Two More Earth-Sized Planets Discovered by Kepler, Orbiting Former Red Giant Star

Credit: S. Charpinet / Univ. of Toulouse

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Amid all of the news last week regarding the discovery by Kepler of two Earth-sized planets orbiting another star, there was another similar find which hadn’t received as much attention. There were two more Earth-sized planets also just discovered by Kepler orbiting a different star. In this case, however, the star is an old and dying one, and has passed its red giant phase where it expands enormously, destroying (or at least barbecuing) any nearby planets in the process before becoming just an exposed core of its former self. The paper was just published in the journal Nature.

The two planets, KOI 55.01 and KOI 55.02, orbit the star KOI 55, a subdwarf B star, which is the leftover core of a red giant star. Both planets have very tight orbits close to the star, so they were probably engulfed during the red giant phase but managed to survive (albeit “deep-fried”). They are estimated to have radii of 0.76 and 0.87 that of Earth, the smallest known exoplanets found so far orbiting an active star.

According to lead author Stephane Charpinet, “Having migrated so close, they probably plunged deep into the star’s envelope during the red giant phase, but survived.”

“As the star puffs up and engulfs the planet, the planet has to plow through the star’s hot atmosphere and that causes friction, sending it spiraling toward the star,” added Elizabeth ‘Betsy’ Green, an associate astronomer at the University of Arizona’s Steward Observatory. “As it’s doing that, it helps strip atmosphere off the star. At the same time, the friction with the star’s envelope also strips the gaseous and liquid layers off the planet, leaving behind only some part of the solid core, scorched but still there.”

The discovery was also unexpected; the star had already been the subject of study using the telescopes at Kitt Peak National Observatory, part of a project to examine pulsating stars. For more accurate measurements however, the team used data from the orbiting Kepler space telescope which is free of interfering atmospheric effects. According to Green, “I had already obtained excellent high-signal to noise spectra of the hot subdwarf B star KOI 55 with our telescopes on Kitt Peak, before Kepler was even launched. Once Kepler was in orbit and began finding all these pulsational modes, my co-authors at the University of Toulouse and the University of Montreal were able to analyze this star immediately using their state-of-the art computer models.”

Two tiny modulations in the pulsations of the star were found, which further analysis indicated could only come from planets passing in front of the star (from our viewpoint) every 5.76 and 8.23 hours.

Our own Sun awaits a similar fate billions of years from now and is expected to swallow Mercury, Venus, Earth and Mars during its expansion phase. “When our sun swells up to become a red giant, it will engulf the Earth,” said Green. “If a tiny planet like the Earth spends 1 billion years in an environment like that, it will just evaporate. Only planets with masses very much larger than the Earth, like Jupiter or Saturn, could possibly survive.” The discovery should help scientists to better understand the destiny of planetary systems including our own.

This finding is important in that it not only confirms that Earth-size planets are out there, and are probably common, but that they and other planets (of a wide variety so far) are being found orbiting different types of stars, from newly born ones, to middle-age ones and even dying stars (or dead in the case of pulsars). They are a natural product of star formation which of course has implications in the search for life elsewhere.

The abstract of the paper is here, but downloading the full article requires a single-article payment of $32.00 US or a subscription to Nature.

Astronomers Find the Most Supermassive Black Holes Yet

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For years, astronomer Karl Gebhardt and graduate student Jeremy Murphy at The University of Texas at Austin have been hunting for black holes — the dense concentration of matter at the centre of galaxies. Earlier this year, they made a record-breaking discovery. They found a black hole weighing 6.7 billion times the mass of our Sun in the centre of the galaxy M87.

But now they shattered their own record. Combining new data from multiple observations, they’ve found not one but two supermassive black holes that each weigh as much as 10 billion Suns.

“They just keep getting bigger,” Gebhardt said.

An artist's impression of the black hole at the centre of the M87 galaxy. Image credit: Gemini Observatory/AURA illustration by Lynette Cook

Black holes are made of extremely densely packed matter. They produce such a strong gravitational field that even light cannot escape. Because they can’t be seen directly, astronomers find black holes by plotting the orbits of stars around these giant invisible masses. The shape and size of these stars’ orbits can determine the mass of the black hole.

