The band of the Milky Way galaxy is visible in dark sky areas at night, pictured here with several Atacama Large Millimeter/submillimeter Array (ALMA) antennas.
When you gaze up at the night sky, far from city lights, you might see a faint, milky band stretching across the darkness. This ethereal glow is our home galaxy, the Milky Way. It’s home to our Solar System, including our Sun and Earth, and to all the stars visible to the naked eye. But just how many stars make up this vast cosmic island? Estimating the number of stars in the Milky Way is a complex but fascinating endeavor.
It’s a challenging task to count stars when we are located inside the galaxy itself. Imagine trying to count trees in a vast forest while standing in the middle of it! However, scientists have developed various methods to arrive at our best estimations. Currently, the most widely accepted estimate is that the Milky Way galaxy contains approximately 100 billion stars. Some studies even suggest this number could be as high as 400 billion stars.
These billions of stars are not scattered randomly but are organized into a massive disk shape, spanning about 100,000 light-years in diameter. To put that size into perspective, light travels at an incredible speed of about 300,000 kilometers per second. A light-year is the distance light travels in one year! Our Solar System resides in what could be considered the suburbs of this galactic city, located roughly 25,000 light-years away from the galactic center. Just as Earth orbits the Sun, our Sun and the entire Solar System are orbiting the center of the Milky Way. One complete orbit takes an astonishing 250 million years – a period known as a galactic year.
Because we are inside the Milky Way, we cannot simply take a photograph of it from the outside to see its overall structure. So how do astronomers know that it’s a barred spiral galaxy and estimate the star count? They rely on several key pieces of evidence and observational techniques.
One of the first clues comes from the milky band we see in the night sky – the very feature that gave our galaxy its name. This band of light is the result of viewing the dense disk of stars that forms the Milky Way from our vantage point within that disk. It immediately tells us that our galaxy is essentially flat and disk-shaped. If the stars were distributed in a spherical shape, like in an elliptical galaxy, we wouldn’t see this band across the sky.
This all-sky image from the DIRBE instrument on COBE shows the flat plane of the Milky Way galaxy, confirming its disk-like structure.
To get a more complete picture, astronomers use telescopes on Earth and in space to take numerous images of the Milky Way’s disk in different directions. This is similar to creating a panoramic photograph with your phone. By piecing together these images, they can map the distribution of stars and gas. The concentration of stars in a band further strengthens the evidence that the Milky Way is a spiral galaxy.
Another vital clue comes from mapping young, bright stars and clouds of ionized hydrogen, known as HII regions, within the Milky Way’s disk. These HII regions are created when young, hot stars emit intense ultraviolet radiation that ionizes the surrounding hydrogen gas, stripping electrons from hydrogen atoms. These regions are significant markers of spiral arms in other spiral galaxies that we observe. By mapping their locations in our own galaxy, astronomers can trace out the spiral structure of the Milky Way. These HII regions are bright enough to be seen across the galactic disk, except where the dense central region of the galaxy obstructs our view.
For years, there was debate about whether the Milky Way had two or four major spiral arms. However, recent data and more precise measurements have solidified the model of a four-armed spiral galaxy. This structure is illustrated in artist’s conceptions based on the distribution of young, massive stars and HII regions.
Based on the measured locations of young, hot stars (red) and ionized hydrogen gas clouds (blue), this artist’s conception illustrates the four-armed spiral structure of the Milky Way galaxy.
Further evidence for the Milky Way’s spiral nature comes from analyzing other properties. Astronomers measure the amount of dust within our galaxy and the dominant colors of light we observe. These characteristics closely match those found in other typical spiral galaxies we see throughout the universe. Combining all these pieces of evidence allows us to construct a comprehensive picture of the Milky Way, even though we cannot step outside of it to view it directly.
While the number of stars in the Milky Way is vast, our galaxy is just one among billions in the observable universe. In fact, only three galaxies beyond our own can be seen without a telescope, appearing as faint, fuzzy patches to the naked eye. The closest of these are the Large and Small Magellanic Clouds, satellite galaxies of the Milky Way visible from the Southern Hemisphere. Even these “nearby” galaxies are about 160,000 light-years away. The Andromeda Galaxy, a larger spiral galaxy, is visible from the Northern Hemisphere under dark skies. At 2.5 million light-years away, it’s our nearest large galactic neighbor and is actually moving closer to us. Scientists predict that in about 4 billion years, the Andromeda Galaxy will collide and merge with the Milky Way in a spectacular cosmic event. Most other galaxies are even more distant and require powerful telescopes to observe.
The Andromeda galaxy, our closest large neighbor galaxy in space, is a spiral galaxy similar to our own Milky Way.
Understanding the number of stars in our galaxy, the Milky Way, is a continuous process of astronomical observation and refinement. While estimates may vary, the current consensus points to hundreds of billions of stars bound together in a magnificent spiral structure, our cosmic home in the vast universe.
