January 27, 2020

Astro-Event of the Week; 02.06.09: An Antares Occultation.

An occultation of a bright star is well placed for observers in the Americas this weekend. The waning gibbous Moon will occult (i.e. pass in front of) the bright star Antares, also known as Alpha Scorpii. The action begins around 10:00 PM EDT and the Moon will cover up Antares for up to an hour and a half, depending on your latitude. Although the Moon will only be 16 hours away from Full, Antares at about magnitude +1 will be an easy naked eye target, and the skies will be completely dark along East Coast longitudes.

The farther west you go, the lower (and in a brighter sky) the pair will be. A cycle of several Antares occultations are happening this year, as it currently lies along the Moon’s path. Observing and recording these events are always fun and beneficial; much of the lunar profile has been mapped this way. But an Antares occultation is special for another reason; a chance to spot its faint companion as it winks in and out! Antares gets its name from the Greek ???????, meaning “rival of Mars,” due to its ruddy Mars-like appearance. I like to think of Antares as the “anti-Mars.” A red giant star an estimated 700 times the diameter of our Sun and about 600 light years distant, it is a good candidate for a galactic supernova one day (Romulans, beware!)…Another cool bit of Antares related scifi trivia; some treatments of the Superman tale place the doomed planet Krypton in orbit of this red giant star! You think Supes’ would mention this impending (for us!) supernova to scientists once in a while… It also has a hot blue companion, at a separation of about 3 arc seconds and a magnitude of +5.5. This companion would not be hard to spot, were it not in Antares glare. Astronomer Professor Burg of Vienna first sighted the elusive companion of Antares during an occultation similar to Saturday’s. The white dwarf companion of Sirius (Sirius B) is one of the few challenging double stars that is tougher. This week’s occultation is a good chance to test the resolution of that new telescope (and the quality of the local seeing!) and replicate this historical feat!

The Astro-term of the week is the Purkinje Effect. Does Antares B look a tad…greenish to you? Observations have described this as one of the very few stars that exhibit a shade of green over the years… this is most likely due to the Purkinje optical effect. In low light conditions, our eyes tend to switch from color sensitive cones to low light rods, kind of like fast and slow speed film (for those of us that remember film!) This is why things can look very black and white under low light conditions…but another consequence of this optical illusion is that some objects and stars can look greenish when contrasted with their red counterparts! The effect can be elusive, and not all folks see the same color contrasts when it comes to double stars. As Antares winks in and out watch for this effect… what differences do you see on film and video?


  1. Albanius says:

    Antares might be 700x the Sun’s diameter, but no known star is anywhere near 700x as massive as Sol.

  2. webmaster says:

    Thanks for the correction; indeed, even Eta Carinae at perhaps 100 Sols, doesn’t even come close!

  3. Eric M. Bram says:

    Regarding your comment that Antares’ may look green because of the Purkinje shift, I have some personal experience on that topic, because I am one of the few human beings who have seen Antares’ companion star by itself, without the light of Antares.

    The occasion was an expedition to the hills of West Virginia one bitter winter in the late 1960s, to view the grazing occultation of Antares. As you know, a lunar occultation of a star occurs when the moon passes in front of the star, and a grazing occultation occurs when just the very edge of the moon grazes the star as seen from the ground. In other words, when the observer happens to be at the northernmost or southernmost limit of the moon’s “shadow” (as cast by the light of the star) as that shadow passes across the Earth.

    In this case it was the southern limit, which is favored by amateur astronomers because the Moon’s southern polar region is rich with mountains, so the star being occulted often appears to blink on and off as the various mountaintops pass in front of it. Amateur astronomers who want to observe this event calculate where the limit will be, and travel large distances to observe the grazing application from that place, with the observers usually stretched along a line laid perpendicular to the limit. The resulting observations can be used to map the “profile” of the Moon’s mountains with much greater resolution than can be done with a telescope.

    I was on a small hillside outside of a farmhouse in the small West Virginia town we chose as our observing site. It was bitterly cold. As I watched Antares waiting for the “graze” to begin, Antares looked as usual in my six-inch Celestron, bright orange with a brilliant emerald green tinge at one side from the unresolvable companion.

    What I did not know, but would soon find out, is that on this particular day (or rather, night) Antares’ companion was to the north of its primary, so there would be times when the mountains of the Moon would block the light of Antares, but not of its companion. It is difficult for a terrestrial telescope to split the pair, and even when it can the primary is so close to the companion, and so bright, that light from its diffraction rings can appear to come from the direction of the companion. But at a distance of some 250,000 miles the Moon made an excellent light baffle and ensured that when it was blocking Antares but not the companion, only the light of the companion will get through to my telescope. At the first occultation event I was amazed and delighted to see the companion star sitting all by itself. The color was an indescribable vivid blue, a pale (yes, vivid but pale) turquoise color that I’ve never seen before or since.

    So why does the companion look green when viewed along with Antares? I do not believe it was a “contrast effect” or some low-light optical effect like the Purkinje shift, since the color did not change over time as my eyes “adapted” to lower light levels when Antares disappeared: there was emerald green at one side of Antares, and then all of a sudden there was that incredible blue. And anyway, the overall light levels did not change since there was always a sizable lit portion of the Moon in my field of view, so my eyes were not particularly dark adapted at that time; I was looking with fairly normal vision at a 5.5 magnitude star through a 6 inch telescope–plenty bright enough to see without dark adaptation.

    So here’s the actual reason, in my opinion, that the companion appears green when observed by us amateur astronomers while viewing Antares: even when a close double star can be resolved, light from the diffraction pattern of each star (multiple diffraction rings, ever decreasing in brightness with distance from the star) extends beyond the Airy disk and onto the other star. The brightness of even the first diffraction disc is much smaller than that of the Airy disk, so this usually makes no difference. But in this case it does, because Antares is more than 60 times brighter than its companion. I believe that the green color of the companion when viewed along with Antares through a telescope is caused by yellow light from the diffraction pattern of the much brighter orange primary mixing with the blue light of the much fainter companion. Remember when you were a kid and mixed yellow and blue paint or crayon? The result is green.

    Yes, yes, I know Antares is orange, and orange and blue are complementary colors so the result should be brown. Maybe Antares isn’t truly orange, maybe the shorter wavelength yellow light from it is diffracted farther, or maybe there is some other reason having to do with bright light from stars mixing together–I don’t know. But I’m pretty sure from my experience that the green color we see is caused by light from Antares mixing with that of its companion, rather than an optical contrast or low-light level effect.


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