North Hawaii News Articles from CFHT

The Depth of Space

In ancient cultures, when people looked up at the sky and saw the stars, they imagined them as lights attached to a giant dome covering the sky. They believed that the stars were, more or less, all at the same distance, like a giant mural painted on a black canvas. Naturally, they connected the dots and imagined the shapes to be animals, people and gods.

It is easy to understand this view: even today, most people looking at the sky have a hard time imagining the third dimension of space, even if they intellectually know it is there. Without some active imagination, the stars still look like dots painted on a black dome. This month provides us an opportunity to look at the sky and contemplate the depth of space.

We will start this meditative journey from the earth, moving outwards, starting with the solar system. The planets in the solar system all orbit the sun in essentially the same plane, as if they were on a record turntable (though with very different speeds for different distances). Standing on the Earth, we see the solar system like a band wrapping across the sky. At this time of month, the plane of the solar system sticks out almost straight overhead. On the star chart, the constellations that run from the upper right (north west) to the lower left (south east) follow the solar system. They have familiar names: Taurus, Gemini, Cancer, Leo, Virgo, Libra. These are six of the zodiac constellations where the planets can be found. At night, look out towards these constellations and imagine the solar system stretching out as a disk away from you.

The next structure in the sky to imagine is our galaxy, the Milky Way. With the dark Big Island sky, it is easy to see the Milky Way, that hazy bright band that stretches across the sky. At this time of year, about an hour after sunset, it runs almost along the horizon, sticking up in the West, and dipping below in the East. The Milky Way is marked on the star chart with a dotted line near the right side.

The Milky Way is not just a random patch of light. It is our view of most of the stars in our galaxy. The Milky Way is a spiral galaxy, with most of the stars and gas in a very thin disk. To get an idea of the shape, stack three CDs on top of each other. The middle CD is roughly the shape of the gas and dust in the galaxy, while the full sandwich is like the stars. If you put a large cotton ball at the center, that would represent a second part of the galaxy which is filled with fairly old stars. The Earth sits about two thirds of the way from the center of the CD to the edge, in the middle of the sandwich.

To see the Milky Way as a galaxy, and not just a patch of light, stand facing south around 8pm. You are looking towards the center of the galaxy, through the disk, with the stars surrounding you on all sides. It is as if you are an ant standing on the CD, looking towards the center. If you look straight up, towards the faint constellation Coma Berenices, you are looking out of the plane of the galaxy. Since the galaxy is a thin disk, nearly all of the stars you see in that direction are quite close, compared to the size of the galaxy.

Galaxies like our Milky Way are not randomly distributed in space. They live in groups, clumps, and clusters. The Milky Way is part of a small cluster which astronomers call the 'Local Group'. When you look towards Coma Berenices, you are looking at a very large cluster of galaxies, the Coma Cluster. This is a collection of hundreds of galaxies, many like the Milky Way, and some larger and brighter. These galaxies are too faint to be seen without a telescope. By coincidence, there is a faint cluster of stars in our galaxy, between us and the Coma Cluster. When you look at Coma, you can faintly see this fuzzy blob of (quite nearby) stars, and imagine you are seeing the much larger, much more distant cluster of galaxies.

Looking a little to the north and east of Coma, towards the bright star which the Greeks called Arcturus and the Hawaiians called Hokulei'a, there is a large region without many galaxies, the Bootes Void. Look in this direction and imagine a vast, empty volume of space, millions of times the volume of the Milky Way. Space if full of structures, and with some imagination, your mind's eye can let you see what your eyes can't.

Eugene Magnier