Observable Universes and Other Oddities

Recently we mused about the “observable universe“, a 10^28 cm radius patch of real estate centered on us, the observer, where the radius of this fanciful sphere should be equal to the theoretical distance light could have traveled since the Big Bang.

Thinking about it, of course, leads to the conclusion that it all depends on what you mean by “observable”. The web references confirm we only mean that light could have reached us from somewhere, at some point in time since the universe became transparent. “Observable” doesn’t directly guarantee us what we can see, and the entire universe is much bigger; last I heard, no one was speculating that the entire universe is bounded and finite.

What we can observe is never really real-time. If the universe is expanding, it is much bigger than the visible boundaries we postulate today. The objects we see out at t minus 10 billion years depict the universe as it appeared in infancy, when it was only a few billion years old. Those objects probably don’t exist any more, “any more” being a completely speculative human construct in the same genus as “time travel”.

We can’t see back to the beginning of time. We can’t even see what’s happening on the sun right now; if it had just gone prematurely supernova, we wouldn’t have an inkling anything was wrong for another 6 minutes.

When I think about the “universe” I get into constant trouble for confusing it with the “observable” universe. What happened to that region of sky where Hubble imaged a galaxy a billion years after the Big Bang? We just won’t know for another 13.7 billion years, I guess. Put that in your tickler file for follow-up.

What is a Blue Supergiant? Just when we thought we were getting comfortable with the properties of red supergiants, current reading reminds us there’s a whole ‘nuther class of star: hot, bright, and huge: 20 to 50 solar masses. These have super-fast but sparse solar winds. They all have the outer shell of expelled mass the suggest a previous red giant phase. A star may swing back and forth between red and blue supergiant, but they still have short lifetimes; the fuel burnout process has already begun, whether red or blue supergiant phase. Rigel is a famous example of the rare blue supergiant.

In 1987, in the Large Magellanic Cloud in the Southern hemisphere, exploding SN1987A produced the most highly visible supernova since the invention of the telescope. Its “progenitor” star, Sanduluk – 69 202a, had been a blue supergiant.

SN1987A (168,000 light years distant) reached a maximum brightness of 3.0, falling well short of the apparent brightness of the Crab Nebula (6,300 light-years distant), which the Chinese described in 1054AD as a daytime “guest star” having a brightness six times greater than Venus and about as brilliant as the full moon ( magnitude -12.6). If you calculated an absolute magnitude for each event, eliminating the 28-fold distance discrepancy, the blue supergiant might just have won out.

But not quite. Cobbing the formula from Wikipedia’s piece on Absolute Magnitude, I worked up a spreadsheet that showed the raw output of the Crab to be about one and a half times as violent as SN1987A. (Warning: the formula actually used in the Wikipedia example for Rigel does not exactly agree with their listed formula for absolute magnitude. I followed the calculation in the example to arrive at the -6.7 absolute magnitude for Rigel, and then applied it to the other objects.)

absolute magnitude calculations

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