Posted on by Tammy Plotner

Messier 59 – the NGC 4621 Elliptical Galaxy

Messier 60, Messier 59 and Messier 58. Credit: Wikisky

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the spiral galaxy known as Messier 59.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects he initially mistook for comets. In time, he would come to compile a list of approximately 100 of these objects, hoping to prevent other astronomers from making the same mistake. This list – known as the Messier Catalog – would go on to become one of the most influential catalogs of Deep Sky Objects.

One of these objects is the elliptical galaxy known as Messier 59 (aka. NGC 4621). This galaxy is located approximately 60 million light-years from Earth in the direction of the southern Virgo constellation. Sitting just a few degrees away Messier 60, and bordered at a distance by Messier 58, this galaxy is visible using smaller instruments, but is best observed using a larger telescope.

Atlas image of Messier 59 obtained by the Two micron All Sky Survey (2MASS). Credit: 2MASS/NASA/UMass

Description:

Located about 60 million light years away and spanning about 90 million light years of space, but what exactly is its type? Says Takao Mizuno (et al) in their 1996 study:

“We decomposed two-dimensionally an elliptical galaxy, NGC 4621, which shows deviations from the brightness distribution law. We have found that its brightness distribution can be reproduced by three components possessing constant ellipticities of the residuals in the circular region of radius. The component obeying the aw has 62% of the total light, and, hence, is the main body of this elliptical galaxy.” So it might not be the biggest or the brightest of the group, but it is home to nearly 2000 globular clusters. This isn’t news when it comes to this galaxy type, but what is news is how they rotate… the wrong way!

“We present adaptive optics assisted OASIS integral field spectrography of the S0 galaxy NGC 4621. Two-dimensional stellar kinematical maps (mean velocity and dispersion) reveal the presence of a 60 pc diameter counter-rotating core (CRC), the smallest observed to date.” says Fabien Wernli (et al), “The OASIS data also suggests that the kinematic center of the CRC is slightly offset from the center of the outer isophotes. This seems to be confirmed by archival HST/STIS data. We also present the HST/WFPC2 V-I colour map, which exhibits a central elongated red structure, also slightly off-centered in the same direction as the kinematic centre. Although the stellar velocities are reasonably fitted, including the region of the counter-rotating core, significant discrepancies between the model and the observations demonstrate the need for a more general model.”

What could account for such unusual behavior? Try a quiet black hole! As J. M. Wrobel (et al) indicated in their 2008 study:

“The nearby elliptical galaxies NGC 4621 and NGC 4697 each host a supermassive black hole. Analysis of archival Chandra data and new NRAO Very Large Array data shows that each galaxy contains a low-luminosity active galactic nucleus (LLAGN), identified as a faint, hard X-ray source that is astrometrically coincident with a faint 8.5-GHz source. The black holes energizing these LLAGNs have Eddington ratios placing them in the so-called quiescent regime. The emission from these quiescent black holes is radio-loud, suggesting the presence of a radio outflow. Also, application of the radio-X-ray-mass relation from Yuan & Cui for quiescent black holes predicts the observed radio luminosities to within a factor of a few. Significantly, that relation invokes X-ray emission from the outflow rather than from an accretion flow. The faint, but detectable, emission from these two massive black holes is therefore consistent with being outflow-dominated.”

The M59 spiral galaxy. Credit: NOAO

History of Observation:

Both M59 and neighboring M60 were discovered on April 11, 1779 by Johann Gottfried Koehler who wrote: “Two very small nebulae, hardly visible in a 3-foot telescope: The one above the other.” Charles Messier would independently recover it four days later and state in his notes:

“Nebula in Virgo and in the neighborhood of the preceding [M58], on the parallel of epsilon [Virginis], which has served for its [position] determination: it is of the same light as the above, equally faint. M. Messier reported it on the Chart of the Comet of 1779.”

While both William and John Herschel would also observe it, it sometimes confounds me that they didn’t seem to notice all the other galaxies around it! Fortunately for historic record, Admiral Smyth did:

“A fine field is exhibited under the eye-piece, which magnifies 93 times, just as this object [M60 with NGC 4647] enters, because the bright little nebula 59 M. is quitting the np [north preceding, NW] verge, and another small one is seen in the upper part, H. 1402 [NGC 4638]: in fact, four nebulae at once.”

Locating Messier 58:

M59 is a telescope-only object and requires patience to find. Because the Virgo Galaxy field contains so many galaxies which can easily be misidentified, it is sometimes easier to “hop” from one galaxy to the next. In this case, we need to start by locating bright Vindemiatrix (Epsilon Virginis) almost due east of Denebola. Then starhop four and a half degrees west and a shade north of Epsilon to locate one of the largest elliptical galaxies presently known – M60.

The location of M59, which sits between M58 and M60 in the direction of the Virgo constellation. Credit: IAU

At a little brighter than magnitude 9, this galaxy could be spotted with binoculars, but stick with your telescope. In the same low power field (depending on aperture size) you may also note faint NGC 4647 which only appears to be interacting with M60. Also in the field to the west (the direction of drift) is the Messier we’re looking for, bright cored elliptical galaxy M59.

In a smaller telescope, do not expect to see much. What will appear at low power is a tiny egg-shaped patch of contrast change with a brighter center. As aperture increases, a sharper nucleus will begin to appear as you move into the 4-6″ size range at dark sky locations, but elliptical galaxies do not show details. As with all galaxies, dark skies are a must!

Enjoy your journey around the Virgo Galaxy Field!

Object Name: Messier 59
Alternative Designations: M59, NGC 4621
Object Type: E5 Galaxy
Constellation: Virgo
Right Ascension: 12 : 42.0 (h:m)
Declination: +11 : 39 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 9.6 (mag)
Apparent Dimension: 5×3.5 (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, 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:

Posted on by Fraser Cain

What is an Electric Sail? Another Exotic Way to Explore the Solar System

What Is An Electric Sail?
What Is An Electric Sail?

We’re all familiar with the idea of solar sails to explore the Solar System, using the light pressure from the Sun. But there’s another propulsion system that could harness the power of the Sun, electric sails, and it’s a pretty exciting idea.

A few weeks ago, I tackled a question someone had about my favorite exotic propulsion systems, and I rattled off a few ideas that I find exciting: solar sails, nuclear rockets, ion engines, etc. But there’s another propulsion system that keeps coming up, and I totally forgot to mention, but it’s one of the best ideas I’ve heard in awhile: electric sails.

Artist concept of a solar sail demonstration mission that will use lasers for navigation. Credit: NASA.
Artist concept of a solar sail demonstration mission that will use lasers for navigation. Credit: NASA.

As you probably know, a solar sail works by harnessing the photons of light streaming from the Sun. Although photons are massless, they do have momentum, and can transfer it when they bounce off a reflective surface.

In addition to light, the Sun is also blowing off a steady stream of charged particles – the solar wind. A team of engineers from Finland, led by Dr. Pekka Janhunen, has proposed building an electric sail that will use these particles to carry spacecraft out into the Solar System.

To understand how this works, I’ll need to jam a few concepts into your brain.

First, the Sun. That deadly ball of radiation in the sky. As you probably know, there’s a steady stream of charged particles, mainly electrons and protons, zipping away from the Sun in all directions.

Visualization of the solar wind encountering Earth's magnetic "defenses" known as the magnetosphere. Clouds of southward-pointing plasma are able to peel back layers of the Sun-facing bubble and stack them into layers on the planet's nightside (center, right). The layers can be squeezed tightly enough to reconnect and deliver solar electrons (yellow sparkles) directly into the upper atmosphere to create the aurora. Credit: JPL
Visualization of the solar wind encountering Earth’s magnetic “defenses” known as the magnetosphere. Clouds of southward-pointing plasma are able to peel back layers of the Sun-facing bubble and stack them into layers on the planet’s nightside (center, right). The layers can be squeezed tightly enough to reconnect and deliver solar electrons (yellow sparkles) directly into the upper atmosphere to create the aurora. Credit: JPL

Astronomers aren’t entirely sure how, but some mechanism in the Sun’s corona, its upper atmosphere, accelerates these particles on an escape velocity. Their speed varies from 250 to 750 km/s.

The solar wind travels away from the Sun, and out into space. We see its effects on comets, giving them their characteristic tails, and it forms a bubble around the Solar System known as the heliosphere. This is where the solar wind from the Sun meets the collective solar winds from the other stars in the Milky Way.

In fact, NASA’s Voyager spacecraft recently passed through this region, finally making their way to interstellar space.

The solar wind does cause a direct pressure, like an actual wind, but it’s incredibly weak, a fraction of the light pressure a solar sail experiences.

This artist's concept shows the Voyager 1 spacecraft entering the space between stars. Interstellar space is dominated by plasma, ionized gas (illustrated here as brownish haze), that was thrown off by giant stars millions of years ago.Credit: NASA.
This artist’s concept shows the Voyager 1 spacecraft entering the space between stars. Interstellar space is dominated by plasma, ionized gas (illustrated here as brownish haze), that was thrown off by giant stars millions of years ago.Credit: NASA.

But the solar wind contains a stream of positively charged protons and electrons, and this is the key.

An electric sail works by reeling out an incredibly thin wire, just 25 microns thick, but 20 kilometers long. The spacecraft is equipped with solar panels and an electron gun which takes just a few hundred watts to run.

