Chandra cuts through the fog

By Phil Plait | April 30, 2009 6:30 am

When you look up on a dark, clear, moonless night, you might just see a faint fuzzy streak cutting across the sky. That’s the Milky Way, called that because it looks like milk has spilled across the sky.

But when you look at it through a telescope, it resolves itself into millions upon millions of stars. Faint and tightly packed, they merge together, forming a fuzzy streak because we lack the resolution in our eyes to separate out the stars.

The same is true when you look at the sky in X-rays. A pervasive X-ray glow permeates the same area of the sky as the Milky Way, and for many years astronomers haven’t known if this is due to a vast ribbon of million-degree hot gas and is truly a fuzzy glow, or if it’s caused by lots and lots of point sources packed together.

NASA’s orbiting Chandra X-ray observatory has finally resolved (haha! I kill me) the issue. Behold:

The image shown is a Spitzer infrared image of a region very near the center of the Milky Way. Chandra stared at the indicated spot for roughly 250 hours (!) — one of the deepest exposures ever taken using the observatory — and instead of a faint, diffuse glow it found hundreds of faint point sources of X-rays. This strongly indicates that the glow in X-rays seen by earlier telescopes is in fact the combined glow of millions of such discrete sources!

This is good news; it hardly seemed possible that there was a source of energy that could keep so much gas heated to millions of degrees. But now we have another question: what the heck are the point sources? Most likely they’re white dwarfs, stars like the Sun whose lives are over, and all that’s left is the hot, dense core. If a white dwarf is in a tight orbit around a normal star, it can draw material off that star. This matter piles up on the dwarf and gets very hot, emitting X-rays. It can also build up to a critical point and undergo massive spontaneous fusion, again blasting out X-rays.

There are most likely other sources of X-rays in there as well, including highly magnetic binary stars, neutron stars, and black holes.

I love this observation. It’s everything science should be: an interesting question, a straightforward observation to determine the solution, a willingness on the part of the observatory directors to take a lot of precious telescope time to make the observation, and a result that is so clean it’s pretty easy to interpret.

If only all things in science were like this. But of course, sometimes the fun is at the edges, where observations are tricky, the physics uncertain, and the results surprising and leading to new vistas to explore.

Simple and clean, or difficult and fuzzy: science has room for both.

Credit: X-ray (NASA/CXC/TUM/M.Revnivtsev et al.); IR (NASA/JPL-Caltech/GLIMPSE Team)

CATEGORIZED UNDER: Astronomy, Cool stuff

Comments (44)

  1. MPG

    Wowzers. So does this mean the estimate of the number of stars in our galaxy is higher now, if we include these “expired” white dwarfs and such?

  2. TobiasTheViking

    So what does this do for Dark Matter?

    will it have any impact at all on the current theories/hypothesis?

    I mean, this must mean there is more normal matter in the universe.. right?

    Though it might be so little that it doesn’t really change anything(i kinda suspect this is the case):

  3. Becca Stareyes


    IANAC*, but I seem to recall some of the measurements of dark matter compute the ratio of ‘baryonic matter’ (protons and neutrons) to ‘all matter’. Since white dwarfs would be made of baryons, they’d be counted in that census.

    Someone with more knowledge or a better memory (or time to google) can (and should) correct me.

    * I am not a cosmologist

  4. How sensitive is this measurement?

    Are we only seeing major X-ray producers like binary white dwarfs and black holes or are we seeing less intense emitters like main sequence stars?

    I would be interested to see this contrasted with a visible light image at the same resolution.

  5. MKremer

    It possibly could affect the count of total stars in our galaxy, but its mass (reflected as # of Solar masses) is already pretty well known and shouldn’t change.

  6. laserfuzz

    “It’s everything science should be” . I love that one question leads to even more questions. Where is the X-ray source coming from? Well right there. What is it? Could be black holes, white dwarfs, neutron stars, well lots of things. Well lets look for those. Then we end up finding even more information with even more questions. It never ends. That might be a good way of getting students involved in science. You want job security? Science has it!

  7. Bill Nettles

    Are these real x-rays (coming from bremstrahlung electrons or discrete electron transitions) or simply em blackbody radiation being emitted by extremely hot objects with wavelengths that are typical of x-rays.

