Scintillating Stars But Not Planets

Aristotle (384 BC – 322 BC) may have been the first person to write that stars twinkle but planets don’t, though our understanding of twinkling has evolved since he explained that “The planets are near, so that the visual ray reaches them in its full vigour, but when it comes to the fixed stars it is quivering because of the distance and its excessive extension.”

John Stedman (1744-1797), a controversial and complicated figure to be sure, writes the following dialog between teacher and student in The Study of Astronomy, Adapted to the capacities of youth (1796):

PUPIL.  How is the twinkling of the stars in a clear night accounted for?

TUTOR.   It arises from the continual agitation of the air or atmosphere through which we view them; the particles of air being always in motion, will cause a twinkling in any distant luminous body, which shines with a strong light.

PUPIL.  Then, I suppose, the planets not being luminous, is the reason why they do not twinkle.

TUTOR.   Most certainly.  The feeble light with which they shine is not sufficient to cause such an appearance.

Still not quite right, but closer to our current understanding. Our modern term for “twinkling” is atmospheric scintillation, which is changes in a star’s brightness caused by curved wavefronts focusing or defocusing starlight.

Scintillation is caused by refractive index variations (due to differences in pressure, temperature, and humidity) of “pockets” of air passing in front of the light path between star and observer at a typical height of about 5 miles. These pockets are typically about 3 inches across, so from the naked eye observer’s standpoint, they subtend an angle of about 2 arcseconds.

The largest angular diameters of stars are on the order of 50 milliarcseconds1 (R Doradus, Betelgeuse, and Mira), and only seventeen stars have an an angular diameter larger than 1 milliarcsecond. So, it is easy to see how cells of air on the order of 2 arcseconds across moving across the light path could cause the stars to flicker and flash as seen with the unaided eye.

The five planets that are easily visible to the unaided eye (Mercury, Venus, Mars, Jupiter, and Saturn) have angular diameters that range from 3.5 arcseconds (Mars, at its most distant) up to 66 arcseconds (Venus, at its closest). Since the disk of a planet subtends multiple air cells, the different refractive indexes tend to cancel each other out, and the planet shines with a steady light.

From my own experience watching meteors many nights with my friend Paul Martsching, our reclining lawn chairs just a few feet apart, I have sometimes seen a principal star briefly brighten by two magnitudes or more, with Paul seeing no change in the star’s brightness, and vice versa.

Stedman’s dialogue next turns to the distances to the nearest stars.

PUPIL.  Have the stars then light in themselves?

TUTOR.   They undoubtedly shine with their own native light, or we should not see even the nearest of them: the distance being so immensely great, that if a cannon-ball were to travel from it to the sun, with the same velocity with which it left the cannon, it would be more than 1 million, 868 thousand years, before it reached it.

He adds a footnote:

The distance of Syrius is 18,717,442,690,526 miles.  A cannon-ball going at the rate of 1143 miles an hour, would only reach the sun in about 1,868,307 years, 88 days.

Where Stedman comes up with the velocity of a cannon-ball is unclear, but the Earth’s rotational speed at the equator is 1,040 mph, close to Stedman’s cannon-ball velocity of 1,143 mph. He states the distance to the brightest star Sirius—probably then thought to be the nearest star—is 18,717,442,690,526 miles or 3.18 light years, a bit short of the actual value of 8.60 light years. The first measurements of stellar parallax lie 42 years in the future when Stedman’s book was published.

1 1 milliarcsecond (1 mas) = 0.001 arcsecond

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The spectral type classification scheme for stars is, among other things, a temperature sequence. A helpful mnemonic for remembering the sequence is Oh, Be A Fine Girl (Guy) Kiss Me Like This, Yes! The O stars have the highest surface temperatures, up to 56,000 K (100,000° F), while the Y infrared dwarfs (brown dwarfs) have surface temperatures as cool as 250 K (-10° F).

Here are the brightest representatives of each of these spectra classes readily visible from the northern hemisphere. Apparent visual magnitude (V-band) is given unless otherwise noted.

Milky Way Supernova Candidates

There is a supermassive binary star in our own Milky Way Galaxy that has the potential to create a super-supernova (hypernova?).  It could go off tomorrow—or a million years from now.  The star system’s name is Eta Carinae.  Currently 4th-magnitude and located some 7,500 ly away in the direction of the southern constellation Carina (“The Keel”), Eta Carinae consists of a 100-200 M star and a 30-80 M star in a highly-eccentric 5.54y orbit with the more massive star undergoing prodigious mass loss.  Eta Carinae never rises above the horizon unless you’re south of latitude 30° N.  So, if Eta Carina ever does go supernova while humans still walk the Earth, you’ll have to travel at least as far as southern Texas or southern Florida to see it.  And it will be an impressive sight, easily visible during the daylight hours.

Closer to home, there are seven prime candidates for the next relatively nearby supernova.  The nearest of these currently is IK Peg.  Keep in mind that over hundreds of thousands of years, stars move quite a lot, so what is close to us now will not necessarily be close to us when a supernova event finally does occur.

IK Pegasi, a binary system comprised of a white dwarf already near the Chandrasekhar limit, and a close-by soon-to-be-giant main-sequence star, lies just 147 to 155 ly away in the direction of the constellation Pegasus, the Winged Horse.  IK Peg appears to us visually as a 6th magnitude star located roughly ⅓ of the way from Delphinus to the Square of Pegasus.  As the giant star expands into the vicinity of the white dwarf, the white dwarf will accumulate enough material to put it over the Chandrasekhar limit, and a Type Ia supernova will ensue.

Spica (α Vir), located at a distance between 237 and 264 ly, is a massive binary system (10 M and 7M), with the two stars orbiting each other every four days.

Alpha Lupi (α Lup) is a massive star (~10 M) located between 454 and 476 ly from our solar system.

Antares (α Sco) is a massive star (~12 M, the supernova progenitor) orbited by another massive star (~7 M).  However, their orbital period is at least 1,200 years.  The Antares system lies between 473 and 667 ly from our solar system

Betelgeuse (α Ori) is a massive star (~12 M) between 500 and 900 ly away.  Incidentally, there is a lot of uncertainty about the distance to Betelgeuse, primarily because it’s angular size (44 mas) is an order of magnitude larger than its parallax (4.5 mas) (Harper et al. 2017).

Rigel (β Ori) is a massive star (~23 M) between 792 and 948 ly distant.

Gamma2 Velorum (γ2 Vel) is a binary system 1,013 to 1,245 ly distant containing two stars which will go supernova in the not-too-distant future.  The system consists of a 28.5 MO7.5 giant star and a 9.0 MWolf-Rayet star (the nearest, incidentally) orbiting each other every 78.5 days.  The Wolf-Rayet star will be the first to supernova, followed later by the O giant star.

Tomorrow—or a million years from now?  We have no way of accurately predicting.  But rest assured, in the unlikely event that any one of these stars goes supernova during our lifetimes, none will be close enough to harm us.  Instead, for a time, we will be treated to a object comparable to the Moon in brightness and visible both day and night.

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