The Canadian asteroid-hunting NEOSSAT is among the fleet of satellites launched on Feb. 25, 2013.Credit: Canadian Space Agency
Early next week, an Indian rocket will launch into space carrying seven satellites on board. Among them will be a small but mighty asteroid-hunting telescope called NEOSSat. Built by the Canadian Space Agency, it will mainly focus on the Atira class of asteroids, which are made up of space rocks within Earth’s orbit, to figure out their size and distribution. The suitcase-sized NEOSSat will orbit approximately 800 kilometers above Earth, searching for near-Earth asteroids that are difficult to spot using ground-based telescopes.
Here’s a full rundown of what’s soaring to space on Monday (Feb. 25), if all goes to plan. Check out the launch from India at this link; it’s supposed to go into space around 7:25 a.m. Eastern (12:25 p.m. UTC).
– NEOSSat (Canada). Short for Near-Earth Object Surveillance Satellite, the satellite is actually split into two different missions. For half the time, it will be keeping a sharp eye out for asteroids that may swing by Earth at some point. The telescope will spend its other science mission watching satellites and space debris in orbit, to better track their movements.
“NEOSSat will discover many asteroids much faster than can be done from the ground alone,” said Alan Hildebrand of the University of Calgary. “Its most exciting result, however, will probably be discovering new targets for exploration by both manned and unmanned space missions.”
– SARAL (India/France). This is by far the largest satellite of the fleet; the rest of the mini sats listed below are hitching a ride to share launch costs. The satellite is supposed to take altimeter measurements of water and ice to watch the movement of waves and to add more data into climate change databases, among other objectives.
– CanX-3BRITE (Canada). The BRIght Target Explorer is billed as the smallest astronomical telescope, at just 8 inches (20 centimeters) across. Unlike bigger observatories that focus on very faint objects, BRITE will — as the name suggests — watch over brighter stars that we commonly use on Earth to connect the dots in constellations. Oddly enough, despite their prominence in our sky, these brighter stars are poorly studied, astronomers said.
– Sapphire (Canada). A military mission, this satellite will keep track of objects orbiting between 3,800 and 25,000 miles (6,000 and 40,000 kilometers) from Earth. The Canadians will share this information with their close military ally, the Americans.
– TUGSat-1 BRITE (Austria). This will be the first Austrian satellite. Like CanX-3, it will investigate bright stars by watching the changes in brightness using a technique called photometry (measuring visible light.) The satellite is equipped with a high-resolution CCD imager to take pictures.
– AAUSat 3 (Denmark). This satellite will test the capabilities of automatic identification of ships (AIS) technology, following the beacons that ships are required to send out with information about their cargo and destination. Most of the testing will focus on the water around Greenland.
– STRaND-1 (United Kingdom). This satellite is literally a screamer, as it will be broadcasting the sound of human screams into space to see if anyone nearby can hear them. (This is to test the oft-repeated phrase that in space, nobody can hear you scream.) Besides monitoring shrieks, the satellite makers will be testing how well the satellite is controlled by a smartphone. The acronym is short for Surrey Training, Research and Nanosatellite Demonstrator.
A young star is spotted firing jets of material out into space (ESA/Hubble & NASA. Acknowledgement: Gilles Chapdelaine)
Like very young humans, very young stars also tend to make a big mess out of the stuff around them — except in the case of stars it’s not crayon on the walls and Legos on the floor (ouch!) but rather huge blasts of superheated material that are launched from their poles far out into space.
The image above, acquired by the Hubble Space Telescope, shows one of these young stars caught in the act.
HL Tau is a relatively newborn star, formed “only” within the past several hundred thousand years. During that time it has scooped up vast amounts of gas and dust from the area around itself, forming a disc of hot, accelerated material that surrounds it. While most of this material eventually falls into the star, increasing its mass, some of it gets caught up in the star’s complex, rotating magnetic fields and is thrown out into space as high-speed jets.
As these jets plow thorough surrounding interstellar space they ram into nearby clouds of molecular gas, ionizing the material within them and causing them to glow brightly. These “shocks” are known as Herbig-Haro objects, after researchers George Herbig and Guillermo Haro who each discovered them independently in the early 1950s.
