A Herschel PACS (Photodetector Array Camera and Spectrometer) image of the center of the Milky Way. The dark line of cool gas is thought to be an elliptical ring surrounding the galactic center. The galaxy’s central supermassive black hole Sagittarius A* (Sgr A*) is labelled. The differential velocity of clouds in the ring may result from interaction with Sgr A*. Credit: ESA/Herschel/NASA/Molinari et al.
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The Herschel Space Observatory scanned the center of the galaxy in far-infrared and found a cool (in all senses of the word) twisting ring of rapidly orbiting gas clouds. The ring is estimated to have dimensions of 100 parsecs by 60 parsecs (or 326 by 196 light years) – with a composite mass of 30 million solar masses.
The ring is proposed to oscillate twice about the galactic mid-plane for each orbit it makes of the galactic center – giving it the apparent shape of an infinite symbol when viewed from the side.
The research team speculate that the ring may be conforming to the shape of a standing wave – perhaps caused by the spin of the central galactic bulge and the lateral movement of gas across the galaxy’s large central bar. The researchers suggest that the combination of these forces may produce some kind of gravitational ‘sloshing’ effect, which would account for the unusual movement of the ring.
The estimated shape of the 100 by 60 parsec ring. Note the oscillating shape from a lateral perspective – and from above, note the ring encircles the supermassive black hole Sagittarius A*, but the black hole is not at its center. Credit: Molinari et al.
Although the ring is estimated to have an average orbital velocity of 10 to 20 kilometers a second, an area of dense cloud coming in close to the galaxy’s central supermassive black hole, Sagittarius A*, was clocked at 50 kilometers a second – perhaps due to its close proximity to Sagittarius A*.
However, the researchers also estimate that Sagittarius A* is well off-centre of the gas ring. Thus, the movement of the ring is dominated by the dynamics of the galactic bulge – rather than Sagittarius A*, which would only exert a significant gravitational influence within a few parsecs of itself.
Dense filaments of gas in the IC5146 interstellar cloud. This image was taken by ESA’s Herschel space observatory at infrared wavelengths 70, 250 and 500 microns. Stars are forming along these filaments. Credits: ESA/Herschel/SPIRE/PACS/D. Arzoumanian (CEA Saclay) for the “Gould Belt survey” Key Programme Consortium.
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Its true there is no sound in empty interstellar space, but the Herschel space observatory has observed the cosmic equivalent of sonic booms. Networks of tangled and tremendously large gaseous filaments seen within clouds of gas and dust between stars are likely to be remnants of slow shockwaves from supernovae, Herschel scientists say. And surprisingly, no matter what the length or density of these filaments are, the width is always roughly the same, about 0.3 light years across, or about 20,000 times the distance of Earth from the Sun. This consistency of the widths demands an explanation, scientists say.
And it’s possible these shockwaves could generate sound within an interstellar cloud – if something were there to hear it.
“Although the density in an interstellar cloud is lower than in a very good vacuum on Earth there are molecules in the order of 10^8 per cm^3” said Goeran Pilbratt, ESA’s Herschel mission scientist. “That should be enough for sound to propagate, apart from the fact that we do not have the instruments to measure it.”
Filaments like this have been sighted before by other infrared satellites, but they have never been seen clearly enough to have their widths measured. Herschel is seeing that the width of these filaments is nearly uniform across three nearby clouds: IC5146, Aquila, and Polaris. The Herschel team, lead by Doris Arzoumanian, Laboratoire AIM Paris-Saclay, CEA/IRFU, made observations of 90 filaments, and found all had nearly identical widths. “This is a very big surprise,” Arzoumanian said.
The network of interstellar filaments in Polaris as seen by Herschel. Credits: ESA/Herschel/SPIRE/Ph. André (CEA Saclay) for the Gould Belt survey Key Programme Consortium and A. Abergel (IAS Orsay) for the Evolution of Interstellar Dust Key Programme Consortium.
Also, newborn stars are often found in the densest parts of these filaments. One filament imaged by Herschel in the Aquila region contains a cluster of about 100 infant stars.
The Herschel team said their observations provide strong evidence for a connection between interstellar turbulence, the filaments and star formation.
“The connection between these filaments and star formation used to be unclear, but now thanks to Herschel, we can actually see stars forming like beads on strings in some of these filaments,” said Pilbratt.
