Astronomy Archives

March 19, 2009December 24, 2015 by Ian O'Neill

The Sun as a White Dwarf Star

Dusty debris around an old white dwarf star (NASA)

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What will happen to all the inner planets, dwarf planets, gas giants and asteroids in the Solar System when the Sun turns into a white dwarf? This question is currently being pondered by a NASA researcher who is building a model of how our Solar System might evolve as our Sun loses mass, violently turning into an electron-degenerate star. It turns out that Dr. John Debes work has some very interesting implications. As we use more precise techniques to observe existing white dwarf stars with the dusty remains of the rocky bodies that used to orbit them, the results of Debes’ model could be used as a comparison to see if any existing white dwarf stars resemble how our Sun might look in 4-5 billion years time…

A comparison of the Sun in its yellow dwarf phase and red giant phase

Today, our Sun is a healthy yellow dwarf star. If you want to be precise, it is a “G V star”. This yellow dwarf will happily burn 600 million tonnes of hydrogen per second in its core for 10 billion years, generating the light that is required to make our planet habitable. The Sun is approximately half-way through this hydrogen burning phase, so it’s OK, things aren’t going to change (for the Sun at least) for a long time yet.

But what happens then? What happens in 4-5 billion years when the supply of hydrogen runs out in the core? Although our Sun isn’t massive enough to entertain the thought of going out in a blaze of supernova glory, it will still go through an exciting, yet terrifying death. After evolving through the hydrogen-burning phase, the Sun will puff up into a huge red giant star as the hydrogen fuel becomes scarce, expanding 200 times the size it is now, probably swallowing the Earth. Helium, and then progressively heavier elements will be fused in and around the core. The Sun will never fuse carbon however, instead it will shed its outer layers forming a planetary nebula.

Once things calm down, a small sparkling jewel of a white dwarf star will remain. This tiny remnant will have a mass of around half that of our present Sun, but will be the size of the Earth. Needless to say, white dwarfs are very dense, intense gravitational pull countered not by fusion in the core (like all Main Sequence stars), but by electron degeneracy pressure.

Relative sizes of IK Pegasi A (left), IK Pegasi B (lower center; a white dwarf) and the Sun (NASA)

When the Solar System reaches this phase in its evolution, what will it look like? What will become of the asteroids, gas giants, moons and rocky planets? I was very fortunate to chat with astrophysicist Dr John Debes, from NASA’s Goddard Space Flight Center, at January’s American Astronomical Society (AAS) conference in Long Beach (California) who is developing an n-body code simulating an evolving Solar System.

After the Sun has stopped hydrogen fusion in its core, it loses mass as it sheds its outer layers after the red giant phase and subsequent planetary nebula formation. It is estimated that the Sun will lose about 50% of its mass during this time, naturally affecting the Solar System as a whole. As the Sun loses mass, the outer planets (such as Jupiter) will drift outwards, increasing their orbital radii. In the simulation, Debes is very careful to ensure there is a gradual reduction in solar mass to ensure stability in the simulation.

What we are left with is an old Solar System, where little is left of the inner planets (it is likely that anything within the orbit of the Earth will have been swallowed by the Sun as it expanded through the red giant phase). Although the future white dwarf Solar System will seem very alien to present day, some things won’t change. Jupiter’s orbit might have receded with the drop in solar mass, it will remain a planetary heavyweight, causing disruption in asteroid orbits. Using known asteroid data, the motion of these chunks of rocks are allowed to evolve, and over millions of years, they may get thrown out of the Solar System, or more interestingly, pushed closer to the white dwarf. Once the whole system has settled down, resonances in the asteroid belt will become amplified; Kirkwood Gaps (caused by gravitational resonance with Jupiter) will widen, and according to Debes’ simulations, the edges of these gaps will become perturbed even more, making more asteroids available to be tidally disrupted and shredded to dust.

Artists concept of shredded asteroid around white dwarf (NASA/JPL-Caltech)

The AAS conference was full of amazing research into white dwarf observations. The reason for this is that there are many white dwarf candidates out there with dusty metallic absorption lines. This means that there used to be rocky bodies orbiting these stars, but became pulverised (by tidal shear) for astronomers to analyse. These white dwarf systems can give us a clue as to what mechanisms could be supplying the white dwarfs with dusty material, even giving us a glimpse into the future of our Solar System.

“We have a physical picture for the link between planetary systems and dusty white dwarfs,” Debes said when describing his model in relation to the mysterious dusty white dwarf observations. “Dusty white dwarfs are truly a mystery! We think we know what might be going on, but we don’t have a smoking gun yet.”