Exploding stars called supernovae often leave behind black holes, but these only weigh as much as the single star. Black holes billions of times the mass of our Sun have grown to be so big. Most likely, an ordinary black hole consumed another, captured huge numbers of stars and the massive amount of gas that they contain, or be the result of two galaxies colliding. The larger the collision, the more massive the black hole.

The supermassive black holes Gebhardt and Murphy have found are at the centres of two galaxies more than 300 million light years from Earth. One weighing 9.7 billion solar masses is located in the elliptical galaxy NGC 3842, the brightest galaxy in the Leo cluster of galaxies 320 million light years away in the direction of the constellation Leo. The other is as large or larger and sits in the elliptical galaxy NGC 4889, the brightest galaxy in the Coma cluster about 336 million light years from Earth in the direction of the constellation Coma Berenices.

Each of these black holes has an event horizon — the point of no return where nothing, not even light can escape their gravity — 200 times larger than the orbit of Earth (or five times the orbit of Pluto). That’s a mind-boggling 29,929,600,000 kilometres or 18,597,391,235 miles. Beyond the event horizon, each has a gravitational influence that extends over 4,000 light years in every direction.

The illustration shows the relationship between the mass of a galaxy's central black hole and the mass of its central bulge. Recent discoveries of supermassive black holes may mean that the black holes in all nearby massive galaxies are more massive than we think. This could signal a change in our understanding of the relationship between a black hole and its surrounding galaxy. Image credit: Tim Jones/UT-Austin after K. Cordes & S. Brown (STScI)

For comparison, the black hole at the centre of our Milky Way Galaxy has an event horizon only one-fifth the orbit of Mercury — about 11,600,000 kilometres or 7,207,905 miles. These supermassive black holes are 2,500 times more massive than our own.

Gebhardt and Murphy found the supermassive black holes by combining data from multiple sources. Observations from the Gemini and Keck telescopes revealed the smallest, innermost parts of these galaxies while data from the George and Cynthia Mitchell Spectrograph on the 2.7-meter Harlan J. Smith Telescope revealed their largest, outmost regions.

Putting everything together to deduce the black holes’ mass was a challenge. “We needed computer simulations that can accommodate such huge changes in scale,” Gebhardt said. “This can only be done on a supercomputer.”

But the payoff doesn’t end with finding these massive galactic centre. The discovery has much more important implications. It “tells us something fundamental about how galaxies form” Gebhardt said.

These black holes could be the dark remnants of previously bright galaxies called quasars. The early universe was full of quasars, some thought to have been powered by black holes 10 billion Solar masses or more. Astronomers have been wondering where these supermassive galactic centres have since disappeared to.

Gebhardt and Murphy might have found a key piece in solving the mystery. Their two supermassive black holes might shed light on how black holes and their galaxies have interacted since the early universe. They may be a missing link between ancient quasars and modern supermassive black holes.

Source: McDonald Observatory Press Release.

Where Have All the Quasars Gone?

Asteroid 2005 YU55: See It For Yourself!

Passage of of 2005 YU55 near Altair from 6:03 p.m. – 6:12 p.m. EST (11:03 – 11:12 UTC)

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It’s already been stated several times here on Universe Today that 2005 YU55, a 400-meter-wide roughly spherical asteroid, will not pose any threat to Earth as it passes by on Tuesday, November 8… even though it will come within 80% of the distance to the Moon. Many experts have come forward to state this fact, including Don Yeomans of JPL’s Near-Earth Object Observation Program and Lance Benner, a radio astronomer with the Deep Space Network in Goldstone, CA.  But it will still be a notable event, being the first time since 1976 such a large object will pass so closely by our planet. So, with the eve of YU55’s approach upon us, let’s turn our curiosity toward another aspect of this cosmic visitation: how can we see it?

Unfortunately there are a couple of factors working against the casual observer being able to witness YU55’s pass. One: it’s a dark object. A very dark object. 2005 YU55 is a C-type asteroid, which means it is composed of carbonaceous material and is thus effectively darker than coal, reflecting less than 1% of the sunlight that it receives. It probably won’t be brighter than magnitude 10. (On the backwards-ranked scale of apparent magnitude, 6 is the limit of best visibility to the average human eye, while -1 or 0 would be a very bright star. Jupiter is about -3 right now, while the full moon would be -12.7. In a typical suburban neighborhood 3 or 4 is the limit of naked-eye visibility.)