Comparing the observations with computer models, the astronomers suggest that filaments are probably formed when slow shockwaves dissipate in the interstellar clouds. These shockwaves are mildly supersonic and are a result of the huge amounts of turbulent energy injected into interstellar space by exploding stars.
They travel through the dilute sea of gas found in the galaxy, compressing and sweeping it up into dense filaments as they go. As these “sonic booms” travel through the clouds, they lose energy and, where they finally dissipate, they leave these filaments of compressed material.
Interstellar clouds are usually extremely cold, about 10 degrees Kelvin above absolute zero, and this makes the speed of sound in them relatively slow at just 0.2 km/s, as opposed to 0.34 km/s in Earth’s atmosphere at sea-level.
Sound travels in waves like light or heat does, but unlike them, sound travels by making molecules vibrate. So, in order for sound to travel, there has to be something with molecules for it to travel through. On Earth, sound travels to your ears by vibrating air molecules. In deep space, the large empty areas between stars and planets, there are no molecules to vibrate.
M31, or the Andromeda Galaxy seen in a variety of wavelengths by the Herschel and XMM-Newton space observatories. Credits: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendle
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To the naked eye, the Andromeda galaxy appears as a smudge of light in the night sky. But to the combined powers of the Herschel and XMM-Newton space observatories, these new images put Andromeda in a new light! Together, the images provide some of the most detailed looks at the closest galaxy to our own. In infrared wavelengths, Herschel sees rings of star formation and XMM-Newton shows dying stars shining X-rays into space.
During Christmas 2010, the two ESA space observatories targeted Andromeda, a.k.a. M31.
Andromeda is about twice as big as the Milky Way but very similar in many ways. Both contain several hundred billion stars. Currently, Andromeda is about 2.2 million light years away from us but the gap is closing at 500,000 km/hour. The two galaxies are on a collision course! In about 3 billion years, the two galaxies will collide, and then over a span of 1 billion years or so after a very intricate gravitational dance, they will merge to form an elliptical galaxy.
Let’s look at each of the images:
Herschel’s view in far-infrared:
Andromeda in far-infrared from Herschel. Credits: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent
Sensitive to far-infrared light, Herschel sees clouds of cool dust and gas where stars can form. Inside these clouds are many dusty cocoons containing forming stars, each star pulling itself together in a slow gravitational process that can last for hundreds of millions of years. Once a star reaches a high enough density, it will begin to shine at optical wavelengths. It will emerge from its birth cloud and become visible to ordinary telescopes.
Many galaxies are spiral in shape but Andromeda is interesting because it shows a large ring of dust about 75,000 light-years across encircling the center of the galaxy. Some astronomers speculate that this dust ring may have been formed in a recent collision with another galaxy. This new Herschel image reveals yet more intricate details, with at least five concentric rings of star-forming dust visible.
XMM Newton’s view in X-rays
XMM Newton's view in X-Ray. Credits: ESA/XMM-Newton/EPIC/W. Pietsch, MPE
Superimposed on the infrared image is an X-ray view taken almost simultaneously by ESA’s XMM-Newton observatory. Whereas the infrared shows the beginnings of star formation, X-rays usually show the endpoints of stellar evolution.
XMM-Newton highlights hundreds of X-ray sources within Andromeda, many of them clustered around the centre, where the stars are naturally found to be more crowded together. Some of these are shockwaves and debris rolling through space from exploded stars, others are pairs of stars locked in a gravitational fight to the death.
In these deadly embraces, one star has already died and is pulling gas from its still-living companion. As the gas falls through space, it heats up and gives off X-rays. The living star will eventually be greatly depleted, having much of its mass torn from it by the stronger gravity of its denser partner. As the stellar corpse wraps itself in this stolen gas, it could explode.
Together, the infrared and X-ray images show information that is impossible to collect from the ground because these wavelengths are absorbed by Earth’s atmosphere. Visible light shows us the adult stars, whereas infrared gives us the youngsters and X-rays show those in their death throes.
Artist's Impression of the Herschel Space Telescope. Credit: ESA
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While the Herschel Space Observatory usually looks at some of the coldest and most distant objects in the Universe, it also has a side mission to study objects within our own solar system, looking for water-related chemistry on some of the other neighboring planets and moons, as well as comets. At the European Planetary Science Congress in Rome, Herschel scientists presented their first results from their observations of Mars, and said their findings may completely revise our understanding of the Red Planet’s atmosphere.
