Ask and Astronomer

This page was originally created by Leo Burtscher, filled with the help of many others, and discussed at Galaxy Coffee on May 7, 2009 in preparation of the 2009 Open Day. We revisited it on the July 19, 2012 galaxy coffee in preparation of the 2012 open day.

Some questions we didn't yet discuss, for others we couldn't come up with good answers. The ones where answers are pending are highlighted.

Planets and Stars

Exoplanets / Life in the Universe

  • Is there life anywhere else in the universe?
    • We don't know, at all. Nevertheless our ignorance can be factored out in the well known Drake equation. This equation has been presented by Frank Drake (a U.S. astrophysicist) in 1960 and serves to estimate the number of technically developped and intelligent civilisations in our galaxy. It contains the factors
      • average rate of star formation per year in our galaxy
        • ~1 Msun/yr. Reasonably well known. What matters is actually the number of main sequence stars that are of suitable spectral type (meaning, not too early and probably also not too late) to support life. Many tens of billions in our galaxy.
      • fraction of those stars that have planets
        • roughly half of all (single stars). Reasonably well known, as of 2012 still consistent with newer Kepler results (40%). High. Hundreds of planets have been found (mostly close in gas giants due to an observational bias). Current data are consitent with *all* stars not in multiple systems having a planetary system of some sort.
        • Ralf and Kees look into it. Quite poorly known. Giant gas planet frequency less than about 5%. Terrestrial planet frequency still unknown and may be much higher.
      • average number of planets that can potentially support life per star that has planets
        • Not well known (yet). Very few earth-like planets have been detectable due to technical limitations. Improving techniques now allow probing into terrestrial domain in some special cases and hint that earth-like planets are common. Ask again in 10 years.
      • fraction of the above that actually go on to develop life at some point
        • Unknown. However, we can speculate :-) the fraction of planets that are able to support very basic life forms (like single-celled, "extremophile" bacteria on earth) is probably high, since we know now that life is able to exist in circumstances that are expected to prevail in (newly formed) terrestrial planets (these circumstances are very different from those that prevail on Earth now, but similar to the "young" Earth). The complex organic molecules that form the building blocks of life have been detected in various astrophysical environments. The fraction of planets that can support complex life is unknown and may be very low
      • fraction of the above that actually go on to develop intelligent life
        • Completely unknown
      • fraction of civilizations that develop a technology that releases detectable signs of their existence into space
        • Strictly speaking unknown, however, depending on your definition of "intelligent" it seems likely that an intelligent species, once come into existence, will learn to build a radio transmitter at some point.
      • length of time such civilizations release detectable signals into space.
        • Depending on the intelligence (in particular, "social" or "ethical" intelligence) of the species this time can be long in some cases, short in others.
  • Why do exoplanets have these boring numbers? Can't the discoverer just give his planet a name?
    • IAU regulation. Discoverers of minor bodies in the SS can name them once the trajectory is known. Number of objects is very large. While for asteroids the discoverer can propose a name (which

can refer to persons other than the discoverer, landmarks, cities, mythological creatures, and much more like e.g. an institute like MPIA itself), comets are named after the discoverer. Exoplanets always get these boring numbers. This is just a convention astronomers have been using. A possible explanation might be that exoplanet research is a comparatively young field and today large groups are involved in designing and building instruments and telescopes to do the observations in the first place and more people are involved in performing the observations, analysing the data and interpreting them.

  • Are there earth-like planets (liquid water and all that) orbiting other stars?
    • 3 potentially habitable candidates so far: Gl 581d (but: needs 7 bar CO2 to be habitable!), GJ 667Cc, HD 85512b. But: minimum masses only! There might be liquid water on Europa (a moon of Jupiter) and there might have been liquid water in the past on Mars (certainly subsurface water). Europa is a good example: not detectable at 10 ly - need to drill down! (Follow-up question: no, the Juice mission apparently does not have a lander.)
  • How many planets are currently estimated to exist in the known universe and what is the percentage for those who have been created before Earth?
    • about 780 known, 100 multiples (see, guess: about half of them are older than the earth... ???

Dangerous Universe

  • Could a fly-by of a planet-like object destroy the solar system order?
    • Most probably not, but if it happened in a very special way it might... ???

It is very unlikely that such an object would visit us.

