Guide to Space Archives

December 30, 2009December 24, 2015 by Fraser Cain

Distance to the Center of the Earth

The Earths interior (University of Chicago)

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The average distance to the centre of the Earth is 6,371 km or 3,959 miles. In other words, if you could dig a hole 6,371 km, you’d reach the center of the Earth. At this point you’d be in the Earth’s liquid metal core.

I said that this number is an average. That’s because the Earth isn’t a perfect sphere, it’s actually an oblate spheroid – a squished ball. The Earth is rotating on its axis, turning around once a day. Points on the equator are moving in a circle more than 1,600 km/hour. This creates a centrifugal force that pulls regions of the equator outward and flattens the poles.

The distance to the center of the Earth from the equator is 6,378 km or 3,963 miles. And the distance to the center of the Earth from the poles is only 6,356 km or 3,949 miles. That’s a difference of 22 km. In other words, if you’re standing on the equator, you’re 22 km further away from the center of the Earth than someone standing on the North Pole.

So if you did want to dig that hole into the Earth, the shortest distance would be from the North or South Pole. Good luck!

We’ve written several articles about the center of Earth. Here’s an article about the radius of the Earth, and here’s an article about the layers of the Earth.

If you’d like more info about the interior of the Earth, check out this article from the University of Nevada, Reno.

We’ve recorded an entire episode of Astronomy Cast about the Earth. Listen here, Episode 51: Earth.

December 30, 2009December 24, 2015 by Fraser Cain

Distance to Alpha Centauri

Alpha Centauri is the closest known star system to the Solar System. Also known as Rigil Kentaurus, Alpha Centauri is actually a multiple star system. It’s certainly a binary star, with two sunlike stars orbiting one another. And there’s also a red dwarf star, Proxima Centauri, which astronomers still argue about whether it’s part of the system.

The closest star in the group is Proxima Centauri, located only 4.243 light-years from the Sun. And then the Alpha Centauri AB stars are located 4.37 light-years away.

With the unaided eye, Alpha Centauri looks like a single star. But then under the power of a telescope, it’s possible to split them and see the individual stars separately. Alpha Centauri is only really prominent in the southern skies, and below the horizon to astronomers in the north.

Alpha Centauri A is slightly larger and more luminous than the Sun, while Alpha Centauri B is smaller and cooler than the Sun. But Proxima Centauri is a tiny red dwarf star, with only 1/8th the mass of the Sun.

We’ve written several articles about the Alpha Centauri system. Here’s an article about how we might be able to detect Earthlike planets around Alpha Centauri, and here’s an article about the sounds of Alpha Centauri.

Here’s a cool image of Alpha Centauri at Astronomy Picture of the Day.

We’ve also recorded an episode of Astronomy Cast about what it might take to travel to Alpha Centauri. Listen here, Episode 145: Interstellar Travel.

December 29, 2009December 24, 2015 by Fraser Cain

How Was Venus Discovered?

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Venus is one of the planets visible with the unaided eye. In fact, Venus is the brightest object in the sky, after the Sun and the Moon. So it’s impossible to know how was Venus discovered. The planet has been known about since prehistoric times. Perhaps a better question is, when did we realize that Venus was a planet?

Thousands of years ago, the Greek astronomers thought that the Earth was the center of the Universe, and everything revolved around us, including the Sun, the Moon, the planets and the stars. But in the in the 1500s, Nicolaus Copernicus developed his theories of a Sun-centered Solar System. Instead of the traditional idea, the Sun was at the center, and the Earth was just another planet like Venus and Mars.

This theory was given a tremendous amount of evidence when Galileo Galilei first turned his rudimentary telescope on Venus, showing that the planet went through phases, like the Moon. This meant that it orbited the Sun, and not the Earth. Galileo also discovered the 4 major moons orbiting Jupiter, demonstrating that not all objects in the Universe orbited the Earth.

