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.

nHI_alt_skyview_big

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.

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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.

M17-HST-Subaru-LLL

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.

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

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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.

References:

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

http://www.upv.es/satelite/trabajos/pracGrupo11/radio/

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.

eso1336a

Credit: ESO

References:

http://www.eso.org/public/spain/news/eso1336/

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