Posted on by Tammy Plotner

The Wall – NGC 7000 Region by Kent Wood

NGC 7000 Wall Region by Kent Wood

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Known as Caldwell 20 to some, NGC 7000 to others and the North America Nebula to most, this diffuse emission/reflection nebula near Deneb can frequently be seen by the unaided eye from a dark location, but the sheer size of this 1600 light year distant gas cloud often confuses people as to the reality of what they are seeing. Let’s take a look at just a few of the bricks in the “Wall”..

Hey you, out there beyond the wall… Is there anybody out there?

In this image taken by Kent Wood, we are looking a just a close-up of the region shaped like the Gulf of Mexico and often referred to as the “Cygnus Wall”. It is here that light from young, energetic stars is taking the surrounding cold gas fields and warming them, causing an ionization front to form – filled with dense and delightfully delicate filaments. This highly energized “shock front” stands out in bold relief against the complex dark gases and streaking dark dustlanes.

What shall we use… To fill the empty spaces? What shall we use… To complete the wall?

Let’s try star formation, eruptive variables, flare stars and T-Tauri types. According to G.W. Marcy: “A slitless spectrographic search for H..cap alpha.. emission stars in NGC 7000 has revealed 18 new examples, most of which are presumably T Tau stars. An examination of all known T Tau stars in these fields has uncovered no events of the FU Ori type, except for that of V1057 Cygni.” All of these make themselves at home in the warm ionized gas in the local interstellar medium. However, it is the properties of this ionized gas that are so curious to study. In this case, in the faint optical emission lines of hydrogen alpha.

Hey you, don’t help them to bury the light…

Along the bright rim of the wall is where the action is at. According to the work of Koji (et al), it is here where most of the star forming action is going on. “We have found small clusters of near-infrared sources having young stellar object (YSO) colors in some of these objects; most of the cluster members are considered to be older than the IRAS point sources and to be pre–main-sequence stars such as T Tauri stars. In at least six bright-rimmed clouds, the clusters are elongated toward the bright-rim tip or the exciting star(s) of the bright rim with the IRAS sources situated near the other end. There is a tendency for bluer (i.e., older) stars to be located closer to the exciting star(s) and for redder (i.e., younger) stars to be closer to the IRAS sources. This asymmetric distribution of the cluster members strongly suggests small-scale sequential star formation or propagation of star formation from the side of the exciting star(s) to the IRAS position in a few times 105 yr, as a result of the advance of the shock caused by the UV radiation from the exciting star(s).”

And all in all it was just a brick in the wall…

But some of the true beauty is the dust and soot laced clouds filled with PAHs. We learned about those Polycyclic Aromatic Hydrocarbons, not long ago and just what they mean. And, we know the Cygnus X region is one of the richest star formation sites in the Galaxy. But what about this structure? This Wall?

The Wall- NGC 7000 (Panorama) by Kent Wood
The Wall- NGC 7000 (Panorama) by Kent Wood

A distant ship, smoke on the horizon…. You are only coming through in waves.

Believe it or not, NGC 7000 was imaged from the lunar surface during the 1972 Apollo 16 mission and continues to be studied for its polarization properties and scattering in h-alpha wavelengths. It has even had its electron temperature taken to prove that interstellar dust is masking the light we see. However, what we do see may be an illusion. From the studies of R.J. Reynolds; “According to photoionization models of the warm ionized medium, these [O i]/Ha ratios suggest that most of the Ha originates from density-bounded, nearly fully ionized regions along the lines of sight rather than from partially ionized H i clouds or layers of H ii on the surfaces of H i clouds.”

Hey you, out there beyond the wall… Is there anybody out there?

Venture into the dark cloud and find out. According to Laugalys (et al) “Magnitudes and color indices of 430 stars down to V Ëœ 17.5 mag in the eight-color Vilnius + I photometric system were obtained in four areas of diameter 20′ within the dark cloud L935 separating the North America and Pelican nebulae. Spectral types, interstellar color excesses, extinctions and distances of stars were determined from the photometric data. The plot of extinction vs. distance shows that the dark cloud begins at a distance of 520±50 pc. About 40 stars in the cloud, mostly K and M dwarfs, are suspected to have Hα emission; these stars also exhibit infrared excesses. Four of them are known pre-main-sequence stars. Our star set contains J205551.3+435225 (V = 13.24) which, according to Camerón and Pasquali (2005), is the O5 V type star ionizing the North America and Pelican nebulae. If this spectral type is confirmed, the star would have an extinction AV between 9 and 10 magnitudes (depending on the accepted extinction law) and a distance which is not very different from the dust cloud distance.”.

How shall I fill the final places? How should I complete the wall?