By shooting electrons off into space, the spacecraft maintains a highly positive charged state. Since the protons from the Sun are also positively charged, when they encounter the positively charged tether, they “see” it a huge obstacle 100 meters across, and crash into it.

By imparting their momentum into the tether and spacecraft, the ions accelerate it away from the Sun.

The amount of acceleration is very weak, but it’s constant pressure from the Sun and can add up over a long period of time. For example, if a 1000 kg spacecraft had 100 of these wires extending out in all directions, it could receive an acceleration of 1 mm per second per second.

In the first second it travels 1 mm, and then 2 mm in the next second, etc. Over the course of a year, this spacecraft could be going 30 km/s. Just for comparison, the fastest spacecraft out there, NASA’s Voyager 1, is merely going about 17 km/s. So, much faster, definitely on an escape velocity from the Solar System.

One of the downsides of the method, actually, is that it won’t work within the Earth’s magnetosphere. So an electric sail-powered spacecraft would need to be carried by a traditional rocket away from the Earth before it could unfurl its sail and head out into deep space.

I’m sure you’re wondering if this is a one-way trip to get away from the Sun, but it’s actually not. Just like with solar sails, a electric sail can be pivoted. Depending on which side of the sail the solar wind hits, it either raises or lowers the spacecraft’s orbit from the Sun.

Strike the sail on one side and you raise its orbit to travel to the outer Solar System. But you could also strike the other side and lower its orbit, allowing it to journey down into the inner Solar System. It’s an incredibly versatile propulsion system, and the Sun does all the work.

Although this sounds like science fiction, there are actually some tests in the works. An Estonian prototype satellite was launched back in 2013, but its motor failed to reel out the tether. The Finnish Aalto-1 satellite was launched in June 2017, and one of its experiments is to test out an electric sail.

We should find out if the technique is viable later this year.

It’s not just the Finns who are considering this propulsion system. In 2015, NASA announced that they had awarded a Phase II Innovative Advanced Concepts grant to Dr. Pekka Janhunen and his team to explore how this technology could be used to reach the outer Solar System in less time than other methods.

The Heliopause Electrostatic Rapid Transit System, or HERTS spacecraft would extend 20 of these electric tethers outward from the center, forming a huge circular electric sail to catch the solar wind. By slowly rotating the spacecraft, the centrifugal forces will stretch the tethers out into this circular shape.

Artist's illustration of NASA's Heliopause Electrostatic Rapid Transit System. Credit: NASA
Artist’s illustration of NASA’s Heliopause Electrostatic Rapid Transit System. Credit: NASA

With its positive charge, each tether acts like a huge barrier to the solar wind, giving the spacecraft an effective surface area of 600 square kilometers once it launches from the Earth. As it gets farther, from Earth, though, its effective area increases to the equivalent of 1,200 square km by the time it reaches Jupiter.

When a solar sail starts to lose power, an electric sail just keeps accelerating. In fact, it would keep accelerating out past the orbit of Uranus.

If the technology works out, the HERTS mission could reach the heliopause in just 10 years. It took Voyager 1 35 years to reach this distance, 121 astronomical units from the Sun.

But what about steering? By changing the voltage on each wire as the spacecraft rotates, you could have the whole sail interact differently on one side or the other to the solar wind. You could steer the whole spacecraft like the sails on a boat.

In September 2017, a team of researchers with the Finnish Meteorological Institute announced a pretty radical idea for how they might be able to use electric sails to comprehensively explore the asteroid belt.

Instead of a single spacecraft, they proposed building a fleet of 50 separate 5-kg satellites. Each one would reel out its own 20 km-long tether and catch the Sun’s solar wind. Over the course of a 3-year mission, the spacecraft would travel out to the asteroid belt, and visit several different space rocks. The full fleet would probably be able to explore 300 separate objects.

This image depicts the two areas where most of the asteroids in the Solar System are found: the asteroid belt between Mars and Jupiter, and the trojans, two groups of asteroids moving ahead of and following Jupiter in its orbit around the Sun.

Each spacecraft would be equipped with a small telescope with only a 40 mm aperture. That’s about the size of a spotting scope, or half a pair of binoculars, but it would be enough to resolve features on the surface of an asteroid as small as 100 meters across. They’d also have an infrared spectrometer to be able to determine what minerals each asteroid is made of.

That’s a great way to find that $10 trillion asteroid made of solid platinum.

Because the spacecraft would be too small to communicate all the way back to Earth, they’d need to store the data on board, and then transmit everything once they came past our planet 3 years later.

The planetary scientists I’ve talked to love the idea of being able to survey this many different objects at the same time, and the electric sail idea is one of the most efficient methods to do it.

According to the researchers, they could do the mission for about $70 million, bringing the cost to analyze each asteroid down to about $240,000. That would be cheap compared to any other method proposed of studying asteroids.

Space exploration uses traditional chemical rockets because they’re known and reliable. Sure they have their shortcomings, but they’ve taken us across the Solar System, to billions of kilometers away from Earth.

But there are other forms of propulsion in the works, like the electric sail. And over the coming decades, we’re going to see more and more of these ideas put to the test. A fuel free propulsion system that can carry a spacecraft into the outer reaches of the Solar System? Yes please.

I’ll keep you posted when more electric sails are tested.

Posted on by Fraser Cain

Where Do Comets Come From? Exploring the Oort Cloud

Where Do Comets Come From? Exploring the Oort Cloud
Where Do Comets Come From? Exploring the Oort Cloud

Before I get into this article, I want to remind everyone that it’s been several decades since I’ve been able to enjoy a bright comet in the night sky. I’ve seen mind blowing auroras, witnessed a total solar eclipse with my own eyeballs, and seen a rocket launch. The Universe needs to deliver this bright comet for me, and it needs to do it soon.

By writing this article now, I will summon it. I will create an article that’ll be hilariously out of date in a few months, when that bright comet shows up.

Like that time we totally discovered a supernova in the Virtual Star Party, by saying there wasn’t a supernova in that galaxy, but there was, and we didn’t get to make the discovery.

Anyway, on to the article. Let’s talk about comets.

Comet C/2014 Q2 Lovejoy, Widefield view, false color. Feb 8, 2015. Credit and copyright: Joseph Brimacombe.
Comet C/2014 Q2 Lovejoy, Widefield view, false color. Feb 8, 2015. Credit and copyright: Joseph Brimacombe.

Comets are awesome. They’re made of gas, dust, rock, and organic materials, smashed together, and existing mostly unchanged since the formation of the Solar System 4.5 billion years ago. Every now and then, some gravitational interaction kicks a comet into an orbit that brings it closer to the Sun.

Because of the increased radiation, the comet’s volatile gas and dust sublimates off the surface, leaving behind a long tail of ice. And this is how we discover them.

In fact, comets are one of the objects in the night sky regularly found by amateurs. And by discovering a comet, you get to have it named after you. Of course many of the comets are named after robotic observatories, just another way the robots are taking human jobs.

The source of comets was originally proposed by Gerard Kuiper in 1951, when he theorized that there must be a vast disk of gas and dust surrounding the Solar System, out beyond the orbit of Pluto.

This “Kuiper Belt”, contains millions of objects, which orbit the Sun, jostling each other with their gravity. These interactions kick these Kuiper Belt comets into orbits that bring them closer to the Sun, where they get their characteristic tails.

Astronomers call these short period comets, since they orbit the Sun relatively often. They’re given names and designations, and astronomers can calculate when the comet will pass near to the Sun and flare up again.

Halley's Comet, as seen by the European Giotto probe. Credit: Halley Multicolor Camera Team, Giotto Project, ESA
Halley’s Comet, as seen by the European Giotto probe. Credit: Halley Multicolor Camera Team, Giotto Project, ESA

The famous Halley’s Comet is a good example, which was known to antiquity, but had its orbit first calculated in 1705 by Edmond Halley. Every 74 to 79 years, Halley’s Comet swings near the Sun, flares up and we get a view of this amazing object. It last passed our area in 1986, and it’s not due to return until 2061. I should be in my third robot body by then.

The long period comets are much more mysterious. These objects come out of nowhere, pass through the inner Solar System or smash into the Sun, and then zip back out into deep space. Now, where do they come from?

The Dutch astronomer Jan Oort calculated that there must be an even vaster cloud of ice even farther out beyond the Kuiper Belt – between 5,000 and 100,000 astronomical units from the Sun. Just a reminder, 1 astronomical unit is the distance from the Earth to the Sun, so we’re talking really really far away.

The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA
The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA

Like, the Voyager 1 spacecraft, which is the most distant and fastest object ever sent out by humanity, will still need about 300 years to reach the edge of the Oort Cloud.

Astronomers think that occasional gravitational nudges in the Oort Cloud cause these long period comets to fall down into the inner Solar System and make their rare appearances. It could take a comet like this hundreds of thousands or even millions of years to complete an orbit around the Sun. I’ll need a few dozen robot bodies for that repeat observation.

Check out this cool picture of Comet C/2017 K2 PANSTARRS, taken by the Hubble Space Telescope. This is a great example of a long-period comet, which is visiting our neighbourhood for the first time in the 4.5 billion-year history of the Solar System.

This is the dimmest, farthest comet ever discovered, first seen when it was out beyond the orbit of Saturn.

This cloud of material around the comet is probably the sublimation of frozen volatile gases, like oxygen, nitrogen, carbon dioxide and carbon monoxide. Astronomers think it started to become active about 4 years ago, and they just discovered it now.