Detail of HH 151’s jet
In this Hubble image HH 151 is visible as a multiple-lobed cone of material fired away from HL Tau, with the leftover glows from previous outbursts dimly illuminating the rest of the scene.
The material within these jets can reach speeds of several hundred to a thousand kilometers a second. They can last anywhere from a few years to a few thousand years.
HH 151 is embedded within the larger star-forming region LDN 1551, located about 450 light-years away in the constellation Taurus. LDN 1551 is a stellar nursery full of dust, dark nebulae, newborn stars… and Herbig-Haro objects like HH 151.
(Hey, if baby stars are going to make a mess at least they can do it in the nursery.)
These unusual shapes on Mars surface are actually cones and inflated lava flows, Credit: NASA/JPL/University of Arizona.
Although these strange features on Mars look a bit like hieroglyphics or geoglyphs such as the mysterious Nazca lines on Earth, they are completely natural features, ones that are found on Earth too.
Called ‘rootless cones,’ they form on lava flows that interact with subsurface water or ice. Their formation comes from an explosive interaction of lava with ground ice or water contained within the regolith beneath the flow. Vaporization of the water or ice when the hot lava comes in contact causes an explosive expansion of the water vapor, causing the lava to shoot upward, creating a rootless cone.
Dr. Alfred McEwen, HiRISE Principal Investigator, described the ancient lava flow as ‘inflated.’ “Lava inflation is a process where liquid is injected beneath the solid (thickening) crust and raises the whole surface, often raising it higher than the topography that controlled the initial lava emplacement,” he wrote on the HiRISE website.
The scene above is located in Amazonis Planitia on Mars, a vast region covered by flood lava. McEwen said if this image were in color, we’e see the surface is coated by a thin layer of reddish dust, which avalanches down steep slopes to make dark streaks.
Similar features are found in Iceland, where flowing lava encountered water-saturated substrates.
Rootless cones (a) on Mars and (b) in Iceland. The scale of the Martian and terrestrial cones are comparable. Credit: University of Hawaii/Mars Orbiter Camera/MSSS.
Just how big are these strange features on Mars and how old are they? “The cones are on the order of a hundred meters across and ten meters high,” Colin Dundas from the US Geological Survey told Universe Today. “The age of these specific cones isn’t known. They are on a mid- to late-Amazonian geologic unit, which means that they are young by Martian standards but could be as much as a few hundred million to over a billion years old.”
If subsurface water or ice was part of their formation, could it still be there, underground?
“The water or ice that led to the formation of these cones was likely within a few meters (or less) of the surface, and so it’s probably not there anymore,” Dundas said. “At this low latitude (22 degrees north), shallow ground ice is currently unstable, and should sublimate on timescales much less than the likely age of the cones.”
Dundas added that since ice stability varies as the obliquity changes, it’s even possible that ice has come and gone repeatedly since the lava erupted.
See more views of this region on Mars on the HiRISE website
Comet 2011 L4 PanSTARRS imaged from Argentina by Luis Argerich on February 13th, 2013. (Credit: Luis Argerich - Nightscape photography. Used with Permission).
Great ready. After much anticipation, we could have the first naked eye comet of 2013 for northern hemisphere observers in early March. As discussed earlier this week on Universe Today, 2013 may well be the Year of the Comet, with two bright comets currently putting on a show in the southern hemisphere and comet C/2012 S1 ISON set to perform the closing cometary act of 2013. But while comet C/2012 F6 Lemmon won’t be visible for northern hemisphere residents until April, Comet C/2011 L4 PanSTARRS (which we’ll refer to simply as “Comet PanSTARRS” from here on out) may well become a fine early evening object in the first two weeks of March.
That is, if it performs. Comets are often like cats. Though we love posting pictures of them on the Internet, they often stubbornly refuse to perform up to our expectations. Some comets have been solid performers, like Hale-Bopp in 1997. Others are often promoted to great fanfare like Comet Kohoutek in 1973-74, only to fizzle and fade into notoriety. Continue reading “Comet PANSTARRS: How to See it in March 2013”
Canadian Space Agency astronaut Chris Hadfield uses a camera to photograph the topography of a point on Earth from a window in the Cupola of the International Space Station. Credit: NASA
It’s not often that people on Earth get to hangout with astronauts in space, but today NASA held the first-ever Google Plus Hangout from the International Space Station. It was a live event, and if you aren’t familiar yet with G+ Hangouts (you really should be by now!) they allow people to chat face-to-face while thousands more can tune in to watch the conversation live on Google+ or YouTube. NASA took questions live from Twitter and G+, but they also took questions submitted previously via You Tube, and we were proud to see that Fraser’s question that he submitted via You Tube was included in the Hangout! You can see the question and astronaut Chris Hadfield’s reply at about 42:00 in the video above.
