MadSci Network: Astronomy |
Let me focus on Seyfert galaxies, since these are spiral galaxies and therefore somewhat akin to our own. A back-of the envelope calculation shows that a Seyfert galaxy doesn't have much of an effect on how much radiation the planet recieves. The average luminosity of a Seyfert Galaxy is 10e36 to 10e38 Watts. Most of this luminosity originates from the Broad-Line Region, a small area near the center of that galaxy which is light days to light weeks in diameter. Let's say a star was located 8.5 kiloparsecs (1 kpc = 3.09e19 m) away from the center of such a galaxy. This is how far the Sun is from the center of the Milky Way. If the source of the Seyfert's emission is 10 light-days across (a happy medium), then the flux received in the vicinity of the star is given by f2 = (r^2/d^2) f1 Where f1 = L/(4*pi*r^2) r is the radius of the broad-line region (5 light-days or 1.296e14 m) L is the luminosity of the galaxy (let's say 10e37 Watts assumed to be all concentrated in this broad-line region or BLR) f1 is the flux at the surface of the BLR d is the distance of the star from the center of the galaxy Plugging in the numbers gives us f1 = 4.73e7 W/m^2 f2 = (1.30e14/8.5*3.09e19)^2 * 4.73e7 W/m^2 = 23.45 W/m^2 Now this is only about 2% of the solar constant (the flux received at the Earth's surface from the Sun). So even if the Earth was located in Seyfert galaxy, the radiation from the central engine would be a small fraction of what the Earth gets from the Sun. Of course, this would be a significant increase as far as life on Earth is concerned. Such an increase or decrease would probably change the weather patterns on the Earth, just as it did during the Maunder minimum, a period in when very few sunspots were observed on the Sun. This coincided with the "Little Ice Age" from 1640-1710, when the global temperature dropped by about 2 degrees Farenheit - causing rivers to ice up and precipitating major crop failures in Europe and other areas of the world. But this would only be the case if the Earth were instantly transported to a Seyfert galaxy. If the planet had been there all along, whatever life had developed there would no doubt have adapted to the higher flux long ago. As far as the last part of your question goes, there is a theory that all spiral galaxies pass through a stage where their nuclei are very active. It is possible (though this is by no means a solid theory) that the Milky Way was a Seyfert galaxy in the past. However, if the Milky Way had gone through such a phase, it would probably have done early in its history, long before life arose on Earth. It is astronomically improbable that mass extinctions on our planet were caused by such a phenomenon. Still, a very interesting question!
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