Category: Astronomy

[/caption]
The Moon is one of the most significant objects in the night sky, second only in brightness to the Sun. So, how big is the Moon?

The diameter of the Moon is only 3,474 km across. Just for comparison, the diameter of the Earth at the equator is 12,756 km. That’s only 27% the diameter of the Earth. The Moon is also the 5th largest moon in the Solar System, after Ganymede, Titan, Callisto and Io.

In terms of volume, the Moon only contains 2.195 x 10¹⁰ km³. That sounds like a lot of cubic kilometers of Moon, but again, that’s only 2% the volume of Earth.

The surface area of the Moon is 3.793 x 10⁷ km². That’s about the same size as Russia, Canada and the United States combined.

The circumference of the Moon is 10,921 km. Again, that’s only a little over a quarter the circumference of the Earth.

We’ve written many articles about the Moon for Universe Today. Here’s an article about the Moon compared to Earth, and here’s an article about the mass of the Moon.

If you’d like more info on the Moon, check out NASA’s Solar System Exploration Guide on the Moon, and here’s a link to NASA’s Lunar and Planetary Science page.

We’ve also recorded several episodes of Astronomy Cast about the Moon. Listen here, Episode 113: The Moon, Part 1.

Object Name: Messier 99
Alternative Designations: M99, NGC 4254, Pinwheel Galaxy
Object Type: Type Sc Spiral Galaxy
Constellation: Coma Berenices
Right Ascension: 12 : 18.8 (h:m)
Declination: +14 : 25 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 9.9 (mag)
Apparent Dimension: 5.4×4.8 (arc min)

Locating Messier 99: As part of the Virgo Cluster of Galaxies, M98 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. This face-on presentation will look like a round hazy patch to small optics and begin revealing its spiral arm pattern with mid-sized telescopes under dark skies.

What You Are Looking At: What’s in an Sc designation when it comes to a spiral galaxy? It means its rotating counterclock-wise. While that sounds very normal, you’ll also notice that M99’s mass seems to be just a little “off center”. What’s going on here? Let’s turn to the research of Victor P. Debattista and J. A. Sellwood: “We show that bars in galaxy models having halos of moderate density and a variety of velocity distributions all experience a strong drag from dynamical friction unless the halo has large angular momentum in the same sense as the disk. The frictional drag decreases the bar pattern speed, driving the co-rotation point out to distances well in excess of those estimated in barred galaxies. The halo angular momentum required to avoid strong braking is unrealistically large, even when rotation is confined to the inner halo only. We conclude, therefore, that bars are able to maintain their observed high pattern speeds only if the halo has a central density low enough for the disk to provide most of the central attraction in the inner galaxy. We present evidence that this conclusion holds for all bright galaxies.”

But what if it wasn’t just the galaxy itself, but a chance merger? “We present high-resolution H I and H? observations of the spiral galaxy NGC 4254. The observations were obtained with the VLA and the Maryland-Caltech Fabry-Perot camera, respectively. NGC 4254 is unusual in having a grand-design spiral structure with a strong m = 1 component for which there is no obvious cause in optical images. Our observations reveal that, in addition to the usual galactic disk component, there are H I clouds superposed on and beyond the H I disk, at velocities up to 150 km s^-1^ from those established for the disk. The mass in these clouds is ~2.3 x 10^8^ M_sun_, and they may be the remnants of an entity that was tidally disrupted by NGC 4254 and is now merging with it. The direct effects of the interaction between the cloud gas and the galaxy are limited to the region where the gas appears to be merging with the disk, where it may be causing a warp.” says Yuichi Terashima (et al).

“But the indirect effects of the infalling gas appear profound: it is the most likely cause for the unusual spiral structure of NGC 4254. If so, the m = 1 spiral structure of NGC 4254 is recent, and an internal amplification mechanism such as swing amplification has played a major role in its evolution. Since NGC 4254 does not appear to be exceptionally deficient in dark matter and is apparently a normal Sc galaxy, the nature of the interaction appears important in determining the susceptibility of the disk to various spiral modes (in particular the m = 1,3, and 5 modes of NGC 4254).”

