Messier 100

Article written: 5 Jan , 2010
Updated: 24 Dec , 2015
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Object Name: Messier 100
Alternative Designations: M100, NGC 4321
Object Type: Type Sc Spiral Galaxy
Constellation: Coma Berenices
Right Ascension: 12 : 22.9 (h:m)
Declination: +15 : 49 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 9.3 (mag)
Apparent Dimension: 7×6 (arc min)


Locating Messier 100: As part of the Virgo Cluster of Galaxies, M100 is best found by returning to our “galaxy hopping” ways we’ve learned. Begin with the bright M84/84 pairing located in the heavily populated inner core of the Virgo Cluster of galaxies about halfway between Epsilon Virginis and Beta Leonis. Once identified, stay at the eyepiece a move your telescope north until you locate M99 and continue at least 3 or 4 more eyepiece fields. This is what is known as “sweeping”. When you reach a star pattern you are certain that you can identify, shift the telescope one eyepiece field to the east and continue northward for several eyepiece fields. If you have not seen the fairly large round patch of M100, continue the process carefully one eyepiece field at a time. (Not all eyepieces have the same apparent field of view, but use your lowest magnification.) M100 is face-on in presentation, so it will be a round of nebulousity that requires dark, clear skies and can be spotted with binoculars.

What You Are Looking At: M100 is a spiral galaxy, very similar to our own Milky Way. The galaxy has two distinct arms of young, hot and massive stars which show photographically as bright blue stars. These stars have formed recently from interactions with neighboring galaxies, but in a slightly odd way. “The total H I distribution is mostly confined to the radius of the optical disk, but a large though faint extension is seen in the H I data at 45” resolution on the SW side of the disk. NGC 4321 is asymmetric in H I and may be called “lopsided.” We have derived a rotation curve which agrees fairly well with what was previously published but shows more detail due to the higher resolution of our new observations. The rotation curve does not decline within the radius of the disk, but important differences are seen between the behavior of the approaching and the receding sides.” says Johan H. Knapen (et al), “These differences are caused by deviations from circular motions in the outer disk that are probably due to a close passage of the companion galaxy NGC 4322, which may also be the cause of the observed asymmetry in the total H I distribution. Deviations from circular motion due to density wave streaming are seen in the inner disk. From skewing of the velocity contours in the central part of NGC 4321, the presence of a nonaxisymmetric potential is deduced. Near-infrared and H? images indicate that a bar is indeed present in this galaxy. The deviations from circular motions seen in the velocity field can be identified with gas streaming around the bar in elongated orbits, in broad agreement with theoretical predictions.”

As one of Lord Rosse’s original 14 “spiral nebula”, Messier 100 seems to employ a perfect spiral shape – one that seems to lack a central bar structure. “We analyse new integral-field spectroscopy of the inner region (central 2.5 kpc) of the spiral galaxy NGC 4321 to study the peculiar kinematics of this region. Fourier analysis of the velocity residuals obtained by subtracting an axisymmetric rotation model from the H? velocity field indicates that the distortions are global features generated by an m= 2 perturbation of the gravitational potential which can be explained by the nuclear bar.” says A. Castillo-Morales (et al). “This bar has been previously observed in the near-infrared but not in the optical continuum dominated by star formation. We detect the optical counterpart of this bar in the 2D distribution of the old stellar population (inferred from the equivalent width map of the stellar absorption lines). We apply the Tremaine–Weinberg method to the stellar velocity field to calculate the pattern speed of the inner bar, obtaining a value of ?b= 160 ± 70 km s?1 kpc?1 . This value is considerably larger than the one obtained when a simple bar model is considered. However, the uncertainties in the pattern speed determination prevent us from giving support to alternative scenarios.”

