232 / Let's visit Space - again ...

Regular readers know, I spend a lot of time in space, many blogs attest to this ... OK, not really in space ... but in my mind, in my imagination and in conjunction with some amateur studies I indulge in. On this occasion I'm smack-bang in the middle of the Horse Head Nebula, and I'm counting dust grains.

I have an essay on COSMOS (from my book en.light.en.ment) where I muse about the consistency of nebulae. It was very hard to find data on this subject matter. Now I have made contact with David Malin, the author of my favourite space picture book The Invisible Universe. "A beautiful oversized volume of more than 50 astronomical photographs created using the most sophisticated telescopes on Earth. David Malin is the Ansel Adams of the sky."

You see, I had always wondered what it meant nebulae were clouds in space - what are they made of (dust and gases, apparently), how large are they? How dense are they? How large is a dust grain? In particular, how does inter-galactic dust compare to earthly dust? In my essay I used information I had scrounged from the internet ... but when I asked David to have a look at the essay, his reply made it necessary for me to make some adjustments; this is his response in its entirety (thank you, David):

"The cosmos is within us throughout us; we are made from its primordial atoms and energies, both enriched and tamed by generations of long-vanished stars. When the Sun dies, in four or five billion years, the same elements of life will be returned to the cosmos, further enriched. After much drifting and mixing, the process will gradually begin again, elsewhere in the Milky Way, itself one galaxy of billions of stars among billions of similar galaxies throughout the Universe. While we and our planet are rare concentrations of matter, the universe itself is mostly empty, and on average contains a few dozen atoms in a room-sized volume. Only within dusty nebulae within the gavitational embrace of a galaxy can life form --- probably.

The Universe is big, very big. Astronomers measure its vastness by how far light travels in one year. Light travels quickly --- 300,000 kilometres a second --- a billion kilometres per hour --- around the Earth seven times a second; in a year it traverses ten trillion kilometres (six trillion miles) and travels from the Sun to Earth in about eight minutes. It travels from the edge of the boundless Universe in 13.7 billion years. The Universe is boundless because it has no centre and no edge: its dimensions are defined by the travel time of light since the Big Bang spirited it into existence, 13.7 billion years ago, a fact that links time and space together into the four-dimensional fabric of spacetime.

Most of the 'stuff' of the Universe is in the form of Dark Matter and Dark Energy, the nature of which remains to be unravelled. The only measurable property of Dark Matter is its mass; it is dark because it does not absorb, reflect or emit any kind of radiation. It remains unseen, and its presence is inferred from its gravitational effects on galaxies and on light, which is bent by gravity. Dark Energy is an even more mysterious force, which is accelerating the expansion of the universe as would negative gravity. Since mass and energy are interchangable we are startled to find that invisible Dark Matter and repulsive Dark Energy together comprise about 95 percent of the Universe. The rest, you and me, the stars and galaxies, our experiences and our lives exist in the remaining five percent.

Of that tiny fraction we can see, the most compelling pictures from space are of the nebulae --- cosmic clouds of dust and gas --- many of which measure many light years across. A few dust particles are a tenth of a millimetre in size, most are a thousand times smaller, more like smoke than the dust of the desert. Its density can reach one particle per cubic centimetre ... it does near the sun, but elsewhere it can be much lower (1 particle/cubic km) or 1000 x higher in a molecular cloud ..."

photo of the Horse Head Nebula by David Malin
the Australian Astronomical Observatory (AAO)