Messier 41 – the NGC 2287 Open Star Cluster

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at the double star known as Messier 41. Enjoy!

During the 18th century, famed French astronomer Charles Messier noted the presence of several “nebulous objects” in the night sky. Having originally mistaken them for comets, he began compiling a list of them so that others would not make the same mistake he did. In time, this list (known as the Messier Catalog) would come to include 100 of the most fabulous objects in the night sky.

One of these objects is the open star cluster known as Messier 41 (aka. M41, NGC 2287). Located in the Canis Major constellation – approximately 4,300 light years from Earth – this cluster lies just four degrees south of Sirius, the brightest star in the night sky. Like most open clusters, it is relatively young – 190 million years old – and contains over 100 stars in a region measuring 25 to 26 light years in diameter.

Description:

Running away from us at a speed of about 34 kilometers per second, this field of about 100 stars measures about 25 light years across. Born about 240 million years ago, it resides in space approximately 2300 light years away from our solar system. Larger aperture telescopes will reveal the presence of many red (or orange) giant stars and the hottest star in this group is a spectral type A.

View of the night sky in North Carolina, showing the constellations of Orion, Hyades, Canis Major and Canis Minor. Credit: NASA

As G.L.H. Harris (et al) explained in a 1993 study:

“We have obtained photoelectric UBV photometry for 100 stars, uvbyb photometry for 39 stars and MK spectral types for 80 stars in the field of NGC 2287. After combination with data from other sources, several interesting cluster properties are apparent. Both the UBV and uvbyb photometry point to a small but nonzero reddening, while our spectral types confirm previous results indicating a high binary frequency for the cluster. Based on our spectral and photometric data for the cluster members, we find a minimum binary frequency of 40% and discuss the possibility that the results may imply a binary frequency closer to 80%. The cluster age is found to be based on both the main-sequence turnoff and the red giant distribution; the width of the turn up region can probably be explained by a combination of duplicity and a range in stellar rotation.”

But there’s more than just red giant stars and various spectral types to be found hiding in Messier 41. There’s at least two white dwarf stars, too. As P.D Dobbie explained in a 2009 study:

“[W]e use our estimates of their cooling times together with the cluster ages to constrain the lifetimes and masses of their progenitor stars. We examine the location of these objects in initial mass-final mass space and find that they now provide no evidence for substantial scatter in initial mass-final mass relation (IFMR) as suggested by previous investigations. This form is generally consistent with the predictions of stellar evolutionary models and can aid population synthesis models in reproducing the relatively sharp drop observed at the high mass end of the main peak in the mass distribution of white dwarfs.”

Messier 41 and Collinder 121. Image: Wikisky

As you view Messier 41, you’ll be impressed with its wide open appearance… and knowing it’s simply what happens to star clusters as they get passed around our galaxy. As Giles Bergond (et al.) stated in their 2001 study:

“Taking into account observational biases, namely the galaxy clustering and differential extinction in the Galaxy, we have associated these stellar overdensities with real open cluster structures stretched by the galactic gravitational field. As predicted by theory and simulations, and despite observational limitations, we detected a general elongated (prolate) shape in a direction parallel to the galactic Plane, combined with tidal tails extended perpendicularly to it. This geometry is due both to the static galactic tidal field and the heating up of the stellar system when crossing the Disk. The time varying tidal field will deeply affect the cluster dynamical evolution, and we emphasize the importance of adiabatic heating during the Disk-shocking. During the 10-20 Z-oscillations experienced by a cluster before its dissolution in the Galaxy, crossings through the galactic Disk contribute to at least 15% of the total mass loss. Using recent age estimations published for open clusters, we find a destruction time-scale of about 600 million years for clusters in the solar neighborhood.”

That means we’ve only got another 360 million years to observe it before it’s completely gone (though some estimates place it at about 500 million). Either way, this star cluster is destined to disappear, perhaps before we are!

History of Observation:

Messier 41 was “possibly” recorded by Aristotle about 325 B.C. as a patch in the Milky Way… quite understandable since it is very much within unaided eye visibility from a dark sky location. Said Aristotle:

“.. some of the fixed stars have tails. And for this we need not rely only on the evidence of the Egyptians who say they have observed it; we have observed it also ourselves. For one of the stars in the thigh of the Dog had a tail, though a dim one: if you looked hard at it the light used to become dim, but to less intent glance it was brighter.”

Messier 41 and Sirius. Image: Wikisky

However, Giovanni Batista Hodierna was the first to catalog it in 1654, and the star cluster became a bit more astronomically known when John Flamsteed independently found it again on February 16, 1702. Doing his duty, Charles Messier also logged it:

“In the night of January 16 to 17, 1765, I have observed below Sirius and near the star Rho of Canis Major a star cluster; when examining it with a night refractor, this cluster appeared nebulous; instead, there is nothing but a cluster of small stars. I have compared the middle with the nearest known star; and I found its right ascension of 98d 58′ 12″, and its declination 20d 33′ 50″ north.”