I guess the last words would be the illuminating source. In a study done by Comerón and Pasquali; “We present the results of a search for the ionizing star of the North America (NGC 7000) and the Pelican (IC 5070) nebulae complex. The application of adequate selection criteria to the 2MASS JH KS broad-band photometry allows us to narrow the search down to 19 preliminary candidates in a circle of 0o 5 radius containing most of the L935 dark cloud that separates both nebulae. Follow-up near-infrared spectroscopy shows that most of these objects are carbon stars and mid-to-late-type giants, including some AGB stars. Two of the three remaining objects turn out to be later than spectral type B and thus cannot account for the ionization of the nebula, but a third object, 2MASS J205551.25+435224.6, has infrared properties consistent with it being a mid O-type star at the distance of the nebulae complex and reddened by AV ≃ 9.6. We confirm its O5V spectral type by means of visible spectroscopy in the blue. This star has the spectral type required by the ionization conditions of the nebulae and photometric properties consistent with the most recent estimates of their distance. Moreover, it lies close to the geometric center of the complex that other studies have proposed as the most likely location for the ionizing star, and is also very close to the position inferred from the morphology of cloud rims detected in radio continuum. Given the fulfillment of all the conditions and the existence of only one star in the whole search area that satisfies them, we thus propose 2MASS J205551.25+435224.6 as the ionizing star of the North America/Pelican complex.”

All in all… It’s just another brick in the wall.

We would like to thank AORAIA member, Kent Wood for the splendid image and the great research challenge!

Posted on by Fraser Cain

Pictures of the Sun

Sun with a huge coronal mass ejection. Image credit: NASA

There are so many beautiful pics of the Sun, it’s almost too difficult to know where to start.

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This is a picture of the Sun captured by NASA’s SOHO spacecraft. It would be a typical day on the Sun, except for the enormous coronal mass ejection blasting out of the upper right-hand side of the Sun. When the Sun is at its most active state, it can release 5-6 of these a day.

STEREO's image of the Sun. Image credit: NASA
STEREO's image of the Sun. Image credit: NASA

This photograph of the Sun was one of the first captured by NASA’s STEREO mission. These twin spacecraft were launched in 2006. One is leading the Earth in orbit, while the other has fallen behind. With both observing the Sun, scientists are given a 3-dimensional view of the Sun.

Sun seen from TRACE. Image credit: NASA
Sun seen from TRACE. Image credit: NASA

This pic of the Sun shows our star on a calm day, believe it or not. When you look close, this is what the surface of the Sun is doing all the time. The TRACE spacecraft was launched in 1997, and helps scientists study the Sun’s magnetic field – and to take beautiful photos like this.

Ultraviolet view of the Sun. Image credit: SOHO
Ultraviolet view of the Sun. Image credit: SOHO

This picture of the Sun was captured by the EIT instrument on board the NASA/ESA SOHO spacecraft. It reveals the normally invisible ultraviolet radiation streaming from the Sun. It’s actually a composite of three different Sun photos captured at different parts of the ultraviolet spectrum and then merged together.

Picture of the Sun in 3-D. Image credit: NASA
Picture of the Sun in 3-D. Image credit: NASA

You’re going to need a set of 3-D glasses to get the most out of this Sun photograph. It’s an image of Sun captured by NASA’s twin STEREO spacecraft. Images like this help scientists understand how the Sun interacts with its local environment, and better predict space weather.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Posted on by Fraser Cain

Sundial

Sundial on Mars. Image credit: NASA

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You take your clock for granted today, but it’s only been in the last couple of centuries that machines (and electronics) have been accurate enough to be used for timekeeping. Before that, people had to use other ways to tell the time of day. One of the most useful and easy to make is a sundial.

In its simplest form, a sundial consists of a style – a thin rod or sharp straight edge – that casts a long shadow onto a flat surface. As the Sun moves in the sky, the shadow moves as well in a perfectly predictable way. By putting marks on the flat surface, you can know what time it is by the position of the shadow.

For a sundial to work, it must be aligned with the axis of the Earth’s rotation. The style must be pointed towards North, and the style’s angle with horizontal must be equal to the sundial’s latitude.

NASA’s Mars Exploration rovers are equipped with miniature sundials on top of their color calibration targets. Scientists use these to fix colors in images based on the known colors in these calibration targets. The sundials are decorative, but they also help locate the Sun’s direction compared to the rovers.

This article on Universe Today talks about the sundial attached to NASA’s Mars Exploration rovers.

Would you like to make your own sundial? NASA has a cool page that shows you how to construct and use your own sundial. This page gives you a template so you can construct your own sundial (warning, it’s a PDF document, not a web page).

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

Posted on by Fraser Cain

Sun Orbit

Position of the Sun in the Milky Way. Image credit: NASA

Everything’s orbiting something it seems. The Moon goes around the Earth, and the Earth orbits the Sun. But did you know that the Sun orbits the Milky Way galaxy?

Astronomers have calculated that it takes the Sun 226 million years to completely orbit around the center of the Milky Way. In other words, that last time that the Sun was in its current position in space around the Milky Way, dinosaurs ruled the Earth. in fact, this Sun orbit has only happened 20.4 times since the Sun itself formed 4.6 billion years ago.