    Some people differentiate the names based purely on wavelengths (or energies), and others (mostly nuclear physicists) differentiate based on process: x-ray is an electron process, gamma-ray is a nucleus process, and then there are high-energy photons (electron-positron annihilation for example).

    I’m not trying to be picky. I just want to understand what an astronomer means when they say x-ray and gamma-ray.


  8. Jason Dick


    So what does this do for Dark Matter?

    Answer: nothing. Our most sensitive measurements of the ratio of dark matter to normal matter stem from the cosmic microwave background. When the CMB was emitted, the physics were much simpler (we didn’t have these dense clumpy things called “galaxies” or “stars”), and thus the relative effects of normal matter and dark matter easier to understand. Put simply, in the early universe, normal matter tended to “bounce” when it fell into a gravitational well, because it felt the pressure of the photons (at the time, the universe was a plasma). Dark matter did not, because dark matter carries no electromagnetic charge. Thus we can detect how much relative dark matter and normal matter there is by measuring how much “bounce” there is, which we can see in the statistical distribution of cool and warm spots on the CMB.

    For a visual representation of how the statistics change based upon these parameters, check out this website:

    If you click the “w_b” and “w_d” buttons on the right side of the picture, you should see an animation that changes the amount of dark matter and normal matter in the universe (the underscore denotes a subscript). Clicking on “w_b” gives you an animation where the amount of normal matter (w_b) is being increased while the amount of dark matter (w_d) is being reduced. Keep your eye on just the top plot, for the moment. Notice that there are two main effects:
    1. The first peak gets larger.
    2. The peaks themselves become more uniform: instead of this strange even-odd behavior, you approach a spectrum that just gets smaller in a consistent manner.

    I don’t recall at the moment why the first peak gets brighter, but the fact that the peaks become more uniform stems from the bouncing of the normal matter: as the first peak is before the first bounce: the matter has just moved into the first set of potential wells. The second peak is when the matter has moved into smaller potential wells, then bounced back. So by transitioning to more normal (baryonic) matter, this bouncing effect becomes large, and there is no difference between even and odd peaks.

    Now, take a look at the “w_d” animation. Here we do the opposite: increase the amount of dark matter while keeping the normal matter constant. Notice that the even-numbered peaks seem to die away almost entirely as the dark matter contribution goes up. This is because dark matter doesn’t bounce. Thus by carefully measuring the ratio between the even-numbered and odd-numbered peaks in this power spectrum (which captures the statistics of the CMB), we get a very accurate measurement of the ratio between normal and dark matter.

    Since none of this calculation relies upon where any of the normal matter is now, this result has no impact.

  9. DrFlimmer

    @ Bill Nettles:

    Well, if an astronomer says “gamma rays” or “X-rays”, then they are gamma and x due to their wavelength (and I bet every physicist says so. If particle physicist say that “x-ray is an electron process”, then they that x-rays are mainly produced by processes including electrons; but the x-rays are still x-rays because they are in the “x-ray”-part of the spectrum).
    The process to create those rays is another thing. Most likely every possible process is involved. In order to analyse which process is responsible, you need a spectrum (it is an easy task to distinguish between a thermal and a non-thermal spectrum). I don’t know if they took a spectrum in this special research, but since they “just” wanted the resolution, I think they didn’t make one. This is another task to find out which process drives the radiation – just what Phil has speculated about at the end.

  10. Mike

    This is cool. I have a hard time believing, though, that there are “that many” white dwarfs in tight orbits …

    @Dr.Flimmer: In medical physics x-ray and gamma ray are used to delinate origin (i.e. x-ray = electron, gamma = nuclear).

  11. thank you for writing in a way that allows those with an interest but not a background in astronomy to understand and in a way that causes us to want to find the answers to our questions.

  12. So will one possible next step be to try and match some of those x-ray sources with their optical counterparts? Or are the stars just too dim or too crowded in that neck of the galaxy to distinguish?

    I have to say, that composite image is way cool. It’s the sort of thing that reminds me of when I was a kid poring over grainy black and white images of the Milky Way in the old astronomy books our library had. Fuzzy and dim, yes, but boy, did it spark the imagination. Makes me want to shift that little circle over a few degrees and see what’s up in that part of the sky!

