Object Name: Messier 95
Alternative Designations: M95, NGC 3351
Object Type: Type SBb Barred Spiral Galaxy
Right Ascension: 10 : 44.0 (h:m)
Declination: +11 : 42 (deg:m)
Distance: 38000 (kly)
Visual Brightness: 9.7 (mag)
Apparent Dimension: 4.4×3.3 (arc min)
Locating Messier 95: M95 is the southernmost in the widefield eyepiece pairing of galaxies which includes M96. With good sky conditons, both M95 and M96 are easy to locate in the belly of the constellation of Leo. Begin by identifying Alpha (Regulus), the brightest, southernmost star in the backwards question mark asterism. Now, look about a fistwidth west where you will see the shallow triangle asterism which marks Leo’s hips. The westernmost of these stars (Theta) is your next marker. Look between the two markers for a faint star in an almost central position. If the skies are right to see this galactic pair, you will also see another star just south of your last marker. M95 and M96 are between these last two stars. The pair can just barely be seen in larger binoculars and although they are faint, perceivable in a small telescope. Larger aperture will bring out far more details. Because these are fainter galaxies, the require a dark sky location and cannot tolerate background glow, such as moonlit nights.
What You Are Looking At: Located about 38 million light years away, M95 was one of the galaxies in the key project of the Hubble Space Telescope for the determination of the Hubble constant: the HST was employed to look for Cepheid variable stars and thereby determine this galaxy’s distance. “To empirically calibrate the IR surface brightness fluctuation (SBF) distance scale and probe the properties of unresolved stellar populations, we measured fluctuations in 65 galaxies using NICMOS on the Hubble Space Telescope. The early-type galaxies in this sample include elliptical and S0 galaxies and spiral bulges in a variety of environments. Absolute fluctuation magnitudes in the F160W (1.6 ?m) filter (MF160W) were derived for each galaxy using previously measured I-band SBF and Cepheid variable star distances. F160W SBFs can be used to measure distances to early-type galaxies with a relative accuracy of ~10%, provided that the galaxy color is known to ~0.035 mag or better. Near-IR fluctuations can also reveal the properties of the most luminous stellar populations in galaxies.” says Joseph Jensen (et al).
“Comparison of F160W fluctuation magnitudes and optical colors to stellar population model predictions suggests that bluer elliptical and S0 galaxies have significantly younger populations than redder ones and may also be more metal-rich. There are no galaxies in this sample with fluctuation magnitudes consistent with old, metal-poor (t>5 Gyr, [Fe/H]<-0.7) stellar population models. Composite stellar population models imply that bright fluctuations in the bluer galaxies may be the result of an episode of recent star formation in a fraction of the total mass of a galaxy. Age estimates from the F160W fluctuation magnitudes are consistent with those measured using the H? Balmer-line index. The two types of measurements make use of completely different techniques and are sensitive to stars in different evolutionary phases. Both techniques reveal the presence of intermediate-age stars in the early-type galaxies of this sample." One of the most beautiful aspects of M95 is it’s bright core, but what goes on inside? “A high-resolution Hubble Space Telescope WFPC2 F218W UV image of the barred spiral NGC 4303 (classified as a LINER-type active galactic nucleus [AGN]) reveals for the first time the existence of a nuclear spiral structure of massive star-forming regions all the way down to the UV-bright unresolved core (size <= 8 pc) of an active galaxy. The spiral structure, as traced by the UV-bright star-forming regions, has an outer radius of 225 pc and widens as the distance from the core increases. The UV luminosity of NGC 4303 is dominated by the massive star-forming regions, and the unresolved LINER-type core contributes only 16% of the integrated UV luminosity. The nature of the UV-bright LINER-type core---stellar cluster or pure AGN---is still unknown." says Luis Colina (et al). “In contrast to NGC 4303, the UV F218W image of the non-AGN barred galaxy NGC 3351 shows a nuclear star-forming ring of 315 pc (semimajor axis) with a faint core. In the ring, the star formation is arranged in clumps of about 60–85 pc in diameter. Each clump consists of a few compact UV-bright clusters embedded in a more diffuse component. The integrated IUE spectrum of NGC 3351 shows the presence of Si IV 1400 A and C IV 1550 A absorption lines, typical features of young, 4–5 Myr old, massive star clusters. The presence of ring and spiral star-forming structures in the nuclear regions of these two barred spirals supports the bar-induced gas-fueling scenario by which bars accumulate gas in the nuclear regions of galaxies, produce nuclear star-forming rings (NGC 3351), and might eventually generate or feed an AGN (NGC 4303).”
History: This pretty galaxy was first discovered by Pierre Mechain in 1781 and cataloged by Charles Messier 4 days later on March 24, 1781. He writes: “Nebula without star, in the Lion [Leo], above star l (53 Leonis): its light is very faint.” On March 11, 1784, Sir William Herschel would also note it: “A fine, bright nebula, much brighter in the middle than at the extremes, of a pretty considerable extent, perhaps 3 or 4′ or more. The middle seems to be of the magnitude of 3 or 4 stars joined together, but not exactly round; from the brightest part of it there is a sudden transition to the nebulous part, so that I should call it cometic.”
It would nearly 100 years later when Admiral Smyth would most aptly decribe M95 as: “A lucid white nebula, on the lion’s ribs, with only two small stars, np [north preceding, NW] and nf [north following, NE], in the field. Its place is almost due east of Regulus, with a distance of 9 deg, where it forms the southern vertex of a triangle nearly equilateral with Gamma and Delta Leonis. This nebula is round and bright, and perhaps better defined on the southern than on the northern limb, a phenomenon worthy to remark, and observable in the great nebula of Andromeda [M31], and other wonderful masses. It was discovered by Mechain in 1781, and registered by Messier as a “feeble nebula, without a star.” Nearly a degree to the eastward of this object, follows another round but not equally well defined nebula, large, and of a pale white colour. It is Messier’s No. 96, and was also discovered by Mechain in 1781; it constitutes the intersecting point of a rectangle formed by five stars, of which the nearest is in the sp [south preceding] quadrant, and of the 11th magnitude.”
Top M95 image credit, Palomar Observatory courtesy of Caltech, , M95 2MASS image, M95 Jacobus Kapteyn Telescope, M95 Spitzer Image, M95 Image from Mayall Telescope and M95 image courtesy of NOAO/AURA/NSF.