As it gets closer to the Sun and warms up, it’ll become a true comet, when its hard-as-rock water ice structure starts to sublimate and earns its tail.

It should make its closest approach in 2022 when it gets about as close to the Sun as Mars.

And this is why we can’t detect out into the Oort Cloud yet. We can barely detect comets outside the orbit of Saturn, not to mention hundreds of times farther than that.

Our Sun isn’t alone in the Milky Way, obviously. It’s a vast swirling storm of hundreds of billions of stars, and over the tens of thousand of years, other stars come much closer to the Sun than we see today.

The European Space Agency’s Gaia spacecraft recently released one of the most detailed maps of stellar positions and motions, and gave us a much better picture of where our Sun is going, and what it’s going to be interacting with in the future.

In order to interact with the Oort Cloud, astronomers have calculated that a star needs to get within about 6.5 light years before it can interact gravitationally, depending on its mass.

Credit: ESA / Gaia / DPAC / A. Moitinho & M. Barros, CENTRA – University of Lisbon.
Credit: ESA / Gaia / DPAC / A. Moitinho & M. Barros, CENTRA – University of Lisbon.

Based on data gathered by the Gaia spacecraft, astronomers charted out the motions of 300,000 stars in our vicinity of the Milky Way in the next 5 million years or so.

Of those stars, 97 will come within 15 light-years of the Sun, and 16 will get closer than 6.5. The most interesting of these is Gliese 710. In 1.3 million years, it’ll pass less than 2.5 light-years away from the Sun, plunging right through the Oort Cloud.

Gliese 710 has about 60% the mass of the Sun, and it’s going about half the speed that stars normally go as they sweep past the Solar System. Which means that it’s going to stick around for a long time, pushing comets around with its mass, and send showers of comets down into the Solar System.

On average, it seems like a star passes within 15 light-years every 50,000 years or so, jostling up our collection of comets.

This is important, because comet impacts could be a cause of past extinction events on Earth. By tracking the movements of stars in our region, astronomers could try to match up past events with times that stars jostled up the Oort Cloud, and predict future events.

Could we ever reach the Oort Cloud and explore it? A few years ago, a space observatory was proposed that could attempt to observe objects as distant as the Oort Cloud. Known as the Whipple Mission, it would orbit in the Sun-Earth L2 point, and watch the sky with a wide field of view.

It would try to detect transiting events when objects as small as a kilometer across passed in front of a more distant star. In theory, the mission would be capable of spotting these transits out as far as 22,000 astronomical units or nearly half a light year. Unfortunately, it hasn’t gotten past the proposal stage.

How the FOCAL mission would see a terrestrial planet. Credit: Geoffrey A. Landis
How the FOCAL mission would see a terrestrial planet. Credit: Geoffrey A. Landis

Another intriguing idea is known as the FOCAL mission, which involves sending a space telescope out to a distance of 550 astronomical units away from the Sun. At this point, the telescope can use the gravity of the Sun itself as an enormous lens, focusing the light from more distant objects.

Actually, you’d need to go farther. At 550 astronomical units, the sunlight drowns out anything the space telescope might try to see. Instead, it needs to go out to a distance of more than 2,000 astronomical units from Earth, when the light focused by the Sun turns into an Einstein Ring around it.

What could you do with a telescope like this? If an exoplanet were to pass behind the Sun, perfectly lined up, you could resolve features as small as 1 kilometer across on a world 35 light-years away.

A telescope like this gives us a very good reason to learn to travel out and explore the Oort Cloud.

The Gaia spacecraft is still hard at work gathering data, and astronomers are expecting another massive data dump in April, 2018. Over time, the spacecraft will map out the position and movements of a billion stars in the Milky Way.

Comets are awesome, and I’d like to see a visible comet in the night sky, but I’d like them to keep their distance.

Posted on by Tammy Plotner

Messier 58 – the NGC 4579 Barred Spiral Galaxy

The galaxies of Messier 58, Messier 59, Messier 60, Messier 87, Messier 89 and Messier 90. Credit: Wikisky

Welcome back to Messier Monday! Today, we continue in our tribute to our dear friend, Tammy Plotner, by looking at the barred spiral galaxy, Messier 58.

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function, becoming one of the first catalogs of Deep Sky Objects.

One of these objects is the intermediate barred spiral galaxy known as Messier 58, which is located approximately 68 million light years away in the Virgo constellation. In addition to being one of just four barred spiral galaxies in the Messier Catalog, it is also one of the brightest galaxies in the Virgo Supercluster. Due to its proximity in the sky to other objects in the Virgo Galaxy Field, it can be seen only with the help of a telescope or a pair of large binoculars.

Description:

This beautiful old barred spiral galaxy located approximately 68 million light-years from Earth. Although it might appear pretty plain, it has some great things going for it… namely an active galactic nucleus. As Marcella Contini indicated in a 2004 study:

“We have modelled the low-luminosity active galactic nuclei (AGN) NGC 4579 by explaining both the continuum and the line spectra observed with different apertures. It was found that the nuclear emission is dominated by an AGN such that the flux from the active centre (AC) is relatively low compared with that of the narrow emission-line region (NLR) of Seyfert galaxies. However, the contribution of a young starburst cannot be neglected, as well as that of shock-dominated clouds with velocities of 100, 300 and 500kms-1. A small contribution from an older starburst with an age of 4.5 Myr, probably located in the external nuclear region, is also found. HII regions appear in the extended regions, where radiation and shock-dominated clouds prevail.

“The continuum SED of NGC 4579 is characterized by the strong flux from an old stellar population. Emissions in the radio range show synchrotron radiation from the base of the jet outflowing from the accretion disc within 0.1 pc from the active centre. Radio emission within intermediate distances is explained by the bremsstrahlung from gas downstream of low-velocity shocks reached by a relatively low radiation flux from the AC. In extended regions the radio emission is synchrotron radiation created by the Fermi mechanism at the shock front. The shocks are created by collision of clouds with the jet. All types of emissions observed at different radius from the centre can be reconciled with the presence of the jet.”

The Messier 58 barred spiral galaxy. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona

Yet where is this gas traveling to and why? According to 2014 study by S. Garcia-Burillo (et al):

“We created a complete gravity torque map of the disk of the LINER/Seyfert 1.9 galaxy NGC 4579. We quantify the efficiency of angular momentum transport and search for signatures of secular evolution in the fueling process from r ~ 15 kpc down to the inner r ~ 50 pc around the active galactic nucleus (AGN). The derived gravity torque budget in NGC 4579 shows that inward gas flow is occurring on different spatial scales in the disk. In the outer disk, the decoupling of the spiral allows the gas to efficiently populate the UHR region, and thus produce net gas inflow on intermediate scales. The co rotation barrier seems to be overcome by secular evolution processes. The gas in the inner disk is efficiently funneled by gravity torques down to r ~ 300 pc. Closer to the AGN, gas feels negative torques due to the combined action of the large-scale bar and the inner oval. The two m=2 modes act in concert to produce net gas inflow down to r ~ 50 pc, providing clear smoking gun evidence of inward gas transport on short dynamical timescales.”

What causes inward transport of gases? Why, a massive gravity pull of course. And what could be more gravitational attractive than a black hole! As Eliot Quataert (et al) indicated in their 1999 study:

“M81 and NGC 4579 are two of the few low-luminosity active galactic nuclei which have an estimated mass for the central black hole, detected hard X-ray emission, and detected optical/UV emission. In contrast to the canonical “big blue bump,” both have optical/UV spectra which decrease with increasing frequency in a plot. Barring significant reddening by dust and/or large errors in the black hole mass estimates, the optical/UV spectra of these systems require that the inner edge of a geometrically thin, optically thick, accretion disk lies at roughly 100 Schwarzschild radii. The observed X-ray radiation can be explained by an optically thin, two temperature, advection-dominated accretion flow at smaller radii.”

Galaxy NGC 4579 was captured by the Spitzer Infrared Nearby Galaxy Survey (SINGS) Legacy Project using the Spitzer Space Telescope’s Infrared Array Camera (IRAC). In this image, the red structures are areas where gas and dust are thought to be forming new stars, while the blue light comes from mature stars. This SINGS image is a four-channel, false-color composite, where blue indicates emission at 3.6 microns, green corresponds to 4.5 microns, and red to 5.8 and 8.0 microns. The contribution from starlight (measured at 3.6 microns) in this picture has been subtracted from the 5.8 and 8 micron images to enhance the visibility of the dust features.

Messier 58 (NGC 4579), as imaged by the Spitzer Infrared Nearby Galaxy Survey (SINGS) Legacy Project using the Spitzer Space Telescope’s Infrared Array Camera (IRAC). Credit: NASA/JPL-Caltech/R. Kennicutt (University of Arizona) and the SINGS Team

History of Observation:

When Charles Messier discovered this one on April 15, 1779, I’m sure he didn’t know he was looking back into time when he wrote:

“Very faint nebula discovered in Virgo, almost on the same parallel as Epsilon, 3rd mag. The slightest light for illuminating the micrometer wires makes it disappear. M. Messier reported it on the chart of the Comet of 1779, which is located in the volume of the Academy for the same year.”

Messier 58 may not have been a comet, but it certainly was another distant cousin of our own Milky Way!