However, Debes is getting close to finding a possible smoking gun, he’s basing his model on some of the key characteristics of these ancient dusty remnants to see what the Solar System could look like in billions of years time.

So, where does this dust come from? As the asteroid orbits are perturbed by Jupiter, they may get close enough to be tidally disrupted. Get too close and they will get shredded by the gravitational shear created by the steep tidal radius of the compact white dwarf. The asteroid dust then settles into the white dwarf. The presence of this dust has a very obvious signature in the absorption lines of spectroscopic data, allowing researchers to infer an accretion rate for metal-rich white dwarfs. In Debes’ model, he has set the upper limit to 10¹⁶ g/year and a lower limit to 10¹³ g/year, consistent with observed estimates.

Spectra of G29-38. Could this resemble the spectra of the Sun after turning into a white dwarf? (NASA/Spitzer)

In his evolved Solar System model, Jupiter’s gravity controls this accretion rate, pushing asteroids toward the white dwarf and, by using a powerful supercomputer to track the perturbations and eventual shredding of known asteroids, there may be an opportunity to arrive at a profound conclusion. Debes is able to use his model to compare observations of known dusty white dwarfs with the simulated outcome of the Solar System. With reference to previous studies (in particularly Koester & Wilken, 2006 in the journal Astronomy & Astrophysics), Debes has found some similar white dwarf “Suns”.

“For G29-38, the canonical dusty white dwarf, they [Koester & Wilken] estimate a total mass of 0.55 solar masses–about what people believe the mass that our own sun will have remaining when it becomes a white dwarf,” Debes added. “But mass estimates are a bit uncertain–I’ve seen estimates ranging from 0.55-0.7 solar masses for this particular white dwarf.”

The Su<span>n's future? The whit</span>e dwarf G29-38 (NASA) — The Sun's future? The white dwarf G29-38 (NASA)

“Another good candidate is a DAZ [a metal-rich white dwarf] called WD 1257+278, which does not show dust but is spot on with the mass expected for the Sun–0.54 M_Sun,” said Debes. “Its accretion rate is also consistent with my model predictions so far assuming an asteroid belt mass and characteristic perturbation timescale that I found in my simulations.”

Debes is continuing to make his model more and more sophisticated, but already the results are promising. Most exciting is that we may already be observing white dwarfs, like G29-38 or WD 1257+278, giving us a tantalizing glimpse of what our Solar System will look like when the Sun becomes a white dwarf star, ripping apart any remaining asteroids and planets as they stray too close to the Sun’s tidal shear. However, it also raises the question: if white dwarfs like G29-38 are being fed by the remains of tidally-blended asteroids, are there massive planets shepherding asteroids in these white dwarf systems too?

March 18, 2009December 24, 2015 by Tammy Plotner

Turning the Tides – NGC 3109 by Ken Crawford

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Somewhere off in the far reaches of Hydra about 4.3 million light years away is a local subgroup of galaxies and a small, barred spiral that just seems to be quietly minding its own business. Or is it? NGC 3109 might not look like much at first, but this little island universe is really turning the tides…

First discovered by John Herschel on March 24, 1835 while in South Africa, NGC 3109 was first classed as an irregular galaxy – the dominating member of a small group of dwarf galaxies believed to be Local Group Member candidates – a determination which isn’t easy to make. “The Local Group dwarf galaxies offer a unique window to the detailed properties of the most common type of galaxy in the Universe.” says Mario L. Mateo, “Our understanding of these galaxies has grown impressively in the past decade, but fundamental puzzles remain that will keep the Local Group at the forefront of galaxy evolution studies for some time.”

What makes NGC 3109 and its little band of followers so interesting? Well, chances are it may not be a member of our Local Group at all, but the nearest of the outsiders. “The small Antlia-Sextans clustering of galaxies is located at a distance of only 1.36 Mpc from the Sun and 1.72 Mpc from the adopted barycenter of the Local Group. The latter value is significantly greater than the radius of the zero-velocity surface of the Local Group that, for an assumed age of 14 Gyr, has R_0=1.18+/-0.15 Mpc.” says Sidney Van den Bergh, “This, together with the observation that the members of the Ant-Sex group have a mean redshift of 114+/-12 km s^-1 relative to the centroid of the Local Group, suggests that the Antlia-Sextans group is not bound to our Local Group and that it is expanding with the Hubble flow. If this conclusion is correct, then Antlia-Sextans may be the nearest external clustering of galaxies.”

