DO I SEE OMEGA CENTAURI?
----------------------
John Pazmino
NYSkies Astronomy Inc
www.nyskies.org
nyskies@nyskies.org
2009 April 27 initial
2016 January 2 current
Introduction
----------
Among the favorite sights for a southern astronomer is the bright
globular cluster in Centaurus, labeled omega Centauri in the Bayer
system of star naming. It also has the designation NGC5139.
An other favorite sight in Centaurus is the Engimatic Galaxy or
NGC5128, among the brighter specimina in the sky. This is also the
Centaurus A radio emissor and seems to be a spherical galaxy wrapped
with a belt of shredded dust.
Small scopes near or south of the equator can spot these targets,
with omega Centauri being a bare-eye object. However, these targets
are generally neglected in New York because they are regarded as only
seen from southern latitudes.
In fact, they are well seen from the southern states of the US,
specially Hawaii. While they are indeed 'southern' they do peep above
our south horizon when they are culminating. At such time NGC5139 is
only 1 degree above the horizon; NGC5128, 5.
Some history
----------
The globular cluster is so bright, in spite of its low altitude
from Europe and classical culture centers, that it was recognized as a
star by Ptolemaeus. Bayer in 1603 gave it the letter omega, the last
in the Greek alphabet, even tho it is hardly near the dimmest of the
Bayer stars of Centaurus. Speculation is that Bayer did not himself
see the cluster but relied on reports from other, not well
experienced, observers.
Its nebulous nature was discovered by Halley in 1677. In the 1830s
John Herschel resolved its stars as his object h3504. Messier missed
it from his location in Paris, but laCaille noted it as entry I-5 in
his 'southern Messier' list.
Messier missed NGC5128 from his Paris observatory and it escaped
notice by laCaille. It was first seen by Dunlop in 1826, who entered
it as Dun482 in his records. John Herschel in 1847 first described the
dark band across it and listed it as h3501. There is no generally used
proper name but because of its peculiar shape and radio emission it is
sometimes named the Enigmatic Galaxy.
Only the far south situation prevents these two targets from being
among the showpieces of the skies of new York.
Challenge
-------
Can we really hope to see the cluster and galaxy from New York? In
addition to their low altitude, making them easily blocked by almost
any skyline, they are dimmed by atmosphere absorption. The globular
cluster is 4th magnitude, but may appear no more lustrous than 5th
magnitude when viewed thru the thick air. Binoculars are required.
Engimatic Galaxy is 7th magnitude, which is bright for a galaxy, may
seem only of 8th magnitude.
The City presents the added impediment of luminous graffiti that
is generally deeper at low altitudes, even tho the overhead skies are
dark. To mitigate this grayed sky we are favored with an ocean to our
south. There is essentially no luminous graffiti in that direction if
we view from the seashore with no intervening builtup land.
There is some gain of altitude by atmosphere refraction. It lifts
omega Centauri about 1/2 degree above its geometric altitude but
affects Enigmativ Galaxy only by about 10 arcminutes. This is how it
is possible, but hardly probable, to see omega from as far north as 43
degrees latitude.
Reward
----
It seems amazing but both targets are rewarding to inspect from
the City. Both have definite structure and angular dimension. They are
no worse in aspect than the majority of deepsky targets elsewhere in
the sky, including those in high sky. We do not resolve any stars out
of NGC5139 nor discern the dark dust band across NGC5128.
Trying to view the two objects can be part of starbrowsing thru
other parts of the spring sky, like the Virgo galaxy cluster. From my
own experience, the globular cluster and the galaxy present as good an
view as other galaxies in higher sky.
Date & hour
---------
Because of the low altitude, the two objects must be searched at
or near their culmination. More than an hour away from the meridian,
they are either down or really too low. You must, by a computer
planetarium program, select dates and hours when culmination occurs.
The two are almost on the same hour circle as Spica and easily
recognized about 1/3 up when it crosses the meridian. Centering Spica
on the meridian puts both challenge objects at or near the meridian.
Since the challenge began in 2009 observers in New York's latitude
noted that the last week of May was too often infected by twilight.
On the other hand April brings the two targets to the meridian within
the normal observing hours before midnight.
Starting in 2016 the omega Centauri challenge season is the two
months of April and May, giving about four more weeks to catch sight
of the galaxy and cluster.
The table here gives approximate combinations of date and hour for
the meridian crossing of omega Centauri, sidereal time 13h28m, for
usual period of most observing. Before March the weather in owl hours
can be terrificly arctic. After May the targets culminate in twilight.
Hours are in EST. Add one hour during the EDST period.
+----+-------+-------+-------+
|day | March | April | May |
+----+-------+-------+-------+
| 10 | 02:12 | 00:10 | 22:12 |
| 20 | 01:33 | 23:27 | 21:29 |
| 30 | 00:53 | 22:47 | 20:50 |
+----+-------+-------+-------+
Preparation
---------
You need an ocean view to the horizon with no obstructions. Altho
you can get such a view from a high inland roof or hill, you are also
looking across builtup land that can throw luminous graffiti into your
sightline. You really need a place fronting the sea, like one of the
beaches around the City's southern flank.
