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mapguy
2016-Mar-17, 02:54 PM
A team of astronomers at Calvin College predicts that a star in Cygnus will go nova within the next few years (http://adsabs.harvard.edu/abs/2015AAS...22541505M). From the abstract:


If the model holds up, this star presents the unprecedented opportunity to perform a set of targeted observations of a luminous red nova progenitor and to follow carefully the course of the merger.

Cougar
2016-Mar-17, 08:20 PM
A team of astronomers at Calvin College predicts that a star in Cygnus will go nova within the next few years (http://adsabs.harvard.edu/abs/2015AAS...22541505M).

That's great! They're predicting an orbiting pair of stars will merge in just a handful of years. I liked this sentence in the abstract:




"While we cannot yet conclude the system will follow the model fit, the prediction it makes will soon be easily falsifiable."

slang
2016-Mar-17, 09:57 PM
I wonder if such a merger would be in detection range of LIGO. How far away is that binary?

Jens
2016-Mar-17, 10:45 PM
I think that LIGO is meant to detect mergers of massive black holes, but here they're talking about two main sequence stars.

slang
2016-Mar-17, 11:20 PM
IIRC any two masses orbiting each other generate gravitational waves. What I don't know how to calculate, and can't quickly find, is how much the amount differs between inspiralling stars and black holes, and how much closer than GW150914 two inspiralling mainstream stars should be to generate something detectable.

01101001
2016-Mar-18, 12:50 AM
Considering that the bulk of the first LIGO "sighting" was the 0.2 seconds prior to merger, it was probably important that the centers of the two 30-solar mass blackholes were able to be in close proximity. With much larger, inspiraling lightweights, the signal may not be quite so strident.

Hornblower
2016-Mar-18, 01:49 AM
As I think I understand it, two merging main sequence stars would initially make a nova, and then settle down as a more massive but still main sequence star. As usual, don't take my remarks as authoritative, but that is the explanation I have seen for "blue straggler" stars in globular clusters, where all of the original blue stars have long since evolved off the main sequence and become red giants and eventually white dwarfs.

ETA: For generating gravitational waves, I would expect this to be a much milder merger than that of two black holes.

George
2016-Mar-18, 06:34 PM
I assume the distance ratio of this binary to the 1.3 billion lightyear observed would give you the amplitude strength for LIGO for at least the distance portion since the amplitude is a linear attenuation. [I didn't see a pdf version of the paper that would give me the star name so I am s.o.l. unless there is another way in for this amateur.]

mapguy
2016-Mar-18, 06:38 PM
The star is designated KIC 9832227.

George
2016-Mar-18, 07:43 PM
The star is designated KIC 9832227.I don't see any distance estimates but one paper shows it at a magnitude 12.369, though it is variable. The effective temperature is about 5400K or so and it is a dwarf (not a giant), so I would estimate the distance to be roughly 900 lightyears, but it could be closer (or farther) if its size is less (or larger) than the Sun. So if the gravity wave's amplitude is one or two millionth that of the bh merger, then it would be detectable. If it is much weaker than that then I would assume not, at least for now.

slang
2016-Mar-18, 08:15 PM
I don't see any distance estimates but one paper shows it at a magnitude 12.369, though it is variable. The effective temperature is about 5400K or so and it is a dwarf (not a giant), so I would estimate the distance to be roughly 900 lightyears, but it could be closer (or farther) if its size is less (or larger) than the Sun. So if the gravity wave's amplitude is one or two millionth that of the bh merger, then it would be detectable. If it is much weaker than that then I would assume not, at least for now.

Thanks George, those are the kinds of numbers that make it somewhat possible to at least think about it. And of course there is not just amplitude, also the shape and frequency of the signal. If it's a much slower event it may fade away more easily in background. Then again.. when you know when to look for something.. Oh well, if it's possible someone is probably already preparing a paper about it :)

StupendousMan
2016-Mar-18, 08:16 PM
Differences between the recently-announced LIGO observation, due to the merger of two black holes, and the postulated merger of two main-sequence stars discussed in this thread:

1. the black holes were within a few km of each other at the time of merger; these stars will be either within a few hundred thousand km (if one speaks of the time when the photospheres really merge), or a few thousand km (if one speaks of the time when the cores merge). Recall that the gravitational forces scale as distance to the power of negative two, so in the latter case, the gravitational forces at the time of merger might be roughly millions of times smaller.

2. In the LIGO case, the two stars were around 30 solar masses each; in this nearby star case, the stars have effective temperatures similar to that of the Sun, so I would guess that they aren't much more than 1 solar mass each. The gravitational force will scale as (mass1)*(mass2), which is a factor of roughly one thousand.

3. The timescales are very different. In the case of the merging black holes, the final few revolutions took place in under a second -- leading to gravitational waves with frequencies of about 10 Hertz -- to which LIGO is very sensitive. In the case of merging main-sequence stars, or even merging main-sequence cores, the final few revolutions will have periods of order 5 to 10 seconds (rough estimate), meaning a frequency of order 0.1 Hertz. LIGO is much less sensitive at such low frequencies, by orders of magnitude.