...but see - you might not even need a fly-by; major collisions could happen just like that...

  • What would happen if Eta Carina were to become a supernova?
    • Eta Carina is one of the most massive stars that is very near to (or has already) exploded as a supernova. At a distance of about 2.3 kpc and with a mass of about 100 solar masses it would probably be as bright as V = -7, i.e. one could easily see it during day time and it would be bright enough to read newspaper during night
  • What about Betelgeuze, then?
    • Much closer at 200 pc, so When it blows, it could be as bright as the full moon. But it's still too far to hurt us. A supernova at less than 10 pc would be nasty, though.
  • HWR asks: What are the chances of their being an undiscovered binary containing a White dwarf close enough for a type Ia supernova to become dangerous to us?
  • What is the chance that an comet from another planetary system hits (or better: passes through) ours?
    • from another? Not very likely. There is no known asteroid with an excentricity > 1 (as would be the case for objects entering our solar system from another system).
  • ...what about comets from our own Solar system then?
    • The chance that an asteroid from our solar system hits the earth is very real. The last big impact freeing more than a thousand times the energy of a nuclear bomb happend in 1908 in Russia. Many more dramatic events can be seen as relic craters such as the Beringer crater in Arizona, U.S. or the Nördlinger Ries in Bavaria, Germany proving that these events happen.
    • Apophis: This one had a fairly worrying probability after its discovery - not for its next encounter with Earth on April 13, 2029 (yes, it's a Friday, of all things), but for the one after that. However, impact probability has now been downgraded to four in a million here.
  • Why are impact craters on the Moon all circular when most asteroids would not come down vertically?
    • Craters are almost always circular, even with high incidence angles, due to explosion which creates it, and subsequent crater wall collapse. Only for impact angle >80 deg from zenith or so do we expect elongation. (Q&A by Coryn)
  • Which planets will be engulfed by the sun in the later stellar evolution phase?
    • Depends on whom you ask and when... Probably all planets until, maybe including, the earth will be engulfed by the sun.

Before that, the sun will get more luminous, making the earth considerably warmer. This is because of hydrogen-helium conversion, the mean particle weight in the core becomes larger, thus increasing density/pressure, causing Hydrogen burning to proceed more efficiently

  • Update 2012:, which seems to be pretty thorough has the Earth not engulfed as the Sun expands, but it gets swallowed later as tidal interactions alter its orbit.
  • Are Cosmic Rays dangerous?
    • Particles in space are called cosmic rays. The highest energy cosmic rays are 10^20 -- 10^21 electron volt, or 10 -- 100 Joules, equal to the kinetic energy of a baseball at speed 100 km/h. These highest energy particles are extremely rare, their arrival rate is only 1 per century per square kilometer.

Little is known about the origin of these highest-energy particles; they are most likely produced in cosmic accelerators outside the Milky Way, possibly active galactic nuclei (AGNs) or gamma-ray bursts (GRBs). There is a well-understood reason why no cosmic rays with energy much above 10^20 electron volt arrive at Earth, known as the GZK cutoff: a particle of that energy would collide with low-energy photons from the cosmic microwave background radiation, quickly losing energy. Only very close to one of these most powerful cosmic accelerators could cosmic rays above the GZK cutoff exist, but another simple estimate (not given here) shows that AGN and GRBs can only accelerate up to ~ 10^20 electron volt. Curiously cosmic ray accelerators run out of steam at the same energy where the universe is no longer transparent. A cosmic ray of 10^20 eV interacts in matter of density 1 g / cm^3 --- like water, or your head :-) --- on average after travelling a distance of ~ 100 cm. This means that only about every 10th UHECR passing through your head will interact. If there is an interaction, on the order of 100 new particles will be created. These again have a chance of 10 % to interact before leaving your head, which means that in total ~ 10 particles will have interacted. These 10 interactions will happen in different cells in your head, destroying one molecule per cell, most likely water molecules. Overall, the damage to your head is negligible. Besides, you would have to be an astronaut, because on Earth you are shielded by the atmosphere.

