Posted on by Steve Nerlich

Astronomy Without A Telescope – Plausibility Check

OK, this looks nice - but let's think it through. You've got two binary stars with angular diameters and spectral properties roughly analogous to our Sun - shining through an atmosphere containing semi-precipitous water vapor (also known as clouds). Plausible? Credit: NASA.

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So we all know this story. Uncle Owen has just emotionally blackmailed you into putting off your application to the academy for another year – and even after you just got those two new droids, darn it. So you stare mournfully at the setting binary suns and…

Hang on, they look a lot like G type stars – and if so, their roughly 0.5 degree angular diameters in the sky suggest they are both only around 1 astronomical unit away. I mean OK, you could plausibly have a close red dwarf and a distant blue giant having identical apparent diameters, but surely they would look substantially different, both in color and brightness.

So if those two suns are about the same size and at about the same distance away, then you must be standing on a circumbinary planet that encompasses both stars in one orbit.

To allow a stable circumbinary orbit – either a planet has to be very distant from the binary stars – so that they essentially act as a single center of mass – or the two stars have to be really close together – so that they essentially act as a single center of mass. It’s unlikely a planet could maintain a stable orbit around a binary system where it is exposed to pulses of gravitational force, as first one star passes close by, then the other passes close by.

Anyhow, if you can stand on a planet and watch a binary sunset – and you are a water-solvent based life form – then your planet is within the star system’s habitable zone where H2O can exist in a fluid state. Given this – and their apparent size and proximity to each other, it’s most likely that you orbit two stars that are really close together.

To get a planet in a habitable zone around a binary system - your choices are probably limited to circumbinary planets around two close binaries - or circumstellar planets around one star in a widely spread binary. Credit: NASA/JPL.

But, taking this further – if we accept that there are two G type stars in the sky, then it’s unlikely that your planet is exactly one astronomical unit from them – since the presence of two equivalent stars in the sky should roughly double the stellar flux you would get from one. And it’s not a simple matter of doubling the distance to halve the stellar flux. Doubling the distance will halve the apparent diameters of the stars in the sky, but an inverse square relation applies to their brightness and their solar flux, so at double the distance you would only get a quarter of their stellar flux. So, something like the square root of two, that is about 1.4 astronomical units away from the stars, might be about right.

However, this means the stars now need a larger than solar diameter to create the same apparent size that they have in the sky – which means they must have more mass – which will put them into a more intense spectral class. For example, Sirius A has 1.7 times the diameter of the Sun, roughly twice its mass – and consequently about 25 times its absolute luminosity. So even at 2 astronomical units distance, Sirius A would be nearly five times as bright and deliver five times as much stellar flux as the Sun does to Earth (or ten times if there are two such stars in the sky).

So, to sum up…

It’s a struggle to come up with a scenario where you could have two stars in the sky, with the same apparent diameter, color and brightness – unless you are in a circumbinary orbit around two equivalent stars. There’s no reason to doubt that a planet could maintain a stable circumbinary orbit around two equivalent stars, that might be G type Sun analogues or whatever. However, it’s a struggle to come up with a plausible scenario where those stars could have the angular diameter in the sky that they appear to have, while still having your planet in the system’s habitable zone.

I mean OK you’re on a desert world, but two stars of a more intense spectral class than G would probably blow away the atmosphere – and even two G type stars would give you a Venus scenario (which receives roughly double the solar flux that Earth does, being 28% closer to the Sun). They could be smaller K or M class stars, but then they should be redder than they appear to be – and your planet would need to be closer in, towards that range where it’s unlikely your planet could retain a stable orbit.

So, at this point you should call shenanigans.

Further reading: Planets Thrive Around Stellar Twins (includes a permitted screen shot from a certain movie).

Posted on by Mike Simonsen

Symbiotic Variable Star On the Verge of an Eruption?

Symbiotic variables are binary pairs in orbit around each other inside a common envelope. Credit: NASA

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November 23rd, astronomers from the Asiago Novae and Symbiotic Stars collaboration announced recent changes in the symbiotic variable star, AX Persei, could indicate the onset of a rare eruption of this system. The last major eruption took place between 1988 and1992. In the (northern hemisphere) spring of 2009, AX Per underwent a short outburst that was the first time since 1992 this star had experienced a bright phase. Now AX Per is on the rise again. This has tempted astronomers to speculate that another major eruption could be in the making. 

Symbiotic variable stars are binary systems whose members are a hot compact white dwarf in a wide orbit around a cool giant star. The orbital periods of symbiotic variables are between 100 and 2000 days. Unlike dwarf novae, compact binaries whose periods are measured in hours, where mass is transferred directly via an accretion disk around the white dwarf, siphoned directly from the surface of the secondary, in symbiotic variables the pair orbit each other far enough away that the mass exchanged between them comes from the strong stellar wind blowing off the red giant. Both stars reside within a shared cloud of gas and dust called a common envelope.

When astronomers look at the spectra of these systems they see a very complex picture. They see the spectra of a hot compact object superimposed on the spectra of a cool giant star tangled up with the spectrum of the common envelope. The term “symbiotic” was coined in 1941 to describe stars with this combined spectrum.

Typically, these systems will remain quiescent or undergo slow, irregular changes in brightness for years at a time. Only occasionally do they undergo large outbursts of several magnitudes. These outbursts are believed to be caused either by abrupt changes in the accretion flow of gas onto the primary, or by the onset of thermonuclear burning of the material piled up on the surface of the white dwarf. Whatever the cause, these major eruptions are rare and unpredictable.

The AAVSO light curve of AX Persei from 1970 to November 2010. In the middle is the eruption of 1988-1992. The precursor outburst is the sudden narrow brightening left of the larger eruption. To the right of the light curve you can see the 2009 brightening event. Is this a precursor to a coming major eruption? Credit: AAVSO

AX Per underwent a short-duration flare about one year before the onset of the major 1988-1992 outburst. Now astronomers are tempted to speculate. Could the 2009 short outburst be a similar precursor type event? The present rise in brightness by AX Per might be the onset of a major outburst event similar to that in 1988-1992. The watch begins now, and professional and amateur variable star observers will be keeping a close eye on AX Per in the coming months.

Ranging from 8.5 to 13th magnitude, AX Persei is visible to anyone with an 8-inch telescope, and if it erupts to maximum it will be visible in binoculars. You can monitor this interesting star and report your observations to the American Association of Variable Star Observers (AAVSO). Charts with comparison stars of known brightness can be plotted and printed using the AAVSO’s Variable Star Chart Plotter, VSP.

The AAVSO comparison star chart for AX Persei