Herschel has observed Mars with its three instruments, the Heterodyne Instrument for the Far Infrared (HIFI), the Photodetector Array Camera & Spectrometer, and the Spectral and Photometric Imaging Receiver (SPIRE). From these observations, Herschel scientists have been able to obtain an accurate globally-averaged temperature profile of the Martian atmosphere which may cause scientists to revise their models about atmospheric circulation on Mars.
Additionally, the first sub-millimeter observation of molecular oxygen on the planet may lead to a completely new picture of the oxygen distribution in the Martian atmosphere.
“Water vapor plays a key role in the Martian atmospheric chemistry and physics,” said Dr. Paul Hartogh of the Max Planck Institute for Solar System Research in Germany.
SPIRE has provided the first continuous spectrum of the Martian atmosphere in the spectral range in the far-IR/sub-millimeter, as well as, the first complete set of water vapor and carbon monoxide (CO) content in this range.
HIFI observed Mars between April 11-16, 2010, and while only a small part of the data has been analyzed up to now, it already has provided some interesting results: A globally averaged temperature profile has been retrieved from the first simultaneous observations of two carbon monoxide isotopes.
“The best fit of the Martian atmospheric model to these observations shows important differences compared to what we were predicting: between 40 and 80 km from the ground, the atmosphere appears to be more than 10 degrees Celsius colder than predicted,” said Hartogh.
Observations (black) and model (red) of molecular oxygen in the Martian atmosphere. Credit: ESA/Herschel team
Scientists also reporedt on the first sub-mm detection of molecular oxygen (O2) on Mars, with an observational accuracy at least 10 times better than was done before.
“Our sub-mm observations provide for the first time a vertical profile of molecular oxygen in the Martian atmosphere. We found that, contrary to the general assumption of a constant O2 content independently of altitude, the Martian atmosphere is richer in oxygen near the ground and then O2 decreases rapidly with altitude,” said Hartogh.
If this profile is confirmed it may imply different oxygen production and loss processes not considered before, leading to new insights about the Martian atmosphere.
“Obviously, much work still needs to be done on the vertical profile of O2 before we draw such conclusions,” he added.
Herschel’s “Water and related chemistry in the Solar System” project, was conceived with the sole aim to determine the origin, evolution, and distribution of water in Mars, the outer planets, Titan, Enceladus and the comets. Herschel will continue exploring our solar system in the next 2–3 years of its planned mission duration.
“We hope that surprises and major breakthroughs in our knowledge will keep coming in, and that at the end we will have gained a unified picture of the origin and evolution of water in the Solar System objects,” says Dr. Hartogh.
The Herschel Space Observatory launched on May 14, 2009.
For more information, the teams’ papers are available here:
First results on Martian carbon monixide from Herschel/HIFI observations, Hartogh, P., Blecka, M., Jarchow, C. et al., 2010, A&A in press, http://arxiv.org/abs/1007.1291.
HIFI observations of Mars: first detection of O2 at submillimetre wavelengths and upper limits on HCl and H2O2, Hartogh, P., Jarchow, C., Lellouch, E. et al., 2010, A&A in press, http://arxiv.org/abs/1007.1301
Water and related chemistry in the Solar System. A guaranteed time key programme for Herschel, Hartogh, P., Lellouch, E., Crovisier, J., et al., 2009, Planet. Space Sci., Vol. 57, Issue 13, Pages 1596-1606
The Herschel-SPIRE submillimetre spectrum of Mars, Swinyard, B., Hartogh, P., Sidher, S. et al., 2010, A&A, 518, L151, doi:10.1051/0004-6361/201014717
A part of a Herschel/HIFI spectral scan (the white curve) overlaid on a background Spitzer Space Telescope image (ESA/HIFI/HEXOS/E. Bergin).
Love to read science papers? Here’s a batch that will keep you busy for a while. 152 papers were released this morning highlighting the Herschel telescope’s first science results. A few papers describe the observatory and its instruments, and the rest are dedicated to observations of many astronomical targets from bodies in the Solar System to distant galaxies. Herschel is the only space observatory to cover a spectral range from the far infrared to sub-millimeter, so there’s a wide range of objects and topics covered, including star formation, galaxy evolution, and cosmology.
And you thought you’d have nothing to do this weekend!
