The Milky Way Galaxy is a spiral galaxy spanning approximately 100,000 light-years across. Our Sun is one of at least 100 billion stars residing within it, arranged in four main arms that orbit a colossal black hole at the galaxy’s center. This black hole is estimated to be four million times the mass of our Sun and is located about 28,000 light-years away from it, representing a third of the distance from this central black hole. Fortunately, this distance keeps Earth safe from any immediate threat. It is believed that most stars within our galaxy have planetary systems, and thousands of these have already been discovered, with more expected in the future. Many of these newly discovered planetary systems are vastly different from our own.

Like other galaxies, the Milky Way is an isolated collection of stars and other materials bound together by gravity. Alongside the stars, there are billions of planets—some part of solar systems, while others drift freely. Between the stars, there are countless nebulae composed of dust and gas, mostly hydrogen and helium. From Earth, the galaxy appears as a band of diffuse, curved light across the night sky. The term “Milky Way” in English comes from the Romans, who imagined it as a streak of spilled milk. Astronomers and philosophers debated the nature of the Milky Way until Galileo Galilei first observed it through a telescope and discovered that its light emanated from countless distant stars that are too faint to see individually. However, their combined light creates the familiar band.

Until the early 20th century, astronomers assumed that the Milky Way contained all the stars in the universe, either stretching to fill the entire cosmos or being limited in size and surrounded by infinite voids. However, in the early 1920s, astronomer Edwin Hubble made detailed observations of the Andromeda Nebula and revealed it to be a separate galaxy, located millions of light-years away from us.

What is the Size of the Galaxy?

The Milky Way is composed of three main parts: the core, the disk, and the halo.

Starting with the core, it is not spherical but elongated in a bar shape, ranging from 5,000 to 20,000 light-years in length. About a quarter of the stars in the Milky Way reside within the core, where a supermassive black hole with a mass 4.1 million times that of the Sun is located. The disk has a radius of between 75,000 and 100,000 light-years but is no more than 1,000 light-years thick. Within the disk are several major spiral arms where the density of stars and gas is higher than average, making these arms stand out in visual observations. Our solar system is situated in the disk about 28,000 light-years from the galactic center, near the inner edge of the Orion Arm.

Beyond the disk lies the Milky Way’s “halo,” a spherical region with a radius of about 100,000 light-years, containing old stars and globular clusters all orbiting the galaxy’s center in random directions. Dark matter extends even farther, up to 400,000 light-years from the center, according to a 2019 study published in the Astrophysical Journal.

The Milky Way orbits along its galactic orbit at an average speed of about 828,000 kilometers per hour, with our solar system taking approximately 230 million years to complete one full orbit around the galactic center.

Where is the Milky Way Located?

The Milky Way has two major satellite galaxies: the Large and Small Magellanic Clouds. It is part of a group of galaxies known as the Local Group, which spans about 10 million light-years and consists of more than 30 galaxies bound together by gravity. Apart from our galaxy, the largest galaxy in this group is Andromeda, our closest neighbor, located 2.5 million light-years away. It appears to be on a collision course with the Milky Way, expected to occur in about four billion years, along with 80 smaller galaxies.

The Structure of the Milky Way

Estimating the true size of the Milky Way is challenging because we reside within it, and clouds of gas and dust obscure our observations. Astronomers estimate the galaxy’s total mass to be about a trillion times that of the Sun. According to NASA, most of this mass is in the form of dark matter, with stars representing about 1% of the galaxy’s mass and interstellar gas only 0.1%. Observations of other galaxies show that stars in the outer regions orbit the galactic centers at the same speed as stars closer to the center, which challenges Newton’s established laws of gravity. Consequently, astronomers have deduced that something other than stars and gas and dust is providing the additional gravity, which is believed to be dark matter—a mysterious substance detectable only through its gravitational effects.

The Local Group is one of many galaxy clusters that are all moving away from each other, with more space appearing between them. This observation led to the development of the Big Bang theory about the universe’s origin. Scientists expect gravity to act as a brake on the expansion, potentially halting or even reversing it. However, in the 1990s, it was discovered that the expansion is actually accelerating, driven by a force called dark energy, whose nature remains unknown. One possibility is that it is the energy inherent in the vacuum of space itself.

Since matter and energy are equivalent (as expressed in Einstein’s famous equation, E = mc²), scientists have concluded that dark energy makes up about 68% of everything in the universe, with dark matter accounting for another 27%, leaving only 5% for protons, neutrons, electrons, and photons—essentially, everything we can directly observe and understand. Scientists estimate that there are at least 100 billion galaxies in the observable universe, each filled with an extensive number of stars.

Exploring the Galaxy

Scientists currently use powerful telescopes on Earth and in space to study distant stars and galaxies. The famous Hubble Space Telescope, which revealed the universe in great detail for the first time, will soon be replaced by the more powerful James Webb Space Telescope. Meanwhile, the Kepler mission has explored part of our galaxy in search of other planets.

NASA also has five robotic spacecraft with sufficient speed to escape our solar system and travel into interstellar space. However, only one spacecraft, Voyager 1, has crossed that boundary, entering interstellar space in 2012. Voyager 2 is likely next. Both spacecraft, launched in 1977, remain in contact with NASA’s Deep Space Network. Additionally, the New Horizons probe flew past Pluto in 2015 and is currently exploring the Kuiper Belt beyond Neptune. It, along with Pioneer 10 and Pioneer 11, will eventually leave our solar system.

Before 1983, confirmed planets were only those within our solar system, though scientists believed many others orbited distant stars in the Milky Way. In 1983, a team discovered a disk believed to be composed of primordial materials for planet formation, providing the first evidence of an exoplanet. The first exoplanet was discovered nine years later in 1992, and the number of known exoplanets has rapidly increased since then.