  • Roberto Decarli adds: About an AGN jet shooting towards a star / planet, it may be worth of mentioning the unique case of 3C321, a radio galaxy whose jet is blasting away the interstellar medium of the companion galaxy: press release


  • Why is Pluto no longer a planet? What is a planet, anyway?
    • Pluto has not cleaned his orbit from other objects as required by the new IAU definitions for a planet. A planet also has to be round and has to be in orbit around the sun... The IAU issued these new definitions in 2006 since, due to ever better observing techniques, more and more so called Kuiper belt objects were discovered and since they were not much different than Pluto one could have either added new planets to the list all the time or redefine a planet to only include "real" planets. Pluto's density is also more comet-like than planet-like.

Pluto's mass was originally grossly over-estimated, leading to a falls planetary status assignment. But we don't know really ;-)

  • What is the age of the sun, and how is this determined?
    • The age of the sun is 4.5 billion years. It is determined by looking at the age of planets and comets (using radio-active dating techniques) and assuming that all these objects were created at the same time together with the sun.
  • Why are there planets around a neutron star?
    • Planets either survive the SN stage, but may also be actually be formed in remnant material surrounding the central object, this may be possible only in systems where a stellar companion provides an additional source of material
  • How would the sky look like on a planet that orbits a star in the center of large stellar cluster?
    • In the disk of the Milky Way the typical distance between two stars is about 1 pc, in the centers of globular cluster the stellar density can reach up to 10^5 stars per pc^3. If planets can form at all with all the close encounters by other stars the planetary system would probably not be very stable, most certainly not stable enough to support the evolution of higher life forms that can look at the sky. That being said, the sky would look bright even in the night with thousands of stars much brighter than 1 mag in the visual (compared to 12 in our sky). Source
  • What are the biggest stars in the current universe and what limits their size?
    • Biggest stars are ones like Eta Car with M = 100 M_sun. The limit on this size is given by the Eddington limit. It was bigger in the past since in earlier times there were less metals and so material around the proto-star could not absorb as much light.

Largest stars (in diameter) can be ~2100 Rsun, i.e. ~10 AU.

  • How many stars would I see if there were no humans on this planet?
    • None. :-) (or about 6000.)
  • We have manned space flight. Are our astronauts in danger?
    • space debris? Getting more dangerous by the day

radiation belts (space weather)? If the sun has an outburst, the astronouts have to stay in :-)

  • How was the moon created?
    • a probably mars-sized impactor hit the earth when the solar system was very young. Light material was flung into orbit and formed the moon (within a couple of days). Heavy stuff is now in the core of the earth. The moon was much closer back then, and has been steadily drifting away due to tidal interaction. Still continues.
  • Does Jupiter help to keep the solar system "clean" of dangerous impacting comets?
    • nowadays it may, in the early solar system as many comets were deflected toward earth as away
  • is it just by chance that there are no massive stars nearby?
    • Massive stars are very rare.
  • which body in the SS is best for living?
    • Holland
  • The magnetic field of the earth, is it changing? On what timescales?
    • polarity of earth magnetic field has changed many times, during change the field is weak and we have lots of energetic particles (stay inside)
  • did the sky look a lot different a million year ago?
    • yes a bit, but qualitatively the same
  • how long does it take to reach the nearest star with the fastest rocket?
    • ~100 Myr

Non-science questions

  • How does astronomical knowledge help our society? More drastically: Isn't it better to spend the $1.5 billion for ALMA on feeding hungry children?
    • General strategy: Do not lead with the (undeniable) economic benefits through innovation, and hold off on the "but other people are wasting even more money).
    • Astronomy or the interest in heavenly events is one of the oldest interests in mankind. It is part of our nature to be curious and to want to find out. And astronomy today is part of our culture and an important part for (higher) education, also to motivate people to study "complicated" subjects like maths or physics in the first place and thus to train people in these fields where specialists are always needed. Compared to other costs in our society, we are not spending a large amount in astronomy, althought single projects might suggest differently. ALMA for example is a world-wide enterprise. In Germany every person spends about 5 Euro per year for astronomy which is a lot less than what is spent per person for coal miners' subsidies. Compare this to other expenses our societies make:
    • There are also the technical spin-offs like ccds and special materials developed for astronomy -> digital cameras, ceran stove-tops for example.
  • Why should we be spending tax money on astronomy?
    • see above
  • Why do we need manned spaceflight?
    • We do not need manned spaceflight for scientific reasons. (What happened to the "Humans are more flexible than robots; this might be an advantage for exploring solar system planets"?) But for political / motivational / philisophical reasons it is highly wanted.

