FABULOUS SHOW, PORRIMA!
-----------------------
John Pazmino
NYSkies Astronomy Inc
nyskies@nyskies.org
www.nyskies.org
2011 March 15 initial
2011 March 21 current
Introduction
----------
Among the most fascinating once-in-TWO-lifetime shows I witnessed
in the 2-thous was that of gamma Virginis (GA-ma VIRR-jih-niss) or
Porrima (PO-rih-ma). This star is a binary like no other in the sky
for its 169-year period in a strongly excentric orbit.
For most of its cycle the star is a wide pair for small
telescopes. As gamma Virginis approached its proximity, when the
components rounded periastron, the pair grew harder and harder to
split thru a given telescope and eyepiece. At some date before the
periastron the stars merged into a single point. It passed beyond the
resolution of the instrument.
This merger happened at different dates depending on the scope,
observer skill and faculties, and state of the air above him. For me
the merger came in the 2003 apparition of Porrima. I just could not
see the star as double any more.
I continued to inspect Porrima thru my own and other scopes with
nothing of a duplicity about it. I also followed reports from other
astronomers who were able to see the two stars well into 2004.
I waited two years to try and capture the pair as it widened. At
first i failed repeatedly. In the 2008 apparition I just barely, with
long study and patience discerned that the one dot was probably two
overlapping ones. By the 2009 apparition the star was definitely, tho
not easily, seen as two in my small scopes.
As background and history of gamma Virginis, please see my prior
articles 'A star on the move' and 'They're closing in!' in the NYSkies
website at 'www.nyskies.org/articles/pazmino/gam-vir1.htm' and
'.../gam-vir2.htm'. This here piece here continues the story of gamma
Virginis thru 2010-2011.
Some geometry
-----------
The two stars of Porrima orbit in Kepler ellipses but seen at a
140-degree angle against our sightline from Earth. The angular closest
approach of the stars, peoximity, is really not the linear closest
approach, periastron, an effect entirely of our perspective.
Periastron is the near end of the orbital ellipse, analogous to
perihelion or perigee in the solar system. Proximity is the angular
closest separation on the sky as seen from Earth. Think of a planet or
comet in apparent conjunction with the Sun.
By the geometry of the Porrima orbit, periastron and proximity are
almost coincident, causing many authors to use them as equivalent.
In the general binary star they are wildly different.
Periastron occurred in about 2005.4, meaning 4/10 way thru year
2005. In double star work we cite dates as a year and decimal, rather
than calendar dates or Julian day numbers.
The inclination of 140 degrees is interpreted like a comet orbit's
inclination. It is retrograde in the sense that the companion star
rotate thru north, then west, south, and east. The direction, position
angle, is conventionally dimensioned from north, thru east, south,
west, so the comes (KO-mess) continuously decreases its position angle.
Apparition
--------
Porrima's apparition follows that of other late spring stars by
coming into the eastern sky in fall after the Sun moved from Virgo
into Libra. It is then in the dawn sky before sunrise. All of this
description applies to New York City, a typical mid-northern latitude.
As the Sun marches eastward thru the zodiac, Virgo, with Porrima,
drifts into night sky in owl-hours and midnight, than into the late
evening sky.
gamma Virginis drifts gradually from east thru south thru west
during spring and summer. It souths during April after nightfall. By
late summer, as the Sun approaches Virgo out of Leo, the star sets
into evening twilight.
The blackout, the Sun gap, spans September thru November when
observations of Porrima are not feasible from the ground. With the
excitement of periastron in the 2-thous many readers watched gamma
Virginis thruout its entire apparition,
In the 2011 apparition Porrima is joined by Saturn as a marker in
poorer skies. The two are a pair of handy targets for demonstrating
astronomy at public stargazing sessions.
Facts and figures
---------------
Here are some facts & figures about Porrima. These are updated
from my prior articles. First up are Porrima's other names:
--------------------------
catalog | designation
-------------+------------
3rd Fundamtl | FK3-477
Aitken | ADS1630
Bayer | gamma1 & gamma2 Virginis
Bonner Durch | BD+00:2601
Boss | GC17170
Flamsteed | 29 Virginis
Henry Draper | HD110379 & HD110380
Hipparchos | HIP61491
Hubble | GSC4949-1113
Pos Pro Mot | PPM400177
Proper name | Porrima, Arich
Smithsonian | SAO138912 & SAO138917
Struve | [Sigma]1670, STF1670
Tycho | TYC4949-1120-2
Washington | WDS12417-0127
Yale | HR4825 & HR4826
Zodiacal | ZC1819 & ZC1821
-------------------------
Because for the most of the 19th and 20th century the stars were
well separated, they commonly each earned their own designation. It
can be ambiguous in catalogs which component is which.
