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Mirages in the Universe INTERNATIONAL SPACE SCIENCE INSTITUTE SPATIUM Published by the Association Pro ISSI No. 32, November 2013 Editorial When it comes to astrophysics, one blur reality, they have evolved in Impressum can’t go past Albert Einstein. His the hands of astrophysicists into a overwhelming intellectual author- powerful tool to detect the pres- ity endowed science with a series ence and distribution of dark of landmarks, of which the Gen- matter or to estimate the age of eral Theory of Relativity perhaps the Universe, just to name two SPATIUM stands out most. Upon publishing notable examples. Published by the the Special Theory of Relativity in Association Pro ISSI 1905 he embarked on an eight-year All those exciting topics made up search for a relativistic theory of the milestones of the talk on gravity. After numerous detours Mirages in the Universe by Profes- and false starts, his work culmi- sor Georges Meylan, Director of nated in the presentation to the the Laboratory of Astrophysics, Association Pro ISSI Prussian Academy of Science in École Polytechnique Fédérale de Hallerstrasse 6, CH-3012 Bern November 1915 of what are now Lausanne for the Pro ISSI audience Phone +41 (0)31 631 48 96 known as the Einstein field equa- on 22 March 2012. It is with great see tions. These equations specify how pleasure that we publish herewith www.issibern.ch/pro-issi.html the geometry of space and time is an issue of Spatium entirely de- for the whole Spatium series ruled by matter and gravity, and voted to one of astronomy’s most form the core of his General The- intriguing aspects, the mirages in President ory of Relativity. the Universe. Prof. Nicolas Thomas, University of Bern Even though the theory is clearly Hansjörg Schlaepfer superior to Newtonian gravity, Brissago, November 2013 Layout and Publisher General Relativity remained some- Dr. Hansjörg Schlaepfer thing of a curiosity among physi- CH-6614 Brissago cal theories for several decades. This didn’t change even when the Printing deflection of starlight by the Sun, Stämpfli Publikationen AG as predicted by General Relativity, CH-3001 Bern was observed by Sir Arthur Stan- ley Eddington on an expedition during the total solar eclipse of 29 May 1919: Einstein became in- stantly famous, while his theory re- mained a Sleeping Beauty. This did change, however, in the 1960’s and 1970’s when ever more precise solar system tests confirmed its predictive power. One of the theory’s most popular forecasts re- fers to gravitational lensing, the effect observed by Eddington in 1919. Even though gravitational lenses tend to create mirages that SPATIUM 32 2 Mirages in the Universe1 by Prof. Georges Meylan, Laboratory of Astrophysics, École Polytechnique Fédérale de Lausanne (EPFL) Introduction Mirages in the Universe? Well, we have heard about mirages on Earth, such as the Fata Morgana, the flickering apparitions in the hot desert. But mirages in the Uni- verse? Do such elusory features ex- Fig. 1 (left): The Moon setting in Fig. 2 (right): The image of the set- ist in space too, and if this is the Casco Bay in Maine. The Moon’s shape ting Sun is perturbed in a similar way. is disturbed near the horizon as a con- This image displays even the elusive case, how do they form, and fur- sequence of refraction effects by the green flash at the top of the solar disk. ther: are they more than just an Earth’s atmosphere. The image of a (Credit: Karina Hamalainen) exotic oddity amusing astro - second Moon rises slowly towards the physicists? first, both eventually merging during setting. (Credit: John Stetson). The answer is yes, of course, and the present issue of Spatium aims age of a second Sun or Moon even- duced by the presence of huge ag- at providing our readers with an tually merging with the first during gregations of mass. What makes insight into this fascinating field of setting. This may lead to the fancy them most valuable for scientists is astronomy. In the framework of Etruscan Vase moonset as shown in the fact that not only the mass of his General Relativity Theory in Fig. 1 and the colourful sunset of visible matter, but also that of in- the mid-1910’s Albert Einstein2 Fig.. 2 visible dark matter gives rise to postulated the existence of mirages mirages. Hence, mirages help open long before they were actually ob- In the Universe, other effects give the window towards the dark served. Today, scientists do not rise to mirages. Here, it is the cur- Universe. only enjoy their beauty but also vature of space-time, a phenome- have learned to exploit their scien- non described first by Albert Ein- Fig. 3: The Horseshoe Einstein