Following suit, other historical astronomers also observed M41 – including Sir John Herschel to include it in the NGC catalog. While none found it particularly thrilling… their notes range from a “coarse collection of stars” to “very large, bright, little compressed”, perhaps you will feel much differently about this easy, bright target!

Locating Messier 41:

Finding Messier 41 isn’t very difficult for binoculars and small telescopes – all you have to know is the brightest star in the northern hemisphere, Sirius, and south! Simply aim your optics at Sirius and move due south approximately four degrees. That’s about one standard field of view for binoculars, about one field of view for the average telescope finderscope and about 6 fields of view for the average wide field, low power eyepiece.

The location of Messier 41 in the Canis Major constellation. Credit: IAU and Sky & Telescope magazine/Roger Sinnott & Rick Fienberg

Because Messier 41 is a large star cluster, remember to use lowest magnification to get the best effect. Higher magnification can always be used once the star cluster is identified to study individual members. M41 is quite bright and easily resolved and makes a wonderful target for urban skies and moonlit nights!

Because you understand what’s there…

Object Name: Messier 41
Alternative Designations: M41, NGC 2287
Object Type: Open Galactic Star Cluster
Constellation: Canis Major
Right Ascension: 06 : 46.0 (h:m)
Declination: -20 : 44 (deg:m)
Distance: 2.3 (kly)
Visual Brightness: 4.5 (mag)
Apparent Dimension: 38.0 (arc min)

We have written many interesting articles about Messier Objects here at Universe Today. Here’s Tammy Plotner’s Introduction to the Messier Objects, , M1 – The Crab Nebula, M8 – The Lagoon Nebula, and David Dickison’s articles on the 2013 and 2014 Messier Marathons.

Be to sure to check out our complete Messier Catalog. And for more information, check out the SEDS Messier Database.

Sources:

Weekly SkyWatcher’s Forecast: February 19-25, 2012

[/caption]

Greetings, fellow SkyWatchers! It’s going to be an awesome week as we watch the planets – Mars, Saturn, Jupiter, Venus and Mercury – dance along the ecliptic plane. You don’t even need a telescope for this show! But that’s not all. We’ll take a look at a wealth of bright star clusters, challenging studies and lots more. I’ll see you in the back yard…

Sunday, February 19 – Today is the birthday of Nicolas Copernicus. Born in 1473, he was the creator of the modern solar system model which illustrated the retrograde motion of the outer planets. Considering this was well over 530 years ago, and in a rather “unenlightened” time, his revolutionary thinking about what we now consider natural is astounding.

Have you been observing retrograde motion while keeping track of Mars? Good for you! You may have also noticed that Mars has dimmed slightly over the last few weeks. Right now it’s around -1.0. Keep track of its many faces!

While we still have dark skies on our side, let’s head for a handful of difficult nebulae in a region just west of Gamma Monocerotis. For binoculars, check out the region around Gamma, it is rich in stars and very colorful! You are looking at the very outer edge of the Orion spiral arm of our galaxy. For small scopes, have a look at Gamma itself – it’s a triple system that we’ll be back to study. For larger scopes? It’s Herschel hunting time…

NGC 2183 (Right Ascension: 6 : 10.8 – Declination: -06 : 13 ) and NGC 2185 (Right Ascension: 6 : 11.1 – Declination: -06 : 13 ) will be the first you encounter as you move west of Gamma. Although they are faint, just remember they are nothing more than a cloud of dust illuminated by faint stars on the edge of the galactic realm. The stars that formed inside provided the light source for these wispy objects and at their edges lay in intergalactic space.

To the southwest is the weaker NGC 2182 (Right Ascension: 6 : 09.5 – Declination: -06 : 20), which will appear as nothing more than a faint star with an even fainter halo about it, with NGC 2170 (Right Ascension: 6 : 07.5 – Declination: -06 : 24) more strongly represented in an otherwise difficult field. While the views of these objects might seem vaguely disappointing, you must remember that not everything is as bright and colorful as seen in a photograph. Just knowing that you are looking at the collapse of a giant molecular cloud that’s 2400 light-years away is pretty impressive!

Monday, February 20 – Today in history celebrates the Mir space station launch in 1986. Mir (Russian for “peace”) was home to both cosmonauts and astronauts as it housed 28 long duration crews during its 15 years of service. To date it is one of the longest running space stations and a triumph for mankind. Spasiba! Today in 1962, John Glenn was onboard Friendship 7 and became the first American to orbit the Earth. As Colonel Glenn looked out the window, he reported seeing “fireflies” glittering outside his Mercury space capsule. Let’s see if we can find some…

The open cluster M41 (Right Ascension: 6 : 46.0 – Declination: -20 : 44) in Canis Major is just a quick drift south of the brightest star in the northern sky – Sirius. Even the smallest scopes and binoculars will reveal this rich group of mixed magnitude stars and fill the imagination with strange notions of reality. Through larger scopes, many faint groupings emerge as the star count rises to well over 100 members. Several stars of color – orange in particular – are also seen along with a number of doubles.

