The big challenge for astronomers is to look at the sky better and further, using both telescopes and measurement sensors. The sky is our observational laboratory and our experimental data are that which emerges from these tools, especially photometry and spectroscopy. Then the laws of physics are deduced from those observational data looking for a mathematical function that approximates them in the most accurate way.
Actually, after the applications of spherical trigonometry for the calculation of stellar positions, after the kinematic theory of Kepler’s orbits and the dynamics of Newton’s orbits that led to the definition of that mathematical science called “celestial mechanics” (a specific application of analytical mechanics), and after the elaboration of Einstein’s relativistic geometrodynamics, there have been no truly crucial new discoveries that have broadened the horizons of classical astronomy (also called astrometry or “astromathematics”). The great discoveries have instead taken place in astrophysics and in cosmology, but to achieve this we have needed very sophisticated measurement instruments, which are becoming more and more performant.
Today the astronomer (a mathematician) and the astrophysicist (a physicist) are exactly the same thing, and deal with the macroscopic part of the physical world. As I see it now, the great challenge of today’s astronomers is basically summarized technically in the achievement of six main objectives: 1) limit magnitude, 2) sensitivity and dynamic range, 3) multi-wavelength, 4) spatial resolution , 5) spectral resolution, 6) time resolution. I would like, especially for students, to try to deal with them one by one very briefly, because they represent the core of today’s astronomical research.
1. Limit Magnitude – It consists in being able to detect the weakest possible signals, which has a double value: to observe objects that are relatively close but very weak in brightness such as brown dwarf stars or any transplutonian planets (KBO), or very bright supernovae located in very distant galaxies and galaxies at cosmological distances. In order to reach these limit values, large-aperture telescopes must be used, where the number of the received photons increases with the square of the telescope diameter, which counterbalances the fact that electromagnetic signals decrease with the inverse of the square of the distance. Then we need extremely effective detectors (appropriately connected to telescopes), such as the latest generation CCD cameras characterized by a high sensitivity to weak signals. All this, in the visible band, pertains to photometry (both CCD mode and photon counting mode as it once was done using photomultipliers), but also falls on spectroscopy because the ability to detect ultra-weak signals allows us to carry out – within certain limits – even spectroscopic analysis of these sources (a dispersing medium removes much light from the original light source).
2. Sensitivity and Dynamic Range – In conventional photography if we take a photograph of a firefly next to a lamppost it happens that in order to try to record the firefly we cause saturation by overexposure of the lamppost while we record little or nothing at all of the firefly. In astrophysics it is fundamental that a sensor (of CCD type, nowadays) is able to spatially detect (on a matrix of many pixels, all extremely small) both very bright and relatively dark areas, without the very bright part is overexposed (as it was the case in conventional photography). This, for example, allows us, always using CCD photometric sensors of the latest generation, to expose together without overexposure both hyper-luminous stars of the background and the emission nebulae in which they are immersed, to detect bridges of matter between a galaxy and anothe other or to detect minimal details in sunspots. But, using today’s CCD cameras, we are also able to detect even a light variation of a part in a million, which is not only very useful to study the variability of stars, but above all to understand using the technique of occultation if an extrasolar planet periodically passes in front of its star, a technique that allowed us to discover many exoplanets, especially thanks to the Kepler space telescope.
