The Milky Way Explained

The Milky Way Explained, talks you through the dramatic scale and shape of the galaxy and identifies the oldest and freshest elements to emmerge.


The Milky Way Explained

The Milky Way galaxy viewed from the outside

The Earth exists in the Milky Way galaxy that contains between 200 and 400 billion stars, depending on the number of faint dwarf stars, that are difficult for us to detect. The majority of stars lie in a disc around 100,000 light years in diameter and, on average, around 1,000 light years thick. These vast distances are very difficult to visualize. A distance of 100,000 light years means that light itself, travelling at 300,000 kilometers (186,000 miles) per second, would take 100,000 years to make a journey across our galaxy. Or, to put it another way, the distance between the Sun and the outermost planet of our solar system, Neptune, is around four light hours – that’s one-sixth of a light day. You would have to lay around 220 million solar systems end to end to cross our galaxy.

One of the intrinsically brightest stars in our galaxy appears as the bright white dot in the centre of this image taken with the NASA/ESA Hubble Space Telescope. Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) was needed to take the picture, because the star is hidden at the galactic center, behind obscuring dust. NICMOS' infrared vision penetrated the dust to reveal the star, which is glowing with the radiance of 10 million suns.

The intense white star at the centre of this image is the Pistol Star, which is the brightest star in our galaxy

Super-Massive Black Hole

At the center of our galaxy, and possibly every galaxy in the universe, there is believed to be a super-massive black hole. Astronomers believe this because of precise measurements of the orbit of a star known as S2. This star orbits around the intense source of radio waves known as Sagittarius A star, that sits at the galactic center. S2’s orbital project is just over fifteen years, which makes it the fastest-known orbiting object, reaching speeds of up to 2 per cent of the speed of light. If the precise orbital path of an object is known, the mass of Sagittarius A* is enormous, at 4.1million times the mass of our sun. Since the star S2 has a closest approach to the object of only seventeen light hours, it is known that Saggitarus A* must be smaller than this, otherwise S2 would literally bump into it. The only known way of cramming 4.1million times the mass of the Sun into a space less than 17 light hours across is as a black hole, which is why astronomers are so confident that a giant black hole sits at the center of the Milky Way. These observations have recently been confirmed and refined by studying a further twenty-seven stars, known as the S-stars, all with orbits taking them very close Sagittarius A*.

The densest star cluster

The Milky Way Explained - Archers Cluster

Artists impressionof Archers Cluster, the densest known cluster of stars in the Milky Way

Beyond the S-stars, the galactic centre is a melting pot of celestial activity, filled with all sorts of different systems that interact and influence each other. The Arches Cluster is the densest known star cluster in the galaxy. Formed from about 150 young, intensely hot stars that dwarf our sun in size, these stars burn brightly and are consequently very short lived, exhausting their supply of hydrogen in just a couple of million years. The Quintuplet Cluster contains one of the most luminous stars in our galaxy, the Pistol Star, which is thought to be near the end of its life and on the verge of becoming a supernova. It is in central clusters like the Arches and the Quintuplet that the greatest density of stars in our galaxy can be found. As we move out from the crowded galactic center, the number of stars drops with distance, until we reach the sparse cloud of gas in the outer reaches of the Milky Way known as the Galactic Halo.

The Milky Way Explained

The oldest object

In 2007, scientists using the Very Large Telescope (VLT) at the Paranal Observatory in Chilie were able to observe a star in the Galactis Halo that is thought to be the oldest object in the Milky Way. HE 1523-0901 is a star in the last stages of its life; known as a red giant, it is a vast structure far bigger than our sun, but much cooler at its surface. HE 1523-0901 is interesting because astronomers have been able to measure the precise quantities of five radioactive elements – uranium, thorium, europium, osmium and iridium – in the star. Using a technique very similar to carbon dating (a method archaeologists use to measure the age of organic material on Earth), astronomers have been able to get a precise age for this ancient star. Radioactive dating is an extremely precise and reliable technique when there are multiple ‘radioactive clocks’ ticking away at once. This dying star turns out to be 13.2 billion years old – which is almost as old as the Universe itself, which began just over 13.7 billion years ago. The radioactive elements in this star would have been created in the death throes of the first generation of stars, which ended their lives in supernova explosions in the first half a billion years of the life of the universe.