First noted telescopically by Giovanni Batista Hodierna in the mid-1500s, ancient texts indicate that Aristotle saw this naked-eye cluster some 1800 years earlier. Like other Hodierna discoveries, M41 was included on Messier’s list – along with even brighter clusters of antiquity such as Praesepe in Cancer and the Pleiades in Taurus. Open cluster M41 is located 2300 light years away and recedes from us at 34km/sec – about the speed Venus moves around the Sun. M41 is a mature cluster, around 200 million years old and 25 light years in diameter. Remember M41… Fireflies in night skies.

Tuesday, February 21 – Tonight is New Moon! Tonight let’s take a journey just a breath above Zeta Tauri and spend some quality time with a pulsar embedded in the most famous supernova remnant of all. Factually, we know the Crab Nebula to be the remains of an exploded star recorded by the Chinese in 1054. We know it to be a rapid expanding cloud of gas moving outward at a rate of 1,000 km per second, just as we understand there is a pulsar in the center. We also know it as first recorded by John Bevis in 1758, and then later cataloged as the beginning Messier object – penned by Charles himself some 27 years later to avoid confusion while searching for comets. We see it revealed beautifully in timed exposure photographs, its glory captured forever through the eye of the camera — but have you ever really taken the time to truly study M1 (Right Ascension: 5 : 34.5 – Declination: +22 : 01)? Then you just may surprise yourself…

In a small telescope, M1 might seem to be a disappointment – but do not just glance at it and move on. There is a very strange quality to the light which reaches your eye, even though initially it may just appear as a vague, misty patch. Allow your eyes to adjust and M1 will appear to have “living” qualities – a sense of movement in something that should be motionless. The “Crab” holds true to many other spectroscopic studies. The concept of differing light waves crossing over one another and canceling each other out – with each trough and crest revealing differing details to the eye – is never more apparent than during study. To observe M1 is to at one moment see a “cloud” of nebulosity, the next a broad ribbon or filament, and at another a dark patch. When skies are stable you may see an embedded star, and it is possible to see six such stars.

Many observers have the ability to see spectral qualities, but they need to be developed. From ionization to polarization – our eye and brain are capable of seeing to the edge of infra-red and ultra-violet. Even a novice can see the effects of magnetism in the solar “Wilson Effect.” But what of the spinning neutron star at M1’s heart? We’ve known since 1969 that M1 produces a “visual” pulsar effect. About once every five minutes, changes occurring in the neutron star’s pulsation affect the amount of polarization, causing the light waves to sweep around like a giant “cosmic lighthouse” and flash across our eyes. M1 is much more than just another Messier. Capture it tonight!!

Wednesday, February 22 – Today in 1966, Soviet space mission Kosmos 110 was launched. Its crew was canine, Veterok (Little Wind) Ugolyok (Little Piece of Coal); both history making dogs. The flight lasted 22 days and held the record for living creatures in orbit until 1974 – when Skylab 2 carried its three-man crew for 28 days.

Since we’ve studied the “death” of a star, why not take the time tonight to discover the “birth” of one? Our journey will start by identifying Aldeberan (Alpha Tauri) and move northwest to bright Epsilon. Hop 1.8 degrees west and slightly to the north for an incredibly unusual variable star – T Tauri.

Discovered by J.R. Hind in October 1852, T Tauri and its accompanying nebula, NGC 1555 (Right Ascension: 4 : 22.9 – Declination: +19 : 32), set the stage for discovery with a pre-main sequence variable star. Hind reported the nebula, but also noted that no catalog listed such an object in that position. His observations also included a 10th magnitude uncharted star and he surmised that the star in question was a variable. On each count Hind was right, and both were followed by astronomers for several years until they began to fade in 1861. By 1868, neither could be seen and it wasn’t until 1890 that the pair was re-discovered by E.E. Barnard and S.W. Burnham. Five years later? They vanished again.

T Tauri is the prototype of this particular class of variable stars and is itself totally unpredictable. In a period as short as a few weeks, it might move from magnitude 9 to 13 and other times remain constant for months on end. It is about equal to our own Sun in temperature and mass – and its spectral signature is very similar to Sol’s chromosphere – but the resemblance ends there. T Tauri is a star in the initial stages of birth!