So, if NGC 3109 can hold those kind of secrets… What other kinds of secrets can it keep to itself? Try a tiny tidally interacting elliptical galaxy discovered in 1999 by Alan Whiting, George Hau and Mike Irwin. It’s called the Antlia dwarf and it was found to be just beyond the zero-velocity surface of the Local Group. “These data increase the number of certain (or probable) Local Group members to 36. The spatial distribution of these galaxies supports Hubble’s claim that the Local Group “is isolated in the general field.” Currently available evidence suggests that star formation continued much longer in many dwarf spheroidals than it did in the main body of the Galactic halo.” says Sidney Van den Bergh, “It is suggested that “young” globular clusters, such as Ruprecht 106, might have formed in now defunct dwarf spheroidals. Assuming SagDIG, which is the most remote Local Group galaxy, to lie on, or just inside, the zero-velocity surface of the Local Group yields a dynamical age >~17.9+/-2.7 Gyr. However, this value is meaningful only if the outer regions of the local Group are in virial equilibrium.”

Take a look at the full-size image done by Ken Crawford and check out all the red and blue super giant stars and scattered HII regions where new stars are forming – along with all the background galaxies. According to the work of D.G. Barnes; “A substantial warp in the disk of NGC 3109 is detected in the H I emission image in the form of an extended low surface brightness feature. We report a positive detection in H I of the nearby Antlia dwarf galaxy and measure its total neutral gas mass to be 6.8+/-1.4×105 M solar. We show the warp in NGC 3109 to lie at exactly the same radial velocity as the gas in the Antlia dwarf galaxy and speculate that Antlia disturbed the disk of NGC 3109 during a mild encounter ~1 Gyr in the past. H I data for a further eight galaxies detected in the background are presented.”

In the meantime, NGC 3109 continues to be an on-going object of study. Its many compact HII regions could be an indicator of planetary nebulae formations that are totally unlike anything we’ve seen before. “The excitation patterns of the PNe in NGC 3109 are very different from the excitation patterns of PNe in other galaxies.” says Miriam Pena, “This would imply that the evolution of PNe depends upon the properties of their progenitor stellar populations, which vary from galaxy to galaxy. This should affect the PN luminosity function and its use as a distance indicator.” And NGC 3109’s unique structure has equally fascinated Sebastian Hidalgo; “Its edge-on orientation (which simplifies the study of a possible halo) and the possibility that it could, in fact, be a small spiral (the smallest in the Local Group) makes its deep analysis of major relevance to understand the properties of dwarf galaxies and the transition from dwarf irregulars to spirals.”

Many thanks to Ken Crawford for this inspiring image!

March 17, 2009December 24, 2015 by Fraser Cain

Cinder Cone Volcanoes

Cinder cone Paricutin. Image credit: USGS

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Picture a volcano in your mind. You’re probably thinking of a cinder cone volcano, the simplest type of volcano. Cinder cone volcanos have steep sides with a bowl-shaped crater at the top.

Cinder cone volcanoes grow from a single vent in the Earth’s crust. Gas-charged lava is blown violently out of the volcano’s central vent, and the ash and rocks rain down around the vent. After multiple eruptions, the volcano takes on the familiar cone shape, with the erupted rubble forming the steep slopes. Cinder cones rarely grow much taller than 300 meters above their surroundings, and they’re common in western North America, and wherever there’s volcanic activity.

Although they can be solitary structures, cinder cones are often associated with other kinds of volcanoes, like shield volcanoes and stratovolcanoes (or a composite volcano). For example, geologists have discovered more than 100 cinder cones on the sides of Hawaii’s Mauna Kea, one of the biggest volcano in the world. Each cinder cone comes from a vent that opened up on the sides of the volcano.

One of the most famous cinder cone volcanoes erupted out of a Mexican corn field in 1943. The volcano erupted for 9 years, and quickly built up the cinder cone to 424 meters, and covered 25 km² of fields in lava flows and rubble. Nearby towns were eventually buried in ash by the eruptions.

We have written many articles about volcanoes for Universe Today. Here’s an article about the biggest volcano on Earth, and here’s one about the largest volcano in the Solar System.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

March 16, 2009December 24, 2015 by Anne Minard

ESO Image Reveals Galaxy Duo in Explosive Dance

The ‘peculiar galaxy’ Arp 261 has been imaged in unprecedented detail, revealing two galaxies in a slow motion — but highly chaotic and disruptive — close encounter.

Arp 261 lies about 70 million light-years distant in the constellation of Libra, the Scales. The new close-up was captured by the ESO’s Very Large Telescope, at the Paranal Observatory in Chile.

Although individual stars are very unlikely to collide in such an interaction, the huge clouds of gas and dust certainly do crash into each other at high speed, leading to the formation of bright new clusters of very hot stars that are clearly seen in the picture. The paths of the existing stars in the galaxies are also dramatically disrupted, creating the faint swirls extending to the upper left and lower right of the image. Both interacting galaxies were probably dwarfs not unlike the Magellanic Clouds orbiting our own galaxy.