And two: the Moon will be close to full on the night of the 8th, and YU55 will be headed in its direction. That sure won’t help visibility.

But, should you be located in a dark area, and should you have a 6″ or larger telescope at your disposal, you may want to give a go at spotting the asteroid that’s caused quite a fuss over the past few months for yourself. It won’t be a simple task, but it’s not impossible – and to help you out teacher, writer and astronomy enthusiast David Dickinson has posted an article about it on his blog, Astro Guyz.

Here’s an exerpt:

Closest approach to Earth occurs at 11:29 UTC/06:29 EST at about 202,000 miles distant, placing it high to the southwest for observers on the US Eastern Seaboard. At its closest approach, 2005 YU55 will glide along at one degree every 7 minutes, easily noticeable after a few minutes of observation at low power. I plan to target selected areas with my GOTO mount, sketch the field, then watch for changes. I may also take some wide-field piggyback stills with the DSLR, but mostly, this one will just be fun to watch.

Visually tracking a Near-Earth asteroid can be thrilling to watch; for example, I’ve actually seen 4179 Toutatis years ago show discernable movement after tracking it for a few moments in the eyepiece!

– David Dickinson

Wide field finder of 2005 YU55 from sunset until 8:30PM EST.

The asteroid will pass through the constellations Aquila, Delphinus, and Pegasus as it heads westward. Interestingly, 2005 YU55 passes within a degree of Altair centered on 6:07:30PM EST only 27 minutes after local sunset, and also makes a very close pass of the star Epsilon Delphini during closest approach. These both make good visual “anchors” to aim your scope at during the appointed time and watch. Keep in mind, the charts provided are rough and “Tampa Bay-centric…”

On an approach as close as this one, two factors muddle the precise prediction coordinates of the asteroid; one is the fact the gravitational field of the Earth will change the orbit of 2005 YU55 slightly, and two is that the position will change due to the position of the observer on the Earth and the effect of parallactic shift. Many prediction programs assume the Earthly vantage as a mere point in space, fine for positioning deep sky objects but not so hot for ones passing near the planet. A good place to get updated coordinates is JPL Horizons website which lets you generate an accurate ephemeris for your exact longitude latitude and elevation.

David goes on to add:

2005 YU55 will pass our Moon at 8 AM Universal Time on November 9th at a distance only marginally closer than it did the Earth, at 140,000 miles. Interestingly, it also transited Sun on November 3rd as seen from the Moon, but would have appeared <1” in size, a tough target for any would-be lunar-based observer. Its next close predicted passage of the Earth won’t be until 2056 at nearly 3 times the distance.

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Excellent information… many thanks to David for sharing with us! (You can read the full article on his website here.) And if you do witness the pass of this asteroid and somehow manage to get some photos of it, you can share them on the Universe Today Flickr group… they may be featured in an upcoming article!

Read more about 2005 YU55’s close pass by Earth tomorrow.

Charts and excerpts by David Dickinson, created with Starry Night and Paint.

 

Three New Planets and a Mystery Object Found Orbiting Dying Stars

A planet about to be consumed by its expanding red giant star. Credit: Mark Garlik/HELAS

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Some interesting new additions to the exoplanet family were announced last week by astronomers from Penn State University. While finding exoplanets these days may be considered “just another day at the office,” astronomers discovered three unique planets and an additional “mystery” object. What’s unique about these planets is the fact that the stars they orbit are all old and dying – red giant stars which have swollen up as they near the end of their lives, which ordinarily would consume any unlucky planets which may be too close to escape…

The three stars are HD 240237, BD +48 738, and HD 96127; the second one also has the mystery object orbiting it, which may be another planet, a low-mass star or a brown dwarf — something whose mass is in between that of a smaller, cooler star and a giant planet.

“We will continue to watch this strange object and, in a few more years, we hope to be able to reveal its identity,” said team leader, Alex Wolszczan.

Wolszczan was the first astronomer to discover exoplanets, three small planets orbiting a pulsar (neutron star) in 1992.

