In Our Skies: Waiting for Andromeda to merge with us

When we look around a dark sky at night with our unaided eye, we generally see stars. The number of stars that we see multiplies greatly when we start examining the sky with a telescope, and meanwhile we also start to see objects that appear to be fuzzy or misty.

These objects are generally called nebulae – plural for nebula, the Latin word for cloud or mist - and some of these, at least, are indeed large clouds of interstellar gas and dust.

A significant fraction of these nebulae, however, exhibit a distinct spiral shape, especially when photographed, and a century ago these were accordingly called spiral nebulae.

There was a tremendous debate within the astronomical community at that time as to just what these things were; some astronomers argued that these were relatively nearby, and perhaps were new planetary systems in the act of forming, whereas other astronomers argued that they were much more distant, and in fact were island universes well outside our own Milky Way.

The brightest of these spiral nebulae is located within the constellation Andromeda, which is now well-placed for viewing in the northeastern sky during the evening hours of these crisp mid-November nights, and which can be seen with the unaided eye from reasonably dark locations. It was while carrying out observations of the Andromeda Nebula that the American astronomer Edwin Hubble finally settled the matter in 1925.

By measuring the brightness of a certain type of star called a Cepheid variable, Hubble identified within the Andromeda Nebula, he determined that its distance from us is at least several hundred thousand light-years – far beyond the limits of our own galaxy.

The conclusion was inescapable: the Andromeda Nebula, and consequently all the various other spiral nebulae around the sky, are separate galaxies in their own right, further indicating that the universe is far larger than was heretofore believed.

Current measurements place the Andromeda galaxy’s distance from us as being 2.5 million light-years; in other words, the light that we see from it now left on its journey to us 21Ž2 million years ago. It is the most distant object that is detectable with the unaided eye. Although a smaller galaxy in the constellation Triangulum, close to Andromeda, lies a similar distance away and can also be detected with the unaided eye from dark rural sites.

Our Milky Way and the Andromeda galaxy are the two largest members of a cluster of galaxies – dubbed the Local Group – that includes various satellites of both galaxies, including the Magellanic Clouds that are visible from the southern hemisphere, the Triangulum galaxy, and two or three dozen very tiny dwarf galaxies. The Andromeda galaxy is somewhat larger than our Milky Way: while our galaxy appears to contain perhaps 200 to 300 billion stars, Andromeda may contain as many as one trillion stars.

Because of our location within the Milky Way we cannot view it from outside, and thus much of what we know about our galaxy’s structure comes from studying Andromeda. We can examine features like spiral arms and dust lanes, and the area surrounding a galaxy’s nucleus. X-ray sensitive telescopes tell us that both the Milky Way and Andromeda contain a large supermassive black hole in their respective centers; the one in our galaxy is some four million times as massive as our sun, whereas the one in Andromeda is perhaps 50 times larger than that.

It turns out that the Milky Way and Andromeda are approaching each other at a relative speed of about 68 miles per second. In about four billion years the two galaxies will start to collide with each other, and over a period of another billion years or so will merge into one giant galaxy that has already been dubbed Milkomeda or Milkdromeda.

While the supermassive black holes at the centers of the two respective galaxies will likely merge together – releasing tremendous amounts of energy in the process – it is extremely unlikely that any stars within the respective galaxies will collide, due to the large relative distances between stars. For example, if our sun were to be represented as a grain of sand, the nearest star would be a similar-sized grain of sand nine miles away.

But our sun – and, of course, its accompanying system of planets and other objects – would still be affected by the gravitational interactions going on between the two large galaxies. It is possible that our sun could be pulled deep into our galaxy’s interior, even conceivably pulled into the central supermassive black hole. It is more likely, however, that our sun would be thrown out far away from the center – perhaps even ejected from the galaxy altogether.

As dramatic as these events may be to witness – and, again, it should be kept in mind that they will take place over a timescale of hundreds of millions, to billions, of years – no one on Earth will witness them. Well before the Milky Way and Andromeda start to merge, our sun will have brightened dramatically and will be well on its way to becoming a red giant star, quite possibly engulfing the Earth in the process. Any descendants of humanity, should any survive until that far-future era, will just have to watch the goings-on from somewhere else.

Alan Hale is a professional astronomer who resides in Cloudcroft. Hale is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His web site is earthriseinstitute.org.