  13. “I love this observation. It’s everything science should be…”

    And it’s worth noting how exhilarating new data is. Just because this affects how we view/think/consider the natural world it’s not being swept under the rug by that imaginary “science conspiracy”. Or explained away simply as ghosts, space faeries or unimportant to our daily lives.

  14. Can the Chandra observatory detect pulsating X-ray sources? I wonder how many of these X-ray points are actually X-ray pulsars.

  15. I have a question, something I have always wondered: How can an orbiting observatory “stare” at one spot in space for 250 hours, or, however long it takes to image a certain spot in space. I have often heard this about Hubble as well? If the platform is orbiting, at some point, won’t the earth block it’s view?

  16. Mike

    @Michael L: Depends which way it’s pointed. If it’s pointed parallel to the Earth’s axis, it can indeed look at the same spot indefinitely. In most cases, however, multiple exposures are integrated to produce the long exposure time.

  17. DrFlimmer

    @ Tom Marking

    Can the Chandra observatory detect pulsating X-ray sources?

    Taking in mind the long exposure time, I guess this is unlikely. Any “pulsating” source should be washed out and should just be visible as a “bright” source. Shorter exposure times are probably possible but the problem would be the worse resolution. This would result in a difficulties in locating the source.

    @ Michael L and Mike

    Yes. The biggest problem, however, is to hold the telescope in the correct position. You need at least two gyroscopes (almost frictionless rotating spheres). The combined angular momentum keeps the telescope steady, but it’s a very difficult process.

  18. Charlie Young

    Just looked it up…Gamma rays have the same energy (wavelength) as x-rays, but the source is the difference. Gamma rays are emitted from within the nucleus of radioactive atoms. X-rays are produced by the interaction of electrons and atoms.

  19. Oh geez. Sure, you can define X-rays vs. gamma rays based on the source, if you’re a loser!

    If it’s an MeV, it’s a gamma ray. If it’s 10 keV, it’s an X-ray. Somewhere in between it switches over, and where it does so depends on whom you ask. However, the Chandra bandpass gives out around 10 keV, these here are definitely X-rays.

    Also, to expand on DrFlimmer’s comment, the length of the exposure doesn’t prevent detecting (slowly) pulsating sources, since the arrival time of each photon is tagged reasonably well (in most observing modes — I’m not sure what was used here). However, detecting pulsations takes more photons than just detecting the presence of a source, so you probably can’t in this image.

    And to be precise, the gyroscopes are used to detect attitude, mainly during slewing between targets. When observing, the telescope (like pretty much all satellites) uses star trackers (small telescopes) to determine where it’s pointing, and actually controls the direction it’s pointing by changing the rotational speed of several “momentum wheels”.

  20. Mike

    Oh geez. Sure, you can define X-rays vs. gamma rays based on the source, if you’re a loser!


    I’ve been developing medical imaging systems for over 30 years at a Big 10 university. Federally funded. Graduate students. The whole ball of wax. But I guess I’m just a loser.

  21. Bill Nettles

    With apologies to Paul Simon (sung to the tune of America:
    “Let us be losers
    We’ll label our photons by sources.
    Gammas emit from disturbed nuclei.
    Electrons accelerate
    Causing x-radiation,
    And discrete ones pop out of electron clouds.”

    I guess it doesn’t make a difference whether that 70 keV photon came from a 153-Eu nucleus or
    mercury K-shell transitions.

  22. Charlie Young

    Typical medical or dental x-ray sources emit photons in the 30 to 50 keV range. It all depends on the amount of voltage applied to the cathode, the exposure time, and the current. This does not change the wavelength, only the number of photons.

    Am I still a loser?

  23. Bah, humbug! When will you astronomers admit that most of the strange s**t you see Out There is artificial in origin? “Hundreds of faint point sources”, indeed! Surely you realize that a galactic-scale civilization means a LOT of interstellar travel, right? Come on, don’t be ashamed, say with a straight face that you’ve found the million-degree exhaust from hundreds of torchships, and then let’s get our act together and beam ’em a complaint for endangering our habitat through Galactic Warming. And then, public service announcements every so often: “Every time you take a swig of that Rigellian green-fuming brandy, Great A’Tuin sheds a tear”, and all that.

  24. NerdBusters

    This little X-ray vs. Gamma ray spat has made me look this stuff up. Apparently there is a section of the spectrum where X-ray and Gamma ray wavelengths overlap where the difference can only be discerned by the source – or is this also in dispute?

