The Impossible Star | Hacker News Reader

The Impossible Star

astrobites.org

63 pointsqwerta11 comments

    "These dips could have been caused by a planet orbiting the
    binary system, but in 2013, a different group proved that
    the third object must be a star. They did this by finding
    variations in the orbital period of the binary, variations
    caused by the changing light travel time as the binary
    orbits a third star."

Translated: the binary star system is co-orbiting a third object, whose pull on the binary is so strong that it must be star-sized rather than planet-sized. How do they measure this pull? It's a Doppler effect -- not the familiar one, a shift in the frequency of waves (like sound) -- but a shift in the frequency of the binary star orbit! This orbit is a clock with a constant 0.258 day period between ticks. When the binary is moving towards earth, these clock ticks are "catching up" [0] with the starlight moving towards earth -- they're compressed together, so the binary looks like it's orbiting faster than it actually is! If you measure this Doppler shift, how it changes as the binary orbits the third star (first moving slightly faster towards earth, then away from it), you can infer the speed of its orbit around the third star -- and from that, the mass of the third star!

I was just reading about this recently: I was going to say (wrongly) this technique was used by in the 17th century to measure the speed of light, by measuring a Doppler shift of the orbital period of Jupiter's moon Io [1]. In fact Ole Rømer measured the phase of Io's orbit, rather than the frequency. You can track Io's orbit phase very exactly by timing the moment it falls behind Jupiter's horizon (a regular eclipse). With Io's orbit as a clock located at Jupiter (the eclipse moment as the "tick"), you can measure differences in light travel time from Jupiter to Earth as they move apart. This is a phase measurement, not frequency; and it depends on the Jupiter-Earth distance, not their relative speed.

[0] Like this GIF: https://en.wikipedia.org/wiki/Doppler_effect#mediaviewer/Fil...

[1] https://en.wikipedia.org/wiki/Rømer's_determination_of_the_s...

(§8.2 says my confusion is a common one, so there!)

(Disclaimer: this has nothing to do with special relativity, despite dealing with the speed of light! Delays in observing an event, due to light travel time, are not what relativity is. This is simpler stuff).

(Also, don't confuse this with doppler spectroscopy [2] -- measuring shifts in the frequency (color) of light, rather than the frequency of orbits. This is the "Doppler" that's used to discover exoplanets!)

[2] https://en.wikipedia.org/wiki/Doppler_spectroscopy

When they say the orbital period is 0.258 days, they mean the stars are moving so quickly around each other that it only takes ~6 hours to complete a full orbit? (It may be a little more complicated than that because both stars are moving with respect to a system-wide center of mass.)

I found that a fascinating thought, so I looked up other binary stars and found there's one with a period of ~17 minutes. http://en.wikipedia.org/wiki/Orbital_period#Binary_stars

"Catalysmic Variable Star" is a good name! :)

https://en.wikipedia.org/wiki/AM_Canum_Venaticorum

Both of the stars stars are white dwarves, a mass of a sun squeezed into the size of a small planet. With a 17-minute orbit, they're incredibly close: one of them is ripping the atmosphere off of the other. Helium from the lighter dwarf is being pulled off and falling onto the more massive one -- pulled so strongly, it impacts the surface at several percent the speed of light. A helium layer builds up, at temperatures and pressures close to that of the interior of a hydrogen bomb. And at some point it does go off, like a hydrogen bomb -- a thermonuclear detonation the size of a planet, the entire surface igniting almost simultaneously. And this keeps repeating!

Couldn't resist; I had to check that "size of a planet" claim: 2.7 * 10^36 erg in tons TNT gives me (on Google) 6.45315488 × 10^19 tons TNT.

The area of Earth is around 5 × 10^8 km^2. So, 'planet size', that's about 10^11 tons TNT per square km, or 2,000 Tsar Bomba's per square km (Tsar Bobma was around 5-6 × 10^7 ton TNT equivalent).

Volume-wise, it would be about one Tsar Bomba per cubic kilometer of Earth.

So yes, I guess you could call that planet sized.

More simply, the white dwarf star is the size of a planet, and its entire surface is detonating all at once. :)

Well, when you work out the size of a few numbers regarding Tsar Bomba, I'd be more than happy to call those numbers indicative of planet sized. For instance.

  The TNT equivalent of the 50 Mt test could be represented by a cube of TNT 312 metres (1023 feet) on a side, approximately the height of the Eiffel Tower.

Which when you carry with your numbers, means that more than 9 (3^3) but much less than than 64 (4^3), (excuse the massive lack of accuracy but I'm doing a fermi estimate here, it seems appropriate) which are both massively less than 2000, which means that, the explosion you would get if you covered the earth with a 10Km 'crust' of TNT, would still not be bigger than this star when it goes off.

Id definitely call that planet sized.

I love their sense of humor:

> These quadruple models yield reasonably good fits to the data, and are physically possible, which is an improvement over the triple model!

> The right panel shows the same basic model, but ignores Kepler’s laws. This produces a much better fit, but has the unfortunate drawback of breaking the laws of physics.

Brilliant. Just noticed the author of the paper is from my alma mater, too.

Fascinating. Presumably it wouldn't be possible for any planets to be orbiting in the vicinity of the 0.258 days orbital period binary. Or maybe, if far enough away?

However, imagine being a species on such a planet, and trying to work out astronomy. Motions of objects in the sky would be rather confusing. To put it mildly.

I wonder whether that's a comparable confusion to our cycles and epicycles when we still assumed that the earth was the centre of the solar system. Movements of objects through the sky wasn't exactly simple either, but there was an explanation that simplified things immensely. I guess figuring out that there is a binary star might just be such a discovery that simplifies explanations for them. Might take a while, but sooner or later someone should think of it, I guess.

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