Locating Messier 58:

Finding M58 requires a telescope or large binoculars, and lots of patience. Because the Virgo Galaxy field contains so many galaxies which can easily be misidentified, it is sometimes easier to “hop” from one galaxy to the next! In this case, we need to start by locating bright Vindemiatrix (Epsilon Virginis) almost due east of Denebola. Let’s hop four and a half degrees west and a shade north of Epsilon to locate one of the largest elliptical galaxies presently known – M60.

At a little brighter than magnitude 9, this galaxy could be spotted with binoculars, but stick with your telescope. In the same low power field (depending on aperture size) you may also note faint NGC 4647 which only appears to be interacting with M60. Also in the field to the west (the direction of drift) is our next Messier, bright cored elliptical M59. Now we will need to continue about an average eyepiece field of view, or a degree further west of this group to bring you to our “galactic twin”, fainter M58.

The location of M58, in the direction of the Virgo constellation. Credit: IAU

In a smaller telescope, do not expect to see much. What will appear at low power is a tiny egg-shaped patch of contrast change. As aperture increases, so does detail and a bright nucleus will begin to appear as you move into the 4-6″ size range and dark sky locations. As with all galaxies, dark skies are a must!

And here are the quick facts on this object to help you get started:

Object Name: Messier 58
Alternative Designations: M58, NGC 4579
Object Type: SBc Galaxy
Constellation: Virgo
Right Ascension: 12 : 37.7 (h:m)
Declination: +11 : 49 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 9.7 (mag)
Apparent Dimension: 5.5×4.5 (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, 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:

Posted on by Tammy Plotner

Messier 57 – The Ring Nebula

Hubble image of the Ring Nebula (aka. Messier 57). Credit: NASA/ESA/ Hubble Heritage (STScI/AURA) – ESA /Hubble Collaboration
Hubble image of the Ring Nebula (aka. Messier 57). Credit: NASA/ESA/ Hubble Heritage (STScI/AURA) – ESA /Hubble Collaboration

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the the Big Ring itself, the planetary nebula known as Messier 57. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is known as Messier 57, a planetary nebula that is also known as the Ring Nebula. This object is located about 2,300 light years from Earth in the direction of the Lyra constellation. Because of its proximity to Vega, the brightest star in Lyra and one of the stars that form the Summer Triangle, the nebula is relatively easy to find using binoculars or a small telescope.

What You Are Looking At:

Here you see the remainders of a sun-like star… At one time in its life, it may have had twice the mass of Sol, but now all that’s left is a white dwarf that burns over 100,000 degrees kelvin. Surrounding it is an envelope about 2 to 3 light years in size of what once was its outer layers – blown away in a cylindrical shape some 6000 to 8000 years ago. Like looking down the barrel of a smoking gun, we’re looking back in time at the end of a Mira-like star’s evolutionary phase.

It’s called a planetary nebula, because once upon a time before telescopes could resolve them, they appeared almost planet-like. But, as for M57, the central star itself is no larger than a terrestrial planet! The tiny white dwarf star, although it could be as much as 2300 light years away, has an intrinsic brightness of about 50 to 100 times that of our Sun.

One of the most beautiful features of M57 is the structure in the ring itself, sometimes called braiding – but scientifically known as “knots” in the gaseous structure. As C.R. O’Dell (et al) indicated in their 2003 study:

“We have studied the closest bright planetary nebulae with the Hubble Space Telescope’s WFPC2 in order to characterize the dense knots already known to exist in NGC 7293. We find knots in all of the objects, arguing that knots are common, simply not always observed because of distance. The knots appear to form early in the life cycle of the nebula, probably being formed by an instability mechanism operating at the nebula’s ionization front. As the front passes through the knots they are exposed to the photoionizing radiation field of the central star, causing them to be modified in their appearance. This would then explain as evolution the difference of appearance like the lacy filaments seen only in extinction in IC 4406 on the one extreme and the highly symmetric “cometary” knots seen in NGC 7293. The intermediate form knots seen in NGC 2392, NGC 6720, and NGC 6853 would then represent intermediate phases of this evolution.”

However, examining things like planetaries nebulae in different wavelengths of light can tell us so much more about them. Behold the beauty when see through the Spitzer Space Telescope! As M.M. Roth explained in a 2007 study:

“Emission nebulae like H II regions, Planetary Nebulae, Novae, Herbig Haro objects etc. are found as extended objects in the Milky Way, but also as point sources in other galaxies, where they are sometimes observable out to very large distances due to the high contrast provided by some prominent emission lines. It is shown how 3D spectroscopy can be used as a powerful tool for observations of both large resolved emission nebulae and distant extragalactic objects, with special emphasis on faint detection limits.”

History of Observation:

This deep space object was first discovered in early January 1779 by Antoine Darquier who wrote in his notes:

“This nebula, to my knowledge, has not yet been noticed by any astronomer. One can only see it with a very good telescope, it is not resembling any of those [nebula] already known; it has the apparent dimension of Jupiter, is perfectly round and sharply limited; its dull glow resembles the dark part of the Moon before the first and after the last quarter. Meanwhile, the center appears a bit less pale than the remaining part of its surface.”

Although Darquier did not post a date, it is believed his observation preceded Messier’s independent recovery made on January 31, 1779 when he states that Darquier picked it up before him:

“A cluster of light between Gamma and Beta Lyrae, discovered when looking for the Comet of 1779, which has passed it very close: it seems that this patch of light, which is round, must be composed of very small stars: with the best telescopes it is impossible to distinguish them; there stays only a suspicion that they are there. M. Messier reported this patch of light on the Chart of the Comet of 1779. M. Darquier, at Toulouse, discovered it when observing the same comet, and he reports: ‘Nebula between gamma and beta Lyrae; it is very dull, but perfectly outlined; it is as large as Jupiter and resembles a planet which is fading’.”

A few years later, Sir William Herschel would also observe Messier Object 57 with his superior telescope and in his private notes he writes:

“Among the curiosities of the heavens should be placed a nebula, that has a regular, concentric, dark spot in the middle, and is probably a Ring of stars. It is of an oval shape, the shorter axis being to the longer as about 83 to 100; so that, if the stars form a circle, its inclination to a line drawn from the sun to the center of this nebula must be about 56 degrees. The light is of the resolvable kind [i.e., mottled], and in the northern side three very faint stars may be seen, as also one or two in the southern part. The vertices of the longer axis seem less bright and not so well defined as the rest. There are several small stars very bear, but none seems to belong to it.”

Admiral Smyth would go on in later years to add his own detailed observations to history’s records:

“This annular nebula, between Beta and Gamma on the cross-piece of the Lyre, forms the apex of a triangle which it makes with two stars of the 9th magnitude; and its form is that of an elliptic ring, the major axis of which trends sp to nf [SW to NE]. This wonderful object seems to have been noted by Darquier, in 1779; but neither he nor his contemporaries, Messier and Méchain, discerned its real form, seeing in this aureola of glory only “a mass of light in the form of a planetary disc, very dingy in colour.”

“Sir W. Herschel called it a perforated resolvable nebula, and justly ranked it among the curiosities of the heavens. He considered the vertices of the longer axis less bright and not so well defined as the rest; and he afterwards added: ‘By the observations of the 20-feet telescope, the profundity of the stars, of which it probably consists, must be of a higher than the 900th order, perhaps 950.'”

“This is a vast view of the ample and inconceivable dimensions of the spaces of the Universe; and if the oft-cited cannon-ball, flying with the uniform velocity of 500 miles an hour, would require millions of years to reach Sirius, what an incomprehensible time it would require to pass so overwhelming an interval as 950 times the distance! And yet, could we arrive there, by all analogy, no boundary would meet the eye, but thousands and ten thousands of other remote and crowded systems would still bewilder the imagination.

“In my refractor this nebula has a most singular appearance, the central vacuity being black, so as to countenance the trite remark of its having a hole through it. Under favourable circumstances, when the instrument obeys the smooth motion of the equatorial clock, it offers the curious phenomenon of a solid ring of light in the profundity of space. The annexed sketch affords a notion of it. Sir John Herschel, however, with the superior light of his instrument, found that the interior is far from absolutely dark. “It is filled,’ he says, ‘with a feeble but very evident nebulous light, which I do not remember to have been noticed by former observers.'”

Since Sir John’s observation, the powerful telescope of Lord Rosse has been directed to this subject, and under powers 600, 800, and 1000, it displayed very evident symptoms of resolvability at its minor axis. The fainter nebulous matter which fills it, was found to be irregularly distributed, having several stripes or wisps in it, and the regularity of the outline was broken by appendages branching into space, of which prolongations the brightest was in the direction of the major axis.

Locating Messier 57:

M57 is a breeze to locate because it is positioned between Beta and Gamma Lyrae (the westernmost pair of the lyre’s stars), at about one-third the distance from Beta to Gamma. While it is easily seen in binoculars, it is a little difficult to identify because of its small size, so binoculars must be very steady to distinguish it from the surrounding star field.

In even a small telescope at minimum power, you’ll quickly notice a very small, but perfect ring structure which takes very well to magnification. Despite low visual brightness, M57 actually takes well to urban lighting conditions and can even be spied during fairly well moonlit nights. Larger aperture telescopes will easily see braiding in the nebula structure and often glimpse the central star. May you also see the many faces of the “Ring”!