The celestial coordinates are, for epoch 2000:
------------------
Right ascension | 12h 41.6m
Declination | -01d 26.9m
Ecliptic long | 190.13
Ecliptic lat | +02.79
Galactic long | 297.77
Galactic lat | +61.33
------------------------
The two stars are nearly equal in size, brilliance, diameter, and
other properties:
--------------------------------------------
Property | A & B | A | B | comments
------------+-----+-------+-------+---------
Diameter | | 1.3 | 1.3 | Suns
Mass | | 1.3 | 1.3 | Suns
Surf temp | | 7,100 | 7,100 | Kelvin
Spectrum | | F0-V | F0-V | yellow-white
App magn | +2.7 | +3.4 | +3.5 |
Abs magn | +2.4 | +3.1 | +3.2 |
Luminos'y | | 4.3 | 4.2 | Suns
Distance | 38.6 | | ly, 11.8pc
Sun magn | +5.2 | | seen from star
Rad Vel | -20 | | km/s
----------------------------------------------
Slight variations in these values occur in the litterature.
What happened?
------------
Porrima is a binary whose orbit hangs the pair for decades on end
near apoastron but whips it around near periastron. The previous
instance, the first witnessed, was in 1836. The instant one, only the
second to be observed, was in 2005. The two stars collapsed into a
single dot thru most home telescopes by 2003.
Periastron occurred in May-June of 2005 with a separation of about
0.35 arcseconds. There is a minor discrepancy among observers in the
exact date, with all within about 40 days.
Only really large telescopes under truly stable and still air
could resolve Porrima into two stars. The star was broken apart by
various interferometric techniques at campus observatories.
By 2008 gamma was pulling apart. First the larger scopes and then
gradually the smaller ones were able to show it as a pair, not just a
congealed image. In 2009 pretty much any good quality home telescope
revealed the star as double.
For the next 160 or so years, gamma Virginis will be a wide double
star, a pleasant sight in any scope. The farthest separation, at
apoastron, is about 6 arcseconds in around year 2090. If you want to
watch the next periastron, hang around until 2174..
Stars in motion!
--------------
Of all the stars in the heavens, only Porrima allowed us to see
within months or years real stars in real gravitational motion. There
are a few other stars that show large proper motion, like Barnard's
star, but in gamma we saw the influence of one star's gravity upon an
other in a vivid manner.
We will not ever see this again in our lifetime and, with a
plausible odd, nor will our grandchildren. Of course, the decades to
come may bring new devices to the home astronomer that can reveal
gravitational stellar motion elsewhere in the sky, but this is
speculation to the max.
The situation of gamma is quite different from other binaries .
Even short period binaries require decades to discern the orbital
movement. It's the cometary nature of the orbit in Porrima that lets
the stars whirl so rapidly near pariastron within a couple years. For
the rest of the orbital period the star changes lazily thru the
decades.
Future behavior
-------------
As weird as it sounds, we had no definitive orbit for gamma
Virginis almost right up thru periastron! In spite of the century and
half of detailed study since the 1836 round, there were several orbits
in circulation. A couple, recited in the last years before the 2005
periastron claimed the closest approach comes by 2008!.
Any one adhaering to such wrong orbits -- issued by hefty
authorities -- may have lost their once and forever chance to observe
gamma during its fast-forward phase. They sleeped thru 2005 intending
to go and look in 2007 or so. Perhaps it is my good fortune not to
personally know of an astronomer fooled by a false prediction. I hear
anecdotes of such astronomers, but not any I know of for myself.
Now with two positively observed periastra (yes, the word got a
proper plural), a good orbit is in hand. Not a one single solution but
several that don't differ all that much. In any case, now there's no
critical phase coming for a LOOoong while off.
The orbits here are from Scardia in 2007-2008 and Mason &a in
2006. The latter is part of an ongoing speckle interferometry project
on binaries at USNO. The figures are within the range of other data in
the litterature.
--------------------------------------------------
parameter | Scardia | Mason &a | comments
------------------+----------+----------+---------
ascending node | 35.36 | 213.79 | degrees
arg of periastron | 255.03 | 73.78 | degrees
lon of periastron | 290.39 | 287.57 | = node + arg
inclination | 140.41 | 148.82 | degrees
excentricity | 0.8815 | 0.8825 |
periastron time | 2005.509 | 2005.438 | 2005 Jun 9-Jul 4
semimajor axis | 3.641 | 3.662 | arcsec
orbital period | 169.101 | 169.93 | years
-----------------------------------------------
Notice how much the longitude of ascending node and argument of
periastron diverge! Yet their sum, longitude of periastron, is quite
the same. This case illustrates how tough it is to suss out the 3D
orientation of a binary's orbit from the one remote viewpoint as a
plane shape on the celestial sphere.
