Our home galaxy might be larger than we first assumed, astronomers have found.

A new model of the Milky Way has revealed that our galaxy is wider than we thought, according to a new paper in the journal Nature Astronomy.

The researchers found that the bulge at the center of our galaxy is less densely packed with stars than they expected.

NASA image of the Milky Way as seen from Earth. Our galaxy might be larger than we thought, research has found. NASA image of the Milky Way as seen from Earth. Our galaxy might be larger than we thought, research has found. NASA

"...we obtained a significantly larger 'size' (defined as the half-light radius) for the Milky Way than that expected," the researchers wrote in the paper.

The Milky Way is a barred spiral galaxy with a central bar-shaped structure and several spiral arms extending from the center. The galaxy has a dense central bulge around the Galactic Center, which is thought to contain a supermassive black hole named Sagittarius A*. The Milky Way contains between 100 billion and 400 billion stars, but the exact number is difficult to determine due to the galaxy's vast size and the presence of dust that obscures our view.

"Our understanding of the Milky Way's structure has improved tremendously through the advancement of galactic observations over recent decades. Thanks to the proximity to our home galaxy, we are able to study the Milky Way's substructures (for example, disk scale heights, spiral arms and bar/X-shape) in great detail. For the same reason, however, a global picture of the galactic structure is still incomplete," the researchers wrote in the paper.

"The sun's position embedded in the disk results in high line-of-sight extinction towards the densest region in the galaxy, and thus, collecting data from large samples of stars over a wide spatial range is costly in terms of observing time. For instance, a surface brightness radial profile across a galaxy, a basic observable of galaxies that contains rich information about their assembly histories and is easily obtainable from their images, has long been missing for the Milky Way galaxy."

In the paper, the researchers describe how they measured the brightness of all parts of our galaxy, and conducted a census of the red giants dotted across the Milky Way. They found that the bulge in the center of the galaxy is not as densely packed as first thought and is also flatter; the Milky Way thus has a greater half-light radius than we knew.

The half-light radius is a measure used in astronomy to describe the size of an astronomical object, defined as the radius within which half of the total light (or luminosity) of the object is emitted. In other words, it is the distance from the center of the object to the point where half of the total light from the object is contained within a sphere of that radius.

NASA illustration of the Milky Way from above. The bulge at the center may be less dense than we first thought. NASA illustration of the Milky Way from above. The bulge at the center may be less dense than we first thought. NASA

"Because the inner disk profile flattens, the half-light radius of the Milky Way is significantly larger than that expected from a picture of the Milky Way's structure with a bulge and single-exponential thick and thin disk components," they wrote.

"We also confirm that the size growth history of the Milky Way is broadly consistent with high-redshift galaxies but with systematically smaller sizes. Our results suggest that the Milky Way has a more complex radial structure and larger size than previously expected."

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