Arp 261 is listed in Halton Arp’s catalogue of Peculiar Galaxies that appeared in the 1960s, with the goal of chronicling objects in the sky that appear strange and may tell rewarding science stories.

The images used to create the new picture of Arp 261 were not actually taken to study the interacting galaxies at all, but to investigate the properties of the inconspicuous object just to the right of the brightest part of Arp 261 and close to the center of the image. This is an unusual exploding star, called SN 1995N, that is thought to be the result of the final collapse of a massive star at the end of its life, a so-called core collapse supernova. SN 1995N is unusual because it has faded very slowly — and still shows clearly more than seven years after the explosion took place.

SN 1995N is also one of the few supernovae to have been observed to emit X-rays. It is thought that these unusual characteristics are a result of the exploding star being in a dense region of space so that the material blasted out from the supernova plows into it and creates X-rays.

Apart from the interacting galaxy and its supernova, the image also contains several other objects at wildly different distances from us. Starting very close to home, two small asteroids, in our Solar System between the orbits of Mars and Jupiter, happened to cross the images as they were being taken and show up as the red-green-blue trails at the left and top of the picture. The trails arise as the objects are moving during the exposures and also between the exposures through different colored filters. The asteroid at the top is number 14670 and the one to the left number 9735. They are probably less than 5 km (3 miles) across. The reflected sunlight from these small bodies takes about 15 minutes to reach Earth.

The next closest object is probably the apparently bright star at the bottom. It may look bright, but it is still about one hundred times too faint to be seen with the unaided eye. It is most likely a star rather like the Sun and about 500 light-years from us — 20 million times further away than the asteroids. Arp 261 itself, and the supernova, are about 140,000 times farther away than this star, but still in what astronomers would regard as our cosmic neighborhood. Much more distant still, perhaps some fifty to one hundred times further away than Arp 261, lies the cluster of galaxies visible on the right of the picture.

Videos of the unusual system are available here and here.

Source: ESO

March 13, 2009December 24, 2015 by Anne Minard

Earth Cyclones, Venus Vortices Have Much in Common

Scientists have spotted an S-shaped feature in the center of the vortices on Venus that looks familiar — because they’ve seen it in tropical cyclones on Earth.

Researchers from the United States and Europe spotted the feature using NASA’s Pioneer Venus Orbiter and The European Space Agency’s Venus Express. Their new discovery confirms that massive, swirling wind patterns have much in common where they have been found — on Venus, Saturn and Earth.

At cloud top level, Venus’ entire atmosphere circles the planet in just about four Earth days, much faster than the solid planet does. Despite this “superrotation,” some dynamical and morphological similarities exist between the vortex organization in the atmospheres of Venus’s northern and southern hemispheres and tropical cyclones and hurricanes on Earth.

Organization of the Venus atmospheric circulation into two circumpolar vortices, one centered on each pole, was first deduced more than 30 years ago from Mariner 10 ultraviolet images. The S-shaped feature in the center of the vortices on Venus was first detected by the Pioneer Venus Orbiter near the northern pole and recently by Venus Express orbiter around the southern pole. It is also known to occur in Earth’s tropical cyclones.

Using an idealized nonlinear and nondivergent barotropic model, lead author Sanjay S. Limaye, of the University of Wisconsin-Madison, and his colleagues are proposing that these S-shaped features are the manifestations of barotropic instability. The feature can be simulated with a barotropic model and, like in the vortices on Venus and in tropical cyclones, it is found to be transient.

Another similarity between the observed features in the vortex circulations of Venus and in terrestrial hurricanes is the presence of transverse waves extending radially outward from the vortex centres. The lack of observations of such features in Earth’s polar vortices is suggestive that the dynamics of the Venus polar vortices may have more in common with hurricanes than their more direct terrestrial counterparts.

Given the challenges in measuring the deep circulation of Venus’s atmosphere, the authors expect that the morphological similarities between vortices on Earth and Venus might help scientists better understand atmospheric superrotation on Venus and guide future observations.

IMAGE CAPTIONS: 1. The ‘eye of the hurricane’ on Venus, taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express. The yellow dot represents the south pole. Credit: ESA 2. An infrared satellite image of Hurricane Howard [1998], showing an S-shaped pattern in the low (warm) clouds in the tropical cyclone’s eye. Credit: Sanjay S. Limaye.