Scardia and Mason also worked out the future aspect of gamma:
-------------------------------
| Scardia || Mason &a
+--------------++---------
year | sep | pos || sep | pos
-------+-------+------++-------+-----
2010.0 | 1.390 | 23.7 || 1.417 | 22.5
2011.0 | 1.592 | 18.5 || --- | ---
2012.0 | 1.779 | 14.4 || 1.807 | 13.6
2013.0 | 1.952 | 11.2 || --- | ---
2014.0 | 2.116 | 8.4 || --- | ---
2015.0 | 2.270 | 6.0 || --- | ---
--------------------------------------
The position angle is the direction from the primary toward the
secondary. With the two stars almost equally bright, it can be hard to
tell which is actually the primary. The position angle may be given as
an strike, like '11.2-191.2' for 2013. This is merely the directions,
180 degrees apart, as seen from each star in turn. Note the retrograde
sense, toward lower angles.
Wild speculations
---------------
As word of the impending closure of Porrima flashed thruout the
astronomy world in the 1830s, a panic set in. What will happen to the
stars?! Mind well that we didn't understand what stars were and
certainly had no concept of a collision between two of them.
Er figured the stars could be like the Sun, only viewed from a
humongous remoteness. Big help that was being that we didn't
understand what the Sun was. A common notion was that the Sun is s
solid globe like Earth with a luminous cloud deck. Sunspots are holes
in the clouds to allow peeks of the dark land under them. So, what
happens if two Suns collided?
Will the stars fuse together in one and never again be seen as a
double star? Will they collide and explode like bombs before our eyes?
Will they interfuse and fluff out into a nebula? Will they snuff out
like capped candles, never to shine again? Will they trigger a
calamity on Earth to end humankind's existence? Will they bounce apart
like billiard balls and distort the Virgo constellation catchfigure?
No one knew. With the small weak scopes of the 1830s the stars did
blend into one dot for a couple years. What's going on with them? A
united relief erupted when by 1838 the two stars emerged from
conjunction! I don't think there was actually a wild cheering or giddy
celebration, but surely we were glad that nothing disastrous occurred.
The blended pair for the year around 1836 did block positive
assessments of the orbit. That's why the time of the 1836 periastron
is still a bit uncertain, altho all estimates fall within the merger
period. This imprecise timing caused the parade of various orbit
models for the next 160 and more years, until the periastron of 2005.
Occultations
----------
Porrima is a favorite star for occultations by the Moon. It sits 2
degrees north of the ecliptic about 60% from Denebola to Spica. Each
component suffers its own occultation, allowing for tight bounds on
their orientation and separation.
We in 2011 are, as luck falls, between seasons ed Porrima
occultations. Most of the events of the previous and next season miss
New York City.
We missed an occultation on 2004 April 4 when the Moon slided next
to the star along her northern limb. A little south of New York there
was a graze occultation. The two components each skimmed thru valleys
and mountains on the north edge of th Moon.
-------------------
UT date | event
------------+------
1997 Aug 8 | first occultation of old season
1998 Jul 1 | first occultation over NYC
2003 Aug 3 | previous occultation over NYC
2004 Apr 4 | close miss in NYC, graze south of NYC
2005 Jan 3 | last occultation of old season
--- | 11 years between seasons
2016 Feb 25 | first occultation of new season
2017 Jan 18 | next occultation over NYC
2022 Mar 19 | last occultation over NYC
2023 Jun 26 | last occultation of new season
--------------------------------------
In 2005 the Moon's descending node migrated too far form Porrima
and the Moon began to miss the star. The ascending node creeps toward
Porrima by 2016 to start the next season of occultations. There were
no occultation right around the periastron to help fill in data at
that critical point.
The circumstances of the 2017 occultation for the City are:
----------------------------------------------------------------
Date EST P %Ph Eln Alt Az Cusp Pos Ver LibL LibB
----------- -------- - --- --- ------ ---- --- --- ---- ----
2017 Jan 18 00:11:04 D 66- 108 15 105 -74N 97 144 +6.1 -3.6
2017 Jan 18 01:18:02 R 65- 108 27 118 +72N 310 352 +5.9 -3.6
-------------------------------------------------------------
P is the phaenoenon with D for disappearance, immersion, ingress
and R for reappearance, emersion, egress. The calculation is for ONE
component with supplemental information about its duplicity.