So it was in the 16th and 17th centuries that astronomers really came to understand that both Venus and Earth were just planets orbiting the Sun.

We’ve written many articles about the discovery of planets for Universe Today. Here’s an article about how Uranus was discovered, and here’s how Neptune was discovered.

And if you’d like more info on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

December 28, 2009December 24, 2015 by Fraser Cain

Moon Rotation

The rotation of the Moon is a strange situation. It takes the same amount of time for the Moon to complete a full orbit around the Earth as it takes for it to complete one rotation on its axis. In other words, the Moon rotation time is 27.3 days, just the same as its orbital time: 27.3 days.

What this means to us here on Earth is that the Moon always presents the same face to the Earth. We only see one side of the Moon, and not the other. And if you could stand on the surface of the Moon, the Earth would appear to just hang in the sky, not moving anywhere.

Astronomers say that the Moon is tidally locked to the Earth. At some point in the past, it did have a different rotation rate from its orbital period. But slight differences in the shape of the Moon caused the Moon to experience different amounts of gravity depending on its position. These differences acted as a brake, slowing the Moon rotation speed down until it matched its orbital period. There are other tidally locked moons in the Solar System. Pluto and its moon Charon are tidally locked to each other, so they always present the same face to one another.

We’ve written many articles about rotation for Universe Today. Here’s an article about the rotation of the Earth, and here’s an article about the rotation of Saturn.

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 an episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.

December 28, 2009December 24, 2015 by Fraser Cain

How Big is the Moon?

Earth Moon Comparison. Image credit: NASA

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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.

December 28, 2009December 24, 2015 by Abby Cessna

Flat Earth Society

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The Flat Earth Society is an organization whose members adhere to the belief that the Earth is flat rather than a sphere. Although it would seem that this organization was founded before the fourth century B.C., when Greek philosophers and scientists advanced the idea of a spherical planet, it was not. Instead, it was founded in 1956 by Samuel Shenton who was a Fellow of both the Royal Astronomical Society and the Royal Geographic Society. He believed that his theory was based on common sense and scientific evidence and did not believe that scientists had enough proof to ascribe to the view of a spherical Earth.

When Shenton died in 1971, the organization was taken over by Charles Johnson and the society’s headquarters were moved from England to California. Johnson inherited much of Shenton’s large library containing flat earth literature when he took over the organization. Johnson claimed that there was a conspiracy against the theory of a flat earth. The new president of the organization also thought that the Sun and Moon were flat discs about 4,800 kilometers from Earth and that the stars were only approximately 6,400 kilometers from our planet.

When Samuel Shenton was presented with the pictures taken by probes from space showing a spherical Earth, he wavered at first. He restated his position though and said that the photographs could easily fool the untrained eye and that they had been faked. The Flat Earth Society also declared that the 1969 Moon landing was a hoax and it had been scripted and filmed. The Flat Earth Society took some of its philosophy from the Bible, claiming that the creation text shows that the Earth was created flat. Johnson in particular quoted biblical passages in his newsletters. Johnson also turned to the United Nations’ flag for proof of a flat earth. The globe portrayed on the flag looked similar to the model of a flat earth as described by the Flat Earth Society.

The Flat Earth Society began to decline in membership during the 1990’s, and it came to an end when Charles Johnson passed away in 2001. After the Flat Earth Society disbanded, several other groups seemed to have made use of its name at one time or another. There are number of flat earth forums and websites up and running. Even though at least one group has borrowed the Flat Earth Society’s name, it may not be directly connected to the old organization.

Universe Today has articles on is the Earth round and who discovered the Earth.

You should also check out the Flat Earth and do they really think the Earth is flat.

Astronomy Cast has an episode on Earth.