Source: Geophysical Research Letters

March 13, 2009December 24, 2015 by Fraser Cain

Shield Volcanoes

Color mosaic of Mars' greatest mountain, Olympus Mons, viewed from orbit. Credit NASA/JPL

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Shield volcanoes are large volcanoes with gently sloping sides. In fact, the largest volcanoes on Earth (and even the Solar System) are shield volcanoes. They form when lava flows of low viscosity build up over long periods of time, creating volcanoes with huge internal volume. The best known shield volcanoes are ones that make up the Big Island of Hawaii: Mauna Loa and Mauna Kea.

The common feature with shield volcanoes is that they’re built up slowly over time from a very stable central summit vent. Flow after flow pours out of the vent, slides down the slopes of the volcano, and builds up the size. The largest volcanoes, like Mauna Loa and Mauna Kea would have been created from thousands of these flows.

Shield volcanoes can be found around the world. In northern California and Oregon, they can be 5-10 km across and about 500 meters high. But in the Hawaiian Islands, the volcanoes were atop very active vents for millions of years. Mauna Loa projects 4,168 meters above sea level, but if you measure it from the base of the ocean to its top, it measures 8,534 meters. (Mount Everest is 8,848 meters tall).

Volcanic activity is linked to plate tectonics, and the most of the world’s volcanoes are located near plate boundaries where subduction is happening. This is where one plate is passing under another plate, sinking into the Earth’s mantle.

The largest shield volcano in the Solar System is Olympus Mons on Mars. This monster measures 27 km above the surface of Mars, and is 550 km in width. It’s believed that Olympus Mons got so big because Mars lacks plate tectonics. A single volcanic hotspot was able to channel lava for billions of years, building up the volcano to such a great size.

We have written many articles about the Earth for Universe Today. Here’s an article about Olympus Mons, and here’s an article about Mauna Kea and Mauna Loa.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

March 13, 2009December 24, 2015 by Tammy Plotner

Weekend SkyWatcher’s Forecast – March 13-15, 2009

Greetings, fellow SkyWatchers! With the Moon gone from the early evening skies, it’s time for a little sky dancing this weekend. Are you ready for a little old stepping out and a little new? Then waltz this way as we check out some very new star clusters and interesting asterisms! Grab your binoculars and telescopes and I’ll meet you in the back yard….

Friday, March 13, 2009 – Today note the 1886 birth of Albert William Stevens, a daring balloonist who took the Explorer II to an altitude of 72,395 feet. He took the first photo showing Earth’s curvature and the first solar eclipse photo of the Moon’s shadow on Earth. Also, salute the 1855 birth on this date of Percival Lowell, who predicted the existence of Pluto (but Clyde Tombaugh was the one who actually discovered it, on Lowell’s 75th birthday!). Sir Percival was a determined soul who spent his life trying to find proof of life on Mars. He founded Lowell Observatory in 1894, where he studied Mars intensively, drawing the Red Planet covered with canals and oases. As Lowell once said: ‘‘Imagination is as vital to any advance in science as learning and precision are essential for starting points.’’

Tonight we’ll look at a bright collection of stars located less than a handspan west of Procyon. Its name is Collinder 106 (RA 06 37 19 Dec -05 57 55). At a combined magnitude of 4.5, this expansive open cluster can be spotted as a hazy patch with the unaided eye and comes to full resolution with binoculars. It contains only around 14 members, but this widely scattered galactic collection has helped scientists determine size scales and dispersion among groups of its type. Viewed telescopically at low power, the observer will find it rich in background stars and a true delight in a low power, wide field eyepiece. If you’d like a challenge, hop a half degree to the northeast to spot Collinder 111 (RA 06 38 42 Dec -06 54 00). While visually only about one-tenth the apparent size of its larger southwestern neighbor, spare little Collinder 111 also belongs to the same class of open clusters. Who knows what may lurk around these understudied clusters?

Saturday, March 14, 2009 – Before dawn, look for the close appearance of Spica and the Moon to celebrate today’s famous astro births, starting with astronaut Frank Borman (b. 1928), a crew member of Apollo 8, the first manned flight around the Moon. Next, astronaut Eugene Cernan (b. 1934), who floated in space for more than 2 hours during the Gemini 9 mission and piloted Apollo 10. How about Giovanni Schiaparelli (1835), the Italian astronomer who described Mars’s ‘‘canali’’ and named its ‘‘seas’’ and ‘‘continents.’’ Schiaparelli’s comet studies demonstrated that meteoroid swarms existed in the path of cometary orbits, and thus predicted annual meteor showers. He was first to suggest that Mercury and Venus rotate and discovered the asteroid Hesperia. Still not enough? Then wish a happy birthday to Albert Einstein (b.1879), the German–American physicist considered the most brilliant intellect in human history!

