JohnD wrote: ↑
Wed May 22, 2019 2:59 pm
Two stars, 12 million miles apart, yet actively merging and already sharing 30% of their mass? And you can show us a sharp image of that situation from 160,000 light years away? Surely those two stars will be in orbit around each other with a very short period? So short that a snap shot would show a blur? And if their surfaces are in contact, they must be collapsing (or were 160K years ago) into each other, like the famous merging Black Holes we've heard about?
I feel as if you are joking, this story sounds so unlikely! But you're Ann, the Queen of Colour! A serious person, who might do an April Fool, but not at the end of May!
PS just looked up the diameter of a typical O-type star. They are so enormous that centres 12M miles apart would make them merge! My mistake! (Ignorance!) But what aboit the rest of my comments? Equally ignorant? Please teach!
Contact binary VFTS 352 in the Large Magellanic Cloud.
Illustration: ESO/L. Calçada.
The center of the massive R136a cluster in the Tarantula Nebula.
ESO/P. Crowther/C.J. Evans
Please note that the picture of the contact binary is an artist's illustration, not an actual photo. Why not?
Take a look at the picture at right, which shows us the center of the crowded R136a supercluster at the heart of the Tarantula Nebula. To my knowledge, this rather blurry (infrared) picture is the best picture we have got of individual stars in the Tarantula Cluster. (Of course, there is certainly this visula light Hubble picture
too - by NASA, ESA, F. Paresce, R. O'Connell, and the Wide Field Camera 3 Science Oversight Committee - http://hubblesite.org/newscenter/archiv ... 2/image/a/
- but as you can see, in the visual light Hubble picture the stars are not better separated than in the ESO/P. Crowther/C.J. Evans one.)
So, long story short: Even relatively wide binaries are not easy to spot in the Tarantula Nebula, which is 160,000 light-years away. And remember that VFTS 352 is an extremely tight contact binary. In my opinion, you wouldn't be able to actually photograph the two components of such a tight contact binary even if they were as close to us as Proxima Centauri, the closest star to us in the sky after the Sun. (But I might be wrong about being unable to photographically separate such contact binaries in visual light if they are as close as Proxima Centauri, so Chris, Art, Geck, anyone, correct me.)
The stars of the contact binary VFTS 352 are 12 million kilometers
apart, not 12 million miles
. The diameter of the Sun is 1,391,000 kilometers, and while I had some trouble finding information on the typical diameter of a main sequence O-type star, I found some info on the O9V star 10 Lacertae. The diameter of 10 Lacertae is, according to John Kaler
, 4.7 times that of the Sun, and if the two components of VFTS 352 are similar in size to 10 Lacertae, their diameters would be about 6.5 million kilometers. Clearly then, if they are separated by only 12 million kilometers, it seems inevitable that they would "touch".
Note that one of both of the components of VFTS 352 may well be larger than 10 Lacertae, because 10 Lac may well be small as O-type stars go. At any rate, 10 Lac is indeed relatively light-weight as O stars go, only 16 solar masses, according to Jim Kaler. The two components of VFTS 352 pack a combined mass of 57 times solar.
The question is, how did the scientist find this star? The only way for them to know that it is a binary star is to look at its spectrum. At left you can see a typical spectrum of a spectroscopic binary star, where the spectrum alone tells us that the star is a binary. (Unfortunately I wasn't able to extract more information from the page where the picture was posted.)
I would guess, too, that the stars of VFTS 352 would be orbiting each other very rapidly. That would make their spectral lines broad and possibly blurred.
How, though, would scientists know that the components of VFTS 352 are only 12 million kilometers apart? You've got me there, I'm afraid. My best guess - and that is a guess, mind you - is that the mere act of physically interacting with another star will alter a star's spectrum.
Artist's impression of Beta Lyrae.
One of the most famous contact binaries in the nearby sky is Beta Lyra. This is what Wikipedia says about this interacting star pair:
Beta Lyrae Aa is a semidetached binary system made up of a stellar class B7 main sequence primary star and a secondary that is probably also a B-type star. The fainter, less massive star in the system was once the more massive member of the pair, which caused it to evolve away from the main sequence first and become a giant star. Because the pair are in a close orbit, as this star expanded into a giant it filled its Roche lobe and transferred most of its mass over to its companion. The secondary, now more massive star is surrounded by an accretion disk from this mass transfer
, with bipolar, jet-like features projecting perpendicular to the disk. This accretion disk blocks humans' view of the secondary star, lowering its apparent luminosity and making it difficult for astronomers to pinpoint what its stellar type is. The amount of mass being transferred between the two stars is about 2 × 10−5 solar masses per year, or the equivalent of the Sun's mass every 50,000 years, which results in an increase in orbital period of about 19 seconds each year. The spectrum of Beta Lyrae shows emission lines produced by the accretion disc. The disc produces around 20% of the brightness of the system.
The variable luminosity of this system was discovered in 1784 by the British amateur astronomer John Goodricke. The orbital plane of this system is nearly aligned with the line of sight from the Earth, so the two stars periodically eclipse each other. This causes Beta Lyrae to regularly change its apparent magnitude from +3.2 to +4.4 over an orbital period of 12.9414 days. The two components are so close together that they cannot be resolved with optical telescopes, forming a spectroscopic binary. In 2008, the primary star and the accretion disk of the secondary star were resolved and imaged using the CHARA Array interferometer and the Michigan InfraRed Combiner (MIRC) in the near infrared H band (see video below), allowing the orbital elements to be computed for the first time.
If the orbital plane of VFTS 352 is aligned with the line of sight of the Earth, its apparent luminosity would certainly be variable from the Earth's point of view.
To summarize: It is impossible to photograph the two components of VFTS 352 in visual light, and it is almost certainly impossible to separate them in infrared light, too. But there are very many hot massive stars in the Tarantula Nebula, and scientists would easily be able to tell that VFTS 352 is just one more of them. The spectrum of this binary would give away its binary nature, and quite possibly the stars would be surrounded by accretion disks of material which they have stolen from each other. (If the accretion disks emit visual light, their spectra would be different than the spectra from the stars.) The binary might well vary in luminosity due to its binary nature (where one component eclipses the other one, completely or partly).
My best guess as to how scientists claim to know that the two components are only 12 million kilometers apart is that they can see from the stars' spectra that they are both main sequence O-type stars and that they are definitely exchanging mass. That gives the scientists a relatively good handle on the stars' true sizes, and if they are indeed interacting and exchanging matter, they can't be separated by a lot more than about 12 million kilometers.
But I'm guessing. Sorry.