Sources:
http://en.wikipedia.org/wiki/Flat_Earth_Society
http://science.howstuffworks.com/space-conspiracy-theory8.htm

December 27, 2009December 24, 2015 by Jerry Coffey

Celestial Body

[/caption]The term celestial body is as expansive as the entire universe, both known and unknown. By definition a celestial body is any natural body outside of the Earth’s atmosphere. Easy examples are the Moon, Sun, and the other planets of our solar system. But those are very limited examples. The Kuiper belt contains many celestial bodies. Any asteroid in space is a celestial body. So, what do you write about with such a broad topic? How about a sampling of five of my favorites and leave it at that for now? Which five, though. Well, let’s cover Ceres, the Kuiper belt, the asteroid Cruithne, Achernar, and Apophis.

Ceres is a celestial body that is by far the largest and most massive asteroid in the belt between Mars and Jupiter. It is approximately the size of Texas or 975km x 909 km with a mass of 9.5×10²⁰. It actually represents 1/3 of all of the mass of the asteroid belt. It has enough mass for self gravity which is a major requirement to be considered a dwarf planet. It revolves around the sun every 1679.819 days with a very small axial tilt. The surface is relatively warm. The high temperature is thought to be in the neighborhood of -38°C(235 K). Ceres has a visual brightness magnitude of +6.9 to +9. When it is at the brightest point possible, Ceres is nearly bright enough to be seen with the naked eye. It can be seen with binoculars whenever it is above the horizon on a completely dark night.

The Kuiper belt contains many a celestial body. It is actually a disk-shaped region in the outer solar system lying beyond the orbit of Neptune and extending to a distance of about 50 astronomical units, containing thousands of small icy bodies, some of which are on highly elliptical orbits, periodically visiting the inner solar system as comets. It is thought to be a collection of the remnants of the formation of the solar system. Who knows what may be found when we are able to send spacecraft to its edges?

As a celestial body, the asteroid Cruithne is sort of small and indistinct until you consider that it is locked in a 1:1 orbit with the Earth. The asteroid is sometimes referred to as the Earth’s second moon. It is not a true moon because the Earth’s gravity does not effect it nor does its effect the Earth. Cruithne’s nearest pass to Earth is .1 AU (40 moon lengths), although right now it never comes closer than .3 AU. The asteroid sort of runs like a corkscrew around the Earth while both are revolving around the Sun. The asteroid Cruithne is in a normal elliptic orbit around the Sun. Its revolution around the Sun, approximately 364 days at present, is almost equal to that of the Earth. Because of this, Cruithne and Earth appear to follow each other in their paths around the Sun.

The celestial body Achernar is a bright, blue, B3-type star of six to eight solar masses lying approximately 144 light years away. It is classified as a dwarf, but it is 3,000 times more luminous than our Sun. It is in the deep southern sky and never rises above 33°N. Achernar is best seen from the southern hemisphere in November; it is circumpolar below 33°S. Achernar spins so quickly that is spherical in shape. The distance along its equator is 50% greater than its polar diameter. It is the brightest star in the Eridanus constellation. It is also the 9th brightest star in the night sky. Of the 10 brightest stars, other than our Sun, it is the hottest and bluest.

The celestial body Apophis is one of the most intriguing, to me. It is the stuff that many sci-fi legends have been based on. Apophis is most famous for the stir it caused in 2004. The asteroid was discovered on its way towards the Earth and was predicted to have a 2.7% chance of impacting the Earth. That in and of itself is not significant. Objects impact the Earth on a yearly basis. The size of Apophis was the major concern. Even a small chance that an asteroid the size of a small town hitting the Earth rightly caused a large commotion. It achieved the highest score ever on the Torino scale and it stayed on an elevated level for longer than any other asteroid ever has. It was eventually studied enough to know that it would not hit the Earth in 2004. The asteroid will pass again in 2029. Scientists predict that it will not hit the Earth, but it may pass through a gravitational keyhole that could alter its orbit enough that it could impact in 2036. The chances are slight, but real. Even if it doesn’t hit a keyhole in 2029 it will return every seven years and may pose a serious threat in the future. Scientists have proposed that Apophis be nudged out of its present orbit into an orbit that takes it further from the keyhole. NASA scientist David Morrison says, “After 2029, the deflection would have to be vigorous enough to miss not just a tiny keyhole but the much larger target of the Earth itself. And such a deflection is far beyond present technology for an asteroid this large.”.