For a moment let’s reflect on Einstein’s Cross, proof of his genius. We can’t observe this Pegasus-based gravitational lens right now, but we can try to understand Einstein’s theory of gravity as an effect of the curvature in space–time. For example, if you draw a line around the center of a ball, the line would be straight, eventually coming back to its point of origin. We don’t see the point until we reach it, but we know it’s there. Einstein knew this dimension existed and predicted any object with mass will bend space and time around it, just like our line around the ball. He predicted light would also follow a curved path around an object… such as a distant quasar located behind a closer galaxy!

Tonight’s object is a ‘‘cross’’ astersim of stars. Begin at Procyon and shift about 10 degrees southwest (or 2 degrees south of 18 Monocerotis) to locate this pretty grouping of stars. Yes it’s true. It’s just an unknown, undocumented, and unnamed asterism, but how fitting to honor all these famous astro figures and a brilliant man who once said: ‘‘The fairest thing in life we can experience is the mysterious. It… stands at the cradle of true art and true science.’’

Now let’s go for a a challenging study. Larger telescopes should look for diminutive Bochum 2 less than half a degree northeast of ‘‘Einstein’s Asterism’’ (RA 06 48 50 Dec 00 22 35). At low power, it’s just a tight configuration of stars, but test the limit your telescope and increase magnification. This young open cluster has been studied for internal kinematics, spectroscopic binaries, and its motion in the galaxy, but its most interesting feature is a trapezium system at its heart. After a 4-year study, two of the members were documented as close binary stars with highly eccentric orbits, and one of the members is leaving as a runaway!

For smaller optics, continue another half degree east for NGC 2301 (RA 06 51 48 Dec -00 28 00). Even telescopes as small as Lacaille’s can see this bright, 2,500 light-year-distant open cluster. Studied for its variable stars, NGC 2301 is also on many binocular deep-sky observing lists!

Sunday, March 15, 2009 – Today marks the 1713 birth of Abbe Nicolas Louis de Lacaille, the French astronomer who named 15 of the 88 constellations. Using only a half-inch refractor, Lacaille made 26 new discoveries and charted 9,776 stars, creating the first southern star catalog. Sharing the date is William Rutter Dawes (b. 1799). ‘‘Eagle-eyed’’ Dawes made exhaustive measurements of binary stars, discovered Saturn’s inner Crepe Ring, and accurately mapped Mars. Dawes also devised the elegantly simple formula (Dawe’s Limit) of dividing the number 11 by the aperture in centimeters to give the arcseconds of resolution required to split a binary star.

Thankfully, somebody was watching the sky at 5:30 p.m. on this date in 1806, because the observed fall of a pristine 6-kilogram chondrite meteor made an indisputable case that chondrites carried carbon-based organic chemicals. Perhaps it was from one of the Corona Australid meteors whose shower peak is tonight after midnight? The fall rate is about 5–7 per hour, and best for our friends in the southern hemisphere!

Tonight let’s return to the Einstein’s Asterism and drop 15 degrees due southeast to study open cluster NGC 2360 (RA 07 17 42 Dec -15 38 00). At a distance of 4,600 light-years, magnificent NGC 2360 contains around 40 members, 7 of which are red giants. You have Caroline Herschel to thank for this lovely cluster… and her birthday is tomorrow!

Now, return to our Einstein’s Asterism and head slightly more than half a degree west to study scattered open cluster Dolidze 25 (RA 06 45 06 Dec -00 18 00). This low power, telescopic only, galactic cluster is a worthy study for those who seek the unusual. Located at the outer edges of our own galaxy, Dolidze 25 may very well be the product of the merger of the Milky Way and the Canis Major Dwarf galaxy. Extremely rich in oxygen and significantly deficient in metals, this huge starforming region contains young stars, pre-main sequence stars, and Delta Scuti types. With its thin veil of nebula, Do25 should prove to be challenging and quite to your liking! Hop another half degree west, and then slightly south for Dolidze 23 (RA 06 43 12 Dec -00 00 00). This telescope-only cluster reveals around a dozen easily resolvable stars at low power. Dolidze 23’s two brighter members are finderscope visible. Locate the cluster at low power, and place it at the south edge of the field of view. Turn off your drive units and allow the field to cruise by naturally as you observe. This allows Dolidze 25 to drift across your line of sight, a technique that often improves your ability to spot fine detail in fainter objects.

Celestial scenery alert on Tuesday, March 17! A few hours before dawn, the Moon and mighty Antares will be nearly touching, separated by only a fraction (0.2) of a degree. For some, this could be a wonderful occultation event, so be sure to check maps and resources! Although the occultation path is limited, even more so is the graze path, just a few kilometers wide. For these lucky viewers, brilliant red Antares may flash in and out of view several times as it moves slowly along behind the lunar mountains.