It is expected that our own Sun will also become a red giant star in another five billion years or so. Not a good thing for us obviously, but still a long ways off thankfully, since at that time, all of the inner planets of the solar system will probably be consumed by the expanding Sun.

The subject of planets orbiting dying stars will also be the focus of an upcoming conference, Planets Around Stellar Remnants, in Puerto Rico next January. It is organized by Penn State’s Center for Exoplanets and Habitable Worlds, and will take place exactly 20 years since Wolszczan made his discovery.

Interesting, since by far most of the exoplanets found so far orbit “normal” stars, like our Sun, which are still in mid-life or younger. But now, they’ve been observed around stars at all different stages of evolution, from the youngest stars, even those still with protoplanetary disks, to the oldest, stars which have already died and burned out, like pulsars. What this seems to indicate is that planets are a normal part of star formation, from beginning to end. The numbers now being found by astronomers, in the thousands and likely millions or billions, also suggest this; a big change from just a few decades ago when it was unknown if there were any other solar systems out there at all. There are, a lot of them.

Source: Penn State University

“Impossible” Star Exists in Cosmic Forbidden Zone

This ancient star, in the constellation of Leo (The Lion), is called SDSS J102915+172927 and has been found to have the lowest amount of elements heavier than helium of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old. Credit: ESO/Digitized Sky Survey 2

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Astronomers say a newly found star should not exist and is in the “forbidden zone” of a widely accepted theory of star formation. The star, called SDSS J102915+172927, is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it. And how should this star be classified? We suggest either ‘easy listening’ or ‘jazz,’ as this star is certainly not heavy metal! But it may be one of the oldest stars ever found.

This faint star is located in the constellation of Leo (The Lion), and has the lowest amount of elements heavier than helium (what astronomers call “metals”) of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old.

“A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed,” said Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Germany and Observatoire de Paris, France), lead author of the paper appearing in this week’s edition of Nature. “It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”

The team found the star with the X-shooter and UVES instruments on the Very Large Telescope. This allowed them to measure how abundant the various chemical elements were in the star. They found that the proportion of metals in SDSS J102915+172927 is more than 20,000 times smaller than that of the Sun.

“The star is faint, and so metal-poor that we could only detect the signature of one element heavier than helium — calcium — in our first observations,” said Piercarlo Bonifacio (Observatoire de Paris, France), who supervised the project. “We had to ask for additional telescope time from ESO’s Director General to study the star’s light in even more detail, and with a long exposure time, to try to find other metals.”

This picture shows the distribution of the light of different colours coming from the remarkable star SDSS J102915+172927 after it has been split up by the X-Shooter instrument on the ESO VLT. Credit: ESO/E. Caffau

The prevailing theory is that hydrogen and helium were created shortly after the Big Bang, together with some lithium, while almost all other elements were formed later in stars. Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is.

“The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found,” adds Lorenzo Monaco from ESO, also involved in the study.

Also very surprising was the lack of lithium in SDSS J102915+172927. Such an old star should have a composition similar to that of the Universe shortly after the Big Bang, with a few more metals in it. But the team found that the proportion of lithium in the star was at least fifty times less than expected in the material produced by the Big Bang.

“It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star.” Bonifacio added.

Is this star one-of-a-kind? Probably not, the researchers say. “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case,” said Caffau.

Read the team’s paper (pdf file)

Source: ESO

Shiny New Supernova Spotted in Nearby Galaxy

Supernova PTF11kly in M101 on August 24, 2011. Credit: BJ Fulton, LCOGT.

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Literally an event of stellar proportions, a new Type Ia supernova has been identified in a spiral galaxy 25 million light-years away! Spotted by Caltech’s Palomar Transit Factory project, this supernova, categorized as PTF11kly, is located 58″.6 west and 270″.7 south of the center of M101. It was first seen yesterday, August 24, 2011.

According to AAVSO Special Notice #250 P. Nugent et al. reported in Astronomical Telegram #3581 that a possible Type-Ia supernova has been discovered by the Palomar Transient Factory shortly after eruption in the galaxy M101 and has been designated “PTF11kly”. The object is currently at a magnitude of 17.2, but may well rise by several magnitudes. The object is well placed within M101 for good photometry, and observations of this potential bright SNIa are strongly encouraged.