T Tauri are all pre-main sequence and are considered “proto-stars”. In other words, they continuously contract and expand, shedding some of their mantle of gas and dust. This gas and dust is caught by the star’s rotation and spun into an accretion disc – which might be more properly referred to as a proto-planetary disc. By the time the jets have finished spewing and the material is pulled back to the star by gravity, the proto-star will have cooled enough to have reached main sequence and the pressure may have allowed planetoids to form from the accreted material.

Thursday, February 23 – If you have an open western horizon, then be out at twilight! Right now the speedy inner planet – Mercury – will make a brief appearance. Depending on your time zone, you might also spot a very young Moon just above it! For curiosity seekers, you can also find asteroid Vesta to the south of the Moon, along with planet Uranus to the south-east. How cool is that?!

In 1987, Ian Shelton made an astonishing visual discovery – SN 1987a. This was the brightest supernova in 383 years. More importantly, before it occurred, a blue star of roughly 20 solar masses was already known to exist in that same location within the Large Magellanic Cloud. Catalogued as Sanduleak -69-202, that star is now gone. With available data on the star, astronomers were able to get a “before and after” look at one of the most extraordinary events in the universe! Tonight, let’s have a look at a similar event known as “Tycho’s Supernova.”

Located northwest of Kappa Cassiopeia, SN1572 appeared so bright in that year that it could be seen with the unaided eye for six months. Since its appearance was contrary to Ptolemaic theory, this change in the night sky now supported Copernicus’ views and heliocentric theory gained credence. We now recognize it as a strong radio source, but can it still be seen? There is a remnant left of this supernova, and it is challenging even with a large telescope. Look for thin, faint filaments that form an incomplete ring around 8 arc minutes across.

Friday, February 24 – Tonight the slender first crescent of the Moon makes its presence known on the western horizon. Before it sets, take a moment to look at it with binoculars. The beginnings of Mare Crisium will show to the northeast quadrant, but look just a bit further south for the dark, irregular blotch of Mare Undarum – the Sea of Waves. On its southern edge, and to lunar east, look for the small Mare Smythii – the “Sea of Sir William Henry Smyth.” Further south of this pair and at the northern edge of Fecunditatis is Mare Spumans – the “Foaming Sea.” All three of these are elevated lakes of aluminous basalt belonging to the Crisium basin.

For telescope users, wait until the Moon has set and return to Beta Monocerotis and head about a fingerwidth northeast for an open cluster challenge – NGC 2250 (Right Ascension: 6 : 32.8 – Declination: -05 : 02). This vague collection of stars presents itself to the average telescope as about 10 or so members that form no real asterism and makes one wonder if it is indeed a cluster. So odd is this one, that a lot of star charts don’t even list it!

Today in 1968, during a radar search survey, the first pulsar was discovered by Jocelyn Bell. The co-directors of the project, Antony Hewish and Martin Ryle, matched these observations to a model of a rotating neutron star, winning them the 1974 Physics Nobel Prize and proving a theory of J. Robert Oppenheimer from 30 years earlier.

Would you like to get a look at a region of the sky that contains a pulsar? Then wait until the Moon has well westered and look for guidestar Alpha Monocerotis to the south and bright Procyon to its north. By using the distance between these two stars as the base of an imaginary triangle, you’ll find pulsar PSR 0820+02 at the apex of your triangle pointed east.

Saturday, February 25 – As the Moon begins its westward journey after sunset in a position much easier to observe. The lunar feature we are looking for is at the north-northeast of the lunar limb and its view is often dependent on libration. What are we seeking? “The Sea of Alexander von Humboldt”…

Mare Humboldtianum can sometimes be hidden from view because it is an extreme feature. Spanning 273 kilometers, the basin in which it is contained extends for an additional 600 kilometers and continues around to the far side of the Moon. The mountain ranges which accompany this basin can sometimes be glimpsed under perfect lighting conditions, but ordinarily are just seen as a lighter area. The mare was formed by lava flow into the impact basin, yet more recent strikes have scarred Humboldtianum. Look for a splash of ejecta from crater Hayn further north, and the huge, 200 kilometer strike of crater Bel’kovich on Humboldtianum’s northeast shore.

When the Moon begins to wester, let’s head for Beta Monocerotis and hop about 3 fingerwidths east for an 8.9 magnitude open cluster that can be spotted with binoculars and is well resolved with a small telescope – NGC 2302 (Right Ascension: 6 : 51.9 – Declination: -07 : 04). This very young stellar cluster resides at the outer edge of the Orion spiral arm. While binoculars will see a handful of stars in a small V-shaped pattern, telescope users should be able to resolve 40 or so fainter members.

Until next week, may all of your journeys be at light speed!

If you enjoy the weekly observing column, then you’ll love the book, The Night Sky Companion 2012 written by Tammy Plotner. This fully illustrated observing guide includes star charts for your favorite objects and much more!