Since the Sun is 26,000 light-years from the center of the Milky Way, it has to travel at an astonishing speed of 782,000 km/hour in a circular orbit around the Milky Way center. Just for comparison, the Earth is rotating at a speed of 1,770 km/h, and it’s moving at a speed of 108,000 km/h around the Sun.

It’s estimated that the Sun will continue fusing hydrogen for another 7 billon years or so. In other words, it only has another 31 orbits it can make before it runs out of fuel.

Are you interested in more articles about the Sun? We have written plenty for Universe Today. Here’s an article that shows how some stars take an erratic journey around the Milky Way, and another article about a ring of stars orbiting the Milky Way.

Here’s an article that describes the process astronomers used to determine the orbit around the Milky Way.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

References:
NASA Imagine the Universe!
NASA Spacemath
NASA Solar System Exploration Page

Posted on by Fraser Cain

Sun and Earth

Sun with a huge coronal mass ejection. Image credit: NASA

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We owe everything we have to the Sun. If it weren’t for the Sun, there’d be no life on Earth. The relationship between Sun and Earth has gone back for 4.6 billion years, and should last for another 7 billion years or so.

As you probably know, the Sun is just a giant sphere of gas. At the core of the Sun, huge quantities of hydrogen are squished together in the intense pressure and temperature of this extreme environment. Hydrogen is converted to helium, and this reaction releases a tremendous amount of energy.

How much energy? Astronomers calculate that there are 600 million tons of hydrogen fused every second. 4 million tons of matter is converted to pure energy every second. This releases 3.86×1026 joules of energy every second. Although most of this energy heads off into space, plenty still falls onto the Earth. In fact, there’s enough energy coming from the Sun to deposit 342 Watts of energy onto every square meter of the Earth (averaged over the year, over the whole planet).

From our perspective, Sun and Earth go hand in hand. This energy from the Sun heats up the planet, preventing us from cooling down to near absolute zero temperatures of space. Our atmosphere traps the energy as heat, keeping the whole planet a nice comfortable temperature.

Plants have been soaking up this energy for millions of years. When you burn gasoline in your car, it comes from oil, which is energy from the Sun that planets have been storing for millions of years.

Sun and Earth are locked in a gravitational dance as well. The mass of the Sun is 2 × 1030 kilograms. This is enough to reach out across space and keep the Earth (and the rest of the planets) locked in orbit around it. We even experience tides from the gravity of the Sun.

Were you wondering how far away the Earth is from the Sun? And the Sun isn’t always trying to help us. Sometimes it’s throwing monster flares at us as well.

Here’s NASA’s Solar System Exploration Guide on the Sun. And here’s the homepage for NASA’s STEREO mission, which is taking amazing pictures and videos of the Sun.

We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.

References:
NASA Sun Earth Day
NASA Cosmicopia: Sun

Posted on by Tammy Plotner

Weekend SkyWatcher’s Forecast – September 12 -14, 2008

Greetings, fellow SkyWatchers! It’s big… It’s bright. It’s undeniably the Moon. So what are we going to do this weekend? Why, study of course! We’ll take a look at some history, some mystery and even some cool variability that can be studied without any special equipment. Are you ready to journey into the night?

Friday, September 12, 2008 – Arthur Auwers was born today in 1838. His life’s work included unifying the world’s observational catalogs. He specialized in astrometry, making very precise measurements of stellar positions and motions and he also calculated the orbits of Sirius and Procyon long before their companions were discovered. Auwers also directed expeditions to measure the transits of Venus and began a project to unify the all available sky charts, an interest that began with his catalog of nebulae which he published in 1862. There’s even a lunar crater named for him!

Also today, in 1959, the USSR’s Luna 2 became the first manmade object to hit the moon. It was the first spacecraft to reach the surface of the Moon, and it impacted the lunar surface west of Mare Serenitatis near the craters Aristides, Archimedes, and Autolycus. Scientifically, Luna 2 is most famous for confirming the earlier detection of the solar wind by Luna 1. However, it’s most famous for what it did after it launched! When it separated from its third stage, the spacecraft released a bright orange cloud of sodium gas, which aided in spacecraft tracking and acted as an experiment on the behavior of gas in space. Can you imagine the sight? Today also celebrates the 1966 Gemini 11 launch – the highest Earth orbit ever reached by an American manned spacecraft (1374 kilometer altitude).

Tonight our primary lunar study is crater Kepler. Look for it as a bright point, slightly north of lunar center near the terminator. Its home is the Oceanus Procellarum – a sprawling dark mare composed primarily of minerals of low reflectivity (low albedo), such as iron and magnesium. Bright, young Kepler will display a wonderfully developed ray system. The crater rim is very bright, consisting mostly of a pale rock called anorthosite. The “lines” extending from Kepler are fragments that were splashed out and flung across the lunar surface when the impact occurred. The region is also home to features known as “domes” – seen between the crater and the Carpathian Mountains. So unique are Kepler’s geological formations that it became the first crater mapped by U.S. Geological Survey in 1962.