3. Multi-Wavelength – Since we started to send telescopes in space 40 years ago and to use infrared telescopes from the ground too, the physical picture of the universe has radically changed. The concept is to observe the same astronomical object simultaneously in multiple wavelength windows, that is in radio waves, in microwaves, in the infrared, in the visible, in the ultraviolet, in the X-rays and in the Gamma rays, in a growing progression of energies. In this way, for example, we are able to understand (in radio waves and X-rays) that a quasar shows bipolar jets of matter that is accelerated to high energies (otherwise almost invisible in the visible band only), that the more internal areas of the accretion disks (in X-rays) in high-mass close binary stars (with neutron star / pulsar as the collapsed component) and in AGN nuclei of quasars reach temperatures of up to 10 million degrees °K and beyond, that supernova remnants contain “blobs” of matter (in X-rays) that are compressed and heated by shock fronts, that the corona of hot stars (in ultraviolet) produce winds of up to 4000 Km/sec, that stars of 100 solar masses explode as “hypernovae” generating very high energies (in Gamma rays), that in well-defined areas of the galactic plane, in the presence of gas and dust, the birth and formation of stars is going on (in the infrared), that pulsars and quasars produce non-thermal synchrotron radiation (in radio waves, above all), that there are molecules in the interstellar space (in radio waves and in microwaves), that there could be intelligent SETI signals (in microwaves). The astronomical research of the past was carried out only using the visible wavelength window, and what we could understand of the physics of the Universe was only one millionth of what really exists. This qualitative leap in the understanding of cosmic physical mechanisms took place thanks (above all) to the fact of having been able to send to space telescopes that are operating in wavelength bands that are different from the visible one, bands that otherwise (except for the infrared band in favorable cases) would be invisible from the ground. In this way an extremely dynamic picture of the Universe emerged, unlike the “harmony of the orbital spheres” of the astronomy of the four centuries ago.
4. Spatial Resolution – It consists in the ability to photometrically distinguish high-definition details of celestial bodies, for example the structure of extended objects such as galaxies, the well-separated stars of the globular clusters, the structure of the planets, the periodic oscillation in angular velocity of planets in orbit around other stars. Large aperture telescopes allow high spatial resolution, since the most minute detail becomes detectable only in these circumstances as it is inversely proportional to the telescope diameter, and this obviously applies to all telescopes at any wavelength they operate, and in any case with photon collecting areas whose diameter is proportional to the wavelength itself. By coupling CCD cameras to very large pixel arrays, we are able to enhance the spatial resolution. This performance is augmented when we are in a condition to use a good “seeing” (1 “-2”, in the most favorable cases) for optical telescopes (i.e. being able to see the stars as point-like light sources and not as enlarged disks), which is why optical telescopes (such as Hubble) are sent to space in order to avoid the effect of atmospheric turbulence in enlarging the star disk, or compensating, using optical telescopes from the ground, the effect of atmospheric seeing using the technique of adaptive optics that correct the wavefront altered by atmospheric turbulence.
5. Spectral Resolution – Spectroscopy was the technique of starlight analysis that led classical astronomy to become astrophysics, in the mid-1800s, thanks to Father Angelo Secchi. In order to acquire a spectrum of a celestial source (in instrumental configurations in which a spectrograph is always directly attached to a CCD camera, in previous times to a luminescence intensifier) means that the light passing through the dispersing element is much less than that which would arrive directly to the CCD camera. Therefore, the more the telescope diameter and the CCD sensor sensitivity are high, the more we can afford to obtain spectra of very distant or intrinsically weak celestial sources, by understanding the (quantum) excitation and ionization levels of celestial objects by studying atomic physics in play. But if the spectral resolution is poor (which inevitably happens for too weak celestial objects) many physical details escape us, precisely because two spectral lines that should be separated even if very close appear to us as only one. For example, if a star produces a strong magnetic field, we expect to see the Zeeman effect, that is, all split spectral lines (literally every line is doubled): if we increase the spectral resolution (if this is allowed) we see the splitting (thus managing to calculate the intensity of magnetic field from the separation in Ångström of the lines), if instead we remain in low resolution we cannot see any line splitting. Moreover, the higher the spectral resolution, the better we are able to study with precision an almost imperceptible periodic variation of the radial velocity by Doppler effect of spectral lines, which allows us to detect the gravitational perturbation exerted by an exoplanet on its star and therefore to discover that planet with precisions that