Until next week, dreams really do come true when you keep on reaching for the stars!

This week’s awesome photos are: Sir Percival Lowell (historical image), Collinder 106 (credit – Palomar Observatory, courtesy of Caltech), Einstein’s Cross (credit – HST/NASA), “Einstein’s Asterism’’ (Credit – Palomar Observatory, courtesy of Caltech), Bochum 2, NGC 2301, Dolidize 25 and NGC 2360 (credit – Palomar Observatory, courtesy of Caltech). Thank you so much!

March 12, 2009October 21, 2016 by Fraser Cain

What is Earth’s Magnetic Field?

You can’t see it, but there’s an invisible force field around the Earth. Okay, not a force field, exactly, but a gigantic magnetic field surrounding the Earth, and it acts like a force field, protecting the planet – and all the life – from space radiation. Let’s take a look at the Earth’s magnetic field.

The Earth is like a great big magnet. The north pole of the magnet is near the top of the planet, near the geographic north pole, and the south pole is near the geographic south pole. Magnetic field lines extend from these poles for tens of thousands of kilometers into space; this is the Earth’s magneto sphere.

The geographic poles and the magnetic poles are far enough apart that scientists distinguish them differently. If you could draw a line between the magnetic north and south poles, you would get a magnetic axis that’s tilted 11.3 degrees away from the Earth’s axis of rotation. And these magnetic poles are known to move around the surface, wandering as much as 15 km every year.

Scientists think that the Earth’s magnetic field is generated by electrical currents flowing in the liquid outer core deep inside the Earth. Although it’s liquid metal, it moves around through a process called convection. And the movements of metal in the core sets up the currents and magnetic field.

As I mentioned at the top of this article, the magnetic field of the Earth protects the planet from space radiation. The biggest culprit is the Sun’s solar wind. These are highly charged particles blasted out from the Sun like a steady wind. The Earth’s magnetosphere channels the solar wind around the planet, so that it doesn’t impact us. Without the magnetic field, the solar wind would strip away our atmosphere – this is what probably happened to Mars. The Sun also releases enormous amounts of energy and material in coronal mass ejections. These CMEs send a hail of radioactive particles into space. Once again, the Earth’s magnetic field protects us, channeling the particles away from the planet, and sparing us from getting irradiated.

The Earth’s magnetic field reverses itself every 250,000 years or so. The north magnetic pole becomes the south pole, and vice versa. Scientists have no clear theory about why the reversals happen. One interesting note is that we’re long overdue for a reversal. The last one happened about 780,000 years ago.

We have written many articles about Earth for Universe Today. Here’s an article about how the geomagnetic reversal doesn’t mean doomsday in 2012, and here’s an article about how a solar storm compressed the Earth’s atmosphere.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Reference:
NASA: Is the Earth’s magnetic field changing?

March 12, 2009December 24, 2015 by Anne Minard

Stars at Milky Way Core ‘Exhale’ Carbon, Oxygen

Carbon exists only in a fine-tuned universe( 'Cat's Eye' Planetary Nebula) — Cat's Eye Nebula. Researchers have found carbon and oxygen in dusty planetary nebulae surrounding stars at the center of the Milky Way. Credit: NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA

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Carbon and oxygen have been spotted in the dust around stars in the center of the Milky Way galaxy, suggesting that the stars have undergone recent disruptions of some kind — and hinting how stars can send heavy elements — like oxygen, carbon, and iron — out across the universe, paving the way for life.

Scientists have long expected to find carbon-rich stars in our galaxy because we know that significant quantities of carbon must be created in many such stars. But carbon had not previously shown up in the clouds of gas around these stars, said Matthew Bobrowsky, an astrophysicist at the University of Maryland and a co-author of a new study reporting the discovery.

“Based on our findings, this is because medium-sized stars rich in carbon sometimes keep that carbon hidden until very near the end of their stellar lives, releasing it only with their final ‘exhalations’,” explained Bobrowsky.

The new results appear in the February issue of the journal Astronomy and Astrophysics.

Bobrowsky and his team, led by J. V. Perea-Calderón at the European Space Astronomy Centre in Madrid, Spain, used the Spitzer Space Telescope to view each star and its surrounding clouds of dust and particles, called a planetary nebulae. The researchers measured the light emitted by the stars and the surrounding dust and were able to identify carbon compounds based on the wavelengths of light emitted by the stars. Looking in an area at the center of the Milky Way called the “Galactic Bulge,” the team observed 26 stars and their planetary nebulae and found 21 with carbon “signatures.”

