We live in a galaxy òÀÓ a star city with several hundred billion residents. Moreover, we know our Milky Way is a spiral galaxy, with prominent arms of bright young stars that spiral outwards from a central bar that is dominated by old stars. These arms are so prominent as they are where the most massive stars are found. Massive stars have short but dramatic lives. Their youthful vigour means they shine brightly. A star ten times as massive as the Sun shines with an intensity a thousand times greater. The spiral arms thus dominate the appearance of a galaxy when seen from the outside.
A recent sketch of how our Galaxy might look like if viewed from outside, based on interpretation of infrared measurements made with NASAòÀÙs Spitzer Space Telescope. Two principal arms (Scutum-Centaurus and Perseus) wrap off the ends of a central bar of stars. Two minor arms (Norma and Sagittarius) are located between the major arms. The Sun lies near a smaller, partial arm known as the Orion Arm, lying between the Sagittarius and Perseus arms. (Image credit: NASA/JPL-Caltech/R. Hurt)
Spiral arms are home to much more than just the brightest stars, however. They are the cauldron for a galactic ecosystem. Driven by flows of energy emanating from stars, a continuous cycle of matter flows backs and forth between the stars and the gas of the interstellar medium.
Spread along the bright spiral arms can be seen prominent dark lanes, clouds of molecular gas and dust which obscure the visible light. Inside these clouds are often found nurseries of infant stars. They are born when gas collapses, under the weight of its own gravity, inside these natal clouds. When their cores get hot and dense enough to start generating nuclear energy from the fusion of protons we say a star is born.
The beautiful spiral galaxy NGC 2997, as imaged by David Malin with the Anglo Australian Telescope. Our own Galaxy would look something like this if it could be viewed from outside. The spiral arms are lit up by the extraordinary luminous young massive stars that are born within the dark clouds of molecular gas that lie along the arms.
(Image credit: ˆéAustralian Astronomical Observatory. www.aao.gov.au)
Particles then stream from their surfaces in stellar winds, enriched in elements produced by nucleosynthesis. The winds sweep up the interstellar gas, which gets compressed and eventually turns into molecular clouds, where star formation can begin once again, so continuing the star-gas cycle. A cycle that has driven the evolution of our Galaxy for the last ten billion years.
How do we know that we live in a spiral galaxy, however, where such an ecosystem plays out? If you look at the sky at night you will see a field of stars, but no spiral arms will be evident in their distribution. If you are lucky enough to see a truly dark sky, however, away from any lights and without the Moon, you will be able to discern a band of stars running across it. This is our Milky Way. Look more closely and dark patches will be seen in the band, apparent voids in space without stars. Today we know that the band is our galaxy, and the voids are clouds of molecular gas and dust where new stars are being formed.
An image of the Milky Way from Northern Ireland showing the dark patches of molecular clouds. (Image credit: Colin Johnston/Armagh Planetarium)
This interpretation òÀÓ of a spiral galaxy with active star formation within it òÀÓ is not obvious. When William Herschel (the discover of Uranus) produced the first map of the Galaxy in 1784, from his 40-foot telescopeˆµ in Slough, west of London (the most unlikely place to now envisage building a telescope!) he came to a different conclusion. By counting the relative numbers of stars he could see with his telescope along different directions in the sky, he produced the map shown below. The òÀÜMilky WayòÀÝ is prominent as the bright band, with relatively few stars found outside it. Some directions in the band do have more stars, and the dark patches that can be seen in it are represented as sightlines with fewer stars.
William Herschel and his first map of the Galaxy, drawn in 1784. The dots represent the relative number of stars seen along each direction. The position of Sun (and ourselves) is represented by the larger dot near the centre. (Image credits: Herschel image: public domain (via Wikimedia.org)/Galactic map from “On the Construction of the Heavens. By William Herschel, Esq. FRS., reproduced with permission)
As described by Herschel òÀÜfrom this figure however, which I hope is not a very inaccurate one, we may see that our nebula, as we observed before, òÀæ. has a very extensive, branching, compound congeries of many millions of starsòÀÝ. A prosaic description òÀÓ congeries of stars òÀÓ but unfortunately one that is quite wrong! This is most clearly evident in where Herschel placed the Sun in the map, near to the centre of the Galaxy. Today we know that the galactic centre lies about 25 000 light years away, in the direction of the constellation Sagittarius.
HerschelòÀÙs 40-foot telescope in Slough, west of London.
In the visible spectrum we can see no further away from us than about 3 000 light years in any direction along the galactic plane. The reason is because of those clouds of molecules. Dust inside them blocks the light. Going to longer wavelengths, such as the infrared, allows one to peer through the clouds and see the full extent of the Galaxy beyond. However in HerschelòÀÙs day the concept of interstellar dust was unknown. Indeed, even infrared radiation was another discovery of HerschelòÀÙs, made when he noticed the increase in temperature of a thermometer that was placed just beyond the red end of the spectrum of light from the Sun, after being spread by out by a prism. There was no possibility of measuring such radiation from the stars.
Even the very concept of what a òÀÜgalaxyòÀÝ was different in HershelòÀÙs day to now. Then it was considered as the entirety of the cosmos, not as an òÀÜisland universeòÀÝ of stars òÀÓ one of countless billions òÀÓ that we understand today. However that tale, about the size scale of the Universe, a tale which shaped much of 20th century astronomy, is another story.