Spiral modes, huh? T. Kranz (et al) knows a lot about that, and before there can be stars there has to be the material to make them – gas. “As a pilot project, we analyzed the data of NGC 4254 (M99). Assuming a constant stellar mass to light ratio, the gravitational potential due to the stellar mass fraction was calculated by direct integration over the whole mass distribution taken from the NIR-image. The mass to light ratio for the maximum disk contribution was scaled by the measured rotation curve. For the dark matter contribution we assumed an isothermal halo with a core. To combine the two components we chose a stellar mass fraction and added the halo with the variable parameters adjusted to give a best fit to the rotation curve.” says Kranz, “We used this potential as an input for the hydrodynamical gas simulations. Figure 2 presents the results for the resulting gas surface density, as it settles in the potenital. The morphology of the gas distribution is very sensitive to the velocity, with which the spiral pattern of the galaxy rotates (pattern speed).”

“Determining individual mass fractions of the luminous and dark matter is not a straightforward task. The rotation curve of a disk galaxy is only sensitive to the total amount of gravitating matter, but does not allow to distinguish the two mass density profiles,” continues Kranz. “Here we would like to exploit the fact, that the stellar mass in disk galaxies is often organized in spiral arms, thus in clearly non-axisymmetric structures.”

“On the other hand, in most proposed scenarios, the dark matter is non-collisional and dominated by random motions. It is not susceptible to spiral structures and distributed like the stars in elliptical galaxies. If the stellar mass dominates, the arms could induce considerable non-circular motions in the gas, which should become visible as velocity wiggles in observed gas kinematics. Using hydrodynamical gas simulations we are able to predict these velocity wiggles and compare them to the observations. Hence the contribution of the perturbative forces with respect to the total forces can be determined quantitatively and can be used to constrain the disk to halo mass ratio.”

History: M99 was discovered on March 15, 1781 by Messier’s colleague and friend, Pierre Mechain, together with the nearby situated M98 and M100. Charles Messier measured its position and included it in his catalog on April 13, 1781. In his notes he writes: “Nebula without star, of a very pale light, nevertheless a little clearer than the preceding [M98], situated on the northern wing of Virgo, and near the same star, no. 6, of Comae Berenices. The nebula is between two stars of seventh and of eighth magnitude. M. Mechain saw it on March 15, 1781.”

While M99 would be observed by both William and John Herschel, it would be Lord Rosse who finally brought it to light. Even though he didn’t truly understand the nature of what he was looking at, he was fascinated with knowing it had a spiral structure and M99 became his second “confirmed kill”. In his notes he writes: “In the following spring [of 1846] an arrangement, also spiral but of a different character [than in M51], was detected in 99 Messier, Plate XXXV. fig 2. This object is also easily seen, and probably a smaller instrument, under favourable circumstances, would show everything in the sketch.”

Top M99 image credit, Palomar Observatory courtesy of Caltech, M99 2MASS image, M99 by Hunter Wilson, M99 Spitzer images, M99 courtesy of Ole Nielsen, Rosse’s historical M99 sketch and M99 image courtesy of NOAO/AURA/NSF.

Object Name: Messier 98
Alternative Designations: M98, NGC 4192
Object Type: Type Sb Barred Spiral Galaxy
Constellation: Coma Berenices
Right Ascension: 12 : 13.8 (h:m)
Declination: +14 : 54 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 10.1 (mag)
Apparent Dimension: 9.5×3.2 (arc min)

Locating Messier 98: As part of the Virgo Cluster of Galaxies, M98 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 west. Now sweep south for several eyepiece fields. If you have not seen the slender scratch of M98, 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.) M98 is edge-on in presentation, so it will be a slender scratch of nebulousity that requires dark, clear skies and at least 4″ in aperture.