Fraser asked how being on the ISS and the special conditions it has (microgravity, harsh exposures, distant objects, weird lighting ) affect photography — and as you know we feature A LOT of ISS photography here on UT.
Hadfield said photography from orbit is quite complex, but the “weird” part about it is that space is so incredibly black and dark. The difficulty is having the dark background of space against the brightness of Earth and trying to balanace that. The advantage is being able to use the really big lenses and have them be weightless — no tripod needed!.
“The best part is,” Hadfield added, “even though we are not photographers by trade, we have really good professional photographers as trainers and a vantage point that is absolutely unparallelled.”
The nebulas in Battlestar: Blood and Chrome make for nice scenery, but they're a lot brighter than the truth, according to a Harvard astronomer. Credit: Battlestar Galactica: Blood and Chrome/Machinima (screencap)
In the Battlestar: Galactica universe, nebulas are a nifty spot to hide from the Cylons that are plotting to kill humanity. There’s just one problem with the hypothesis, though — these diffuse areas of gas in our universe are actually very faint, even if you get close up. Probably too faint for a hiding spot.
Prequel Battlestar Galactica: Blood and Chrome (released on DVD this week) shows the young William Adama flying around the universe with pretty nebulas in the background. That’s not anywhere near the truth, Harvard astronomer Peter Williams told Universe Today.
In an e-mail, Williams explained that bright nebulas are a common misperception seen in Star Wars, Star Trek and a host of other sci-fi series.
The big issue is that nebulae are just too faint for the human eye to see. And while it’s tempting to think that they’d look brighter from up close, in fact this isn’t actually true — they actually look just as bright from any distance! This is a law of optics, known in the jargon as the “conservation of surface brightness”. The key is that there are two competing effects in play. Imagine that you can see a nebula that’s, say, the size of the full moon.
Yes, if you get closer, your eye will receive more total power from the nebula. But the nebula will also look bigger, so that energy will be spread out over a larger visual area (technically: “solid angle”). The physics tells you that the power per solid angle in fact stays exactly the same, and this quantity is precisely the “brightness” of an object. So if nebula are too faint for to see from Earth with the naked eye — and they are — getting up close and personal doesn’t help any.
The opening sequence in Battlestar: Galactica shows the ships hiding in a bright nebula. Credit: Battlestar Galactica/SciFi (screencap)
Further, Williams, explains, the bright colors we’re used to seeing in Hubble Space Telescope images are just an approximation of what a nebula actually looks like.
Reproduced images of nebulae don’t portray their colors accurately. As you may know, some astronomical images use “false color” to represent wavelengths of light that humans can’t even see. This does happen with images of nebulae, but nebulae really are colorful, and many nebula images try to reproduce those colors faithfully. No current reproduction, however, can be truly accurate.
The Crab Nebula. Image credit: NASA/Hubble Space Telescope.
The problem is that the colorful nebular emission comes from reactions that produce light at a few, specific wavelengths; meanwhile, our inks and pixels emit over much broader wavelength ranges. We can mix these broad ranges in ways that approximate the narrow ones, but the results aren’t quite the same.
For an entertaining look at the science of nebulas, Williams recommends this entertaining video by astronomer Phil Plait, a long-time friend of Universe Today who is best known for his Bad Astronomy blog (now at Slate). “If you were inside [the nebula and looked down], you wouldn’t see it,” Plait says in this 2008 clip.