To study M100 is to take a look back into its growth and history… a history that apparently isn’t “going quietly into that good night”. Astronomers are still able to observe the remains of a star which exploded in 1979 – still shining as brightly in X-rays now as when it was first observed. This in itself is unusual because most supernova events fade fairly quickly in a period of just a few months. Dr. Stefan Immler at NASA’s Goddard Space Flight Center in Greenbelt, Md., led this observation using the European Space Agency’s XMM-Newton observatory. The star explosion (supernova), called SN 1979C, shows no sign of letting up, he said. By observing with the XMM-Newton optical/UV image of the galaxy M100 and supernova SN 1979C obtained with the Optical Monitor in the B, U, and UVW1 filters we’ve taken one of our deepest looks ever. The position of SN 1979C is marked by a white circle. The streak across the image is from an artifact caused by a dead detector column. The scale bar is 2 arc min, corresponding to 32,600 light years.

“This 25-year-old candle in the night has allowed us to study aspects of a star explosion never before seen in such detail,” Immler said. “All the important information that usually fades away in a couple of months is still there.” Among the many unique finds, Immler said, is the history of the star’s stellar wind dating back 16,000 years before the explosion. Such a history is not even known about our Sun. Also, the scientists could measure the density of the material around the star, another first. The lingering mystery, though, is how this star could fade away in visible light yet remain so radiant in X-rays. The results appear in The Astrophysical Journal. How is this accomplished? Through a composite XMM-Newton X-ray image of the galaxy M100 in soft (0.3-1.5 keV, red), medium (1.5-4 keV, green) and hard (4-10 keV, blue) X-rays. The image shows large amounts of diffuse X-ray emission from hot gas in the galaxy (red), various point-like X-ray sources and supernova SN 1979C south-east of the nucleus of M100 (marked by a white line). “We can use the X-ray light from SN 1979C as a ‘time machine’ to study the life of a dead star long before it exploded,” Immler said.

History: Messier 100 was originally discovered by Pierre Mechain on March 15, 1781. It was later confirmed and cataloged by Charles Messier on April 13, 1781 who wrote in his notes: “Nebula without star, of the same light as the preceding [M99], situated in the ear of Virgo. Seen by M. Mechain on March 15, 1781. The three nebulae, nos. 98, 99 & 100, are very difficult to recognize, because of the faintness of their light: one can observe them only in good weather, and near their passage of the Meridian.”

It would be observed and cataloged by both Herschels, but it was Admiral Smyth who described it the best: “A round nebula, pearly white, off the upper part of the Virgin’s left wing, and certainly at a great distance from Virgo’s ear of corn, where the Connaissance des Temps places it [actually Messier’s position is quite close]: indeed, the true site will be hit upon just one-fifth the way from Beta Leonis towards Arcturus. This is a large but pale objects, of little character, though it brightens from its attenuated edgestowards the centre; and is therefore proved to be globular. It was discovered by M. Méchain in 1781, and is accompanied by four small stars, at a little distance around it; besides minute points of light in the field, seen by occasional gleams.

We are now in the broad grand stratum of nebulae, which lies in a direction almost perpendicular to the Galaxy [Milky Way], and passes from the south, through Virgo, Berenices Hair, Canes Venatici, and te Great Bear, to the Pole, and beyond. This glorious but mysterious zone of diffused spots, is an indisputable memorial to all future times, of the unwearied industry and indomitable scientific energy of Sir William Herschel. Yet has this unrivaled contributor to knowledge been disparagingly described, as a man indulging in “speculations of no great value to astronomy, rather than engage in computations by which the science can really be benefited.” Save the mark! This is said of a philosopher of zeal and application hitherto unequaled: one whose contributions to the Philosophical Transactions prove the bold but circumspect grandeur of his conceptions, his consummate mechanical resources, and the exactness of his elaborate calculations. Herschel’s labor, however, transcended those of the ages in which he was cast, although he gave such animation and bias to sidereal astronomy that his mantle was caught at.”

May you, too, “save the mark”!

Top M100 image credit, Palomar Observatory courtesy of Caltech, M100 Hubble Image, Issac Newton Telescope True-color image of M100, M100 XMM Newton Images and M100 image courtesy of N.A.Sharp/NOAO/AURA/NSF.


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