The location of Messier 57 in the Lyra Constellation. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

And here are the quick facts on Messier 57 to help you get started:

Object Name: Messier 57
Alternative Designations: M57, NGC 6720, the “Ring Nebula”
Object Type: Planetary Nebula
Constellation: Lyra
Right Ascension: 18 : 53.6 (h:m)
Declination: +33 : 02 (deg:m)
Distance: 2.3 (kly)
Visual Brightness: 8.8 (mag)
Apparent Dimension: 1.4×1.0 (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, 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:

Posted on by Fraser Cain

Where Do We Go Next? Building the Deep Space Gateway

Where do we go next? The Deep Space Gateway
Where do we go next? The Deep Space Gateway


I don’t have to tell you that the vision of human space exploration in the Solar System has kind of stalled. Half a century ago, humans set foot on the Moon, and we haven’t been back since. Instead, we’ve thoroughly explored every cubic meter of low Earth orbit, going around and around the Earth. In fact, back in 2016, the International Space Station celebrated 100,000 orbits around the Earth.

The space shuttle was the last US vehicle capable of taking humans up into orbit, and it was retired back in 2011. So things look pretty bleak for sending humans out to explore the Solar System.

Earlier this year, however, NASA announced their next great step in their human space exploration efforts: the Deep Space Gateway. And if all goes well, we’ll see humans living and working farther from Earth, and for longer periods than ever before.

After the space shuttle program was wrapped up, NASA had a bunch of challenges facing it. Perhaps the greatest of these, was what to do with the enormous workforce that built and maintained the space shuttle fleet. Thousands were laid off, and moved to other aerospace jobs and other industries, but the space agency worked to develop the next big launch system after the shuttle.

Originally there were the Ares rockets, as part of the Constellation Program, but these were canceled and replaced with the Space Launch System. We’ve done a whole episode on the SLS, but the short version is that this new rocket will be capable of lifting more cargo into orbit than any rocket ever.

The first version, known as the Block 1 will be capable of lofting 70,000 kg into low-Earth orbit, while the upcoming Block 2 will be able to carry 130,000 kg into LEO – more than the mighty Saturn V rocket.

What are you going to do with a rocket this powerful? Launch new space telescopes, robotic missions to the outer Solar System, and put humans into space, of course.

In addition to the SLS, NASA is also working on a new capsule, known as the Orion Crew Module. This Apollo-esque capsule will be capable of carrying a crew of 4 astronauts out beyond low-Earth orbit, and returning them safely back to Earth.

But if you can send astronauts out beyond low-Earth orbit, where will they go?

Artist's impression of the Deep Space Gateway, currently under development by Lockheed Martin. Credit: NASA
Artist’s impression of the Deep Space Gateway, currently under development by Lockheed Martin. Credit: NASA

The Deep Space Gateway.

The plan is to put a brand new space station into a cis-lunar orbit. Specifically, it’s known as a near-rectilinear halo orbit. It won’t actually be orbiting the Moon, but it’ll be on an orbit that allows it to serve as a stepping stone to the Moon. Sort of a bridge between Lagrange points. This station will range in distance from 1,500 to 70,000 km from the Moon in a way that keeps it relatively easy to reach.

From the outside, it’ll look like a smaller version of the International Space Station, with a group of 4 pressurized modules connected together: a power module, habitation module, cargo logistics pod, and an EVA module.

Space inside the Gateway will be cramped, with astronauts needing to share their living quarters, reconfiguring the space as necessary. Seriously, the ISS is going to feel like a luxury hotel after spending time in the Gateway.

Artist illustration of Habitation Module. Credit: Lockheed Martin
Artist illustration of Habitation Module. Credit: Lockheed Martin

The station will be solar powered, with arrays providing 40 kW of energy. It’ll also have 12 kW ion thrusters which will be used for station keeping, as well as traditional hydrazine thrusters. The first habitation module will be capable of supplying the astronauts for 30-60 days, but a later cargo logistics pod will extend the length of missions.

Right now, there are a group of contractors being considered to build the Deep Space Gateway. The designs I’m showing you come from Lockheed Martin, but things could change.

The goal of the Deep Space Gateway will be to keep humans alive in space outside the Earth’s protective magnetosphere for at least a year, studying the effects of deep space on the human body.

But in the long term, the Gateway will serve as a stepping stone to Mars. The astronauts will assemble the future Deep Space Transport, a spacecraft that will carry humans to the Red Planet. But more on that later.

On the International Space Station, astronauts are protected by the Earth’s magnetosphere from solar radiation and cosmic rays. But on board the Deep Space Gateway, there’ll be no such protection. Instead, the station will need to be reinforced with radiation protection. At the same time, the region actually has less space junk, so it won’t need to same kind of micrometeorite protection.

In addition to being a science platform, the DSG will serve as a base of operations for exploring the Moon. In the near term, NASA is planning new lander and rover missions to the Moon. The Gateway could serve as a dock for missions blasting off from the Moon, where astronauts could unload science samples, and refurbish a rover for another mission down on the lunar surface.

Another intriguing idea is that the Deep Space Gateway could be used as a place to study samples from Mars without a risk of contaminating Earth. Under the current planetary contamination guidelines, samples from Mars need to be sterilized before they can be brought to Earth.

It’s hard to search for life in your samples, when you need to kill all life in your samples. But I’m sure the astronauts would be willing to take the risk of catching Martian flu for a chance to discover there’s life on Mars.

When will we actually see the Deep Space Gateway?

Not for a few years, sadly. Building the Gateway is going to require a few launches of the SLS, and there are already a bunch of missions queued up to use this new launcher.

SLS Block 1 Expanded View. Credit: NASA
SLS Block 1 Expanded View. Credit: NASA

The first launch of SLS will be an uncrewed test with an Orion capsule, sometime in 2019, known as EM-1. This will be followed by the launch of the Europa Clipper mission, also in 2019.

Once those missions are out of the way, the first crewed launch with SLS blasts off some time between 2021 and 2023. Designated as EM-2, this is when the construction of the Deep Space Gateway begins. 4 astronauts will spend 3 weeks beyond low Earth orbit, delivering the first module to the Deep Space Gateway: the Solar Power Electric Bus.

In 2024, EM-3 will have another crew of 4 blast off with the Deep Space Gateway’s Habitation Module. EM-4 should lift off by 2025 with the Logistics module. Finally, some time around 2026, mission EM-5 will deliver the station’s Airlock module.

SLS Block 2. Credit: NASA
SLS Block 2. Credit: NASA

What comes next? After the Deep Space Gateway, there’ll be the Deep Space Transport. If you’ve seen The Martian, think of the Hermes spacecraft that ferries the crew to and from Mars. The details are thin right now, but if all goes well, the pieces of the Transport will launch to the Gateway by 2027.

The various components will be assembled by the astronauts over the course of several launches, and once completed, the Deep Space Transport would make a series of 1-3 year missions to and from Mars. It’ll carry a crew of a six astronauts in a large habitation module and keep them alive for the journey.

The first mission could head out in 2033, with a human flyby of Mars. Side note, wouldn’t it be heartbreaking to get that close to Mars, and not actually be able to set foot on the surface? Anyway, future missions to Mars will include landings, and perhaps a visit to the SpaceX luxury Martian hotel where the astronauts can relax and apologize to each other for what they did when they all got space madness.

But this is so far in the future, it’s pretty hard to even wrap my mind around it yet.

Of course, these are all long term plans. And as I’ve mentioned in previous episodes, long term plans have a tendency of getting canceled. Who knows if the Deep Space Gateway actually get constructed, or if NASA will shift its support to private missions to Mars.

We’ll just have to stay tuned.

Posted on by Tammy Plotner

Messier 56 – the NGC 6779

Messier 56 and Messier 57 (the Ring Nebula). Credit: Wikisky

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the the globular star cluster known as Messier 56. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is Messier 56, a globular star cluster located in the small northern constellation of Lyra, roughly 32,900 light years from Earth. Measuring roughly 84 light-years in diameter, this cluster has an estimated age of 13.70 billion years. It is also relatively easy to spot because of its proximity to well-known asterisms like the celestial Swan, the Northern Cross, and the bright star Vega.

Description:

Spanning about 85 light years in diameter, this incredible ball of stars is moving towards planet Earth at a speed of 145 kilometers per second… yet still remains about 32,900 light-years away. As one of the less dense of the Milky Way’s halo globulars, it is also less dense in variable stars – containing only perhaps a dozen. But out of that twelve, there a very special one… a Cepheid bright enough to be followed with amateur instruments. However, astronomers never stopped looking for the curious – and they found what they were looking for!

NASA/ESA Hubble image of the globular star cluster known as Messier 56. Credit: NASA/ESA/HST/Gilles Chapdelaine

The CURiuos Variables Experiment (CURVE) was performed on M56 in 2008. As P. Pietrukowicz (et al) wrote of the cluster in the accompanying study:

“We surveyed a 6.5’×6.5′ field centered on the globular cluster M56 (NGC 6779) in a search for variable stars detecting seven variables, among which two objects are new identifications. One of the new variables is an RRLyrae star, the third star of that type in M56. Comparison of the new observations and old photometric data for an RV Tauri variable V6 indicates a likely period change in the star. Its slow and negative rate of -0.005±0.003 d/yr would disagree with post-AGB evolution, however this could be a result of blue-loop evolution and/or random fluctuations of the period.”