  25. DrFlimmer

    I think, most people define X and gamma rays by their wavelength. If some guys do not (they are not losers) they should always make clear what definition they use. If you are not communicating your “special” definition, it will be considered being the wavelength you’re talking about.
    I am (or am about to become) an astrophysicist and I never used another definition than the wavelength. Gamma rays are the high energetic friends produced by synchrotron radiation, SSC-scattering (a very interesting process, btw 😉 ), pion-decay, etc. If you detect them you can be pretty sure that something VERY interesting and very violent is going on :)

  26. Mike

    It depends on your point of view. Both viewpoints are “special”. Neither view point is right or wrong. There is no “official” definition. It’s the culture of the people you work with.

    It is useful to classify by wavelength if your original data is undifferentiated by source (you detect a photon, but you do not know the mechanism by which it was produced without applying a model (i.e. you were not present when it was produced)), whereas if you are a device person creating your own photons, it makes sense to differentiate by source.

    Charlie – An x-ray tube produces a spectrum of photon energies. For example, a tube operated at 50 kV will produce photons at “all” energies (the very low energy ones will not make it through the vacuum envelope of the tube) up to 50 keV. The range of wavelengths produced is dependent on the voltage. The number produced is dependent on the voltage, current and exposure time.

  27. Charlie Young

    Thanks for the clarification, Mike. I am a practicing dentist and haven’t had to spout this information for about 20 years since my oral radiology classes. The info still floats up there, but I don’t typically go back to the old text (unless I have to try to look smart on these boards!)

  28. Mike

    I guessed you were a dentist, Charlie. The voltages you give are dental. Mammography voltages are also typically 30 kV. All other medical imaging is significantly higher. For example, CT is often done at 120 kV. Fluoroscopic cardiac imaging (my field) ranges from 60 kV for small people to 120 kV for large people (you need a lot more umph to get through all that tissue).

  29. Charlie Young

    …and yes, Mike, you had it right. I did go back to the text and a full spectrum of energies are produced. The graph for dental x-rays peaked in the 30-50keV range with other photons of lesser or greater energies in much smaller quantities. Increasing the kVp (voltage across the cathode) gives the same type of energy distribution, but more photons are emitted.

  30. Bill Nettles

    Okay, when speaking with astronomers, astrophysicists, and cosmologists, I’ll recognize “x-ray” as referring to a generally low energy photon slightly more energetic than the UV region (which stops where, around 1 keV?).

    Now…are these x-ray sources primarily brehmstrahlung, high temp blackbody, or we don’t know? Seems like you could measure the x-ray spectrum between 1 keV and 10 keV, do a shape fit and figure it out. Shouldn’t the spectrum shapes be different? Also, brehmstralung wouldn’t require as high a temp as an x-ray blackbody.

    O-class stars (30 kK surface temp) aren’t that hot…you need about a 1 nm wavelength to get a 1 keV photon, and shorter for higher…and they peak around 100 nm. At 1 nm they tail out to an infinitesimally small radiance (something like e^-300). Of course, a 15 MK temp would give a fabulous 1 keV glow. Those photons still have to fight their way out and get subdidivded by all the Compton events. Don’t know many stars with a 15MK surface temp.

    Are their other candidates?

  31. Bill Nettles

    Then there are all those 511 keV annihilation photons from your PET work, Mike.

  32. @Bill Nettles “Some people differentiate the names based purely on wavelengths (or energies), and others (mostly nuclear physicists) differentiate based on process: x-ray is an electron process, gamma-ray is a nucleus process”

    Surprisingly, most astrophysical sources of gamma rays have nothing to do with direct nuclear processes (e.g., gamma rays emitted from a nucleus). Rather, they are the result of charged particles spirally around magnetic field lines and emitting synchrotron radiation. If we use the nuclear source criterion then there are no gamma rays from pulsars, black holes, galactic jets, etc.

    Most astrophysicists use an energy or wavelength breakdown:

    soft X-rays: 0.3 – 30 nanometer wavelength (0.04 – 4 keV)
    hard X-rays: 0.03 – 0.3 nanometer wavelength (4 – 40 keV)
    soft gamma rays: 0.003 – 0.03 nanometer wavelength (40 – 400 keV)
    hard gamma rays: 400 keV)

  33. TJ

    Re making the observation:

    If you look at the observation times on the Chandra page, you’ll see that the observations will be made over several months this upcoming May through September.