There are currently no comparison stars available in VSP for this field; please indicate clearly the comparison stars that you use for photometry when reporting observations to AAVSO. Please retain your images and/or photometry for recalibration when comparison star magnitudes are available.

Need coordinates? The (J2000) coordinates reported for the object are RA: 14:03:05.81 , Dec: +54:16:25.4. Messier 101 is located in the constellation of Ursa Major at RA: 14h 03m 12.6s Dec: +54 20′ 57″

Charts for PTF11kly may be plotted with AAVSO VSP. You should select the DSS option when plotting, as the galaxy will not appear on standard charts. This object has been assigned the name “PTF11kly” for use with AAVSO VSP and WebObs; please use this name when reporting observations until it is conclusively classified as a supernova and a proper SN name is assigned.

Image of M101 and PTF11kly by Joseph Brimacombe.

Type Ia supernovae are the result of a binary pair of mismatched stars, the smaller, denser one feeding on material drawn off its larger companion until it can no longer take in any more material. It then explodes in a catastrophic event that outshines the brightness of its entire galaxy! Astronomers believe that Type Ia supernovae occur in pretty much the same fashion every time and thus, being visible across vast distances, have become invaluable benchmarks for measuring distance in the Universe and gauging its rate of expansion.

The fact that this supernova was spotted literally within a day of its occurrence – visibly speaking, of course, since M101 is 25 million light-years away and thus 25 million years in our past – will be extremely handy for astronomers who will have the opportunity to study the event from beginning to end and learn more about some of the less-understood processes involved in Type Ia events.

“We caught this supernova earlier than we’ve ever discovered a supernova of this type. On Tuesday, it wasn’t there. Then, on Wednesday, boom! There it was – caught within hours of the explosion. As soon as I saw the discovery image I knew we were onto something big.”

– Andy Howell, staff scientist at Las Cumbres Observatory Global Telescope

It’s a big Universe and there are a lot of stars and therefore a lot of supernovae, but getting a chance to study one occurring so recently in a galaxy so relatively close to our own is something that is getting many astronomers very excited.

So, get those CCD camera out and best of luck!

Keep up with the latest news on PTF11kly on the rochesterastronomy.org site, and check out Phil Plait’s informative article on his BadAstronomy blog. Also read the press release from the University of California here.

Tammy Plotner also contributed to this article.

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Jason Major is a graphic designer, photo enthusiast and space blogger. Visit his website Lights in the Dark and follow him on Twitter @JPMajor and on Facebook for more astronomy news and images!

Hubble Finds “Oddball” Stars in Milky Way Hub

Astronomers using the Hubble Space Telescope to peer deep into the central bulge of our galaxy have found a population of rare and unusual stars. Dubbed “blue stragglers”, these stars seem to defy the aging process, appearing to be much younger than they should be considering where they are located. Previously known to exist within ancient globular clusters, blue stragglers have never been seen inside our galaxy’s core – until now.

The stars were discovered following a seven-day survey in 2006 called SWEEPS – the Sagittarius Window Eclipsing Extrasolar Planet Search – that used Hubble to search a section of the central portion of our Milky Way galaxy, looking for the presence of Jupiter-sized planets transiting their host stars. During the search, which examined 180,000 stars, Hubble spotted 42 blue stragglers.

Of the 42 it’s estimated that 18 to 37 of them are genuine.

What makes blue stragglers such an unusual find? For one thing, stars in the galactic hub should appear much older and cooler… aging Sun-like stars and old red dwarfs. Scientists believe that the central bulge of the Milky Way stopped making new stars billions of years ago. So what’s with these hot, blue, youthful-looking “oddballs”? The answer may lie in their formation.

Artist's concept of a blue straggler pair. NASA, ESA, and G. Bacon (STScI)

A blue straggler may start out as a smaller member of a binary pair of stars. Over time the larger star ages and gets even bigger, feeding material onto the smaller one. This fuels fusion in the smaller star which then grows hotter, making it shine brighter and bluer – thus appearing similar to a young star.