What You Are Looking At: M98 is nearly edge-on in presentation and displays a disturbed, misty elongated disk. There are some blue regions of new star formation, as well as a massive quantity of occulting dust which reddens the appearance of the small, bright nucleus. But where did all this dust come from?

“Debris sent into the intergalactic medium during tidal collisions has received much attention as it can tell us about several fundamental properties of galaxies, in particular their missing mass, both in the form of cosmological Dark Matter and so-called Lost Baryons. High velocity encounters, which are common in clusters of galaxies, are able to produce faint tidal debris that may appear as star–less, free floating HI clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous, dark matter (DM) dominated, objects which for some reason never managed to form stars. VirgoHI21, in the Virgo Cluster, is by far the most spectacular and most discussed Dark Galaxy candidate so far detected in HI surveys. We show here that it is most likely made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC 4254 during its fly–by at about 1000 km s?1 by a massive intruder. Our numerical model of the collision is able to reproduce the main characteristics of the system: in particular the absence of stars, and its prominent velocity gradient. Originally attributed to the gas being in rotation within a massive dark matter halo, we find it instead to be consistent with a combination of simple streaming motion plus projection effects.” say Piere Alain Duc.

“Based on our multi-wavelength and numerical studies of galaxy collisions, we discuss several ways to identify a tidal origin in a Dark Galaxy candidate such as optical and millimetre–wave observations to reveal a high metallicity and CO lines, and more importantly, kinematics indicating the absence of a prominent Dark Matter halo. We illustrate the method using another HI system in Virgo, VCC 2062, which is most likely a Tidal Dwarf Galaxy . Now, whereas tidal debris should not contain any dark matter from the halo of their parent galaxies, it may exhibit missing mass in the form of dark baryons, unaccounted for by classical observations, as recently found in the collisional ring of NGC 5291 and probably in the TDG VCC 2062. These “Lost Baryons” must originally have been located in the disks of their parent galaxies.”

So is it dust that dims M98’s core or is it something else? Something like maybe a Low Luminosity Active Galactic Nuclei (LLAGNs)? “Low-luminosity active galactic nuclei (LLAGNs) comprise 30% of all bright galaxies (B?12.5) and are the most common type of AGN . These include LINERs, and transition-type objects (TOs, also called weak- [OI] LINERs). These two types of LLAGNs have similar emission line ratios in [OIII]/HB, [NII]/H?, and [SII]/H?, but [OI]/H? is lower in TOs than in LINERs. LLAGNs constitute a rather mixed class and different mechanisms have been proposed to explain the origin of the nuclear activity, including shocks, and photoionization by a non-stellar source, by hotstars or by intermediate age stars.” says Rosa M. Gonzalez Delgado (et al). “Because we do not know yet what powers them and how they are related to the Seyfert phenomenon, LLAGNs have been at the forefront of AGN research since they were first systematically studied by Heckman (1980). Are they all truly “dwarf” Seyfert nuclei powered by accretion onto nearly dormant supermassive black holes (BH), or can some of them be explained at least partly in terms of stellar processes? If LLAGNs were powered by a BH, they would represent the low end of the AGN luminosity function in the local universe and would also establish a lower limit to the fraction of galaxies containing massive BHs in their centers. If, on the contrary, LLAGNs were powered by nuclear stellar clusters, their presence would play an important role in the evolution of galaxy nuclei. Therefore, it is fundamental to unveil the nature of the central source in LLAGNs.”