  34. MKremer

    It certainly would be interesting to see any ‘intermediate’ images (if available) during the buildup to the final point-source image – watch the fuzzy patterns resolve into individual patches and then the final ‘dots’.

    Also, imagine a future, more sensitive X-ray telescope mission able to scan that whole region in the Spitzer view! Immense head-scratching, hair-pulling, and excitement (and work, lots of work) trying to quantify and explain everything imaged!

  35. WMDKitty

    ::stares… open-mouthed…::

    I feel so very small, now.

  36. @BA: “When you look up on a dark, clear, moonless night, you might just see a faint fuzzy streak cutting across the sky.”

    … or if you live somewhere with less light-polluted skies, you can see a great swathe of light, so clear that you can make out – without difficulty, even in-between streetlights while walking home – dark nebulae like the Coalsack, and the dust along the Milky Way. 😉 I’m happy I have good skies 8)

  37. @ Tom Marking

    Can the Chandra observatory detect pulsating X-ray sources?

    to which DrFlimmer replied:

    Taking in mind the long exposure time, I guess this is unlikely.

    This is not correct. The detectors on Chandra are CCDs that are read out every 3.2seconds (in the case of the ACIS; see This is not too fast, but for weak sources such as the ones detected here this is sufficient to ensure that only one photon is detected per readout cycle (for bright sources the CCDs can go much faster). So, yes, you can detect pulsating X-ray sources, provided they have pulse periods of longer than about 10s, and many X-ray binaries have pulse periods longer than that. However, given how weak the sources are, it will be not easy to detect periodicities, and since these sources are stars, I doubt that any of them will be periodic anyway…

    The images shown are produced by taking the list of measured photons and then backprojecting the photons onto the sky (that’s easy for Chandra since its spatial resolution is so good). So, yes, MKremer, it is possible to make a movie of the “build up” of the image, but overall for most Chandra images this movie will be rather boring.

    @Bill Nettles: Yes, we usually try to determine the origin of the radiation by fitting the observed X-ray spectrum with spectral models. For late type stars (with masses around that of our Sun and less), you are right in that the spectra are fairly soft and the spectrum can be well described by a bremsstrahlung spectrum plus emission lines from highly ionized ions. The X-rays in these stars come from their coronae, i.e., a very hot (millions of K) gas that is presumably heated by acoustic waves from the surface of the star, similar to what’s happening in our Sun. This heating mechanism does not work for early type stars (O- and B), since these do not have an outer convection zone and thus their surface is not as “bubbly”. However, these stars have strong stellar winds, and there are very strong shocks in these winds that result in the emission of X-rays.

    @everybody discussing the difference between X-rays and gamma-rays: While some textbooks still use the old distinction that gamma-rays are nuclear and all other high energy photons should be called X-rays, Tom Marking has hit the nail on the head, i.e., photons measured at energies less than a few 100keV are usually called X-rays by astronomers and photons at energies above that are called gamma-rays. This is pretty much a matter of taste, however, and no astrophysicist or nuclear physicist I know takes the distinction between these two bands too serious.

  38. Dov Henis

    Why Stars Are Born
    New Star Coming

    A. From “Stay tuned: New star coming in 1 million years”
    Radio observations of a dark, dusty cloud in a nearby star-forming region have revealed one of the earliest phases of star formation and may reveal new insights on starbirth.

    “Gravity ultimately transforms many such starless, cold cores into protostars, stellar embryos that release tremendous amounts of heat as they pack on more and more material. Eventually, a protostar becomes dense enough to ignite nuclear reactions at its core, a sign that a bona fide star has been born.

    “How these objects condense out of the surrounding gas in the galaxy is something that we have not fully solved,” notes Bergin. “If you want to understand how stars are born, prestellar cores are the objects that will unlock those secrets,”

    B. How they condense is not yet fully solved, but what drives them to condense is suggested at the two following brief notes

    28Dec09 Implications Of E=Total[m(1 + D)]

    Cosmic Evolution Simplified

    Dov Henis
    (Comments From The 22nd Century)

  39. Radiation fears after Japan blast, as if not enough …


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