Galaxies & Cosmology

Black Holes

  • Is it proven that black holes do exist?
    • Observations of the motions of stars at the center of our galaxy leave no other possibility than to assume that there is a black hole. X-Ray binaries are an even stronger argument for (stellar mass) black holes. In X-Ray-measurements in these sources (Done et al.) one does not see a reflection, therefore it must have an event horizon.
  • How dense is a black hole?
    • Taking Schwarzschild geometry gives roughly 1.84e19 (M_sun/M)^2 kg/m^3, i.e. stellar mass black holes have densities comparable to the density of a nucleus. 10^9 M_sun Super Massive Black Hole (as found in the centers of big elliptical galaxies) have average densities lower than the density of water. Black Holes don't necessarily have extreme densities. However, note that this is just the average density of the black hole. It does not tell anything about the density structure inside the event horizon.
  • So I've heard that there's a black hole at the center of the Milky Way. Why aren't we being sucked in?
    • Cf. "What would happen to the orbit of the earth if we were to replace our sun by a black hole of 1 M_sun?" ... Nothing. (OK, it gets very cold, and pretty much all the living beings on Earth freeze to death, but you know what we mean: Earth orbits on.)
  • How long would it take for a central black hole of a galaxy to eat all stars of the galaxy?
    • 1 star / 10 000 years gets disrupted near the black hole. If you waited forever, most stars would be thrown out of the galaxy though and not end up in the center.
  • What happens when two black holes collide?
    • They get closer and closer due to radiation of gravitational waves (the efficiency of this braking mechanism goes with 1/R^5) until their horizons touch. Then there's a plunge phase that can only be described using 3D simulations, and a ring-down phase wherein the regularly shaped, merged black hole makes a transition to Kerr geometry.
  • How many black holes are floating around in our galaxy unseen? (the person was referring to the number of stellar black holes created by all the massive stars until now)
    • Taking the Salpeter IMF with xi(M) = 0.03 (M/M_sun)^-1.35 where xi(M) is the relative probability that a star with mass M is formed and integrating this for M > 8 M_sun (where stars form to black holes) one gets that 5% of the inital stellar mass is now in black holes. Taking the stellar mass of the milky way as 10^11 M_sun and a typical stellar black hole of a few M_sun one gets that there are about 10^9 stellar mass black holes in the milky way. 75% of them will be in a binary system.
  • Are artificially produced black holes dangereous?
    • No. First of all it is not at all sure that black holes will be produced artificially at a particle collider. If so, then the best theoretical expectation is that they evaporate before hitting any atom that they might devour. And even if this is not the case, according to calculations, it takes more than the lifetime of the earth for such a black hole to accrete a few kg of matter. From the existence of old neutron stars (age can be measured through the change of the rotation period of pulsars for example) that have been bombarded with much more energetic cosmic rays since a long time, one can further infer that the process of creating black holes by particle collisions, if it happens at all, cannot be catastrophic for the body on which it happens.

Neutron stars

  • How come neutron stars have a magnetic field? Don't you need electrical currents (that is, charged particles on the move) to create such a field?