Saturday, September 13, 2008 – Today in 1922, the highest air temperature ever recorded on the surface of the Earth occurred. The measurement, taken in Libya, burned in at a blistering 136° F (58° C) – but did you know that the temperatures in the sunlight on the Moon double that? If you think the surface of the Moon is a bit too warm for comfort, then know that surface temperatures on the closest planet to the Sun can reach up to 800° F (430° C) at the equator during the day! As odd as it may sound, and even as close to the Sun as Mercury is, it could very well have ice deposits hidden below the surface at its poles.

Get out your telescope, because tonight we’re going to have a look at a lunar feature that goes beyond simply incredible – it’s downright weird. Start your journey by identifying Kepler, and head due west across Oceanus Procellarum until you encounter the bright ring of crater Reiner. Spanning 30 kilometers, this crater isn’t anything showy…just shallow-looking walls with a little hummock in the center. But, look further west and a little more north for an anomaly – Reiner Gamma.

Well, it’s bright. It’s slightly eye-shaped. But what exactly is it? Having no appreciable elevation or depth, Reiner Gamma could very well be an extremely young feature caused by a comet. Only three other such features are known to exist – two on the lunar far side and one on Mercury. They are high albedo surface deposits with magnetic properties. Unlike a lunar ray, consisting of material ejected from below the surface, Reiner Gamma can be spotted during the daylight hours – when ray systems disappear. And, unlike other lunar formations, it never casts a shadow.

Reiner Gamma is also a magnetic deviation on a barren world that has no magnetic field, so how did it form? Many ideas have been proposed, such as solar storms, volcanic activity, or even seismic waves. But the best explanation? It is the result of a cometary strike. Evidence exists that a split-nucleus comet, or cometary fragments, once impacted the area, and the swirl of gases from the high-velocity debris may have somehow changed the regolith. On the other hand, ejecta from such an impact could have formed around a magnetic “hot spot,” much like a magnet attracts iron filings.

No matter which theory is correct, the simple act of viewing Reiner Gamma and realizing it is different from all other features on the Moon’s Earth-facing side makes this journey well worth the time!

Sunday, September 14 – With a nearly Full Moon, skies are light-trashed tonight, so if you’d like to visit another object that only requires your eyes, then look no further than Eta Aquilae (RA 19 52 28 37 Dec +01 00 20), about one fistwidth due south of Altair…

Discovered by Pigot in 1784, this Cepheid variable varies by over a magnitude in a period of 7.17644 days. During this time it will reach of maximum of magnitude 3.7, and then decline slowly over five days to a minimum of 4.5… Yet it only takes two days to brighten again! This period of expansion and contraction makes Eta unique. To help gauge these changes, compare Eta to Beta on Altair’s same southeast side. When Eta is at maximum, it will about equal Beta in brightness.

Wishing you clear skies and a super weekend!!

This week’s awesome images are Kepler Crater by Wes Higgins, Luna 2 courtesy of NASA, Reiner Gamma from the Clementine Lunar Browser and Eta Aquilae – Credit: Palomar Observatory, courtesy of Caltech. Many thanks!

Posted on by Fraser Cain

Ten Interesting Facts About the Sun

The Sun as viewed by the Solar and Heliospheric Observatory (NASA/SOHO)

Think you know everything there is to know about the Sun? Think again. Here are 10 facts about the Sun, collected in no particular order. Some you might already know, and others will be totally new to you.

1. The Sun is the Solar System
We live on the planet, so we think it’s an equal member of the Solar System. But that couldn’t be further from the truth. The reality is that the mass of the Sun accounts for 99.8% of the mass of the Solar System. And most of that final 0.2% comes from Jupiter. So the mass of the Earth is a fraction of a fraction of the mass of the Solar System. Really, we barely exist.

2. And the Sun is mostly hydrogen and helium
If you could take apart the Sun and pile up its different elements, you’d find that 74% of its mass comes from hydrogen. with 24% helium. The remaining 2% is includes trace amounts of iron, nickel, oxygen, and all the other elements we have in the Solar System. In other words, the Solar System is mostly made of hydrogen.

3. The Sun is pretty bright.
We know of some amazingly large and bright stars, like Eta Carina and Betelgeuse. But they’re incredibly far away. Our own Sun is a relatively bright star. If you could take the 50 closest stars within 17 light-years of the Earth, the Sun would be the 4th brightest star in absolute terms. Not bad at all.

4. The Sun is huge, but tiny
With a diameter of 109 times the size the Earth, the Sun makes a really big sphere. You could fit 1.3 million Earths inside the Sun. Or you could flatten out 11,990 Earths to cover the surface of the Sun. That’s big, but there are some much bigger stars out there. For example, the biggest star that we know of would almost reach Saturn if it were placed inside the Solar System.

5. The Sun is middle aged
Astronomers think that the Sun (and the planets) formed from the solar nebula about 4.59 billion years ago. The Sun is in the main sequence stage right now, slowly using up its hydrogen fuel. But at some point, in about 5 billion years from now, the Sun will enter the red giant phase, where it swells up to consume the inner planets – including Earth (probably). It will slough off its outer layers, and then shrink back down to a relatively tiny white dwarf.