The black spot in the green-tinged cloud near the top of the image is a hole blown through NGC 1999 by the jets and winds of gas from the young stellar objects in this region of space. Credits: ESA/HOPS Consortium
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There is a black patch of space in NGC 1999, and for years astronomers have thought it was just a dense cloud of gas and dust, blocking light from passing through. But the Herschel infrared space telescope – which has the ability to peer into these dense clouds — has made an unexpected discovery. This black patch is actually a hole that has been blown in the side of the nebula by the jets and winds of gas from the young stellar objects in this region of space. “No-one has ever seen a hole like this,” said Tom Megeath, of the University of Toledo in the USA. “It’s as surprising as knowing you have worms tunneling under your lawn, but finding one morning that they have created a huge, yawning pit.”
Any previous descriptions of NCG 1999 said that the ominous dark cloud in the center was actually a condensation of cold molecular gas and dust so thick and dense that it blocks light. And astronomers had no reason to believe otherwise, until Herschel’s powerful infrared eyes took a look from space. A Hubble image of NCG 1999 showing the dark patch. Credit: Hubble Heritage Team (STScI) and NASA
When Herschel looked in the direction of this nebula to study nearby young stars, the cloud continued to look black. But, that should not be the case. Herschel’s infrared eyes are designed to see into such clouds. Either the cloud was immensely dense or something was wrong.
Investigating further using ground-based telescopes, astronomers found the same story however they looked: this patch looks black not because it is a dense pocket of gas but because it is truly empty. Something has blown a hole right through the cloud.
Stars are born in dense clouds of dust and gas. Although jets and winds of gas have been seen coming from young stars in the past, it has always been a mystery exactly how a star uses these to blow away its surroundings and emerge from its birth cloud. With Herschel, this may be the first time we can see this process.
The astronomers think that the hole must have been opened when the narrow jets of gas from some of the young stars in the region punctured the sheet of dust and gas that forms NGC 1999. The powerful radiation from a nearby mature star may also have helped to clear the hole. Whatever the precise chain of events, it could be an important glimpse into the way newborn stars disperse their birth clouds.
The galactic bubble RCW 120. Image credit: ESA/PACS/SPIRE/HOBYS Consortia
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Just days before the first anniversary of the Herschel space observatory’s launch, the first full science results – along with some very pretty images – were released at a symposium in the Netherlands. “Herschel is a new eye on a part of the cosmos that has been dark and buried for a long time,” said the mission’s NASA project scientist, Paul Goldsmith at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.
Above, Herschel’s observation of the star-forming cloud RCW 120 has revealed not only the huge blue bubble of gas, but also the small white spot is what some astronomers have called an “impossible” star.
It already contains eight to 10 times the mass of the sun and is still surrounded by an additional 2,000 solar masses of gas and dust from which it can feed further.
“This star can only grow bigger,” says Annie Zavagno, Laboratoire d’Astrophysique de Marseille in France. Massive stars are rare and short-lived. To catch one during formation presents a golden opportunity to solve a long-standing paradox in astronomy. “According to our current understanding, you should not be able to form stars larger than eight solar masses,” says Zavagno.
A region the the galactic center in the Eagle constellation. Credits: ESA/Hi-GAL Consortium
This image is taken looking towards a region of the Galaxy in the Eagle constellation, closer to the Galactic center than our Sun. Here, we see the outstanding end-products of the stellar assembly line. At the center and the left of the image, the two massive star-forming regions G29.9 and W43 are clearly visible. These mini-starbursts are forming, as we speak, hundreds and hundreds of stars of all sizes: from those similar to our Sun, to monsters several tens of times heavier than our Sun.
These newborn large stars are catastrophically disrupting their original gas embryos by kicking away their surroundings and excavating giant cavities in the Galaxy. This is clearly visible in the ‘fluffy chimney’ below W43.
Click the images for larger versions.
Learn more in this video released by the ESA, or see this ESA website
Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA
When you look up into the night sky, it seems like you can see a lot of stars. There are about 2,500 stars visible to the naked eye at any one point in time on the Earth, and 5,800-8,000 total visible stars (i.e. that can be spotted with the aid of binoculars or a telescope). But this is a very tiny fraction of the stars the Milky Way is thought to have!
So the question is, then, exactly how many stars are in the Milky Way Galaxy? Astronomers estimate that there are 100 billion to 400 billion stars contained within our galaxy, though some estimate claim there may be as many as a trillion. The reason for the disparity is because we have a hard time viewing the galaxy, and there’s only so many stars we can be sure are there.