First Curiosity Drilling Sample in the Scoop. This image shows the first sample of powdered rock extracted by the rover's drill after transfer from the drill to the rover's scoop. The sample will now be sieved and portions delivered to the Chemistry and Mineralogy instrument and the Sample Analysis at Mars instrument. The scoop is 1.8 inches (4.5 centimeters) wide. The image was taken by Curiosity's Mastcam 34 camera on Feb. 20, or Sol 193.The image has been white-balanced to show what the sample would look like if it were on Earth. Credit: NASA/JPL-Caltech/MSSS
Newly received images from the surface of Mars confirm that NASA’sCuriosity rover successfully extracted the 1st ever samples collected by drilling down inside a rock on another planet and transferred the pulverized alien powder to the robots processing scoop, thrilled mission scientists announced just hours after seeing visual corroboration.
Collecting the 1st particles bored from the interior of a rock on a planet beyond Earth marks a historic feat in humankind’s exploration of the cosmos – and is crucial for achieving Curiosity’s goal to determine whether Mars ever could have supported microbial life, past or present.
The essential next step is to feed carefully sieved portions of the precious gray colored material into the high powered duo of miniaturized analytical chemistry labs (CheMin & SAM) inside the rover, for thorough analysis and scrutiny of their mineral content and to search for signatures of organic molecules – the building blocks of life as we know it.
Curiosity is drilling into ancient bedrock and hunting for clues to the planet’s habitability over the eons and that preserve the historical record – perhaps including organics.
The rover team believes that this work area inside Gale Crater called Yellowknife Bay, experienced repeated percolation of flowing liquid water long ago when Mars was warmer and wetter – and therefore was potentially more hospitable to the possible evolution of life. See our Yellowknife Bay worksite and drill hole photo mosaics below by Ken Kremer & Marco Di Lorenzo, created from rover raw images.
Curiosity accomplished Historic 1st drilling into Martian rock at John Klein outcrop on Feb 8, 2013 (Sol 182), shown in this context mosaic view of the Yellowknife Bay basin taken on Jan. 26 (Sol 169) where the robot is currently working. The robotic arm is pressing down on the surface at John Klein outcrop of veined hydrated minerals – dramatically back dropped with her ultimate destination; Mount Sharp. Credit: NASA/JPL-Caltech/Ken Kremer (kenkremer.com)/Marco Di Lorenzo
“We collected about a tablespoon of powder, which meets our expectations and is a great result,” said JPL’s Scott McCloskey, drill systems engineer for Curiosity, at a NASA media briefing on Feb. 20. “We are all very happy and relieved that the drilling was a complete success.”
The gray colored tailings from the rocky interior offer a startlingly fresh sight of Mars compared to the red-orangey veneer of rusty, oxidized dust we are so accustomed to seeing globally across what we humans have referred to for centuries as the “Red Planet”.
“For the first time we are examining ancient rocks that have not been exposed to the Martian surface environment, and weathering, and preserve the environment in which they formed,” said Joel Hurowitz, Curiosity sampling system scientist of JPL.
This is a key point because subsequent oxidation reactions can destroy organic molecules and thereby potential signs of habitability and life.
“The tailings are gray. All things being equal it’s better to have a gray color than red because oxidation is something that can destroy organic compounds,” said John Grotzinger, the Curiosity mission’s chief scientist of the California Institute of Technology.
On Feb. 8, 2013 (mission Sol 182), Curiosity used the rotary-percussion drill mounted on the tool turret at the end of the 7 foot (2.1 meter) long robotic arm to bore a circular hole about 0.63 inch (16 mm) wide and about 2.5 inches (64 mm) deep into a red colored slab of flat, fine-grained, veiny sedimentary bedrock named “John Klein” that formed in water.
“Curiosity’s first drill hole at the John Klein site is a historic moment for the MSL mission, JPL, NASA and the United States. This is the first time any robot, fixed or mobile, has drilled into a rock to collect a sample on Mars,” said Louise Jandura, Curiosity’s chief engineer for the sampling system.
“In fact, this is the first time any rover has drilled into a rock to collect a sample anywhere but on Earth. In the five decade history of the space age this is indeed a rare event.”
“The rock drilling capability is a significant advancement. It allows us to go beyond the surface layer of the rock, unlocking a time capsule of evidence about the state of Mars going back 3 or 4 Billion years.”
“Using our roving geologist Curiosity, the scientists can choose the rock, get inside the rock and deliver the powdered sample to instruments on the rover for analysis.”