But could other things exist inside M56? Events, perhaps, like nova? As astronomer Tim O’Brien wrote:

“Classical nova outbursts are the result of thermonuclear explosions on the surface of a white dwarf star in a close binary system. Material from the other star in the system (one not unlike our own sun) falls onto the surface of the white dwarf over thousands of years. The pressure at the base of this layer of accreted material builds up until thermonuclear reactions begin explosively. An Earth’s mass or more of material is ejected from the surface of the white dwarf at speeds of a few hundred to a few thousand kilometres per second. Old novae are therefore surrounded by shells of ejected matter illuminated by the light from the central binary system.”

And as M.E.L. Hopwood (et al.) wrote in a 2000 study:

“We report the possible detection of diffuse X-ray emission in the environment of NGC 6779, and find the emission to be well aligned with the proper motion of the cluster. The position of the emission suggests we are observing heated ISM in the wake of the cluster that could be the result of an interaction between the intracluster medium and the halo gas surrounding it.”

Globular cluster Messier 56 in Lyra. Credit: Wikipedia Commons/Hewholooks

History of Observation:

Charles Messier first discovered M56 on January 23rd, 1779. As he wrote of his discovery at the time:

“Nebula without stars, having little light; M. Messier discovered it on the same day as he found the comet of 1779, January 19. On the 23rd, he determined its position by comparing it with the star 2 Cygni, according to Flamsteed: it is near the Milky Way; and close to it is a star of 10th magnitude. M. Messier reported it on the chart of the comet of 1779.”

However, it would be Sir William Herschel who revealed its true nature in 1807. In his private notes he writes: “The 56th of the Connoiss. is a globular cluster of very compressed and very small stars. They are gradually more compressed towards the centre.” His son John would go on to observe it many times, even after cataloging it! His best description reads: “Large; round; very gradually brighter toward the middle. I see the stars which are very small and of different sizes. It fades gradually away to the borders.”

As always, it would be Admiral Smyth who would be perhaps a bit more descriptive when he included in his observing notes:

“A globular cluster, in a splendid field, between the eastern joke of Lyra’s frame and the Swan’s head: it is 5 1/4 deg distant from Beta Lyrae, on the south-east line leading to Beta Cygni, which is about 3 1/2 deg further. This object was first registered by M. Messier in 1778, and, from his imperfect means, described as a nebula of feeble light, without a star. In 1784, it was resolved by Sir William Herschel, who, on gauging, considered its profundity to be of the 344th order.”

Messier 56 location. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Locating Messier 56:

Finding M56 isn’t too hard since it’s located about half-way between Beta Cygni (Albireo) and Gamma Lyrae. In both binoculars and finder scope, you will see a triangle of stars when progressing from Gamma towards the southeast that will almost point directly at it! Because M56 isn’t particularly large or bright, it does require dark skies – but makes a great object for both binoculars and small telescopes.

Enjoy this pincushion of stars! And here are the quick facts on this Messier Object to help you get started”

Object Name: Messier 56
Alternative Designations: M56, NGC 6779
Object Type: Class X Globular Cluster
Constellation: Lyra
Right Ascension: 19 : 16.6 (h:m)
Declination: +30 : 11 (deg:m)
Distance: 32.9 (kly)
Visual Brightness: 8.3 (mag)
Apparent Dimension: 8.8 (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, 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:

Posted on by Tammy Plotner

The Crux Constellation

The constellation Crux, aka the Southern Cross. Credit: ESO/ Yuri Beletsky Nightscapes.

Welcome to another edition of Constellation Friday! Today, in honor of the late and great Tammy Plotner, we take a look at the “Southern Cross” – the Crux constellation. Enjoy!

In the 2nd century CE, Greek-Egyptian astronomer Claudius Ptolemaeus (aka. Ptolemy) compiled a list of all the then-known 48 constellations. This treatise, known as the Almagest, would be used by medieval European and Islamic scholars for over a thousand years to come, effectively becoming astrological and astronomical canon until the early Modern Age.

One of these constellations is known as Crux, a small constellation located in the southern skies. Despite its size, it is one of the most well-known constellations in the southern hemisphere due to its distinctive cross-shape. Today, it has gone on to become one of the 88 modern constellations currently recognized by the International Astronomical Union (IAU).

Name and Meaning:

For the people of the Southern Hemisphere, the Crux constellation has a great deal of cultural significance. The Incas knew the constellation as Chakana (Quechua for “the stair”), and a stone image of the stars was found in Machu Picchu, Peru. To the Maori, the constellation was known as Te Punga, or “the anchor”, due to the important role it played in maritime navigation.

The “Emu in the Sky”, an important constellation recognized by the Aborigines of Australia. Credit: RSAA/ANU

To the Aborigines of Australia, the cross and the Coalsack Nebula together represented the head of the Emu in the Sky. This mythical bird is associated with several Aborigine creation myths and is one of the most important constellations in their astronomical traditions. Because of this significance, the Southern Cross is represented on the flags of Australia, Papua New Guinea, New Zealand Brazil, and Brazil.

The first recorded instance of Crux being named is believed to have occurred in 1455, when Venetian navigator Alvise Cadamosto made note of an asterism in the southern skies that he called carr dell’ostro (“southern chariot”). However, historians generally credit Portugese astronomer Joao Faras with the discovery, which occurred in 1500 when he spotted it from Brazil and named it “Las Guardas” (“the guards”).

By the late 16th century, Crux began to be depicted as a separate constellation on celestial globes and maps. In these and subsequent maps, the name Crux was used (Latin for “Cross”), referring to the constellation’s distinct shape.

History of Observation:

Crux was originally considered to be part of Centaurus, but as the precession of the equinoxes gradually lowered these stars below the European horizon, they were lost sight of, and so was the memory of these stars. At one time, around 1000 BCE, the stars of Crux were visible to the northern hemisphere, but by 400 CE they had slipped below the horizon for most populated areas.

The constellation Crux as it can be seen by the naked eye. Credit: Till Credner/AlltheSky.com

Even though it was originally plotted on Ptolemy’s Almagest, it first appeared as “Crux” on the charts of Petrus Plancius and Jodocus Hondius in 1598 and 1600 – both navigators. It is known that Amerigo Vespucci mapped the stars of Crux on his expedition to South America in 1501, and with good reason!

Two of the stars of Crux (Alpha and Gamma, Acrux and Gacrux respectively) are commonly used to mark due south. Following the line defined by the two stars for approximately 4.5 times the distance between them leads to a point close to the Southern Celestial Pole. A definite point needed for navigation! In 1920, Crux was included among the 88 modern constellations recognized by the IAU.

Notable Objects:

Of the major stars in Crux, Alpha Crucis (Acrux) is the brightest, and the 12th brightest star in the night sky. It is located approximately 320 light years away and is a multiple star system composed of Alpha-1 Crucis (a B class subgiant) and Alpha-2 Crucis (a B class dwarf). Both stars are very hot and their respective luminosities are 25,000 and 16,000 times that of the Sun.

Beta Crucis (Becrux, or Mimosa) is the second brightest star of the Southern Cross and the 20th brightest star in the night sky. It is approximately 350 light years distant, is classified as a Beta Cephei variable, and is a spectroscopic binary composed of two stars that are about 8 AU apart and orbit each other every five years. The name Mimosa refers to its color (blue-hued).

Gamma Crucis (Gacrux) is a red giant that is approximately 88 light years distant from Earth. It is the third brightest star in the Crux constellation and the 26th brightest star in the sky. Located about 400 light years distant from Earth, this binary star is composed of a M4 red dwarf star and a A3 white dwarf star.

Crux is also associated with several Deep Sky Objects, the most notable of which is the Coalsack Nebula. This object is easily seen as a dark patch in the southern region of the Milky Way (hence the name) and crosses into the neighboring constellations of Centaurus and Musca. It is located about 600 light years from Earth and is between 30 and 35 light years in radius. In Aboriginal astronomy, the nebula represents the head of the Emu.

Then there’s the Kappa Crucis Cluster (aka. the “Jewel Box” or “Herschel’s Jewel Box”), an open star cluster that is located approximately 6,440 light years from Earth. It contains roughly 100 stars and is one of the youngest clusters ever discovered (only 14 million years old). To the naked eye, the cluster appears like a star near Beta Crucis.

Finding Crux:

The constellation itself consists of four bright, main stars and 19 stars which have Bayer/Flamsteed designations. It is bordered by the constellations of Centaurus and Musca. At present, Crux is visible at latitudes between +20° and -90°. While it is fairly circumpolar for the southern hemisphere, it is best seen a culmination during the month of May.

The location of the Crux constellation. Credit: IAU

Now, let’s take out binoculars and examine its stars, started with Alpha Crucis, the “a” shape on our map. Its proper name is Acrux and it is the twelfth brightest star in the night sky. If you switch your binoculars out for a telescope, you’ll find that 321 light year distant Acrux is also a binary star, with components separated by about 4 arc seconds and around one half stellar magnitude difference in brightness.

The brighter of the two, A1 is itself a spectroscopic binary star – with a companion that orbits no further away than our own Earth, yet is around 14 times larger than our own Sun! Needless to say, there’s a very good chance this star may one day go supernova. While you’re there, take a look an addition 90 arc seconds away for a third star. While it may just be an optical companion to the Acrux system, it does share the same proper motion!

