Eclipsing Binaries

With the advent of relatively inexpensive CCD cameras, amateur astronomers with modest-sized telescopes are in an excellent position to contribute valuable scientific data to the astronomical community.  One type of object that can be very interesting and useful to observe is the eclipsing binary.  And there are a lot of them.

Due to a sometimes fortuitous alignment of the orbital plane of a binary star along or near our line of sight, one or both stars pass directly in front of the other periodically, and this type of object is known as an eclipsing binary.

The brightest eclipsing binary in our sky is Algol (Beta (β) Persei).  Known to vary in brightness since antiquity, astute ancient Arab astronomers gave Beta Persei the name “al Ghul” which, loosely translated, means “the Demon Star”.  Today, we know that Algol’s brightness variations are caused by a hot blue B8V star (Algol A) going behind and in front of its cooler and less massive but larger K0IV companion (Algol B).  Since the two stars orbit each other once every 2.867328 days (they are very close, separated by just a little over 5½ million miles), every 2 days, 20 hours, 48 minutes, and 57 seconds Algol B passes in front of much-brighter Algol A for a few hours, and the single point of light we see from Earth dims by 1.3 magnitudes.  This is the primary eclipse.  A secondary eclipse also occurs half a period before or after each primary eclipse.  When Algol A passes in front of Algol B, the brightness of the point of light we see drops by only 0.05 magnitude.  This shallow secondary minimum occurs because Algol B is not nearly as bright as Algol A.

Eclipsing binaries like Algol (which are close enough to each other to form an interacting pair) are interesting subjects for amateur astronomers to monitor.  Periods can change, phases can shift, and unexpected events can occur, such as when Dr. Jim Pierce (now Emeritus Professor of Astronomy at Minnesota State University in Mankato) and I were the first to observe ultraviolet flare events from the eclipsing binary V471 Tau at Iowa State University’s Erwin W. Fick Observatory in 1978.

So, how do you know when eclipses will occur, how deep they will be, and how long to monitor the star before, during, and after the event?  A great starting point is the Eclipsing Binary Ephemeris Generator by Shawn Dvorak which shows you a number of stars that will be in eclipse and observable from your location on any given night.  The Timing Database at Krakow (TIDAK), maintained by Jerzy M. Kreiner at the Mt. Suhora Astronomical Observatory in Poland, is another great source of eclipsing binary information.

A schedule, if you will, of eclipsing binary primary eclipses (like other astronomical events) is called an ephemeris.  Eclipsing binary ephemerides look like this one for Algol:

HJD = 2452500.21 + E × 2.867315

Here, HJD is the heliocentric Julian date of minimum light.  Julian date is a continuous count of days and fractions thereof elapsed since an arbitrary starting date of noon Universal Time (UT) on January 1, 4713 B.C.  The heliocentric Julian date removes the orbital motion of the Earth from the ephemeris calculations, centering the times of events on the Sun rather than the Earth.  An event could be observed to occur as much as 8.3 minutes earlier or later than calculated depending on where the Earth is in her orbit relative to the star.  The first number in the equation above, in this case 2452500.21, refers to the heliocentric Julian date of some arbitrary starting minimum.  The E stands for epoch, simply a consecutive integer count of successive minima, and the second number, in this case 2.867315, refers to the orbital period of the eclipsing binary in days.  The Kreiner website takes the chore out of choosing the appropriate value of E for the time you want to observe by calculating the HJDs (and corresponding Earth-based UT dates and times) of the eclipsing binary you choose over the next several days.

You should monitor a star before, during, and after the eclipse, so having a rough of idea of what object you should observe and when does not require you convert heliocentric Julian date to the Julian date at the telescope. However, any event times from data you record at the telescope must be converted to HJD for it to be useful.  There is an online tool to do this for you.  Of course, you not only need to know the UT date and time of an event, but also the equatorial coordinates (right ascension and declination) of the object you were observing to calculate the heliocentric Julian date.

