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Reading history of T CrB. (Yes, a little obsessed - there will be a new star for a couple days, and it's rare and it's only been noticed and reported on a couple times before!)
T CrB is a double star about 2625 light years away: a white dwarf and a red giant. (These are always the most fun doubles, it seems.) They orbit each other with a period of about 8 months, and they're a little closer to each other than Venus is to the Sun. (They don't eclipse or anything; we're looking roughly at their north or south poles.)
The red giant is 10th magnitude (small telescope needed) - red giants are bright things (this one's 600x the Sun's brightness) but 2625 is a long distance. The giant's far brigher than the white dwarf. (The Sun at this distance would be about 15th magnitude - that would take a 12" or bigger telescope aperture to glimpse.)
But the dwarf has a much higher surface gravity - it's actually more massive than the giant and its mass is compressed into a much smaller volume. And red giants are big; the white dwarf is almost skimming the outermost layer of the red giant. So the dwarf is stealing matter - mostly hydrogen, because the outer layer of any star is mostly hydrogen.
Normal stars of course spend their time doing nuclear fusion in their cores. Normal stars are fusing mostly hydrogen into helium; hydrogen's way easier to fuse than anything else. A white dwarf, though, is old. And being old, it's enormously compressed, because the core of a white dwarf has used up its hydrogen and helium (and in this case carbon), but lacks the mass to fuse heavier elements, so it's only glowing from residual thermal energy and it's held up by electron degeneracy - so it's far more compressed than a younger star is.
But in this close binary system, because it's stealing matter from its companion, there's an accreting cloud of hydrogen or mostly-hydrogen around this particular dwarf, which - remember - has an incredibly high surface gravity. The hydrogen is being pulled down hard into the star.
And so, every so often, there's so much hydrogen compressed onto the surface of the white dwarf that the hydrogen on the surface undergoes runaway fusion.
At which point, boom. The white dwarf gets ten magnitudes - 10000x - brighter. For a couple days... during which time there's a new star in the sky.
We only know of about a dozen of these long-period catastrophic variables in our galaxy, from what we've so far been able to observe. And of those dozen, T CrB is the nearest, the longest-period, and the brightest one we know - the only one to get easily visible to the naked eye.
We observed it for sure in 1866 and 1946 - and we're pretty sure that we saw it in 1787. Somebody also recorded something that might have been it in 1217.
Which means this is kind of exciting - we know a lot about how these happen now, but we do not know enough to know exactly when it will happen, as in what day or even week. The regularity is still sort of inferred. And its last two eruptions were 80 years apart, but we think this one's going to happen after only 78.
But we're pretty sure. People were getting careful and precise about observations by the 1930s, and they started seeing T CrB brighten a little in 1938, then dim for a while in 1945, before going boom in 1946 - and we've seen it brighten a little in 2016, and then dim for a while in 2023, so we're pretty sure what's coming. Sometime. Between May and September.
The excitement, for me, is in some ways similar to that for a comet. Seeing an unexpected change in the heavens was always a momentous, mysterious, and feared-to-be-disastrous (hence the name: "dis-aster", "hell star") event to our forebears. Those people relied on the constancy and predictability of the stars as their calendar, clock, compass, reminder alarms, and even GPS. And they were pretty sure you could tell far more than that from how the planets moved, even if they were mostly wrong about that part.
But for "unexpected" events - from Halley forward - knowing the cause, predicting the occurrence, and then getting to see the predicted event occur is one of those things symbolizing the triumph of reason and investigation over superstition, and that's one of the pure things I have always loved about the study of astronomy.
T CrB is a double star about 2625 light years away: a white dwarf and a red giant. (These are always the most fun doubles, it seems.) They orbit each other with a period of about 8 months, and they're a little closer to each other than Venus is to the Sun. (They don't eclipse or anything; we're looking roughly at their north or south poles.)
The red giant is 10th magnitude (small telescope needed) - red giants are bright things (this one's 600x the Sun's brightness) but 2625 is a long distance. The giant's far brigher than the white dwarf. (The Sun at this distance would be about 15th magnitude - that would take a 12" or bigger telescope aperture to glimpse.)
But the dwarf has a much higher surface gravity - it's actually more massive than the giant and its mass is compressed into a much smaller volume. And red giants are big; the white dwarf is almost skimming the outermost layer of the red giant. So the dwarf is stealing matter - mostly hydrogen, because the outer layer of any star is mostly hydrogen.
Normal stars of course spend their time doing nuclear fusion in their cores. Normal stars are fusing mostly hydrogen into helium; hydrogen's way easier to fuse than anything else. A white dwarf, though, is old. And being old, it's enormously compressed, because the core of a white dwarf has used up its hydrogen and helium (and in this case carbon), but lacks the mass to fuse heavier elements, so it's only glowing from residual thermal energy and it's held up by electron degeneracy - so it's far more compressed than a younger star is.
But in this close binary system, because it's stealing matter from its companion, there's an accreting cloud of hydrogen or mostly-hydrogen around this particular dwarf, which - remember - has an incredibly high surface gravity. The hydrogen is being pulled down hard into the star.
And so, every so often, there's so much hydrogen compressed onto the surface of the white dwarf that the hydrogen on the surface undergoes runaway fusion.
At which point, boom. The white dwarf gets ten magnitudes - 10000x - brighter. For a couple days... during which time there's a new star in the sky.
We only know of about a dozen of these long-period catastrophic variables in our galaxy, from what we've so far been able to observe. And of those dozen, T CrB is the nearest, the longest-period, and the brightest one we know - the only one to get easily visible to the naked eye.
We observed it for sure in 1866 and 1946 - and we're pretty sure that we saw it in 1787. Somebody also recorded something that might have been it in 1217.
Which means this is kind of exciting - we know a lot about how these happen now, but we do not know enough to know exactly when it will happen, as in what day or even week. The regularity is still sort of inferred. And its last two eruptions were 80 years apart, but we think this one's going to happen after only 78.
But we're pretty sure. People were getting careful and precise about observations by the 1930s, and they started seeing T CrB brighten a little in 1938, then dim for a while in 1945, before going boom in 1946 - and we've seen it brighten a little in 2016, and then dim for a while in 2023, so we're pretty sure what's coming. Sometime. Between May and September.
The excitement, for me, is in some ways similar to that for a comet. Seeing an unexpected change in the heavens was always a momentous, mysterious, and feared-to-be-disastrous (hence the name: "dis-aster", "hell star") event to our forebears. Those people relied on the constancy and predictability of the stars as their calendar, clock, compass, reminder alarms, and even GPS. And they were pretty sure you could tell far more than that from how the planets moved, even if they were mostly wrong about that part.
But for "unexpected" events - from Halley forward - knowing the cause, predicting the occurrence, and then getting to see the predicted event occur is one of those things symbolizing the triumph of reason and investigation over superstition, and that's one of the pure things I have always loved about the study of astronomy.