6. The Sun has layers
The Sun looks like a burning ball of fire, but it actually has an internal structure. The visible surface we can see is called the photosphere, and heats up to a temperature of about 6,000 degrees Kelvin. Beneath that is the convective zone, where heat moves slowly from the inner Sun to the surface, and cooled material falls back down in columns. This region starts at 70% of the radius of the Sun. Beneath the convection zone is the radiative zone. In this zone, heat can only travel through radiation. The core of the Sun extends from the center of the Sun to a distance of 0.2 solar radii. This is where temperatures reach 13.6 million degrees Kelvin, and molecules of hydrogen are fused into helium.

7. The Sun is heating up, and will kill all life on Earth
It feels like the Sun has been around forever, unchanging, but that’s not true. The Sun is actually slowly heating up. It’s becoming 10% more luminous every billion years. In fact, within just a billion years, the heat from the Sun will be so intense that liquid water won’t exist on the surface of the Earth. Life on Earth as we know it will be gone forever. Bacteria might still live on underground, but the surface of the planet will be scorched and uninhabited. It’ll take another 7 billion years for the Sun to reach its red giant phase before it actually expands to the point that it engulfs the Earth and destroys the entire planet.

8. Different parts of the Sun rotate at different speeds
Unlike the planets, the Sun is great big sphere of hydrogen gas. Because of this, different parts of the Sun rotate at different speeds. You can see how fast the surface is rotating by tracking the movement of sunspots across the surface. Regions at the equator take 25 days to complete one rotation, while features at the poles can take 36 days. And the inside of the Sun seems to take about 27 days.

9. The outer atmosphere is hotter than the surface
The surface of the Sun reaches temperatures of 6,000 Kelvin. But this is actually much less than the Sun’s atmosphere. Above the surface of the Sun is a region of the atmosphere called the chromosphere, where temperatures can reach 100,000 K. But that’s nothing. There’s an even more distant region called the corona, which extends to a volume even larger than the Sun itself. Temperatures in the corona can reach 1 million K.

10. There are spacecraft observing the Sun right now.
The most famous spacecraft sent to observe the Sun is the Solar and Heliospheric Observatory, built by NASA and ESA, and launched in December, 1995. SOHO has been continuously observing the Sun since then, and sent back countless images. A more recent mission is NASA’s STEREO spacecraft. This was actually two spacecraft, launched in October 2006. These twin spacecraft were designed to watch the same activity on the Sun from two different vantage points, to give a 3-D perspective of the Sun’s activity, and allow astronomers to better predict space weather.

We have recorded an episode of Astronomy Cast all about the Sun called The Sun, Spots and All.

References:
NASA Science
NASA SOHO
NASA Stereo

Posted on by Nancy Atkinson

Star Endured Unique Explosion That Didn’t Destroy

Eta Carinae Credit: Gemini Observatory artwork by Lynette Cook

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There’s ‘smoked but didn’t inhale,’ ‘promised but didn’t deliver,’ and now there’s ‘exploded but didn’t destroy.’ Eta Carinae, the galaxy’s biggest, brightest and perhaps most studied star after the sun, appears to be driven by an entirely new type of stellar explosion that is fainter than a typical supernova and does not destroy the star. Astronomer Nathan Smith proposes that Eta Carinae’s historic 1843 explosion was, in fact, an outburst that produced a fast blast wave similar to, but less energetic than, a real supernova. This well-documented event in our own Milky Way Galaxy is probably related to a class of faint stellar explosions in other galaxies recognized in recent years by telescopes searching for extragalactic supernovae.

“There is a class of stellar explosions going off in other galaxies for which we still don’t know the cause, but Eta Carinae is the prototype,” said Smith, a UC Berkeley postdoctoral fellow.

Eta Carinae (η Car) is a massive, hot, variable star visible only from the Southern Hemisphere, and is located about 7,500 light years from Earth in a young region of star birth called the Carina Nebula. In 1843, observers saw Eta Car brighten immensely. Visible now is the resulting cloud of gas and dust, known as the Homunculus nebula, wafting away from the star. A faint shell of debris from an earlier explosion is also visible, probably dating from around 1,000 years ago.

But these shells of gas and dust are moving relatively slowly at 650 kilometers per second (1.5 million miles per hour) compared to the blast shell of a regular supernova.

Presumably blown off by the star’s fierce wind, the shells of gas and dust are moving slowly – at speeds of 650 kilometers per second (1.5 million miles per hour) or less – compared to the blast shell of a supernova. But new observations by Smith show filaments of gas moving five times faster than the debris from the Homonuculus, which would equal speeds of materials accelerated fast blast wave of a supernova explosion.

The fast speeds in this blast wave could roughly double earlier estimates of the energy released in the 1843 eruption of Eta Carinae, an event that Smith argues was not just a gentle surface eruption driven by the stellar wind, but an actual explosion deep in the star that sent debris hurtling into interstellar space. In fact, the fast-moving blast wave is now colliding with the slow-moving cloud from the 1,000-year-old eruption and generating X-rays that have been observed by the orbiting Chandra Observatory.