  • How can the universe be expanding without there being a center?
    • The problem in imagining an expanding universe without a center only arises when you picture it as an explosion expanding in something. But this is not the case. The universe looks the same everywhere we look indicating that there is no preferred point that might be the center. You can think of the galaxies in the expanding universe as dots on a balloon that you fill with air. The bigger the balloon becomes the greater will the distance between the points be without any of the points being in the center. Another analogy is that of being stuck in an infinitely large piece of raisin bread that is expanding.
  • Will earth and moon at some point no longer be gravitationally bound due to the accelerated expansion of the universe?
    • In the currently accepted standard model, they will stay bound. The acccelerated expansion of the universe can be understood as a force term that is proportional to q_0 * H_0^2 and therefore constant in a de-Sitter universe. For a matter- or lambda-dominated universe, whether or not the system stays bound indefinitely or participates in the Hubble flow is purely a matter of scale. A proton and an electron with a separation of less than the radius of Neptune's orbit stay bound. A solar system with a diameter of less than 400 ly stays bound as well (cf. On the other hand, for p/rho<-1 (corresponding to a "phantom energy" equation of state where expansion is accelerated even more than for a universe dominated by a cosmological constant - not part of the current standard model!) would there be a "big rip" - eventually, the solar system, then the earth, then atoms, would get ripped apart (cf.
  • Radiation gets redshifted. But matter can be seen as a de-Broglie-wave (matter-wave). Why don't protons have less energy / mass today than at earlier times?
    • The redshift effect is a relativistic effect that only appears when particles travel at relativistic speeds (i.e. near the speed of light) with respect to the Hubble flow (e.g. the galaxies), i.e. the expansion of the universe. Most baryonic particles (e.g. protons) are bound in their galaxies and therefore we don't see this relativistic effect with them. If we were able to detect ultra-high energy protons from galaxies at significant redshifts, their energy should indeed be reduced by the expansion of the universe.
  • Do we live in a multiverse?
    • The best explanation for the early phase of our universe (chaotic inflation) makes it plausible that we just live in one of many universes. (cf. Max Tegmark's article John Everett: Quantum multiverse (a new universe gets created everytime a wavefunction collapses). However, this question is not scientifically accessible at the moment.
  • What was before the big bang?
    • We don't know. The answer depends not only on which theorist you ask but also on what you call "the big bang". Linearly extrapolating the expansion of the universe backwards leads to some point t = 0 which is referred to as "big bang". But what we can really only infer from observations is that the universe at a time approximately 13.8 billion years before now was much smaller, much denser and much hotter. Thanks to an ever increasing knowledge in particle physics we can understand the state of matter until fractions of a second after the "big bang" but since it does not even make sense to talk about the time before these fractions of a second it certainly does not make sense to talk about what happened before the big bang, we simply don't know and have no means of finding it out at the moment.
  • What is outside the universe? If the universe is curved, in which (higher dimensional?) space is it curved?
    • Curvature does not require an embedding in a higher dimensional space. Consider the surface of a sphere: While we can only picture it when embedded in three dimensions (e.g. as a globe), the surface is a two-dimensional object (2 coordinates, e.g. longitude and latitude, suffice to fully describe any point on the surface). Even an ant that effectively lives only in those two dimensions could determine the curvature without any reference to the three-dimensional outside, simply by constructing, and measuring the properties of, triangles.
    • Cosmological note: Local curvature does not fix the topology: A flat universe, for instance, could be infinitely extended, think of a two-dimensional plane, or it could be compact, think of the two-dimensional surface of a torus.
    • There is an outside to the observable universe.
  • How do we know that the universe is still expanding, while all light that we receive shows us the past?
    • We don't know but it is very likely that the universe is still expanding. Current observations suggest that the universe is homogeneous and that we don't live in any special place in the universe (the so called cosmological principle) which means that by observing distant galaxies we also learn about what's happening more locally.
  • What causes the cosmological redshift?
    • Light gets stretched as the cosmic scale factor changes, just as the distances between far-away galaxies do. That's why the redshift is so simple (1+z proportional to ratio at time of emission and absorption). If you want to follow the photon on its way from one nearly-inertial coordinate system to the next, you should get the same result adding up the Doppler shifts between all those systems, but that's much more complicated.
  • Where does the energy go in an expanding universe? As the universe expands, photons get redshifted, lose energy... Related: Where does the energy go when a photon is gravitationally redshifted?
    • It's not at all simple to define what global energy/mass even is in GR - and certainly not in a situation where spacetime isn't asymptotically flat (isolated system). Actually, that is where the following-the-photon-from-one-nearly-inertial-system-to-the-next (see previous question) could come in handy - it shows that the "loss of energy" is no more real than the fact that a photon (and, for that matter, moving particles etc.) will appear to have different amounts of energy when observed from two different systems in relative motion. Same (even simpler) for gravitational redshift, which can be derived by looking at two appropriately chosen systems in free fall.
  • How sure can we be that the redshift we measure is really due to the expansion of the universe and not caused or modified by other influences (e.g. "tired light" hypothesis, summary effect of gravitational redshift caused by galaxies and clusters)?
    • Take the time dilation effect observed in the lightcurves of distant supernovae: This has nothing to do with the individual photons getting tired. (E.g.
  • Is there a considerable amount of antimatter in the universe? (relation to CP symmetry breaking, abundances of matter/antimatter)
    • While there may be large amounts of dark matter in today's universe where the intergalactic medium is very thin, this would be problematic in the early universe where densities were much higher.
  • At what redshift did the earth ; our galaxy form?
    • 0.4; over a long time / still forming