I wouldn't put money on LIGO detecting the merger, but I'd be happy to lose such a bet.

slang
2016-Mar-18, 08:50 PM
Thanks. That whoosh was the sound of deflating hope. :) As I recall the detected signal started when they were hundreds km apart, so maybe that gives us back one power of 10. Still, just 1 and 2 together already make it hundreds of millions to billions times weaker. Might as well fedex a sundae now. The difference betweek weak and strong field is .. well, stupendous.

publiusr
2016-Mar-18, 09:10 PM
So what would it take to get Hubble to be pointed there right as the two merged?
Or would an amateur scope be given that assaingnment--to be always pointed at the duo whenever possible?

01101001
2016-Mar-18, 10:03 PM
This chart shows likely targets of various detectors. LIGO sweet spot is distant from most of binary background.

Caltech: Gravitational Wave Spectrum (http://www.tapir.caltech.edu/~teviet/Waves/gwave_spectrum.html)

selvaarchi
2016-Mar-19, 12:35 AM
So what would it take to get Hubble to be pointed there right as the two merged?
Or would an amateur scope be given that assaingnment--to be always pointed at the duo whenever possible?
Well if it waited to 2020 then the Chinese Hubble could help out :o

Sent from my SM-G900F using Tapatalk

selvaarchi
2017-Jan-11, 02:57 AM
The merger of a pair of binary stars is predicted to cause an enormous explosion that will make it visible in the night sky about 2022.

http://m.economictimes.com/news/science/new-star-predicted-to-adorn-night-sky-by-2022/articleshow/56437031.cms

"A pair of binary stars is set to merge and explode in 2022, increasing in brightness by ten thousand times to become one of the brighter objects in the night sky for a time, scientists have predicted.

Researchers including those from Apache Point Observatory and the University of Wyoming in the US said that the change to the night sky will be visible to the naked eye.

"It's a one-in-a-million chance that you can predict an explosion. It's never been done before," said Larry Molnar, professor at Calvin College in the US. "

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slang
2017-Jan-11, 03:46 PM
Now predicted (http://www.calvin.edu/academic/phys/observatory/MergingStar/MolnarEtAl2017.pdf) (PDF) to happen in 2021 or 2022.

BigDon
2017-Feb-28, 10:22 PM
So there is a third star in the mix?

That's going to make a gorgeous planetary nebula if a few thousand years.

Ken G
2017-Mar-01, 01:27 AM
They're certainly onto something there-- if it does explode, they have a front-row seat of the buildup and event. If it doesn't, they can still watch the period evolve and ask "why not?", or update their predictions. It's a hit either way!

bruno
2017-Mar-01, 01:57 AM
Would this not cause them to become a blackhole?

01101001
2017-Mar-01, 02:57 AM
Would this not cause them to become a blackhole?

No. 1.395 solar mass plus 0.318 solar mass just isn't very much mass.

BigDon
2017-Mar-01, 04:02 PM
Can anybody guess a time scale for events after they begin once their photospheres come in contact?

Will the newly merged star be blue?

How many years will the new nascent star be likely obscured by all the fire works?

Any hope for anything in the goldilocks zone of KIC 9832227 C in its 500 and something day orbit surviving the event? (Okay, not a real question. Snowball, meet blast furnace.)

If you have a mature solar system around such an event, will the planets far enough out not to be part of the initial "explosion" have their orbits jerked inward, relaxed outward or would they stay the same?

Noclevername
2017-Mar-01, 05:01 PM
If you have a mature solar system around such an event, will the planets far enough out not to be part of the initial "explosion" have their orbits jerked inward, relaxed outward or would they stay the same?

That one I can answer. Their orbits will stay the same.

BigDon
2017-Mar-01, 05:56 PM
Are you sure?

(Ancient DM threat.)

Won't the resulting new body, though having the same mass, be in a much more compact volume?

Hornblower
2017-Mar-01, 06:55 PM
Are you sure?

(Ancient DM threat.)

Won't the resulting new body, though having the same mass, be in a much more compact volume?The external gravitational signature of a spherical body is independent of its volume. It would attract planets in the same manner as if the total mass was concentrated at the center. In its current state, with two stars rapidly orbiting each other and nearly in contact, there will be a bit of wiggling and jiggling out where we might find planets. That would go away as the merged object settles into a sphere.

The final state after the merger is a bit counterintuitive. The final star will be larger in diameter and less dense than the precursors. The internal dynamics of a main sequence star are such that it stabilizes in such a state.

Ken G
2017-Mar-01, 07:12 PM
Won't the resulting new body, though having the same mass, be in a much more compact volume?It should end up with somewhat less mass, as the nova will involve an ejection of some mass. I don't know how much though, most normal novae on white dwarfs don't eject much mass at all but a red nova is rather brighter and could be more violent. The two stars will readjust quite a bit to accomodate their new combined mass (and as Hornblower said, the single star will take up quite a bit more volume than the sum of the original two) and things might get interesting in the process. But perhaps the mass loss will not be significant enough to make the planet orbital radius increase very much (you calculate how much by conserving orbital angular momentum and making the new orbit consistent with the lower stellar mass). The volume of the stars won't matter, only their mass, since the planet orbits will be well away from the stars. Since the mass loss will happen fast, the planets will acquire a slightly elliptical orbit, and could even be put onto parabolic escaping orbits if the star loses more than half its mass (but I strongly doubt that).