The universe is an unfathomable expanse filled with countless stars and celestial bodies, many of which form massive structures known as galaxies. The galaxy that houses our solar system, including Earth, is called the Milky Way galaxy.

Beyond the Milky Way galaxy, the universe contains hundreds of millions of other galaxies, known collectively as extragalactic galaxies.

Our planet, Earth, resides within the inner solar system, which itself is situated in the vast Milky Way. The Milky Way is a spiral galaxy, a majestic structure with a diameter of about 100,000 light-years. To grasp the scale of this, consider that light travels at an incredible speed of 300,000 kilometres per second. A light-year—the distance light covers in one year—amounts to approximately 9.46 trillion kilometres. Therefore, the Milky Way's 100,000 light-year diameter represents an almost inconceivable distance, vast beyond our comprehension.

Stars, the building blocks of galaxies, are made of hot gas and are typically spherical or quasi-spherical in shape. These stellar bodies emit their own light and are the primary sources of light in the universe. Interestingly, a star's mass directly influences its lifespan; more massive stars burn through their fuel quickly, leading to shorter lifespans, while less massive stars can endure for billions of years. Most stars are between 1 billion and 10 billion years old, though some may be even older.

Within the Milky Way, there are an estimated 100 billion to 400 billion stars. But stars are not the only inhabitants of our galaxy; they also contain black holes, vast clouds of interstellar gas, and dust. These components add to the complexity and mystery of the Milky Way.

One of the most intriguing questions that has long puzzled scientists is the total mass of the Milky Way. While the stars, black holes, gas, and dust represent enormous amounts of matter, they account for only a small fraction of the galaxy's total mass. This discrepancy has led scientists to hypothesize the existence of dark matter, an elusive substance that does not emit, absorb, or reflect light, making it invisible to current instruments. However, dark matter does have mass and exerts gravitational influence, making it a key factor in understanding the Milky Way's total mass.

Dark matter is believed to constitute about 85 percent of the universe's total mass. However, the exact proportion of dark matter varies from galaxy to galaxy, and determining how much dark matter exists in the Milky Way is an ongoing challenge for astronomers. They must measure and calculate this percentage carefully to better understand the true nature and composition of our galaxy.

The sheer size of the Milky Way is daunting, with a diameter of 100,000 light-years and a mass greater than 10,000 times that of the Sun. When we gaze up at the night sky, especially during the summer months, we are treated to a stunning view of stars. The stars we see are all part of the Milky Way, and their light reaches us thanks to Earth's atmosphere, which scatters the starlight back to our eyes. Without a telescope, the stars visible to the unaided eye are all within our galaxy, and their combined light forms the familiar starry band we recognize as the Milky Way.

Our nearest galactic neighbours, the Large Magellanic Cloud and the Small Magellanic Cloud, are visible from the southern hemisphere. On Earth, the number of stars we see in the night sky is influenced by our geographical latitude. At high latitudes, near the North and South Poles, the visible portion of the sky is limited. At the North Pole, only stars in the northern half of the sky are visible, while at the South Pole, only those in the southern half can be seen.

As one moves closer to the equator, the visible area of the sky increases, allowing us to see more stars. From the equator, the entire sky—both northern and southern hemispheres—is visible, offering a full view of the cosmic expanse above.