius A* — is about 26,000 light-years.

The Milky Way galaxy, which hosts our planet, is a massive, disk-shaped galaxy with spiral arms. It’s much wider than it is tall, and depending on the way you envision it, there is much more to our galaxy than most depictions give it credit for.

As a result, estimating the size of our galaxy depends on the definition of our galaxy. If you use some of the wider definitions of what constitutes the Milky Way, it gets a little harder to say exactly where the edge is, since some definitions include the threshold where dark matter becomes less dense.

Plus, there are stars that sit beyond the spiraled arms of our galaxy, and it’s unclear exactly how far out they reach. “It’s very hard to see stars that are that far away, individually,” says Mike Boylan-Kolchin, an astronomer at the University of Texas at Austin.

The Size of Our Galaxy and Sagittarius A*

Perhaps the first threshold to discuss when talking about our galaxy is the very center of it — the thing that the rest of the Milky Way orbits around. Sagittarius A* is a supermassive black hole about 4.3 million solar masses.

Black holes are extremely compact, but based on its mass, scientists estimate that Sagittarius A* is about 14 million miles across, or 7 million miles from the center to the event horizon — the point of no return.

“You don’t want to get in there,” says Boylan-Kolchin.

How Many Light-Years Would It Take To Leave The Milky Way?

The next frontier of our galaxy is likely the one that most people would envision when looking at depictions of the Milky Way — the stellar disk. The stellar disk is the frisbee-like shape in which most of the solid matter of our galaxy resides. It’s much wider than it is tall — Boylan-Kolchin says the width of the Milky Way stretches about 80,000 to 100,000 light-years from end to end, or 40,000 to 50,000 from the center to the outside.

For context, Earth is about 26,000 light-years from Sagittarius A*, the center of our galaxy. In other words, it sits a little past halfway to the edge of the stellar disk if you were travelling outwards from the center.

The height is comparably very thin. From top to bottom — if indeed you envision our galaxy as a frisbee in flight — the Milky Way is only about 1,000 light-years tall.

“It’s a pretty thin frisbee,” Boylan-Kolchin says. But the center also has a bulge, which is about 10,000 light-years in diameter, according to the Las Cumbres Observatory.

Read More: The Seven Sisters Star Cluster Is 20 Times Larger Than We Thought

How Big Is our Stellar Halo?

Most stars sit within the stellar disk, either in the arms that spiral out from the center or in the space between the arms. But there are more stars that float in the space beyond the limits of the stellar disk, and also above and below the stellar disk. The farther you get from the stellar disk, the sparser these stars are.

“Those are typically older stars that may have formed early in the galaxy’s history,” says Boylan-Kolchin.

But the stellar halo, as this roughly ball-shaped, less dense grouping of stars is called, stretches the limits of the Milky Way out to about 400,000 to 600,000 light-years across. Although it all depends on how you define the stellar halo.

Recent research has identified stars that are very distant from the stellar disk. If these were considered to mark the edge of the stellar halo, then it could be as much as 2 million light-years across, according to dual studies from The Astrophysical Journal.

How Big Is Our Galaxy’s Dark Matter Halo?

The final frontier for astronomers seeking to chart the limits of the Milky Way is the dark matter halo. This is a fuzzy kind of frontier based on the average area where dark matter density dissipates down to a level that is more in keeping with the average density of dark matter across the universe.

Dark matter is typically much denser as you get closer to the center of a galaxy. But at a certain point, the density of dark matter begins to fall off precipitously. This is the edge of what’s known as the dark matter halo.

The size of this sphere is vast — almost 2 million light-years in diameter, says Boylan-Kolchin, who was part of a team of researchers who also estimated the mass of the dark matter halo to be around 1 trillion solar masses. The team published those findings in Monthly Notices of the Royal Astronomical Society.

The size is so big that the dark matter halo almost touches the dark halo of our nearest galactic neighbor, Andromeda.

The center of Andromeda is only about 2.5 million light-years from Sagittarius A* at the center of our galaxy. They are moving towards each other and will eventually collide. This event, while still billions of years away, will likely have a big impact on distance, no matter which way you measure it. Some have taken to calling the anticipated joint galaxy Milkomeda.

But if you include the dark matter halo of both galaxies, Milkomeda may already be close to forming on our furthest frontier, Bolyan-Kolchin says.

Read More: How Many Suns Exist In The Milky Way? It’s Complicated

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in Monthly Notices of the Royal Astronomical Society: Sizing from the smallest scales: the mass of the Milky Way
• This article references information from a study published in The Astrophysical Journal: The Next Generation Virgo Cluster Survey. XXXVII. Distant RR Lyrae Stars and the Milky Way Stellar Halo Out to 300 kpc
• This article references information from a study published in The Astrophysical Journal: The Discovery of Milky Way Halo RR Lyrae Stars and Distant Quasars in the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) Deep Fields Database
• This article references information from Las Cumbres Observatory: The Milky Way Galaxy