%Ph is the lighted percent of the lunar disc. Eln is the downrange
distance of the Moon from the Sun along the ecliptic.
Alt & Az are the altitude and azimuth of the Moon at each event.
Cusp is the angle around the lunar limb from the north or south
horn to the star on the dark side (-) or light side (+).
Pos is the position angle CCW around the limb to the star from
celestial north.
Ver is the vertex angle CCW around the limb to the star from the
top of the Moon.
LibL & LibB are the lunar libration in longitude and latitude or
the selenographic lon-lat of the center of the lunar disc.
Other components
--------------
Besides the two bright components, A and B, Porrima has at least
four other possible members. They were measured only sporadicly,
leaving their attachment to the main pair still in doubt. The values
here are form different years but the scanty reports seem to indicate
they didn't change much thruout the 20th century.
--------------------
# | magn | sep | pos
--+------+-----+----
C | 15.1 | 53 | 159
D | 12.2 | 124 | 88
E | 13? | 252 | 180
F | 13? | 482 | 269
--------------------
I could find no stated brightness for the E and F star.
Photographs of the field show they are in the 13th magnitude range.
Porrima chasing?
--------------
The periastron of Porrima is an event litterally as rare as a
transit of Venus. You can easily live your life between events without
personally witnessing one. There were in 2004 many trips and special
viewings to watch the Venus transit. Were there any to chase after
gamma Vitginis?
Unlike the transit, or a solar eclipse, there was no need to be at
a specific place at a specific time to see the periastron. It was
visible thruout the world and at any time within its apparition.
On the other hand, Porrima could have been an extra treat during a
solar eclipse trip because such trips routinely offer stargazing. As
far as I found in ads in the 2000-2005 timeframe, there was no mention
of gamma Virginis as an accessory feature of eclipse trips.
It was also possible to organize a viewing of Porrima from a site
blessed with large telescopes and still quiet air. There were trips
for photography and imaging, without a particular astronomy event, in
such locations. The regularly scheduled starparties during this period
could have highlighted the chance to inspect Porrima. Again, I found
no word about gamma Virginis to attract astronomers to these trips.
A wisely traveler on an astronomy trip in the mid 2-thous could
have alerted others with him to the spectacle of Porrima. It would be
a surprise feature of the trip. While I suppose there were such good
folk who called attention to gamma, I have no personal accounts of
any. And I heard really few anecdotes of any.
An example of a surprise treat was a partial solar eclipse during
a halley's comet trip I was with in Australia. In preparing for the
trip I had a timetable for this eclipse to watch at one of the camp
grounds along the itinerary.
On the afternoon of the eclipse I set up my scope with solar
filter and let the other trippers view the Sun. Apart form not knowing
about the eclipse as part of the comet chase, many travelers remarked
that this was the very first solar eclipse they ever personally saw.
All in all, those following Porrima did so from what ever location
and with whatever equipment they had to hand, with possibly only
weak effort to move to a better site or gear.
Why Porrima?
----------
There are many short-period binaries whose motions can be
appreciated within an astronomer's career. What makes Porrima so
special? The two reasons are the strongly excentric orbit and the
closeness of the star to us.
Porrima is only 39 lightyears away. Its stars are angularly set
apart to see. At a separation of 5 arcseconds, suppose Porrima is
removed to ten times its distance, to 390 lightyears. The star would
have separation of only 1/2 arcsecond, out of reach of all but the
strongest home telescopes. Since this is near the widest stance of the
members, Porrima would then drop from a target list of double stars
for small scopes.
An other scenario is that Porrima is 390 lightyears away and still
stands 5 arcseconds apart. The linear distance is then ten times
greater and the orbital period is about 32 times longer. Porrima would
then take some 5,300 years to complete one round. It resembles most
other binaries with little detectable motion in several lifespans.
The elongated orbit, the other reason, makes the companion behave
like a comet, lingering near apoastron for many decades and whipping
around periastron within one decade.
A more circular orbit would still make Porrima a fascinating
target with a gentle circulation noticeable within a lifespan. It's
the intense activity near periastron, like that of a comet near the
Sun, that sets gamma Viriginis apart from other binaries.
Other rapid binaries
------------------
During and specially since the 2005 periastron readers asked if
there are other rapidly changing binaries. There are none so amazing
as gamma Virginis but there are several normal binaries to excite you.
The gotcha is that you better be in astronomy for the duration, like a
couple decades.
Dr Martin Gaskell, University of Nebraska, in 1995 answered this
question with some stars to follow.
I include those of his that are also in my own list of targets for
New York City. We differ in selection because of the changing aspect
of the stars. 'year' is the epoch of the separation and position.
