The Triangulum Galaxy is the third largest galaxy in the Local Group after the Andromeda Galaxy and our own Milky Way. It is a spiral galaxy containing around 30 to 40 billion stars, relatively small compared to the 200 to 400 billion stars in our own galaxy and one trillion in the Andromeda Galaxy. Moving closer at a rate of 200 km/s, there is a possibility of a spectacular collision with the Milky Way - but not for another four billion years.

The name comes from the constellation Triangulum (meaning ‘triangle’), which was known to Ptolemy as early as the second century. The Galaxy itself was first documented by the Sicilian astronomer and priest Giovanni Hodierna in the 17th century, but his works received little attention and the galaxy was forgotten until it was rediscovered in 1764 by Charles Messier and designated as Messier object 33.

Many galaxies are ‘spiral’, but some are ‘barred spiral’ if they display an obvious ‘bar’ across the galactic centre. The orientation of this image is perfect for careful examination. Its current classification is ‘spiral’, but recent analysis suggests the presence of a weak bar extending about 10% of the way from the nucleus to the edge of the galaxy.

It is home to the largest known stellar mass black hole, weighing in at 15.7 times the mass of our sun. These are the most common type of black hole and are formed by the collapse of a dying star that no longer has enough fuel, and therefore pressure, to withstand gravity.

The Triangulum Galaxy contains many HII rich areas of star formation. You can see these as the bright pinky-red areas in the spiral arms. An Hα filter was used to capture these areas in more detail and the data was added to the red channel of the image.