“We couldn’t all be happier as Curiosity drilled her first hole on Mars,” said Jandura.
Over the next few days, the powdery gray scoop material will be shaken and moved through Curiosity’s sample processing device known as CHIMRA, or Collection and Handling for In-Situ Martian Rock Analysis and sieved through ultra fine screens that filter out particles larger than 150 microns (0.006 inch) across – about the width of a human strand of hair.
Figure shows the location of CHIMRA on the turret of NASA’s Curiosity rover, together with a cutaway view of the device. The CHIMRA, short for Collection and Handling for In-situ Martian Rock Analysis, processes samples from the rover’s scoop or drill and delivers them to science instruments. Credit: NASA/JPL-Caltech
Drilling goes to the heart of the mission. It is absolutely indispensable for collecting and conveying pristine portions of Martian rocks and soil to a trio of inlet ports on top of the rover deck leading into the Chemistry and Mineralogy (CheMin) instrument and Sample Analysis at Mars (SAM) instrument .
The sieving process is designed to prevent clogging downstream into the chemistry labs.
The pair of state-of-the-art instruments will then test the gray rocky powder for a variety of inorganic minerals as well as both simple and complex organic molecules.
Samples will be dropped off first to CheMin and then SAM over the next few days. Results are expected soon.
The data so far indicate the drilled rock is either siltstone or mudstone with a basaltic bulk composition, said Hurowitz. The CheMin and SAM testing will be revealing.
The high powered drill was the last of Curiosity 10 instruments still to be checked out and put into full operation and completes the robots commissioning phase.
“This is a real big turning point for us as we had a passing of the key for the rover [from the engineering team] to the science team,” said Grotzinger.
Curiosity has discovered that Yellowknife Bay is loaded with hydrated mineral veins of calcium sulfate that precipitated from interaction with aqueous environments.
I asked how was the drill target hole selected?
“We wanted to be well centered in a large plate of bedrock where we knew we could place the drill into a stable location on an interesting rock,” Hurowitz told Universe Today.
“The drill did not specifically target the veins or nodular features visible in this rock. But these rocks are so shot through with these features that it’s hard to imagine that we would have been missed them somewhere along the travel of the drill.”
“We will find out what’s in the material once we get the materials analyzed by SAM and CheMin.
“We will consider additional drill targets if we think we missed a component of the rock.”
“We believe the white vein material is calcium sulfate based on data from ChemCam and APXS but we don’t yet know the hydration state.” Hurowitz told me.
Regarding the prospects for conducting additional sample drilling and soil scooping at Yellowknife Bay, Grotzinger told me, “We have to take it one step at a time.”
“We have to see what we find in the first sample. We are discovery driven and that will determine what we do next here,” Grotzinger said. “We have no quotas.”
The long term mission goal remains to drive to the lower reaches of Mount Sharp some 6 miles away and look for habitable environments in the sedimentary layers.
Curiosity executed a flawless and unprecedented nail-biting, pinpoint touchdown on Aug. 5, 2012 to begin her 2 year long primary mission inside Gale Crater. So far she has snapped over 45,000 images, traveled nearly 0.5 miles, conducted 25 analysis with the APXS spectrometer and fired over 12,000 laser shots with the ChemCam instrument.
Image collage show Curiosty’s first bore hole drilled on Feb. 8, 2013 (Sol 182). Credit: NASA/JPL-Caltech/MSSS/Marco Di Lorenzo/KenKremer (kenkremer.com)Curiosity’s First Sample Drilling hole is shown at the center of this image in a rock called “John Klein” on Feb. 8, 2013, or Sol 182 operations. The image was obtained by Curiosity’s Mars Hand Lens Imager (MAHLI). The sample-collection hole is 0.63 inch (1.6 centimeters) in diameter and 2.5 inches (6.4 centimeters) deep. The “mini drill” test hole near it is the same diameter, with a depth of 0.8 inch (2 centimeters). Credit: NASA/JPL-Caltech/MSSS
NASA scientist, Dr. David S. McKay has passed away. He may be best known for his paper about a Martian meteorite, ALH84001, which presented an argument that it contained evidence for life on Mars. McKay had been battling serious cardiac health problems for some time, according to an announcement from Johnson Space Center, and he died peacefully in his sleep in the early morning hours of February 20, 2013.