“These observations force us to modify our interpretation of what happened in the 1843 eruption,” he said. “Rather than a steady wind blowing off the outer layers, it seems to have been an explosion that started deep inside the star and blasted off its outer layers. It takes a new mechanism to cause explosions like this.”

If Smith’s interpretation is correct, supermassive stars like Eta Carinae may blow off large amounts of mass in periodic explosions as they approach the end of their lives before a final, cataclysmic supernova blows the star to smithereens and leaves behind a black hole.

“Looking at other galaxies, astronomers have seen stars like Eta Carinae that get brighter, but not quite as bright as a real supernova,” he said. “We don’t know what they are. It’s an enduring mystery as to what can brighten a star that much without destroying it completely.”

Source: EurekAlert

Posted on by Tammy Plotner

The Dragon Slayer – NGC 5985, NGC 5982, NGC 5981 by Ken Crawford

Draco Trio - By Ken Crawford

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There are wonderful tales which surround the circumpolar Draco constellation. According to Greek legend, Draco represents the dragon killed by Cadmus before founding the city of Thebes – or perhaps it represented the dragon which guarded the golden fleece and was eventually killed by Jason and his famous Argonauts. To the Romans, it was simply a creature killed by Minerva and tossed into the sky as stars to be remembered. The Egyptians called it Tawaret. But the most famous of all representations of Draco was one of the twelve labors that Hercules had to overcome. Many of us will never see the jewels that hide within the boundaries of this sprawling constellation, but thanks to the Herculean efforts of Ken Crawford – we can share in its mysteries…

To deep sky observers, the group of NGC 5985, NGC 5982 and NGC 5981 is commonly known as the “Draco Trio”. Two barred spirals at different angles and a face on elliptical all in the same field of view is a rare sight and makes for a beautiful celestial portrait. The beautiful spiral is NGC 5985. The proper designation for the elliptical galaxy is NGC 5982. The catalog number for the edge-on is NGC 5981. While these galaxies span huge amounts of light years apart, they share telescopic space at RA: 15h 38m 40s Dec: +59°21’22” as a center and share photons in the eyepiece at around 25 arc minutes. While the Draco group is far too small to be considered its own galaxy cluster and has never been classified as a compact group, oddly enough all three are around 100 million light years away from the Sol System.

I did mention there were mysteries here, didn’t I? Then let’s explore them…

Take a closer look at the grand spiral, NGC 5985. It’s a Seyfert. According to research done by Simões Lopes (et al) it may also harbor a wonderful black hole right in there with its active galactic nucleus. “This result demonstrates a strong correlation between the presence of circumnuclear dust and accretion onto the central, supermassive black hole in elliptical and lenticular galaxies. Current estimates suggest the dust settling or destruction time is on order of 108 yr, and therefore the presence of dust in ~50% of early-type galaxies requires frequent replenishment and similarly frequent fueling of their central supermassive black holes. The observed dust could be internally produced (via stellar winds) or externally accreted, although there are observational challenges for both of these scenarios. Our analysis also reveals that approximately one-third of the early-type galaxies without circumnuclear dust have nuclear stellar disks. These nuclear stellar disks may provide a preferred kinematic axis to externally accreted material, and this material may in turn form new stars in these disks. The observed incidence of nuclear stellar disks and circumnuclear dust suggests that episodic replenishment of nuclear stellar disks occurs and is approximately concurrent with the fueling of the central AGN.”

But that’s not all, because there’s a quasar there, too. According to a 2001 study done by one of my heroes – Halton Arp and David Russell; “The distribution on the sky of clusters of galaxies shows significant association with relatively nearby, large, active galaxies. The pattern is that of clusters paired equidistant across a central galaxy with the apparent magnitudes and redshifts of their constituent galaxies being closely matched. The clusters and the galaxies in them tend to be strong X-ray and radio emitters, and their redshifts occur at preferred redshift values. The central, low-redshift galaxies often show evidence of ejection in the direction of these higher redshift clusters. In all these respects the clusters resemble closely quasars which have been increasingly shown for the last 34 years to be similarly associated with active parent galaxies. New, especially significant pairings of quasars are presented here, which are, at the same time, associated with Abell clusters of galaxies. It is argued here that, empirically, the quasars are ejected from active galaxies. They evolve to lower redshift with time, forming stars, and fragmenting at the end of their development into clusters of low-luminosity galaxies. The cluster galaxies can be at the same distance as their lower redshift parents because they still retain a component of their earlier, quasar intrinsic redshift.”

Now, let’s take a look at the quiet little elliptical – NGC 5982. Just this year it was studied by Del Burgo (et al) for its dust shell. According to the report: “Shells in Ellipticals are peculiar faint sharp edged features that are thought to be formed by galaxy mergers. We use Spitzer data in the wavelength range from 3.6 to 160 μm and HST/ACS optical data. After subtracting the galaxy models, residual images are used to identify the shells. We detect for the first time shells from mid-infrared data. The very different distributions of dust, warm gas and HI gas together with the presence of shells and a kinematically decoupled core suggest a minor merger in NGC 5982.”

