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

References
Aristotle, De Caelo, Book 2, chap.8, par. 290a, 18
Crumey, A., 2014, MNRAS, 442, 2600
Dravins, D., Lindegren, L., Mezey, E., Young, A. T., 1997a, PASP, 109, 173
Ellison, M. A., & Seddon, H., 1952, MNRAS, 112, 73
Stedman, J., 1796, The Study of Astronomy, Adapted to the capacities of youth

OBAFGKMLTY

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.

Brightest Event Ever Observed

On June 14, 2015, perhaps the intrinsically brightest event ever recorded was detected at or near the center of the obscure galaxy APMUKS(BJ) B215839.70−615403.9 in the southern constellation Indus, at a luminosity distance of about 3.8 billion light years.

ASASSN-15lh (All–Sky Automated Survey for SuperNovae), also designated SN 2015L, is located at α2000=22h02m15.45s, δ2000=-61° 39′ 34.6″ and is thought to be a super-luminous supernova—sometimes called a hypernova—but other interpretations are still in play.

Let’s put the brightness of SN 2015L in context.  Peaking at an absolute visual magnitude of -24.925 (which would be its apparent visual magnitude at the standard distance of 10 parsecs), SN 2015L would shine as bright as the Sun in our sky if it were 14 light years away—about the distance to van Maanen’s Star, the nearest solitary white dwarf.  SN 2015L would be as bright as the full moon if it were at a distance of 8,921 light years.  SN 2015L would be as bright as the planet Venus if it were at a distance of 333,000 light years.  Since the visible part of our galaxy is only about 100,000 ly across, had this supernova occurred anywhere in our galaxy, it would have been brighter than Venus.  If SN 2015L had occurred in M31, the Andromeda Galaxy, 2.5 million light years away, it would take its place (albeit temporarily) as the third brightest star in the night sky (-0.47m), after Sirius (-1.44m) and Canopus (-0.62m), but brighter than Alpha Centauri (-0.27m) and Arcturus (-0.05m).

The Open Supernova Catalog (Guillochon et al. 2017) lists three events that were possibly intrinsically brighter than SN 2015L.  Two events were afterglows of gamma ray bursts GRB 81007 and GRB 30329: SN 2008hw at -25.014m and SN 2003dh at -26.823m, respectively.  And the other event was the first supernova detected by the Gaia astrometric spacecraft, Gaia 14aaa, 500 Mly distant, shining perhaps as brightly as -27.1m.

References
Chatzopoulos E., Wheeler J. C., Vinko J., et al., 2016, ApJ, 828, 94
Dong S., Shappee B. J., Prieto J. L., Jha S. W., et al., 2016, Science, 351, 257
Guillochon J., Parrent J., Kelley L. Z., Margutti R., 2017, ApJ, 835, 64

The Nearest Stars

Within 5 light years (ly) of the Earth, there are 4 stars known (just the Sun and the Alpha Centauri system).  Within 10 ly, there are 15.  Within 15 ly, there are 58 stars.  The number goes up—rapidly!  Undoubtedly, more stars will be discovered within 15 light years of the Sun.

And, cool is the rule when it comes to nearby stars.  Of the 58 known stars within 15 ly of Earth, an amazing 37 (64%) are class M stars.  The remaining 36% include one A star, one F star, three G stars, six K stars, one L infrared dwarf, five very cool T infrared dwarfs, one extremely cool Y infrared dwarf, and three white dwarfs.

The hottest (and bluest) star within 15 light years of the Sun is none other than Sirius (α Canis Majoris)—the brightest star in the night sky—just 8.65 light years distant.  Sirius A is an A1V (main-sequence) star, twice as massive as our Sun, 71% wider, 25 times more luminous, and only 237 to 247 million years old—just a single orbit around the galactic center.  Sirius rotates much faster than the Sun, too, spinning around once on its axis every 5.4 days.  Think about all these things the next time you look up and see Sirius chasing Orion across the meridian these late-winter eves.  And that Sirius has a white dwarf companion that orbits it once every 50 years, too.

All but two of the nearest 48 stars that are not white dwarfs or infrared dwarfs have a luminosity class of V, meaning they are dwarf or main-sequence stars.  The first exception is Procyon (α CMi A).  Its luminosity class of IV-V indicates it is bright for its temperature and spectral type (F5) and beginning to evolve into a subgiant star on its way towards becoming a giant star.  The other exception is Kapteyn’s Star, a red subdwarf star of spectral type and luminosity class M2VI.  A subdwarf star is underluminous for its temperature and spectral type.  This is caused by low metallicity.  The scarcity of elements other than hydrogen and helium in the star results in a more transparent stellar photosphere and thus a star that is a little smaller than it normally would be.  Incidentally, the fact that we have three white dwarf stars within just 15 light years of us suggests that white dwarfs are copious throughout our galaxy.

You might be wondering how many planets have been discovered orbiting these 58 nearest stars.  Beyond the eight planets orbiting our Sun we find another eighteen confirmed planets, plus at least three more unconfirmed planets.  This is a rapidly advancing field and no doubt many more planets will be added to the list in the coming decade.

The masses of the confirmed planets include one 55% more massive than Jupiter, one a little more massive than Neptune, one a little less massive than Uranus, thirteen super-Earths (1.14 M up to 7.7 M), and two less massive than Earth (0.75 M and 0.98 M).  Their orbital periods range from 2 up to 636 terrestrial days, and then one planet (the super-Jupiter) orbiting once every 6.9 years.  Orbital eccentricities range from circular (0.00) to 0.55, with the super-Jupiter in a very elliptical orbit having an eccentricity of 0.702.  The super-Jupiter is orbiting Epsilon Eridani (K2V, 10.48 ly), with all the rest of the confirmed exoplanets orbiting M-dwarf stars except for the four close-in planets orbiting Tau Ceti (G8.5V, 11.89 ly).

References
NASA Exoplanet Archive https://exoplanetarchive.ipac.caltech.edu.
“The Nearest Stars” by Todd J. Henry, Observer’s Handbook 2019, RASC, pp. 286-290.