McKay had been the Chief Scientist For Astrobiology at NASA and searched for evidence of past life on Mars using Martian meteorites and terrestrial analogs. He performed original research on lunar soils, lunar pyroclastics, and space weathering.
McKay joined NASA in June of 1965 and participated extensively in astronaut training up until the Apollo 11 mission. He was named a Principal Investigator to study the first returned lunar samples and continued as a lunar sample PI for the next 20 years. He started many of the laboratories for the Lunar Sample Facility at Johnson Space Center and managed the NASA space resources program out of JSC during much of the 1980s.
McKay published more than 200 peer-reviewed papers on lunar samples, space resource utilization, cosmic dust, meteorites, astrobiology and Mars topics, and NASA said his “body of work includes many contributions to our understanding of the development and evolution of the lunar regolith and space weathering processes.”
Most notably, he was the lead author on the 1996 paper in Science on the ALH84001 Martian meteorite that was found in Antarctica and argued that it contains evidence for life on Mars.
“Although that claim was highly controversial, there can be no question that the appearance of that paper sparked significant changes in martian and planetary science, shaped the direction of the Mars Exploration Program to the present day, and prompted the establishment of the NASA Astrobiology Institute,” said the JSC announcement. “Whether one accepts their arguments or not, it has led, directly or indirectly, to investigations seeking and finding signs of life in the most extreme environments. History will judge the value of that rather serendipitous outcome, but it seems clear that its significance is, and will remain, great.”
Sunspot 1678 in Hydrogen alpha light, taken on February 19, 2013. Credit and copyright: Paul Andrew.
On February 19 and 20, 2013, scientists watched a giant sunspot form in under 48 hours. It has grown to over six Earth diameters. This image by astrophotographer Paul Andrew shows a detailed, close-up view of this sunspot group, named AR 1678, imaged with a hydrogen alpha filter.
NASA said the spot quickly evolved into what’s called a delta region, which has a magnetic field that harbors energy for strong solar flares. NOAA forecasters estimate a 45% chance of M-flares and a 15% chance of X-flares during the next day.
Below is an image from the Solar Dynamics Observatory of this region on the Sun:
This image of AR 1678 combines images from two instruments on NASA’s Solar Dynamics Observatory (SDO): the Helioseismic and Magnetic Imager (HMI), which takes pictures in visible light that show sunspots and the Advanced Imaging Assembly (AIA), which took an image in the 304 Angstrom wavelength showing the lower atmosphere of the sun, which is colorized in red. Credit: NASA/SDO/AIA/HMI/Goddard Space Flight Center
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This illustration shows the disk of our Milky Way galaxy, surrounded by a faint, extended halo of old stars. Astronomers using the Hubble Space Telescope to observe the nearby Andromeda galaxy serendipitously identified a dozen foreground stars in the Milky Way halo. They measured the first sideways motions (represented by the arrows) for such distant halo stars. The motions indicate the possible presence of a shell in the halo, which may have formed from the accretion of a dwarf galaxy. This observation supports the view that the Milky Way has undergone continuing growth and evolution over its lifetime by consuming smaller galaxies.
Illustration Credit: NASA, ESA, and A. Feild (STScI)
Like tantalizing tidbits stored in the vast recesses of one’s refrigerator, astronomers using NASA’s Hubble Space Telescope have evidence of a shell of stars left over from one of the Milky Way’s meals. In a study which will appear in an upcoming issue of the Astrophysical Journal researchers have revealed a group of stars moving sideways – a motion which points to the fact our galaxy may have consumed another during its evolution.
“Hubble’s unique capabilities are allowing astronomers to uncover clues to the galaxy’s remote past. The more distant regions of the galaxy have evolved more slowly than the inner sections. Objects in the outer regions still bear the signatures of events that happened long ago,” said Roeland van der Marel of the Space Telescope Science Institute (STScI) in Baltimore, Maryland.
As curious as this shell of stars is, they offer even more information by revealing a chance to study the mysterious hidden mass of Milky Way – dark matter. With more than a hundred billion galaxies spread over the Universe, what better place to get a closer look than right here at home? The team of astronomers led by Alis Deason of the University of California, Santa Cruz, and van der Marel studied the outer halo, a region roughly 80,000 light years from our galaxy’s center, and identified 13 stars which may have come to light at the very beginning of the Milky Way’s formation.