Ah, ha! So, it’s always the quiet ones that get ya’, huh? Then it might interest you to know that NGC 5982 may also contain its own black hole, a peculiar population of stars, a low luminosity active galactic nucleus and may have even been a product of a black hole merger! What more, new globular clusters may have formed during these interactions without the benefits of gaseous materials. Simply too cool…

Now… How about the wild looking edge-on, NGC 5981? Science loves to examine what it just can’t quite see and in the case of this highly inclined spiral, we’ve found out that the stellar disc just might be cut off – or foreshortened. According to a 2007 work done by Florido (et al); “This is the first work reporting observations of the truncation of a stellar disc, in both the optical and the NIR spectral ranges. No galaxy has been observed at both wavelengths with the required depth. The optical radial profiles of spiral galaxy discs seem to suggest a double exponential behaviour, whilst NIR profiles seem to show a real truncation. NGC 6504 has a real truncation in both the optical and the NIR radial profiles. A double exponential does not fit the observed optical profile. The truncation radius is larger in the V band than in the NIR by ~10 arcsec, about 3 kpc (equivalent to about 10%).”

But, just because its equipment is a little shorter than most, does that mean it doesn’t produce as many stars? Not hardly. It just means its peanut-shaped central bulge may be embedded in a dark halo. Thanks to the work of Joop Schaye who also took a look at NGC 5981, we know a little more about these properties. “We study global star formation thresholds in the outer parts of galaxies by investigating the stability of disk galaxies embedded in dark halos. The disks are self-gravitating, contain metals and dust, and are exposed to UV radiation. We find that the critical surface density for the existence of a cold interstellar phase depends only weakly on the parameters of the model and coincides with the empirically derived surface density threshold for star formation. Furthermore, it is shown that the drop in the thermal velocity dispersion associated with the transition from the warm to the cold gas phase triggers gravitational instability on a wide range of scales. The presence of strong turbulence does not undermine this conclusion if the disk is self-gravitating. Models based on the hypothesis that the onset of thermal instability determines the star formation threshold in the outer parts of galaxies can reproduce many observations, including the threshold radii, the column densities, and the sizes of stellar disks as a function of disk scale length and mass.”

While we’ll never see the Draco Trio in the telescope eyepiece as well as what this incredible image by Ken Crawford presents, we welcome the Dragon Slayer for the opportunity it gives us to take a closer look at another cosmic mystery. Is the Draco Group really a galaxy group? Perhaps. According to independent research papers done by both Giuricin and Garcia, this small group of friends collectively known as the NGC 5866 Group (because it’s the brightest) is located to the northwest of both the M101 Group and its companion galaxies which makes it proximity. Also nearby is the M51 Group, home to the Whirlpool Galaxy, the Sunflower Galaxy, and several others. The distances to these three groups was gathered by studying their individual members and science has found they are similar – and perhaps part of a much larger, more loose association than we’ve yet discovered.

But we’re learning…

Many thanks to AORAIA member Ken Crawford for the use of the spectacular image and the awesome research challenge it posed! My gratitude for the inspiration and the learning challenge…

Posted on by Tammy Plotner

The Fire Cracker Galaxy – NGC 6946 by Dietmar Hager

The FireCracker Galaxy - NGC 6946 by Dietmar Hager

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It’s time to take a look back to what was happening 210 years ago on the night of September 9th. Sir William Herschel was at the eyepiece of his telescope in Slough. While he was viewing in real time, what he was viewing occurred more than 10 million years ago – the fireworks that ignited in NGC 6946.

At one time, it was widely believed that NGC 6946 was a member of our Local Group; mainly because it could be easily resolved into stars. There was a reddening observed in it, believed to be indicative of distance – but now know to be caused by interstellar dust. But it isn’t the shrouding dust cloud that makes NGC 6946 so interesting, it’s the fact that so many supernova and star-forming events have sparkled in its arms in the last few years that has science puzzled! So many, in fact, that they’ve been recorded every year or two for the last 60 years…

Most normally, bursts of star formation happen in galaxies which have nearby companions to lend materials. Yet, NGC 6946 appears to be alone in the field. According to a 2000 study done by Pisano (et al) ” Such gas-rich companions could include material left over from the galaxy assembly process which could persist into the current day around an isolated galaxy such as NGC 6946. NGC 6946 is prolifically forming stars, has a nuclear starburst, and has widespread high-velocity clouds associated with the disc. All of these features could be explained by the accretion of low-mass Hi clouds by NGC 6946. Our survey recovered two previously detected dwarf galaxies associated with NGC 6946, but otherwise found no signatures of interactions in the NGC 6946 system. The companions are small enough, and distant enough from NGC 6946 that they should have minimal effect on the main galaxy. Some tidal debris may be expected due to interaction between the two dwarf galaxies, but none is observed. This could be because it is at low column densities, or because the dwarf galaxies are more separated than they appear on the sky. This study of the system suggests that NGC 6946 is a gravitationally bound system with two dwarf galaxies in stable orbits about the larger primary galaxy.”

