Galaxies

Galaxies

The cold and vast vacuum, the everlasting night of intergalactic space, is a place so desolate that, by comparison, planets and stars and galaxies seem achingly rare and lovely.

Before us is the Cosmos on the grandest scale we know. We are in the realm of the nebulae, eight billion light-years from Earth, halfway to the edge of the known universe.

From an intergalactic vantage point we would see, strewn like sea froth on the wave of cosmos, innumerable faint, wispy tendrils of light. These are galaxies. There are some hundred billion or more galaxies, each with, on average, a hundred billion stars.

A galaxy reminds me more of a collection of lovely found objects – seashells, perhaps, or corals, the productions of Nature laboring for aeons in the cosmic ocean. Image: © Megan Jorgensen (Elena)

Some galaxies are solitary wanderers, most inhabit communal clusters, huddling together, drifting endlessly in the great cosmic dark.

Within a galaxy are stars and worlds and, it may be, a proliferation of living things and intelligent beings and spacefaring civilisations

Suicide Rate Among Galaxies

The unstructured blubs of irregular galaxies, the arms of spiral galaxies an the torus of ring galaxies exist for only a few frames in the cosmic motion picture, the dissipate, often to be reformed again.

Our sense of galaxies as ponderous rigid bodies is mistaken. They are fluid structures with 100 billion stellar components. Just as a human being, a collection of 100 trillion cells, is typically in a steady state between synthesis and decay and is more than the sum of its parts, so also is a galaxy.

Must we devise ingenious and intricate methods to aid the Earthlings to escape from the Earth, their cradle? (Quotations from Megan Jorgenson). Red Diffraction. Image by © M. Jorgenson (Elena)

The suicide rate among galaxies is high. Some nearby examples, tens of hundreds of millions of light-years away, are powerful sources of X-rays, infrared radiation and radio waves, have extremely luminous cores and fluctuate in brightness on time scales of weeks. Some display jets of radiation, thousand-light-year-long plumes, and disks of dust in substantial disarray. These galaxies are blowing themselves up.

Black holes ranging from millions of billions of times more massive than the Sun are suspected in the cores of giant elliptical galaxies such as NGC 6251 and M87. There is something very massive, very dense, and very small ticking and purring inside M87 – from a region smaller than the solar system. A black hole is implicated. Billions of light-years away are still more tumultuous objects, the quasars, which may be the colossal explosions of young galaxies, the mightiest events in the history of the universe since the Big Bang itself.

Lonely Stars, Lonely Planets

Lonely Stars, Lonely Planets

The stars are lonely. They may be surrounded by planets, but these worldlets are rocky and lifeless, they are planetary systems frozen at some early stage at their evolution. Perhaps, many stars have planetary systems rather like our own: at the periphery, great gaseous ringed planets and cold icy moons. And nearer to the center, small, warm, blue-white, cloud-covered worlds, surrounded by an infinite grey and violet silence.

On some of these lonely planets, intelligent life may have evolved, reworking the planetary surface in some massif engineering enterprise. These worlds, if they do exist, they are our brothers and sisters in the Cosmos.

The Milky Way contains some 400 billion stars of all sorts moving with a complex and orderly grace. And all of these brilliant and enormous bundles of fire are lonely, very lonely.

Loneliness versus solitude. Image: © Megan Jorgensen (Elena)

Island in Space

Island in space

Each star system is an island in space

Our overwhelming impression, even between the spiral arms, is of stars streaming by us – a vast array of exquisitely self-luminous stars, some as flimsy as a soap bubble and so large that they could contain ten thousand Suns or trillion Earths. Others are the size of a small town and a hundred trillion times denser than lead.

Some stars are solitary, like the Sun. Most have companions and their systems are commonly double, two stars orbiting one another. But there is a continuous gradation from triple systems through loose clusters of a few dozen stars to the great globular clusters, resplendent with a million suns. Some double stars are so close that they touch, and starstuff flows between them. Most are separated as Jupiter is from the Sun.

Each star system is a lonely island in space and we grow up in isolation. Image : © Megan Jorgensen (Elena)

Can We Fall Into the Sky?