However they were formed, just finding the blue stragglers was no simple task. The stars’ orbits around the galactic core had to be determined through a confusing mix of foreground stars within a very small observation area. The region of the sky Hubble studied was no larger than the width of a fingernail held at arm’s length! Still, within that small area Hubble could see over 250,000 stars. Incredible.

“Only the superb image quality and stability of Hubble allowed us to make this measurement in such a crowded field.”

– Lead author Will Clarkson, Indiana University in Bloomington and the University of California in Los Angeles

The discovery of these rare stars will help astronomers better understand star formation in the Milky Way’s hub and thus the evolution of our galaxy as a whole.

Read more on the Hubble News Center.

Image credit: NASAESA, W. Clarkson (Indiana University and UCLA), and K. Sahu (STScI)

Southern Cross Constellation

Southern Cross Constellation
Herschel's look at the Southern Cross. Credits: ESA and the PACS consortium

[/caption]For the lucky residents of the Southern Hemisphere, or those fortunate enough to enjoy a vacation in Hawaii or Cancun, there’s a stellar delight that few Northerners know about. It’s called the Southern Cross, a small but beautiful constellation located in the southern sky, very close to the neighboring constellation of Centaurus. Originally known by the Latin name Crux, which is due to its cross shape, this constellation is one of the easiest to identify in the night sky. For centuries, it has served as a navigational beacon for sailors, an important symbol to the Egyptians, and played an important role in the spiritual beliefs of the Aborigines and many other cultures in the Southern Hemisphere.

The first recorded example of Crux’s discovery was around 1000 BC during the time of the Ancient Greeks. At the latitude of Athens, Crux was clearly visible, though low in the night sky. At the time, the Greeks identified it as being part of the constellation Centaurus. However, the precession of the equinoxes gradually lowered its stars below the European horizon, and they were eventually forgotten by the inhabitants of northern latitudes. Crux fell into anonymity for northerners until the Age of Discovery (from the early 15th to early 17th centuries) when it was rediscovered by Europeans. The first to do so were the Portuguese, who mapped it for navigation uses while rounding the southern tip of Africa. During this time, Crux was also separated from Centaurus, though it is not altogether clear who was responsible. Some attribute it to the French astronomer Augustin Royer who did it in 1679 while others believe it was Dutch astronomer PetrusPlancius who did the deed in 1613. Regardless, it is believed to have taken place in the 17th century, placing it within the context of European expansion and the revolution that was taking place in the sciences at the time.

In terms of cultural significance, the Crux, like all constellations, played an important role in the belief system of many cultures. In the ancient mountaintop village of Machu Picchu, a stone engraving exists which depicts the constellation. In addition, in Quechua (the language of the Incas) Crux is known as “Chakana”, which literally means “stair”, and holds deep symbolic value in Incan mysticism (the cross represented the three tiers of the world: the underworld, world of the living, and the heavens). To the Aborigines and the Maori, Crux is representative of animist spirits who play a central role in their ancestral beliefs. To the ancient Egyptians, Crux was the place where the Sun Goddess Horus was crucified, and marked the passage of the winter season. The Southern Cross is also featured prominently on the flags of several southern nations, including Australia, Brazil, New Zealand, Papua New Guinea, and Samoa.

We have written many articles about the Southern Cross constellation for Universe Today. Here’s an article about Crux, and here’s an article about constellations.

If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.

We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?

References:
http://en.wikipedia.org/wiki/Crux
http://en.wikipedia.org/wiki/Age_of_Discovery
http://library.thinkquest.org/C005462/scross.html
http://www.windows2universe.org/the_universe/crux.html
http://www.ancientworlds.net/aw/Article/941062

Twinkle Twinkle Little Missing Stars, How I Wonder Where you are?

Why is Our Galaxy Called the Milky Way
Why is Our Galaxy Called the Milky Way

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‘Twinkle twinkle little star, how I wonder what you are?’ This nursery rhyme is one of the best loved around the World. For astronomers though, stars can be a bit more of a nightmare, not only in understanding their complex evolutionary processes but also and perhaps more simply, figuring out how many there are. Until now there has been a gross mismatch between the number of stars that are found within our galaxy, the Milky Way and the amount that astronomer think should be there. In short, where are the missing stars?