But that’s not all that’s hiding inside M98. Now let’s try type II LINERS. ” We present ASCA observations of low-ionization nuclear emission-line regions (LINERs) without broad H? emission in their optical spectra. The sample of “type 2″ LINERs consists of NGC 404, 4111, 4192, 4457, and 4569. We have detected X-ray emission from all the objects except for NGC 404; among the detected objects are two so-called transition objects (NGC 4192 and NGC 4569), which have been postulated to be composite nuclei having both an H II region and a LINER component. The images of NGC 4111 and NGC 4569 in the soft (0.5-2 keV) and hard (2-7 keV) X-ray bands are extended on scales of several kiloparsecs. The X-ray spectra of NGC 4111, NGC 4457, and NGC 4569 are well fitted by a two-component model that consists of soft thermal emission with kT ~ 0.65 keV and a hard component represented by a power law (photon index ~2) or by thermal bremsstrahlung emission (kT ~ several keV). The extended hard X-rays probably come from discrete sources, while the soft emission most likely originates from hot gas produced by active star formation in the host galaxy. We have found no clear evidence for the presence of active galactic nuclei (AGNs) in the sample.” says Yuichi Terashima (et al). “Using black hole masses estimated from host galaxy bulge luminosities, we obtain an upper limit on the implied Eddington ratios less than 5 × 10-5. If an AGN component is the primary ionization source of the optical emission lines, then it must be heavily obscured with a column density significantly larger than 1023 cm-2, since the observed X-ray luminosity is insufficient to drive the luminosities of the optical emission lines. Alternatively, the optical emission could be ionized by a population of exceptionally hot stars. This interpretation is consistent with the small [O I] ?6300/H? ratios observed in these sources, the ultraviolet spectral characteristics in the cases where such information exists, and the X-ray results reported here. We also analyze the X-ray properties of NGC 4117, a low-luminosity Seyfert 2 galaxy serendipitously observed in the field of NGC 4111.”

History: M98 was originally discovered by Pierre Mechain on March 15, 1781 and reported to Charles Messier who confirmed and logged it on April 13, 1781. In his notes he writes: “Nebula without star, of an extremely faint light, above the northern wing of Virgo, on the parallel and near to the star no. 6, fifth magnitude, of Coma Berenices, according to Flamsteed. M. Mechain saw it on Mar 15, 1781.”

Sir William Herschel would catch this great galaxy on December 30, 1783 with much detail. In his unpublished notes he writes: “”The difference [of Messier’s and Mechain’s observations on one hand, and Herschel’s on the other] will appear when we compare my observation of the 98th nebula with that in the Connoissance des Temps for 1784, which runs thus: [Messier’s description follows in French, as translated above]. My observation of the 30th December, 1783, is thus: A large, extended fine nebula. Its situation shews it to be M. Messier’s 98th; but from its description it appears, that that gentleman has not seen the whole of it, for its feeble branches extend above a quarter of a degree, or which no notice is taken. Near the middle of it are a few stars visible, and more suspected. My field of view will not quite take in the whole nebula.”

Obviously, taking your time and really “looking” at M98 makes a huge difference, as Admiral Smyth would point out about a hundred years later: “A fine and large, but rather pale nebula, between Virgo’s left wing and Leo’s tail; with the bright star, 6 Comae Berenices, following [East] in the next field exactly on the parallel. M. [Messier], who discovered it in 1781, merely registered it as “a nebula without a star, with an extremely faint light;” but on keeping a fixed gaze it brightens up towards the centre. It is elongated, in the direction of two stars, the one np [noth preceding, NW] and the other sf [south following, SE] of the object; with another star in the nf [north following, NE] quadrant pretty close. Differentiated with Beta Leonis, which star it follows by 6deg 1/2 in the direction of Arcturus; it lies on the outskirts of the vast region of nebulae that adorns the Virgin’s wing.”

Let your galaxy hunting skills take wing tonight!

Top M98 image credit, Palomar Observatory courtesy of Caltech, M98 2MASS image, M98 Spiral Structure (AANDA) M98 by Adam Block/NOAO/AURA/NSF, M98 Wikipedia courtesy of Ole Nielsen and M98 image courtesy of NOAO/AURA/NSF.