The air must also be really clean and dry, free of all haze and
mist. These don't come often in thee City and they may miss the
culmination period. Which of the meridian crossing days and hours you
try is a function of your life style and daily routine.
Assemble the same gear as for regular stargazing, with the
addition of a good starchart of the area south of Spica. The one HERE
is passable but you may want to cut a more detailed one.
Be on site about a hour before the meridian crossing to get your
bearings, set up your equipment, do some stargazing elsewhere in the
sky.
Travel
----
The waterfront sites are a bit removed from streets, preventing
you from driving your car up to them. You have to park several blocks,
a half kilometer, away. At night there should be open street parking.
Park at a lighted place on a busy street. Avoid dark empty areas.
Commercial parking is expensive for the couple hours you'll be
stargazing.
There are many suitable sites at or near transit services. The
subways run all night but with longer headways late at night. Routes
may be altered from those in evening or early night. You may take
which ever train is running when you go home and transfer to your
proper route some where down the line.
Many bus routes end service in late night. Check your specific
route before riding to your observing site! Getting stranded in no
joke! It could mean a long walk in chilly air to an alternate bus
service.
The Rockaway sites are within a block or two of stations on the
IND Rockaway line. Aquarium and Brighton Beach are on the BMT Brighton
line, Coney Island is served by the Brighton, Sea Beach, Culver, and
West End lines. Midland Beach and South Beach are within a couple
blocks of buses in Hylan Bv. The Staten Island Railroad is rather too
far inland.
Most of the suitable sites are close to shops, eateries, shelter,
restrooms. You should scout these out before hand since each site has
a unique geography. Mind that many facilities close by late night.
Weather
-----
If you look in the early night, before midnight, the only month
that puts the galaxy and cluster on the meridian in those hours is
May. May in New York is a transition month when weather can be from
cool and dry all the way to torrid and humid.
May does bring really transparent skies for the City, allowing for
the newer second season for spotting the Milky Way. The season
complements the autumn season in September thru November. The spring
viewing window is in May and June.
Even if the day is warm, expect cooler air at night, specially on
the seashore. Bring a jacket or sweater. A bottle of fruit or
vegetable juice is hwlpful, too.
If the weather is whacking around, you better be ready for rain. A
brief shower can ruin your starbrowsing with soaked equipment, ground,
furniture, and you. Have plastic bags to stuff your gear into. Bring a
rain shedder or poncho.
Instruments
---------
Only small scopes that can be packed into rolling luggage or a
backpack are needed, Bring binoculars to scout around the target area,
maybe actually see the cluster, and do general starbrowsing.
A go-to scope is very handy but hardly at all necessary. You're
better off with an alt-azimuth scope that you manually operate. Make
sure you're focused on a recognizable star before hunting down the
deepsky targets. You may miss them for being out of focus! A nebulous
object is all the more nebulous if not focused.
Low to medium power with ample field of view is a must. You won't
need or apply high magnification on either target. Make sure your
eyepieces are clean.
You need a pocket lamp. On the darkest of City nights, there is
not enough downlight from the sky to read by. It's really tough to
squint and strain at your map or equipment markings. Wrap the lamp
lens in red cloth or tissue to temper its glare.
Starhopping
---------
The path to omega Centauri and Enigmatic Galaxy is surprisingly
direct from Spica. Drop due south from Spica to gamma Hydrae. This
star seems to have no widely used proper name. From gamma, drop
farther south to iota Centauri. If you end up at Menkent, theta
Centauri, in the stead, slide west to iota to regain your path.
Continue straight south from iota into NGC5128, the galaxy, or a
bit more, almost on the horizon, to NGC5139. In binoculars you will
not spot the galaxy and procede to the cluster. From the City I've
seen omega as a soft fuzzball, definitely nonstellar, so it should be
recognizable when you walk by it.
The galaxy is found in the scope after centering on the cluster.
Nudge the scope four degrees straight north (up, because the targets
are at the meridian). You should hit a soft misty spot, round with
even glow thruout. That's it.
Transparency
----------
The clarity of the air near the south horizon is the key to
success. What's going on overhead doesn't matter. It can in New York
be amazingly dark in the zenith, to a transparency of 5th magnitude!,
yet hardly anything can be seen along the horizon.
The area south of Spica has only dim stars. If you do see a
sprinkling of stars under Spica, you have the chance for the
challenge. A blank area warns of almost certain failure.
The air after a cold front blows thru can be remarkably clear and
dark. This is the time to check for the Milky Way! The summer Milky
Way isn't high enough in premidnight hours. You have to look again in
the owl hours, with Deneb well up in the northeast.