But the scientists did not just find carbon around these stars; they also found oxygen in these 21 dust clouds, revealing a surprising mixture of ingredients for space dust. They report in their paper that this is likely due to a thermal pulse where a wave of high-pressure gas mixes layers of elements like carbon and oxygen and spews them out into the surrounding cloud.

The finding of carbon and oxygen in the dust clouds surrounding stars suggests a recent change of chemistry in this population of stars, according to the authors.

“Stars in the center of the Milky Way are old and ‘metal-rich’ with a high abundance of heavy elements,” Bobrowsky said. “They are different in chemical composition than those found in the disc, farther out from the center.”

Studying the chemistry of the stars helps scientists learn how the matter that makes up our earth and other planets in our galaxy left its stellar birthplaces long ago.

As a star burns hotter and hotter, the hydrogen gas that originally made up almost all of its mass is converted, through nuclear fusion, first to helium, and then to progressively heavier elements. The hottest region in the core fuses together the heaviest elements. And these can reach the surface of the star only when its life is almost over.

“The Big Bang produced only hydrogen and helium,” Bobrowsky said. “Heavier elements like carbon and oxygen only come from getting ‘cooked up’ in stars. Nuclear reactions in stars created the heavier elements found in ‘life as we know it’.”

In the last 50,000 years of their 10 billion-year lives, sun-sized stars expel carbon atoms along with hydrogen and helium to form a surrounding cloud of gas that soon disperses into space, perhaps to eventually become the stuff of new stars, solar systems, or perhaps even life on some earth-like planet. Much larger stars expel their heavier matter in massive explosions called supernovae.

“All the heavy elements [which astronomers call ‘metals,’ and include all elements heavier than hydrogen and helium] on Earth were created by nuclear fusion reactions in previous generations of stars,” said Bobrowsky. “Those earlier stars expelled those elements into space and then our solar system formed out of that gas containing all the heavy elements that we now find in Earth and in life on Earth.”

LEAD IMAGE CAPTION: Cat’s Eye Nebula. Researchers have found carbon and oxygen in dusty planetary nebulae surrounding stars at the center of the Milky Way. Credit: NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA)

Source: Astronomy & Astrophysics and Spitzer, via AAS

March 12, 2009December 24, 2015 by Fraser Cain

Earth, Sun and Moon

From our perspective, the three objects that have the greatest impact on our lives are the Earth, Sun, and Moon. The Earth, of course, is the planet beneath our feet. Without it, well, we wouldn’t have anything at all. The Sun warms our planet, and with the Moon, creates the tides.

The Moon orbits the Earth and in turn, the Earth orbits the Sun. We see the Universe from a platform that is both rotating on its axis, and traveling in an elliptical orbit around the Sun. The Earth’s rotation on its axis makes the Sun rise in the east and set in the west, and is a big part of why the Moon rises and sets too; although the Moon takes 29 days to complete an orbit around the Earth as well.

The average distance from the Earth to the Moon is 384,403 km. And the average distance from the Earth to the Sun is 149,597,887 km. If you divide these two numbers, you get approximately 389. Now, if you divide the diameter of the Sun (1.4 million km) by the diameter of the Moon (3,474 km), you get 403. Those two numbers are pretty close. This is why the Moon and the Sun appear to be the same size in the sky; it’s a total coincidence.

Because they appear to be the same size in the sky, the Sun, Earth and Moon work together to create eclipses. When the Moon is directly in between the Earth and Sun, we see a solar eclipse. The Moon appears to pass in front of the Sun and darken it completely. And in the opposite situation, when the Earth is in between the Sun and the Moon, the Earth’s shadow darkens the Moon. This is a lunar eclipse. We don’t see eclipses every month because the Moon’s orbit it tilted slightly away from the Earth’s orbit around the Sun. Sometimes the Moon is above this orbit and sometimes it’s below, so it doesn’t block the light from the Sun, or get caught in the Earth’s shadow.

The Sun and the Moon work together to create the tides we experience here on Earth. Most of the rise of the tides comes from the gravitational pull of the Moon, but a small amount comes from the Sun. When the two objects are on the same side of the Earth, we get the highest and lowest tides, and when they’re on opposite sides of the Earth, the tides are less extreme.

The brightest object in the Sky is the Sun. Astronomers measure its apparent magnitude as -26.73. This makes it 449,000 times brighter than the full Moon. The brightness of the Moon is only -12.6. Of course all of the Moon’s brightness is just reflected light from the Sun.

We have written many articles about the Earth for Universe Today. Here’s a more detailed article about the Sun and the Moon.

Want more resources on the Earth? Here’s a link to NASA’s Human Spaceflight page, and here’s NASA’s Visible Earth.

We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.

Reference:
NASA Earth Observatory