But, that was some 8 years ago and 16 events into the past. According to studies done by Eva Schinnerer (et al) in 2006, NGC 6946 has been “Caught in the Act” as a Bar-driven Nuclear Starburst Galaxy. “The data, obtained with the IRAM Plateau de Bure Interferometer (PdBI), allow the first detection of a molecular gas spiral in the inner ~10” (270 pc) with a large concentration of molecular gas (MH2~1.6×107 Msolar) within the inner 60 pc. This nuclear clump shows evidence for a ringlike geometry with a radius of ~10 pc as inferred from the position-velocity diagrams. Both the distribution of the molecular gas and its kinematics can be well explained by the influence of an inner stellar bar of about 400 pc length. A qualitative model of the expected gas flow shows that streaming motions along the leading sides of this bar are a plausible explanation for the high nuclear gas density. Thus, NGC 6946 is a prime example of molecular gas kinematics being driven by a small-scale, secondary stellar bar.”

Now, for the really cool part – understanding barred structure. Thanks to the Hubble Space Telescope and a study of more than 2,000 spiral galaxies – the Cosmic Evolution Survey (COSMOS) – astronomers understand that barred spiral structure just didn’t occur very often some 7 billion years ago in the local universe. Bar formation in spiral galaxies evolved over time. A team led by Kartik Sheth of the Spitzer Science Center at the California Institute of Technology in Pasadena discovered that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with nearly 70 percent of their modern counterparts. This makes NGC 6946 very rare, indeed… Since its barred structure was noted back in Herschel’s time and its age of 10 billion years puts it beyond what is considered a “modern” galaxy.

Science believes bars in galaxies have been forming steadily over the last 7 billion years, more than tripling in number. “The recently forming bars are not uniformly distributed across galaxy masses, however, and this is a key finding from our investigation,” Sheth explained. “They are forming mostly in the small, low-mass galaxies, whereas among the most massive galaxies, the fraction of bars was the same in the past as it is today.” The findings, Sheth continued, have important ramifications for galaxy evolution. “We know that evolution is generally faster for more massive galaxies: They form their stars early and fast and then fade into red disks. Low-mass galaxies are known to form stars at a slower pace, but now we see that they also made their bars slowly over time,” he said. Bars form when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. “The tiny elongations in the stars’ orbits grow and they get locked into place, making a bar,” explained team member Bruce Elmegreen of IBM’s research Division in Yorktown Heights, N.Y. “The bar becomes even stronger as it locks more and more of these elongated orbits into place. Eventually a high fraction of the stars in the galaxy’s inner region join the bar.”

Added team member Lia Athanassoula of the Laboratoire d’Astrophysique de Marseille in France: “The new observations suggest that the instability is faster in more massive galaxies, perhaps because their inner disks are denser and their gravity is stronger.” Bars are perhaps one of the most important catalysts for changing a galaxy. They force a large amount of gas towards the galactic center, fueling new star formation, building central bulges of stars, and feeding massive black holes. “The formation of a bar may be the final important act in the evolution of a spiral galaxy,” Sheth said. “Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars.” (HubbleSite News Release)

Yet the studies of NGC 6946 haven’t stopped. In 2005, Gemini II also took a look at this crazy galaxy. “In order to sustain this rate of supernova activity, massive, quickly evolving stars must form or be born at an equally rapid rate in NGC 6946,” said Gemini North Associate Director, Jean-René Roy. “Its stars are exploding like a string of firecrackers!” And with it in 2007, hydrogen halos… Says Rense Boomsma: “A halo of neutral hydrogen is found around an increasing number of spiral galaxies. It is not well understood how hydrogen halos are formed. The orientation of nearby spiral galaxy NGC 6946 enables us to measure vertical gas velocities in the disk of the galaxy and therefore measure how the gas gets into the halo. We find hydrogen with high velocities toward regions where stars are formed. This correlation suggests that the formation of a hydrogen halo is related to massive star formation. A similar close connection is seen in the nearby spiral galaxy NGC 253. For some hydrogen clouds in NGC 6946 we have indications that they have been accreted from outside the galaxy.”

Will we ever understand everything there is to know about galaxies like NGC 6946? Perhaps not in our lifetimes. However, one of the best parts is knowing that it is a galaxy that you can observe and study with larger backyard telescopes. Located in the constellation of Cepheus (RA 20:34.8 Dec +60:09) and billed at magnitude 8.9 (but beware, it’s low surface brightness!), this small barred spiral will show some structure in 10″ or larger scopes with decent skies. Who knows what your night may reveal?

Our many thanks to AORAIA member, Dr. Dietmar Hager of Stargazer Observatory for the use of this incredible image and the challenge of researching the information!

Link to original full size image.