today is already under the meter per second. For the same reason we are able to accurately map the BLR (Broad Line Region) zones in the quasars, where the spectral lines are enlarged due to the very high rotation rate of matter in the accretion disks around black holes of hundreds of millions of solar masses, and if the spectral resolution is particularly high we are also able to distinguish distinct areas at different Keplerian rotation velocities, exactly as in the smaller scale case of accretion disks around young protostars. A high spectral resolution then allows us to better map the rotational regime (Keplerian in predictions) in galaxies and to see precisely how much this regime differs from the Keplerian one due to the presence of dark matter (baryonic or non-baryonic). And when we take advantage of the spectral resolution in the microwave wavelength window (using radioastronomy in this specific case), for example, then using multichannel spectrometers (such as Serendip V) able to provide us in only one shot up to a billion frequencies distributed over as many channels, we are potentially capable of detecting any intelligent SETI signals, or of being at least able to map with extreme precision the radial velocity of neutral hydrogen clouds in this and other galaxies. The spectral resolution gives us, due to its excellence, direct information on the physics of the Universe, up to the cosmological scale: for example, an improvement in spectral resolution allows us to obtain more precise values of the radial velocity (deduced from the Doppler Effect of spectral lines) of receding galaxies due to Big Bang expansion, which then leads us to measure better values of the Hubble constant, values once affected by a notable error due to the extreme weakness of the signal produced by very distant light sources and to the previous use of small-aperture telescopes and to not very sensitive photographic sensors (at that time).
6. Time Resolution – Being able to detect variations up to values of one nanosecond (one billionth of a second) of the light of celestial bodies allows us to know many crucial dynamics. For example, using a special CCD camera called Pixelated Detector Array (PDA) it is possible to scan large slices of the sky and detect variations on the nanosecond timescale possibly caused by localized laser emissions of an artificial nature and of intelligent origin (Optical SETI). But, worse than that, it is possible, in conjunction with many of the other performances discussed here, to build up very high time resolution light curves of celestial sources whose morphology is indirectly attributable to very precise geometries of the light-emitting body (intrinsic or reflected). In the case of solar physics using high time resolution it is possible to study minimal periodic variations of an helioseismological nature. And in the case of high mass close binary stars it is possible to see an accretion disk “frying” around a neutron star or a black hole that sucks mass from a companion star of O or B spectral type. A very high time resolution light curve can also allow us to discover atmospheres in extrasolar planets: from the way in which the light decreases (after removing the phenomenon of limb darkening of the stellar photosphere) at the moment of occultation by another celestial body – linear or exponential and with various slopes – it is possible to trace back to value of the density of that planetary atmosphere, and if this procedure is coupled with spectroscopy, it is also possible to ascertain the chemical composition of that atmosphere from periodic variations in the intensity of some absorption spectral lines.
I just did some simple examples – especially addressed to students in their last year of high school who may wish to enroll in the astronomy degree course – in order to show what an astronomer has in mind while thinking and acting. It is therefore evident that the six fundamental objectives of today’s astrophysicists are all interacting with each other in order to obtain performances that really allow us to advance our knowledge of the physics of the Universe. That is why, before making mathematical models, today first of all we are committed to building increasingly advanced telescopes both on the ground and in space, and related sensors and analyzer devices. Only using measurement instrumentation we can truly understand how stuff works, and this is exactly true also for the investigation of anomalous luminous atmospheric phenomena such as the Hessdalen one, for example. Major projects are in preparation, with short-term achievements expected, such as the European Extra Large Telescope (E-ELT) optical and infrared telescope with 39 meters of aperture to be placed in Chile at ESO (with a powerful contribution from Italy ), the new optical James Webb Space Telescope (JWST), the newest radio telescopes operating with the SKA (Square Kilometer Array) technique and new space telescopes in all the other wavelength bands that will replace or join the glorious IUE, Einsten, Rosat, Spitzer, Newton, Chandra, Beppo-Sax and others. Astrophysics is a wonderful and compelling subject, and those with physical and mathematical skills, a great passion and a desire to work even on weekends will find it a source of great fun.
Well … this is a short article typically signed by Dr. Jekyll during the day, while at night he turns into Mr. Hide who, although totally and always in solidarity with Dr. Jekyll, wants to push science even further.