What’s so special about this group of geriatric suns? In this case, it’s their movement. Instead of cruising along in a radial orbit, these elderly stars show tangential motion – an unexpected observation. Normally halo stars travel towards the galactic center, only to return outwards again. What could cause this double handful of stars to move differently? The research team speculates there could be an “over-density” of stars at the 80,000 light year mark.
As intriguing as these stars are, this strange shell was discovered somewhat by accident. Deason and her team winnowed out the outer halo stars from a seven year study of archival images taken by the Hubble telescope of the Andromeda galaxy. While looking some twenty times further away at our neighboring galaxy’s stars, these strange moving stars came to light as foreground objects… objects that “cluttered” the images. While these halo stars were bad for that particular study, they were very good for Deason and the team. It gave them the chance to take a really close look at the motion of the Milky Way’s halo stars.
However, seeing these stars wasn’t easy. Thanks to Hubble’s incredible resolution and light gathering power, each image contained more than 100,000 individual stars. “We had to somehow find those few stars that actually belonged to the Milky Way halo,” van der Marel said. “It was like finding needles in a haystack.”
So how did the astronomers separate the shell stars from those that belonged to the outer fringes of the Andromeda? The initial observations picked the stars out based on their color, brightness and sideways motion. Thanks to parallax, our halo stars seem to move far faster simply because they are closer. Through the work of team member Tony Sohn of STSci, these revolutionary stars were identified and measured. Their tangential motion was observed and recorded with five percent precision. Not a speedy process when you consider these shell stars only move across the sky at a rate of about one milliarcsecond per year!
“Measurements of this accuracy are enabled by a combination of Hubble’s sharp view, the many years’ worth of observations, and the telescope’s stability. Hubble is located in the space environment, and it’s free of gravity, wind, atmosphere, and seismic perturbations,” van der Marel said.
What makes the team so confident in their findings? As we know, stars at home in our galaxy’s inner halo have highly radial orbits. When a comparison was made between the sideways motion of the outer halo stars with the inner motions, the researchers found equality. According to computer simulations of galaxy formation, outer stars should continue to have radial motion as they move outward into the halo, but these new findings prove opposite. What could cause it? A natural explanation would be an accretion event involving a satellite galaxy.
To further substantiate their findings, the team compared their results with data taken by the Sloan Digital Sky Survey involving halo stars. It was a eureka moment. The observations taken by the SDSS revealed a higher density of stars at roughly the same distance as the shell-shocked travelers. And the Milky Way isn’t alone. Other studies of halo stars involved in both the Triangulum and Andromeda show a large number of halo stars existing to a certain point – only to drop off. Deason realized this wasn’t just a weird coincidence. “What may be happening is that the stars are moving quite slowly because they are at the apocenter, the farthest point in their orbit about the hub of our Milky Way,” Deason explained. “The slowdown creates a pileup of stars as they loop around in their path and travel back towards the galaxy. So their in and out or radial motion decreases compared with their sideways or tangential motion.”
As exciting as these findings are, they aren’t news. Shell stars have been observed in the halos of other galaxies and were predicted to be part of the Milky Way. By nature, they should have been there – but they were simply to dim and too far-flung to make astronomers positive of their presence. Not any more. Now that astronomers know what to look for, they are even more anxious to dig into Hubble’s archives. “These unexpected results fuel our interest in looking for more stars to confirm that this is really happening,” Deason said. “At the moment we have quite a small sample. So we really can make it a lot more robust with getting more fields with Hubble.” The Andromeda observations only cover a very small “keyhole view” of the sky.
So what’s next? Now the team can paint an even more fine portrait of the Milky Way’s evolutionary history. By understanding the motions and orbits of the “shell” of stars in the halo, they might even by able to give us a accurate mass. “Until now, what we have been missing is the stars’ tangential motion, which is a key component. The tangential motion will allow us to better measure the total mass distribution of the galaxy, which is dominated by dark matter. By studying the mass distribution, we can see whether it follows the same distribution as predicted in theories of structure formation,” Deason said.