Structure of the Milky Way:
Why can we only see so few of these stars? Well, for starters, our Solar System is located within the disk of the Milky Way, which is a barred spiral galaxy approximately 100,000 light years across. In addition, we are about 30,000 light years from the galactic center, which means there is a lot of distance – and a LOT of stars – between us and the other side of the galaxy.
Artist’s impression of the Milky Way Galaxy. Credit: NASA
To complicate matter further, when astronomers look out at all of these stars, even closer ones that are relatively bright can be washed out by the light of brighter stars behind them. And then there are the faint stars that are at a significant distance from us, but which elude conventional detection because their light source is drowned out by brighter stars or star clusters in their vicinity.
The furthest stars that you can see with your naked eye (with a couple of exceptions) are about 1000 light years away. There are quite a few bright stars in the Milky Way, but clouds of dust and gas – especially those that lie at the galactic center – block visible light. This cloud, which appears as a dim glowing band arching across the night sky – is where our galaxy gets the “milky” in its name from.
It is also the reason why we can only really see the stars in our vicinity, and why those on the other side of the galaxy are hidden from us. To put it all in perspective, imagine you are standing in a very large, very crowded room, and are stuck in the far corner. If someone were to ask you, “how many people are there in here?”, you would have a hard time giving them an accurate figure.
Now imagine that someone brings in a smoke machine and begins filling the center of the room with a thick haze. Not only does it become difficult to see clearly more than a few meters in front of you, but objects on the other side of the room are entirely obscured. Basically, your inability to rise above the crowd and count heads means that you are stuck either making guesses, or estimating based on those that you can see.
A mosaic of the images covering the entire sky as observed by the Wide-field Infrared Survey Explorer (WISE), part of its All-Sky Data Release. Credit: NASA/JPL
Imaging Methods:
Infrared (heat-sensitive) cameras like the Cosmic Background Explorer (aka. COBE) can see through the gas and dust because infrared light travels through it. And there’s also the Spitzer Space Telescope, an infrared space observatory launched by NASA in 2003; the Wide-field Infrared Survey Explorer (WISE), deployed in 2009; and the Herschel Space Observatory, a European Space Agency mission with important NASA participation.
All of these telescopes have been deployed over the past few years for the purpose of examining the universe in the infrared wavelength, so that astronomers will be able to detect stars that might have otherwise gone unnoticed. To give you a sense of what this might look like, check out the infrared image below, which was taken by COBE on Jan. 30th, 2000.
However, given that we still can’t seem them all, astronomers are forced to calculate the likely number of stars in the Milky Way based on a number of observable phenomena. They begin by observing the orbit of stars in the Milky Way’s disk to obtain the orbital velocity and rotational period of the Milky Way itself.
Estimates:
From what they have observed, astronomers have estimated that the galaxy’s rotational period (i.e. how long it takes to complete a single rotation) is apparently 225-250 million years at the position of the Sun. This means that the Milky Way as a whole is moving at a velocity of approximately 600 km per second, with respect to extragalactic frames of reference.
Infrared image of the Milky Way taken by NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech
Then, after determining the mass (and subtracting out the halo of dark matter that makes up over 90% of the mass of the Milky Way), astronomers use surveys of the masses and types of stars in the galaxy to come up with an average mass. From all of this, they have obtained the estimate of 200-400 billion stars, though (as stated already) some believe there’s more.
Someday, our imaging techniques may become sophisticated enough that are able to spot every single star through the dust and particles that permeate our galaxy. Or perhaps will be able to send out space probes that will be able to take pictures of the Milky Way from Galactic north – i.e. the spot directly above the center of the Milky Way.
Until that time, estimates and a great deal of math are our only recourse for knowing exactly how crowded our local neighborhood is!
We have written many great articles on the Milky Way here at Universe Today. For example, here are 10 Facts About the Milky Way, as well as articles that answer other important questions.
Astronomy Cast did a podcast all about the Milky Way, and the Students for the Exploration and Development of Space (SEDS) have plenty of information about the Milky Way here.
And if you’re up for counting a few of the stars, check out this mosaic from NASA’s Astronomy Picture of the Day. For a more in-depth explanation on the subject, go to How the Milky Way Galaxy Works.