Back to binoculars an on to Beta Crucis – the “B” shape on the map. Mimosa is located about 353 light years away from our solar system and it is also a spectroscopic binary star. This magnificent blue/white giant star is tied at number 19 as one of the brightest stars in the sky, and if we could put it side by side with our Sun it would be 3000 times brighter. Mimosa is also a multiply-periodic Beta-Cephi type star, too, fluxing by about 1/10 of a magnitude in as little as hours. What’s going on? Inside Beta Crucis the iron content is only about half that of Sol and it’s nearing the end of its hydrogen-fusing stage. When the iron core develops? Watch out! It’s supernova time….

Now hang on to your binoculars and head north for Gamma Crucis, the “Y” shape on the map. Gacrux, is a red giant star approximately 88 light-years away from Earth. Did you notice its optical companion about 2 arc minutes away at an angle of 128 degrees from the main star? While the two look close together in the sky, the secondary star is actually 400 light years away! Gacrux shows its beautiful orange coloring to prove it has evolved off of the main sequence to become a red giant star, and it may even be evolving past the helium-burning stage.

The Coalsack Nebula and Kappa Crucis Cluster. Credit: A. Fujii

Move on now to Delta Crucis – the figure “8” on our map. Decrux is a red giant star located about 360 light years away from our vantage point. Delta Crucis is also Beta Cephei variable and changes its brightness just a tiny bit over a period of about an hour and 20 minutes. Another cool factoid about Delta Crucis is that it’s a fast rotator – spinning at a speed of at least 194 kilometers per second at the equator and making a full rotation in about 32 hours.

This massive star also produces a massive stellar wind, shooting off 1000 times more material than our own Sun every second of every day! Or try R Crucis… It’s also a Beta-Cephi type variable star, but it changes by nearly a full stellar magnitude in just a little over five days!

Keep your binoculars handy and head back to Beta and sweep south a degree and a half for the Kappa Crucis star cluster. This beautiful galactic cluster of stars commonly known as the Jewel Box (NGC 4755). After you see its glittering collection of multi-colored stars, you’ll understand how it got its name! It is one of the youngest clusters, perhaps only a few million years old.

Kappa Crucis is also right on the edge of a dark void in the sky called the “Coal Sack”. While you’re looking around, you’ll notice that there seem to be very few stars in this area. That’s because they are being blocked by a dark nebula! The Coal Sack is a large, dark dust cloud about 500 light years away and it’s blocking out the light from stars which lie beyond it. The few stars you do see are in front of the cloud and much nearer to the Earth.

The Jewel Box – the Kappa Crucis Cluster. Credit: ESO/NASA/ESA/Digitized Sky Survey 2/Jesús Maíz Apellániz (Instituto de Astrofísica de Andalucía, Spain)

Now it’s telescope time. Head to Alpha Crucis and slightly less than 2 degrees east for NGC4609. Also on the edge of the Coalsack, this large, fairly condensed open cluster contains about 40 members and they are well spread across the sky. The pattern somewhat resembles the constellation of Orion (in the imagination, of course!). Mark you observing notes for Caldwell 98. Next stop? Back to Delta and less than 3 degrees south/southwest for NGC4103, another open cluster on the edge of night. With a little bit of imagination, this grouping of stars could appear to look like a celestial water tower!

We have written many interesting articles about the constellation here at Universe Today. Here is What Are The Constellations?, What Is The Zodiac?, and Zodiac Signs And Their Dates.

Be sure to check out The Messier Catalog while you’re at it!

For more information, check out the IAUs list of Constellations, and the Students for the Exploration and Development of Space page on Canes Venatici and Constellation Families.

Sources:

Posted on by Tammy Plotner

Messier 55 – the NGC 6809 Globular Star Cluster

The globular star cluster Messier 55 in the constellation of Sagittarius (The Archer) was obtained in infrared light with the VISTA survey telescope at ESO’s Paranal Observatory in Chile. This vast ball of ancient stars is located at a distance of about 17 000 light-years from Earth. Credit: ESO/J. Emerson/VISTA

Welcome back to Messier Monday! We continue our tribute to our dear friend, Tammy Plotner, by looking at the “Summer Rose Star”, other known as the globular star cluster of Messier 55. Enjoy!

In the 18th century, while searching the night sky for comets, French astronomer Charles Messier kept noting the presence of fixed, diffuse objects in the night sky. In time, he would come to compile a list of approximately 100 of these objects, with the purpose of making sure that astronomers did not mistake them for comets. However, this list – known as the Messier Catalog – would go on to serve a more important function.

One of these objects is Messier 55, a globular star cluster located in the Sagittarius Constellation. Also known as the “Summer Rose Star”, this cluster is located 17,600 light-years from Earth and spans about 100 light-years in diameter. While it can be seen with binocular, resolving its individual stars can only be done with a small telescope and finderscope.

Description:

Located some 17,300 light years from planet Earth and spanning nearly 100 light years in diameter, this loose appearing ball of stellar points may not seem concentrated – but its home to tens of thousands stars. Does anyone really take the time to count them? You bet. M.J. Irwin and V. Trimble did just that during their 1984 study of Messier 55:

“We report star counts, as a function of position and apparent magnitude, in the rich, relatively open southern globular cluster NGC 6809 (M55). Three AAO 150arcsec plates were scanned by the Automatic Plate Measuring System (APM) at the Institute of Astronomy, Cambridge, and 20825 images were counted by its associated software. Previously known features of rich globular clusters which appear in the raw counts include a flattening of the luminosity function, increased central concentration of bright stars relative to faint ones (normally interpreted as mass segregation), and mild deviations in radial profile from King models. Crowding of the field, which causes the counting procedure to miss faint stars preferentially near the cluster center, contributes to all of these, and may be responsible for all of the apparent mass segregation, but not for all of the other two effects.”

Globular cluster Messier 55 (M55, or NGC 6809) in the constellation Sagittarius, as imaged by the ESO 3.6-metre telescope on La Silla. Release date: 3 December 2009. Credit: ESO

But just want good does counting the stars do? Well, knowing how many stars are within a given area helps astronomers compute other things as well, like chemical abundances. Said Carlos Alvarez and Eric Sandquist in their 2004 study:

“We have compiled the asymptotic giant, horizontal, and upper red giant branch (AGB, HB, and RGB) stars in the globular cluster M55 (NGC 6809). Using the star counts and the R-parameter we compute the initial helium abundance. The ratio is unusually high for a globular cluster, being almost 2 away from the predicted values, and the highest recorded for a massive globular cluster. We argue that M55’s particular HB morphology and metallicity have produced long-lived HB stars that are not too blue to avoid producing AGB stars. This result hints that we are able to map evolutionary effects on the HB. Finally, although we find no evidence of variations in HB morphology with distance from the center of the cluster, the red HB stars are significantly less concentrated than the majority of HB stars, and the bluest HB stars are more centrally concentrated.”

Studying globular clusters photometrically also gives astronomers the advantage of comparing them to others, to see how each evolves. As P. Richter (et al) indicated in their 1999 study:

“We present Stroemgren CCD photometry for the two galactic globular clusters M55 (NGC 6809) and M22 (NGC 6656). The difference between M55 and M22 may resemble the difference in integral CN band strength between M31 globular clusters and the galactic system. The colour-magnitude diagram of M55 shows the presence of a population of 56 blue-straggler stars that are more centrally concentrated than the red giant-branch stars.”

And viewing globular clusters like Messier 55 in a different wavelength of light other than optical reveals even more stunning details – like the vision of the XMM-Newton. As N.A. Webb (et al) said in their 2006 study:

“Using the new generation of X-ray observatories, we are now beginning to identify populations of close binaries in globular clusters, previously elusive in the optical domain because of the high stellar density. These binaries are thought to be, at least in part, responsible for delaying the inevitable core collapse of globular clusters and their identification is therefore essential in understanding the evolution of globular clusters, as well as being valuable in the study of the binaries themselves. Here, we present observations made with XMM-Newton of globular clusters, in which we have identified neutron star low mass X-ray binaries and their descendants (millisecond pulsars), cataclysmic variables and other types of binaries. We discuss not only the characteristics of these binaries, but also their formation and evolution in globular clusters and their use in tracing the dynamical history of these clusters.”

History of Observation:

M55 was originally discovered by Abbe Lacaille on June 16th, 1752, when he was observing in South Africa. In his notes, he wrote: “It resembles an obscure nucleus of a big comet.” Of course, our own comet hunter, Charles Messier, would search for a good many years before he recovered it to add to his own catalog. By July 24th, 1778, he found the object and recorded it as follows in his notes:

“A nebula which is a whitish spot, of about 6′ extension, its light is even and does not appear to contain any star. Its position has been determined from zeta Sagittarii, with the use of an intermediate star of 7th magnitude. This nebula has been discovered by M. l’Abbe de LaCaille, see Mem. Acad. 1755, p. 194. M. Messier has looked for it in vain on July 29, 1764, as reported in his memoir.”

Messier 55 in Sagittarius. Credit: Hewholooks/Wikipedia Commons

Johann Elert Bode, Dunlop and Caroline Herschel would follow, but it would be Sir William Herschel who would be first to glimpse the resolvability of this great globular cluster. In his private notes he writes:

“A rich cluster of very compressed stars, irregularly round, about 8 minutes long. By the observation of the small 20 feet telescope, which could reach stars 38.99 times as far as the eye, the profundity of this cluster cannot be much less than of the 467th order: I have taken it to be of the 400th order.”

