A fish jumping in shadows

While most of the nocturnal sky looks like a dark ceiling sprinkled with stars, there is a broad band of light arching across the night sky from horizon to horizon. For a long time, it has been a mystery often ignored by the astronomers who considered it as exhalations of the Earth floating in the sky. In the winter of 1609, Galileo pointed its telescope at it and discovered a swarm of stars where the naked eye could only see clouds of faint light. For the astronomers today, this band is a galaxy, a gravitationally bound collection of roughly a hundred billion stars. Our Sun is one of these stars and is located roughly 24,000 light years from the center of this galaxy. What are we talking about today?

Right, this article is about the Milky Way, the name given in english (with its equivalent in most of the European languages) to this band of light. It is true that it looks like milk spilled across the curtain of darkness. In the Greek mythology, it is definitely milk, though the source of the milk and its drinker are not known for certain. It could have been formed by the milk of Hera, which flowed when she realized that she had been giving milk to Hercules and thrust him away. Others have said that Hera intended with her milk to anoint and feed Dionysus in order to heal his madness. Others say that it was the milk of Rhea, which flowed at the time when she presented the stone to Cronos.

For many civilizations, heavens are where the souls would go after death, and the Milky Way is the path to be followed for this journey from Earth to God, playing the same role at night as the rainbow during the day. In the Seminole language for instance, the Milky Way is called so-lo-pi he-ni,  "spirit way", and is a broad pathway in the night sky which leads to the City of the West where the souls of good Indians go when they die.

Some legends coming from very different places are surprisingly similar. A dog which ran from a corn mill in the south towards the north, dropping meal as he ran, is given as the origin of the Milky Way in Scandinavian legends. The Cherokees, faced with a giant dog stealing their corn at night, chased after him making the loudest noises they could. It ran to the top of a hill, leaped into the sky, the cornmeal spilling out the sides of its mouth, and ran across the black night sky until it disappeared from sight. But the cornmeal that had spilled from its mouth made a path way across the sky called "gi li' ut sun stan
un' yi", "the place where the dog ran".

The Milky Way galaxy has three major components: - A thin disk consisting of young and intermediate age stars, which also contains gas and is actively forming new stars. There are spiral arms in this disk, which are regions of active star formation. - A bar of older stars - An extended dark halo whose composition is unknown. Since the matter in the halo does not consist of luminous stars, it's not directly observed. Its existence is inferred from its gravitational pull on the visible matter. This dark matter could even account for 95% of our galaxy...

All the objects in the Milky Way galaxy orbit their common center of mass, called the Galactic Center, where a black hole could be located. As a galaxy, the Milky Way is actually a giant, as its mass is probably between 750 billion and one trillion solar masses. It belongs to the Local Group, a smaller group of 3 large and over 30 small galaxies, and is the second largest (after the Andromeda Galaxy) but perhaps the most massive member of this group. This local group is really local, if you remember that there are billions of galaxies in the universe...

We are now at the end of this week astronomy column, and it is time to explain its title. I'a-lele-i-aka, a fish jumping in shadows, is one of the designations of the Milky Way in the hawaiian language. One night, leave your home, find a dark place, take some time to forget about our artificial lights, and enjoy the silent show of this fish jumping in the shadows of the magnificent Hawaiian sky.

No picture of our whole Galaxy can be better than the sight of the Milky Way  arching across the night sky from horizon to horizon in an clear Hawaiian night. Here, you see only its very center, called the Galactic Center, as observed at CFHT using an instrument, the Adaptive Optics, which removes most of the  blurring caused by the atmosphere. By observing the same area over a few years, we can determine the motion of the stars around the Galactic Center. From their apparent velocities (some are above 2000 kilometers per seconds) we infer that there is something huge, probably a black hole, at the center of our galaxy. Don't worry though, it is very far away.

Christian Veillet
Canada-France-Hawaii Telescope