The Milky Way is joined by about 30 other galaxies that make up our local group of galaxies, including the Andromeda Galaxy and according to current theories there should be about 100 billion stars in each. The calculations are based on the rate of star birth in the Milky Way, about 10 new stars per year. But according to Dr Jan Pflamm-Altenburg of the Argelander Institute for Astronomy at the University of Bonn “Actually, it would give many more stars than we actually see” and therein lies the problem.

The recent study by Dr Pflamm-Altenburg and Dr. Carsten Weidner of the Scottish St. Andrews University suggests that perhaps the estimated rate of star birth being used to calculate the number of stars could simply be too high. With galaxies in our Local Group its relatively easy to just count the number of new stars that can be seen but for more distant galaxies, they are too far away for individual stars to be seen.

By studying the nearby galaxies, Pflamm-Altenburg and Weidner discovered that for every 300 young small stars, there seems to be one large massive new star and fortunately this seems to be universal. Due to the unique nature of the massive young stars, they leave a tell tail sign in the light of distant galaxies so even though they cannot be individually identified they can still be detected and the strength of the signal determines the number of massive stars. Multiply by the number of massive stars by this ratio of 300 and the actual rate of stellar birth can be calculated.

It seems though that this rate has varied over the history of the Universe and dependent on the amount of ‘space’ available in the vicinity of the star formation. If there is a baby boom in star formation then a higher number of heavies seem to form in a theory called ‘stellar crowding’. When stars form, they form as clusters rather than individual stars but it seems that the overall mass of the group is the same, regardless of how many star embryo’s there really are. When star birth is at a high rate, space can be limited so larger more massive stars tend to form compared to smaller stars.

Massive galaxies like this where star birth is booming are called “ultra-compact dwarf galaxies” (UCDs). Sometimes its possible in these galaxies that young stars can even fuse together to form larger stars so the large to small ratio can be around 1:50 instead of 1:300. This means we have been using the wrong figure and estimating far too high.

Using this new found figure, Pflamm-Altenburg and Weidner have recalculated the number of stars that ‘should’ be in a galaxy and compared to those that we can see and rather pleasantly, the numbers match! It seems that the conundrum of the missing stars that has been perplexing astronomers for decades has finally been resolved.

Source: University of Bonn

Dissolving Star Systems Create Mess in Orion

For young stars, stellar outflows are the rule. T Tauri stars and other young stars eject matter in generally collimated jets. However, a region in Orion’s giant molecular cloud known as the Becklin-Neugebauer/Kleinmann-Low (BN/KL) region, appears to have a clumpy, scattered set of outflows with “finger-like” projections in numerous directions. A new study, led by Luis Zapata at the National Autonomous University of Mexico, explores this odd region.

To conduct their study, the team used the Submillimeter Array to trace the motion of carbon monoxide gas in the area. Flying away from this region are three massive and young stars. Tracing their paths back, astronomers had previously determined that these stars likely had a common origin as members of a multiple system that for some reason, broke apart an estimated 500 years ago. Likely related to this, the new study discovered several new fingers of gas moving away as well with velocities that implied they came from the same point of origin near the same time. But what could send stars and gas hurtling outwards?

Nearby, the team also discovered a “hot core” of material as well as a “bubble” of empty space near the point of origin of the event. To explain the combination of these three events, the team proposes that an close interaction between the three stars (or perhaps more) occurred. At that time, the interaction tore apart any potential binary system throwing the stars outwards.

Since the stars are young and still embedded in a nebula, the team suggests it was likely they also contained circumstellar disks that had not yet formed planets. During the interaction, the outer portions which would be least strongly bound, were thrown outwards, creating the finger-like projections. Material that was bound more tightly but just enough to be torn off, “would find itself with an excess of kinetic energy, and will start to expand” creating the apparent bubble. If that bubble, expanding supersonically for the local medium, encountered a region that was overly dense, it would collide, heating the region and potentially forming the hot core.

This new discovery presents a potential first for the discovery of one or more destroyed circumstellar disks. Such findings could help impose new constraints on how planetary systems form since most stars form in open clusters and associations in which such interactions may be commonplace. Yet, the very fact that such destroyed systems have never been found until now imply that interactions sufficiently close to cause such disruption are rare. Regardless, such things will help astronomers form a better picture of the formation of planets.