Locating Messier 55:

M55 is by no means easy to find. One of the best ways to locate it is to begin at Theta 1 and Theta 2 Sagittarius, where you’ll find it approximately two finger widths northwest of this pair approximately four degrees. Both Thetas are on the dim side for the unaided eye – about magnitude 4 and 5 respectively, but you’ll recognize them when you find two stars separated by less than half a degree and oriented north/south.

For average binoculars, this will put M55 about a binocular field away to the northwest. For average image correct finderscopes, place the Thetas in the 8:00 position at the edge of the finderscope field and go to the eyepiece with the lowest possible magnification to locate it.

Messier 55 location. Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

Although it has a high visual brightness, M55 has low surface brightness so it isn’t suitable to urban or light polluted skies. With dark sky conditions, binoculars will see it as a round hazy patch – like a diffuse comet, while small telescopes can begin to resolve individual stars. Larger aperture telescopes will pick out the fine grain of low magnitude stars quite easily!

Enjoy your own resolvability of this great globular cluster!

And as always, here are the quick facts on this Messier Object:

Object Name: Messier 55
Alternative Designations: M55, NGC 6809
Object Type: Class XI Globular Cluster
Constellation: Sagittarius
Right Ascension: 19 : 40.0 (h:m)
Declination: -30 : 58 (deg:m)
Distance: 17.3 (kly)
Visual Brightness: 6.3 (mag)
Apparent Dimension: 19.0 (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, 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:

Posted on by Tammy Plotner

Messier 54 – the NGC 6715 Globular Cluster

Hubble image of Messier 54, a globular cluster located in the Sagittarius Dwarf Galaxy. Credit: ESA/Hubble & NASA

Welcome back to Messier Monday! In our ongoing tribute to the great Tammy Plotner, we take a look at globular cluster known as Messier 54!

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 these objects so 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 globular cluster known as Messier 54. Located in the direction of the Sagittarius constellation, this cluster was once thought to be part of the Milky Way, located about 50,000 light years from Earth, In recent decades, astronomers have come to realize that it is actually part of the Sagittarius Dwarf Galaxy, located some 87,000 light-years away.

What You Are Looking At:

Running away from us at a speed of 142 kilometers per second, this compact globe of stars could be as wide as 150 light years in diameter and as far away as 87,400 light years. Wait… Hold the press… Almost 90 thousand light years? Yeah. Messier 54 isn’t part of our own Milky Way Galaxy!

In 1994 astronomers made a rather shocking discovery… this tough to resolve globular was actually part of the Sagittarius Dwarf Elliptical Galaxy. As Michael H. Siegal (et al) said in their study:

“As part of the ACS Survey of Galactic Globular Clusters, we present new Hubble Space Telescope photometry of the massive globular cluster M54 (NGC 6715) and the superposed core of the tidally disrupted Sagittarius (Sgr) dSph galaxy. Our deep (F606W ~ 26.5), high-precision photometry yields an unprecedentedly detailed color-magnitude diagram showing the extended blue horizontal branch and multiple main sequences of the M54+Sgr system. Multiple turnoffs indicate the presence of at least two intermediate-aged star formation epochs with 4 and 6 Gyr ages and [Fe/H]=-0.4 to -0.6. We also clearly show, for the first time, a prominent, ~2.3 Gyr old Sgr population of near-solar abundance. A trace population of even younger (~0.1-0.8 Gyr old), more metal-rich ([Fe/H]~0.6) stars is also indicated. The Sgr age-metallicity relation is consistent with a closed-box model and multiple (4-5) star formation bursts over the entire life of the satellite, including the time since Sgr began disrupting.”

Inside its compact depths lurk at least 82 known variable stars – 55 of which are the RR Lyrae type. But astronomers using the Hubble Space telescope have have also discovered there are two semi-regular red variables with periods of 77 and 101 days. Kevin Charles Schlaufman and Kenneth John Mighell of the National Optical Astronomy Observatory explained in their study:

“Most of our candidate variable stars are found on the PC1 images of the cluster center – a region where no variables have been reported by previous ground-based studies of variables in M54. These observations cannot be done from the ground, even with AO as there are far too many stars per resolution element in ground-based observations.”

The globular cluster Messier 54. Credit: NASA

But what other kinds of unusual stars could be discovered inside such distant cosmic stellar evolutionary laboratory? Try a phenomena known as blue hook stars! As Alfred Rosenberg (et al) said in their study:

“We present BV photometry centered on the globular cluster M54 (NGC 6715). The color-magnitude diagram clearly shows a blue horizontal branch extending anomalously beyond the zero-age horizontal-branch theoretical models. These kinds of horizontal-branch stars (also called “blue hook” stars), which go beyond the lower limit of the envelope mass of canonical horizontal-branch hot stars, have so far been known to exist in only a few globular clusters: NGC 2808, Omega Centauri (NGC 5139), NGC 6273, and NGC 6388. Those clusters, like M54, are among the most luminous in our Galaxy, indicating a possible correlation between the existence of these types of horizontal-branch stars and the total mass of the cluster. A gap in the observed horizontal branch of M54 around Teff = 27,000 K could be interpreted within the late helium flash theoretical scenario, which is a possible explanation for the origin of blue hook stars.”

But with the stars packaged together so tightly, even more has been bound to occur inside of Messier 54. As Tim Adams (et al) indicated in their study:

“We investigate a means of explaining the apparent paucity of red giant stars within post-core-collapse globular clusters. We propose that collisions between the red giants and binary systems can lead to the destruction of some proportion of the red giant population, by either knocking out the core of the red giant or by forming a common envelope system which will lead to the dissipation of the red giant envelope. Treating the red giant as two point masses, one for the core and another for the envelope (with an appropriate force law to take account of the distribution of mass), and the components of the binary system also treated as point masses, we utilize a four-body code to calculate the time-scales on which the collisions will occur. We then perform a series of smooth particle hydrodynamics runs to examine the details of mass transfer within the system. In addition, we show that collisions between single stars and red giants lead to the formation of a common envelope system which will destroy the red giant star. We find that low-velocity collision between binary systems and red giants can lead to the destruction of up to 13 per cent of the red giant population. This could help to explain the colour gradients observed in PCC globular clusters. We also find that there is the possibility that binary systems formed through both sorts of collision could eventually come into contact perhaps producing a population of cataclysmic variables.”

Messier 54, as imaged by the VLT Survey Telescope at ESO’s Paranal Observatory in northern Chile. Credit: ESO

But the discoveries haven’t ended yet…. Because 2009 studies have revealed evidence for an intermediate mass black hole inside Messier 54 – the first known to have ever been discovered in a globular cluster.

“We report the detection of a stellar density cusp and a velocity dispersion increase in the center of the globular cluster M54, located at the center of the Sagittarius dwarf galaxy (Sgr). The central line-of-sight velocity dispersion is 20.2 ± 0.7 km s-1, decreasing to 16.4 ± 0.4 km s-1 at 2farcs5 (0.3 pc). Modeling the kinematics and surface density profiles as the sum of a King model and a point-mass yields a black hole mass of ~9400 M sun.” says R. Ibata (et al), “However, the observations can alternatively be explained if the cusp stars possess moderate radial anisotropy. A Jeans analysis of the Sgr nucleus reveals a strong tangential anisotropy, probably a relic from the formation of the system.”

History of Observation:

On July 24, 1778 when Charles Messier first laid eyes on this faint fuzzy, he had no clue that he was about to discover the very first extra-galactic globular cluster. In his notes he writes: “Very faint nebula, discovered in Sagittarius; its center is brilliant and it contains no star, seen with an achromatic telescope of 3.5 feet. Its position has been determined from Zeta Sagittarii, of 3rd magnitude.”

Years later Sir William Herschel would also study M54, and in his private notes he writes: “A round, resolvable nebula. Very bright in the middle and the brightness diminishing gradually, about 2 1/2′ or 3′ in diameter. 240 shews too pretty large stars in the faint part of the nebulosity, but I rather suppose them to have no connection with the nebula. I believe it to be no other than a miniature cluster of very compressed stars.”

Countless other observations would follow as the M54 became cataloged by other astronomers and each would in turn describe it only as having a much brighter core and some resolution around the edges. Have fun trying to crack this one!

Locating Messier 54:

M54 isn’t hard to find… Just skip down to Zeta Sagittarii, the southwestern-most star of Sagittarius “teapot” and hop a half degree south and a finger width (1.5 degrees) west. The problem is seeing it! In small optics, such as binoculars or a finder scope, it will appear almost stellar because of its small size. However, if you just look for what appears like a larger, dim star that won’t quite come into perfect focus, then you’ve found it.

In smaller telescopes, you’ll get no resolution on this class III globular cluster because it is so dense. Large aperture doesn’t fare much better either, with only some individual stars making their appearance at the outer perimeters. Because of magnitude and size, Messier 54 is better suited to dark sky conditions.

The location of Messier 54 in the Sagittarius constellation. Credit: IAU/Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)

And here are the quick facts on this Messier Object to help you get started:

Object Name: Messier 54
Alternative Designations: M54, NGC 6715
Object Type: Class III Extragalactic Globular Cluster
Constellation: Sagittarius
Right Ascension: 18 : 55.1 (h:m)
Declination: -30 : 29 (deg:m)
Distance: 87.4 (kly)
Visual Brightness: 7.6 (mag)
Apparent Dimension: 12.0 (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: