(Inter)stellar chemistry

When we want to know about what a material is made of, the first thing we need is, obviously, to have a certain amount of the material we want to study. Once we get it, we go to that area of the scientific knowledge, often hated by students, which is chemistry. Chemistry, as Linus Pauling defined it, is the science that study substances, its structure, its properties and the reactions that transform them in other substances.

Within chemistry, there is an area in charge of telling us what the chemical composition of the substance we want to study is:  the analytical chemistry. To achieve its objective, the analytical chemistry uses a set of methods that depending on its nature can be pure chemical methods, based on the reactions that a substance has in the presence of other ones, or physical chemistry methods, which depend on how some substances physically interact with other ones.

But, how do we do to study the chemical composition of something that we do not have any amount at hand?. This is the situation that appears, without exception, when we want to know the composition of stars or the interstellar medium. It could seem to be impossible but it is true that we know it better and better. As a proof of that, it has been recently searched for ethanethiol in the Kleinmann-Low region inside the Orion nebula.



Kleinmann-Low region in the Orion nebula (Source: NASA APOD, CISCO, Subaru 8.3 m telescope, NAOJ)

Then, how do we do it? We need to go to different areas of science, among which we have chemistry (analytical chemistry), astrophysics and astronomy (concretely astronomical instrumentation)

We mentioned before that the analytical chemistry is in charge of studying the chemical composition and for that purpose it uses different methods. One of them is the spectrometric method, which consists in studying the interaction of the electromagnetic radiation (in every wavelength of the electromagnetic spectrum) with the matter it impacts. Spectrometry uses spectrometers (also known as spectroscopes or spectrographs) which are devices that separate de light in the wavelengths that is made of. The most simple spectroscope (in which functional principle are based all of them) is a simple prism. Using a prism, Newton managed to split the white light coming from the sun in the colors it was made of, getting in such a way the first spectrum in history. However, it was Kirchhoff and Bunsen who invented the first spectroscope by adding a graduated scale allowing them to identify the wavelength of the spectral lines observed when the light passed through the prism. When the image is registered on a device, either electronic or a simple photographic plate, we used to speak about a spectrograph.

Chemical substances can be found in the form of atoms or molecules. In the first one, individual atoms are not bounded to other atoms. In the second one, atoms, which can be of the same kind or different, are bonded amongst them through chemical bonds giving rise to molecules. The electrons that are inside atoms, or that are bond to make up a molecule, can be in different energy states. If there is not any radiation impacting on them (whatever the wavelength), electrons are in their lowest energy state. When radiation impact on them, the electrons jump to a higher energy state. However, due to the electrons tend to be in their lowest energy state, once the radiation stops impacting on them they return to their lowest energy state and, for it, they have to get rid of the energy excess provided by the incoming radiation, emitting the excess of energy in the form of radiation. The difference between the energy of the initial and final energy states tells us the wavelength of the emitted radiation.

Each atom or molecule has different energy states that are characteristic and, therefore, depending on the incoming energy, the transitions between energy states will be different and thus the emitted radiation will be different too. These energy states can be of different types and include vibrational states (due to the vibration of the atom or molecule) or rotational states (due to the rotation of the atom or molecule). On another hand, each energy state cannot be a single one, but can be split in different states in presence of, for instance, a magnetic field, thus enabling additional transitions and the possibility of the emission of the excess of energy in additional wavelengths.



Transitions between energy states that give rise to spectra (Source: monografías.com)

Analytical chemists use, amongst other methods, the spectrometric method to study the structure of these atoms or molecules and their interaction with radiation. Because each atom and molecule has a specific structure of energy levels different to the rest, and that it interacts with radiation in a specific way depending on the type of radiation (and the environmental conditions, such as the presence of magnetic fields), it can be created a catalogue of spectra, so that the next time we see the same spectrum in other place, we could identify the substance we are dealing with.

Some of the places where we can find spectra are stars and the interstellar medium. The problem we have is that we cannot directly access to the star, gather some amount of matter and take it back the laboratory to study it. What we can do is to use our optical telescopes or radio telescopes, to equip them with spectrographs to give us the chance to observe the spectra, with sensors appropriate for the type of radiation (wavelength) we want to observe and point them towards the region of the sky we want to study. The analysis of the spectra through its comparison with the spectra obtained in the laboratory could give us the chemical composition of our object under study. And not only that, we are going to be able to get much more information such as the rotation and translation speeds that have the object (through the measure of the Doppler effect, because the spectral lines will appear displaced, with regards to its position in the laboratory, to the spectrum part where there are longer or shorter wavelengths depending on whether the object is approaching to or recessing from us) or even the intensity of the magnetic field that could be in the region we are looking at.

As always, reality is always much more complex, but we can always rely on the human will and the capacity that scientists have to look for solutions to the problems that the universe presents. And as it has been seen, what for me is the most important thing, we can rely on the collaboration between different scientific areas in the search for those solutions, appearing, in some cases, new scientific fields as result of such collaboration. This is the case of Astrochemistry, which is basically the scientific area that has been addressed in this post.



Stars’ home

A few weeks ago we talked about radio telescopes and said that they were very important for the study of different astrophysical phenomena. Today, we are going to talk about the interstellar medium and will see that, in some cases, radio telescopes are useful to study it.

When we look at the sky, we see with a naked eye a lot of stars and still there are many more. In some cases, we can distinguish other objects that have a magnitude enough to see them with a naked eye, like nebulae or galaxies, but if we look at them without knowing what we are looking at, we may mix them up with unremarkable stars due to we cannot distinguish their shape and extension. However when we look at a region where we don’t see anything, we may think that it is an empty region, but actually it is not empty at all.

Between stars, it exists what it is called the interstellar medium and, although we don’t see it, it is impressive and deserves being studied because of what it implies: it is the place where the stars are born.

Interstellar medium is primarily made of gas, concretely hydrogen gas which is the main component; although it also contains traces of other “heavier” chemical components like helium, carbon, nitrogen or oxygen among others, which are in very small quantities. The reason why these heavier elements exist is that the interstellar medium is not only the place where stars are born, but also the place where they die. When a star evolves, it generates heavier elements in its interior through nuclear fusion processes. When the star dies, like in the case of a supernova, it spreads these elements that are incorporated to the interstellar medium.

The hydrogen we find in the interstellar medium can be in three different states: neutral hydrogen or HI, molecular hydrogen or H2 and ionized hydrogen or HII. To understand these three states, we have to know that hydrogen is the simplest atom because it only has a nucleus made of one proton and an electron bound to it. When hydrogen has this simple structure it is called neutral hydrogen and when it is ionized, that is, when the atom has been given enough energy to provoke that the electron is released from the electrical attraction of the proton is called ionized hydrogen. The third state, the molecular hydrogen, is formed when two hydrogen atoms are bound sharing their respective electrons.

The presence and abundance of these states determines the existence of three types of regions, which are named as: atomic gas regions or HI regions, molecular gas regions or H2 regions and ionized gas regions or HII regions.

HI regions are very cold areas (with minimum temperatures around 30K) which are studied using the 21 cm line of the electromagnetic spectrum which is the range of radio wavelengths, and thus studied using radio telescopes. There can be regions in the sky where, when observing them in visible wavelengths, we don’t see anything, but if we observe them in the 21 cm line, we see that wherever we point the radio telescope we will always detect a signal.

This signal corresponds with a photon emitted when the spins of the electrons and protons are return to a state where they are not aligned after having been aligned, for instance, because of a collision between atoms. The fact that this line can be observed, regardless of the direction we observe, is a proof that atomic hydrogen is everywhere.

We can also use the Doppler Effect to determine how HI regions move. If the 21 cm line is shifted to the part of the spectrum where there are longer wavelengths, it means that the region is approaching us and if it is shifted to the shorter wavelengths part, it means that it is moving away from us. These observations provide us with information, for example, about the rotation of the Galaxy around its centre.


The sky in the 21cm line (Source: NASA APOD. Credits: J. Dickey (UMn), F. Lockman (NRAO), SkyView)

Also, when we observe the sky in the visible part, we see that there are areas densely populated with stars, but between them it seems that there are empty spaces and areas completely dark. These regions are actually molecular hydrogen and dust clouds, being the dust the responsible for the darkness. Molecular hydrogen regions are even colder than HI regions (around a minimum temperature of 10 K, but more dense). These regions are very important because it is inside them where the stars are born. Sadly, there is not a specific line, as in the case of HI region, to observe them. In fact, it is quite difficult to observe molecular hydrogen because H2 is a molecule without a dipolar moment and does not present lines similar to the 21 cm line (concretely it does not present rotational lines. It does have vibrational lines, but it is necessary a very high energy to produce transitions that generate these lines. These conditions are not present in every part of the cloud, only in the proximity of stars being formed which provides very little information about the rest of the cloud).

If the dust prevents us to observe in the visible part of the spectrum and there is not a clear line to observe in the radio wavelengths, how can we observe these regions?

As we have mentioned before, there other heavier elements in the interstellar medium and these elements form molecules that, although its abundance is lower, they let us observe the interior of these clouds. One of these molecules is the Carbon monoxide (CO) which has a net dipolar moment and thus emits rotational lines that can be observed using radio telescopes. Ammonia (NH3) also helps us to look at the inside of these clouds. In this way we can study the environment where the stars are born through its density and temperature, for example.


Barnard 68. A molecular cloud (Source: NASA APOD. Credits. FORS Team, 8.2-meter VLT Antu, ESO)

When stars, in the interior of molecular clouds, are being formed, they are young and with a lot of energy and thus they emit very high energy radiation (in the range of ultraviolet wavelengths) what ionizes the hydrogen in the clouds and becomes HII. HII regions are thus very hot. Even though, the interstellar dust does not permit us to observe the interior of the clouds and we have to, again, use radio telescopes. The radiation these brand new stars emits provokes, in the HII regions, the generation of bremsstrahlung radiation (braking radiation). This radiation appears when an electron approaches an atom of ionized hydrogen which makes that the electron is deviated from its trajectory emitting a radiation that, because there is a lot of electron approaching a lot of protons at different distances, makes to appear a continuous spectrum of radiation. In this case the bremsstrahlung radiation is studied in the range of X-Rays and thus we do not use radio telescopes to study it, however combining the X-rays information with the one in X-Rays when studying molecular clouds, the information obtained is very valuable.


Messier 17 or Omega Nebulae. An HII region (Source: NASA APOD. Crédits: Subaru Telescope (NAOJ), Hubble Space Telescope, Color data: Wolfgang Promper, Processing: Robert Gendler)

As we have seen, there are many things that our eyes cannot see with a naked eye or even using conventional telescopes. The interstellar medium is, in many aspects, an unsolved mystery. Either using radio telescopes or any other type of detectors, we still have a long way to go. Meanwhile we can enjoy some of the beautiful images that other telescopes have gathered throughout the years, like one of my favorites The Orion Nebula.


M42 or Orion Nebula (Source: NASA APOD. Credits: NASA, ESA, M. Robberto (STScI/ESA) et al.)

Antennae to observe the Universe

Who has not looked at the sky in a clear sky night, in a place apart from the city lights and has not asked himself if there is anything more beautiful than a starry sky? It is almost sure that all of us who decided to study the universe started in this way and, in fact, besides being delighted with the beauty of the sky, we started to wonder why everything was like it was and not in another way.

Also, it is almost sure, that we all started to ask our parents for a telescope, that it was always smaller than the one we finally got.

Later on, besides the telescope, we wanted books about how to observe the sky, what objects could be observed, when to observe them and that, in addition they had cool pictures of galaxies, nebulae, globular clusters, and all in all, any object out there.

In those books there were also pictures of telescopes, and why not, it is almost sure that we wanted them all, refractors, reflectors…, but for many of us, the surprise was to learn that there were telescopes without any hole to look through. In fact, they did not look like conventional telescopes. They were similar to the parabolic antennae that some people had to watch several television channels. What was that? Was it possible to observe the Universe with those antennae?

These antennae are actually radio telescopes, and yes, the Universe can be observed with them. In fact, it is a must to observe the Universe with them.

 Yebes_40m40m radio telescope of the IGN in Yebes (Source: IGN)

Conventional telescopes, with a hole to look through, usually observe the Universe in the visible part of the spectrum. Of all the electromagnetic spectrum, they can only see the wavelengths corresponding to the visible light, which are the same as the ones we can see with our eyes. However, radio telescopes are capable of detecting other wavelengths, longer than those of the conventional telescopes. These wavelengths are in the radio part of the spectrum.

A radio telescope is, in general terms, a large parabolic surface (paraboloid of revolution) which acts as a radio waves collector. Having a parabolic form, the incoming waves are reflected by the surface and concentrated in point known as primary focus. In this point, two things can happen. One is that in this point there is a receptor in charge of transmitting the reflected radiation to the instruments that will measure it. The other one is that in the primary focus there is a sub reflector that reflects the radiation to a receptor located in the collector and, from there, transmit it to the instruments. Both options are feasible, but the second one permits accessing to the receptor when it is needed to perform maintenance tasks and it also allows the receptor to be heavier.

 EsquemaElements of a radio telescope (Source: Wikipedia Commons)

Radio telescopes are antennae that can be very large, reaching diameters of 100 m or even more like the 300 m of the Arecibo radio telescope. The size impacts on the resolution of the information gathered. The larger is the size, the larger the resolution. The main problem is that it is practically impossible to build antennae with several kilometres to get a large resolution. This does not mean that the smaller radio telescopes, 100 meters and below, are useless because of the lack of sufficient resolution. Several important discoveries have been made using these radio telescopes. But as any other scientist, astronomers and astrophysicists always want more, especially when for each answer new questions arise.

 Effelsberg_total2100 m radio telescope of the Max Planck Institute in Effelsberg (Source: Wikipedia Commons)

 Arecibo_Observatory_Aerial_View305 m radio telescope in Arecibo (Source: Wikipedia Commons)

To give an answer to these new questions, not only larger radio telescopes are built, but several smaller radio telescopes are built and connected amongst themselves, either in a physical manner so that the radiation gathered by all of them are sent to the same analysis center at the moment of its reception or “virtually” so that each radio telescope gathers its own information and later sends it to other remote centers where it is analyzed together with the information gathered by other radio telescopes.

This is possible by using interferometry techniques. Interferometry consists in combining the radiation gathered by several sources (several radio telescopes) in a way that the resolution of the information being received is increased. Interferometry it is based on the fact that radiation is an electromagnetic wave. To understand what interferometry is, let’s talk about a classical experiment in the history of Physics: the double slit experiment.

When a source of light is located in front of a screen, and between them it is placed a plate which does not let the light go through it but with two thin slits drilled on it, the light when it goes through both slits it is diffracted and follows different paths. When the light impacts on the screen, the diffracted light coming from each slit interferes because it comes from different directions and at different moments. This interference makes that dark lines can be observed where the light has destructively interfered and bright lines where the light has constructively interfered. It can be also seen that when the source of light is a point (a small source) the contrast between bright and dark lines is bigger and when the light source is wide, the contrast is diffuse.

 quantum-double-slitDouble slit experiment (Source: Wikipedia Commons)

Interferometry using radio telescopes follows the same principle. Radio waves arrive to the radio telescopes that are separated a certain distance at different moments (the time difference is very small but noticeable with precise systems for the measure of time). This makes that the signal of the radio waves measured by all of the radio telescopes follows generates an interference pattern. By studying the pattern and the contrast between bright and dark signals measured, the form and characteristics of the source of radio waves can be reconstructed.

ALMAArtist rendering of ALMA (Atacama Large Millimeter/submillimeter Array) for long baseline interferometry (Source: ESA)

Using interferometry techniques we can increase the resolution of the image because, although we have small radio telescopes separated a few meters or kilometers away in the case they are physically connected (also known as long baseline interferometry) or several kilometers in the case they are “virtually” connected (also known as very long baseline interferometry), the final outcome will be as if we had a radio telescope of the size of the maximum separation of the smaller radio telescope. This technique can be used even with radio telescopes in orbit.

Radio telescopes are very useful to study different phenomena occurring in the Universe that we cannot observe with conventional telescopes as the birth of stars as well as the interstellar medium where stars are born. How radio telescopes are used and the physics behind the phenomena they observe will be explained in another post.


PARTNeR Project: http://partner.cab.inta-csic.es/


The beginning of the research on cosmic rays

Everything in life has a beginning and science, and all of its areas, has a beginning as well. This is the case of cosmic rays research too, but what are cosmic rays? In a simply way, they are subatomic particles, smaller than atoms such as their constituents like protons, that come from the outer space moving at speeds close to the speed of light

Victor Hess

Victor Hess

While studying radioactivity at the beginning of the 20th century, it was found that when an electroscope, that is a devise to determine whether body has electrical charge and its sign (positive or negative), was put close to a radioactive source, the air was ionized, that is that the atoms and molecules of the air were charged electrically. If the electroscope was put far from the radioactive source, it was fount that the air was also ionized, therefore it was thought that it was due to the existence of natural radioactive sources in the surface or the interior of the earth ant that this ionization should decrease at higher altitudes.


How an electroscope works

In 1910, Austrian physicist Victor Hess, climbed up the Eiffel tower in Paris with an electroscope in order to try to determine at what altitude ionization was negligible or non-existent. The result was amazing, because instead of decreasing, ionization increased with altitude. As with any other scientific result, that has to be supported with multiple evidences and various experiments repeated, when possible, in different conditions, Hess repeated its experiment but an altitude of 5000 m! For it, in 1912 he used a hot air balloon but this time with an ionization chamber.

An ionization chamber basically is an instrument with a gas inside between two metallic plates, which are applied a voltage. When the gas inside the instrument is hit by, for instance, a cosmic ray, the ions generated inside the gas move towards the metallic plates because of the voltage in a way that an electrical current, that can be measured, is generated.

The results that Hess obtained were the same as those in the Eiffel tower, thus he arrived to the conclusion that the radiation causing the ionization of the air was not coming from the ground but from above. The name of cosmic rays, is not from those days but from 1932 when Robert Millikan named in this way the radiation coming from the outer space as he thought that they were gamma rays, the most penetrating electromagnetic radiation known to date, although later was discovered that it was not electromagnetic radiation but mostly particles with mass.

From Hess’ discovery, the history of cosmic rays has advanced a lot until nowadays.

Dimitri Skobelzyn used the cloud chamber to detect the first traces of the products of cosmic rays in 1929, as well as Carl Anderson did in 1932 to discover the positron, which is the electron anti-particle (but it is not the proton, because, although it has positive charge, it is 2000 times heavier than the electron).

Later in 1938, Pierre Auger, having placed detectors in various distant points in the Alps, detected that the arrival of particles in both detectors was simultaneous, so he found that the impact of high energy particles in the higher layers of the atmosphere generated secondary particle showers.

particle shower

Secondary particle shower generated in the atmosphere by the impact of a cosmic ray

Currently the detectors used to study cosmic rays are more sophisticated and, because the intensity of the particles coming from space is higher at higher altitudes, they are located in mountains and elevated areas as it is the case of the Pierre Auger observatory in the Pampa Amarilla in Argentina with an average altitude over the mean sea level of 1400 m or the MAGIC experiment in the Roque de los Muchachos observatory in the Palma Island of the Canary Islands (Spain).


Telescopios del Experimento MAGIC en el Roque de los Muchachos

Our detectors are even in the space like the Alpha Magnetic Spectrometer, also known as AMS-02 installed in the International Space Station whose objective is to measure the antimatter of cosmic rays to search for eviden of dark matter.


Arqueros F. Rayos Cósmicos: Las Partículas más Energéticas de la Naturaleza. Revista “A Distancia (UNED), 1994.



Galaxies, distances and the expansion of the Universe

When we look at the sky during a dark night, far from the city lights, we can see so many stars that we can feel overwhelmed because of the amount of stars. When we look towards specific areas we can see an almost continuous belt of dust, similar to the trace that someone leaves when a spills a bottle of milk. This trace of milk is our Galaxy, the Milky Way. However, the Milky Way does not cover everything that exists, the Universe extends beyond our galactic home.

The Milky Way is one in the hundreds of thousands of millions of galaxies that share the Universe with us, each one being an enormous set of stellar systems on their own.

From the Earth, to the naked eye and depending on the region of the sky we are observing, we can easily see three galaxies. It is the Local Group, which includes the Milky Way the Large Magellanic Cloud, and the Small Magellanic Cloud. The third member of the trio is the Andromeda galaxy in the homonymous constellation.

Imagen guardada con los ajustes integrados.

Magellanic Clouds and the Milky Way

In the 18th century, the French Charles Messier, who was a comet hunter, scrutinized the sky with his telescope (less than 20 cm of diameter) until he saw a blurry spot. When he found any, he took note of the position in a stellar map. The night after he aimed his telescope to the same point to see whether the spot was still there. If the spot had moved from there, it was a comet if not it was other thing. By that time, those spots were known as nebulae, latin word meaning ‘mist’ or ‘cloud’. In 1774, Messier had catalogued 45 nebulae together with their celestial coordinates and in 1784 the catalogue included 103 objects already.

A German born musician, William Herschel, who worked in the second half of his life in the construction of large telescopes together with his sister Caroline aimed his instruments towards the objects discovered by Messier, and given the ‘power’ of his telescope (4 times bigger than the one of Messier) discovered in seven years more than 2000 objects.

With this catalogue, Herschel tried to make a celestial map including all these objects. From the study of the nebulae, Herschel proposed that if the Milky Way was observed from a distance far enough it would look like a nebula itself.

Because of the power of Herschel telescope, he could resolve some of the blurry spots into globular clusters. In the decade of 1840, William Parsons started to build a telescope 16 m long with a mirror of 2 m of diameter, which was bigger than the one of Herschel.

Parsons aimed his telescope towards one of the objects of the Messier catalogue, concretely M51, and his surprise was immense when he saw a spiral structure, that later was named as the Whirpool galaxy because of this characteristic. He couldn’t find individual stars in it but discovered other nebulae with the same spiral structure.

Processed with MaxIm DL

M51. The Whirpool galaxy

At this point, the question arose: did these nebulae belong to the Milky Way? To answer it, it was necessary to know the size of the Milky Way and the distance to the nebulae.

Shortly before this discover, astronomers knew already the parallax method to measure the distance to close stars, but due to the large distances of the nebulae, this method was useless. After the initial development of methods in spectroscopy the English astronomer William Huggins aimed, in 1867, his telescope equipped with a spectroscope to the brightest star in the night sky, and applying the Doppler effect theory, developed by the Austrian Christian Doppler 20 years before, he found a slight red shift in the spectrum of the star. He calculated that Sirius was moving away from us at a speed of 50 km/s in the line of sight. In the same way, he calculated the speed of a large number of stars. It was only the beginning of the use of the Doppler effect technique in astronomy. Some years later it was found a way to use this method to calculate the distance of stars.

In the early 20th century, the observatory of the Harvard College was making tedious stellar observations from photographic and spectroscopic plates. Women who were deemed by that time appropriate for the tedious and repetitive work, showing machismo behaviour, did the measures and calculations earning less money than men. Several of these women made important contributions but, among all, stressed Henrietta Swan Leavitt.


Henrietta Swan Leavitt

In a number of photographic plates of the Small Magellanic Cloud, Leavitt observed multitude of stars, which changed their brightness periodically because they ‘pulse’, i.e. they expand and shrink regularly. These stars are known as Cepheid variables because the firsts Cepheid discovered is in the homonymous constellation.

Leavitt compiled more than one thousand of Cepheid in the Small Magellanic Cloud and, at least, 16 of them appeared in several photographic plates what enabled her calculating their periods.

She found that the stars were brighter when their periods where longer and established that the period and the brightness were related and that it was possible to graphically show the relation between the period and the luminosity, i.e. Leavitt had found a relation between the apparent magnitude of a variable star with a measure that was independent of the distance to the star: the change in the brightness. Leavitt had discovered a connexion between the period and the absolute magnitude, i.e. its actual magnitude.

Being these stars in the same region of the Small Magellanic Cloud, it could be assumed that all were almost at the same distance from Earth.

The difference between the absolute magnitude in the Cepheid of the Small Magellanic Cloud and their apparent magnitude could be then used to calculate the distance to the star using the inverse squared law: A star, as any other light source, will only show a quarter of its brightness if the distance to the observer is doubled, a sixteenth if the distance is increased fourfold, etcetera.

Since the relation discovered by Leavitt is applied to the Cepheid in general, the fact that determining the absolute magnitude would enable estimate the absolute magnitude of the others and it could be used the period-luminosity scale to find the absolute magnitude of any Cepheid variable star, and thus the distance to the star.

The problem was how to create a distances pattern from the behaviour of Cepheid because the closer Cepheid was farther enough so as to measure its distance using the parallax method.

Leavitt was set aside from the tasks she was doing because the boss in the observatory thought that her job was to gather data and not making calculations, but Ejnar Hertzsprung, in the observatory close to Berlin, took charge of it.

Hertzsprung studied the proper motion of stars, the motion in the space of the star and our Sun, of thirteen Cepheid close to the Sun, and using statistics calculated the ‘average’ distance to the local Cepheid, as well as an ‘average’ apparent magnitude. With these values he was able to calculate an ‘average’ absolute magnitude for an ‘average’ period Cepheid.

Perhaps there were too many ‘averages’, but what Hertzsprung did next was to choose a Cepheid in the Small Magellanic Cloud with the same period as his ‘average’ star. He compared the photographic brightness of the Cepheid in the Cloud with absolute magnitude it should have and calculated the distance: 3000 light-years. This distance put the Small Magellanic Cloud within the Milky Way. It is thought that it was a typo and that the distance should have been 30000 light-years. Even though the distance was well below the actual one.

Why this discrepancy? Actually it was an experimental error. The Cepheid in the Small Magellanic Cloud had been photographed using plates sensitive to the blue light while the local Cepheid had been photographed using plates sensitive to red light. This provoked a difference in the apparent brightness making that the Cepheid in the Small Magellanic Cloud look brighter and thus closer.

The North American astronomer Harlow Shapley knew how to understand the astronomical meaning of Cepheid. Working in the Mount Wilson observatory in Los Angeles, with the 1.5 m telescope, Shapley studied the globular clusters and discovered Cepheid in them. Using the Hertzsprung’s technique, and refining it, he determined the distance to the clusters, being between 50000 and 220000 light-years. It was thought that the clusters were within the Milky Way, but it also was thought that the Milky Way had a diameter of 30000 light-years so the real diameter should be larger than thought. Shapley estimated a diameter of the Milky Way of 300000 light-years being the galactic centre in the direction of Sagittarius.

Astronomers were cautious about this result, in part because they considered as unreliable the Hertzsprung’s method.

At the same time, telescopes were aiming at spiral nebulae and many astronomers suggested that they were galaxies comparable to the Milky Way full of stars because, when the light was passed through a spectroscope it was similar to the light of the stars and not to the one of a gas cloud.

In 1912, Vesto Slipher, in the Lowell observatory, took a detail look at a spiral galaxy in the Andromeda constellation and could measure its Doppler shift. The value he found impressed everybody: it was approaching at a speed of 300 km/s. Later, Slipher observed 15 more spiral galaxies and discovered that 13 of them were moving away from the Earth even faster than the approaching speed of Andromeda.


M31. Andromeda galaxy

In 1919, after having received education as lawyer and after getting a Ph.D. in astronomy and return from war, Edwin Hubble started trying to classify nebulae. Using the new 2,5 m telescope in Mount Wilson he hoped to resolve stars in the spiral galaxies, concretely in Andromeda.

Hubble focused his attention in a points of light known as novae, stars that suffer recurrent mass explosions provoking their luminosity to change (don’t mix it up with supernovae where the explosion of the full star occurs)

Through the comparison of photographic plates showing the same region of the sky, what he initially thought to be a nova he later realised that a star was increasing and decreasing its brightness periodically. It wasn’t a nova but a Cepheid!


Plate where Hubble annotated that it was a variable Cepheid and not a nova

Using the Hertzsprung’s technique, as refined by Shapley, he calculated the distance to Andromeda and got a value of 900000 light-years, what was larger that the size of the Milky Way as Shapley calculated. Andromeda was itself a galaxy!

Because he found Cepheid in spiral galaxies, Hubble made the Universe size to considerably increase. Hubble used the Cepheid to develop distance indicators for galaxies in the same way Shapley did for globular clusters.

While this was occurring in Mount Wilson, in Lowell, Slipher was still measuring Doppler shifts for spiral galaxies, including the ones where Hubble used his technique to calculate the distance.

Milton Humason joint Mount Wilson to work as assistant because of his father-in-law. When one night the telescope operator was ill he worked in his position in such a successful way that he was appointed as permanent operator as well as assistant to Hubble permanently. Humason got enough information about additional Doppler shifts from other galaxies. Hubble gathered all these data to establish a relation between the red shifts and the distances. The relation was simple: except for the closest galaxies, the farther a galaxy the faster it moved away. The rate is now known as the Hubble’s constant.

Although the values that Shapley or Hubble found by their time were rough, the precision has been increased now and currently we know that the Milky Way has a diameter of 100000 light-years and that the Andromeda galaxy is 2.5 million light-years away from us. Though the values are slightly different, the important thing is to remember that the efforts to understand the Universe made possible the development of techniques and methods that, even nowadays, are being used by the modern astronomers and astrophysicists.

As Hubble said:

But with increasing distance our knowledge fades, and fades rapidly, until at the last dim horizon we search among ghostly errors of observations for landmarks that are scarcely more substantial. The search will continue. The urge is older than history. It is not satisfied and it will not be suppressed.


Galaxias. Time Life Folio

Astrofísica. Manuel Rego, María José Fernández


ALMA and the birth of a Star

Last week while I was watching the TV news, the journalist called my attention when she said that Chilean researchers had observed, for the first time, the birth of a star. My first impression was, great! Genuine ‘stelar’ news on TV. Later, while I was looking at the video accompanying the news, listening to what the presenter was saying and calming down after my first reaction, the thing changed a bit.

The information was making reference to the results published in a paper in The Astrophysical Journal: ‘ALMA observations of the HH 46/47 Molecular Outflow’ by Hector Arce from Yale University and his colleagues.

ALMA stands for ‘Atacama Large Millimeter/submillimeter Array’. It is a large set of radio telescopes funded and developed by a large collaboration between European, North-American and East Asian research institutions in partnership with the Republic of Chile. ALMA is located 50 km away from San Pedro de Atacama in Chile in a large plateau in Los Andes, because its altitude and meteorological conditions are appropriate for astronomy including astronomy using radio telescopes.

Imagen: ESO/C.Malin

Credit: ESO/C.Malin

The stellar object observed is HH 46/47 located 1400 light years away from us. HH stands for Herbig-Haro object which are nebulae developed in the proximity of new born stars. Young and hot stars eject high speed gas outflows that collide with the gas and interstellar dust existing in region where the star is born. These collisions ionize the gas producing the characteristics emissions of the Herbig-Haro Objects.

The information was not so misguided after all: it was true that it was related to Chilean researchers (at least some of them, including the research leader mentioned above) who used a radio telescope located in Chile and it was related to a stellar birth, but… it was not the first time that a stellar birth had been observed actually. In fact the HH 46/47 object is known since some years and there are several pictures of it. The interesting information derived from the research is the observation with an extraordinary definition, thanks to ALMA capabilities (in only 5 hours of observation time) of the ejected outflows. These observations have enabled, on the one hand, to measure that the gas outflows are more energetic than expected and, on the other hand, that it exists another outflow which had not been previously observed due to it was hidden to the visible wavelengths by the gas and dust where the star is immersed.

Radio telescopes work in longer wavelengths than optical telescopes and these wavelengths are not visible to the human eye. Therefore we had not been able to observe the new gas outflow before.

Although the pictures shown in the news and in several internet websites are conventional photographic images, they are actually a composition of different images gathered in different wavelengths, including radio the maps elaborated based on the information gathered in the radio wavelengths. The images taken by radio telescopes, concretely the one taken by ALMA of HH 46/47, may look like this one:

Mapa de radio HH4647

The fact that these images contain information more useful from a scientific perspective than a simple photograph does not devalue the latter which are really beautiful and blow the mind of the one who looks at it away.

Despite of the lapse of the journalist, probably because it was what she was told to say, it is very nice that between information about the economic crisis and more economic crisis, some scientific information is presented in the news.

I left you with the composed image of the HH 46/47 object, including the information gathered by ALMA. I hope it blows your mind away as it did with mine.


Credit: ESO



Arce, Hector G. et al., ALMA Observations of the HH 46/47 Molecular Outflow. The Astrophysical Journal.