MadSci Network: Astronomy

Re: So When will our galaxy be struck

Date: Wed Jan 19 22:20:56 2005
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Astronomy
ID: 1104822494.As

Some sources say that the 2 magellanic Clouds be absorbed by our galaxy in 100 million years, while other say it will be billions of years, What makes the time estimates differ so much?

Well, there are several reasons for the uncertainty. I can provide at least three.

First, we aren't exactly sure which way the Magellanic Clouds are moving, and how fast they are going. There are three directions the Clouds could be going: North/South on the sky, East/West on the sky, and towards/away from us. The last one is not too hard to measure: we can break the light from stars in the Clouds into spectra, and look for shifts in the wavelengths of lines in the spectra. If the lines are shifted to short wavelengths, then the Clouds are coming towards us; if they are shifted towards long wavelengths, the Clouds are moving away from us. An article by van der Marel and coauthors in 2002, in Astronomical Journal vol 124, p. 2639, states that the LMC is moving away from us at 262 +/- 3 kilometers per second.

Now, to figure out the transverse motion of the LMC, we need to watch the stars in it over a period of many, many years. If we wait long enough -- decades at least -- we can see the tiny shifts in their apparent positions, North or South, East or West, due to their motion. Each star may have a small random motion within the LMC, but the average motion of all the stars together shows us which way the LMC galaxy is going. It's harder to measure this "transverse" velocity, and our best measurements have much larger uncertainties than the radial component: the LMC is apparently moving about 406 km/s to the east-north-east, but with a significant uncertainty of +/- 60 km/s or so.

Even if we did know exactly how the LMC and the Small Magellanic Cloud (SMC) were moving through space, we still wouldn't know exactly what to expect them to do in the future because we don't know exactly how strong the gravitational pull of the Milky Way is. We don't know the mass of our own galaxy, strangely enough, or its distribution (round, or squashed). The best guesses at the mass of the Milky Way have a very large uncertainty. A recent paper by Sakamoto et al. (Astronomy and Astrophysics vol 397, p 899 (2003) estimates the mass of the Milky Way (out to the distance of the Magellanic Clouds) is roughly 550 billion times the mass of our Sun, but with an uncertainty of about 10 percent. If we don't know how hard the Milky Way pulls on the Clouds, we can't tell exactly how they will orbit around or through our galaxy in the future.

The final problem is that, even if we DID know the exact velocity of the Clouds, and even if we DID know the amount of mass in the Milky Way and its distribution, we still wouldn't be able to state what would happen if the Clouds were to fly through the disk of the Milky Way. Would they be torn apart completely, or merely distorted, or left almost intact? We do see some long, thin streams of stars within our Milky Way which look peculiar: they may be the remnants of some other unfortunate satellite galaxy which long ago passed through the Milky Way and was ripped apart. That might well happen to the Magellanic Clouds, if they were to pass too close, but we aren't sure if it would take just one pass or several to destroy them.

I'm sorry that I can't give you any more exact answers. Astronomers are making pretty good progress in measuring the mass and structure of the Milky Way, however, so if you were to ask again in 5 or 10 years, we might have a more precise reply for you.

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