We’re not even going to get into barycentric Julian date (BJD), or the fact that the distance between the Sun (or the barycenter of the solar system) and the eclipsing binary of interest is growing (radial velocity > 0) or shrinking (radial velocity < 0), and that this means that the period we measure is not exactly the same as the true orbital period of the system.  But it is very close.

Fermilab

Fermilab is a name well known to all physicists.  When I was an astrophysics undergraduate student at Iowa State University in Ames, Iowa in the mid-to-late 1970s, I remember that several members of our large high energy physics group made frequent trips to Fermilab, including Bill Kernan and Alex Firestone.  At the time, it was the best place in the world to do high energy physics.  What is high energy physics?  Basically, it is the creation and study of new and normally unseen elementary particles formed by colliding subatomic particles into one another at very high velocities (kinetic energies).

Wilson Hall, Fermilab, March 4, 2018.  Photo by Lynda Schweikert

On Sunday, March 4, a group of us from the Iowa County Astronomers met up at Fermilab for an afternoon tour of this amazing facility.  We were all grateful that John Heasley had organized the tour, and that Lynda Schweikert photo-documented our visit.

Our group at Fermilab. Our wonderful tour guide is third from left, and club organizer John Heasley is sixth from right. Photo by Lynda Schweikert, fourth from left.

Our afternoon began with an engaging talk by Jim Annis, Senior Scientist with the Experimental Astrophysics Group: “Kilonova-2017: The birth of multi-messenger astronomy using gravitational waves, x-rays, optical, infrared and radio waves to see and hear neutron stars”.  Here he is showing a computer simulation of an orbiting  pair of neutron stars coalescing, an event first observed by the LIGO and Virgo gravitational wave detectors on 17 August 2017 (GW170817), and subsequently studied across the entire electromagnetic spectrum.

Dr. Jim Annis describing the neutron star merger detected on 17 Aug 2017. Photograph by Lynda Schweikert.

One of the amazing factinos I remember from his talk: even though neutrinos were not directly detected from the GW170817 event, the matter in colliding neutron stars is so dense that neutrinos push material outwards in what is called a neutrino wind.  Yes, these are the same neutrinos that could pass through a light year of solid lead and only have a 50% chance of being absorbed or deflected, and pass through your body at the rate of 100 trillion every second with nary a notice.

Even though CERN has now eclipsed Fermilab as the world’s highest-energy particle physics laboratory, Fermilab is making a new name for itself as the world’s premier facility for producing and studying neutrinos.  This is a fitting tribute to Enrico Fermi (1901-1954)—after whom Fermilab is named—as Fermi coined (or at least popularized) the term “neutrino” for these elusive particles in July 1932.

Basic research is so important to the advancement of human knowledge, and funding it generally requires public/government funding because practical benefits are often years or decades away; therefore such work is seldom taken up by businesses interested in short term profit.  However, as our tour guide informed us, the equipment and technology that has to be developed to do the basic research often leads to practical applications in other fields on a much shorter time frame.

Main Control Room at Fermilab. Photograph by Lynda Schweikert.

Thoughts Inspired by Leon Lederman: A Footnote

I had the great privilege in October 2004 of attending a talk given by Leon Lederman (1922-), winner of the 1988 Nobel Prize in Physics and director emeritus of Fermilab.  I listened intently and took a lot of notes, but what I remember best besides his charm and engaging speaking style was his idea for restructuring high school science education.  The growing scientific illiteracy in American society, and the growth of dogmatic religious doctrine, is alarming.  Lederman advocates that all U.S. high school students should be required to take a conceptual physics & astronomy course in 9th grade, chemistry in 10th grade, and biology in 11th grade. Then, in 12th grade, students with a strong interest in science would take one or more advanced science courses.

Teaching conceptual physics (and astronomy) first would better develop scientific thinking skills and lay a better groundwork for chemistry, which in turn would lay a better groundwork for biology.  Whether or not a student chooses a career in science, our future prosperity as a society depends, in large part, on citizens being well-informed about science & technology matters that affect all of our lives.  We also need to be well-equipped to assimilate new information as it comes along.

