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
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.