All stars are nearby, within 100 lightyears, and have orbits less
excentric than Porrima's. The orbit of mu Cygni is almost edgeon to
our line-of-sight. It has two proximities per lap, when the companion
is at inferior and then superior conjunction as seen from Earth.
-------------------------------------------------------------
Cns Star RtAsc Decl Mag Mag colors sep pos year per
hr mn deg mn A B sec deg years
--- ---- ------- ------ --- ---- ------- ---- --- ---- ------
CMa alp 06 45.1 -16 43 1.4- 8.5 whi whi 4.6 150 2000 50.1
Gem alp 07 34.6 +31 53 1.9 3.0 whi whi 4.6 57 2009 420.
UMa xi 11 18.2 +33 06 4.3 4.8 yel yel 1.8 273 2000 59.8
Sco xi 16 04.4 -11 22 4.9 7.3 yel 7.5 46 2005 45.7
Oph 70 18 05.5 +02 30 4.1 6.0 ora whi 5.3 136 2006 88.1
Cyg mu 21 44.1 +28 45 4.8 6.9 y-w yel 1.8 311 2005 507.5
Aqr zet 22 28.8 -00 01 4.3 4.5 y-w whi 2.2 174 2009 856.
----------------------------------------------------------------
All of these stars demand good clean collimated optics, good acute
clear eyesight, good steady quiet air. Alpha Canis Majoris, Sirius, is
a challenge star because the comes is usually veiled by the glare of
the primary.
Please understand that a short period implies a smaller linear
distance between the components. Even at their closeness to Earth the
stars can be angular tight in a small scope. Thee stars do need a
larger scope, generally 200mm aperture or more, to see a significant
part of their orbital motion.
I remind that binaries are evolving targets. Due to their short
periods, the aspect of these stars changes quickly over the years. You
better seek current news about the stars before observing them. In
some cases, like for Porrima, the components may close in beyond
resolution of your equipment.
Cardboard micrometer
------------------
Dr Gaskell promotes a micrometer for double star work made from a
disc of cardboard. I did say 'cardboard'. He reminded me about it when
I assembled material for my first Porrima article 'A star on the
move'. I made a unit during the rampup of excitement for Porrima in
2000. I explain it in 'The cardboard diffraction micrometer' in the
NYSkies website at 'www.nyskies.org/articles/pazmino/diffmic.htm'.
I tried it on gamma Virginis in 2001 and 2002. In the latter year
it was tough to visually split the star. Believe it or not, this
effing thing really worked! It does need the highest power the scope,
eye, air can stand and the double must actually be resolved opticly.
But with this cardboard contraption competent and valid measurements
of separation and position angle are feasible.
I didn't try it since then, thru 2010-2011 when the stars are
getting well separated. In fact, I misplaced the gadget and have to
hunt for it. Or just make a new one.
Observing tips
------------
Naively the stars at periastron should be resolvable in a scope of
480mm aperture, based on (resolution in arcsec) = (120 arcsex.mm) /
(aperture in mm) rule. This is an ideal value with the stars as
perfect optical images. The atmosphere never allows such perfect
images. The stars may blur to a half to a full arcsecond diameter,
thoroly smearing them together.
The practical limit on resolution is generally taken as one
arcsecond in normal air. In addition, this limit requires good quality
optics, well maintained and collimated. It helps to have acute and
clear vision.
You must have a quiet and stable air above you. Viewing gamma
under a turbulent air will likely end in failure while the stars are
still close together. Beyond 2012 Porrima becomes an easy binary star
in the typical small scope under typical air stability.
To better appreciate the motion of the stars, keep to a particular
set of scope and eyepiece,.else the angular extent in a this and that
instrument can not be correlated properly.
The angular rotation is harder to appreciate because of field and
optical rotation. Unless you fix the field orientation, you won't know
which way the stars are turning. This situation will for sure prevail
when inspecting Porrima in telescopes other than your own.
Conclusion
--------
If you are new to astronomy, and missed the Porrima show, you
still have time to witness the star in action. It is a slower one,
yes, but still appreciable thru small scopes. Year by year the pair
separates gently farther apart. It becomes easier to make out the two
individual stars, using the same telescope and eyepiece.
Perhaps with your rig the star still looks single in 2011.
Eventually with that gear you'll be surprised to look and see two
stars where in the previous apparition you could make out only one.
For those who watched Porrima close in, coalesce, than break
apart, you are a very privileged person. You're one of the fellowship
of astronomers who can recount the tales of stars on the move. You are
one with the original witnesses who walked the planet when the United
States had only half of its states,