Can We Fall Into the Sky?

Black holes couldn’t have resulted from stellar collapse, from the normal evolution of massive star systems, because they were too small. Maybe they were primordial, left over from the Big Bang, captured by some unimaginable starship and towed to their destinated stations. Or maybe they were made from scratch.

There is a disk of glowing hydrogen rotating about the center of the Galaxy, and within it a ring of molecular clouds rushing outward toward the periphery of the Milky Way. You can see the ordered motions in the giant molecular cloud complex Sagittarius B2, which had for decades been a favourite hunting ground for complex organic molecules by radio-astronomical experts on Earth. Closer to the center, another giant molecular cloud is located, and then Sagittarius A West, an intense radio source.

And just adjacent at the very center of the Galaxy, locked in a passionate gravitational embrace, is a pair of immense black holes. The mass of one of them is five millions suns. Rivers of gas the size of solar systems are pouring down its mow.

Illustration: Elena

These two colossal, two supermassive black holes are orbiting one another at the center of the Galaxy.

In one second Cygnus A produces more energy than the Sun does in 40,000 years. Two enormous jets of gas, fleeing apart at almost the speed of light, are making a complex web of Rankine-Hugoniot shock fronts with the thin intergalactic gas – and producing in the process a radio beacon that shone brightly over most of the universe. All the matter in this enormous structure, 500,000 light-years across, was pouring out of a tiny, almost inconspicuous point in space exactly midway between the jets.

Bata Shoe Museum. I half-remember one summer’s night, when I was a girl. I had feared I would fall into the sky. (Quotations from M. Jorgensen). Image : © Megan Jorgensen (Elena)

Well-Mannered Galaxies

Well-Mannered Galaxies

Even a galaxy so seemingly well-mannered as the Milky Way has its stirrings and its dances. Radio observations show two enormous clouds of hydrogen gas, enough to make millions of suns, plummeting out from the galactic core, as if a mild explosion happened there every now and then.

A high-energy astronomical observatory in Earth orbit has found the galactic core to be a strong source of a particular gamma ray spectral line, consistent with the idea that a massive black hole is hidden there. Galaxies like the Milky Way may represent the staid middle age in a continuous evolutionary sequence, which encompasses, in their violent adolescence, quasars and exploding galaxies: because the quasars are so distant, we see them in their youth, as they were billions of years ago.

The stars of Milky Way move with systematic grace. Globular clusters plunge through the galactic plane and out the other side, where they slow, reverse and hurtle back again. If we could follow the motion of individual stars bobbing about the galactic plane, they would resemble a froth of popcorn. We have never seen a galaxy change its form significantly only because it takes so long to move.

The stars of Milky Way move with systematic grace, but the spiral pattern persists. Image : A Blue and Nuances Swirl by © Elena

The Milky Was rotates once every quarter billion years. If we were to speed the rotation, we would see that the Galaxy is a dynamic, almost organic entity, in some ways resembling a multy-cellular organism. Any astronomical photograph of a galaxy is merely a shapshot of one stage in its ponderous motion and evolution (but this is not quite true. The near side of a galaxy is tens of thousands of light-years closer to us than the far side; thus we see the front as it was tens of thousands of years before the back. But typical events in galactic dynamics occupy tens of millions of years, so the error in thinking of an image of a galaxy as frozen in one moment of time is small).

The inner region of a galaxy rotates as a solid body. But beyond that, like the planets around the Sun following Kepler’s third law, the outer provinces rotate progressively more slowly.

The arms have a tendency to wind up around the core in an ever-thightening spiral, and gas and dust accumulate in spiral patters of greater density, which are in turn the locales for the formation of young, hot, bright stars, the stars that outline the spiral arms. These stars shine for ten millions years or so, a period corresponding to only 5 percent of a galactic rotation. But as the stars that outline a spiral arm burn out, new stars and their associated nebulae are formed just behind them, and the spiral pattern persists. The stars that outline the arms do not survive even a single galactic rotation; only the spiral pattern remains.