Many things can make a celestial body interesting. Everyone has their favorites. Mine happen to be the five I have listed. Each can be further researched here on Universe Today. Follow these links to find what you need: Ceres, the Kuiper belt, the asteroid Cruithne, Achernar, and Apophis. Astronomy Cast offers a good episode about the mind-boggling possibility of multiple universes. Here’s a list of the 10 brightest stars.

Sources:
http://planetary.org/explore/topics/asteroids_and_comets/ceres.html
http://userpages.umbc.edu/~gwilson/kuiper.html
http://www.astro.uwo.ca/~wiegert/3753/3753.html
http://en.wikipedia.org/wiki/Achernar
http://neo.jpl.nasa.gov/apophis/

December 24, 2009December 24, 2015 by Tammy Plotner

Messier 99

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.

December 24, 2009December 24, 2015 by Tammy Plotner

Messier 98

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.

December 24, 2009December 24, 2015 by Tammy Plotner

Messier 97

Object Name: Messier 97
Alternative Designations: M97, NGC 3587, Owl Nebula
Object Type: Type 3a Planetary Nebula
Constellation: Ursa Major
Right Ascension: 11 : 14.8 (h:m)
Declination: +55 : 01 (deg:m)
Distance: 2.6 (kly)
Visual Brightness: 9.9 (mag)
Apparent Dimension: 3.4×3.3 (arc min)

Locating Messier 97: Locating Messier 97 is fairly easy. You’ll find it one third the distance in a mental line drawn between Beta and Gamma Ursa Majoris and just slightly south of that line towards a dim star. Yep. The problem isn’t finding the Owl Nebula… It’s seeing it! Despite its billed combined magnitude of 9.9, this is one low surface brightness object and requires pristine skies to be seen with an average 4″ telescope. Nebula and light pollution filters do help, but sky conditions truly dictate. (This author has seen it in 16X65 binoculars, but from a guarded dark sky site.) What you are looking for is about the same diameter that Jupiter would be in the given eyepiece you are using and under average skies will appear only as the faintest contrast change. Large aperture, fast focal ratio telescopes improve your chances marginally.

What You Are Looking At: Messier 97 is a very unusual and dynamic planetary nebula whose shape may be considered that of a cylindrical torus shell viewed on the oblique. What we see photographically (and sometimes physically) as the “Owl’s Eyes” may be the projected matter-poor ends of the cylindrical shape, while the head could be a low ionization shell. Inside this 6,000 year old denizen of the night is a dying, now 16th magnitude star with a little bit more than half the mass of our own Sun. A star which – oddly enough – can sometimes be glimpsed easier than the nebula itself!

Why? Perhaps density? “We are able to evaluate the variation of excitation and electron density over the projected envelope of the source. We propose that the Owl Nebula consists of four primary shells: an internal, tilted, barrel-like component responsible for higher excitation emission; two much more uniform, spherically symmetric structures, CSCI and CSCII. These, finally, are enveloped by a much lower intensity, lower excitation halo, dubbed CSCIII. A large proportion of the low-excitation emission appears to be associated with the periphery of CSCI, and it is conceivable that this is, physically speaking, a relatively thin-shelled structure.” says L. Cuesta (et al). “[S II] density mapping appears to indicate that ne is preferentially enhanced toward the northern periphery of the shell, in a regime where low-excitation line strengths are also preferentially enhanced. We suggest that such trends may arise through northerly shocking of the shell CSC.”