It is in this context that I was delighted to read Leon Lederman’s commentary, “Science education and the future of humankind” as the last article in the first biweekly issue of Science News (April 21, 2008). He writes:

Can we modify our educational system so that all high school graduates emerge with a science way of thinking?  Let me try to be more specific.  Consider Galileo’s great discovery (immortalized as Newton’s First Law): “An isolated body will continue its state of motion forever.”  What could be more counterintuitive?  The creative act was to realize that our experience is irrelevant because in our normal experience, objects are never isolated—balls stop rolling, horses must pull carts to continue the motion.  However, Galileo’s deeper intuition suspected simplicity in the law governing moving bodies, and his insightful surmise was that if one could isolate the body, it would indeed continue moving forever.  Galileo and his followers for the past 400 years have demonstrated how scientists must construct new intuitions in order to know how the world works.

I’d like to take Lederman’s comments one step further.  Whether it be science, politics, economics, philosophy, or religion, we must realize that most ignorance is learned.  We all have blind spots you could drive a truck through.  Our perceptions masquerade as truth but sometimes upon closer inspection prove to be faulty.  Therefore, we must learn to question everything, accepting only those tenets that survive careful, ongoing scrutiny.  We must learn to reject, unlearn if you will, old intuitions and beliefs that are harmful to others or that have outlived their usefulness in the world.  We must develop new intuitions, even though at first they might seem counterintuitive, that are well supported by facts and that emphasize the greater good.  We must, all of us, construct new intuitions in order to make our world a better place—for everyone.

Illumination Levels: Then and Now

The following excerpts are from the 1911 and 1925 editions of A Text-Book of Physics by Louis Bevier Spinney, Professor of Physics and Illuminating Engineering at Iowa State College (now Iowa State University) in Ames, Iowa.

From the 1911 edition…

ILLUMINATION

516. The intensity of illumination of any surface is defined as the ratio of the light received by the surface to the area of the surface upon which the light falls.  A unit of intensity which is oftentimes employed is known as the foot candle, and is defined as the intensity of illumination which would be present upon a screen placed at a distance of one foot from a standard candle.  The meter candle is a unit of intensity which is employed to some extent.

The table below gives a number of values of illumination such as are commonly observed, the intensity of illumination being expressed in foot candles.

Suitable for drafting table    .    .    .    .    .    5 to 10

Suitable for library table   .    .    .    .    .   .    3 to 4

Suitable for reading table   .    .    .    .    .   .  1 to 2

Required for street lighting   .    .    .    .    .  0.05 to 0.60

Moonlight (full moon)    .    .    .    .    .    .   .  0.025 to 0.03

 

And from the 1925 edition…

ILLUMINATION

532.  The eye has a remarkable power of adaptation.  In strong light the pupil contracts and in weak light expands, so that we are able to use our eyes throughout a range of illumination which is really quite astonishing.  However, the continued use of the eyes under conditions of unfavorable illumination causes discomfort, fatigue, and even permanent injury.  Experiment and experience show that eye comfort, efficiency, and health considerations demand for each kind of eye work a certain minimum illumination.  Some of these illumination values taken from tables recently compiled are given below.

FOOT-CANDLES

Streets    .    .    .    .    .    .    .    .    .     .    .    .    .    .    1/20 to 1/4

Living rooms; Halls and passageways    .    .    1 to 2

Auditoriums; Stairways and exits;
Machine shops, rough work    .     .    .    .    .    .  2 to 5

Classrooms; Laboratories; Offices;
Libraries; Machine shops, close work    .    .    5 to 10

Engraving; Fine repairing work; Drafting;
Sewing and weaving, dark goods  .    .    .    .     10 to 20

 

By comparing the 1911 and 1925 data with the illumination levels recommended today by IESNA, we can see that recommended light levels for streetlighting have increased anywhere from 40% to 380% since 1925.  A cynic might say that we need more light than our ancestors did to see well at night.  As you may have noticed, light levels have been steadily creeping upward, everywhere, over the last few decades.