ring. tific potential as a powerful tool to stein, that distorts our vision. This The gravity of the luminous yellow galaxy in the foreground has distorted observe the Universe. First, let us may take such fascinating forms as the light coming from a much more dis- contemplate some mirages we are the famous Einstein ring of which tant blue galaxy. In this case, the align- all familiar with. an example is shown in Fig.. 3 The ment is so precise that the background amazing blue ring is not a real galaxy is imaged as a nearly perfect ring or a horseshoe. As such lensing effects On Earth, mirages may appear physical entity in space, but rather were predicted by Albert Einstein, rings when light is bent by layers of air the image of one single object be- like this are now known as Einstein with different temperatures over hind the yellow galaxy in the fore- Rings. (Credit: NASA/ESA) land or over the sea. Light propa- ground. Its enormous mass causes gates in air with a speed that de- the light rays from the blue back- pends on the air density and hence ground object to be bent in such a its temperature. Warm air imme- way as to form a regular, nearly diately above the ocean or the perfect ring around the galaxy. terrain may cause the Sun or Moon’s rays to be mirrored up- We conclude that mirages exist in wards producing the inverted im- the Universe and that they are in- 1 The present issue of Spatium reports on the lecture given by Prof. Meylan for the Pro ISSI Association on 22 March 2012. The notes were taken by Dr. Hansjörg Schlaepfer. 2 Albert Einstein, 1879, Ulm – 1955, Princeton, New Jersey, Swiss-US physicist, Nobel prize laureate in physics, 1922. SPATIUM 32 3 Stars come also in entire families The Mechanism as in the case of the cluster of stars of Mirages of the Pleiades that contain a few thousand stars, Fig. 5. As this is a young cluster of stars, there is still much remaining gas in between. Let us now come to the facts that help us to understand the physical Other stars come in entire globu- processes that create mirages in the lar clusters that contain not a few Universe and to appreciate their thousand but rather a few million value for astrophysicists. To this Fig. 4: Beta Albireo. The two bright stars, such as Omega Centauri, end, we begin with enumerating stars of Albireo in the Constellation of Fig. 6. the Swan (Cygnus) are some 380 light- the forms and types of matter in years away. It is the fifth brightest star the Universe. system in the sky and easily visible to Much visible matter in the Uni- the unaided eye. They circle around verse is also contained in dust and each other in about 75,000 years. gas in large picturesque structures, (Credit: Giuseppe Donatiello) Matter in Space such as for instance the Orion neb- ula, Fig. 7 on the opposite page. From our everyday experience, we axies. A good example of a star is know what astrophysicists call (or- of course our Sun, but also Beta Fig. 7: The Great Orion Nebula is an immense, nearby star-birth region, lo- dinary) baryonic matter. It consti- Albireo in the constellation of cated in the same spiral arm of our tutes the visible performers of the Cygnus, see Fig.. 4 It is actually a Galaxy as the Sun. The various colours theatre of the Universe. Those are double star where the two partners highlight the emission of different gases. the stars with their planets, clusters have different temperatures and The nebula contains many stellar nurseries where gas and dust are cur- of stars, galaxies with a lot of gas hence different colours in the rently forming hot young stars3. (Credit: and dust and entire clusters of gal- visible. NASA/ESA) Fig. 5: The Pleiades can be seen with the naked eye in the Fig. 6: Omega Centauri: the globular star cluster NGC 5139 Orion Constellation. It is one of the brightest clusters in the is over 10 billion years old. It holds nearly than ten million sky counting over 3,000 stars. Huge clouds of dust and gas stars in a sphere some 150 light-years in diameter. (Credit: surround the stars constituting the raw material that eventu- ESA/NASA) ally will build up new stars. (Credit: NASA/ESA) 3 See Spatium no. 6: From Dust to Planets by Willy Benz, October 2000. SPATIUM 32 4 Then, we have galaxies of all types: came aware of this problem and Fig. 9: Andromeda: Our nearest mixed forms like the stunning was among the first to stipulate the large galactic neighbour. Two ESA 5 observatories have combined forces to Sombrero M104 (Fig. 8) or beauti- existence of dark matter , a strange show the Andromeda Galaxy in a new ful spiral galaxies, like Andromeda form of matter that cannot be seen light.
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