Other targets
-----------
There is a thick and deep legacy of stargazing litterature that
cuts off about 10 to 15 degrees above the southern horizon in the
middle zone of the United States. This is a historical glitch dating
to the colonial era of America. Astronomy in the United States derived
from England, 10-15 degrees farther north.
Stargazing books printed there, and brought to the US, naturally
reflected the horizon of typicly London at latitude 51 degrees north.
New York is at 41 degrees north, so the legacy books have a vacant
kerf extending 10 degrees above the southern horizon.
Until the late 20th century native American stargazing books more
or less copied the English model. It wasn't until American astronomers
took to overseas travels, like for solar eclipses, that American
authors covered the southern skies.
As long as you're in the area south of Spica, check out other
targets. A few bordering the meridian of Spica and omega Centauri are
tabulated below.
Double stars
--------------------------------------------------------------
Designatn RA2000 DC2000 Cns MagA MagB Colors Sep PA Year
--------- -------- ------ --- ---- ---- ------- ----- --- ----
N 11 32.3 -29 16 Hya 5.7 5.8 9.3 1967
delta 12 29.9 -16 31 Crv 3.0 9.2 whi 24 214 --
gamma 12 41.7 -01 27 Vir 3.5 3.5 y-w y-w 1.4 260 2000
theta 13 09.9 -05 36 Vir 4.4 9.4 whi 7.1 343 --
3 13 51.8 -33 00 Cen 4.6 6.1 7.9 108 1975
4 13 53.2 -31 56 Cen 4.8 8.4 15 185 --
tau1 14 26.1 -45 13 Lup 4.6 9.3 148 204 --
54 14 46.0 -25 27 Hya 5.1 7.1 8.6 126 1975
mu 14 49.3 -14 09 Lib 5.8 6.7 whi 1.8 355 --
alp1-alp2 14 50.9 -16 02 Lib 2.8 5.2 234 314 1924
pi 15 05.1 -47 03 Lup 4.6 4.7 b-w 1.4 73 --
kap1-kap2 15 11.9 -48 44 Lup 3.9 5.7 b-w whi 27 144 1968
iota 15 12.2 -19 47 Lib 5.1 9.4 58 111 --
mu AC 15 18.5 -47 53 Lup 5.1 7.2 b-w 24 130 --
mu AB 15 18.5 -47 53 Lup 5.1 5.2 b-w 1.2 142 1965
--------------------------------------------------------------
gamma Virginis in the 20-thous rounded its periastron and was
inseparable for a few years. By 2009 the pair parted enough to be seen
as two stars in small scopes.
Galaxies
------------------------------------------------------------
Designatn RA2000 DC2000 Cns Size Magn Other name
--------- ---------- --------- --- --------- ---- ----------
NGC 4594 12 39 59.3 -11 37 23 Vir 8.9'X4.1' 8.3 M104
NGC 4697 12 48 35.8 -05 48 02 Vir 6.0'X3.8' 9.3
NGC 4699 12 49 02.2 -08 39 52 Vir 3.5'X2.7' 9.6
NGC 4753 12 52 22.1 -01 12 02 Vir 5.4'X2.9' 9.9
NGC 5128 13 25 27.8 -43 01 06 Cen 27'X20' 7.0 Enigmatic Galaxy
NGC 5236 13 37 00.2 -29 52 04 Hya 11'X10' 8.5 M83
-----------------------------------------------------
Globular clusters
--------------------------------------------------------
Designatn RA2000 DC2000 Cns Size Magn Other name
--------- ---------- --------- --- ----- ---- ----------
NGC 4590 12 39 28.0 -26 44 34 Hya 12.0' 7.7 M68
NGC 5139 13 26 45.9 -47 28 37 Cen 36.3' 3.5 omega Centauri
NGC 5824 15 03 58.4 -33 04 04 Lup 6.2' 7.8
NGC 5897 15 17 24.5 -21 00 37 Lib 12.6' 8.6
---------------------------------------------
Some of these are a greater challenge to spot than omega Centauri
or Enigmatic Galaxy! There are no good globular nebulae, open nebulae,
or open clusters in this part of the sky.
Conclusion
--------
Observing the stars from New York is often dismissed as a fool's
pursuit. It's deemed better that the novice astronomer take up some
other hobby or make long trips to properly dark skies. As asinine as
this assertion is, it is sometimes applied by inattentive authors.
It is a bit of challenge to spot deepsky targets within the City,
or in the surrounds out to many tens of kilometers. Yet with care and
sensible preparation, galaxies can be enjoyed here. Routinely from
Central Park, about a dozen of the galaxies in the Virgo cluster can
be shown to the public, as well as the M81-82 pair and a couple in
Coma and Canes Venatici.
Because the two challenge targets are actually bright they could
be included in a City astronomer's stargazing. The view they offer is
no worse than that of other targets in high sky.
On the other hand, the impediments of horizon observing will
likely result in repeated failures before a good hit is secured.
Try it!