Recommended Illumination Levels for Streetlighting

Year        Minimum    Average    Maximum

1911             0.05                ???               0.60

1925           0.05               0.25               ???

1996          0.07                1.20               ???

Have you ever noticed how well you can see at night when the full moon is lighting the ground?  The full moon provides surprisingly adequate non-glaring and uniform illumination at just 0.03 footcandles!  For inspiration, take a look at the following text from an Ames, Iowa city ordinance, dated July 8, 1895:

“The said grantees shall keep said lamps in good condition and repair, and have the same lighted every night in the year from dark until midnight, and from 5:00 a.m. until daylight, except such moonlight nights or fractions of the same as are not obscured by clouds, and as afford sufficient natural light to light the streets of said city.”

This was originally published as IDA Information Sheet 114 in November 1996, and authored by David Oesper.

Deep Penumbral Lunar Eclipse – Friday, February 10, 2017

The Moon is Full on Friday, February 10, but that’s not all.  It will plunge deeply into the penumbral shadow of the Earth, not quite touching the umbral shadow.  The penumbral shadow is the part of the Earth’s shadow where you would see the Earth partially eclipsing the Sun.  Normally, penumbral lunar eclipses are no big deal, as they are very difficult or impossible to discern, but this time you should be able to see a noticeable darkening of the full moon from left to right as the eclipse progresses towards maximum penumbral shading, and then brightening from lower right to upper left as the Moon exits the Earth’s penumbral shadow, as shown in this video.  Of course, how much of this you will be able to see will depend on both your local moonrise and when evening twilight ends.

Here are local circumstances for Dodgeville, Wisconsin:

Event Time Moon Altitude
Penumbral Eclipse Begins 4:34:14 p.m. below horizon
Penumbral Eclipse First Visible? 5:14 p.m.? below horizon
Moonrise 5:21:03 p.m. 0° @ 72° (ENE)
Sunset 5:25:43 p.m.
Civil Twilight Ends 5:55:09 p.m.
Nautical Twilight Ends 6:28:36 p.m. 11°
Maximum Penumbral Shading 6:43:54 p.m. 13°
“Dark Enough” 6:45:07 p.m. 14°
Astronomical Twilight Ends 7:01:33 p.m. 16°
Penumbral Eclipse Last Visible? 8:14 p.m.? 29°
Penumbral Eclipse Ends 8:53:29 p.m. 36°

For those of us in SW Wisconsin, I wouldn’t bother looking much before 6:30 p.m., because evening twilight is likely to be too bright.  The best time to look will probably be at 6:43 p.m., just a little over a minute before twilight ceases to become any real concern1.  Evening twilight officially ends at 7:01 p.m., and you will probably notice some shading on the Moon until about 8:14 p.m.

The Moon will be inching closer towards Regulus during the penumbral eclipse (and, in fact, all night long), so watch for that.

For the record, a penumbral eclipse this deep (when there wasn’t also a partial or total lunar eclipse) hasn’t happened since March 14, 2006 (which was even deeper), and won’t happen again until January 10, 2085, though we need only wait until January 31, 2018 and January 20, 2019 for the next two lunar eclipses and they will both be total lunar eclipses—far more impressive than any penumbral lunar eclipse could ever be.  We’ll be seeing only the beginning partial phases of the 2018 eclipse here because the eclipsed moon will be setting during bright morning twilight.  Fortunately, we’ll have a front-row seat to the entire 2019 eclipse as all of it will occur high in the sky after dark, with totality ending conveniently before midnight.

1My late friend Joe Eitter (1942-2014), who was the observatory manager at Iowa State University’s Erwin W. Fick Observatory during its entire existence, used to say that by the time the Sun got down to 15° below the horizon, it is “dark enough”.