Milky Way may be bigger than thought, research shows

Astronomers have discovered that our galaxy may be bigger than we first thought.

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 Natural astronomy.

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

“…we found a significantly larger ‘size’ (defined as the radius of half the light beam) for the Milky Way than expected,” the researchers wrote in the paper.

The Milky Way is a spiral galaxy with a central bar-shaped structure and several spiral arms extending out 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 enormous size and the presence of dust that obstructs our view.

“Our understanding of the structure of the Milky Way has improved greatly due to the advances in galactic observations in recent decades. The proximity of our home galaxy allows us to study the substructures of the Milky Way (e.g. disk scale heights, spiral arms, and bar/X shape) in great detail. However, for the same reason, a global picture of the galactic structure is still incomplete,” the researchers wrote in the paper.

“The position of the Sun in the disk results in high line-of-sight extinction toward the densest region in the galaxy, and so collecting data from large samples of stars over a wide spatial range is costly in terms of observing time. For example, a radial profile of the surface brightness across a galaxy, a basic observable of galaxies that contains rich information about their assembly history and is easily obtained from their images, has long been lost for the Milky Way.”

In the paper, the researchers describe how they measured the brightness of all parts of our galaxy and performed a census of the red giants scattered throughout the Milky Way. They found that the bulge at the center of the galaxy is not as densely packed as previously thought and is also flatter; the Milky Way therefore has a larger half-radius than we knew.

The half-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 brightness) 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 in a sphere of that radius.

“Because the inner disk profile flattens, the half-radius of the Milky Way is significantly larger than 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 growth history of the Milky Way is largely consistent with high-redshift galaxies, but with systematically smaller sizes. Our results suggest that the Milky Way has a more complex radial structure and a larger size than previously expected.”

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