  • How many galaxies have you discovered? How many are there in the universe?
    • Me personally...? In the Hubble Ultra Deep Field the Hubble Space Telescope was directed to a more or less 'empty' region in space (few foreground stars) and astronomers integrated light for about 11 days. In a field of 11 square minute-of-arc they found about 10 000 galaxies. Extrapolating this number to the whole sky (assuming that galaxies are more or less evenly distributed on large scalaes) one gets about 135 billion galaxies that at exist in our observable universe.
  • Why are most of the galaxies so thin?
    • This is a combined effect of gravity, centrifugal force and friction (viscosity)...
  • Are there situations where the light is bent so strongly that we can see our own sun?
    • Probably no
  • If the length scale is stretched by relativistic effects, does it hurt?
    • No.
  • What happens when the jet from an AGN hits a star?
    • A first order estimate is to compare with the solar wind which hits the earth all day. The AGN jet density of about 1.000 - 10.000 particles/cm^3 has to be compared with the solar wind density of <10 particles/cm^3, and the jet velocity of 0.1c to the solar wind speed 0.001c. So an AGN jet hitting the earth would correspond to a extremly strong solar wind with corresponding auroras and breakdown in power supply lines. Thanks to the earth magnetosphere, the jet would not be desastreous for life, I think. Dynamically, calculating the force on earth, the dynamic pressure p=rho*v^2 and force F=Area_earth*p, the acceleration of the earth would be a=10^-12 cm/s^2. This compares to the gravitational acceleration of the earth by the sun of a = 0.6 cm/s^2.
  • How are the Galactic arms created and could it be that the sun belonged to another arm at an earlier time?
    • Spiral galaxies show spiral arms, but these arms are not actually the same matter that constantly rotates around. If it were these arms would eventually just completely wind up because the matter at the center of the galaxy rotates faster than the edge. After a few orbits the arms would wind up and be gone. Instead spiral arms are believed to be created by density waves. Imagine a wave in the water - floating matter bobs up and down as the wave passes and then is still again. Density waves pass through the galaxy and the stars enter regions of higher density. The situation is similar to a traffic jam. Cars move through the jam and the density increases at the jam, but the jam itself does not move a great deal. In fact within a galaxy the jam and the stars are all moving. The speed of the jam is called the pattern speed and is the speed the spiral arm density wave is moving. Galaxies have one position, called corotation, where the stars at that position rotate at the same speed as the pattern. They 'co-rotate'. Our sun is at about 7.5 kpc from the galaxy center. Corotation of the MW is probably around this position. If we are slightly interior to corotation we will be moving faster than the spiral density waves and will eventually move into another spiral arm. If we are slightly beyond corotation then the density wave is moving faster than us and eventually another arm will come upon us. If, however, we are exactly at corotation it means that we are travelling at the exact same speed as the arm and so we will remain in this spiral arm (called the Orion Arm).
    • The Galactic arms are spiral arms as in other spiral galaxies. They are produced by density waves running around the MW, induced by instabilities in the MW disk. The extra density in the arm is not much higher (HOW MUCH???) than the regions in between the arms, but arm are mainly more prominent due to extra star formation induced by the slightly denser gas in the arm region at all times. What we see is the higher light output of young stars compared to older stars.
  • Are the laws of physics the same everywhere? Have they always been? Do fundamental constants (G, fine structure constant (alpha)) change in time? Related: Could it be that there are elements in outer space that are not known to us?
    • There is a lot of discussion with some claimed very small values for e.g. alpha but no agreement yet. (more details?)
  • 'What are the highest energies of astrophysical particles and what happens if this particle hits a solid like my head?
    • Highest energetic particles have about 10^20 eV which is a few Joule of energy, a macroscopic value! Probably not much would happen if it hit your head, though. While it may collide with atoms in your head and thereby create showers of particles and antiparticles that radiate all kinds of harmful energy the length scale of this process would be so large that only very few reactions would actually happen in your head. [more details: pending]

Other AAA pages