So what gives with the holes we call eyes? Let’s ask R. L. M. Corradi (et al): “The haloes have been classified following the predictions of modern radiation-hydrodynamical simulations that describe the formation and evolution of ionized multiple shells and haloes around PNe. According to the models, the observed haloes have been divided into the following groups: (i) circular or slightly elliptical asymptotic giant branch (AGB) haloes, which contain the signature of the last thermal pulse on the AGB; (ii) highly asymmetrical AGB haloes; (iii) candidate recombination haloes, i.e. limb-brightened extended shells that are expected to be produced by recombination during the late post-AGB evolution, when the luminosity of the central star drops rapidly by a significant factor; (iv) uncertain cases which deserve further study for a reliable classification; (v) non-detections, i.e. PNe in which no halo is found to a level of ?10?3 the peak surface brightness of the inner nebulae.”

And what’s going on with the central star? “Einstein, EXOSAT, and ROSAT X-ray observations of planetary nebulae detected soft photospheric X-ray emission from their central stars, but the diffuse X-ray emission from the shocked fast stellar wind in their interiors could not be unambiguously resolved. The new generation of X-ray observatories, Chandra and XMM-Newton, have finally resolved the diffuse X-ray emission from shocked fast winds in planetary nebula interiors.” says Mart?n A. Guerrero. “Furthermore, these observatories have detected diffuse X-ray emission from bow-shocks of fast collimated outflows impinging on the nebular envelopes, and unexpected hard X-ray point-sources associated with the central stars of planetary nebulae. Here I review the results of these new X-ray observations of planetary nebulae and discuss the promise of future observations.”

Is it possible this is just one big planetary nebula bubble? According to Adam Frank and Garrelt Mellema: “We have presented radiation-gasdynamic simulations of aspherical Planetary Nebula (PN) evolution. These simulations were constructed using the Generalized Interacting Stellar Winds scenario where a fast, tenuous outflow from the central star expands into a toroidal, slow, dense circumstellar envelope. We have demonstrated that the GISW model can produce aspherical flow patterns. In particular we have shown that by varying key initial parameters we can produce a variety of elliptical and bipolar forward shock configurations. The dependence of the shock morphology on the initial parameters conforms to the expectations of analytical models (Icke 1988). We have demonstrated that including radiation-transfer, ionization, and radiative heating and cooling does not drastically alter the global morphologies. Radiative cooling does slow the evolution of the forward shock by removing energy from the hot bubble. The evolution of the forward shock configuration is independent of the ionization of the undisturbed slow wind. Also, radiation heating and cooling does change the temperature structure of the shocked slow wind material compressed into the dense shell.”

History: M97 was discovered by eagle-eyed Pierre Mechain on February 16, 1781. (That was back in the day where if you were complaining about light pollution that you asked your neighbor to “put out their candle”.) It was logged into record by Charles Messier on March 24, 1781 where he notes: “Nebula in the great Bear [Ursa Major], near Beta: It is difficult to see, reports M. Mechain, especially when one illuminates the micrometer wires: its light is faint, without a star. M. Mechain saw it the first time on Feb 16, 1781, and the position is that given by him.”

It was later noted by Sir William Herschel in his own celestial wanderings as: “The arguments that the nebulous matter is in some degree opaque which is given in the 25th article, will receive considerable support from the appearance of the following nebulae; for they are not only round, that is to say the nebulous matter of which they are composed is collected into a globular compass, but they are also of a light which is nearly of an uniform intensity except just on the borders. I give these nebulae in two assortments (incl. M97). Number 97 of the Connoissance is “A very bright, round nebula of about 3′ in diameter; it is nearly of equal light throughout, with an ill defined margin of no great extent.”

Top M97 image credit, Palomar Observatory courtesy of Caltech, M97 2MASS Image, M97 IR (NOAO), Owl Nebula – SEDS, “Owl Nebula” – Karen Kwitter (Williams College), Ron Downes (STScI), You-Hua Chu (University of Illinois) and NOAO/AURA/NSF, M97 (AANDA) and M97 images courtesy of NOAO/AURA/NSF.